Wikipedia:Reference desk/Archives/Science/2009 December 18

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December 18

Lone pairs

Why do lone pairs affect the shape of a molecule? --71.144.122.81 (talk) 00:53, 18 December 2009 (UTC)

See VSEPR. Because the lone pairs are still electrons and the still repel other electrons. Lone pair electrons will push away bonding electrons just as well as bonding electrons push each other away. In fact, lone pairs have a greater repulsive effect than do bonding pairs because the lone pairs are more localized around the central atom, and thus more "concentrated" and thus have a greater repulsive force associated with them. Thus, you have a situation where the bonding angle in a tetrahedral system like CH₄ is about 109.5^o, while in H₂O, which is still a tetrahedral system like CH₄, the bonding angle is 103.9^o; the lone pairs around the oxygen have more repulsive force than do the bonding pairs between the oxygen and hydrogen. --Jayron32 03:19, 18 December 2009 (UTC)

As part of that idea, they also represent the same sort of "directionality" (taking up position and space) as if they were a bond. So ammonia, NH₃, still has four pairs like methane does, and therefore still has approximately tetrahedral geometry. DMacks (talk) 05:16, 18 December 2009 (UTC)

Getting your car hot.

Here is an ongoing debate at my local car club - which is rising to annoying proportions! The question is as follows:

The inside of my car is really cold - I turn the "heat" control to max - but how should I set the blower to get the car warm quickly? It seems to me that if I turn the fan on really high - then the cold air won't be in contact with the heater core long enough to get as warm as it could be - and the high air flow is pushing whatever warm air is in the car out and replacing it with cold outside air. On the other hand, if the fan speed is too slow, the air in the heater core would mostly be close to the same temperature as the core - and Newton's law of cooling says that it won't pick up as much energy as it could if it were cooler.

I bet we don't have an article about that - and I don't think it's possible to come up with a solid answer to this based on theory alone...so we're left with experiments.

Now - I've done the experiment every work day for the last 10 days (as best one can with varying ambient temperatures) - and according to my measurements - keeping the airflow low gets the temperature up faster than having the fan on the maximum setting (the opposite of what I guessed - which is interesting). A couple of other people found the same thing (albeit without resorting to actual stop-watches and thermometers as I did) - but since we're a MINI Cooper car club - and our cars are all pretty much identical - that doesn't prove a general point.

So - here's the question - it this finding true of all cars? Does anyone here (with a different kind of car) have some more data points? Is anyone interested in doing some actual science? (OK - this is going to be WP:OR of the worst kind...forgive me!)

SteveBaker (talk) 01:03, 18 December 2009 (UTC)

My understanding is that there's no point having the fan on until the thermostat opens up. All you're doing is blowing cold air into the car. --Trovatore (talk) 01:09, 18 December 2009 (UTC)

That's true - but if the car has been sitting out all night (which is an unstated assumption here), the interior is as cold as the outside air - so there shouldn't be any harm in putting the fan on so that as soon as the thermostat opens up, it'll blow warm air. This actually touches on another question that came up in our club's mailing list...In cold weather, I habitually drive in a lower gear than usual to get the engine up to temperature sooner. This certainly works - but does it waste fuel (because you're running higher revs than you need to) or does it save gas (because the engine runs inefficiently while it's running cold - so getting it up to temperature sooner is good)? On my MINI - which has twin turbo's - driving one gear lower until the car gets warm does indeed save gas - the jury is still out on non-turbo MINI's. SteveBaker (talk) 01:27, 18 December 2009 (UTC)

Well, the interior won't be quite as cold as the outside air — if nothing else, some of your own body heat will warm it up. Not a large effect, but in a small car, especially with passengers, it might not be negligible. More important from a practical point of view is that moving cold air will suck heat from your body faster than still cold air. --Trovatore (talk) 02:09, 18 December 2009 (UTC)

I'd love to help you Steve, I've got a thermometer and a stopwatch... but it's summer and I don't have an air-conditioned garage. --203.202.43.54 (talk) 01:49, 18 December 2009 (UTC)

and yes, it does use more fuel to drive in a lower gear. But you can't call it a waste if you want the heat. It should be obvious by the fact that you're warming the engine more quickly - that heat has to come from somewhere. The somewhere is the increased friction from faster moving engine parts... unless your mini has frictionless bearings running in vacuum. As an aside, apparently your lowest fuel usage (on flat road) comes at the lowest throttle setting (that doesn't lead to the engine lugging) in your highest gear. --203.202.43.54 (talk) 01:53, 18 December 2009 (UTC)

Oops, I didn't read the question properly. As a comparison to running with a cold engine for longer, it's hard to say - I presume your car has a near instantaneous fuel efficiency display, so you should be able to compare fuel used in the first couple of kilometers of a trip following the same path using each method. --203.202.43.54 (talk) 02:13, 18 December 2009 (UTC)

SO I'm sitting here reading through this long, detailed question and the entire time I'm thinking man, SteveBaker will be all OVER this! and then I get to the end and I see that... he was the one who asked it. I'll be damned! 218.25.32.210 (talk) 03:16, 18 December 2009 (UTC)

My personal experience - and I've had several cars: Chevrolet Caprice, Ford Mustang, Pontiac Sunbird, Volkswagon Rabbit, Saturn SC1, and a Toyota Corolla (I've excluded motorcycles as they don't apply)... The heater won't be hot until the engine warms up. That is not necessarily a given. The heater core could heat without the engine running as it is electric itself - but my experience is that it doesn't. Only in the Caprice (which was modified as a police car) did the heater blow how air without the engine running. So, that is the first thing to check. Will your heater blow hot without the engine running? If not, then the heat is obviously relative to the engine temperature. The heat of the engine is relative to the RPMs since it is obvious that the engine gains heat with every tiny little explosions inside each cylinder. However, driving the car in cold weather cools the engine - cold air blows through the grill and across the engine. So, to get heat as soon as possible, I let the engine idle. Most engines (all that I've had) run in a "choked" mode when they are cold and idling. The engine idles at a slightly higher RPM than normal. So, it warms up rather quickly. I can time it because it takes me 10 minutes to drive from home to work (and 10 minutes from home to WalMart, 10 minutes to Target, 10 minutes to Piggly Wiggly, etc... Everything is approximately 10 minutes away in different directions). If I immediately start driving, I don't get any hot air before I'm done driving. If I let the car warm for about 5 minutes (I'm a bit OCD, so I actually watch the clock and feel weird if I don't let it wait for a multiple of 5 minutes), the air blows hot shortly after I start driving - well under 10 minutes. The weird thing is that none of the cars vary much as to how long it takes to blow hot - and the engines are all very different. So, that is something you can experiment with. Judge how long it takes to get hot (not warm) air from your heater while driving. Then, wait 5 minutes with the engine running and then start driving. Does it get hotter quicker (including the 5 minutes waiting to start driving)? I must note that I do the 5 minute warmup easily because I go start the car with one kid, go back to get the other kid, go back to get whatever junk needs to go on the trip, and then start driving. -- kainaw™ 03:35, 18 December 2009 (UTC)

You say [t]he heater core could heat without the engine running as it is electric itself -- I don't believe that's true for any of the listed vehicles. Generally the heater runs off hot coolant coming out of the engine, before (well, I expect it's before, as it makes more sense to me) it goes into the radiator. Or maybe the heater just redirects the air going through the radiator itself? But in any case it's not electric. I suppose an electric car would have to have an electric heater, if it had any heater at all. --Trovatore (talk) 03:40, 18 December 2009 (UTC)

