Is relativity of simultaneity measurable?

[OnlyMe]

You had said E-synch does not work without isotropic OWLS. My point was that it does.

E-synch uses light signals between the two clocks. If the OWLS is not isotropic, the clocks cannot be accurately synchronized using light signals. The synchronization would be asymmetric depending on which clock you started with. Only if OWLS is isotropic can synchronization from either clock return the same results.

In the embankment frame, observer M is always located at the midpoint between the events.

This is true.

In the train frame, observer M' is always located at the midpoint between the events.

This is not quite true. The lightning strikes occur in the embankment frame when M' and M are lined up... But the train is moving so the observer at M' does not record the flashes, while M and M' are lined up but somewhat later. Observing the flash from the lightning strike at B before the flash from A.

Einstein's hypothetical in his 1920 book is set up over several pages, in a conversational manner. Below is a quote of the relevant part addressing the above interpretation. That portion specifically addressing the location of M' when the flashes are observed is in bold.

Let M' be the mid-point of the distance A—B on the travelling train. Just when the flashes of lightning occur, this point M' naturally coincides with the point M, but it moves towards the right in the diagram with the velocity v of the train. If an observer sitting in the position M' in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A.​

No, the hypothetical was constructed so that the lightning strikes were only simultaneous in the embankment frame. There is no reason whatsoever to think they were simultaneous in the train frame.

Again not so. The strikes were simultaneous in both frames, but the flashes by which the observers recorded the strikes were not.

The section of Einstein's hypothetical leading up to the last quote, lays out the comditions that establishes the lightning strikes, as simultaneous in both frames. The portion relevant to the train is in bold.

When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A—B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the mid-point of the distance A—B on the travelling train.​

Here Einstein indicated that points A, B & M, on the embankment, have corresponding points on the train A, B and M', which were lined up each with their counterpart at the instant of the lightning strikes. This establishes that the lightning strikes were in fact simultaneous in both frames. The later reference (quoted above) establishes that they were observed or measured to be simultaneous only in the embankment frame.

To finish up the above section concludes with,

Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result:

Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity).​

This is the most important part of his presentation of RoS. Everything that lead up to this conclusion was in essence a teaching tool. And it is the implications of this conclusion that, leads to my contention that it is not necessary to prove two events to be simultaneous in any absolute manner, as was done in Einstein's hypothetical, to prove RoS. All that should be required is to demonstrate that two events which are observed to be simultaneous in one frame, are observed to be sequential in another.

For that we needn't even consider synchronized clocks. All we need is two good clocks in two frames of reference separated by distance of relative velocity, measuring the timing or order of the same two events.

The attempt to determine whether two spatially separated events are simultaneous, with respect to each other, is where synchronized clocks are required. Though simultaneous lightning strikes were stipulated in Einstein's hypothetical, I don't believe they are required to demonstrate and/or prove RoS...

 

[ash64449]

Again not so. The strikes were simultaneous in both frames, but the flashes by which the observers recorded the strikes were not.

Don't confuse yourself. Embankment frame sees the two lightnings as simultaneous.
Recall that the two lightning as two events
Always think about the Lorentz transformation. What does it actually helps us to find?

If you know the coordinates of two events in one frame(in the above embankment frame if you know the coordinates of those two lightning along with the time of happening)

Then you can find what are the coordinates of those two events in another frame.(in the above trains frame which sees two lightnings as non-simultaneous)

 

[OnlyMe]

Don't confuse yourself. Embankment frame sees the two lightnings as simultaneous.
Recall that the two lightning as two events
Always think about the Lorentz transformation. What does it actually helps us to find?

If you know the coordinates of two events in one frame(in the above embankment frame if you know the coordinates of those two lightning along with the time of happening)

Then you can find what are the coordinates of those two events in another frame.(in the above trains frame which sees two lightnings as non-simultaneous)

The lightning strikes and the flashes are two separate things.

The lightning strikes are simutaneous in thier proper frame, a frame of reference common to both strikes, which in the hypothetical happens to coincide with the embankment frame. The fact that they are events that occur in a common frame of reference, makes them, the lightning strikes, simutaneous in all frames.

What is not simultaneous in all frames, is when the flashes that the lightning strikes create, are observed or recorded by different observers.

