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November 16

"After"-Talk

Is it just a myth or a fact that certain types of bugs can make it possible to listen what was talked in a room upto an even an hour after the talk has ended. I mean the device wasn't there when the conversation was on and installed, say, many minutes after the talkers had left the premises.  Jon Ascton  (talk) 05:13, 16 November 2017 (UTC)

Just a myth. Perhaps someone can link to a reference that debunks this fanciful notion? Dbfirs 08:43, 16 November 2017 (UTC)

(edit conflict) it is hard to debunk the general concept of after-talk-listener - there is no rock-solid physical principle that says you cannot, like it would be the case for a claim that you can listen before the talk happens. But any specific implementation I can imagine would be easily debunked.

For instance, "picking up the attenuated sound waves bouncing off the walls by a strong microphone" is next-to-impossible: (1) since the sentence spoken at t is bouncing off when the sentence at t+Δt is spoken, it will need a whole lot of deconvolution that may or may not be possible and will anyways surely worsen the signal-to-noise ratio; (2) except at resonant frequencies of the room, sound attenuates quite fast (i.e. the Q factor is low) (test: shout at the top of your lungs, and listen if you hear anything a few seconds after you stopped: you don't, which means the decibels drop fairly quick); (3) microphones are not much more sensitive than the human ear and way less complex as far as signal processing go (see e.g. [1], [2]), so if you cannot hear something it is usually a good guess that a microphone next to you cannot either. (I remember someone saying that the acoustic noise generated by air at room temperature was not far below the threshold of human hearing and some people with Hyperacusis could hear it, but I could not track a source to that effect - anyone else can, or is that just another myth?) Tigraan^{Click here to contact me} 09:07, 16 November 2017 (UTC)

Methinks it would require some kind of Echo chamber. But unless it could reverberate for a very long time, the OP's concept wouldn't work. Also, you'd likely have echoes of different parts of the conversation going on all at once, and it would require some tedious work to separate it out. ←Baseball Bugs ^{What's up, Doc?} carrots→ 09:26, 16 November 2017 (UTC)

The events were recorded when they occurred (and could then have been relayed later). There are many ways to record people which they are not necessarily aware of... The belief however reminds me of "wall memory", a spiritualist belief that objects, walls and houses could have memory which they could echo later in particular circumstances to explain paranormal encounters etc. —PaleoNeonate – 10:55, 16 November 2017 (UTC)

The device doesn't have to be inside the room to hear what's going on. See laser microphone. 82.13.208.70 (talk) 11:16, 16 November 2017 (UTC)

Perhaps the users of such devices spread rumours about recording with a device installed long after the event, just to hide how sensitive their real-time devices really are. Dbfirs 12:27, 16 November 2017 (UTC)

Oh, it's a myth, but it would be useful if you could provide a source/link for the original claim. It would be much easier for us to provide sources to debunk a particular and specific assertion, rather than just throwing open the field to try to prove a general negative.

For a thought experiment, though, consider the speed of sound is about 300 meters per second, and a good-sized meeting room might be 10 meters across. In one second, a sound originating from a point in the room will have bounced back and forth off the walls of the room 30 times. (It's even worse if you remember that rooms have ceilings about 3 meters high; that's 100 floor-ceiling bounces per second.) A minute later, a single short, clear sound will have bounced off at least a couple of thousand surfaces, getting spread out, attenuated, and jumbled into a bit of molecular jiggling indistinguishable from heat. A hard surface like concrete, drywall, or glass might reflect as much as 98% ([3]) of the sound energy that hits it back into the room—an echo. If we do the math for 2000 ideal 98% bounces, we get...2*10^-18 times the original intensity. And that's your best-case, because it doesn't account for the presence of soft sound-absorbing objects in the room, like chair cushions, drapes, or people, and it doesn't account for the miserable nuisance of multiple paths interfering with each other.

If I fire a pistol in an office with a closed door, and then open the door a minute later, the guys out in the hall don't hear a 'bang' when the door opens. Forget trying to get information about something like a conversation. TenOfAllTrades(talk) 13:48, 16 November 2017 (UTC)

There's some novel where "the words used by Jesus to raise Lazarus from the dead", I think, are recorded in a ceramic vibrations of a potter's wheel or some such. But sound can be very weak (the ear is said to be able to sense an eardrum motion on the order of a single atom's radius) and these fictional scenarios are usually only that. Although yes, spies can record conversations by bouncing lasers off office tower windows and watching the vibration. You can certainly picture some loopy scenario where such a wiggling of an office tower window reflecting a distant bright object gets recorded on a security camera or something. But the easiest fiction is for a cop to tell someone of average intelligence -- for criminal defendants -- that they have such a device, and that he can get a lesser charge by talking before they have to bring it out to the scene. Wnt (talk) 16:53, 19 November 2017 (UTC)

• I just read of a sci-fi story from the '40's (can't remember if it's a book or movie) where in the future, all prior actions can be reconstructed by trace imprints and vibrations, so the murderer concocts a plot entirely in his head, and befriends his intended victim to allay suspicion. It's killing me I can't think of the name, remember the source, or find it on google. It may have been mentioned in the Oct Discover or Sci Am magazines, which I returned to the library last week. In any case, the idea is nonsense simply based on chaos theory, namely nonlinear feedback and path independence. Most of the relevant info is quickly overwhelmed by signal noise, destroyed by entropy. or simply not recorded in the first place. For example, leave two glasses of water in the fridge an hour apart, then try to find out which one was placed in first the next day? The temperature will give you no clue, and other hints will also very swiftly disappear. μηδείς (talk) 22:06, 19 November 2017 (UTC)

Medeis: ask in Sci Fi Stack Exchange if you want to identify it. – b_jonas 11:39, 24 November 2017 (UTC)

How far from the center has a bound electron of the observable universe ever reached?

Out of zillions of atoms one has had an electron reach the most picometers from the center since the Big Bang. This distance should be estimatable right? Maybe it's "more wrong" to think of electrons this way than as clouds but you're only probabilistically estimating, not observing and changing actual electrons. Sagittarian Milky Way (talk) 08:57, 16 November 2017 (UTC)

Even before you get into quantum weirdness, your question is poorly defined. Say there are only 2 protons and 2 electrons in the universe. If the two electrons are both closer to proton A than to proton B, do you have two hydrogen atoms, or a positive hydrogen ion and a negative hydrogen ion? i.e. when an electron is far enough away from the atom, it ceases to be meaningful to define it as an atom (and that's before you get to issues regarding all electrons being interchangeable, and not having defined positions unless measured). MChesterMC (talk) 09:35, 16 November 2017 (UTC)

The greater issue then is that what you really have is just a set of data describing the location of charge centers in relation to each other and their relative movement. Concepts like "electron" and "proton" and "ion" and "atom" are human-created categorizations to make communicating about complex data like this meaningful to us. We define the difference between an atom and an ion based on our own (arbitrary but useful) distinctions. What makes something a thing is that we set the parameters for that thing. There is no universal definition for that thing outside of human discussion. --Jayron32 11:50, 16 November 2017 (UTC)

Also there is no such thing as the center of the Universe.--Shantavira|^{feed me} 11:14, 16 November 2017 (UTC)

True, but I read it as being from the centre of the atom. My quantum mechanics isn't up to the task, but it should be possible to estimate a probable maximum distance just by multiplying the probability density function from the Schrödinger equation by the number of atoms being considered, perhaps just for hydrogen atoms. Whether such a distance could ever be measured in practice is questionable, but the mathematics based on a simple model should provide a very theoretical answer. Do we have a quantum mechanic able to offer an estimate? Dbfirs 12:22, 16 November 2017 (UTC)

You have to define your probability limits. The maximum distance is infinite if you don't define a limit, like 90% probability, or 99% probability, or 99.99% probability. If you set the probability to 100%, you get a literally infinitely large atom. --Jayron32 12:36, 16 November 2017 (UTC)

Yes, of course, that's what the probability density function gives, but if you find the distance at which the probability is ten to the power of minus eighty, then we have a theoretical figure for the maximum expected distance since there are about ten to the power of eighty hydrogen atoms in the observable universe. Statisticians might be able to refine this estimate, and I agree that it might bear little relevance to the real universe. Dbfirs 12:50, 16 November 2017 (UTC)

In the ground state, the distance you are asking about is ~100 times the Bohr radius of a hydrogen atom. However, in principle there exist an infinite number of potential excited states with progressively increasing orbital sizes. Very large orbitals involve energies very close to but slightly below the ionization energy of the atom. In that case the electron is only very weakly bound. Aside from the general problem that the universe is full of stuff that will interfere, there is no theoretical reason why one couldn't construct a very lightly bound state with an arbitrarily large size. Dragons flight (talk) 14:12, 16 November 2017 (UTC)

The important thing to remember here is that energies are quantized but distances are not. This is all what the uncertainty principle is about. You can't handwave some "yeahbuts" around; the position of electrons with a well-defined momentum are fundamentally unknowable which means that the chance of finding that electron at any arbitrary point in the universe is not zero. In a single-hydrogen-atom universe, we can construct a hydrogen atom of any arbitrarily large radius by asymptotically approaching the ionization energy of the atom (this is akin to the escape velocity in a gravitationally bound system). As the energy level of an electron approaches asymptotically close to the ionization energy, the radius of that atom tends towards infinity. Well, sort of. The radius itself is not a well defined thing, but any given radius definition (such as the Van der Waals radius) will tend to increase arbitrarily large values as one approaches ridiculously high energy levels. There are an infinite number of energy levels below the ionization energy, so you can get arbitrarily close to it without passing it. That's what DF is on about. In a real universe with other atoms, highly excited electrons are able to absorb enough energy from a stray photon to excite it past the ionization energy, so in practical terms in a real universe, there are practical limits to the size of atoms, but those are imposed by factors external to the atom, not factors based on internal forces within the atom. Purely as a system-unto-itself, there is no limit to the distance that a bound atom cannot remain bound. Only an energy limit. --Jayron32 15:15, 16 November 2017 (UTC)

There's no way you can find an answer to "there's a 50% chance a bound electron has not been x far from its atom center" in some way not really applicable to the real universe? (like if you were to measure the position of every electron once to good accuracy (clearly not possible for many reasons i.e. sentient life postdated atoms) there should be a 50% probability one of the bound electrons are x far, the most likely electron speed is y (which has to have an answer since superheavy elements get relativistic effects from the electrons moving that fast) it takes z time for an electron at the most likely or 50th percentile distance to move a reasonable distance away from where it was at that speed (say 1 radian, yes they don't really orbit), there's been w of these time periods since the percent of hydrogen atoms that were non-ionized is similar to now (does that even cover most of the time since stars?) and that could be taken as w more chances for an electron to get far so you can then calculate the distance with w times more atoms each being measured once? So if (numbers for example, not accurate) there were 10⁸⁰ atoms and there have been 10³⁴ periods z long so far you'd find the 50% probability maximum for 10¹¹⁴ atoms being measured once? (since good positional accuracy would screw with trying to measure the real universe's electrons' positions quadrillions of times per second) If cosmological weirdness has made the amount of (normal) matter within the observable boundary vary a lot I wouldn't mind if that was ignored to make the math easier) Sagittarian Milky Way (talk) 19:27, 16 November 2017 (UTC)

There's some fun stuff that happens with low-probability statistics and indistinguishable particles.

The probability that an electron is measured at a distance of a thousand light-years radially from the proton it orbits is very low but non-zero.

But - if you set up a detector, and you register one "count", can you prove that the electron you observed is the one you were trying to measure?

No, you cannot. Your detector might have measured noise - in other words, it might have been lit up by a different electron whose interaction with your detector was more or less likely than the interaction with your super-distant electron. Actually, the probabilities and statistics don't matter at all, because we have a sample-size defined exactly as one event. Isn't quantization fun?

In order to prove that it was the electron you were hoping to measure, you need to repeat the experiment and detect a coincidence. The probability that you will measure this is very low, but non-zero, squared - in other words, it won't be measurable within the lifetime of the universe.

Here's what Plato Encyclopedia has to say on this topic: Quantum Physics and the Identity of Indiscernibles.

Take it from an individual who has spent a lot of time counting single electrons - even with the most sophisticated measurement equipment, my electron looks exactly like your electron, and I can't prove which electron was the one that hit my detector.

Nimur (talk) 19:53, 16 November 2017 (UTC)

I think the weirdness is weirder than that. I mean, the question is how far any electron in the history of history has ranged away from its nucleus and returned. But electrons are ... a cloud of probability. At any instant you could measure the electron and find it anywhere. So if you measured any one electron at every possible instant, an infinite number of instants, it should range pretty much an infinite distance away. Except... that measuring the electron changes it! If you find the electron a light year away from its nucleus, you won't find it at a Bohr radius a nanosecond later away, because the probabilities are all changed. And the reverse is also true. So you can't talk about where you would have measured an electron, but only where you did measure it.

But even if you rephrase the question to where an electron has been found in the history of history, it's still a problem ... because if you measured its position, it is not possible to measure its momentum accurately, to determine if it was truly "in orbit" or if it had simply been ejected from the atom! And so you can't actually give a distance; you can only give a probability function that may extend out an arbitrarily large distance.