I think prestige cars are a little different if they have fancy climate control systems, but in the vast majority of cars the heater generally has it's own little heat transfer "radiator" separate from the car's main radiator. It's something to note if you are going to be driving through a desert or very hot climate, if your car is in danger of overheating and you are desperate to keep driving you should turn the heater on as high as possible because it actually helps with cooling the engine. Obviously you'll want to point all the vents away from yourself and open all the windows, but it actually has a noticeable effect on the running temp of the engine if it is already getting hot. Vespine (talk) 04:35, 18 December 2009 (UTC)

Police cars have electric heaters because they spend a lot of time parked with the engine off. That explains why the Caprice did that - it had been modified. Most cars only get heat from the water circulating around the engine. The heater core is just like the car's main radiator. In fact, if your engine is overheating, turning the heater on full blast may well get you home without a dead engine! SteveBaker (talk) 05:14, 18 December 2009 (UTC)

I meant to relay that since the heater is an "electrical" device in the car and you can turn it on without the engine running, it is not obvious that it won't heat up without the engine running. But, the main point is that I've experienced the heater getting hotter quicker by idling in the "choked" mode where the engine idles a bit higher than when I'm driving. Every car I've had initially idles high just like when I pull the choke on a tractor or old motorcycle. I assume it is doing the same thing - running a little high to adjust for the cold engine. -- kainaw™ 05:56, 18 December 2009 (UTC)

All this text and no answer? The answer is the faster the fan the better. Newton's law of cooling states that the rate of transferring heat from one object to another depends on the difference in their temperature. i.e. transferring heat from a 10° object to a 30° is MORE than twice as fast as from 10° to 20°. (For how much more, measure temperature in kelvin.) So, when you blow the fan fast, the air spends less time in contact with the heater - yes. But at the same time, it's absorbing heat faster, because the difference between the air and the heater is maximized. Contrast to a slow fan - it spends more time on the heat, but the difference in temperature is less (since the air warms up from the heater), so it's absorbing the heat slower. Ariel. (talk) 08:16, 18 December 2009 (UTC)

Except that you're wrong. In my tests, the car got warmer faster with a slower air flow. I agree about the law of cooling - and I mentioned that in my question - but that effect is being fought by the fact that a high fan speed is moving more cold air through the heater core and pushing already warmed out out of the car. So you're losing energy overall because of that. The problem is whether my finding is true of all cars - or just cars with the exact properties of the MINI. SteveBaker (talk) 13:34, 18 December 2009 (UTC)

All (most? some?) cars have a recirculation mode - it's usually called "Max A/C", but works just fine for heating too (if you have a button to turn your A/C on/off). But even without that, I think your test is being affected by your own body heat, because unless the temperature from the vent is colder than the air inside the car (and how could that be possible, since the vent air is what heated it in the first place), you can never loose by getting more of it. At full speed you are still getting warmer air than the air inside the car - except if your own body is heating the space. Ariel. (talk) 18:21, 18 December 2009 (UTC)

Without recirculation the interior of the car could never get hotter than the air coming from the vent. So if the air with the heater on max blower goes from -10 C (outside) to 20 C then you never get warmer than 20 C. By contrast, turning down the blower might allow the air to go from -10 C to 30 C in the heater, in which case the car could warm to 30 C. It would take longer, but ultimately get warmer. This simple example shows that max blower can't always be the answer for maximum warming. Dragons flight (talk) 18:30, 18 December 2009 (UTC)

More than twice as fast? I don't follow. Since Newton's law involves temperature differences, we don't need an absolute temperature scale. As far as the subquestion of "what is the heat flow rate in the air coming off the heating element" goes, that's just $F(v)=(T(v)-T_{0})cAv$, where the temperature of the heated air depends on its speed (because slow air has more time to heat up), $T_{0}$ is the air's initial temperature (which doesn't count for heating the car), c is its volumetric heat capacity, and A is the duct area. If we use a zero-dimensional model for the air-heater interface, $T(v)=T_{h}+(T_{0}-T_{h})e^{-Kl/v}$, where K describes the quality of contact between the heater and the air and l is the length of that contact (so that $l/v=t$). Of course, $\lim _{v\rightarrow 0}T(v)=T_{h}$ and the heat flow drops to 0 when the velocity does. At very high velocities, let $s=1/v$ and expand in a Taylor series: $F(1/s)=(T(1/s)-T_{0})cA/s=(T_{h}+(T_{0}-T_{h})(1-Kls)-T_{0})cA/s=(T_{h}-T_{0})KlcA$, where the velocity doesn't matter. We get a maximum (if not at infinite velocity) when $0=F'(v)=cA(T'(v)v+T(v)-T_{0})$, so $0=(T_{0}-T_{h})Kle^{-Kl/v}/v+(T_{0}-T_{h})(e^{-Kl/v}-1)=(T_{0}-T_{h})((Kl/v+1)e^{-Kl/v}-1)$, so $e^{Kl/v}=Kl/v+1$. But that never happens except when $Kl/v=0$, so running the air faster is always better (in this simple model). By using the same $T_{0}$ in the expressions for $F(v)$ and $T(v)$, I'm assuming that you're recirculating inside air, which is obviously better in realistic scenarios where the car starts at the outside air temperature. --Tardis (talk) 15:34, 18 December 2009 (UTC)

I wasn't trying to be mathematically exact, I was just trying to explain a concept. I did write that for the actual amount use kelvin. Ariel. (talk) 18:21, 18 December 2009 (UTC)

Box model. I am going to approximate the air in the heater as having a uniform temperature. Though not entirely true since there will be a temperature gradient between the front of the heater and the end of the heater, it is close enough to true to allow one to examine the dynamics.

Let there be four temperatures: outside air (T_out), air in heater (T_warming), air in car (T_car), and the heating element (T_heater). Further, let the volumetric air flow rate be R, the density of air be ρ, the specific heat of air be c, the volume of the heater by V_warming, and the volume of the car by V_car, the thermal transfer coefficient between the heater and the air in the heater be k_heater, and the thermal transfer between the car and the outside be k_car. Then we have:

$\rho V_{warming}c{\partial T_{warming} \over \partial t}=\rho Rc(T_{out}-T_{warming})+k_{heater}(T_{heater}-T_{warming})$

$\rho V_{car}c{\partial T_{car} \over \partial t}=\rho Rc(T_{warming}-T_{car})+k_{car}(T_{out}-T_{car})$

In the quasi-static limit then:

$T_{warming}={\rho RcT_{out}+k_{heater}T_{heater} \over \rho Rc+k_{heater}}$

and you maximize the heat flow into the car by maximizing:

$R(T_{warming}-T_{car})=R({\rho RcT_{out}+k_{heater}T_{heater} \over \rho Rc+k_{heater}}-T_{car})$

Which implies:

${{\partial \over \partial R}R({\rho RcT_{out}+k_{heater}T_{heater} \over \rho Rc+k_{heater}}-T_{car})}=0$

Which after a bunch of math implies the maximum heating occurs if you keep:

$R={k_{heater} \over c\rho }({\sqrt {T_{heater}-T_{out} \over T_{car}-T_{out}}}-1)$

Which has some interesting limits. It says that if the temperature inside and outside are the same then the best solution is to turn the blower up to maximum, but that to maintain optimal warming one needs to turn the blower down as the car starts to warm. Also, the optimum air flow is higher when the heater is already warmed up than when the heater also starts out cold. (Note that I haven't considered the ability of the air flow to affect the temperature of the heater.) Dragons flight (talk) 08:10, 18 December 2009 (UTC)

