The Relativity of Simultaneity: A Fundamental Concept in Special Relativity

[mangaroosh]

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.

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.

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?

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.

 

[mangaroosh]

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?

 

[mangaroosh]

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.

hey DS, I'm just wondering if you could explain how this pertains to the question about RoS under Einsteinian relativity, because I can't make the connection.

 

[James_Harford]

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?

Length contraction, time dilation, and RoS are all consequences of the Lorentz transformations, which in turn are derived from Einstein's postulates, which includes the constancy of the speed of light. Therefore, if any of these effects did not occur, Einstein's postulates would be violated.

Time dilation and Lorentz contraction lack the completeness of Einstein's postulates. They cannot be used as postulates to derive the RoS, constant speed of light, etc.

-Regards

 

[harrylin]

[..]

again, YES! Let's get the proper theory explained, then it
will become clear why older theories were inadequate. [..]

The older theory that you seem to refer to is Lorentz theory of electrons; and this is rather well explained in Einstein's 1907 paper which is discussed in a parallel thread. And there is already a parallel thread about other theories. So yes, please let's not mix those topics!

 

[mangaroosh]

Length contraction, time dilation, and RoS are all consequences of the Lorentz transformations, which in turn are derived from Einstein's postulates, which includes the constancy of the speed of light. Therefore, if any of these effects did not occur, Einstein's postulates would be violated.

Time dilation and Lorentz contraction lack the completeness of Einstein's postulates. They cannot be used as postulates to derive the RoS, constant speed of light, etc.

-Regards

Thanks James, this is somewhat clearer. There are still a few questions arising for me though, one which I have repeated but haven't really had a discernable answer to yet.


You say that Length contraction, time dilation and RoS are all consequences of the Lorentz transformations; but Lorentzian relativity uses the same transformations doesn't it? If so, then RoS is not necessarily a consequence of the Lorentz transformations, because RoS is not a part of Lorentzian relativity, which includes length contraction and clock retardation, due to mechanical effects; clock retardation appears to be almost the exact same thing as time dilation except for a different metaphysical explanation.


The differences appear to be:
- time dilation in Einseinian relativity; but mechanical retardation of a clock in Lorentzian
- RoS in Einsteinian relativity; absolute relativity in Lorentzian.


There appears to be some correlation between time dilation and RoS, is that a fair assessment?

 

[harrylin]

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?

No - and that answer is already contained in my posts #31 and #34.

 

[James_Harford]

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.

Consequence means "derivable from". RoS is not derivable from the "lorentz contraction" and/or "time dilation", without also assuming the constancy of the speed of light.

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.

Correlation means little. By this one can equally argue that contractions are a consequence of RoS.

 

[mangaroosh]

No - and that answer is already contained in my posts #31 and #34.

I referenced post #31 in #48.

 

[mangaroosh]

Consequence means "derivable from". RoS is not derivable from the "lorentz contraction" and/or "time dilation", without also assuming the constancy of the speed of light.

Is it possible to assume the constancy of the speed of light without assuming "lorentz contraction" and/or "time dilation"?

Correlation means little. By this one can equally argue that contractions are a consequence of RoS.

I would say not, if the simultaneity of an event is contingent on the time co-ordinate provided by a clock.

 

[James_Harford]

You say that Length contraction, time dilation and RoS are all consequences of the Lorentz transformations; but Lorentzian relativity uses the same transformations doesn't it? If so, then RoS is not necessarily a consequence of the Lorentz transformations, because RoS is not a part of Lorentzian relativity, ...

Stop right there! RoS is most assuredly an effect of Lorentzian relativity. The predictions of the two theories are exactly the same. In either theory two observers can disagree on the order of two distant events. LET claims that one observer is wrong and the other right, but doesn't know which. LET claims that one definition of now is the "right one" but doesn't know which. In other words, the differences between the two theories are non-physical, or metaphysical. LET is SR with metaphysical baggage.

- Regards

 

[harrylin]

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?

