# Relativity of Simultaneity

by mangaroosh
Tags: relativity, simultaneity
P: 3,188
 Quote by Agerhell If you first assume that the speed of light is c in relation to the guy standing on the platform. The guy on the platform would classically expect that it takes longer time for the light from the guy on the train to reach the back and front wall and bounce back towards the guy on the train than if the train had not been moving. You can do the math, it is simple. However, in Special Relativity the Time Dilation formula kicks in and it so happens that if the guy on the platform would expect it to take a factor k longer for the light to reach the back and front wall and get back to the guy on the train again, then the time for the guy on the train slows down with the same factor k so he thinks the two way speed of light has the same value, c, no matter what the velocity of the train relative to the ground is... In this way the "two-way speed of light is the same in all inertial frames". I belive this is how it is always explained...
The way you describe it, you probably mean the ceiling and the floor. Else you should add length contraction to the mix...
And to top it off (in view of the topic!), one usually makes the one-way speed equal to the two-way speed by means of the Poincare-Einstein synchronization procedure.
P: 67

 Quote by mangaroosh this might seem like yet another basic question, but I was wondering about RoS. The impression that I got from reading about relativity was that relativity of simultaneity was a consequence of Lorentz contractions, primarily time dilation.

 Quote by mangaroosh ... the trouble I have is understanding how one interpretation [SR] includes RoS while the other [LET] includes absolute simultaneity. George suggested that it is down to the different assumptions about the propagation of light, but I don't see how that would lead to RoS without some intermediary step(s).
are answered in my earlier post. I guess I missed that you have a third question:

 Quote by mangaroosh I don't think we're too concerned with Lorentzian relativity in this particular instance, what we're looking for, or rather, what I'm trying to understand, is what phenomena must occur in order for the speed of light to remain constant for all observers, regardless of their motion realtive to the source of the light...
May I suggest that we work on them one at a time? Bouncing back and forth between all of them and adding time dilation and lorentz contractions and LET makes all of this a rather poisonous mix of unrelated factors. Please go back to my earlier post and note that it fully answers your first two questions, as stated. The only valid questions about that post are
1. Is it a correct analysis?
2. Do you understand it?

If you do not, then, I would not go on to the third question until you do.
If you do, then you have just the third question.

That question, unlike the others, is about the ratio of a time and a space measurement, and this unavoidably requires some level of understanding of the Lorentz transforms themselves, which is bad news for you. Give up the notion that time dilations and Lorentz space contractions are sufficient. You will fail. You need a complete coordinate transformation, because only that gives exactly the differences in how the various observers make these measurements.

I would recommend that you start playing around with pencil and paper with various XT coordinate systems, and see if you can find (by trial and error) two coordinate systems in which the path of a photon (a slanted line) has the same speed. Whether you succeed or not, the exercise will get you past a psychological obstacle and improve your chances of grasping what your friends here are telling you.

 Quote by mangaroosh One thing he might disagree on is that Lorentzian relativity necessarily includes the superfluous concept of an aether.
I doubt it.

- Regards
P: 5,632
 I hope I'm using the right phraseology here when I say that, under Galilean transformations we would expect the moving observer to measure a different speed of light to the pulse operator, but under Lorentz transformations this isn't the case; is that accurate?
yes. Galileo would have EXPECTED to measure a different speed, but if he had the proper apparatus, would have been dumbfounded when he actually observed the fixed speed of light! And probably so would have Newton.

Dalespam:
 I don't think that anything productive will come from a discussion of Lorentzian relativity. It is a defunct interpretation which asserts the reality of undetectable entities. All of the confusion with LET comes from the simple fact that the aether is undetectable, so in LET you continually have to make careful distinctions between measurements and what is happening in (undetectable) reality. It is a rabbit hole not worth going down.
YES!!! YES!!!! .....I was just about to post similar when I realized there is this last page.

Harford:
 May I suggest that we work on them one at a time? Bouncing back and forth between all of them and adding time dilation and lorentz contractions and LET makes all of this a rather poisonous mix of unrelated factors.
again, YES!!!! Let's get the proper theory explained, then it
will become clear why older theories were inadequate.