I think... Wnt (talk) 17:04, 19 November 2017 (UTC)

In other words, shut up and calculate. --47.138.163.207 (talk) 09:45, 20 November 2017 (UTC)

ethanol fermentation

A gram of sugar has 4 calories and a gram of alcohol has 7 calories. Would anyone be able to tell me what the approximate conversion rate of sugar to alcohol in ethanol fermentation is in calories? Eg if you put 400 calories of sugar into the reaction how many calories of ethanol do you get out? I've tried reading the article but all the equations are way over my head, sorry. Thanks for your time. — Preceding unsigned comment added by Oionosi (talk • contribs) 10:21, 16 November 2017 (UTC)

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

translates into

180g sugar → 2x 46 g alcohol + 2x 44g CO2,

(those weight values can be found at respective article of glucose, ethanol and CO2)

4x 180 calories sugar (=720) -> 7x 2x 46 calories alcohol (=644) + 76 calories lost

As you see, this was not are way over my head, you underestimate yourself — Preceding unsigned comment added by 185.24.186.192 (talk) 11:36, 16 November 2017 (UTC)

That looks like a valid calculation (though the one-significant-figure calories given by the OP lack the precision to come up with an accurate "calories lost" figure). I'll add a note of explanation though. With fermentation the idea is to take something in an intermediate redox state and split it up into more oxidized and reduced components. In a very loose sense it is like the opposite of a combustion reaction, though one rarely thinks of burning something (even something greasy) in carbon dioxide! So you start with a sugar, i.e. -CHOH-. Not counting any ends, the formal oxidation state of the carbon is -1 from the direct hydrogen and +1 from the oxygen bonds adding to 0; the oxygen is -2 as usual. Such a compound could be produced by the oxidation of -CH2- (which has carbon at -2) with one oxygen. As a result it has less energy than -CH2-, assuming an oxygen-containing atmosphere. (On Saturn or early Earth it would have more energy than -CH2- since that won't do much in regard to methane) Oxidation can take a terminal -CH2OH (alcohol, -1) and convert it to -CHO (aldehyde, +1), then -COOH (carboxylic acid, +3), then separate it as CO2 (carbon dioxide, +4, with a -1 change in oxidation state on the other end as a hydrogen replaces this carbon on the decarboxylated main chain). Reduction can convert -CH2OH to -CH3 (methyl, -3). By such manipulations, we see that the initial six +0 CHOH carbons become two +4 CO2s, two -3 methyls, and two -1 alcohols. As a result, the "burnability" of six carbons is concentrated into four carbons, while the other two are now completely burned to CO2. This doesn't quite match the 7/4 ratio because the energy of compounds is more complicated than that (after all, oxidation state is a rather crude approximation that assumes absolute differences based on quantitative differences in electronegativity, among many other things) but I think it is qualitatively useful. Wnt (talk) 01:14, 20 November 2017 (UTC)

Telephone lines

Can someone explain how telephone lines "work", i.e. how it is that they can carry multiple conversations simultaneously, rather than being busy for all the people on the exchange whenever any subscriber is on the phone? I looked at telephone and telephone line without seeing anything, and Google wasn't helpful either. I would imagine that the line would carry electrical pulses just one at a time (as on a party line), and multiple conversations would cancel each other out, but obviously our whole telephone system wouldn't work properly if that were the case. Nyttend (talk) 12:23, 16 November 2017 (UTC)

This has a pretty good explanation, getting down to how signals are encoded for travelling down the wire. --Jayron32 12:34, 16 November 2017 (UTC)

Hm, so nowadays it's electronic when going from exchange to exchange; not surprised, but I wasn't aware of that. And I didn't realise that there was a completely separate wire from every subscriber to the exchange, or I wouldn't have wondered. But before electronics, how was it possible for two subscribers from the same exchange to talk simultaneously with two subscribers from the other exchange, rather than one person taking it up and preventing other callers? Does Myrtle have multiple individual wires going to every nearby town's exchange, and a massive number of individual wires going to some nationwide center in order to allow Fibber to phone someone halfway across the country? Nyttend (talk) 12:49, 16 November 2017 (UTC)

Andy gives a good summary below. In the early days of telephone systems there really was a direct electrical connection that had to be made between each pairs of callers, and no one else could use the same wires at the same time, so each hub on the network had to have many separate wires available that could be variously connected to make one complete circuit for each call. However, we have long since abandoned that approach. Nowadays everything is generally digitized and travels over packet switched networks. Depending on where you live, and who provides the phone line, the digitization may happen inside your home or at some regional or central exchange. Dragons flight (talk) 14:25, 16 November 2017 (UTC)

• There are several methods used historically.

• Party line systems were connected simply in parallel. Only one could be used at a time.
• Underground cables used a vast number of conductor pairs, one for each circuit. 100 pair cables were common, far more than could ever be used by overhead cables, which were mostly single pairs to each visible cable (the first telegraph signals used a single copper conductor for each visible wire, so a telephone circuit might need two wires and pairs of china insulators). Cables for the 'local loop' from the exchange to the telephone used a single pair for each phone. Cables between exchanges were of course circuit switched to only need enough pairs for the calls in progress (not the number of phones) and many calls would be local, within the same exchange.
• Analogue multiplexing[4] was used from the 1930s (rarely) to the 1980s. Like a radio, this was a broadband system that packed multiple separate signals down the same cable by multiplexing them. Frequency division multiplexing was used, like an AM radio. Each telephone signal only needed a narrow bandwidth of 3kHz: 300Hz to 3.3 kHz. This meant that the largest trunk lines could carry several MHz signals over a coaxial copper tube conductor, several to a cable, and these could each carry thousands of voice phone calls - or a single TV signal, for the early days of national TV in the 1950s-1960s.
• In the 1980s, PCM (pulse code modulation) came into use, where analogue phone signals were digitised, then distributed as circuit-switched digital signals. Usually this was done in the telephone exchange, but commercial switchboards[5] began to operate digitally internally and so these could be connected directly to the digital network, through ISDN connections (64kbps or 2Mbps). There was some movement to placing concentrators in cabinets in villages, where the local phones were digitised and then connected to a local town's exchange via such a connection (all phones had a digital channel to the exchange). This allowed simpler cables (such as a single optical fibre) to the village, but was less complex than an exchange in the village.
• In the 1990s, the Internet became more important and packet switching began to replace circuit switching for digital connections between exchanges and commercial sites. The domestic telephone was still an analogue connection, rarely ISDN, and anyone with home internet access used a modem over this.
• By 2000 the analogue telephone was no longer as important as the digital traffic. Also IP protocols from the computer networking industry replaced the mix of digital protocols (ATM, Frame Relay) from the telephone industry. Analogue phones became something carried over an IP network, rather than digital traffic being carried by analogue modems. BT in the UK began to implement the 21CN, as a total reworking of their legacy network. Andy Dingley (talk) 13:27, 16 November 2017 (UTC)

Thank you; this really helps. I don't much understand how radio works, but the idea of broadcasting at different frequencies I understand, so using a different frequency for each telephone conversation makes sense. Could you add some of this information to telephone line and/or telephony, since the first is small and the second mostly talks about digital? Nyttend backup (talk) 15:06, 16 November 2017 (UTC)

What he said about 100 circuits is the source for the old "all circuits are busy" message. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:30, 16 November 2017 (UTC)

Very rarely. It was exchange equipment that ran out first, not cables.

Telephone exchanges are obviously complex, but for a long time and several generations of technology pre-1980 (and the introduction of stored program exchanges, i.e. single central control computers) they consisted of line circuits, junctors and a switching matrix between them. Line circuits were provided for each local loop (i.e. each customer phone). Obviously the amount of equipment per-customer was kept to an absolute minimum, and as much as possible was shared between several subscribers. Typically[6] a rack of subscribers' uniselectors was provided, each one handling 25 lines. Several sets were provided, so each subscriber might be connected to 5, or even 10 on a busy exchange. When a subscriber picked up their phone, the next free uniselector would switch to their line (and only then the dialling tone was turned on). So no more than 1 in 5 people could make a call at the same time - any more than that and you didn't get dial tone (and maybe did get a busy tone or message instead).

Exchanges are connected together by cables, and the switching circuit for these is called a junctor (Junctor is a useless article). Again, these are expensive so the equipment is shared and multiple sets are provided, but not enough to handle a call over every cable at once. Traffic planning and the Erlang were important topics for telephone network design. For a pair of exchanges (called "Royston" and "Vasey") where all of their traffic is between the two exchanges and they don't talk to people from outside, then enough junctors might be provided to meet the full capacity of that one cable. Usually though, enough equipment was provided to meet the "planned" capacity for a cable and the "planned" capacity for the exchange, and the equipment racks (the expensive and more flexible aspect) would be the one to run out first. Only in exceptional cases would all the traffic land on a single cable, such that it was the cable which maxed out.

One aspect of more recent and packet switched systems, rather than circuit switched, is that they become more efficient at load sharing, thus "equipment busy" becomes rarer. Also we demand more, and the hardware gets cheaper, so it's easier to meet this demand. Andy Dingley (talk) 17:27, 16 November 2017 (UTC)

Thanks Andy, great stuff! I agree that junctor is a fairly poor article, and I'm interested to learn more about them. If anyone has any references on those please post them here, maybe we can use them to improve our article :) SemanticMantis (talk) 02:00, 17 November 2017 (UTC)

See also the phantom circuit, later reused in a patented method for supplying power, known as Power over Ethernet (PoE). --Hans Haase (有问题吗) 11:11, 17 November 2017 (UTC)

Many years ago I went to an "Open Day" at the local telephone exchange. There was a historical talk and we saw lots of metal rods with ratchets which moved up and down to make connections, making a clicking noise. I accepted an offer from the presenter to call my house - he didn't get through, but when I mentioned this later my mother said that the phone had been ringing intermittently all evening. 82.13.208.70 (talk) 16:36, 17 November 2017 (UTC)

See Strowger exchange Andy Dingley (talk) 17:23, 17 November 2017 (UTC)

What is a 'double domed appearance'?

"Double domed" cranium

Relating to an animal's (in this case, an elephant's) head. 109.64.135.116 (talk) 19:49, 16 November 2017 (UTC)

It looks like an Asian elephant looks. Henry^Flower 19:54, 16 November 2017 (UTC)

A picture is worth several words →

2606:A000:4C0C:E200:C9A:4B44:2E28:1611 (talk) 05:48, 17 November 2017 (UTC)

• There also the scene towards the end of Alexander DeLarge on stage reaching up but unable to grab a pair of double domes due to his "treatment" in Kubrick's adaptation of A Clockwork Orange. But that's NSFW or for children, so I'll just allude to what occurred to me when I read the header. μηδείς (talk) 00:32, 18 November 2017 (UTC)
  • Oddly enough, that connects with the question on another desk, about the connection between Beethoven and Hitler. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:52, 18 November 2017 (UTC)

Sun's helium flash

Judging by the article about helium flash, our Sun will apparently exhibit this when it starts fusing helium. Do we know how bright this flash will be? Will it affect life on Earth? Is this the part where the Sun engulfs Mercury and Venus? Also, I suppose there will be a process of dimming once the hydrogen supply is exhausted while the Sun is collapsing. Is this near-instantaneous or will it take minutes/days/millenia? 93.136.10.152 (talk) 20:40, 16 November 2017 (UTC)

You can read Sun#After_core_hydrogen_exhaustion. Ruslik_Zero 20:50, 16 November 2017 (UTC)

Thanks, didn't think to look there. So the compression of the core is more or less gradual as Sun reaches the end of the red giant branch, until the moment of the helium flash? 93.136.10.152 (talk) 21:15, 16 November 2017 (UTC)

There's also a carbon flash with three heliums kung-powing into carbon and so on in stars with enough mass (more than the Sun). If the star's massive enough it can reach 1 million sunpower with only tens of sun mass and build up central iron ash till it loses structural integrity (since stars can't run on heavy elements). The star collapses till 200 billion Fahrenheit, bounces off, and explodes with the light of billions of Suns (and up to about a trillion sunpower of neutrino radiation). When the center reaches the density of a supertanker in a pinhead it becomes extremely resistant to further collapse (but not invincible) since there's only so many neutrons that can fit in a space unless it can force a black hole (or possibly get crushed into smaller particles). Sagittarian Milky Way (talk) 00:01, 17 November 2017 (UTC)

Will it affect life on earth? No, because by that time all life on earth will have died off. 2601:646:8E01:7E0B:5917:3E80:D859:DF69 (talk) 06:43, 17 November 2017 (UTC)

Relevant quote from the article: In the case of normal low mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand (a processes requiring so much energy, it is roughly equal to the total energy released by the helium flash to begin with), and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable to observation, and is described solely by astrophysical models. "Flash" in this context is meant in the sense of "in a flash", since the helium fuses extremely rapidly, not in the sense of a visible flash of light. The Sun will engulf Mercury and Venus when it transitions to a red giant. A helium flash happens in stars that have already been red giants for a long time. As for what happens once the core's hydrogen is exhausted, Stellar evolution#Mid-sized stars seems to answer this. --47.138.163.207 (talk) 09:41, 20 November 2017 (UTC)

November 17

An odd ball [possible insect nest identification]

Odd ball

What is it?

It's not a wasp hive. It seems pretty solid when you break it open. It is heavy and hard. The one pictured is around 25cm high. They are all over the place in Hainan. Ants seem to like crawling around that one.

What is it?

Anna Frodesiak (talk) 06:09, 17 November 2017 (UTC)

COuld it be a bird nest of some sort? Swallows build nests of mud, though usually on cliff faces or under building eaves. --Jayron32 11:57, 17 November 2017 (UTC)

The acrobat ant builds nests like this (looks similar to me). Alansplodge (talk) 12:24, 17 November 2017 (UTC)

Looks like an ant colony to me too. I was a bit surprised to not see any ants in the photo, but OP says ants are all around, so that's also pretty good evidence. Tropical termites can also build roughly similar nests, but I think they usually also build covered galleries. Further destructive sampling would probably clear this up rather quickly, at least to determine ant or non-ant. SemanticMantis (talk) 15:06, 17 November 2017 (UTC)

Just a note - it's not wise to mess with these things. Sometimes insects swarm out and sting you to death - or if you're rock climbing you fall to your death. 82.13.208.70 (talk) 15:54, 17 November 2017 (UTC)

I think that's a bit melodramatic, but better safe than sorry I suppose. For the version found in the USA, "acrobat ants are usually of minimal nuisance to people" University of Florida Entomology & Nematology. Alansplodge (talk) 21:01, 17 November 2017 (UTC)

Thank you, all. I'm still perplexed. There are never openings large enough for a bird. It is nearly solid inside. No ants swarm out when disturbed. The surface is made of mostly whole leaves glued on. There are ants visible on the thing and stem below, but not that many. There are no openings where any bug comes and goes, or none easily seen. They come in all different sizes, always enveloping branches quite symmetrically. An odd ball indeed. Anna Frodesiak (talk) 22:34, 17 November 2017 (UTC)

• Some sort of mud-dauber ant or wasp nest. Remember, ants are just a subgroup of the wasps. See this google image search for "dauber ant nest china". μηδείς (talk) 00:04, 18 November 2017 (UTC)

Ants, eh? That does sound possible. It's made of whole leaves so I can't see wasps carrying those. Ants could because they work as a team.