The combination with the first postulate leads to the conclusion that the operationally defined speed of light must be the same constant in every inertial reference system (did you carefully read Einstein's description?). And most textbooks as well as some already given replies here provide the answer to your question. I'm afraid that you think that a theory can be learned from merely having discussions on a discussion forum; however, that's just a waste of time of the people here. It's even not an efficient use of your own time.

 

[James_Harford]

Is it possible to assume the constancy of the speed of light without assuming "lorentz contraction" and/or "time dilation"?

Of course it is. And from that follows,

1. the lorentz contraction
2. time dilation
3. and RoS (as shown in post 42) .

I would say not, if the simultaneity of an event is contingent on the time co-ordinate provided by a clock.

Simultaneity requires no clock!
Again, see post 42.

 

[harrylin]

Hi Harry, the conversation with DaleSpam is in reference to mathematical transformations, while your reply in post #28 was with respect to clocks; unfortunately I don't have the nous to make the connection between the two, [...]

I find that an astonishing comment; for it means that you did not understand (if you indeed read) the introduction in Einstein's 1905 paper to which I referred earlier.

I had a quick glance at the reference in post #31, but statement immediately following the link was something you had mentioned before, with respect to detecting absolute simultaneity (or the time on a distant clock) and which I had addressed in #18;
[..]
so that affected my judgement of the necessity to go through it in detail. Is there a specific part that I can jump to that would address the issue?

It addresses how and why "local" simultaneity first emerged, independent of the concept of time dilation (of which the possibility also is suggested); and I already pointed to it. I would have to look up another paper to direct you to how this next lead to the concept of "relative" simultaneity. However only reading one part is not the correct way to surely understand a discourse - and jumping around between text fragments and parts of explanations of people here isn't a good way to learn a topic. The proper way would be to first study a textbook, do some exercises, and check out the explanations in some of the original papers.

Post #20:

The issue being raised appears to be the idea of detecting absolute simultaneity; but that isn't necessarily an issue that needs to be addressed. We don't need to figure out how to synchronise clocks to say that if all clocks remained synchronised then there would be absolute simultaneity; it's somewhat of a tautology. [...]

Only if with you mean "absolute simultaneity" in an operational sense without the implication of "true" simultaneity. I would call that "universal simultaneity", and it's what one effectively does in descriptions of the universe as a whole.
ADDENDUM: Perhaps you meant with " remained", the method of slow clock transport. Then my last remark doesn't apply. Instead, the clarifications of PAllen apply: slow clock transport is a way to naturally approximate the same outcome as is achieved with the Poincare-Einstein synchronization. And it illustrates in which way time dilation and relativity of simultaneity are not fully independent in SR. However, this will hardly be possible to understand without first learning SR; and we can't do that for you.

 

[mangaroosh]

Einstein's 1905 paper

I find that an astonishing comment; for it means that you did not understand (if you indeed read) the introduction in Einstein's 1905 paper to which I referred earlier.

It addresses how and why "local" simultaneity first emerged, independent of the concept of time dilation (of which the possibility also is suggested); and I already pointed to it. I would have to look up another paper to direct you to how this next lead to the concept of "relative" simultaneity. However only reading one part is not the correct way to surely understand a discourse - and jumping around between text fragments and parts of explanations of people here isn't a good way to learn a topic. The proper way would be to first study a textbook, do some exercises, and check out the explanations in some of the original papers.

Only if with you mean "absolute simultaneity" in an operational sense without the implication of "true" simultaneity. I would call that "universal simultaneity", and it's what one effectively does in descriptions of the universe as a whole.
ADDENDUM: Perhaps you meant with " remained", the method of slow clock transport. Then my last remark doesn't apply. Instead, the clarifications of PAllen apply: slow clock transport is a way to naturally approximate the same outcome as is achieved with the Poincare-Einstein synchronization. And it illustrates in which way time dilation and relativity of simultaneity are not fully independent in SR. However, this will hardly be possible to understand without first learning SR; and we can't do that for you.