Where this gets confused and clock synchronization gets involved is when, one tries to put observers at A and B in both frames, to prove simultaneity. With ideally synchronized clocks, both observers on the train would agree that the lightning strikes were simultaneous, since A and B on the train were aligned with A and B on the embankment, at the time the lightning strikes occurred. Trying to add that proof to a test of RoS is what stands in the way of success... Because we cannot ideally synchronize clocks separated spatially.

Again, neither of the observer's at M and M' record the lightning strikes when they actually occur. Both must wait until the flashes reach them. By that time the train has moved on toward B and records the flashes sequentially first the flash from B and then the flash from A. Both of which were created by lightning strikes when A and B on the train were inline with A and B on the embankment.

Don't confuse the lightning strikes with the time of light delay associated with the flashes. It is the flashes that are observed.

The Lorentz transformation does not play into Einstein's construction of the hypothetical. The train was not said to be moving relativistically and there is no attempt to put any number to the times or distances.

 

[ash64449]

The lightning strikes and the flashes are two separate things.

The MOST important thing in relativity is that it deals with 'Events'. If you are thinking of common frame of reference and saying lightning strikes are simultaneous in all frames but flashes make every thing delayed as a result they see events as non-simultaneous,you are using the interpretation of LET. I am using the interpretation of SR. So discussing about this is not quite right since you and i use different interpretation. You must understand how you are using the LET interpretations.

The lightning strikes are simutaneous in thier proper frame, a frame of reference common to both strikes, which in the hypothetical happens to coincide with the embankment frame. The fact that they are events that occur in a common frame of reference, makes them, the lightning strikes, simutaneous in all frames.

It seems to me that your thinking about absolute frame of reference or you are displaying how Lorentz transformation actually works.

There are two main interpretation of SR. One is interpretation of Einstein and the other LET(Lorentz Ether Theory).

The fact that you are saying about common frame of reference is a direct indication that you are using LET too and also i think you are also using interpretation of SR.

Be careful, Don't mix both interpretations. You are not allowed to say common frame of reference if you are following interpretations of SR.

In SR. Only those who are (observed) in the same state of motion can agree on same definition of simultaneity.

What is not simultaneous in all frames, is when the flashes that the lightning strikes create, are observed or recorded by different observers.

yes. this is right if you are following LET. But if you are following interpretation of Einstein,you are not allowed to say this because Einstein assumes speed of light is same in both frame of reference.

in LET,It gives supreme importance to a frame of reference which is "common frame of reference" in your post.

And as a result you established that lightning strikes are simultaneous in all frames.(by that also you mean "actually" but interpretation of SR does not have the word "actually")

You seemed that i am the one confusing because you have mixed both interpretation.

EDIT: I am not saying that what you said is incorrect. Your statements are correct as long as you are using LET only.

 

[OnlyMe]

Ash64449, I am not confusing SR and LET, which are indistinguishable.., but that is another discussion entirely.

I see no way to clarify this misunderstanding without quoting a large portion of Einstein's hypothetical and pointing out the significant parts. The following is taken from a project Gutenberg copy of, "Relativity: The Special and General Theory". Those portions, of added bold emphasis, are not intended to be read out of context, only to emphasize their significance.

(Portions of the quoted text, were presented in a conversational manner, as a hypothetical discussion between himself and the reader...)

Beginning in the section just proceeding the section on RoS,

8. ON THE IDEA OF TIME IN PHYSICS

Lightning has struck the rails on our railway embankment at two places A and B far distant from each other. I make the additional assertion that these two lightning flashes occurred simultaneously.​

Above, he (Einstein) clearly establishes his assertion that the lightning strikes are simultaneous in the embankment frame.

"Your definition would certainly be right, if only I knew that the light by means of which the observer at M perceives the lightning flashes travels along the length A → M with the same velocity as along the length B → M. But an examination of this supposition would only be possible if we already had at our disposal the means of measuring time. It would thus appear as though we were moving here in a logical circle."

"I maintain my previous definition nevertheless, because in reality it assumes absolutely nothing about light. There is only one demand to be made of the definition of simultaneity, namely, that in every real case it must supply us with an empirical decision as to whether or not the conception that has to be defined is fulfilled. That my definition satisfies this demand is indisputable. That light requires the same time to traverse the path A → M as for the path B → M is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own freewill in order to arrive at a definition of simultaneity."​

​

Above he stipulates his assumption that the speed of light is isotropic.., the same between A → M as it is between B → M. This section was intended to establish a definition of simultaneous events and did include ideally synchronized clocks, but the clock synchronization is not important to the next section and RoS. Only to defining simultaneous events.

