Talk:Special relativity

Let's work out revisions to the Transverse Doppler effect section

@Gregor4: I understand what you are trying to do. But what you have written is verbose, rather confusing, and not written at a level appropriate for the target audience, which would be high school seniors to first year college students. Let's try and work out a better approach. I would recommend that we first review the presentation in Relativistic_Doppler_effect#Transverse_Doppler_effect which covers many of the same points that you wish to address. Thanks! Prokaryotic Caspase Homolog (talk) 08:28, 2 November 2021 (UTC)

Answer by Gregor4 (talk) 02:40, 9 November 2021 (UTC) Sorry, I had not seen the document Relativistic_Doppler_effect#Transverse_Doppler_effect which gives a good explanation. I think, we should refer to that page, and I have rewritten a contribution for the page Special Relativity below.

When I originally wrote the current short, highly abbreviated section on TDE in the Special relativity article, I had deliberately covered only the circular cases. Discussing the linear diagrams, as I did in Relativistic_Doppler_effect#Transverse_Doppler_effect, introduces a lot of complications. As I work on this section below, it keeps on getting bigger...and bigger... I'm not sure that what I'm creating here is an appropriate level of detail for Special relativity. Prokaryotic Caspase Homolog (talk) 18:08, 13 November 2021 (UTC)

@Gregor4: Here is the result of my re-write. I don't like it. The level of detail seems out of proportion to what should be in an introductory article for Special relativity, although appropriate for Relativistic_Doppler_effect#Transverse_Doppler_effect. Prokaryotic Caspase Homolog (talk) 14:00, 15 November 2021 (UTC)

I tried a new version. What do you think? Gregor4 (talk) 04:29, 17 November 2021 (UTC)

@Gregor4: We need to emphasize Einstein's original formulation of relativistic Doppler shift, with the receiver pointed directly at where it perceives the image of the source to be at its closest point. Ninety-nine percent of all TDE experiments are devoted to this case. Start by reversing (B) and (A). Prokaryotic Caspase Homolog (talk) 14:35, 17 November 2021 (UTC)

I have added a note about Einstein's formulation in the description of case (2). I do not want to change the order of A and B because the case (1) happens before case (2). Gregor4 (talk) 22:30, 17 November 2021 (UTC)

Your 5-3a is way too busy. Since this illustration describes the situation in the frame of the source, the analysis should be an almost trivial application of time dilation. You do not need to illustrate any blueshift as the distance decreases in this diagram, because then you have redshift some time after the distance increases. You just confuse the reader. If you want to describe the point of zero Doppler shift, you should do so in a separate section via a separate diagram. Prokaryotic Caspase Homolog (talk) 04:29, 18 November 2021 (UTC)

I have slightly revised Fig 5-3(a) and have rewritten the explanation for his case. I hope you lie it. Gregor4 (talk) 23:30, 21 November 2021 (UTC)

Transverse Doppler effect

Figure 5–3. Transverse Doppler effect: Variant scenarios

The transverse Doppler effect (TDE) is one of the novel predictions of special relativity. Assume that a source and a receiver are both approaching each other in uniform inertial motion along paths that do not collide.

At the beginning, when the observer approaches the light source, the observer sees a blueshift, and later, when the distance with the source increases, he sees a redshift. The transverse Doppler effect describes the situation when the light source and the observer are close to each other. At the moment when the source is geometrically at its closest point to the observer, one may distinguish

1. the light that arrives at the observer,
2. the light that is emitted by the source, and
3. the light that is at half distance between the source and observer.

The situation of case (1) is shown in Fig. 5-3(a) in the rest frame of the source. The frequency observed by the observer is blueshifted by the factor γ because of the time delation of the observer (as compared with the rest frame of the source). The dotted blue image of the source shown in the figure represents how the observer sees the source in his own rest frame.