Hmmm - that's interesting. So my finding that a slower fan speed works best seems reasonable - but may not be the optimum solution. Clearly, I need to do some more experiments and try varying the fan speed from max to min as the car grows warmer. Don't you just love this science stuff? Hypothesis, prediction, experiment, better hypothesis, prediction, experiment, theory! Resulting in warmer feet on the way to work. SteveBaker (talk) 13:34, 18 December 2009 (UTC)

Don't Minis have the "recirculate the inside air" feature? Clearly recirculating inside air as fast as possible is the best solution. --Sean 14:30, 18 December 2009 (UTC)

I have always done the same thing as you, Steve. Low fan speed and burn a little extra fuel to warm things up. I do know that I have always practiced keeping the heater fan on low speed simply because it makes the car feel even colder to trun it up on high right away, bascially because of the intial pushing of cold air against me. 10draftsdeep (talk) 14:41, 18 December 2009 (UTC)

Internally recirculate the air? How utterly reasonable. Design a better mousetrap why don't you. Dragons flight (talk) 14:44, 18 December 2009 (UTC)

Just to muddy the waters a bit more, we need to keep in mind that there are at least two scenarios that we need to account for regarding why the car is being warmed. In scenario one, you want the car to warm up while you're outside the car scraping off the windows and sweeping off the snow. In scenario two, you're inside the car and just need the temperature to become tolerable before you head out on your merry way. If you're outside the car, you want to crank the fans and everything else before you get started. The cold air that initially will be blowing around won't bother you as you're outside the car and as soon as the air is slightly warmer it will begin to act on the interior fog/frost on the windows. If you're inside the car, having freezing air blowing at you is distinctly uncomfortable, so you should keep the fans down; getting the car marginally warmer (maybe!) is of limited value if you're just spent the last few minutes in a 0°C wind tunnel. Matt Deres (talk) 15:14, 18 December 2009 (UTC)

i think that you should keep the fan off untill the water in the radiator is hotter by the engine , because while you'r in the car isolated from the outside heat exchange will be as slow as it could be,therefore you'r body would'nt lose much heat (you wan't feel colder),if you turn the fan on cold air,(colder than you'r skin tempreture),air will contact you'r skin and gain heat from you making you feel cold,so the wright thing to do is to wait until the cooling water is gaining much heat from the engine then you can turn the fan on,and start to heat you car--Mohammed jaafreh (talk) 16:24, 18 December 2009 (UTC) . —Preceding unsigned comment added by Mohammed jaafreh (talk • contribs) 16:22, 18 December 2009 (UTC)

I believe there's a facet to this argument that hasn't been brought up yet.

For a fixed engine temperature, and a single fan speed, there is a maximum amount of heat transfer from the heater core to the air passing over it, because a constant amount of air is involved for any single fan speed. (At high speeds, there is less heat transfer to the moving air -- Steve mentioned this in his opening paragraph.) Does this not also mean that there will thus be a maximum amount of heat "available" from a given fan speed? If so, the only way to get the heat above that point is to lower the fan speed.

(Still, that said, I fail to understand why this is a problem in Texas.) --DaHorsesMouth (talk) 00:18, 19 December 2009 (UTC)

I take it you've never lived there. Texas can get pretty chilly in the winter. I've told the story before about one day when Toronto was starting to cool off, and I looked up the weather in Denton, TX (my previous home), and it was significantly colder there.

Of course the worst plausible cold snap is much less bad in Texas, and it doesn't spend as much time being cold. --Trovatore (talk) 01:08, 19 December 2009 (UTC)

Yes - exactly. We had snow just a couple of weeks ago - and we see snow around christmas time nearly every year. But it rarely gets more than an inch or two deep and it usually melts within 24 hours. There is a photo of my house with actual Texas snow here. But we've had weather cold enough to need the car heater to be on for about a month now. SteveBaker (talk) 18:22, 19 December 2009 (UTC)

The assumption that driving a vehicle in a lower gear causes more fuel consumption is a common misconception. The reality is that fuel consumption is directly proportional to how far you push your foot down on the accelerator. It's true, look at the fuel maps next time you have your Mini tuned.161.165.196.84 (talk) 04:49, 21 December 2009 (UTC)

If each spatial dimension extends in two dirrections, why does the time dimension only go in one?

We live in a three spacial dimenmsion and one time dimension world (I mean dimensions that we see and experience, I know that according to string theory there are curled up six dimensions everywhere) and when we talk about dimensions we say that one is forwards and backwards, one up and down, one goes to the left and then to the right, and then forwards through time. What about backwards through time? I know there is a heated debated about time travel; but i am not asking is it possible or not, i am asking why when we talk about time as a dimension do we only considering it in one direction while all the others extend in both ways (forwards and backwards, left and right vs. just forward for time). Why is it that we say that youy can go in either direction on a spaciasl dimension but not on a temporal one, that on a temporal one there is only forwards? Is it possible that we actually live with a different number of dimensions that we experience than four? Could it be that we live in a seven dimensional world where each dimensoin is only one direction along a line insteasd of two (instead of one spacial dimension being forwards and backwards it is actually two, forwards is its own dimension and is backwards is its own, up is its own dimension down is its own dimension instead of up and down being one, this would explain why we only experience time forwards, mabey there is a seperate backwards time dimension that our world does not include, hence our world might have seven diomensions instead of four, up is one, down is two, right is three, left is four, forward is five, backwards is six, and forwards through time is the seventh and are univers does not copntain the backwards in time dimension). --74.67.89.61 (talk) 23:52, 19 December 2009 (UTC)Connor Goham--74.67.89.61 (talk) 23:52, 19 December 2009 (UTC)

Talking about time as a "dimension" is more of a mathematical convenience than anything physically real. This alone is a complete answer to your question...if time and space aren't the same kind of thing - then there is no reason to expect them to behave similarly. But if you absolutely insist on calling time a proper "dimension" then indeed, you should say it goes forwards and backwards - just like space goes left AND right. We measure time as a 'signed quantity' - it can be positive or negative. You can even argue that we don't really travel "through" time at all - we merely exist at over a span of time just like we exist over a span of space. The only odd thing is that our brains only have memories from the past...this imparts an "arrow" to time that we don't have in space. But there is a physical direction too - because entropy behaves differently in one direction than the other. SteveBaker (talk) 01:34, 18 December 2009 (UTC)

Steve, time is one of the coordinates of the Minkowski manifold (space-time). There is no reason to exclude it from the list of dimensions of the universe. It is not just a mathematical convenience. Dauto (talk) 04:37, 18 December 2009 (UTC)

You just described a mathematical convenience! SteveBaker (talk) 05:08, 18 December 2009 (UTC)

Measuring an interval of 5 feet is just as real and meaningful as measuring an interval of 5 seconds. Both time and space can be usefully thought of as dimensions, though they don't behave the same in all circumstances. Dragons flight (talk) 08:35, 18 December 2009 (UTC)

Certainly it's just as meaningful to measure distance and time - and also mass, temperature, voltage or any of a gazillion other things. In that sense, mass and pretty much any other quantity you fancy can be treated as coordinates in some kind of phase space - but that's just a convenience - there isn't a physical reality underlying space and time being considered to be "the same kinds of thing". Certainly, one should not be tempted into trying to make analogies between space-like and time-like "dimensions" just because we happen to use the same name for them and can sometimes find it convenient to label points with four dimensions. But one shouldn't confuse a mathematical convenience with reality. Sometimes, when doing computer graphics, I find it convenient to treat points as fourteen dimensional objects with properties like color being "extra dimensions" - but that doesn't make me wonder whether I can treat color like I do space. SteveBaker (talk) 13:19, 18 December 2009 (UTC)