9. THE RELATIVITY OF SIMULTANEITY

Up to now our considerations have been referred to a particular body of reference, which we have styled a "railway embankment." We suppose a very long train travelling along the rails with the constant velocity v and in the direction indicated in Fig 1. People travelling in this train will with a vantage view the train as a rigid reference-body (co-ordinate system); they regard all events in reference to the train. Then every event which takes place along the line also takes place at a particular point of the train. Also the definition of simultaneity can be given relative to the train in exactly the same way as with respect to the embankment. As a natural consequence, however, the following question arises :​

​

Above he introduces:

1. A very long train moving relative to the embankment frame.
2. That every event taking place on the embankment corresponds to a particular point on the train.
3. And that the definition of the simultaneity of those events can be applied to the train exactly as it is to the embankment.

This is referring to the events of the lightning strikes, not the observation and recording of those events by observers at M and M', which at the instant of the strikes are line up with each other and midway between points A and B in their respective frames.

The lightning strikes are simultaneous in both frames.

When we say that the lightning strokes A and B are simultaneous with respect to be embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A → B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the mid-point of the distance A → B on the travelling train. Just when the flashes (as judged from the embankment) of lightning occur, this point M' naturally coincides with the point M but it moves towards the right in the diagram with the velocity v of the train. If an observer sitting in the position M' in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result:

Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). ...​

​

Then as he winds up, he explains what simultaneous means relative to observers at M and M', which is dependent on the flashes of light created by the lightning strikes, not the instantaneous observation of the strikes. The observer at M records the flashes as simultaneous and as a result of the train's relative velocity with respect to the embankment, the observer at M' records them as occurring sequentially, first recording the flash from B and then the flash from A.

Neither of the observers at M or M', have clock's or observers at A and B in their frames. They have only the timing and order they observe the flashes to work with. Thus for the observer at M, the flashes are simultaneous, while for the observer at M' they are sequential.., they are not simultaneous. Both observers record the flashes as events and without greater information assume the lightning strikes to be either simultaneous or sequential. If they had all the information and a full understanding of SR and Lorentz transformations, they could both determine that the lighting strikes were simultaneous, in both frames. That is not what this section is intended to demonstrate. It demonstrates that different observers obverse the same remote events to occur in different orders. Observing remote events to be simultaneous is relative.., and subject to delays in the time of light (or even time of sound) traveling between distant locations.
____________

This returns me to my earlier statement, that proving two events to be in fact simultaneous, is not necessary to proving RoS, because RoS is the result of delays in the transmission of information, between events and remote observers. In the case of the hypothetical, flashes of light.

To prove RoS requires only that one demonstrate that one observer records two events to be simultaneous, while another observer records them to be sequential.

To prove that two events are simultaneous, is a separate issue and is the problem that has complicated the earlier discussion of RoS in this thread and many others. Proving two events to be simultaneous requires ideally synchronized clocks.., which leads to the earlier debate. Einstein did include this in the construction of his hypothetical, but it was in defining time in physics, establishing that the strikes were simutaneous in both frames and reinforcing his assertion that OWLS is isotropic... And it functions as a convincing teaching tool, where the reader can see that the strikes are simultaneous while the resulting flashes are not recorded to be simultaneous by all observers.

Earlier in the thread I made a reference to using the flash of light and sound of thunder, from a lightning strike as a proof of RoS. I still maintain that it would be a valid proof. Each (the flash and sound) being a separate event, an observer or recording devices at the origin of the flash and thunder clap, would record the two to be simultaneous. While any observer, some distance away would record them as sequential. The fact that one travels at the speed of sound and the other at the speed of light is not significant. The two represent separate sequential events when observed or recorded from a distance and simultaneous events when observed or record at their point of origin. Setting that aside...

Even with light and technology available today, if one sets aside the desire to prove absolute simultaneity, RoS is provable. Part of the confusion is in thinking of it as a measurement in and of itself. Any experimental proof of RoS is not a direct measurement, it is a comparrison of measurements made by observers or recoding devices separated by distance and/or relative velocity. To be an analog of Einstein's hypothetical all one needs is two events recorded as simultaneous in one location and sequential in another.