The situation of case (2) is shown in Fig. 5-3(b) in the rest frame of the observer. This light is received later when the source is not any more at closest distance, but it appears to the receiver to be at closest distance. The observed frequency of this light is redshifted by the factor γ because of the time delation of the source (as compared with the rest frame of the observer). This situation was Einstein's original statement of the TDE

In the situation of case (3), the light will be received by the observer without any frequency change.

Whether an experiment reports the TDE as being a redshift or blueshift depends on how the experiment is set up. Consider, for example, the various Mössbauer rotor experiments performed in the 1960s. Some were performed with a rotating source while others were performed with a rotating receiver, as in Fig 5‑3(c) and (d). Fig 5‑3(c) and (b) are corresponding scenarios, as are Fig 5‑3(d) and (a).

References

The effect's "novelty" is exaggerated

The "transverse Doppler" phenomenology isn't as novel to SR as you might think. A similar effect seems to show up in almost any theory where the motion of the emitter has at least some influence on how light propagates.

Take nasty old ballistic emission theory as an example. If an object moving through the lab throws light at what it believes to be "90 degrees" to its relative motion vector, a lab onlooker will see that ray to be advancing at the same rate as the object, and therefore angled to point slightly forward. If the lab onlooker aims a narrow-angle detector at lab-90 degrees to the path of the object, the light that registers on the detector does not belong to the transverse-aimed ray, but a different ray that was originally aimed slightly to the rear, and is therefore expected to include a recession redshift component.

As a result, emission theory predicts a similar (actually stronger) redshift to SR's, and pretty much any dragged-light or dragged-aether model that predicts a transverse-aimed ray being deflected forward in the lab frame will predict that the ray seen at 90 degrees in the lab frame will be seen to be redshifted. ErkDemon (talk) 21:38, 27 August 2023 (UTC)

"In Galilean relativity, length..between two events [does] not change when observed from different frames of reference."

That's not correct. The length of an object is invariant in Galileo's world, but the distance/length between events is not invariant (when two frames are moving with respect to each other). This is an error I've seen before. Johanley (talk) 11:02, 2 April 2023 (UTC)

Indeed, good catch.

That is why a note is sticking to the expression $\Delta r$: "In a spacetime setting, the length of a rigid object is the spatial distance between the ends of the object measured at the same time." (emphasis added).

For clarity and precision, I have changed that to: "In a spacetime setting, the length of a moving rigid object is the spatial distance between the ends of the object measured at the same time. In the rest frame of the object the simultaneity is not required." In Galilean relativity, the simultaneity in the "moving frame" implies that in the rest frame of the object.

I have also changed the phrase ...length and temporal separation between two events... to the more precise an object's length and the temporal separation between two events...'

Change diff: [1] - DVdm (talk) 13:53, 2 April 2023 (UTC)

Einstein's mechanics

Special relativity is occasionally referred by this name, both in educational resources and in research papers. Is it common enough to mention this alternative name in the beginning and to make a redirect? I ask it here so it's not lost in the edit history. Tarnoob (talk) 10:49, 26 July 2023 (UTC)

I don't think it is common enough name to be mentioned in the lead. A redirect can certainly be made, but should probably point to Relativistic mechanics instead of this article. Jähmefyysikko (talk) 11:22, 26 July 2023 (UTC)

Special relativity postulates

I think it would be interesting that a citation and comment of the following article would be inserted: https://doi.org/10.1119/1.10490 It shows that the Lorentz transformations and the existence of an invariant speed can be derived based on the principle of relativity and homogeneity of space–time, isotropy of space–time, group structure, causality condition. It is quite an impressive result that there should be a "limit speed" based on these hypotheses onuly. In this presentation, light does not play such an important role in the elaboration of the theory. 88.180.38.188 (talk) 09:26, 7 February 2024 (UTC)

Old hat. Already covered in section Special relativity#Relativity without the second postulate. - DVdm (talk) 18:11, 7 February 2024 (UTC)

ok noted. There is no reference to the paper by Levy-Leblond, however. 88.180.38.188 (talk) 08:20, 8 February 2024 (UTC)