But time is a dimension just as the three spacial ones not just a mathematical convenience, hence the spacetime continium, Einstien coined the phrase because space and time are so closely connected to each other, it is regarded as the fourth dimension by scientists. —Preceding unsigned comment added by 74.67.89.61 (talk) 20:09, 19 December 2009 (UTC)

Your speed moving through space is how far you go in a given time. If I go north, as time passes, I'm further north. Going through time would mean how far you end up in the future after a certain amount of time. You always move through time at a rate of 1. This is because you're measuring in terms of time. You could just as well measure in terms of north. A line extending north-east and south-west would be moving east in terms of north. If you walk north, you're going forward in time in terms of north. If you walk south, you're going back in time in terms of north. In short, time is different because you decided to measure everything in terms of time. If you measured everything in terms of north, time would be normal, but you'd only be able to go forward in north. — DanielLC 07:02, 18 December 2009 (UTC)

No, no, no. If I go south, I do not end up at a universe with lower entropy. The reason time only moves in one direction is because the state of the universe's entropy only moves in one direction. If I redefine my coordinate system to a moving coordinate system as you describe I don't move backwards in time because redefining my coordinate system does not affect the second law of thermodynamics. --Jayron32 07:12, 18 December 2009 (UTC)

And furthermore - time is not unique in that regard. You might just as well pick some other property and measure that as a function of distance. You could rewrite what DanielLC said with time replaced by some other property and end up with a statement that still makes about the same amount of sense. I dunno - pick "air pressure" which varies with altitude above the surface of the earth - so I'll also change north/south into up/down. Do that and you get this curious (but just as meaningful) statement:

"Your pressure gradient moving through space is how far you go for a given pressure change. If I go up, as the pressure changes, I'm further up. Going through pressure variation would mean how much you end up at a different pressure after a certain amount of pressure change. You change pressure through pressure at a rate of 1. This is because you're measuring in terms of air pressure. You could just as well measure in terms of up. A line extending up-and-east and down-and-west would be moving east in terms of up. If you climb up, you're going down in air pressure in terms of up. If you climb down, you're going up in pressure in terms of up. In short, air pressure is different because you decided to measure everything in terms of air pressure. If you measured everything in terms of up, pressure would be normal, but you'd only be able to go forward in north."

SteveBaker (talk) 13:19, 18 December 2009 (UTC)

Something else to think about: It's only a matter of convention that we use "up/down", "left/right" and "further/nearer" as our three dimensions. You could have picked any set of three non-parallel directions instead. You could even use cylindrical or spherical coordinates as your three spatial dimensions ("azimuth, elevation, range" for example). The point being that absolutely all such systems for establishing a position require exactly three numbers - you can't devise a system that needs 4 numbers (without one turning out to be redundant) or one that needs only 2 numbers. The "threeness" of space is completely fundamental, no matter which dimensions you choose to use - you always need three. That's what makes space "three-dimensional" - not that we happen to choose a set of three orthogonal axes to use as our measuring stick. Considered like that, we should not think of three very specific dimensions - but just of a more general 'threeness'. Time is a similar deal - you only need one number to specify a time - and you can't use two numbers without one of them turning out to be redundant. So time is inherently "one-dimensional". SteveBaker (talk) 14:24, 18 December 2009 (UTC)

Steve, IF your model of space and time where space has an intrinsic three-ness associated to it and time has an intrinsic one-ness associated to it were correct THAN you would be right that a four-dimensional space-time would be (at best) a mathematical convenience. The problem here is that your model is more than 100 years out of date. In fact space DOESN'T HAVE a three-ness associated to it and time DOESN'T HAVE an one-ness associated to it but space and time together DO HAVE a four-ness associated to them. An observer can measure the time of two events t₁ and t₂ and find t₁ > t₂ while a different observer would measure the time of those events and find t₁' < t₂'. That is inconsistent with your one-dimansional time model. Dauto (talk) 19:27, 18 December 2009 (UTC)

Well, clearly time is distinguished from the other dimensions by the signature of the manifold — the inner product of a timelike vector with itself is negative, whereas the inner product of a spacelike vector with itself is positive (or vice versa depending on your convention). So there are distinguished three dimensions and a distinguished lone dimension. They aren't picked out uniquely, of course.

The interesting thing to me is how easily Steve, who affects to disdain metaphysics, ends up discussing it :-) --Trovatore (talk) 19:37, 18 December 2009 (UTC)

Oh yeah... Time IS distict from space. As you said they have different sign signature. That's a good thing since we know experimentally that time is different from space (hence the OP's question). But steve's treatment where time is delt with separately from space is inconsisten with experiment. Dauto (talk) 04:30, 19 December 2009 (UTC)

Hmm? I don't see how. Experiments rarely put much constraint on how we choose to interpret or describe observations and predictions; those are matters for metaphysics. I don't say that disparagingly. Metaphysics is very important, and there is at least sometimes a fact of the matter to metaphysical questions, even though they can't be settled experimentally.

Let's see if this comes up blue: neo-Lorentzian. --Trovatore (talk) 23:55, 18 December 2009 (UTC)

That's not a matter of metaphysics. The 1-dimentional time model requires that if an observer compares the timing of two different events and finds that t₁ > t₂ than a different observer must find t₁' > t₂' which is inconsistent with experiment. Compare that with a different physical quantity such as electrical charge. The electrical charge follows a 1-dimentional model. If an observer measures two different electric charges and finds out that q₁ > q₂ than it is true (and born by experiment) that if a different observer measures the same charges she/he will find out that q₁' > q₂'. Dauto (talk) 04:30, 19 December 2009 (UTC)

What? No, it doesn't require that at all. Why should any two observers agree on the order, merely because time is 1-dimensional? You haven't come remotely close to justifying that assertion. --Trovatore (talk) 06:10, 19 December 2009 (UTC)

I find that this video (http://www.youtube.com/watch?v=8Q_GQqUg6Ts) helps a bit. Zhatt 21:17, 18 December 2009 (UTC)

What is this object ?

What is this screen like object on the right side of obama?--yousaf465' 04:59, 18 December 2009 (UTC)

A teleprompter. DMacks (talk) 05:03, 18 December 2009 (UTC)

It's not positioned quite right for a teleprompter. I think it may just be a reflector, to improve the illumination of Obama in the photograph. Dcoetzee 05:09, 18 December 2009 (UTC)

It's a teleprompter. Tempshill (talk) 05:18, 18 December 2009 (UTC)

Indeed. It's a teleprompter like in this photo. Dismas|^(talk) 05:53, 18 December 2009 (UTC)

Since teleprompters came up [1] [2] Nil Einne (talk) 06:23, 18 December 2009 (UTC)

And see this as well [3] on Obama's heavy reliance on teleprompters.