 

[Neddy Bate]

Neither of the observers at M or M', have clock's or observers at A and B in their frames. They have only the timing and order they observe the flashes to work with. Thus for the observer at M, the flashes are simultaneous, while for the observer at M' they are sequential.., they are not simultaneous. Both observers record the flashes as events and without greater information assume the lightning strikes to be either simultaneous or sequential. If they had all the information and a full understanding of SR and Lorentz transformations, they could both determine that the lighting strikes were simultaneous, in both frames.

When you claim, "The lighting strikes were simultaneous in both frames," you are claiming the lightning strikes occurred simultaneously in the train frame. Except for that, I think you and I agree on everything else:

1. We can assume the lightning strikes leave burn marks on the front and rear of the train.
2. According to the train frame, the observer at M' is always located at the midpoint of those two burn marks
3. The light from the lighting strikes arrives at M' sequentially, (not simultaneously)
4. Assuming the one-way speed of light is isotropic M', must determine that the lighting strikes actually occurred sequentially, (not simultaneously), according to the reference frame of the train

If you disagree with any of those, please tell me which, and explain why.

_____________________


Now, what other "information" would be required for M' to determine that the lightning strikes occurred simultaneously in the train frame? Are you saying M' should know that the speed of light was not isotropic, but rather traveled at a speed of $$(c-v)$$ from the rear of the train to M' and traveled at a speed of $$(c+v)$$ from the front of the train to M'?

 

[OnlyMe]

When you claim, "The lighting strikes were simultaneous in both frames," you are claiming the lightning strikes occurred simultaneously in the train frame. Except for that, I think you and I agree on everything else:

1. We can assume the lightning strikes leave burn marks on the front and rear of the train.
2. According to the train frame, the observer at M' is always located at the midpoint of those two burn marks
3. The light from the lighting strikes arrives at M' sequentially, (not simultaneously)
4. Assuming the one-way speed of light is isotropic M', must determine that the lighting strikes actually occurred sequentially, (not simultaneously), according to the reference frame of the train

If you disagree with any of those, please tell me which, and explain why.

There is no disagreement with the list of everything else. Re: the simultaneity of the lightening strikes I refer you back once more to the bold portion of the opening paragraph of section 9, quoted below.

9. THE RELATIVITY OF SIMULTANEITY

Up to now our considerations have been referred to a particular body of reference, which we have styled a "railway embankment." We suppose a very long train travelling along the rails with the constant velocity v and in the direction indicated in Fig 1. People travelling in this train will with a vantage view the train as a rigid reference-body (co-ordinate system); they regard all events in reference to the train. Then every event which takes place along the line also takes place at a particular point of the train. Also the definition of simultaneity can be given relative to the train in exactly the same way as with respect to the embankment.​

​

Since Einstein had already established that A, B and M' of the train frame, were lined up with A, B and M of the embankment frame, at the time of the lightning strikes, the lightning strikes were simultaneous in both frames, though the recording of the flashes was not.

The lightning strikes are events which share a common proper time and so are simultaneous in all frames, but are not observed to be simultaneous by all observers. RoS is about the observation of simultaneity by remote observers.

Now, what other "information" would be required for M' to determine that the lightning strikes occurred simultaneously in the train frame? Are you saying M' should know that the speed of light was not isotropic, but rather traveled at a speed of $$(c-v)$$ from the rear of the train to M' and traveled at a speed of $$(c+v)$$ from the front of the train to M'?

This is not really important to RoS, but is very often raised in discussions of RoS. RoS is only about a direct comparrison of the sequential record of the same event(s), as recorded from different frames.

However, since it does come up. If the the observer on the train had all of the information.., pause for a moment.., Einstein though he developed the lightning strikes as simutaneous in both frames, by describing the relationship between A, B and M' in the train frame and A, B and M in the embankment frame, he did not, within the context of the hypothetical, give that information to the observers at M and M'. They were only aware of the order in which they record the flashes, which leads to RoS... Back to the question.., had either or both observers had all of the information and a knowledge of SR and Lorentz transformations, they could both have known that the lightning strikes were in fact simultaneous. The transform would have gone from the train frame to the embankment frame, and shown that that was a proper frame for the simultaneity of the lightning strikes. This also assumes that they are aware that the train is moving relative to a stationary embankment frame, but that much would have been an obvious conclusion, always available to both M and M'.