The HISTORY section here explains why such confusion reigned before Einstein....nobody had Einstein's physical insights.....

http://en.wikipedia.org/wiki/Minkowski_space
P: 359
 Quote by Agerhell If you first assume that the speed of light is c in relation to the guy standing on the platform. The guy on the platform would classically expect that it takes longer time for the light from the guy on the train to reach the back and front wall and bounce back towards the guy on the train than if the train had not been moving. You can do the math, it is simple. However, in Special Relativity the Time Dilation formula kicks in and it so happens that if the guy on the platform would expect it to take a factor k longer for the light to reach the back and front wall and get back to the guy on the train again, then the time for the guy on the train slows down with the same factor k so he thinks the two way speed of light has the same value, c, no matter what the velocity of the train relative to the ground is... In this way the "two-way speed of light is the same in all inertial frames". I belive this is how it is always explained...
 Quote by harrylin The way you describe it, you probably mean the ceiling and the floor. Else you should add length contraction to the mix... And to top it off (in view of the topic!), one usually makes the one-way speed equal to the two-way speed by means of the Poincare-Einstein synchronization procedure.
Does this not just verify the point of the OP that RoS is just a consequence of Lorentz contractions, and isn't necessarily a separate, third aspect of Einsteinian relativity?
P: 359
 Quote by DaleSpam That is fine. If you don't understand then ask new questions that will help me get at your root concern. But if you merely repeat the same incorrect claim that I answered already then all I can do is as I did above and refer back to my previous answers. You are correct, I was travelling for about two weeks with reduced time and access. However, I did find it frustrating to come back and find you repeating the same mistake that I already addressed weeks ago.
Apologies DS, I tend to treat each conversation individually and try not to take it for granted that every poster reads every post, so I tend to restate certain points for the purpose of the conversation. A number of the times I have restated the point, however, has either been in somewhat different context or for the purpose of reformulating for clarity.

I would argue, though, that the times I have restated it it hasn't necessarily been superfluous to the two-way conversation. I can understand how it might be frustrating to read the same point repeated, particularly when you are under the impression that you have already answered it, but, as mentioned, I had subsequent questions which were taken up by someone else; this lead me to repeat and/or reformulate the point for the purpose of that discussion.

 Quote by DaleSpam I don't think that anything productive will come from a discussion of Lorentzian relativity. It is a defunct interpretation which asserts the reality of undetectable entities. All of the confusion with LET comes from the simple fact that the aether is undetectable, so in LET you continually have to make careful distinctions between measurements and what is happening in (undetectable) reality. It is a rabbit hole not worth going down.
The purpose of this thread isn't necessarily to discuss Lorentzian relativity, I think it is sufficient to discuss it in the competing theories thread; the purpose of bringing up Lorentzian relativity here, was solely to juxtapose it with Einsteinian relativity for the purpose of clarification; namely that it appears that the same transformations can result in both absolute simultaneity and RoS. The question that is begged from that is, what is the difference between the two; why RoS under one interpretation, but absolute relativity in the other?

Please forgive the repetition there, I'm using it solely to emphasise that the question hasn't been addressed just yet.

I'm not sure the specific example of the transform you gave fully addresses the question, because the question is more general. It might be the case that length contraction and time dilation do not lead every single event to be relatively simultaneous, but if time dilation and/or length contraction never occurred at all, anywhere, would RoS still prevail?

 Quote by DaleSpam Try the reply without the invalid reference. Btw, I saw the reference, and if that is the kind of material that you are trying to learn relativity by reading then it is no wonder that you are confused.
It's not so much that I'm confused, rather I haven't made the logical connection yet as to how RoS is separate, stand-alone aspect of Einsteinian relativity and not just a consequence of Lorentzian contractions.

It's probably also worth pointing out that I don't try to learn relativity from sources such as the one posted; I learn relativity largely from discussions with people on sites like this, from the references they post and from my own searches; however, I tend not to simply accept as gospel what I am told, and try to subject such things to critical inquiry, which usually serves the purpose of developing a better understaning; on that basis I will consider alternative viewpoints as long as they stand up to reason, and of course experiment.