Wow how value laden was that! "Heavy" reliance? No more so than politicians in the UK, and I wouldn't be surprised to find that it's no more so than other US politicians. It goes with the territory these days. --TammyMoet (talk) 08:54, 18 December 2009 (UTC)

Well, not always. David Cameron delivered his 2007 End-of-Conferrence speech without a script or teleprompter[4]. It lasted more than an hour; he had a list of bullet points on a chair that he glanced at now and then. Quite impressive. Going back a few years, Harold MacMillan was noted for speaking without notes. Alansplodge (talk) 10:41, 18 December 2009 (UTC)

Yes, I have seen Shirley Williams give a 45 minute talk on data privacy without notes. Using notes used to be for wimps let alone teleprompters but I guess people didn't record and were more forgiving. --BozMo talk 11:11, 18 December 2009 (UTC)

I note you've quoted opposition politicians here. I suppose it could be said that it's more forgivable for politicians whose pronouncements have no chance of being enacted to speak off the cuff! --TammyMoet (talk) 16:15, 18 December 2009 (UTC)

Incidentally, Bozo when was this golden age of memorized speeches? President Lincoln used notes to deliver the Gettysburg Address, and that's only about two paragraphs long! APL (talk) 06:39, 19 December 2009 (UTC)

That cartoon is OK, but this bit from the Onion News Network is better. APL (talk) 16:49, 18 December 2009 (UTC)

Not that there's almost certainly one to his left as well. Most politicians have adopted this style of having a teleprompter to either side of the speaker. Typically positioned so that he can look all the way to the left side of the audience, and all the way to the right side of the audience and still be able to read his lines. APL (talk) 16:49, 18 December 2009 (UTC)

Quite a useful device for likes of Obama. --yousaf465' 05:12, 19 December 2009 (UTC)

Virtually everyone who routinely makes speeches uses them nowadays. Especially if they're going to be on TV. It looks a lot better during TV closeups if you can be looking up at your audience through your entire speech instead of looking down at your podium every couple of sentences. (I don't care how good your hair is, you don't want any closeups of the top of your head when you're giving a speech.)APL (talk) 06:39, 19 December 2009 (UTC)

@yousaf: In the interests of not Obama-bashing...

SteveBaker (talk) 13:39, 19 December 2009 (UTC)

Why don't they use the HUD or similar technology ?--yousaf465' 13:27, 20 December 2009 (UTC)

It sort of is a hud. You'll notice in those picture (Except the Clinton one, for some reason) that the prompter is a clear glass plate. The speaker can see the audience through the glass, and they can see him. He just sees his speech super-imposed over the audience.

I suppose it'd be simple to use a wearable HUD instead, but the president probably doesn't want to look like a Borg. APL (talk) 19:19, 20 December 2009 (UTC)

It absolutely IS a HUD! In a fighter aircraft, there is exactly the same tilted sheet of semi-mirrored glass with a bright video projector underneath shining the image up onto the glass. It wouldn't surprise me in the slightest if they used the exact same technologies in fact. SteveBaker (talk) 02:12, 21 December 2009 (UTC)

So what is so expensive in it ? Simple optics.--yousaf465' 03:24, 21 December 2009 (UTC)

Anything that's made in very small quantities for a specialized application tends to be expensive. There aren't many jet fighters in the world - and there aren't many teleprompters either - so they cost big $$$. When I worked in flight simulation, we had the choice to buy real aircraft HUD units or make our own - we could make them for a fraction of the price of the 'official' unit. They were basically not much more than a sheet of lightly silvered glass and a regular projection TV unit with the lens swapped out with a lens we designed especially for the job. SteveBaker (talk) 21:12, 21 December 2009 (UTC)

They're not that expensive. A quick Googling shows that The on-camera kind can be had for less than a grand. The free standing ones can go as cheap as twelve hundred. Not super cheap, but for a specialized piece of equipment, that's not bad at all. Some people pay more for a TV. However if all that's too rich for your blood, perhaps you need a web-cam teleprompter so you can look your audience in the eye during your State of the Blog addresses, that'll run you a mere fifty bux. (sixty for the laptop version.) APL (talk) 01:53, 22 December 2009 (UTC)

Do you really need a teleprompter on a computer using the webcam. I have seen these on camera types used by Local news channel. Also SteveBaker how did you constructed one ? any plan details.--yousaf465' 03:55, 22 December 2009 (UTC)

anyone here really knowledgeable on Circadian rhythms?

I want to ask a few questions on photoperiodism as well as M and E cells in drosophila, but I want make sure there are specialists on the topic here first. John Riemann Soong (talk) 05:13, 18 December 2009 (UTC)

Reference Desk protocol is to just ask the questions and not ask for experts first. Tempshill (talk) 05:19, 18 December 2009 (UTC)

(ec) I know a good bit about circadian rhythms, but won't necessarily be able to answer highly detailed questions about drosophila in particular. Looie496 (talk) 05:21, 18 December 2009 (UTC)

We'll do what we can :) -- what is the question? --Dr Dima (talk) 05:44, 18 December 2009 (UTC)

I don't really get the evolutionary as well as mechanistic basis for why SCN would split in hamsters in LL conditions; other than the fact that you see it on an activity graph and that SCNs cut horizontally (as opposed to coronally) show bimodal behaviour in culture.

Also I'm having trouble understanding selective M and E cell ablation experiments. If you knock out M cells, you knock out the source of synchronising pdf -- so why do E cells still induce evening anticipation behaviour? Also, when will ablating E cells affect morning anticipation behaviour? John Riemann Soong (talk) 11:06, 18 December 2009 (UTC)

I can't address the second question, but for the first question I think the answer is that LL conditions do not occur in nature (except in the far polar regions), and the SCN activity they induce is probably just pathological. Are you aware that in rhythm splitting the left and right SCN run 180 degrees out of phase with each other, and that rhythm splitting of this sort is only seen in hamsters? Looie496 (talk) 17:20, 18 December 2009 (UTC)

vector or scalar quantity

Is area a vector or sceler quantity? —Preceding unsigned comment added by Munupulu (talk • contribs) 06:38, 18 December 2009 (UTC)

Vector quantities have value and direction, while scalar quantities only have value. Does the concept of "area" have any directionality to it? Answer that question, and you can answer your own homework question here. I would direct you to our Wikipedia articles on scalar and vector, but they are written primarly for reference by people with advanced mathematics degrees, and are nearly completely incomprehensible to anyone else. Just remember that vectors have direction, while scalars do not. --Jayron32 07:00, 18 December 2009 (UTC)

There are examples of both: for instance, a differential vector area in a surface integral, and a scalar area of the Earth (by symmetry). Of course, curved surfaces lack a unique normal direction. --Tardis (talk) 07:16, 18 December 2009 (UTC)

(ec) The answer is context-dependent. Typically, area simply refers to a scalar value, but in the case of an oriented surface, it is very common to express the orientation as a vector with magnitude equal to the (differential or total) area. Nimur (talk) 07:35, 18 December 2009 (UTC)

The absence of sections of those articles suitable for people who don't have advanced mathematics degrees seems like a failure, in an encyclopedia. I'd improve the articles myself, but I don't have an advanced mathematics degree. If I did, that would probably remove the motivation. Shame. 81.131.30.43 (talk) 11:43, 18 December 2009 (UTC)

Try vector area. Gandalf61 (talk) 11:56, 18 December 2009 (UTC)

That's just the surface normal vector. Area is a scalar quantity. SteveBaker (talk) 12:58, 18 December 2009 (UTC)

I was responding to the previous poster's comment that the other articles mentioned were too difficult to understand. Area can be either a scalar or a vector quantity, depending on context. It would be nice if we had two separate terms, like the distance/displacement distinction for the 1D case, but we don't, so there is some ambiguity. Gandalf61 (talk) 17:47, 18 December 2009 (UTC)

We also have speed and velocity as two separate terms to help distinguish the scalar from the vector. I just want to add a minor technical neatpick that area is in fact a pseudovector. Dauto (talk) 19:34, 18 December 2009 (UTC)

Autonomous behaviour on Guitar Hero

If you play Guitar Hero or Rock Band for any length of time you tend to progress as follows:

1. You start off by consciously watching the notes and painstakingly learning the finger positions for the keys.
2. You've memorised the finger positions but you still consciously process the scrolling notes.
3. You reach a point where you can play by just watching the notes scroll down. You think about them automatically without having to concentrate.
4. Like #3, but MORE. You don't even think any more and tend to completely "zone in", it's as if you are subconsciously processing the notes and playing them before they even register on your brain. This is especially true of complex note sequences where your hands are definitely working faster than your brain, you only realise what note sequence you've played after it has already passed or if it repeats.