But again this has no real connection to RoS. It involves aspects of SR developed later in the text. Nothing in the RoS section or that protion of the book leading up to it involves Lorentz transformations. The first mention of Lorentz transformations comes in section 11, two sections later.

 

[ash64449]

OnlyMe,
That book is itself very confusing. I know that book.. That is the one i bought. But there are things that can confuse anyone.

So i recommend a very excellent book.. It is called Space-Time Physics written by Taylor and Wheeler.

You will understand Relativity perfectly from that book. The original book written by Albert Einstein is very tough to grasp and can cause many confusions,like the one you are telling now.. i cannot reply further.. it is tough to reply when you quote very large sentences.

 

[ash64449]

Nothing in the RoS section or that protion of the book leading up to it involves Lorentz transformations. The first mention of Lorentz transformations comes in section 11, two sections later.

No.. There is a very good connection!! it is just that he didn't mentioned that.. if you carefully look into Lorentz Transformation,you will find out. If you want to understand then i recommend the book i mentioned in the above post.

And don't consider my post in anyway else.

 

[OnlyMe]

OnlyMe,
That book is itself very confusing. I know that book.. That is the one i bought. But there are things that can confuse anyone.

So i recommend a very excellent book.. It is called Space-Time Physics written by Taylor and Wheeler.

You will understand Relativity perfectly from that book. The original book written by Albert Einstein is very tough to grasp and can cause many confusions,like the one you are telling now.. i cannot reply further.. it is tough to reply when you quote very large sentences.

No.. There is a very good connection!! it is just that he didn't mentioned that.. if you carefully look into Lorentz Transformation,you will find out. If you want to understand then i recommend the book i mentioned in the above post.

And don't consider my post in anyway else.

Contemporary texts like Taylor and Wheeler's, don't really begin from the same knowledge base that Einstein's book did. This may make the explanations and descriptions more accessible to today's audience. It does not invalidate the intent of the earlier text... Other than how our understanding of the world has grown in the past 100 years.

Einstein described RoS before introducing Lorentz transformations. RoS itself does not require any knowledge of Lorentz transformations.., but it does set the stage for the introduction of Lorentz transformations. Just as each section of the the book leading up to the section on RoS, contributes in some way to the construction and understanding of RoS. RoS contributes to those sections that follow, including the introduction of Lorentz transformations.

Yes, we can create constructions that would involve Lorentz transformations. This only adds to and complicates the basic and fundamental requirements necessary, to demonstrate RoS. RoS only requires a comparrison of the information recorded by the two observers at M and M', as they detect the flashes of light, from the lightning strikes. All other information about the construction of the hypothetical is for the benefit of the reader... And it does play into the further developement of SR, in subsequent sections.

The whole point of RoS is not whether two events are in fact and absolutely simultaneous. It is about whether events that are recorded as simutaneous in one frame are simultaneous in all frames.

Lorentz transformations become one further component of SR that allows the obserbers at M and M', to transform their measurements between frames. It allows what is recorded in one frame to be reconciled with what is recorded in another frame.

If you begin with those transformations there is no RoS..., in fact by adding the tramsforms into the RoS hypothetical you emphasize the fact that simutaneity cannot be determined with certainty, because it requires ideal clocks. Something that can be imagined in a hypothetical but cannot be obtained with certainty in practice... And is not required to demonstrate RoS.

RoS does not itself require any mention of Lorentz transformations, but it does influence the developement of Lorentz transformations, within the context of Einstein's 1920 book.

Technically the thread title asks a question that is erroneous. The answer to the question, "Is relativity of simutaneity measurable?", is no! RoS is not a measurement in and of itself. It is a comparrison of the naive measurement of the same two events by two observers.., in the hypotheical at M and M'. That does not mean that it cannot be demonstrated or even proven.

The hypothetical Pete presented in the OP, because it involves synchronized clocks and stated velocities, does raise the question of Lorentz transformations. I believe that if it is considered from the naive perspective of the observers at M and M' in Einstein's construction, it does demonstrate RoS. The problem is, because the time difference and velocity is incorporated, it also opens interpretation to the application of Lorentz transformations.., which leads the discussion away from the intent of OP.