The "reference" posted seems to make quite a reasonable point about the propagation of light, which I haven't seen considered elsewhere. Unfortunately I'm not in a position to subject it to the level of critical reasoning required so I can only present it as it is and see if there are arguments against it. However, as it strikes me as being quite reasonable, and conforms to [more reliable] information I have encountered previously, I would lean more towards accepting it (without accepting it fully).

Incidentally, I posted the reply without the reference in the other thread.
P: 359
Hi James, if possible I'd like to change the first question; I didn't formulate it in reply to yourself, but did in reply to Agerhall.

Your explanation was based on the second postulate, the constancy of c in every reference frame, regardless of the motion relative to the source. The question that arises from that is, what phenomena have to occur to allow for this possibility? Ordinarily, with the addition of velocities we would expect the moving observer to measure a different speed of light; what phenomena occur that leads to him measuring the speed of light to be the same as the other observer?
P: 359
 Quote by DaleSpam I don't think that anything productive will come from a discussion of Lorentzian relativity. It is a defunct interpretation which asserts the reality of undetectable entities. All of the confusion with LET comes from the simple fact that the aether is undetectable, so in LET you continually have to make careful distinctions between measurements and what is happening in (undetectable) reality. It is a rabbit hole not worth going down.
DS, just wondering if you would agree with what George said was accurate, that Lorentzian relativity had been divested of all the trappings except for the absolute rest frame?
P: 359
 Quote by PhilDSP It would probably be fair and correct to say that the mathematics going into and coming out of the Lorentz Transformation and their usage is very different comparing Einstein-Minkowski and Lorentz-Poincare relativity. While the Lorentz Transformation itself is obviously common. For the most part, Lorentz worked with vectors, differential equations and their initial and boundary conditions. Einstein and Minkowski built a platform where those things were abstracted away in which tensors and 4-vectors are preferred.
Thanks Phil.

The points being raised appear to pertain to the Lorentz transformation; does it make sense then to ask why the transformation under one interpretation has RoS while under the other interpretation has absolute simultaneity?
P: 359
 Quote by harrylin Then, evidently, you did not understand it. As Dalespam suggests, you can simply re-post the reformulation of your question without including a link to a crank(?) site.
I could be completely wrong Harry, but based on your posts and the fact that you mentioned you prefer to base your understanding on Einstein's original German paper I would guess that either English or German is your native language - of course you could be form a bilungual family. But if one of them isn't your native language, then you presumably have experience in learning a foreign language. If so, I'm sure you will have noticed that there is usually a disconnect between how much a person can understand and how much they can express themselves, such that it is possible to understand something without necessarily being able to express it [adequately] in your own words. The inability to express it in your own words wouldn't necessarily mean that you haven't understood something that has been said.

The post has been re-posted without the link. As to whether or not the site is a "crank" site, I prefer to examine the content and judge it on that basis.
Mentor
P: 17,541
 Quote by mangaroosh I'm not sure the specific example of the transform you gave fully addresses the question, because the question is more general. It might be the case that length contraction and time dilation do not lead every single event to be relatively simultaneous, but if time dilation and/or length contraction never occurred at all, anywhere, would RoS still prevail?
I specifically addressed that in the math above. In one transform I showed there can be no length contraction nor time dilation at all, anywhere, and there is still RoS.

The opposite claim is that the RoS is a consequence of length contraction and time dilation, and I disproved that claim also by providing a transform that had length contraction and time dilation, but no RoS.

Both claims have been disproven by counter-example. I don't know why you persist in them.
Mentor
P: 17,541
 Quote by mangaroosh DS, just wondering if you would agree with what George said was accurate, that Lorentzian relativity had been divested of all the trappings except for the absolute rest frame?
He and I disagree about LET, but it is such a useless topic that I do not intend to dispute it, neither with him nor with you.
P: 359
 Quote by DaleSpam I specifically addressed that in the math above. In one transform I showed there can be no length contraction nor time dilation at all, anywhere, and there is still RoS. The opposite claim is that the RoS is a consequence of length contraction and time dilation, and I disproved that claim also by providing a transform that had length contraction and time dilation, but no RoS. Both claims have been disproven by counter-example. I don't know why you persist in them.
I have a few questions relating to that, so if it is possible to proceed from here I might be able to get my head around it.