I am acutely aware that I might be exhausting peoples' patience here, but I am genuinely trying to understand this. I appreciate your advice that consulting a textbook would be a good place to start, and if you could recommend a worhtwhile one, I would eagerly consult it. I do however believe that people on here, and indeed elsewhere, give a relatively relaible representation of the concepts and phenomena contained in many textbooks, because from discussions I've had with people, the information they've presented has been almost entirely representative of the textbook-like resources that I have encountered - through their references. The added benefit of discussing it in a forum is that it offers the chance to question what is meant by certain terminology, something that isn't possible with a textbook.

I appreciate people taking the time to post detailed replies, but because of my lack of a scientific or mathematic background, I am not always able to make the logical connections between points that some people might think is obvious - for example, DaleSpams non-Lorentzian transformation example, which didn't pertain to Einsteinian relativity, when I was working on the assumption that it was Lorentzian transformations, under Einsteinian relativity, that we were talking about. Unfortunately, in such instances, unless it is spelled out for me, I can't see the logical connection between the two.


the 1905 Paper
I did indeed read, and understand, the introduction to the paper you posted; but I'm still unsure as to how DaleSpams example relates to it; it is more the maths used by Dalespam that I don't understand than the introduction to the paper, I would say.

If it would be possible to proceed slowly on the basis of Einstein's definition of simultaneity in that paper, I can give my understanding and if everyone hasn't put me on ignore by then, maybe, just maybe, someone can point out where it is I'm going wrong.

 

[mangaroosh]

Of course it is. And from that follows,

1. the lorentz contraction
2. time dilation
3. and RoS (as shown in post 42) .

Of course, that should have been obvious. I can see how 1 & 2 follow, but I can't yet see how 3 is separate from 1 & 2.

Simultaneity requires no clock!
Again, see post 42.

Is it not required for assigning the time co-ordinate of an event?

 

[mangaroosh]

Stop right there! RoS is most assuredly an effect of Lorentzian relativity. The predictions of the two theories are exactly the same. In either theory two observers can disagree on the order of two distant events. LET claims that one observer is wrong and the other right, but doesn't know which. LET claims that one definition of now is the "right one" but doesn't know which. In other words, the differences between the two theories are non-physical, or metaphysical. LET is SR with metaphysical baggage.

- Regards

Neo-Lorentzian theory appears to have been divested of a lot of that metaphysical baggage, as George mentioned and as the person who posted the widipedia enty also maintains (assuming they're not one and the same person):

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. As a result, the term "Lorentz ether theory" is sometimes used today to refer to a neo-Lorentzian interpretation of special relativity

The remaining metaphysical baggage appears to be the "postulate of a unique absolute rest frame", which could probably be done away with, without the assumption that reference frames are at rest in the ether, as George has suggested Einsteinian relativity has.

 

[Dale]

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.

hey DS, I'm just wondering if you could explain how this pertains to the question about RoS under Einsteinian relativity, because I can't make the connection.

You have repeatedly made the mistaken assertion that RoS is not a separate feature of the Lorentz transform, but rather is somehow automatically implied by LC and TD. You have even made incorrect conclusions based on that assumption by considering LC and TD and assuming that RoS was included and your conclusions were identical to what the Lorentz transforms would predict.

IF your assertion were correct, then all transforms which included LC and TD would automatically also include RoS and would therefore be equivalent to the Lorentz transform. I have provided counter examples which demonstrate that there are transforms (which are not the Lorentz transform) which have TD and LC but not RoS and vice versa.

The connection is that, by considering LC and TD but neglecting RoS, you are unwittingly using one of these alternate transforms, instead of the Lorentz transforms. Thus you are reaching incorrect conclusions.

Is that clear?

 

[harrylin]

[..] the 1905 Paper
I did indeed read, and understand, the introduction to the paper you posted; but I'm still unsure as to how DaleSpams example relates to it; it is more the maths used by Dalespam that I don't understand than the introduction to the paper, I would say.