RoS is about a comparrison of how events are recorded by observers in different frames of reference. Lorentz transformations are a tool used to reconcile those observations.

 

[Neddy Bate]

Dear Ash64449,

Do you still maintain that RoS is not measurable? If you understand Einstein's thought experiment as well as you seem to, then you should understand that M measures the lightning strikes as simultaneous, and M' measures the lightning strikes as non-simultaneous. That is RoS being measured, don't you agree?

 

[ash64449]

RoS is about a comparrison of how events are recorded by observers in different frames of reference. Lorentz transformations are a tool used to reconcile those observations.

If this above statement means that Lorentz Transformation helps to find out what are the co-ordinates in one-reference frame when you know the co-ordinates in another reference frame or if you say that Lorentz Transformation connects different inertial reference frame, i do agree.

I cannot understand what we are actually discussing!! every thing that you say seems fine.

 

[ash64449]

Dear Ash64449,

Do you still maintain that RoS is not measurable? If you understand Einstein's thought experiment as well as you seem to, then you should understand that M measures the lightning strikes as simultaneous, and M' measures the lightning strikes as non-simultaneous. That is RoS being measured, don't you agree?

Ok. Do you agree that a single observer can measure the co-ordinate time?

 

[OnlyMe]

Ok. Do you agree that a single observer can measure the co-ordinate time?

What role is it that you are assigning to coordinate time, with respect to RoS? I ask only so everyone is on the same page and referring to the same measurement.

 

[hansda]

What role is it that you are assigning to coordinate time, with respect to RoS? I ask only so everyone is on the same page and referring to the same measurement.

I think "co-ordinate time" can not be measured but it can only be calculated from "proper time" measured by clocks in two different frames.

 

[PhysBang]

The lightning strikes are simutaneous in thier proper frame, a frame of reference common to both strikes, which in the hypothetical happens to coincide with the embankment frame. The fact that they are events that occur in a common frame of reference, makes them, the lightning strikes, simutaneous in all frames.

Thats simply crazy talk that assumes that SR is false. There is never any proper frame for any event, we can use whatever system of coordinates we wish. We may be able to determine a system of coordinates in which we can assign the same time coordinate to two events, but this does not guarantee that we will assign the same time coordinate to these events in every system of coordinates unless we add this as an additional stipulation. If we do this, then we must abandon the principle that the speed of light is the same when measured against any (well-formed) system of coordinates.

What is not simultaneous in all frames, is when the flashes that the lightning strikes create, are observed or recorded by different observers.

Where this gets confused and clock synchronization gets involved is when, one tries to put observers at A and B in both frames, to prove simultaneity. With ideally synchronized clocks, both observers on the train would agree that the lightning strikes were simultaneous, since A and B on the train were aligned with A and B on the embankment, at the time the lightning strikes occurred. Trying to add that proof to a test of RoS is what stands in the way of success... Because we cannot ideally synchronize clocks separated spatially.

The train examples are examples where we imagine that the ideal clock synchronization happens to the relevant clocks. This demonstrates the difference in simultaneity in different frames between the coordinates assigned to events, not merely to observations of those events.

Don't confuse the lightning strikes with the time of light delay associated with the flashes. It is the flashes that are observed.

Indeed, you are confusing these two things.

Einstein may have poorly worded his train example, but he uses it to show that the actual events of the lightning strikes are not simultaneous in both frames.

 

[eram]

Sorry, I haven't been following this thread lately. And it seems to have deviated far from its original topic. Can someone provide a gist? Thanks.

 

[Neddy Bate]

Dear Ash64449,

Do you still maintain that RoS is not measurable? If you understand Einstein's thought experiment as well as you seem to, then you should understand that M measures the lightning strikes as simultaneous, and M' measures the lightning strikes as non-simultaneous. That is RoS being measured, don't you agree?

Ok.

I hope your, "Ok" means that you do agree that RoS is being measured.

Do you agree that a single observer can measure the co-ordinate time?