Were the transformations you gave for just one event EDIT: between two reference frames?

Would the same transformations be used in Lorentzian relativity and not include RoS in any of them?

In the example of the transform where there was no RoS, would we conclude that absolute simultaneity prevails, right across the universe, under the Einsteinian interpretation?
Mentor
P: 17,541
 Quote by mangaroosh Were the transformations you gave for just one event?
They were for all events.

 Quote by mangaroosh Would the same transformations be used in Lorentzian relativity and not include RoS in any of them?
Those transforms are not the Lorentz transform, so it is not used in LET.

 Quote by mangaroosh In the example of the transform where there was no RoS, would we conclude that absolute simultaneity prevails, right across the universe, under the Einsteinian interpretation?
Again, those transforms are not the Lorentz transform, so it is not used by SR.
P: 359
 Quote by DaleSpam They were for all events. Those transforms are not the Lorentz transform, so it is not used in LET. Again, those transforms are not the Lorentz transform, so it is not used by SR.
sorry, you've thrown me with the last 2 comments; I thought we were talking about Lorentz transformations under Einsteinian relativity.

EDIT: that might be where the confusion is arising from.
P: 67
 Quote by mangaroosh Apologies DS, ... I can understand how it might be frustrating to read the same point repeated, particularly when you are under the impression that you have already answered it, but, as mentioned, I had subsequent questions which were taken up by someone else; this lead me to repeat and/or reformulate the point for the purpose of that discussion.
Repeating a question that has already been answered to your satisfaction "for the purpose of that discussion" is puzzling, to put it mildly.

 Quote by mangaroosh ...the purpose of bringing up Lorentzian relativity here, was solely to juxtapose it with Einsteinian relativity for the purpose of clarification; namely that it appears that the same transformations can result in both absolute simultaneity and RoS. The question that is begged from that is, what is the difference between the two; why RoS under one interpretation, but absolute relativity in the other?.
In terms of predictive capability, there is no difference between the two. The only difference is of interpretation. Lorentz assumed, in accordance with the conventional belief of that time in the aether, that a preferred frame of reference existed. Einstein' noted that since no such frame was detactable, it is a superfluous assumption. That, in a capsule, is all you need to know about Lorentz Ether Theory (LET).

 Quote by mangaroosh ... but if time dilation and/or length contraction never occurred at all, anywhere, would RoS still prevail?
Yes. A simple example of your hypothetical question is a Euclidean space spacetime. It also has RoS, but unlike Minkowski spacetime of SR, moving objects undergo the opposite effects, i.e. time contraction and space dilation. So if you want to insist that such effects "explain" RoS, you must include these as well. Learning Euclidean spacetime is, relatively speaking (!), a snap, so you might want try out your questions on this spacetime first, perhaps with pencil and paper. Hint : the axis of every coordinate system in a Euclidean spacetime are at right angles. You will see exactly how RoS interacts with space dilation and time contraction, and having done this, you will have some idea of how to adapt what you have learned to actual relativistic, or Minkowski, spacetime.

 Quote by mangaroosh It's not so much that I'm confused, rather I haven't made the logical connection yet as to how RoS is separate, stand-alone aspect of Einsteinian relativity and not just a consequence of Lorentzian contractions.
You don't know that it is a consequence, so why assume that it is? That no one can explain your belief should tell you that maybe this duck can't fly. Indeed, it cannot.

- Regards
P: 67
 Quote by mangaroosh Hi James, if possible I'd like to change the first question; I didn't formulate it in reply to yourself, but did in reply to Agerhall.
Then you should have done so.