And careful about calling ants "just a subgroup of the wasps". They already have wing envy, and if they hear that they get very upset and start running around in different directions. :) Anna Frodesiak (talk) 05:08, 18 November 2017 (UTC)

Beware when they stop doing the conga and take off their tutti-frutti Carmen Miranda hats. ;) 2606:A000:4C0C:E200:C9A:4B44:2E28:1611 (talk) 05:57, 18 November 2017 (UTC)

Ants are generally considered party animals, and that is why the conga line is huge with them. (By "huge" I mean quite small because they're ants.) Anna Frodesiak (talk) 07:52, 18 November 2017 (UTC)

I would guess that this is man(women)-made "ball" (or weight?). Birds or small animals would use other materials and better locations for a nest in a wood and colony insects would not include complete leaves in their construction like in the picture. Other interesting details are that this seems to be a young tree, that it is very straight and the added info that its in China, which could be a hint towards the ancient chines art of forming plants (Bonsai etc.) for some (later) purpose. --Kharon (talk) 06:23, 18 November 2017 (UTC)

Not man-made, no way, impossible. They're everywhere and people would not make those things. They're too busy farming. Anna Frodesiak (talk) 07:52, 18 November 2017 (UTC)

Maybe they plant something in there and farm them later. You know the rich Chinese cuisine, right? --Kharon (talk) 14:08, 18 November 2017 (UTC)

Have you google-imaged the topic "nests"? It looks sort of like a hornets' nest. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:51, 18 November 2017 (UTC)

Bugs, I actually looked at the source data on Anna's photo; saw the location was Hainan, China; google imaged "dauber aunt nest china" and posted the link in my bulletted response above, so people can just click there and use the related image link if not satisfied. μηδείς (talk) 17:17, 18 November 2017 (UTC)

Hi μηδείς. I'm stuck with Bing here in China. And yes, the pic is from Hainan. So, Bing shows dauber wasps, but I cannot find dauber ants at search engines or at Wikipedia. I'd like to find a species at Wikipedia then add it to the disambiguation page Dauber.

Anyhow, every single picture of a dauber nest at search engines shows them made of stuff like hummus but not with lots of whole leaves. Seeing that the leaves in my image are the same as the living ones on the plant, they could be just bent over and hummussed rather than the leaves gathered from the floor and carried. If the latter, then it must be ants because wasps cannot carry huge leaves.

Next time I'm in the forest, I'll get someone to cut one open and I'll take a picture of the inside. I'll tap on it first to ensure it's abandoned. Anna Frodesiak (talk) 23:46, 18 November 2017 (UTC)

Well, wasps will build cells reminiscent of honeybee combs, although maybe not as well-structured. Tapping may not be enough assurance, I would put it in a sealed bag with a rag soaked in insect killer--bug spray can also be toxic to humans; follow the instructions. You could also just go to a local university with a reasonable biology department, as they will know the genus, if not the species. μηδείς (talk) 00:02, 19 November 2017 (UTC)

μηδείς, thank you kindly for the suggestions. However, 1, I will not kill them, and 2, the university thing would be a dead end for a hundred hilarious reasons. :) But thank you. I'll take my chances with a knock-then-run-then-return technique. :) Anna Frodesiak (talk) 00:12, 19 November 2017 (UTC)

Hornets' nests come in a wide variety of styles, some of which resemble your mystery object (e.g.), but a cursory image search didn't find any that incorporated whole leaves; they tend to make their own "paper" covering from chewed fiber and saliva (the leaves on this one seem incidental). Note: don't mess with Chinese hornets! —2606:A000:4C0C:E200:C9A:4B44:2E28:1611 (talk) 03:47, 19 November 2017 (UTC)

Yes, this doesn't seem to be papery. Sort of dauby with leaves.

And I have encountered those giant wasps. They are absolutely huge! Anna Frodesiak (talk) 04:54, 20 November 2017 (UTC)

Evolution of anticoagulants

Why did some blood-sucking animals evolve anticoagulants if they suck in a manner where the blood doesn't have enough time to coagulate? E.g. mosquitos or leeches pierce the skin and suck directly, so the blood goes steadily into them, similar to injection needle which prevents coagulation. My guess is that anticoagulants were present before they evolved the necessary organs, allowing them to feed on spilled blood. Brandmeister^talk 16:30, 17 November 2017 (UTC)

I'm not sure I'm following your question. Does it help to know that ordinary saliva contains enzymes like amylase and lipase that start the pre-digestion of food? In other words, ordinary saliva is already pretty decent at breaking things up. Now, something like a vampire bat has even more advanced enzymes working for it (at least, according to our citation), but it was clearly a case where evolution took something that was already good at a certain activity and then enhanced it (to some extend, a form of exaptation). Matt Deres (talk) 18:26, 17 November 2017 (UTC)

As I understand, digestive enzymes and anticoagulants here are different stuff. Digestive enzymes may be needed to break down blood, but anticoagulants look redundant if the sucked blood goes straight for digestion anyway. Per clotting time, blood starts to coagulate in about 8 minutes, far more time than required for a mosquito to feed and fly away. Brandmeister^talk 18:45, 17 November 2017 (UTC)

As usual, Matt Deres has it with exaptation. As long as a preëxisting circumstance (digestive enzymes in the saliva) provide a little benefit, and the benefits of producing stronger enzymes is possible (chemically and given the right mutations) and outweigh the costs, evolution will move in that (in this case anticoagulant) direction.

Or, to further address Bradnmeister, other chemicals which have anticoagulant properties may become expressed in the saliva due a change in the regulation of gene expression. (Look at the ubiquity of melanin and cholesterol-type chemicals and their various roles.) Eventually these will be classified as different classes of chemicals.

μηδείς (talk) 23:49, 17 November 2017 (UTC)

I am concerned that there may be some unexamined and potentially incorrect assumptions here about the clotting process. Geometry matters—clotting will proceed more quickly in the very long, narrow proboscis of a mosquito compared to the relatively much wider tubes used to measure clotting time in the lab. For instance, this presentation shows how long bleeding persists in a rat experiment where a needle is inserted and removed from the tail vein of the animal (we're interested in the control – "vehicle" – bar at the extreme left). Even with a direct puncture of a vein, bleeding from the much narrower opening stops in less than 90 seconds—not 8 minutes. The size of the needle wasn't obviously specified in that slide deck, but even a relatively fine 30-gauge needle still has outer and inner diameters of roughly 300 and 150 microns, respectively. That's several times the diameter of the mosquito proboscis; see the figures in this paper.

There's no need to invoke exaptation or other phenomena when we already know that blood clots faster in narrow tubes. TenOfAllTrades(talk) 01:31, 18 November 2017 (UTC)

That last comment makes no sense to me at all. Exaptation is the use of a pre-existing element--saliva enzymes--for a new purpose; anti-coagulant. What in the world does that have to do with probosces? Especially since even non-proboscised vampire bats have anticoagulants. I suspect you may have misundertood me, but I did not even mention the feeding organ in my post. μηδείς (talk) 02:04, 18 November 2017 (UTC)

You're right; I completely misread what you were saying about exaptation.

That said, there's actually another somewhat subtler misunderstanding at work here, about how mosquito saliva inhibits coagulation. The potent anticoagulant agents in the saliva of mosquitoes (and many other blood-sucking creatures, for that matter) generally aren't enzymes at all; they're polypeptides that bind to regulatory and/or active sites and act as inhibitors of pro-clotting enzymes. That is to say, they don't inhibit clotting by digesting clots, they inhibit clotting by preventing clot formation in the first place. There's a bit of an overview in this PNAS snippet, which looks at anophelin (we need an article...), a thrombin inhibitor in Anopheles mosquitoes.

Numerous animal species, from insects (mosquitoes) to mammals (vampire bats), feed primarily or exclusively on fresh blood from their prey. These parasites produce some of the most potent antagonists of the blood clotting system known, which are critical for their hematophagous lifestyle. Many of these compounds are small polypeptides that inhibit the proteolytic enzymes of the clotting cascade, notably thrombin....

The overwhelming majority of proteinaceous inhibitors of proteolytic enzymes works by physically blocking access of the substrate to the active site....

I would certainly be intrigued if someone could locate some good references regarding the evolutionary roots of these sorts of polypeptide inhibitors. Given that the anophelins have no intrinsic enzymatic activity and don't share any conspicuous sequence similarity to any enzymes (digestive or otherwise, based on a quick BLAST) I don't find the hypothesis of digestive enzyme exaptation particularly compelling.... TenOfAllTrades(talk) 03:21, 18 November 2017 (UTC)

I agree, I was making no actual claim that it was, say, specifically amylase that mosquito saliva developed from, but some pre-existing substance. Small polypeptides can also easily evolve from large polypeptides with a frame-shift mutation or two that deactivates all but the part of a gene that was already producing a digestive enzyme, leaving just a small polypeptide that blocks coagulation. The step from necrophagy to sarcophagy to hemophagy seems not to have to overcome to many barriers in the evolutionary landscape. You are right, we need sources, and this (vampyrism) is not a specific field I have studied or have books on. μηδείς (talk) 03:37, 18 November 2017 (UTC)

Again, though, there's no particular evidence (at least none provided here) that small(ish) polypeptides like anophelin originated with any sort of digestive enzyme, given their lack of similarity with even fragments of existing, known proteases. Yes, there's lots of ways to mutate the gene for a larger protein which will result in a much smaller product, but I'm still not clear on the fixation in this thread on the idea that a digestive enzyme in particular would be the likely starting point. TenOfAllTrades(talk) 21:59, 18 November 2017 (UTC)

Read this sentence again (emphasis added): "I was making no actual claim that it was, say, specifically amylase that mosquito saliva developed from, but some pre-existing substance." μηδείς (talk) 22:46, 18 November 2017 (UTC)

I read the whole comment, actually. I even think I understood it this time. The second sentence says (emphasis added): "Small polypeptides can also easily evolve from large polypeptides with a frame-shift mutation or two that deactivates all but the part of a gene that was already producing a digestive enzyme, leaving just a small polypeptide that blocks coagulation." If you – or anyone – don't want me to talk about how you're talking about digestive enzymes, stop talking about digestive enzymes. TenOfAllTrades(talk) 02:02, 19 November 2017 (UTC)

But you are totally dropping the context that I had just said I was using it, "say", as an example of "some prexisting substance". Would I be justified in assuming that you are arguing that an entirely new polypeptide, that just happens to be expressed in the mosquito's saliva, appeared out of nowhere de novo with all the regulatory mechanisms in place to generate it and release it at the proper time? Of course not. So please retain the full context of what I write (notice I use paragraphs) and stop cherrypicking words when my full meaning is clear from my entire post. μηδείς (talk) 02:34, 19 November 2017 (UTC)

The third paragraph of Coagulation begins: “Coagulation begins almost instantly after an injury to the blood vessel has damaged the endothelium lining the vessel.” Assuming a bite damages the endothelium, that appears sufficient to answer OP’s question, but here are a few related facts and sources that may provide perspective. From leach: “An externally attached leech will detach and fall off on its own when it is satiated on blood, which may be anywhere from 20 minutes to two hours or more.” From Mosquito#Saliva: “Universally, hematophagous arthropod saliva contains at least one anti-clotting, one anti-platelet, and one vasodilatory substance.” The book Mosquito, by Andrew Spielman and Michael D’Antonio, pp.14,15 mentions that a mosquito will bite up to 20 times before finding a blood vessel and each time it will inject “a chemical that inhibits your body’s ability to stop any bleeding that might begin.” (Hematophagy#Mechanism_and_evolution does not appear to directly address the question.)--Wikimedes (talk) 21:39, 18 November 2017 (UTC)

The most recent free paper I found on anophelin is this, which classifies it as an atypical serine protease inhibitor. The paper doesn't try to guess which (if any) it came from. Two important things to note is that anophelin is fast-evolving even within Anopheles, creating new relevant sites in some versions absent from others, and it is a largely disordered protein, exposing those sites readily, which means that there is less stabilizing selection because one amino acid doesn't have to stay the same to contact another. The result is that it is going to be very hard to tell which way this train went by looking at the track. I would not presume to say it is impossible, though. BTW the clotting enzyme (thrombin) which it inhibits is described as an atypical chymotrypsin/trypsin like enzyme, so you can say that the clotting it inhibits is an exapted digestive enzyme if you want. ;) Wnt (talk) 16:37, 20 November 2017 (UTC)

A question about g-force

How much g-force a pilot who fly an airplane at 510 knots will endure? 37.142.17.66 (talk) 18:09, 17 November 2017 (UTC)

If they fly straight and level, then zero.

They only feel g forces if they manoeuvre, usually by pulling vertically (relative to the airframe) upwards (the wings can generate more lift than any yaw or roll forces). So loops or tight turns. Andy Dingley (talk) 18:37, 17 November 2017 (UTC)

I'm particularly talking about United Airlines Flight 175. 37.142.17.66 (talk) 18:49, 17 November 2017 (UTC)

I don't recall any particularly hard manoeuvring on that day. Also these were airliners, which just aren't built to pull many g. Andy Dingley (talk) 20:06, 17 November 2017 (UTC)1;

I am confused. Do you mean how much G-force was generated by the abrupt stop (crash)? --Lgriot (talk) 20:08, 17 November 2017 (UTC)

No. before the crash. according to National Transportation Safety Board the pilot nosedived more than 15,000 feet in two and a half minutes. so how much the g-force was on the pilot, if he really fly at 510 knots? 37.142.17.66 (talk) 20:44, 17 November 2017 (UTC)

That is not enough information to say. G-forces are caused by acceleration, i.e. how fast the velocity is changing. A descent of 15,000 feet in 2.5 minutes corresponds to a vertical speed component of 100 ft/s. The question is how rapidly they transitioned from level flight to 100 ft/s descent. If they took x seconds that would be an acceleration of −100/x ft/s², or approximately −3/x gees. If x is small, the G-force is large. If x is large, meaning a gradual transition into descent, the G-force is small. --69.159.60.147 (talk) 21:22, 17 November 2017 (UTC)

It depends on how quickly the pilot noses over, and that data isn't specified here. But first, some ballparks:

Let's do some math (mostly trig). 510 kts is 860 feet per second. In 2.5 minutes at that speed, the plane covers 129,000 feet and, per the above, descends 15,000 feet. It's obvious even there that 15 is only a small proportional component of 129; the trig works out to a grade of 7%.

This Boeing publication discusses pitch rates on takeoffs and shows a nominal flight case of ~3 degrees per second of pitch rotation to a maximum 15 degree pitch. That takes five seconds and is performed on virtually every single flight. Of course, that's at a lower speed than 500 kts.

On the other hand, we can check out how these planes usually descend. Per the FAA, a 767-200 flies a standard climb and descent rate of 3500 feet per minute, roughly half of the 6,000 feet per minute discussed here. Assuming standard descent happens at more or less cruising speed, that's about a 4% grade.