The current little section is properly sourced from a textbook and another journal article, so there's no need to add another source. - DVdm (talk) 10:46, 8 February 2024 (UTC)

"Special relativity (simplified)" listed at Redirects for discussion

The redirect Special relativity (simplified) has been listed at redirects for discussion to determine whether its use and function meets the redirect guidelines. Readers of this page are welcome to comment on this redirect at Wikipedia:Redirects for discussion/Log/2024 October 2 § Special relativity (simplified) until a consensus is reached. 1234qwer1234qwer4 13:57, 2 October 2024 (UTC)

the section Twin paradox

Figure 4-4. Doppler analysis of twin paradox

I disagree with the statement "in order for the two observers to compare their proper times, the symmetry of the situation must be broken: At least one of the two observers must change their state of motion to match that of the other." And this is depicted in Figure 4.4 when the traveling twin (which I'll call #2) reaches the destination (3 light-years away) and heads back home.

But actually, #2 doesn't need to do anything more after he reaches the destination. In the 1st diagram, #1 sends his 2nd annual message, which will arrive at the destination when #1 has aged 5 years (#1 time).  #2 also knows this, but when he receives the message at the destination, he has aged only 4 years (#2 time).

Similarly, in the 2nd diagram, when #2 sends his 4th message (from the destination), #1 receives it in his 8th year (#1 time), and subtracting the 3-year propagation delay, he knows that he had aged 5 years (#1 time) when #2 sent the message (after only 4 years of #2 time).
Bob K (talk) 16:32, 6 November 2024 (UTC)

The statement is properly sourced. Our personal analysis and views are really off-topic here. See WP:TPG. - DVdm (talk) 17:18, 6 November 2024 (UTC)

I am quoting just our article, which is someone's interpretation of the source. Where is the policy that says it's "off topic" to question an editor's interpretation? I am also an editor, and my interpretion of the figure presented as evidence does not support the statement.
--Bob K (talk) 23:56, 6 November 2024 (UTC)

I am the principal author of this particular section, so I am of course concerned in instances where I may have failed to express myself with perfect clarity. Perhaps you would prefer if I rephrased the sentence, "in order for the two observers to perform side-by-side comparisons of their proper times, the symmetry of the situation must be broken: At least one of the two observers must change their state of motion to match that of the other"? Your proposed counterexamples are not side-by-side comparisons of proper time, but rather #1's and #2's respective calculations of what they think would be observed by the other. Prokaryotic Caspase Homolog (talk) 04:18, 7 November 2024 (UTC)

Thank you. I think we need more discussion. For the side-by-side method, all those messages are irrelevant. Just do the journey and compare the two clocks, side-by-side. The messages are what intrigued me. And when I figured out how to interpret the diagrams, I realized that only the outbound trip is necessary to create the paradox. The change of direction is unnecessary... but our article states (or at least implies) otherwise.

To clarify what you mean by "calculations of what they think would be observed by the other", the paradox is based on classical physics. One calculation is that they expected the two clocks to agree at the end of the entire trip. Another is that they expected the clocks to agree halfway through. Your quibble simply boils down to questioning the assumed 3 year propagation time of messages across a 3 ly distance. If that is in question, the article should say so.

What I would like to see clarified is:

1. The diagram without the messages (side-by-side comparison of clocks) reveals the paradox. The change of direction only explains the changes in message arrivals.
2. The bottom half of diagram 1 (with messages) reveals the paradox. The bottom 8/10 of diagram 2 also reveals the paradox.
   

--Bob K (talk) 16:42, 7 November 2024 (UTC)

I think I just did a pretty good job, but in case I was too verbose, let me point out that if communication between #1 and #2 was instantaneous, the paradox would be still be there and their clocks could be compared as often as they wished. The detail of a communication delay is an obfuscation of the main point of the paradox.
--Bob K (talk) 17:21, 7 November 2024 (UTC)