Is relativity of simultaneity measurable?

[OnlyMe]

Several times in this thread it has seemed to me that the simultaneity of the event(s) of the lightning strikes is being confused with the simultaneity, of when the rays of light reached the observers. The two lightning strikes and the rays of light, both represent simultaneous events for the embankment.., but not the train. For the train the rays of light are not simultaneous, while the lightning strikes are... Note, I am not saying that M' (the observer on the train) has any way of knowing that the lightning strikes were simultaneous, without additional information.

Perhaps another reference, apart from Einstein's book, may help, http://www.pitt.edu/~jdnorton/Goodies/rel_of_sim/

The breakthrough was his recognition of the relativity of simultaneity: judgments of the simultaneity of events will vary according to the state of motion of the observer.​

​

The above quote is from Step 1, of the above link.

This is where I lose some...

In Einstein's hypothetical, the lightning strikes share a common coordinate time in the rest frame of the embankment, which means the events of the lightning strikes (not the rays of light) are simultaneous, for both M and M', but only observed to be simultaneous by M.

 

[PhysBang]

Several times in this thread it has seemed to me that the simultaneity of the event(s) of the lightning strikes is being confused with the simultaneity, of when the rays of light reached the observers.

Perhaps, but this has nothing to do with your fundamental difficulty in reading the very lines from Einstein that you cut and paste. The actual simultaneity of the lightning strikes themselves is to be determined by whether or not the light from each strike reaches the midpoint between those strikes at the same time. That's the definition Einstein gives in both Chapter 8 and Chapter 9 in plain and direct language.

The two lightning strikes and the rays of light, both represent simultaneous events for the embankment.., but not the train.

Well at least you admit this now!

In Einstein's hypothetical, the lightning strikes share a common coordinate time in the rest frame of the embankment, which means the events of the lightning strikes (not the rays of light) are simultaneous, for both M and M', but only observed to be simultaneous by M.

M and M' are locations. That means that they exist at multiple times. We can talk of an observer that is stationary at M or M', but we cannot talk of an event being simultaneous with a location.

 

[Neddy Bate]

But observers can only measure "proper" time and Not "co-ordinate" time.

Imagine you are at rest in an inertial fame, and a clock travels a circle around you. The time shown on the traveling clock is not your proper time, but you can still measure the time shown on the moving clock. Agreed?

In the both cases,there are two observers and two observers are measuring their proper time.

Yes, and in the embankment frame, the proper time of the left lighting strike is $$t$$, and the proper time of the right lighting strike is also $$t$$, making the lighting strike events simultaneous in that frame.

And in the train frame, the proper time of the left lighting strike is $$t'_L = \gamma(t - \frac{vx_L}{c^2})$$, and the proper time of the right lighting strike is $$t'_R = \gamma(t - \frac{vx_R}{c^2})$$, making the lighting strike events NON-simultaneous in that frame.

Now all you have to do is let the embankment frame measure the times shown on the moving clocks $$t'_L = \gamma(t - \frac{vx_L}{c^2})$$, and $$t'_R = \gamma(t - \frac{vx_R}{c^2})$$, even though those are not proper times for the embankment, and you have measured RoS. Agreed?

_________________


By the way, if you want to continue to claim RoS is not measurable, you need to jump on the "c might not be isotropic" bandwagon that everyone else has jumped on. Claiming coordinate time is not measurable is not working.

 

[OnlyMe]

Perhaps, but this has nothing to do with your fundamental difficulty in reading the very lines from Einstein that you cut and paste. The actual simultaneity of the lightning strikes themselves is to be determined by whether or not the light from each strike reaches the midpoint between those strikes at the same time. That's the definition Einstein gives in both Chapter 8 and Chapter 9 in plain and direct language.

M' is only at the midpoint of A and B when the lightning strikes occur. If there were clocks on the train at M' and coincident with A and B when M' is at that midpoint, all three clocks would register the same time. That is when the lightning strikes occurred. And the only time that the lightning strikes as events, are simultaneous with anything. When M' records the rays of light, the lightning strike events are in its past.