The main thing to note here is that the calculated 7% grade isn't a remarkable number. There's lots of wiggle room, and for that, we go to the g-acceleration calculator for curved paths.

• If the plane rotates at 3 degrees per second at 260 m/s (~500 kts), that's a curve of radius 5000 m (260*30 / (pi/2)) and yields -1.4 g (so -0.4 g observed). Well, that's substantial, though not debilitating. But that's making the whole pitch transition in two seconds; that's really fast.
• 1 degree per second still accomplishes starting the descent in 7 seconds and drops the effect to -0.45 g (so 0.55 g observed). That's close to what a plane might reasonably experience on a standard flight in abnormal circumstances.
• Go to 0.5 degrees per second (still just 15 seconds to transition into the dive) and the force from the dive is just -0.2 g (so 0.8 g observed). Totally reasonable.

So, absent additional evidence, there's no cause to expect that a controlled descent of 15000 feet in 150 seconds at cruising speed would impose g loads with any significant immediate consequence. — Lomn 21:43, 17 November 2017 (UTC)

[e/c]

Also, it depends on which axis (G_x, G_y, G_z) the G-force is applied before G-LOC occurs (unconsciousness due to hypoxia). — (Speculation) It is likely on a commercial airliner that the plane would experience a catastrophic failure (like a wing falling off) before the pilot experiences unconsciousness. 2606:A000:4C0C:E200:C9A:4B44:2E28:1611 (talk) 21:09, 17 November 2017 (UTC)

Direction as well as axis. A nose-over won't lead to G-LOC in the way that a pull-up will. Andy Dingley (talk) 21:55, 17 November 2017 (UTC)

It depends on the Pilot and what he is trying to do! Very good Pilots manage to do 1G-barrel rolls in soaring planes and Airline Pilots aim to give their Passengers a gentle pleasant journey by default so they probably try to keep the G-Forces at 1 +- 0.2 at all times except at take off, landing and emergencies of course. --Kharon (talk) 06:40, 18 November 2017 (UTC)

The whole point of a barrel roll (rather than an aileron roll) is that it's a low-g manoeuvre. Although neither puts many g on the pilot in the central fuselage, the forces needed to aileron roll a large aircraft will easily overload the wing spars. This is why the Boeing 707 [7] and the Avro Vulcan were barrel rolled, not aileron rolled. Andy Dingley (talk) 20:43, 18 November 2017 (UTC)

My whole point of mentioning a 1G barrel roll was just to show that aviation knows some tricks to circumvent the "simple" physical logic applied here in prior answers using (2-dimensional) direction change per second formulas to calculate a (g)force. --Kharon (talk) 00:43, 19 November 2017 (UTC)

Is that a Very Good Pilot? I know nothing about this, but I picture some turbulence or mechanical issue putting that glass of water all over the top of the cockpit, dripping down into any electronics etc., all while he's trying to recover from a barrel roll under already pear-shaped conditions. Is that as bad a thing as I imagine, and if so, I wonder if that is really such a bright idea. I mean, he could have gotten more hits drawing a cartoon penis. ;) Wnt (talk) 16:26, 20 November 2017 (UTC)

• Just to be pedantic, the pilot experiences a force of 1G downward when the plane is not accelerating. A descent of 15000 feet in 150 seconds is not accelerating even as much as a free fall. In free fall, the pilot would experience 0G. -Arch dude (talk) 07:06, 20 November 2017 (UTC)
• G-forces on the pilot are caused by forces on the aircraft, plus gravity forces on the pilot. Oversimplifying, the aircraft experiences four forces: thrust, drag, lift, and gravity. On a commercial aircraft the strongest of these is lift (supplied by the wings), which is why a plane banks in order to turn. On a commercial aircraft, the wings will break off before the pilot has (other) troubles related to G-forces. -Arch dude (talk) 07:21, 20 November 2017 (UTC)

So to answer the OP's question directly, unless he was doing something unnecessary, the pilot would have experienced a force slightly less than 1G during that descent? --Lgriot (talk) 16:39, 20 November 2017 (UTC)

Yes. But so "slightly" that the word "endure" is wholly inappropriate.

Even for a suicidal terrorist trying to impact a building, there is no need to fly heavy-handedly. If they do so, there's also a risk of a large aircraft responding badly to that. Andy Dingley (talk) 16:44, 20 November 2017 (UTC)

November 18

Back EMF in DC Motors

Most materials that I have read about the dc motors mostly point out the back emf generated in in the armature windings. Does this mean that no back emf is generated in the field coils? — Preceding unsigned comment added by Adenola87 (talk • contribs) 07:45, 18 November 2017 (UTC)

Back emf, also known as Counter-electromotive force, in a motor armature winding is voltage produced by relative motion between the armature and the magnetic field produced by the motor's field coils. (This voltage opposes the applied voltage, or allows the motor to act as a generator). Stationary field coils that carry a constant current produce a constant field so there is no back emf of this kind on them, only Ohm's law and ohmic heating apply to the DC current and voltage here. A transient change in field coil current may be detected when the motor speed or load changes. This is due to transformer coupling to the changing armature current and is not a back emf effect. Blooteuth (talk) 16:03, 18 November 2017 (UTC)

Incidentally, that article isn't particularly good. See Brushed DC electric motor for a more comprehensive treatement of the subject. Tevildo (talk) 19:25, 18 November 2017 (UTC)

When the DC motor is running, there is a back emf in the armature windings, since the current through them is interrupted by the commutator. There are usually small-value capacitors fitted across the commutator brush terminals to reduce or suppress this back emf, which causes excessive sparking, brush wear and radio interference if not suppressed. The current in the armature windings is not interrupted while the motor is running, so there's no back emf in them, but they do experience a back emf when the motor is switched off, since this is a 100% change of current. At all times when the motor is running, there is however, electromagnetic induction into the field windings caused by the magnetic field in the nearby rotating armature. "Back emf" is reserved for self-induced voltage in a coil when the current through it is changed abruptly - in almost all practical applications it is used when referring to the complete removal of current through the coil. "Induced emf" or simply "induction" is used to refer to the voltage induced in a coil by an external magnetic field whose strength is changing, as developed in the field windings already discussed. This magnetic field may be from a simple magnet moving nearby, or another nearby coil which is in motion with direct current flowing in it or a stationary coil with alternating current flowing in it. This latter AC application is the principle on which the transformer is based. Akld guy (talk) 01:46, 19 November 2017 (UTC)

The article Counter-electromotive force explains that the expression can apply equally validly either to voltage caused by relative motion of the armature in the surrounding magnetic field (my response) or to self-induced voltage that opposes a change in current through an inductance (Akid guy's response). @Tevildo thank you for linking to Brushed DC electric motor which is a good article. Blooteuth (talk) 14:43, 19 November 2017 (UTC)

Nevertheless, I would caution against using "back emf" for anything except a self-induced voltage in a single coil when the current through it is changed abruptly. It's a term that's almost exclusively used to refer to the undesirable very high voltage developed across a coil when the current is turned off, such as the several hundred volts that can be developed across a relay winding at the moment of switch-off when the relay was operating at a much lower voltage, such as 12 volts. The term would never be used to describe the voltage induced in the secondary of a transformer, for example. Its very name ("back") implies that the voltage is in the reverse polarity to that of the original operating voltage, and this is indeed the case in a single coil back emf situation. In the armature-to-field scenario, the induced voltage is reverse-polarity and aiding-polarity as the armature rotates toward and away from the field, so "back emf" doesn't seem to be a correct term. Akld guy (talk) 21:30, 19 November 2017 (UTC)

@Akld guy Let's be aware that the two usages of "back emf" are equally valid because they describe the same phenomenon of a magnetic field and a conductor in relative motion. The difference is which one is stationary. In my response it is the field that is stationary. In your response it is the conductor (coil) that can be stationary while its own-produced field collapses around it. Since the OP asks about motors (presumably not employed as generators) the armature Counter-electromotive force is indeed a nett "back emf" that opposes the driving voltage, except at start-up or when the motor is stalled. With a commutated armature each winding moves through a virtually constant field strength during the small rotation angle when it is in circuit. Blooteuth (talk) 13:06, 20 November 2017 (UTC)

Please don't refer to me as Akid guy. If you look at my page you'll see why I'm named AKLD_GUY. I'm not going to argue with you as I've said all I wish to say and we will not agree. Akld guy (talk) 20:24, 20 November 2017 (UTC)

I apologize for misreading your name and have fixed my mistake. You have an issue of disagreement with the article Counter-electromotive force which is the result of many editor's work. You should try engaging constructively with them at Talk:Counter-electromotive force. If obduracy prevents understanding that a back emf arises from an armature's movement through the magnetic field, the obdurate one may have difficulty explaining why the current drawn by a DC motor decreases with increasing r.p.m. Blooteuth (talk) 10:46, 21 November 2017 (UTC)

Easier to convert to natural gas: diesel or otto?

What motor is easier to convert to natural gas? --Hofhof (talk) 11:35, 18 November 2017 (UTC)

Otto engines are very easy to adapt for running on gas. Converted Vehicles usually even have a simple switch for gasoline or gas which you can savely switch while you drive. Diesel can not be converted completely because Diesel engines usually work with self ignition on pressure max, but there are systems where a gas tank is added to lower the use of diesel. In theory that is. I never saw any Diesel-Gas mixture engines besides the military ones, that basically make the engines capable to run on anything, from crude oil to gas.

I actually owned a car with an Otto engine and a gasoline- aswell as a gas-tank. Worked great and saved me some thousands cash for fuel during the time i owned it. The only flaw was the installed 90l gas tank that took away half of the trunk space but i never needed all the trunk space anyway. --Kharon (talk) 13:50, 18 November 2017 (UTC)

Your car didn't run on natural gas. Andy Dingley (talk) 20:33, 18 November 2017 (UTC)

Potayto, potahto. Its a mixture of carbone fuel gases in both cases. It doesnt make a difference for an otto engine: See Natural_gas_vehicle#Differences between LNG and CNG fuels. --Kharon (talk) 00:55, 19 November 2017 (UTC)

But still, your car didn't run on natural gas. Nor would your car have been able to store a useful quantity of natural gas (as it can't be liquefied by simply pressurising it, it would require substantial refrigeration). Andy Dingley (talk) 01:01, 19 November 2017 (UTC)

Yes and there are about 5,000 (genetical different!) potato varieties worldwide. You need a different tank for LPG than for CNG, so what? The question was about combustion engines, not about gas storage tanks. --Kharon (talk) 01:12, 19 November 2017 (UTC)

" You need a different tank for LPG than for CNG, so what? "

The "so what" is that there's no way to make a useful CNG tank that will fit in a car. Andy Dingley (talk) 11:26, 19 November 2017 (UTC)

Really? This seems eminently practical to me ;-) Alansplodge (talk) 19:39, 19 November 2017 (UTC)

One could just skim wp for "CNG" and find a whole list of vehicles. Even a tiny car can be CNG fitted.--TMCk (talk) 19:53, 19 November 2017 (UTC)

The Fiat Panda is a good example of how difficult it is to make this work. The TwinAir engine was designed for bifuel CNG from the outset, but the provision of car size gas cylinders is still such a problem (energy / volume is only something like a sixth of petrol). The Panda has to triple its volume fuel capacity to manage this, which involved raising the suspension (the LPG duel fuel version doesn't need this) to make space. There's also a serious price premium for the Natural Power - something like 20%. Andy Dingley (talk) 21:50, 19 November 2017 (UTC)

Are you seriously still claiming "...there's no way to make a useful CNG tank that will fit in a car..."???--TMCk (talk) 22:28, 19 November 2017 (UTC)

... the difficulty is in fitting the passengers and luggage around the tank. Dbfirs 22:49, 19 November 2017 (UTC)

Difficult but not impossible as claimed. Gosh, batteries are taking up more space and they managed to make it work just as they managed to make CNG work.--TMCk (talk) 22:55, 19 November 2017 (UTC)
And no, no trunk space used in case of the Panda.--TMCk (talk) 22:59, 19 November 2017 (UTC)

Not for a conversion, no. And this is a question about conversions.

The Panda tanks needed to squeeze in tankage that's twice the size of the existing petrol tank (and the petrol tank is still needed), which needed major rearrangement and a suspension lift beneath. They also cost a couple of thousand, just for the tanks, and they give the car likely limited lifetime of only 10 years (current pressure vessel regulations require their replacement then, and neither inspection nor economic replacement seem practical for a 10 year old car).

This new car design is right on the edge of what's practical - and conversions aren't. Andy Dingley (talk) 23:19, 19 November 2017 (UTC)

• Obvious link: octane rating. Natural gas (i.e. methane) has a RON of 120, making it even more knock-resistant to engine knocking than high-grade gasoline, and even worse than that at diesel combustion (where you need the injected mass to auto-ignite as fast as possible).

Now, of course there are incovenients to running on gaseous fuels - fuel tank size can be a problem as hinted above, for instance. But those are not really a diesel vs. gasoline thing. Tigraan^{Click here to contact me} 14:26, 18 November 2017 (UTC)

Courtesy link: Otto engine -- Are there currently any vehicles using this? 2606:A000:4C0C:E200:C9A:4B44:2E28:1611 (talk) 19:20, 18 November 2017 (UTC)

It is the normal 4-stroke petrol engine! -- Q Chris (talk) 10:53, 20 November 2017 (UTC)

Lol, that's not clear from the article -- it only shows old-timey machines. 2606:A000:4C0C:E200:E958:86E3:541F:E7F1 (talk) 04:39, 21 November 2017 (UTC)

Just to clarify, the Otto engine was the first to use the Otto cycle. Tevildo (talk) 07:01, 21 November 2017 (UTC)

Thanks. I took the liberty of adding that to article (here). —[dynamic IP]:2606:A000:4C0C:E200:E958:86E3:541F:E7F1 (talk) 17:12, 21 November 2017 (UTC) ... note: my edit was reverted; anybodiy care to clarify this in the article?

• In practical terms, neither. Conversions are possible, but they're somewhere between "Well I wouldn't start from here if I were you" and "There's not much left of the Ship of Theseus by the time you've finished". As such natural gas engines require either gas pipelines to supply them, or heavy pressurised storage tanks, then these are large engines and so they're built from industrial or truck diesel engines - but they're built from scratch as gas engines, and although the cylinder blocks might start out the same, there's little in common when they're finished. Most of the larger ones (for electricity generation, rather than mechanical output) are gas turbines on the Brayton cycle, not piston engines. Larger ones are CCGT where the gas turbine exhaust has its heat recovered by a steam boiler and turbine (something like half of the UK's electricity is coming from these at present [8]).