If it would be possible to proceed slowly on the basis of Einstein's definition of simultaneity in that paper, I can give my understanding and if everyone hasn't put me on ignore by then, maybe, just maybe, someone can point out where it is I'm going wrong.

Yes it may be better if you restart on that basis!

 

[Agerhell]

Thanks James, this is somewhat clearer. There are still a few questions arising for me though, one which I have repeated but haven't really had a discernable answer to yet.


You say that Length contraction, time dilation and RoS are all consequences of the Lorentz transformations; but Lorentzian relativity uses the same transformations doesn't it? If so, then RoS is not necessarily a consequence of the Lorentz transformations, because RoS is not a part of Lorentzian relativity, which includes length contraction and clock retardation, due to mechanical effects; clock retardation appears to be almost the exact same thing as time dilation except for a different metaphysical explanation.


The differences appear to be:
- time dilation in Einseinian relativity; but mechanical retardation of a clock in Lorentzian
- RoS in Einsteinian relativity; absolute relativity in Lorentzian.


There appears to be some correlation between time dilation and RoS, is that a fair assessment?

Special Relativity basically says:

1. There is no way to measure the one-way speed of light, one can only measure the two way speed of light.
2. The two way speed of light is the same for all inertial observers.

Length contraction and time dilation is then used to explain how the two way speed of light is the same for all observers.

That is all there is to it.

Yes in LET you assume a universal preferred frame and there is no "relativity of simultaneity".
"Relavity of simultaneity" occurs when you decide that all inertial observers should get the same result when they measure the speed of light.

It has nothing to do with time dilation per se.

Yes LET uses the same formulas for time dilation and length contraction but it does not state that the speed of light is the same in all inertial systems and thus has no need for relativity of simultaneity.

 

[harrylin]

Special Relativity basically says:

1. There is no way to measure the one-way speed of light, one can only measure the two way speed of light.
2. The two way speed of light is the same for all inertial observers.

Length contraction and time dilation is then used to explain how the two way speed of light is the same for all observers.

That is all there is to it.[...]

That's almost all there is to it. However some people here find it important to state "the obvious" and it does relate to the topic of this thread: the one-way speeds in an inertial reference system can be made equal to the two-way speeds by means of appropriate clock synchronization (and next one can "measure" that they are indeed equal. ). That can be easily understood as a mathematical theorem about averages.

 

[James_Harford]

The connection is that, by considering LC and TD but neglecting RoS, you are unwittingly using one of these alternate transforms, instead of the Lorentz transforms. Thus you are reaching incorrect conclusions.

Is that clear?

Well stated.

 

[mangaroosh]

You have repeatedly made the mistaken assertion that RoS is not a separate feature of the Lorentz transform, but rather is somehow automatically implied by LC and TD. You have even made incorrect conclusions based on that assumption by considering LC and TD and assuming that RoS was included and your conclusions were identical to what the Lorentz transforms would predict.

IF your assertion were correct, then all transforms which included LC and TD would automatically also include RoS and would therefore be equivalent to the Lorentz transform. I have provided counter examples which demonstrate that there are transforms (which are not the Lorentz transform) which have TD and LC but not RoS and vice versa.

The connection is that, by considering LC and TD but neglecting RoS, you are unwittingly using one of these alternate transforms, instead of the Lorentz transforms. Thus you are reaching incorrect conclusions.

Is that clear?

Ah, sorry, I understand your rationale now, but I still don't understand the maths.

the part I don't understand is the initial equations; I read $t'=t-vx$ as meaning $t'$ equals t minus the velocity along the X-axis, but I don't understand why the velocity comes into it.

and $x'=x-vt$ I read as $x'$ equals x minus the velocity multiplied by the time - which makes a bit more sense to me [without understanding how it demonstrates length contraction]

While I don't doubt that your examples demonstrate transformations which include LC and TD but not RoS, and RoS but not LC and TD I can't discern how they do so. I also have difficulty relating them to the physical phenomena which they represent. That is why I find it easier to discuss the physical phenomena affecting clocks than the maths.