Well at least you admit this now!

I have never intended to be saying anything different. The way I have been describing the evolution of the hypothetical obviously has not been clear. At least not clear to some. But it is the evolution of the hypothetical that I have been attempting to clarify.

M and M' are locations. That means that they exist at multiple times. We can talk of an observer that is stationary at M or M', but we cannot talk of an event being simultaneous with a location.

Einstein assigned observers to both locations. We can talk about events that are simultaneous with an observer that passes through a location. As it happens in Einstein's hypothetical, the observer at M' passed trough a loction coincident with the midpoint between the lightning strikes (and coincident with M), as they ( the lightning strikes) occurred. That makes the lightning strikes as events simutaneous in the train frame... But simutaneity in SR is determined by when the rays of light are recorded....

The lightning strikes were simultaneous relative to the location of M' at the time of the strikes... But because M' was moving toward B with velocity v, the rays of light were observed sequentially, instead of simultaneously, first the ray from B and then the ray from A.

*** this situation could only be presented in a hypothetical. In any practical manner whether two spatially separated events are simultaneous cannot be measured. In principle they might be, as Pete pointed out, but in practice not yet.

 

[OnlyMe]

The actual simultaneity of the lightning strikes themselves is to be determined by whether or not the light from each strike reaches the midpoint between those strikes at the same time.

This was the test Einstein used to validate his assertion that the lightning strikes were simultaneous.

As I mentioned earlier, if an observer records two flashes as simultaneous, they are simultaneous by definition in SR, but without all of the hypothetical setup Einstein gave, it says nothing about the order in which the events that caused the flashes occurred.

 

[PhysBang]

M' is only at the midpoint of A and B when the lightning strikes occur.

No. M' is always at the midpoint of the spatial locations of A and B as determined relative to the reference frame in which the train is stationary. If you cannot accept this very basic fact, then there is no point discussing relativity theory whatsoever.

If there were clocks on the train at M' and coincident with A and B when M' is at that midpoint, all three clocks would register the same time.

Only if these clocks were set to be coordinated according to the reference frame in which the embankment is stationary. You seem to have back-slided into denial of relativity theory and what you have cut and pasted of Einstein's work.

When M' records the rays of light, the lightning strike events are in its past.

M' is a location, it exists at all times. You are saying something that makes no sense.

I have never intended to be saying anything different.

You just did in that very post! Perhaps you have some sort of associative personality disorder.

The way I have been describing the evolution of the hypothetical obviously has not been clear. At least not clear to some. But it is the evolution of the hypothetical that I have been attempting to clarify.

You seem only to add confusion.

Einstein assigned observers to both locations. We can talk about events that are simultaneous with an observer that passes through a location. As it happens in Einstein's hypothetical, the observer at M' passed trough a loction coincident with the midpoint between the lightning strikes (and coincident with M), as they ( the lightning strikes) occurred. That makes the lightning strikes as events simutaneous in the train frame.

Why are you rejecting the actual definition of what simultaneous means? The only acceptable definition of simultaneous is that the light from the flashes reaches the midpoint at the same time. And this is clearly false from the frame associated with the train. You keep insisting that simultaneous in the frame associated with the embankment means simultaneous in the frame associated with the train. This is not good.

Your flip-flopping on this point is not a sign of great character.

.. But simutaneity in SR is determined by when the rays of light are recorded....

And again you seem to flip-flop.

The lightning strikes were simultaneous relative to the location of M' at the time of the strikes...

Only in the reference frame associated with the embankment.

But because M' was moving toward B with velocity v, the rays of light were observed sequentially, instead of simultaneously, first the ray from B and then the ray from A.

Yes, we know the order in which the rays of light reached M' because of what we know in the reference frame associated with the embankment. So we know that, in the reference frame associated with the train, it fails to be the case that the lightning strikes were simultaneous, because the sole criterion for simultaneity fails in that frame.