What you may be thinking of instead are the far more common autogas conversions of spark ignition petrol Otto engines to run on LPG (propane), rather than Natural Gas (methane). This is easier to store (it liquefies at a manageable low pressure), so the dense liquid stores in a similar volume to a petrol tank, often replacing the spare wheel storage space. It's also very easy to convert the engine for its use. A typical 1990s-onward installation uses an extra set of fuel injectors, one per cylinder drilled into the inlet manifold before the inlet valves. The control of these is then taken from the existing (and highly developed) manufacturer's own petrol injection system, by using the petrol injector timing and multiplying that by an adjustable factor, set during conversion. The complex mapping of ignition timing and fuel volume from all of the engine parameters has already been done by the car maker - the gas injection just runs at a fixed multiple of this. The engine is started and warmed through on petrol, then switches automatically. Such conversions are popular in Europe (my Volvo 940 has such a conversion) where petrol is heavily taxed but Autogas LPG is far cheaper (half the volume price). Filling stations are available at the "one per small town" and every motorway or large road service station level. Andy Dingley (talk) 20:31, 18 November 2017 (UTC)

Natural gas vehicle seems to state that vehicles capable of running on either natural gas or gasoline are not unheard of. Here in Southern California it seems just about all the public transit buses were modified to run on CNG in the 2000s. Granted, I don't know what's involved in that. --47.138.163.207 (talk) 09:29, 20 November 2017 (UTC)

Several things are needed. Mostly a damned good reason for doing so. In Europe, and a few US cities, that reason is the reduction of diesel particulate pollution from idling buses. That justifies the costs for bus replacement. Buses are large, expensive anyway, and can justify the cost of the on-board tankage. A further problem is that of refilling them - this is slow (several hours) and requires dedicated filling bays (not just one shared pump) with protection for the equipment and cheap overnight electricity. That's far easier to do in a well organised bus garage than in a domestic flat with an ad hoc parking space outside. Andy Dingley (talk) 10:41, 20 November 2017 (UTC)

• Isuzu sell them commercially. https://www.isuzu.com.au/truck-range/cng-trucks/ as do Freightliner . Approximately 7% of Australian truck sales are CNG. My impression is that they are diesel based. Greglocock (talk) 06:20, 19 November 2017 (UTC)

... fitted to the same type of vehicle as diesel engines, but the combustion is much more like that in a petrol (gasoline) engine. Dbfirs 22:24, 19 November 2017 (UTC)

November 19

Smallest black hole to eat Earth

Inspired by these articles, what would have to be the smallest mass a black hole placed at the center of the Earth would have to have to destroy the Earth or at least wreak some noticeable havoc before it perishes from Hawking radiation? 93.136.4.186 (talk) 02:25, 19 November 2017 (UTC)

I don't know the numbers, but any black hole which would suck matter in faster than it evaporates would eventually destroy all Earth if placed at the center thereof. 2601:646:8E01:7E0B:404:F3D3:C557:159A (talk) 11:03, 19 November 2017 (UTC)

If you look at your links, you will see that the Earth will be heated into a plasma, before being swallowed. Is vapourising the Earth counted as destroying it? Graeme Bartlett (talk) 00:34, 20 November 2017 (UTC)

Stellar_black_hole#Properties cite: "There are no known processes that can produce black holes with mass less than a few times the mass of the Sun.[...].As of April 2008, XTE J1650-500 was reported by NASA[6] and others to be the smallest-mass black hole currently known to science, with a mass 3.8 solar masses and a diameter of only 15 miles (24 kilometers). However, this claim was subsequently retracted. The more likely mass is 5–10 solar masses.". --Kharon (talk) 01:13, 20 November 2017 (UTC)

Hmmm, what if you shot two black holes at each other at particle-accelerator speeds? One might speculate the black hole merger should be absolute, yet the angular momentum this imposes would make the resulting hole a naked singularity. Is there any theory that suggests they break back apart, perhaps into some kind of compex black hole shrapnel, so the pieces keep their event horizons? Wnt (talk) 01:21, 20 November 2017 (UTC)

If you believe in Hawkins theories of "Micro black holes" read Micro black hole#Minimum mass of a black hole. In that case also please try to answer the Question what process should produce and apply enough force to make one when even whole suns, much much bigger than ours (a multiple of the resulting smallest black hole atleast - lets say minimum ~20 times our sun's mass), only have a small theoretical chance to produce a black hole. Looks to me like an ant thinking if it could push a planet on another Orbit by keep jumping in one place. --Kharon (talk) 01:40, 20 November 2017 (UTC)

I was originally thinking of collisions among multiple black holes that may pile up near the galactic center. But regarding your reply... that article suggests that large accelerators might produce black holes. Now if what is needed is 22 micrograms in a Planck radius, the first question that comes to my mind regards the Compton radius of ridiculously relativistic particles with that combined mass, which presumably one collides from opposite directions. The particles are surely foreshortened to a very narrow distance, but do they remain as fuzzy laterally as when they are at rest? I would think they'd have to, in which case they usually pass through each other without forming a singularity. (Of course, you also need a ridiculously accurate means of alignment...) Wnt (talk) 04:04, 20 November 2017 (UTC)

Odd... Compton radius directs to classical electron radius, not Compton wavelength. I'll have to look later and see if there's a plausible reason for that... Wnt (talk) 16:20, 20 November 2017 (UTC)

OP here, @Graeme Bartlett, vaporizing Earth counts, I'm aware that complete digestion is impossible. Basically, I'm assuming that there is a certain mass m, where black holes with smaller mass will tend to lose mass to Hawking radiation faster than they can accrete and thus evaporate, while black holes with a higher mass will tend to accrete material fast enough to grow. @Kharon this is just a "what if" scenario, I'm aware that there's p=0 probability of this happening. 93.139.55.105 (talk) 04:16, 20 November 2017 (UTC)

Yeast as food

This might be a stupid question but why isn't yeast used as a meat replacement dietary protein source?

I've been reading about world protein shortages and about how farming animals for meat won't be sustainable in the long term to feed the growing population. Various alternative protein sources like farming insects and plant based proteins are discussed. There doesn't seem to be any attention paid to yeast though.

Yeast is easy to grow and is a compete protein. It requires no light so can be grown pretty much anywhere. Is there a reason that it has been overlooked as a dietary protein source? How much space would be required to grow enough yeast to feed one person?

Thanks Temic3300 (talk) 08:15, 19 November 2017 (UTC)

No, this is not a stupid question. And yes, yeast is used for food, see yeast extract and vegemite. There's also some yeast in leavened bread and unfiltered beer. Dr Dima (talk) 09:12, 19 November 2017 (UTC)

and we also have an article on Nutritional yeast. Dr Dima (talk) 09:18, 19 November 2017 (UTC)

As well as the actual yeast in beer (at least in Real ale, as Dr Dima alludes), the alcohol produced by the yeast also has nutritional value, per Alcoholic drink#Food energy, hence it sometimes being called "liquid bread". {The poster formerly known as 87.81.230.195} 94.0.37.45 (talk) 10:37, 19 November 2017 (UTC)

Have a look at the article on Quorn which is a meat substitute derived from a type of fungus. The advantage of the particular fusarium fungus is that it produces hyphae with a similar structure to muscle fibres, and can therefore be processed to give a more meat-like texture than would be possible with yeast. Wymspen (talk) 14:36, 19 November 2017 (UTC)

See also Single-cell protein. Various yeast are mentioned, but it's they're just some of several suggestions. Of course some suggest other alternatives like Entomophagy or plant proteins for various reasons (including the issues with keeping a sterile culture and yield). See also [9]. Of course you also have to convince people to eat the thing, that's one of the reasons why in the short term at least, most of these are ending up as animal feed. Nil Einne (talk) 15:13, 19 November 2017 (UTC)

• There is also the issue that people like the taste of meat. --Jayron32 12:19, 20 November 2017 (UTC)
  • Except for vegetarians maybe. To cite the old saying, "All the more for us!" ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:49, 20 November 2017 (UTC)

One reason for not eating large amounts of yeast is that it is high in purines which can bring on attacks of gout in people who are susceptible. RJFJR (talk) 22:19, 22 November 2017 (UTC)

Feynman Lectures. Exercises. Exercise 14-21 JPG archive

In previous discussion if we take into account the conservation of momentum law then there is no a paradox with energy needed for acceleration from 0 to 1 m/s and from 100 to 101 m/s.

$T_{\text{probe 1}}+T_{\text{earth 1}}+U_{\text{probe -earth 1}}=T_{\text{probe 2}}+T_{\text{earth 2}}+U_{\text{probe -earth 2}}$

${\tfrac {m{v_{1}}^{2}}{2}}+{\tfrac {M{V_{1}}^{2}}{2}}-{\tfrac {GMm}{R}}={\tfrac {m{v_{2}}^{2}}{2}}+{\tfrac {M{V_{2}}^{2}}{2}}-{\tfrac {GMm}{\infty }}$

$MV_{1}+mv_{1}=MV_{2}+mv_{2}=V_{0}(m+M)\Rightarrow$
$V_{1}=V_{0}(1+m/M)-v_{1}m/M;$
$V_{2}=V_{0}(1+m/M)-v_{2}m/M$
$\therefore {\tfrac {m{v_{1}}^{2}}{2}}+{\tfrac {M}{2}}{\Big [}{\Big (}V_{0}(1+m/M){\Big )}^{2}+(v_{1}m/M)^{2}-2V_{0}v_{1}{\Big (}m/M+(m/M)^{2}{\Big )}{\Big ]}-{\tfrac {GMm}{R}}={\tfrac {m{v_{2}}^{2}}{2}}+{\tfrac {M}{2}}{\Big [}{\Big (}V_{0}(1+m/M){\Big )}^{2}+(v_{2}m/M)^{2}-2V_{0}v_{2}{\Big (}m/M+(m/M)^{2}{\Big )}{\Big ]}$

$(m/M)^{2}\to 0$
${\tfrac {m{v_{1}}^{2}}{2}}+{\tfrac {M}{2}}{\Big [}{\cancel {{\Big (}V_{0}(1+m/M){\Big )}^{2}}}+0-2V_{0}v_{1}{\Big (}m/M+0{\Big )}{\Big ]}-{\tfrac {GMm}{R}}={\tfrac {m{v_{2}}^{2}}{2}}+{\tfrac {M}{2}}{\Big [}{\cancel {{\Big (}V_{0}(1+m/M){\Big )}^{2}}}+0-2V_{0}v_{2}{\Big (}m/M+0{\Big )}{\Big ]}$
${\tfrac {m{v_{1}}^{2}}{2}}-V_{0}v_{1}m-{\tfrac {GMm}{R}}={\tfrac {m{v_{2}}^{2}}{2}}-V_{0}v_{2}m$
Last equation is a law relating probe start and final velocities ($v_{1},v_{2}$) for sun reference frame. E.g. for $v_{2}=42{\text{km/sec}}$ (which is the escape velocity for the sun from 1 AU distance) it gives $v_{1}=46.3{\text{km/sec}}$, which coincides with 16.3 km/sec in earth reference frame.

Why can't we solve an equation like this : $T_{\text{probe 1}}+T_{\text{earth 1}}+T_{\text{sun 1}}+U_{\text{probe-earth 1}}+U_{\text{probe-sun 1}}+U_{\text{sun-earth 1}}=T_{\text{probe 2}}+T_{\text{earth 2}}+T_{\text{sun 2}}+U_{\text{probe-earth 2}}+U_{\text{probe-sun 2}}+U_{\text{sun-earth 2}}$
Username160611000000 (talk) 20:11, 19 November 2017 (UTC)

Isn't this just the three-body problem? Rmhermen (talk) 00:48, 20 November 2017 (UTC)

No, because no need to know a trajectory. We know the start position and the final position. To solve a problem of escape velocity from solar system we use 2-step method (1st step is an overcoming the Earth gravitation, 2nd step is an overcoming the Sun gravitation from Earth orbit), shown in article. I wonder is it possible to solve the problem directly. Username160611000000 (talk) 05:08, 20 November 2017 (UTC)

I wouldn't usually bother trying to answer such a malformed question, but I think the OP is trying to calculate the escape velocity from the Earth's surface to interstellar space. I don't know why it is done in two steps, perhaps https://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics) explains in enough detail. Greglocock (talk) 06:01, 20 November 2017 (UTC)

No, the article Sphere_of_influence is not about my question.Username160611000000 (talk) 06:32, 20 November 2017 (UTC)

As you like. I've just done a MOOC in Space Mission Design and Operations and I can assure you that SOI was fundamental to the three stages of planning the delta V needed for an interplanetary mission. Greglocock (talk) 09:19, 20 November 2017 (UTC)

As I see from the article the Sphere of influence (SOI) is an approximate imaginary surface, Feynman said nothing like that. And again I do not care about space dynamics. The exercise is to calculate initial speed to make the probe guaranteed to move at infinity with a residual speed v. I simplify the exercise to zero residual speed .
All we know and all we should use is lectures 1-14. And 2 -step method was explained by ToE here and here. The 2-step method is next. When the probe starts from the Earth with speed 11 km/sec it then overcomes Earth gravity and is flying in solar system with speed 30 km/sec in Sun ref.frame and with 0 km/sec in Earth ref.frame. To escape it should have 42 km/sec. So the excess speed = 42 - 30 = 12 km/sec and excess energy = 0.5m(12 km/sec)².
It was not clear why excess energy isn't calculated like 0.5m(42 km/sec)² - 0.5m(30 km/sec)². But when I have counted the conservation of momentum law and got formula ${\tfrac {m{v_{1}}^{2}}{2}}-V_{0}v_{1}m-{\tfrac {GMm}{R}}={\tfrac {m{v_{2}}^{2}}{2}}-V_{0}v_{2}m$, this confirmed that at an initial speed of 46 km/sec the final speed would be 42 km/sec.
Username160611000000 (talk) 12:26, 20 November 2017 (UTC)

Feynman didn't mention SOI because he was only considering two bodies. Duh. You want three bodies (probe earth sun), so SOI becomes an important concept. Out. Greglocock (talk) 18:44, 20 November 2017 (UTC)

Feynman has mentioned all planets in lecture 9, sec. 7 and showed a way to calculate positions at any moment with any wished accuracy. But the exercise 14.21 is to lectures 13, 14 on Energy. Feynman never proposed exercises that go beyond the material of lectures. In this exercise it is not asked to find all coordinates of the probe. I will use SOI and the numerical methods when in an exercise it will be asked. Username160611000000 (talk) 19:02, 20 November 2017 (UTC)

I don't want to go on scribd, just on account of not liking the site. But the equation shown seems unnecessarily complicated. The sun and the earth aren't changing - their relative kinetic and potential energy will add up to some constant. Using the sun as our fixed frame of reference, the probe will have a positive kinetic energy because it moves with the Earth's surface, and a negative potential energy relative to the sun, and a negative potential energy relative to the earth. Oddly all of these change during a day before launch because the surface spins with or against the orbital velocity, the probe gets nearer and further from the sun, and the tides increase and decrease the Earth's gravity. But just pick a launch window and stick with it -- I assume the best time is roughly when the probe, if released, would spin out away from the sun rather than toward it? It makes sense that the increasing and decreasing tide would do work on the probe as it stands on the launch pad, I think... Anyway, if you can tot all that crap up you ought to get an answer for pure kinetic energy given a certain distance from the sun. Wnt (talk) 00:01, 22 November 2017 (UTC)

November 20

How hard can ice be?