You mentioned that the example of the transform you gave wasn't a useful one in physics, but was useful for demonstrating that RoS wasn't a consequence of LC and TD; I'm just wondering if the examples you gave correspond to physical phenomena, that might help with understanding them?

Just on the point "by considering LC and TD but neglecting RoS, you are unwittingly using one of these alternate transforms, instead of the Lorentz transforms". Are the transforms you used based on the assumption of the constancy of c, as with Einsteinian relativity?

 

[mangaroosh]

Special Relativity basically says:

1. There is no way to measure the one-way speed of light, one can only measure the two way speed of light.
2. The two way speed of light is the same for all inertial observers.

Length contraction and time dilation is then used to explain how the two way speed of light is the same for all observers.

That is all there is to it.

Yes in LET you assume a universal preferred frame and there is no "relativity of simultaneity".
"Relavity of simultaneity" occurs when you decide that all inertial observers should get the same result when they measure the speed of light.

It has nothing to do with time dilation per se.

Yes LET uses the same formulas for time dilation and length contraction but it does not state that the speed of light is the same in all inertial systems and thus has no need for relativity of simultaneity.

Thanks Ager.

My understanding of that explanation would be:

RoS is a consequence of the assumption that the speed of light is the same for all inertial observers.

The assumption about the constancy of c requires Length contraction and/or time dilation to explain it, therefore RoS is a consequence of length contraction and/or time dilation.

If Length contraction or time dilation didn't occur, then the speed of light would not be c for all observers and there would be no RoS.

i must stress, that's just according to that explanation.

EDIT: to summarise, it appears as though length contraction and time dilation are a necessary intermediate step before we can arrive at the conclusion the RoS is a consequence of the constancy of c.

 

[mangaroosh]

That's almost all there is to it. However some people here find it important to state "the obvious" and it does relate to the topic of this thread: the one-way speeds in an inertial reference system can be made equal to the two-way speeds by means of appropriate clock synchronization (and next one can "measure" that they are indeed equal. ). That can be easily understood as a mathematical theorem about averages.

Does the clock synchronisation rely on the constancy of the one way speed of light?

 

[kmarinas86]

What on Earth is a two-way speed of light?

Speed is the magnitude of velocity. Speed is the velocity SANS the directional component.

Edit: HMMM. WEIRD:

http://en.wikipedia.org/wiki/One-way_speed_of_light

One-way vs. two-way speed of light
[edit] The two-way speed

The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place only one clock is needed to measure the total time, thus this speed can be experimentally determined independently of any clock synchronization scheme. Any measurement in which the light follows a closed path is considered a two-way speed measurement.

Experiments have shown within tight limits that in an inertial frame the two-way speed of light is independent of the closed path considered.

Since 1983 the meter has been defined as the distance traveled by light in vacuum in 1⁄299,792,458 second.[11] This means that the speed of light can no longer be experimentally measured in SI units, but the length of a meter can be compared experimentally against some other standard of length.
[edit] The one-way speed

Although the average speed over a two-way path can be measured, the one-way speed in one direction or the other is undefined (and not simply unknown), unless one can define what is "the same time" in two different locations. To measure the time that the light has taken to travel from one place to another it is necessary to know the start and finish times as measured on the same time scale. This requires either two synchronized clocks, one at the start and one at the finish, or some means of sending a signal instantaneously from the start to the finish. No instantaneous means of transmitting information is known. Thus the measured value of the average one-way speed is dependent on the method used to synchronize the start and finish clocks. This is a matter of convention.

The Lorentz transformation is defined such that the one-way speed of light will be measured to be independent of the inertial frame chosen.[12]

wow... -They- really don't want to show this stuff very often, do -they-?

 

[mangaroosh]

What on Earth is a two-way speed of light?

Speed is the magnitude of velocity. Speed is the velocity SANS the directional component.

I think it refers to the round-trip speed of light.

 

[Dale]

I also have difficulty relating them to the physical phenomena which they represent. That is why I find it easier to discuss the physical phenomena affecting clocks than the maths.