 

[Neddy Bate]

Unfortunately, OnlyMe's misunderstandings about RoS are not uncommon. Below is a link to a study called, "Student understanding of time in special relativity: simultaneity and reference frames."

http://arxiv.org/abs/physics/0207109

From the conclusion on page 19:

"This investigation has identified widespread difficulties that students have with the definition of the time of an event and the role of intelligent observers. After instruction, more than 2/3 of physics undergraduates and 1/3 of graduate students in physics are unable to apply the construct of a reference frame in determining whether or not two events are simultaneous. Many students interpret the phrase “relativity of simultaneity” as implying that the simultaneity of events is determined by an observer on the basis of the reception of light signals. They often attribute the relativity of simultaneity to the difference in signal travel time for different observers. In this way, they reconcile statements of the relativity of simultaneity with a belief in absolute simultaneity and fail to confront the startling ideas of special relativity."​

And this quote, from pages 11 and 12, seems to describe OnlyMe's behavior exactly:

"During the interview, many students seemed to resist thinking about simultaneity in terms of emission, rather than reception, of the signals. As the interviews progressed, we realized that part of the difficulty was that they believed strongly that, in every reference frame, the two events occurred at the same instant. Many seemed to treat the non-simultaneity of the reception of the signals as a way of reconciling this belief with what they thought they had learned about the relativity of simultaneity."​

 

[OnlyMe]

No. M' is always at the midpoint of the spatial locations of A and B as determined relative to the reference frame in which the train is stationary. If you cannot accept this very basic fact, then there is no point discussing relativity theory whatsoever.

It seems you have not read the two sections.

Though in discussion, we have been talking about A and B in the train frame, even myself, Einstein never assignepointing or locations, in the train frame the labels A and B. All he did is say that when the lightning strikes occur A and B of the embankment frame correspond to locations on the passing train.

Until you do, read the text we are discussing, there is no point in further discussion, as you only think you know how the hypothetical is constructed.

 

[OnlyMe]

Unfortunately, OnlyMe's misunderstandings about RoS are not uncommon. Below is a link to a study called, "Student understanding of time in special relativity: simultaneity and reference frames."

http://arxiv.org/abs/physics/0207109

From the conclusion on page 19:

"This investigation has identified widespread difficulties that students have with the definition of the time of an event and the role of intelligent observers. After instruction, more than 2/3 of physics undergraduates and 1/3 of graduate students in physics are unable to apply the construct of a reference frame in determining whether or not two events are simultaneous. Many students interpret the phrase “relativity of simultaneity” as implying that the simultaneity of events is determined by an observer on the basis of the reception of light signals. They often attribute the relativity of simultaneity to the difference in signal travel time for different observers. In this way, they reconcile statements of the relativity of simultaneity with a belief in absolute simultaneity and fail to confront the startling ideas of special relativity."​

And this quote, from pages 11 and 12, seems to describe OnlyMe's behavior exactly:

"During the interview, many students seemed to resist thinking about simultaneity in terms of emission, rather than reception, of the signals. As the interviews progressed, we realized that part of the difficulty was that they believed strongly that, in every reference frame, the two events occurred at the same instant. Many seemed to treat the non-simultaneity of the reception of the signals as a way of reconciling this belief with what they thought they had learned about the relativity of simultaneity."​

Neddy, I will read over the paper you link and get back to you. It is too long to respond just based on the two quotes in your post.

 

[PhysBang]

It seems you have not read the two sections.

Though in discussion, we have been talking about A and B in the train frame, even myself, Einstein never assignepointing or locations, in the train frame the labels A and B. All he did is say that when the lightning strikes occur A and B of the embankment frame correspond to locations on the passing train.

Einstein specifically says that M' lies at the midpoint between the positions of A and B on the train. A and B in themselves are not locations, they are events that have locations in every (well-formed) frame of reference. Einstein also explicitly (and foolishly) also uses "A" and "B" to refer to positions on the train. Are you desperately trying to lie about this, or do you really have so bad an understanding?