A plastic bag containing a 3 to 4 kg chunk of ice fell of a table and cracked a terracotta floor tile. The ice showed almost no damage. Was the tile substandard or can ice really be harder than terracotta? The ice temperature was estimated at about -10 to -15 Celcius. It was frozen in a walk-in freezer at a meat packing plant set to -20C. Roger (Dodger67) (talk) 17:14, 20 November 2017 (UTC)

There's much more at work here than merely "hardness"; among other things, there is also the shape of the two pieces and the manner of impact. If you hold up a piece of aluminum foil, you can deform it with a cotton ball or feather, but you wouldn't normally say that either of them were "harder than" aluminum. Consider also that there probably were small cracks in the ice, but these essentially can "heal" if they don't immediately result in fracture. Matt Deres (talk) 17:38, 20 November 2017 (UTC)

The temptation is to answer this question by referencing the famous Mohs scale of mineral hardness, where ice is a 2 and terracotta is usually closer to a 5. But that's not the kind of "hardness" we're looking for here. It just means that terracotta will easily scratch a piece of ice, while ice will not easily scratch terracotta, which I think we knew.

We're really looking for a measure of material strength. Probably either Toughness or Fracture toughness.

I don't have an exact answer for you, but check out this mystifying chart.

If I'm reading it correctly, terracotta, being a porous non-industrial ceramic, should be tougher than ice, but not a lot tougher.

So why did your tile break but not the ice? Probably luck. Ice could certainly be heavy enough to smash tile. The mystery is really only why the tile broke first. That probably comes down to how they landed. Whether there was a pressure point or a weak spot, etc. (Also, Are you sure the ice was undamaged? If a chip came out of the ice at high speed it could absorb a lot of the energy.)

I don't think anyone can give you an exact mathematical answer without about a bunch of measurements and stuff. Sorry. ApLundell (talk) 17:39, 20 November 2017 (UTC)

Per ApLundell, there are different measures of hardness, it's a vague term and asking how "hard" something is depends on what you mean by hardness. None of these, however, strictly applies to the scenario being described. One can break a steel container with nothing but the force of air pressure, and yet air is not "hard" by any definition. The relevent thing here is not how "hard" the ice is, per se, but with how much force it strikes the tiles, over what area, and over what period of time. Higher forces concentrated in smaller areas over shorter periods of time are more likely to exceed the forces necessary to break the tiles, regardless of what provides that force. --Jayron32 17:45, 20 November 2017 (UTC)

There is probably a bunch of measurements and stuff for ice at http://www.tms.org/pubs/journals/JOM/9902/Schulson-9902.html and refs (in particular #39), but I am a bit short on time right now to look at them. Tigraan^{Click here to contact me} 18:10, 20 November 2017 (UTC)

• You can break a floor tile with a rubber mallet, if it's not perfectly bedded.

Tiles are brittle. They are hard and strong, but they will not bend. If you support the tile on two sides with a wide gap in the mortar beneath, then any load on the top can cause them to bend, thus break. A soft impact (ice or mallet) might not give a sharp point, but if the ice is concentrated over an area smaller than the mortar gap, then even if the ice is crushed, the tile can still break. Andy Dingley (talk) 18:20, 20 November 2017 (UTC)

You may also be interested in Pykrete, a mixture of ice and sawdust which is bullet-proof and was seriously considered for the construction of a giant aircraft carrier called Project Habakkuk. Alansplodge (talk) 18:31, 20 November 2017 (UTC)

Mammoth Size

Why were woolly mammoths estimated to be smaller than other mammoths and elephants if they lived in cold climates? wouldn't they survive more easily in the cold if they were larger? אדנין (talk) 19:40, 20 November 2017 (UTC)

The premise is questionable. Woolly mammoth states that they were about the same size as the African elephant, the largest elephant species alive today. When it comes to other mammoths, Mammoth says that "most species of mammoth were only about as large as a modern Asian elephant", so that means that wooly mammoths were larger than most mammoths. - Lindert (talk) 19:49, 20 November 2017 (UTC)

According to this the Wooly Mammoth was about the middle in terms of size of Elephantidae. This notes "Mammoths and modern elephants overlap significantly in body mass." This also has a size chart that puts the Woolly Mammoth right at the middle in terms of average size. --Jayron32 20:08, 20 November 2017 (UTC)

But still, the Columbian mammoth and straight-tusked elephant lived in warmer climates, right? and they were still bigger than the woolly mammoth. אדנין (talk) 06:57, 21 November 2017 (UTC)

Quality and quantity of available food might have been a significant factor. Note that the last surviving woolly mammoths, on Wrangel Island, had become dwarfed: Island dwarfing is a phenomenon attributed partly to lower availability of food resources in a geographically restricted habitat. Assuming that woolly mammoths were woolly because they generally lived in colder climates than other mammoths, their available food resources are likely to have been poorer, too. {The poster formerly known as 87.81.230.195} 94.0.37.45 (talk) 08:43, 21 November 2017 (UTC)

Big size has advantages and disadvantages. Big animals are less threatened by predators but they usually also reproduce much slower. One theory in the science debate about Quaternary extinction event is that many species became distinct because of human hunting. If that was the truth, size did not matter much in sense of big elephant or small elephant because they where the delicious meat burgers which could not hide anyway. --Kharon (talk) 10:01, 21 November 2017 (UTC)

Despite their mammoth hide. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:20, 21 November 2017 (UTC)

The presumption that larger animals live in colder climates seems an odd belief to have, given the billions of counterexamples where it isn't true. --Jayron32 16:33, 21 November 2017 (UTC)

The presumption seems to be a misunderstanding of Bergmann's rule. B8-tome (talk) 18:49, 21 November 2017 (UTC)

November 21

Does physics have axioms?

Does physics have some kind of axioms? Should we treat as a given at least the perception of basic units? For example: movement, change, or distance? Or how should we call the basic conceptual units? --B8-tome (talk) 11:24, 21 November 2017 (UTC)

Yes, most science has axioms. Much of the more theoretical end of their researches is about either removing such axioms, or at least clearly defining them in their minimal form. An experimental scientist might see axioms as a failure of science thus far, as some form of deus ex machina. A mathematical theoretician though sees them as the basis for a formalised and axiomatic system, an approach that has been powerful in mathematics. For physics, see Hilbert's sixth problem and Hilbert space. Andy Dingley (talk) 11:38, 21 November 2017 (UTC)

Causality (physics) is often considered axiomatic in physics. See Axiom of Causality, which is a bit weak for a Wikipedia article, sometimes this is called the "Causality principle". --Jayron32 12:14, 21 November 2017 (UTC)

• Most of physics is grounded in mathematical theories, which entail the acceptance of the axioms of the underlying mathematical theory.

About experimental sciences (rather than strictly physics), Asimov has a good quote: I believe in evidence. I believe in observation, measurement, and reasoning, confirmed by independent observers. I'll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be. (The end of it is merely a wordy explanation of the concept of a Bayesian probability, but the beginning is an assumption that knowledge can be gained by observation, which is not trivial philosophically speaking - see problem of induction.) The foundation of the experimental method is the belief that any well-designed experiment ought to be repeatable at least in some statistical sense. For instance, if I throw a coin many times, each result might be random, but the average result over a large number should depend only on my throwing technique, whether the coin is rigged, etc., and not on extra variables (such as the time of the day or the location on Earth where I perform the experiment) which ought not to matter. If it does turn out that there is some variation, we will ascribe it to extraneous variables that we failed to take in consideration rather than to a variation of the law of physics with space/time.

Accepting the above, a lot of experimental stuff can be tricky to classify. For instance consider the Huygens-Fresnel principle. It is "simple" (in an Occam's razor sense), and matches experimental data, but we do not really have a clue about why it is so. Does it count as an axiom, or is it a logical conclusion of the experimental evidence plus the "axioms of experimental method"? Tigraan^{Click here to contact me} 13:15, 21 November 2017 (UTC)

Well, if you really want to get metaphysical, anything beyond simple solipsism requires us to accept the evidence of our own senses and intellect as axiom. There's no way to derive, from sufficient logic, that we can trust our own senses and intellect based on outside evidence, we have to work under the assumption we can trust them to some point. --Jayron32 13:20, 21 November 2017 (UTC)

Well, yeah, pretty much any knowledge-seeking activity will rely on assumptions that the external world exists, that basic logic rules work, etc. But assuming that the experimental method "works" is different. The modern scientist (à la Bacon, Galileo, Newton etc.), when confronted with a phenomenon they do not understand, assume that there is some reason for why it happens with explanatory power (i.e. the reason does not just fit the current data, it also tells you stuff about other possible experiments or the later repetition of that experiment).

Consider radioactive decay which seems to follow some statistics (Poissonian process). We are not sure why it is so, and it may well be that we will never know of any explanation that is further up in the chain of causes and consequences; yet we take the belief that any radioactive decay phenomenon follows those laws, not just the ones we observed. Maybe the immediately-preceding root cause is that some deity is playing dice for each atom at each instant; but we still assume that the dice are thrown the same way when we are and when we are not looking (rather than the deity stacking the results when experiments are made in a lab). Or to take another example: nowadays, we have some reasonable clue as to how thunder strikes; a couple millenia back, "Thor made thunder follow yet-to-be-known laws" was a scientific assertion, when "thunder strikes when and where Thor pleases" was not. Tigraan^{Click here to contact me} 13:59, 21 November 2017 (UTC)

Some aspects of the answers are going too far. Math axioms will be valid within physics, but they are not a part of it. I also don't want to enter into the philosophical question whether we are in a simulated universe like The Matrix. I mean axioms within physics. You can break down physical concepts like speed into the rate of distance and time. Rate is a mathematical concept. Distance and time are physical concepts. Can these be broken down in even smaller stuff? Does it stop somewhere? B8-tome (talk) 17:16, 21 November 2017 (UTC)

The lead of Special relativity says

In Albert Einstein's original pedagogical treatment, it is based on two postulates:

The laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).

The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

See also Postulates of special relativity. Loraof (talk) 18:17, 21 November 2017 (UTC)

• Also Mathematical formulation of quantum mechanics#Postulates of quantum mechanics. Loraof (talk) 18:32, 21 November 2017 (UTC)

The Laws of thermodynamics are axioms or conceptional laws of physics. But since logic is officially part of math you could as well argue that all axioms are mathematical concepts, in the end pushing even philosophy from its holy throne and instead crowning math as highest and most basic of all sciences. However real science does not care about crowns and domination. What it really wants is harmony so it does not matter if an axiom is physics or math, it matters that an axiom works for as many parts of science as possible. --Kharon (talk) 06:37, 22 November 2017 (UTC)

Using a transistor rated for 45 V in a 60 V circuit

According to my thinking, I should be able to use a BC337 transistor rated for a maximum of 45 V across the collector and emitter in a circuit which uses 60 V to power LED filaments because, when the transistor is off/closed, the LEDs (there are about 18 in series, I suppose) should drop some of the voltage themselves.

I don't yet have my LED filaments so I did an experiment with four regular LEDs in series with the transistor and applied 8 V. With the transistor off/closed, I measured the voltage across the transistor to be 2.2 V and the voltage across the LEDs to be 2.4 V even though the voltage across the transistor AND the LEDs was 8 V and 2.2 + 2.4 != 8. I was advised (on an electronics forum) that this discrepancy is because the DMM itself is passing current and affecting the measurement. I was also told that I should just use a transistor rated for 80 V but I think this was a lazy response that doesn't conisder the voltage being dropped by the LEDs. --185.216.48.85 (talk) 16:18, 21 November 2017 (UTC)

• A DMM or "valve voltmeter" shouldn't pass significant current. It's the function of their high impedance input to reduce this to a low level that won't load a circuit like this. However, when your transistor is switched off the circuit's impedance will itself rise to a level that it again becomes comparable to that of the meter, and your meter will (probably, depending on the meter) become a significant load again, as the forum advised.

As to the breakdown voltage, then the V_CEO limit for a BC337 depends on the current, but it also has a sharp cutoff at around 45V. The transistor is assumed to fail above this, whether any current is flowing or not. The idea that the LEDs will drop voltage is only true if they're flowing current. In the zero-current situation like this, the transistor voltage will still float high, toward the supply rail voltage (and the transistor fails). Of course, it might work. It might well do, but fail early. It's impossible to know without serious testing, but you are working past the limits of the datasheet, so any failure would be an "I told you so". I'd probably look at using an MPSA42 rather than a BC337. Andy Dingley (talk) 16:47, 21 November 2017 (UTC)

Okay, thanks for the recommendation; they're pretty cheap on AliExpress. --Seans Potato Business 17:49, 21 November 2017 (UTC)

See data sheet for BC337. This transistor type, manufactured in quantity with a spread in collector-emitter breakdown voltage, is tested and sorted thus: If V_CEO is less than 45V it may be marked BC338, if V_CEO is over 45V it can be marked BC337. If you test individually a batch of BC338 you may be able to extract a minority with a higher V_CEO than 45V, but why bother? There are lots of good fish in the sea, consider a 2N2484. See Breakdown voltage#Diodes and other semiconductors in Wikipedia. Blooteuth (talk) 13:28, 22 November 2017 (UTC)

Pomegranate pith

From an evolutionary view, do the inedibility of pomegranate flesh (pith) and its relatively thick envelope mean to protect it from harmful birds in the same way as poisonous berries do? Does it also mean that because of that pomegranate relies more on pollination rather than seed dispersal? 212.180.235.46 (talk) 16:30, 21 November 2017 (UTC)

This seems to be a good start at answering some of your questions. --Jayron32 16:32, 21 November 2017 (UTC)

Computing with magnetic knots

This is the typical sort of thing that takes but a moment to daydream but so much more to figure out...