You mentioned that the example of the transform you gave wasn't a useful one in physics, but was useful for demonstrating that RoS wasn't a consequence of LC and TD; I'm just wondering if the examples you gave correspond to physical phenomena, that might help with understanding them?

They do not correspond to physical phenomena, that is precisely why they are not useful and why your unwittingly using them is such a problem.

 

[kmarinas86]

Does the clock synchronisation rely on the constancy of the one way speed of light?

No. The following is why:

http://en.wikipedia.org/wiki/One-way_speed_of_light

The "one-way" speed of light from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again. Albert Einstein chose a synchronization convention (see Einstein synchronization) that made the one-way speed equal to the two-way speed. The constancy of the one-way speed in any given inertial frame, is the basis of his special theory of relativity although all experimentally verifiable predictions of this theory do not depend on that convention.[1][2][3][4][5][6][7][8]

Experiments that attempted to probe the one-way speed of light have been proposed, but none has succeeded in doing so.[9] It was later shown that these experiments are in fact measuring the two-way speed.[1][10]

The 'speed of light' in this article refers to the speed of all electromagnetic radiation in vacuum.

Other clock synchronization conventions need not hold this assumption.

References [1] through [8]:

^ a b Yuan-Zhong Zhang (1997). Special Relativity and Its Experimental Foundations. World Scientific. ISBN 9789810227494.
^ Anderson, R.; Vetharaniam, I.; Stedman, G. E. (1998), "Conventionality of synchronisation, gauge dependence and test theories of relativity", Physics Reports 295 (3-4): 93–180, Bibcode 1998PhR...295...93A, doi:10.1016/S0370-1573(97)00051-3
^ Conventionality of Simultaneity entry by Allen Janis in the Stanford Encyclopedia of Philosophy, 2010
^ Mathpages: Conventional Wisdom and Round Trips and One-Way Speeds
^ a b Edwards, W. F. (1963). "Special Relativity in Anisotropic Space". American Journal of Physics 31 (7): 482–489. Bibcode 1963AmJPh..31..482E. doi:10.1119/1.1969607.
^ Winnie, J. A. A. (1970). "Special Relativity without One Way Velocity Assumptions". Philosophy of Science 37: 81–99, 223–38. JSTOR 186029.
^ Rizzi, Guido; Ruggiero, Matteo Luca; Serafini, Alessio (2004). "Synchronization Gauges and the Principles of Special Relativity". Foundations of Physics 34 (12): 1835–1887. arXiv:gr-qc/0409105. Bibcode 2004FoPh...34.1835R. doi:10.1007/s10701-004-1624-3.
^ Sonego, Sebastiano; Pin, Massimo (2008). "Foundations of anisotropic relativistic mechanics". Journal of Mathematical Physics 50 (4): 042902-042902-28. arXiv:0812.1294. Bibcode 2009JMP...50d2902S. doi:10.1063/1.3104065.

 

[mangaroosh]

They do not correspond to physical phenomena, that is precisely why they are not useful and why your unwittingly using them is such a problem.

I'm not sure that I am unwittingly using them; maybe my reply to Agerhell can clarify what I was trying to get at

EDIT: the point being made by a number of people is that RoS is a consequence of the constancy of c; but the constancy of the speed of light does not cause length contraction and time dilation, length contraction and time dilation must occur in order for all observers to measure the speed of light to be c - hence RoS is a consequence of them - in terms of real world phenomena, as opposed to hypothetical mathematics that doesn't correspond to physical phenomena.

 

[James_Harford]

Of course, that should have been obvious. I can see how 1 & 2 follow, but I can't yet see how 3 is separate from 1 & 2.

Since, 3, Relativity of Simultaneity [RoS] can be demonstrated simply and directly without requiring the introduction of Lorentz contractions, time dilations, measuring rods, or clocks, it is explained most simply without them.

Is [RoS] not required for assigning the time co-ordinate of an event?

Yes. But simultaneity is not, as you said, "contingent on the time coordinate of a clock", as I showed in the earlier post.

- Regards.