Besides the fact that I'm spending WAY too much time playing games, what is happening at #4? It is not a matter of memorisation either. Last night I played a new song with a tricky note sequence and automatically snapped into autonomous mode. Where does the processing take place in this case? Also, are there any WP articles or studies out there (especially related to GH rather than computer games in general). Zunaid 09:38, 18 December 2009 (UTC)

No real answer, I think it's called getting in the zone. I try and pay attention whilst driving but still have noticed a couple of times that my hands and feet are doing something before I've realized exactly what it is I'm responding to. Rather strange feeling really, I think I've even got a sense of 'God you're crap, let me do my job' from it as I think about doing something wrong and I've autonomously done the right thing. Dmcq (talk) 11:19, 18 December 2009 (UTC)

Might be interesting to see what happens in the language center over this learning process. I reckon the difference in processing is that you start out treating the symbols with more general-purpose strategies for decoding and interpretation, strategies which might be suitable for any written language or set of symbols, and the "automatic" behaviour is a matter of using less general-purpose strategies, which could also be called thinking about it less. 81.131.30.43 (talk) 11:50, 18 December 2009 (UTC)

See procedural memory and muscle memory. Gandalf61 (talk) 11:54, 18 December 2009 (UTC)

For what it's worth, one of the programmers who worked on Guitar Hero is in the office next to mine at work. He says that they worked very hard to get that kind of progression through the course of the game. It's no accident - it's the result of a lot of very clever design work and careful tuning of difficulty levels, choosing the order of song 'unlocking' and such. SteveBaker (talk) 12:55, 18 December 2009 (UTC)

Utilization behavior also seems pertinent, or at least funny. 81.131.30.43 (talk) 13:07, 18 December 2009 (UTC)

Great question...and I must say that getting to #4 is why I play the game. cheers, 10draftsdeep (talk) 14:19, 18 December 2009 (UTC)

Flow (psychology) seems relevant to this conversation. 206.252.74.48 (talk) 14:50, 18 December 2009 (UTC)

Some good answers, but not quite there yet. Muscle memory and procedural memory are more to do with practice and repetition, which explains how your fingers can snap to exactly the right positions, but I'm really after an explanation of that autonomous thing where somewhere between your eyes and your fingers something happens that just bypasses your conscious thought completely. It feels almost but not quite like instinct. Maybe I'm under-thinking it though...a lot of the songs actually do have standard "chord" and finger changes so perhaps it is just muscle memory combined with some pattern recognition? Steve, what does your buddy say? Did they actually study this phenomenon in any detail when designing the game? Also, if there are any reasearch papers on this I'd like to have a read-through. My google-fu is failing me. Zunaid 15:01, 18 December 2009 (UTC)

Didn't see the Flow (psychology) link until after I posted. It seems to be what I'm after but doesn't actually explain the mechanism by which it works. And sadly it devotes exactly one line to its use in game design. Zunaid 15:07, 18 December 2009 (UTC)

A single line, with a citation, which leads to an extensive online essay with diagrams and everything. 81.131.31.130 (talk) 16:22, 18 December 2009 (UTC)

Unfortunately there isn't a single universally used term for the phenomenon -- maybe "overlearning" and "automatization" are the most commonly used. For what it is worth, here is a link to a recent paper that examines what happens in the brain during the process. Looie496 (talk) 17:53, 18 December 2009 (UTC)

For what it's worth, I get the same thing playing an instrument I've been playing for many years: the information seems to go straight from the page or the sound in my head to my fingers (and respiratory system, where relevant). So there might be stuff written about musicians which would be relevant: I know there's a lot written about how musicians read music. 86.178.164.251 (talk) 18:13, 18 December 2009 (UTC)

I'm with 86.178... The same sort of thing happens with playing a real guitar. Early on, I spent lots of time concentrating on making the chord shapes and learning strumming patterns. Now, I instinctively know, by listening to the rhythm of the song, when to strum and what patterns to use, without thinking about it. Its kinda automatic. Plus, you get better with things like chord progressions, I can now pretty much do the same thing with my left hand work. If I know the key a song is in, and I hear it once or twice, I can pretty much follow along with the changes automatically, without thinking of it. --Jayron32 18:58, 18 December 2009 (UTC)

Let me quote the first paragraph of the paper I cited above: "Motor skill acquisition is characterized by three main phases: a highly attention-demanding early phase, an intermediate phase characterized by more established performance levels, and an automatic phase (Fitts and Posner, 1967; Schneider and Shiffrin, 1977; Schmidt and Lee, 1999). Both accuracy and stability of the to-be-learned task improve rapidly across the early phase and then advance more gradually until asymptotic performance is reached. Extended practice beyond this point is defined as the automatization or overlearning phase, which is characterized by feedforward instead of feedback control (Wolpert et al., 1998; Miall et al., 2001), allowing the performer to divert attention to other tasks (Schaffer, 1975; Nissen and Bullemer, 1987; Duncan, 1995; Passingham, 1996)" Looie496 (talk) 20:00, 18 December 2009 (UTC)

Just popping back to say the paper Looie linked is quite brilliant and seems to provide the answers I was looking for. Also the citation link to video game design in Flow (psychology) is exactly what SteveBaker was talking about earlier, although it is only tangentially related to my question. Steve you might be interested in reading it if you haven't come across it already. Thanks everyone for the answers thus far. Zunaid 22:06, 18 December 2009 (UTC)

Hmmm - yes - it's certainly an interesting article. While I am a Game programmer, my sub-discipline is Computer graphics - so I don't personally get involved with that psych stuff - we have Game designers who do that. They happily babble on about story arcs and character arcs and skill arcs and game economies and flow and all sorts of weird stuff like that...they even draw scientific-looking graphs and diagrams and such. But in the end, they tell us to please tone down the ambient light, move that building six inches to the left and make the rain fall a little more heavily because that will make the game more fun - and that's what we do. If they do their jobs right, the game is fun and compelling (or instructive or relaxing or whatever the heck it's supposed to be). If I do my job right, it looks good too! SteveBaker (talk) 18:13, 19 December 2009 (UTC)

Science help desk thank you for Christmas

tree

Well it probably breaks all the rules but as thanks for the advice on the last tree here is a photo of the sun rising this very morning behind a chestnut tree in the front lawn. Its the first year I have really been following the reference desk and there is a WWI tradition for ignoring rules on your first Christmas. Clearly it just snowed and as a non-artistic nerd I kind of liked the photo. The other one is behind the children-proof fencing round the moat in the background. Merry Christmas and thanks to everyone. --BozMo talk 10:06, 18 December 2009 (UTC)

You're calling a christmas truce? :) 81.131.30.43 (talk) 11:56, 18 December 2009 (UTC)

Chocolate will be exchanged. Soccer will be played - trolls versus regulars. Machine-gunning of the former by the latter will resume bright and early on the 26th - except in the Humanities desk where the truce will continue into the new year. SteveBaker (talk) 12:51, 18 December 2009 (UTC)

One wonders whether trolls will be kept out by a moat, or attracted to it for all the underbridges. --Sean 14:36, 18 December 2009 (UTC)