1) Can you model a finite state machine or some useful computing task in the unravelling of a trivial knot, i.e. one that can be pulled out to a simple loop, perhaps with the output of the task stored in the final shape of an interlocked simple loop that would be forced into a trivially knotted state in the process?

2) Can a frictionless trivial knot definitely be unravelled to a loop by simply applying a force?

3) I know a magnetic field can be knotted. [10][11] Some of what I'm thinking might somehow be related to magnetic skyrmions... I'm not sure. But is a knot in a magnetic field frictionless?

4) Is there a way to control how readily magnetic reconnection occurs in a sample region, so that a complex trivial magnetic knot (and "answer" knot) be set up in an arbitrary state, then the system is changed so that if pulled on it will unravel rather than reconnecting/decaying?

Extra: If magnetic reconnection occurs near Earth (as the article says) does that mean that there are magnetic knots wandering through our bodies as we sit here, as subtle small scale variations in the Earth's magnetic field or something? Is there a way to "harvest" these, trap them as they pass through some kind of collection screen? Could you use them for energy, clandestine communication, or some other high weirdness? Wnt (talk) 21:03, 21 November 2017 (UTC)

For the Extra, these magnetic effects that you talk about are in plasma. This is not the state of matter in our bodies or the Earth's atmosphere, so we only have more distant effects of electric currents in the ionosphere or magnetosphere. These could have some dramatic effects in a geomagnetic storm. But the fields will be large, hundreds of kilometers in size. Graeme Bartlett (talk) 22:16, 21 November 2017 (UTC)

@Graeme Bartlett: Magnetic field lines from the Earth's magnetic field pass through us right now. Why can't they be knotted in or near our bodies? Wnt (talk) 23:44, 21 November 2017 (UTC)

You can use Maxwell's equations to get a relationship between the magnetic field and the electric field and any currents. There are electric currents in the body, but they are minuscule. Read Bioelectromagnetics. The forces due to magnetic fields in the body will not be large enough to move material around, and mechanical or chemical forces will predominate. This is different to a plasma, where the material is free to move, and conducts, and is highly affected by electromagnetic fields. The overall magnetic field around us is to the Earth's magnetic field modified by ionospheric currents. (see Ionospheric dynamo region and Magnetospheric electric convection field.) You may be interested in Geomagnetic secular variation and Geomagnetic excursion. You will find that the sizes of these magnetic structures are huge - planetary sized. Graeme Bartlett (talk) 00:35, 22 November 2017 (UTC)

You may also be interested in https://books.google.com.au/books?id=6JgFyk-zku8C and Knot theory. I think the answer to (2) is no, as you can get a tangle formed. Graeme Bartlett (talk) 00:43, 22 November 2017 (UTC)

(ec) Hmmm... Maxwell's equations famously describe the motion of magnetic and electric fields through hard vacuum! Are such oddities completely impossible at less than lightspeed, or could some strange coiled path retain them at a lower speed? And I'm also not sure why the tenuous nature of the atmosphere and the relative uninterestingness of our bodies wouldn't simply make the knots weaker when examined at a human scale. Still, I have to admit what you say seems to match common sense. Wnt (talk) 00:47, 22 November 2017 (UTC)

Those magnetic knots etc will only be possible if there are also electric currents. So that means they are happening in some material, which would likely have friction. Perhaps in a superconductor there is no friction, but when magnetic field lines move through that there would be some kind of friction. Graeme Bartlett (talk) 01:00, 22 November 2017 (UTC)

A paper about knots being used for computation http://www.mdpi.com/2073-8994/7/3/1289/htm I don't know if it is any good though, or if you can understand it. Maxwell's equations describe the fields though materials too. Dielectric constant and electric currents may vary in a substance. Graeme Bartlett (talk) 07:19, 22 November 2017 (UTC)

MDPI is kinda-sorta-not-really predatory. Tigraan^{Click here to contact me} 09:57, 22 November 2017 (UTC)

You might like [12], though there the knots are to protect from noise. Dmcq (talk) 12:23, 22 November 2017 (UTC)

Like Magnetic-core memory? --Kharon (talk) 23:42, 22 November 2017 (UTC)

Between Turing's conception of a computer in 1936 and the introduction of Transistor computers, Elliot sold computers that used magnetism to calculate. They exploited the magnetic hysteresis property of hard ferrites to create elements of a computer such as logic gates and memory. Blooteuth (talk) 11:36, 23 November 2017 (UTC)

Medieval chemistry

How did medieval alchemists make sulfuric acid? 2601:646:8E01:7E0B:404:F3D3:C557:159A (talk) 23:47, 21 November 2017 (UTC)

Oil of vitriol was made from vitriol. For more see [13] (you can find a PDF download) which I think should cover it. Wnt (talk) 00:06, 22 November 2017 (UTC)

The sulfuric acid article gives particular credit to roasting green vitriol (Iron(II) sulfate) in an iron retort. The iron (II) sulfate occurs as a hydrate, so there is plenty of water to crack apart, yielding Fe(OH)2 + H2SO4, I presume. <--- actually the source below says it is FeO + H2SO4. Any neutral H2SO4 formed would instantly evaporate, thus being removed from the reaction and driving equilibrium toward the product, and could be condensed elsewhere. Still, I would think the presence of some other acidic component in the reaction would greatly assist this process by increasing the fraction of SO42- that is protonated... so I don't think I've told everything. Wnt (talk) 00:38, 22 November 2017 (UTC)

This source seems to be adequately researched, but otherwise unverified. It includes Biringuccio's description of the process. 2606:A000:4C0C:E200:E958:86E3:541F:E7F1 (talk) 00:42, 22 November 2017 (UTC)

Interesting! What kind of equipment did they use -- I presume that the dry distillation had to be carried out at red heat? 2601:646:8E01:7E0B:1984:C5C9:D8FC:B368 (talk) 06:13, 23 November 2017 (UTC)

November 22

Why aren't odd carbon number fatty acids more common in biology?

At least for plants and animals. Are odd fatty acids more common in other kingdoms? Why aren't there more short fats like triformin and triacetin in bulk fat/oil? Why are only a few omegas common like omega-3, -6, -7, and -9? Sagittarian Milky Way (talk) 04:11, 22 November 2017 (UTC)

Fatty acid synthesis would be a good starting point for helping you research your own answers to this question.--Jayron32 04:17, 22 November 2017 (UTC)

Fatty acid#Synthesis is more to-the-point. tl;dr: in most organisms the synthesis pathway creates fatty acids by adding two carbon atoms at a time. --47.157.122.192 (talk) 09:14, 22 November 2017 (UTC)

As for Why are only a few omegas common like omega-3, -6, -7, and -9?, hmm, Omega-3 fatty acid#Structure might be informative, particularly that huge image caption (which really should be moved into the article body). It might have something to do with their properties vis-a-vis cell membranes. The omega nomenclature is a means of classifying different types of polyunsaturated fatty acids; the relative amount of saturated-to-unsaturated fatty acids in cell membranes affects their properties. In particular, DHA is a very important component of brain neuron cell membranes. But I'm not sure of an exact answer to your question; maybe someone who knows more about biochemistry can help. --47.157.122.192 (talk) 21:18, 22 November 2017 (UTC)

It's clear that there is some biological specificity here that is dominating, especially in organisms like humans that scavenge what PUFAs they can from external sources, but chemistry may also be a factor. To put the double bond at omega-2, for example, would make the end an allylic carbon, which would be more susceptible to oxidation ([14] ; true, from this it is apparent that other positions are also vulnerable, but perhaps those are protected to some degree by the membrane structure?). Exposing the alkene directly would also invite troublesome reactions. Within the lipid, adding adjacent conjugated double bonds would give it a color, causing it to absorb light, which might lead to unwanted reactions or otherwise prove undesirable. (If you only had two conjugated it might not be visible but it could still absorb more UV). Wnt (talk) 11:53, 23 November 2017 (UTC)

General Relativity questions

I recently watched a youtube playlist on General Relativity titled "Curved SpaceTime and General Relativity" and I have a couple questions.

The playlist is a very quick overview of some of the major muscle movements regarding General Relativity. It builds up to the conclusion that 4-dimensional spacetime is not flat but curved, which brings me to my first question: The curvature of a 4-dimensional object implies that there is a fifth dimension, correct? If so, is there any scientific understanding of what this fifth dimension is?

Second, one of the videos in the playlist, titled Is Gravity An Illusion?, speaks to the difference between Newton and Einstein's description of gravity. The main point being that Newton viewed gravity as an actual force and Einstein viewed it as an implied force. The video seems to claim that Newton would view an apple falling to earth as an apple falling down toward a earth & Einstein viewed an apple falling to earth as the apple being more stationary and the earth moving up toward the apple. I suspect this depiction is less intent on being scientifically accurate than it is to get people to stop relying on their "common sense" notions of science - as a necessary step for grasping General Relativity.

So, can anyone confirm which is actually the case regarding the apple? The playlist as a whole seems to be saying that falling objects are under the influence of spacetime curvature not the Newton concept of gravity. That said, what of a falling apple on earth? Does curved spacetime bring the apple towards earth or earth towards the apple? If it is the earth towards the apple, can anyone point me to good explanations of how that is so because I'm still not clear on that point.

128.229.4.2 (talk) 14:35, 22 November 2017 (UTC)

Of course, any curved or flat space-time can be immersed in a space-time of a higher dimension. However such an immersion is not a necessary condition for the existence of space-time curvature. As to an apple and the Earth, you should note that any motion in General Relativity is relative. Therefore, which object moves and which stays at rest, depends entirely on your choice of reference frame.

In General Relativity all free floating objects move along geodesics. In curved space-time they are such that two object, which are initially at rest in some reference frame, will move towards each other. Ruslik_Zero 16:46, 22 November 2017 (UTC)

Extra link for the win: geodesics in general relativity. --47.157.122.192 (talk) 21:04, 22 November 2017 (UTC)

To answer your first part, an N-dimensional curved space can always be mathematically embedded in an (N+M)-dimensional flat space. However, M is not necessarily one. The Whitney embedding theorem guarantees that M is no more than N, but it isn't obvious if 4D spacetime is embeddable in a 5D flat space or how many extra dimensions would be required. (This rather depends on how complicated the topology of spacetime actually gets.) That said, whether such an embedding has any physical meaning is an unanswerable question at present. As far as we know, there is no method of interacting with anything outside of 4-dimensional spacetime, and by extension there is no way of probing the properties of anything outside it. As long as that remains true, the question of whether higher dimensions have any physical meaning is largely unanswerable. (String theory, M-theory, and some of the other hypothetical extensions of general relativity posit additional accessible dimensions, but so far such effects are untested.) Going further, though a flat embedding space does exist (at least as a mathematical construct) it isn't necessary to refer to such a space in order to study the geometry and effects a curved spacetime. The broad field of differential geometry has developed methods of studying curved spaces purely from the properties as they exist in the space itself (e.g. how objects in the space move relative to each other), without the need to reference any higher dimension. The field of general relativity generally applies these methods to do calculations within 4d spacetime without the need to talk about a fifth dimension. Dragons flight (talk) 17:17, 22 November 2017 (UTC)

Thank you Ruslik_Zero & Dragons flight (talk). I find this stuff fascinating & I appreciate your help in progressing my understanding!

128.229.4.2 (talk) 20:05, 22 November 2017 (UTC)

Civil engineering and programme management

In civil engineering and programme management, which type of organisation is seen as the most prestigious to work for? Client, contractor, design consultant or programme management consultant? 193.240.153.130 (talk) 16:22, 22 November 2017 (UTC)

Define "prestigious". ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:52, 22 November 2017 (UTC)

Which is the one most people want to work for? 82.17.228.129 (talk) 18:05, 22 November 2017 (UTC)

The one that pays the best? ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:17, 22 November 2017 (UTC)

This question has been asked and answered here some weeks ago. Please visite the archive. --Kharon (talk)

Yes, by this same user, several times this year. It might help if he would explain why he's asking these questions, and maybe he could get more satisfactory answers. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:34, 23 November 2017 (UTC)

November 23

Science or Fiction ?

In one of his marvels, Crichton, (methinks it was State of Fear), mentions a machine known as "Cavtier", (forgiveness if I misspelled) which we are told is illegal in almost all countries. The great thing about it, which is empowered by very powerful generators required to be carried along in several very big vehicles, can dig a cavity in even the of hardest grounds. Another great feature about it is that you don't even have to bother with the dug-out, which would be heap(s) of mountainous sizes when you will be needing a well of about (say) 20 feet diameter that should be (let's suppose) 60 or 70 feet deep, (dimensions those, as told or implied to the reader, are perfectly within it's capabilities!) as all that stuff (the dug-out) will get pressed (talk of pressure!) into the "walls" of the brand-new, perfectly dug anti-cylinder of 20' diameter, 60 'or 70' deep. Do such machines actually exist in real life ?  Jon Ascton  (talk) 06:26, 23 November 2017 (UTC)

Well you can produce a miniature version of your cylindrical hole by driving a fence-post into the ground with a sledge-hammer or a post-driver. Pile drivers are a slightly bigger version, but I've never seen a giant version on the scale that you mention. I think a conventional excavator is more energy-efficient and environmentally friendly. On the scale that you mention, I would be worried about triggering earthquakes and shaking the foundations of nearby buildings. In most areas, soil is not sixty feet deep. Dbfirs 08:12, 23 November 2017 (UTC)

These factual articles describe present tunnel boring equipment and latest methods. Blooteuth (talk) 11:16, 23 November 2017 (UTC)

Yes, I was thinking of vertical cylinders, but horizontal cylinders of a much bigger size are common. I've walked through some of them. Dbfirs 16:21, 23 November 2017 (UTC)

The word you are looking for, @Jon Ascton: is cavitation, and we have several related articles on supercavitation and cavitation hammer and soforth. Crichton is obviously vague on the details. μηδείς (talk) 19:07, 23 November 2017 (UTC)

Impossible asteroidial speed?