I don't understand why you want to restrict this to a single observer. There is an observer (M) located at rest with the embankment, and there is an observer (M') located at rest with the train. One of them (M) receives the light from both lighting strikes simultaneously, and the other (M') receives the light from the stikes at two different times. Are you suggesting that observer M has no way of "measuring" that the light from the lightning strikes would be received by observer M' at two different times? If so, then that is not correct. Of course M could determine that.

 

[Neddy Bate]

Sorry, I haven't been following this thread lately. And it seems to have deviated far from its original topic. Can someone provide a gist? Thanks.

People were debating the isotropy of the one-way speed of light, and whether or not it could be measured. Some folks seem to believe that if the isotropy of the one-way speed of light cannot be measured, then Einstein synchronization of clocks cannot be trusted, and therefore RoS cannot be measured in Pete's thought experiment at the beginning of this thread, (since it depends on synchronized clocks).

I tried to explain that Einstein had already thought of that in 1905, because he clearly explained that he simply DEFINED simultaneity in a certain way, regardless of whether the one-way speed of light is isotropic or not. This defeats the above argument quite nicely, but in true web forum style, no one changed their minds about anything. Since then, the topic has veered back to Einstein's original thought experiment in which there are no synchronized clocks. Of course the same arguments apply in that case as well, its just that we don't have to worry about synchronized clocks in that case.

 

[OnlyMe]

Thats simply crazy talk that assumes that SR is false. There is never any proper frame for any event, we can use whatever system of coordinates we wish. We may be able to determine a system of coordinates in which we can assign the same time coordinate to two events, but this does not guarantee that we will assign the same time coordinate to these events in every system of coordinates unless we add this as an additional stipulation. If we do this, then we must abandon the principle that the speed of light is the same when measured against any (well-formed) system of coordinates.

The train examples are examples where we imagine that the ideal clock synchronization happens to the relevant clocks. This demonstrates the difference in simultaneity in different frames between the coordinates assigned to events, not merely to observations of those events.

Indeed, you are confusing these two things.

Einstein may have poorly worded his train example, but he uses it to show that the actual events of the lightning strikes are not simultaneous in both frames.

I have gone over this several times now. The simultaneous lightning strikes are part of Einstein's construction. He even asserts that the simultaneity of the strikes applies equally to the embankment and train frames. Go back and look at that part carefully. And remember this is a hypothetical. No one is saying that we could reproduce the conditions of the hypothetical in any practical sense. To do so would require ideally syncronized clocks.., but then that portion of the hypothetical was not available to M and M', so it is not important to RoS. It just makes the hypothetical more convincing and establishes conditions which will become important later in the book.

The observers at M and M' do not use synchronized clocks. In Einstein's construction of the hypothetical, as it relates to RoS there are no other clocks and no other observers. Neither of the observers at M or M', are recording when the lightning strikes actually strike the embankment. Both are only recording when they see, observer or measure the resulting flashes to occur, which is sometime after the actual event of the lightning strikes.

This is why I said do not get the lightning strikes and the flashes confused. You the reader have all of the information that goes into the construction of the hypothetical, the observers at M and M', have only when they record the flashes and that the train is moving relative to the embankment. It is the disagreement in their observations of what has been established as simultaneous events that is the foundation of RoS... Not the simutaneous events themselves.

All of my comments unless specifically mentioned otherwise, refer only to Einstein's hypotheical as constructed in his 1920 little book. This is for consistency and to clarify some of the confusion that results from adding, additional synchronized clocks, observers at A and B in both frames, relativistic velocities, burn marks on the embankment and train.., etc.

After one fully understands the basics of RoS, as described in Einstein's hypothetical, it may then be appropriate to look at other constructions.

I am not going to go back and re-quote, the original text. There should be no need. There is nothing in what I have presented that is inconsistent with SR.., as long as you remember I am interpreting a "hypotheical", not a real experiment.

RoS can be confirmed and if you define measure as it seems Neddy is, it can be measured. The measurements made by observers at M and M' of the when the record the flashes, represent coordinate times for those flashes within the context of their respective coordinate systems.

If you work backwards, using information available to the reader, not available to observers at M and M'.., and a full understanding of SR you will find that the lightning strikes were simultaneous in the embankment frame, they share a common time coordinate in that frame. And as Einstein pointed out they were also simultaneous in the train frame, sharing a common time coordinate, within that coordinate system. However, the lightning strikes are not what either observer records. They record only when they "see" the flashes from the lightning strikes.