I know many people disagree, and others think I am being pedantic, but to me "chestnut tree" means a tree of the genus castanea which produces the edible nuts known as "chestnuts". It is true that the expression "horse chestnut" is colloquially used to refer to various species of the genus aesculus, quite unrelated but with inedible nuts of superficially similar appearance; nevertheless I feel that using the word "chestnut" for these trees is misleading. Thus I would say "the sun rising this very morning behind a conker tree", but would accept "behind a horse chestnut tree". Happy Christmas anyway. JamesBWatson (talk) 15:13, 18 December 2009 (UTC)

So how can you tell from the picture (i.e. ignoring BozMo's revelation below) that this an Aesculus rather than a Castanea? (Pulls out own pedantic hat - the names of genera are always capitalized.) Without leaves and covered in snow I'm not sure how you would identify the tree. -- 128.104.112.87 (talk) 15:56, 18 December 2009 (UTC)

Ah, trees. This could be a long thread. Certainly this is an Aesculus but locally anyway the edible ones are known as "Spanish chestnuts" or "Sweet chestnuts". In war time people discovered nuts of aesculus can be eaten too, but they have to be cut very fine and boiled for four or five hours first I believe. I have planted a Spanish chestnut (there are fine ones in Ickworth House nearby so I know they do grow here, but the garden has eight mature Aesculi). Sadly I think this tree has only a dozen years of life left, you can see bracken fungus on the left of the truck, a replacement Lime tree is off shot to the left. There are a lot of big dead beech trees in the garden too (cause unknown). --BozMo talk 15:37, 18 December 2009 (UTC)

Well since people are already talking nonsense... I have to complain that the photo hardly seems Christmasy. It's some tree covered in snow! Who has snow during Christmas?? That's winter stuff and not something anyone wants to think of during Christmas! Please use a more appropriate picture next time, one more suited to a typical summer Christmas. You can have rain if you want. But no snow please! Nil Einne (talk) 16:11, 18 December 2009 (UTC)

Dare I point out that more people live in the temperate climates of the northern hemisphere then the southern? Googlemeister (talk) 16:16, 18 December 2009 (UTC)

Yes, per WP:NPA and WP:Civility you really shouldn't be pointing out that most people are stupid. You know that, I know that, no need to rub their noses in it, eh? Nil Einne (talk) 16:40, 18 December 2009 (UTC)

Can we point out on that basis "not being stupid" correlates with "having convicts as ancestors" then? :). Although to be fair my own great grandfather was a gold miner in New Zealand but came back... --BozMo talk 16:50, 18 December 2009 (UTC)

I think you're confusing us with some other country. While some convicts were sent here [5], most Kiwis** don't have convicts as ancestors (except for those from that other country who invaded, but we accept them as our own since we're a generous people) at least not any more then the average person (which probably means most do). It's true that people from this other country are largely decendent from convicts (don't worry that this isn't true [6], they're all still sleeping and won't understand this anyway) and it shows, but we try[7] not [8] to [9] hold [10] it [11] against [12] them (not that it matters [13] [14]). At least no more then we hold it against those rebel colonists who were always a bunch of odd people and carried out one of the first recorded acts of large scale water pollution and like to play big bully if you don't let some of their ships in. **You're welcome to bring up a bird in reply to this discussion. But if anyone brings up any fruit whose name should under no condition be shortened to remove the fruit suffix, it's assumed they've decided to end participation in this discussion Nil Einne (talk) 18:22, 18 December 2009 (UTC)

I had no idea the name of that fruit was such a contentious issue. To avoid potentially causing offense, I think we should all revert to using the transliteration of an earlier Chinese name, and call them hairy bush fruit. 213.122.49.177 (talk) 19:22, 18 December 2009 (UTC)

It's not so much contentious as an obviously silly shortening. It's like going to the fruit market and asking for a star, dragon, passion, jack, bread or egg. Or perhaps a snake. Nil Einne (talk) 20:12, 18 December 2009 (UTC)

If its any consolation the first version three years ago of the Schools Wikipedia had Brussels Sprouts listed as a place name (see [15]). --BozMo talk 20:25, 18 December 2009 (UTC)

What are you all talking about? Kiwi is a brand of boot polish. And it always snows at Christmas - in pictures, Xmas cards, movies etc, it's one of the rules of Christmas. SpinningSpark 03:21, 19 December 2009 (UTC)

Where was the photo taken please? Was it in Britain, as we've had some rather rare snow recently? Nice pic. 78.147.233.150 (talk) 16:55, 18 December 2009 (UTC)

Barnardiston Suffolk UK at 0908 this morning. Just added one to that article taken of church at same time. --BozMo talk 17:00, 18 December 2009 (UTC)

Nice, but it will look silly in August. And since we only get laying snow on 2 or 3 days a year on average in that region, then a photo of less than 1% of the year is not a true representation. 89.243.91.31 (talk) 21:16, 18 December 2009 (UTC)

Why isn't the sky bright white at night?

If there are an infinate number of stars then the sky ought to be blazing white at night, as a star will be at every point in the sky. Even if there are not an infinite number of stars, there are still very many of them and would produce a similar effect. I recall this paradox was made known by a Victorian gentleman, and that there is a modern explaination(s). Can anyone supply more details please? 89.241.43.33 (talk) 13:30, 18 December 2009 (UTC)

Olber's paradox. Dragons flight (talk) 13:37, 18 December 2009 (UTC)

The modern answer is that the universe might be infinite in extent (but maybe not, we aren't sure) - but it's definitely not infinitely old. Since light takes time to get here - the further away you look, the further you're looking back in time. At some distance from the earth, you're looking back to the time before stars and galaxies had ever formed - and then, all you see is the cosmic microwave background - and at some yet larger distance, all you see is the big bang itself - and because there is no "before" that event, no light can possibly reach us from more than 13.5 billion lightyears away. That places a firm limit on the number of stars we can see - and it's far from infinite. The fact that the sky is black represents a rather elegant demonstration of truth of the big bang theory. SteveBaker (talk) 14:03, 18 December 2009 (UTC)

Well kind of. There are lots of other ways it could work too: non uniform density of stars for example is sufficient to resolve the paradox. But I certainly would argue with it being compatible... --BozMo talk 14:12, 18 December 2009 (UTC)

Yes - I picked the word "demonstration" rather than "proof" for that reason. SteveBaker (talk) 14:33, 18 December 2009 (UTC)

Actually even if the universe were infinitely large and infinitely old we would still only see a limited part of it. The red shift due to the expansion of the universe means that after a certain distance the light is red-shifted out of the visible spectrum. What is more, because longer-wavelength photons have less energy, the total energy we receive declines, not just the energy in the visible spectrum. JamesBWatson (talk) 14:59, 18 December 2009 (UTC)

That's if it was infinitely large, and getting bigger, right? 81.131.31.130 (talk) 16:07, 18 December 2009 (UTC)

Yes, but we know it's getting bigger. (Well, it's expanding - if it's infinite then its size isn't actually changing, but don't think about that too much, you'll get a headache!) --Tango (talk) 18:52, 18 December 2009 (UTC)

Um, a universe that is expanding can't be infinitely old unless you posit some dynamics besides the run of the mill cosmology. Just run the movie backward and a currently expanding universe will contract to zero apparent size in finite time. Dragons flight (talk) 19:08, 18 December 2009 (UTC)

The old steady-state theory had matter popping into existence out of nowhere to explain that. Then, as you "ran the movie backwards" matter would pop out of existence at the same rate the universe shrank and an infinite universe could therefore keep doing that forever without ever shrinking to a point. However, we have the big bang theory - and we know it's correct - so this is no more than a brain fart - which we can ignore for the sake of explaining why the sky is black. SteveBaker (talk) 13:19, 19 December 2009 (UTC)