How could ʻOumuamua have moved 103 km/s relative to one star, TYC4742-1027-1, quite far (0.25 light year), and only 26 km/s relative to our star? 103 km/s is already about half of the orbital speed of the stars here. --Mortense (talk) 11:06, 23 November 2017 (UTC)

Not sure what problem you are imagining? In our local region of space, the stellar velocity distribution has a standard deviation of ~30 km/s. A relative change of 77 km/s from one star to another would be somewhat unusual, but not so unusual as pose any sort of a fundamental problem. Dragons flight (talk) 11:33, 23 November 2017 (UTC)

[Edit Conflict] :Because that star has a very different Space velocity to the Sun. Whilst on average other stars in our Sun's vicinity are orbiting the Milky Way at roughly similar velocities (i.e. speeds and directions), many are not. {The poster formerly known as 87.81.230.195} 90.208.173.186 (talk) 11:40, 23 November 2017 (UTC)

I am imagining that 103 km/s is not the correct figure. That there is an error somewhere. --Mortense (talk) 14:00, 23 November 2017 (UTC)

103 km/s matches the cited source [15]. If you think the source is wrong, there is an email address on their paper, so you could ask the author. Otherwise, I'm not sure what we can tell you. I am assuming that no one here is likely to rerun their simulation to try and check it. Dragons flight (talk) 14:16, 23 November 2017 (UTC)

There are definitely stars (belonging to the halo or thick disk) that have high velocities relative to the Sun. One example is Barnard's star. So, a velocity of 103 km/s is not that unusual. Ruslik_Zero 18:51, 23 November 2017 (UTC)

Which is well known in astronomical circles for being likely the fastest star in the universe (in degrees per millennium, it moves 1 Full Moon width per 2 centuries or so relative to most stars (the ones too far, slow and/or non-tangentially moving to have much proper motion). By star standards a degree/360 years is runaway. Sagittarian Milky Way (talk) 20:44, 23 November 2017 (UTC)

The fastest proper motion in degrees per millennium depends on the relative velocity being high and the star being relatively close. It's irrelevant to the question of relative velocity alone. --69.159.60.147 (talk) 06:34, 24 November 2017 (UTC)

May I direct attention once again to the link I provided earlier: namely Stellar kinematics#High-velocity stars. These are stars "moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighbourhood" [my italics]. Stars moving up to 100km/s relative to the local average (see Local standard of rest) are not even considered high-velocity stars. Those relative velocities are not all in the same direction (and the Sun itself is moving about 20km/s relative to that average motion – see Solar apex), so obviously a particular body can have differing velocities relative to two different stars of this order of magnitude. {The poster formerly known as 87.81.230.195} 90.208.173.186 (talk) 09:38, 24 November 2017 (UTC)

Phone camera science

Phone cameras have come a long way but why is it that the one area they consistently fail against DSLRs continue to be low light conditions? DSLRs also generally seem to be superior on colour contrast, depth and detail. 82.132.232.28 (talk) 13:03, 23 November 2017 (UTC)

Camera lenses, amongst other things, have to capture light. The bigger the front element (the hole at the front), the more light they can potentially capture. So phones lose out.

OTOH, the smaller the aperture (which is related) the greater the depth of focus (see Ansel Adams for what you can achieve with that technique). So phones also gain from this, and the problem of focussing is easier. Andy Dingley (talk) 13:41, 23 November 2017 (UTC)

Aperture? Isn't it the f-number? The focal length can't be very long in a phone, thus a small f-number, thus lower depth of focus. Or is there more to this? --Mortense (talk) 14:15, 23 November 2017 (UTC)

Yes, f-number (the ratio between aperture and focal length) would be a more general measure here. I was trying to keep it simpler. Andy Dingley (talk) 15:41, 23 November 2017 (UTC)

According to which metric do you believe phone cameras "fail" relative to a DSLR? If you can specify, I would be happy to provide a well-referenced explanation and lots of further reading material. The quality of a complete camera product is a composite of its parts (including the sensor, other electronics, and lens); its software and control; and the skill and subjective design choices of its designers; and it is constrained by its energy-, mass-, volume-, and monetary cost- budgets.

A fun place to start reading is the product line-up website of Sony Semiconductors, the commercial conglomerate that manufactures many of the image sensors in both large- and small- consumer cameras. They publish short-form "datasheet" flyers for "mobile" product markets and for "camera" product markets. A well-informed reader can easily compare and contrast the sensor technical specifications and draw some conclusions.

Nimur (talk) 21:18, 23 November 2017 (UTC)

How are 'bits' of information placed on magnetic tape

In a traditional magnetic tape (like cassette) how are the 'bits' of magnetic impulses placed along it? Are they just a single row along the plastic film? The tape head seems to have a single metal reader, so I wonder whether it could read in parallel at all. But if they are just a row, it seems a waste of space. --B8-tome (talk) 14:44, 23 November 2017 (UTC)

See history of multitrack recording. Gandalf61 (talk) 15:00, 23 November 2017 (UTC)

On the compact cassette there are either two mono tracks (side A and side B) or four tracks for stereo, indeed just running parallel to the tape. It is somewhat inefficient, but technologies like helical scan come with complexity and expense, and due to greater accuracy they may be poorly suited to a portable recorder (jogging with your Walkperson). And the C cassette is analog, so no bits. 91.155.192.188 (talk) 15:36, 23 November 2017 (UTC)

• Cassette is analog, that's way I said 'bits' and not bits. How would you refer to the unit of information on them?B8-tome (talk) 16:47, 23 November 2017 (UTC)

An analog signal is a continuous stream, which in the case of audio has instantaneous amplitude proportional to air pressure variation. The pattern of magnetism put on a compact cassette has superimposed cycles of an ultrasonic frequency. This is an inaudible "bias" signal that overcomes distortion caused by the magnetic hysteresis of the ferric oxide tape coating. Blooteuth (talk) 22:46, 23 November 2017 (UTC)

If the tape were narrower, with narrower tracks, wouldn't the noise to signal ratio be even higher than on the usual cassette tape? Wnt (talk) 16:02, 23 November 2017 (UTC)

Also see Digital Audio Tape and Digital Data Storage. Basically they worked like hard drives tho with different and much less delicate mechanics and thus way cheaper and more sturdy. The earliest ones only managed to record a few megabytes of data on one tape but it was a revolution since everyone could suddenly copy and record music, and later even video, with little cost. --Kharon (talk) 16:37, 23 November 2017 (UTC)

In case there's some confusion, tapes for digital storage are still in regular use. LTO-8 (Linear Tape-Open) is the final stages of development with 12TB cartridges (raw capacity) [16] [17]. IBM's proprietary TS1155 has a 15 TB raw capacity [18]. Many small scale operations are abandoning their use of tape for backups/archiving but the cloud storage providers they're turning to may themselves use tape at some level [19] Nil Einne (talk) 19:26, 23 November 2017 (UTC)

• See Magnetic tape data storage. There have been many different ways to do this. Usually, but not always, there are one or more parallel tracks along the tape. When there are parallel tracks, they have been either treated as independent tracks, or read and written in parallel. One alternative to parallel tracks was the helical scan, which was implemented for video signals (VHS) and then adapted for digital signals. The most recent standard is LTO, which used independent parallel tracks. -Arch dude (talk) 04:24, 24 November 2017 (UTC)

Nerve flossing

Can these sensory tracts move relative to each other?

I encountered some sites claiming that "nerve flossing" is a thing to deal with nerve pain. E.g. [20] [21] The idea is at least superficially plausible; the nerve is to be slid this way and that in the hope of somehow working it into a less annoyed position. The specifics, though, in some cases include extending and flexing the head in order to help shift the position of the sciatic nerve, which sends my battered bullshitometer some distance into the red. (It missed the peg on account of the figure at right - I don't actually know the sciatic sensory tract can't move relative to the others, though such a thing would imply a level of lubrication to rival a synovial joint, I think! Also it is possible just that part is bullshit but the lower part isn't.) Searching it on PubMed pulled up one paper about "neurodynamic sliders" that used "neural flossing" as a keyword; there are also a handful of mentions of "neural sliders". The sources tend to be self-help sites and chiropractors, but at the same time, I note the free dissemination of information and the lack of an obvious profit angle from recommending the exercise. So at the moment I'm on the fence between whether this is a science-free delusion or an interesting idea for which I missed the evidence. Can anyone weigh in on its reality? Wnt (talk) 15:50, 23 November 2017 (UTC)

The flexing of the head is useful in order to alleviate the pressure and muscular tension from the upper back and shoulders area toward below. I do not think this part is intended for having a direct effect on the sciatica, but that it will be useful for the purpose of the exercise however. Calling the thing "flossing" is perhaps not the uppermost delicate idea indeed ( as it tends to become complicated of getting kinesiatrics everytime it may seem needed, it's not surprising if self-help sites tend to proliferate, possibly starting polluting subjects and their vocabularies as well ). --Askedonty (talk) 12:23, 24 November 2017 (UTC)

November 24

Can ʻOumuamua be a starship?

ʻOumuamua 117.200.195.57 (talk) 00:05, 24 November 2017 (UTC)

Please see Fermi paradox. It would be very unlikely, considering the distance in time and space separating plausibe intelligent life in the universe. Assuming that the object was not of natural origin, imagine the time required for the probe to reach us and the time required for its acquired data to be available to its owner (even at light speed). —PaleoNeonate – 01:23, 24 November 2017 (UTC)

Besides, we took a spectrum of it and it's made of rock -- not a very likely material for a spaceship! 2601:646:8E01:7E0B:1984:C5C9:D8FC:B368 (talk) 02:55, 24 November 2017 (UTC)

Rock might make a pretty good material for such a ship. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:36, 24 November 2017 (UTC)

The spectrum of 30 Rock (not to be confused with 30 Rock) has a lot of rock but that doesn't mean it's a rock. Sometimes there's rock inside though. For instance, each time the Saturday Night Live musical guest is rock. Sagittarian Milky Way (talk) 04:49, 24 November 2017 (UTC)

We don't know that it's "made of rock." We only know that its surface reflects a light spectrum similar to particular types of rocky asteroids, but a civilization capable of making an interstellar spaceship could trivially easily paint it to simulate such a spectrum :-). Also, as Bugs suggests, making a large space habitat or craft by hollowing out an existing asteroid has long been a staple in scientific speculation. {The poster formerly known as 87.81.230.195} 90.208.173.186 (talk) 09:48, 24 November 2017 (UTC)

This isn't really an answerable question since we can't falsify starship status. We can say that, so far, it hasn't accelerated, fired phasors, exhibited surface artwork, turned Jupiter into a pumpkin etc. We can also say that as starships go, so far it appears to be a damn slow one. (The "20,000 years to leave the Solar System" figure compares quite unfavorably with Voyager 2 and Voyager 1). But a starship is defined by intent, and intent is defined by consciousness, and we don't have an accepted theory of qualia and free will, which means that if an asteroid belt spits out an asteroid after a series of gravitational and physical interactions, we can't actually say that the multi-body interactions involved didn't plan that asteroid to come out and leap between systems; we can't prove that stars and oceans and ten thousand other things don't have some measure of intent to what they do. So for now, the less philosophical people call it a rock, and a handful of more philosophical people call it a rock but imply all sorts of wonderful things that might mean but almost surely doesn't mean. ;) Wnt (talk) 03:36, 24 November 2017 (UTC)

The OP asked "can" it be, so the answer must be "yes". Then you get into probabilities, and the probability seems to be very low. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:16, 24 November 2017 (UTC)

As usual xkcd hits it on the head. 196.213.35.146 (talk) 06:31, 24 November 2017 (UTC)

Galaxy level objects at relativistic velocities?

We observe very far away galaxies moving away from us at relativistic velocities using Doppler red shift. Everything local, though, seems relatively well behaved within our solar system. It seems reasonable that the galaxy interactions required to create spiral galaxies would create objects that are relatively close with velocity/mass shifts that are not within normal solar system bounds. Have we ever observed large objects moving at larger relativistic speeds than normal objects? Do we know which objects created specific bands/arms in the galaxy and what their relative velocity is? --DHeyward (talk) 07:37, 24 November 2017 (UTC)

Optimal shape

A body, let's take steel, has a weight of 1 ton. It is dropped into water, let's say 10 km deep. What is the optimal shape for the fastest sinking/descending possible? Will the surface have to be smooth, dimpled or grooved? Every physics argument welcome. GEEZER^{nil nisi bene} 08:23, 24 November 2017 (UTC)

The streamlined shape is generally the best way to reduce drag.

Rounded at the front, tapered to a point in the back, sort of lack a teardrop. See "streamlined" at right. This the best way to reduce drag, which means your object will be able to fall faster. Whether adding dimples would also help, I don't know. Dimples help reduce drag on round objects, by reducing the wake turbulence, but the tapered shape already serves a similar purpose. Dragons flight (talk) 08:52, 24 November 2017 (UTC)

The idea of different shapes that Dragons flight mentions is discussed article about drag coefficient. We're in three dimensions and with a fixed volume, so we need to consider whether the cross-section in the direction of motion should be circular (the object is a solid of revolution of the streamlined shape) or oval (like an airplane wing) and also the aspect ratio in terms of elongation in the direction of motion. Interestingly, our drag-coefficient article says a "flat plate" has a c_d of 0.001–0.005, which is better than any of the thick bodies. I don't know if that means to be as elongated as possible and/or to be more oval cross-section is better. DMacks (talk) 09:47, 24 November 2017 (UTC)

Probably it should be built with some sort of tailfins, as you see on most airplanes, air-dropped bombs, arrows, and fish. They provide stability of orientation so that the thing will keep moving lengthwise. See Fletching; this talks about air, but I think the same sort of considerations apply in water. --69.159.60.147 (talk) 10:06, 24 November 2017 (UTC)

I doubt tail fins will be necessary for a solid, streamlined object, as long as its centre of gravity will be forward of its centre of lift it should be self stabilizing. The reason that the other things you mention do need stabilizers is that their centre of mass isn't forward of their centre of lift. Rmvandijk (talk) 11:13, 24 November 2017 (UTC)

• It's a fixed weight, not a fixed volume. So general arguments about shape have already been given, but the proportions could well optimise to different proportions depending on the volume and internal dimensions needed. Is this "1 ton" a small solid block, a large hollow shell, or half a ton of shell and half a ton of "works" inside it, needing some minimum dimension? Andy Dingley (talk) 11:07, 24 November 2017 (UTC)

In the thought experiment a solid body. GEEZER^{nil nisi bene} 13:15, 24 November 2017 (UTC)