 Quote by mangaroosh Your explanation was based on the second postulate, the constancy of c in every reference frame, regardless of the motion relative to the source. The question that arises from that is, what phenomena have to occur to allow for this possibility? Ordinarily, with the addition of velocities we would expect the moving observer to measure a different speed of light; what phenomena occur that leads to him measuring the speed of light to be the same as the other observer?
The general phenomena that you are looking for is that every object defines a proper reference frame in which all of the laws of physics, including the speed of light, are the same as that of a stationary observer. These frames are related by the Lorentz transformations derived from Einstein's postulates. All relativistic effects can be obtained from these transformations. Therefore learn about these transformations and their derivation.
P: 359
 Quote by James_Harford Repeating a question that has already been answered to your satisfaction "for the purpose of that discussion" is puzzling, to put it mildly.
The issue is that it hadn't been answered to my satisfaction, in the sense that I didn't fully understand it; hence I repeated the question and/or reformulated it in discussion with someone else, to see if they could highlight where my misunderstanding lay.

Addressing subsequent questions by referring back to the original answer which lead to those subsequent questions doesn't address those subsequent questions - yes that sounds complicated, but that is precisely what appears to me to be happening.

 Quote by DrewD In terms of predictive capability, there is no difference between the two. The only difference is of interpretation. Lorentz assumed, in accordance with the conventional belief of that time in the aether, that a preferred frame of reference existed. Einstein' noted that since no such frame was detactable, it is a superfluous assumption. That, in a capsule, is all you need to know about Lorentz Either Theory (LET).
According to George and wikipedia
 the last vestiges of a substantial ether had been eliminated from Lorentz's "ether" theory, and it became both empirically and deductively equivalent to special relativity. The only difference was the metaphysical[C 7] postulate of a unique absolute rest frame, which was empirically undetectable and played no role in the physical predictions of the theory
current status

It is probably even possible to get rid of the notion of an absolute rest frame also, which appears to be an oft cited reason why Einsteinian relativity is preferred.

 Quote by DrewD Yes. A simple example of your hypothetical question is a Euclidean space spacetime. It also has RoS, but unlike Minkowski spacetime of SR, moving objects undergo the opposite effects, i.e. time contraction and space dilation. So if you want to insist that such effects "explain" RoS, you must include these as well. Learning Euclidean spacetime is, relatively speaking (!), a snap, so you might want try out your questions on this spacetime first, perhaps with pencil and paper. Hint : the axis of every coordinate system in a Euclidean spacetime are at right angles. You will see exactly how RoS interacts with space dilation and time contraction, and having done this, you will have some idea of how to adapt what you have learned to actual relativistic, or Minkowski, spacetime.
I'm not sure I understand the point re: Euclidean spacetime; it appears to suggest that RoS prevails because effects very similar to time dilation and length contraction occur. I have difficulty seeing how that demonstrates that RoS under Einsteinian relativity is not a consequence of Lorentz contractions.

Quite a few explanations have been provided thus far as to how RoS prevails without length contraction and time dilation, but I'm not sure of the relevance to the question being asked. I thought I was discussing Lorentzian transformations according to Einsteinian relativity, but the answers being provided appear to relate to anything but that. Unfortunately I don't immediately see the relevance of such answers to the question in hand, so that may be part of the reason for the general frustration and annoyance in this thread; people are answering a question in a manner they believe addresses the question, but I am having trouble seeing how it does.

If we stick with Einsteinian relativity, however, would RoS still prevail if time dilation and length contraction didn't manifest anywhere?

 Quote by DrewD You don't know that it is a consequence, so why assume that it is? That no one can explain your belief should tell you that maybe this duck can't fly. Indeed, it cannot.
The impression I got was that it was a consequence and thus far I haven't encountered an explanation which clarifies why that impression is inaccurate.

For example, if we take your explanation involving the pulse operator and the moving observer, your explanation was based on the constancy of c, but, to my understanding, in order for the speed of light to be c in all reference frames regardless of the motion relative to the source, then length contraction and/or time dilation have to occur; which again would suggest that RoS, under Einsteinian relativity is a consequence of contractions.
P: 359
 Quote by James_Harford Then you should have done so. The general phenomena that you are looking for is that every object defines a proper reference frame in which all of the laws of physics, including the speed of light, are the same as that of a stationary observer. These frames are related by the Lorentz transformations derived from Einstein's postulates. All relativistic effects can be obtained from these transformations. Therefore learn about these transformations and their derivation.
I'll try a more direct question: if length contraction and/or time dilation did not occur, would an observer moving relative to another observer, and a light source, measure the same speed of light as the other observer?

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