Unless it contracted at a slower and slower rate, under which scheme it would hit some kind of limit, a lower bound of the size. This would mean the universe began at a certain size an infinitely long time ago, and then its expansion accelerated infinitely slowly until we reached present conditions, infinitely later. 213.122.49.177 (talk) 19:36, 18 December 2009 (UTC)

In addition to the fact you seem to have trouble grasping the meaning of infinity, that description is definitely "some dynamics besides the run of the mill cosmology", meaning you'd need to seriously modify general relativity to get the universe to do anything even remotely like that. Dragons flight (talk) 20:40, 18 December 2009 (UTC)

Did anyone say otherwise? I was talking about the universe being infinite in size, not duration. I must correct one thing, though - the universe didn't necessarily start at zero size. If it is infinite now then it was infinite at the big bang - it would have also had infinite density, so the volume of what is now the observable universe would have been zero. --Tango (talk) 21:04, 18 December 2009 (UTC)

Yes, two posts above yours, James suggests infinite in size and age. Dragons flight (talk) 21:58, 18 December 2009 (UTC)

Ah, yes. Please indent your comments just one step more than the comment you are replying to, it makes it much easier to understand what is going on. --Tango (talk) 23:32, 18 December 2009 (UTC)

My intuition isn't satisfied by this explanation. Certainly within a radius of fewer than a dozen billion light years of Earth can be found stars in all directions? Imagine Reason (talk) 06:17, 19 December 2009 (UTC)

Er - hold on - let me step outside and take a look...Nope! I wasn't blinded.

Intuition is often a poor tool for making scientific arguments. In this case, your intuition is wrong. But we can toss in some more arguments that may help you here:

The thing is that those stars are an incredibly long way away. So due to the effects of perspective, they are very, very small disks. They only look as big as they do because they are relatively bright - and the resolution of our eyes is too poor to see them as the tiny dots that they really are. Even when you look at a star with the best telescopes we have - they are still mostly single-pixel dots. It would take a spectacular number of them to completely cover the sky - and there just aren't that many.

Think about it like this: The apparent size of any object decreases linearly with distance...that's simple perspective. So the angular area that the star covers as seen from Earth decreases as the square of the distance from us. The average number of stars within a given spherical volume of space is proportional to the cube of the radius. So the angular area of the sky that's covered by the stars only increases roughly linearly with the distance that we can see. But worse still - once you get outside of our galaxy - they are all grouped together in bunches ("galaxies") and the galaxies are grouped into galactic clusters. That means that those tiny disks tend to overlap each other to a greater degree than random chance would suggest if they were evenly distributed - and that cuts down the area they can cover so that the growth in area covered as a function of range is somewhat less than linear...and that alone is enough to give us a darker sky than you might expect in an infinite universe that has a finite age.

But we can do even better than that: There are also dark clouds of dust and gas that obscure our view of the stars - and the density of that stuff and the amount of visible light it blocks (and re-radiates as low grade infra-red) grows in the same proportion as the number of stars. Hence the number of blocking dust clouds increases at the same rate as the number of stars resulting in the amount of light reaching us being roughly constant no matter the distance we can see.

Now, we have to be very careful: This 'blocking dust cloud' argument was used for a long time as the explanation for why the sky should be dark in an infinite and static universe for many years before the big bang theory became popular - but the argument has a 'thermodynamics' flaw in that case: in a truly infinite (in space and time) universe, it breaks down because that interstellar dust/gas would gradually heat up until it glowed with the same brightness as the stars themselves. However, in a finite (in time) universe, there isn't an infinite amount of light impinging on those dust clouds and they can therefore radiate the heat away in low grade IR and remain cool and invisible.

SteveBaker (talk) 13:14, 19 December 2009 (UTC)

This APOD shows that even with the best telescopes, there aren't stars everywhere you look. --Ayacop (talk) 16:42, 19 December 2009 (UTC)

Meteorites from Vesta and Mars?

The Vesta article mentions meteorites found on earth from Vesta. I've also heard of meteorites from Mars. How can scientists identify where they come from? And since when have they been able to do this? 78.147.233.150 (talk) 16:58, 18 December 2009 (UTC)

If we know what the composition of the meteorite is and what the composition of the asteroid/planet/etc is, then we can compare them. The reference for that statement (here) has more specific info (gotta pay, but the abstract says enough). We can also track movements after collisions, and even follow orbits backwards through time. ~ Amory (u • t • c) 18:36, 18 December 2009 (UTC)

We have an article on the Mars meteorites. The killer piece of evidence is that inclusions within the meteorites exactly match the composition of the martian atmosphere. There is no other body in the solar system with even a remotely similar atmosphere, and if they had come from outside the solar system, that would be even more remarkable. SpinningSpark 20:45, 18 December 2009 (UTC)

How do people know what the composition of Vesta is? And how is it different from everywhere else in the solar system, all 228,203 planets of it. See List of minor planets. 89.243.91.31 (talk) 21:19, 18 December 2009 (UTC)

The evidence for 4 Vesta is not quite so slam-dunk as it is for Mars. Most of the evidence comes from spectroscopy of the planetoid. Vesta has no atmosphere to test of course, but it does have a unique geology, our article says that it is the only known planetoid whose surface has been melted and recrystallized. It is also known to have been in a major collision from which there are many fragments in the asteroid belt showing a good match match to Vesta's spectrum. It is some of these fragments, called HED meteorites, that are believed to be here on earth. Although Vesta is the only surving planetoid in this condition now, it is believed there were others in the past, now destroyed, so their is still the possibility that at least some of the HED meteorites come from elsewhere. You might be interested in Meteorites and their parent planets, a book on this subject. SpinningSpark 02:10, 19 December 2009 (UTC)

One more point that was raised by Amory above, it is certainly true that we can track back asteroid orbits and determine that they were probably involved in a Vesta collision. That is, we can for fragments that are still in orbit, but not the ones that are here on Earth. We have no way of discovering what their orbit was in space once they have impacted the Earth. The only way to do that is to track them before they hit. According to the MOID article the only asteroid we have ever succeeded in tracking before it hit was 2008 TC3. At least, that was the case when I wrote the MOID article some time ago, perhaps there have been others since. I think it is supremely ironic that such a unique asteroid is doomed never now to be given a proper name and will be forever known by its temporary designation 2008 TC3 because, since it impacted the Earth, it is no longer a Solar System body. SpinningSpark 02:30, 19 December 2009 (UTC)

Hawking and the LHC

How does Hawking Radiation stop black holes from forming in the Large Hadron Collider? MMS2013 17:46, 18 December 2009 (UTC)

It won't necessarily stop them, it will just likely cause them to dissipate in a near-instant. The micro black holes the LHC could theoretically create wouldn't have enough mass to last. The top of Hawking radiation basically says it all. ~ Amory (u • t • c) 18:48, 18 December 2009 (UTC)

will hairdye

burn yer skin if u touch a spill on the floor thats been out for a few mounths

Probably best to use gloves when cleaning up any chemical spill, even old ones. Dragons flight (talk) 20:42, 18 December 2009 (UTC)

No, dried hair dye does not normally burn skin, but it can alter its colour. Fresh hair dye may contain hydrogen peroxide or ammonia which are not good for skin, but evaporate, the dyes may be carcinogenic or poisonous, but not burning. Graeme Bartlett (talk) 22:09, 19 December 2009 (UTC)