 

[mangaroosh]

Since, 3, Relativity of Simultaneity [RoS] can be demonstrated simply and directly without requiring the introduction of Lorentz contractions, time dilations, measuring rods, or clocks, it is explained most simply without them.

Is that in relation to hypothetical maths that don't correspond to real world phenomena though?

Rather than repeating the point, I'm just wondering if the clarification of my point in post #100 makes any more sense?

Yes. But simultaneity is not, as you said, "contingent on the time coordinate of a clock", as I showed in the earlier post.

I could be wrong, but that sounds a bit like a category mistake.

Is RoS not the term applied when the time co-ordinates of an event are different across reference frames; and are those time co-ordinates not supplied by local clocks? My understanding is that if the time co-ordinate of an event, as supplied by a local clock, is different than the time co-ordinate for the same event, supplied by a remote clock, then the events are not absolutely simultaneous, but relatively simultaneous.

To refer back to an earlier comment, that "simultaneity requires no clock", that is of course true, but given that there are clocks, and if we assume that they measure time, then if absolute simultaneity prevailed, all "reliable" clocks should measure the same time - is that accurate enough? I think that is roughly what the introduction to [the English translation of] Einstein's 1905 paper said, or at least I think it can be deduced from it.

In order for RoS to arise, something would have to happen. What is that something?


As mentioned, hopefully the clarification of the point pertaining to the constancy of the speed of light is clearer in post #100.

 

[James_Harford]

Is that in relation to hypothetical maths that don't correspond to real world phenomena though?

No, because the demonstration that RoS is relative uses no "hypothetical math". Just Einstein's postulate. You can answer this question and the others in your post yourself. Again, no clock is required. You don't seem to believe me. Go back, look at the post and see for yourself.

It's a really simple demonstration: post #42 (42 = The Answer to Life, the Universe, and Everything).

In order for RoS to arise, something would have to happen. What is that something?

Einstein's postulate. All else follows. You are unlikely to find a simpler answer anywhere. :-)

- Regards.

 

[mangaroosh]

No, because the demonstration that RoS is relative uses no "hypothetical math". Just Einstein's postulate. You can answer this question and the others in your post yourself. Again, no clock is required. You don't seem to believe me. Go back, look at the post and see for yourself.

It's a really simple demonstration: post #42 (42 = The Answer to Life, the Universe, and Everything).

Einstein's postulate. You are unlikely to find a simpler answer anywhere. :-)

Thanks James, I've read post #42 and others have also raised the issue of the second postulate, of the constancy of the speed of light, most recently Agerhell. Indeed, Ager's formulation was quite simple and straight forward, and so the point is probably most easily addressed from that formalism.

The contention appears to be that RoS is a consequence of the constancy of the speed of light. The issue, as I see it is, that the speed of light doesn't cause length contraction and/or time dilation; rather, the observation of the speed of light to be constant, by all observers, is a consequence of length contraction and/or time dilation; that is, if length contraction and/or time dilation did not occur, then observers would not measure the speed of light to be the constant c; and so RoS is a consequence of length contraction and/or time dilation. That is solely based on the explanation using the constancy of the speed of light.

Saying that RoS is a separate and distinct aspect of Einsteinian relativity appears to be a category mistake.That would be my understanding anyway, am I going wrong somewhere there?DaleSpams examples didn't, I don't think, refer to real world phenomena, so I'm not immediately able to see their relevance.

 

[harrylin]

It may be useful to highlight a subtle point that has perhaps not been brought up. Some of the replies can look contradictory because "relativity of simultaneity" has a technical meaning as well as an extended meaning (just like for example "democracy"):

- there is technical relativity of simultaneity, as illustrated by dalespam. Such relative simultaneity does not necessarily imply the PoR.

- there is relativity of simultaneity in the context of relativity theory, implying the PoR.
The expression then acquires the additional meaning that no Newtonian "absolute simultaneity" can be established ("simultaneity is relative"). That is explained by SR with such effects as length contraction and time dilation.

Does that help?

Harald