Until you do, read the text we are discussing, there is no point in further discussion, as you only think you know how the hypothetical is constructed.

It's funny to see you write that, given that you made such a large error above.

 

[OnlyMe]

Einstein specifically says that M' lies at the midpoint between the positions of A and B on the train. A and B in themselves are not locations, they are events that have locations in every (well-formed) frame of reference. Einstein also explicitly (and foolishly) also uses "A" and "B" to refer to positions on the train. Are you desperately trying to lie about this, or do you really have so bad an understanding?


It's funny to see you write that, given that you made such a large error above.

Read it again. Remember it was translated from German and published in 1920. If you are still having trouble look at the diagram. A and B are fixed locations where the lightning strikes the embankment. The train and M' have a velocity v relative to the embankment, but every other instance where A and B are mentioned, it's in reference to the location of the lighting strikes. Example; M' is moving toward A and away from B. if they were points or locations on the train they would be moving with the train. In this construction there are no fixed points A and B on the train, there are positions on the train that correspond to the locations of the lightning strikes..., and M' is midway between those locations when the lightning strikes.

 

[PhysBang]

Read it again. Remember it was translated from German and published in 1920. If you are still having trouble look at the diagram. A and B are fixed locations where the lightning strikes the embankment. The train and M' have a velocity v relative to the embankment, but every other instance where A and B are mentioned, it's in reference to the location of the lighting strikes. Example; M' is moving toward A and away from B. if they were points or locations on the train they would be moving with the train. In this construction there are no fixed points A and B on the train, there are positions on the train that correspond to the locations of the lightning strikes..., and M' is midway between those locations when the lightning strikes.

This is really pathetic.

Let's take a look at text in the citation you gave at Project Gutenberg: "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. "

All you have to do is read. And apologize.

 

[eram]

Several times in this thread it has seemed to me that the simultaneity of the event(s) of the lightning strikes is being confused with the simultaneity, of when the rays of light reached the observers.

And who was the one doing the confusing? You!

 

[ash64449]

Imagine you are at rest in an inertial fame, and a clock travels a circle around you. The time shown on the traveling clock is not your proper time, but you can still measure the time shown on the moving clock. Agreed?

Not yet. Proper Time is same for observers in inertial motion. No way proper time can change when some thing moves relative to something else.

Proper time can only change in non-inertial state of motion. And in inertial frame of reference proper time is "invariant". You need to know that. So no clock disagrees with any clock in inertial frame of reference.

Yes, and in the embankment frame, the proper time of the left lighting strike is $$t$$, and the proper time of the right lighting strike is also $$t$$, making the lighting strike events simultaneous in that frame.

And in the train frame, the proper time of the left lighting strike is $$t'_L = \gamma(t - \frac{vx_L}{c^2})$$, and the proper time of the right lighting strike is $$t'_R = \gamma(t - \frac{vx_R}{c^2})$$, making the lighting strike events NON-simultaneous in that frame.

Now all you have to do is let the embankment frame measure the times shown on the moving clocks $$t'_L = \gamma(t - \frac{vx_L}{c^2})$$, and $$t'_R = \gamma(t - \frac{vx_R}{c^2})$$, even though those are not proper times for the embankment, and you have measured RoS. Agreed?

No. Because The lorentz Transformation are not for "proper" time but "are" for "coordinate" time. That's where you are mistaken. Now have you got the clue what might be RoS about?

_________________

By the way, if you want to continue to claim RoS is not measurable, you need to jump on the "c might not be isotropic" bandwagon that everyone else has jumped on. Claiming coordinate time is not measurable is not working.

No. I don't have too. This way to Speed of Light is isotropic. RoS is based on isotropic in all direction. And note that we "cannot" measure "One-way" speed of light. Only Round Trip Of light.

RoS is based on One-way Speed Of light. That can also be the reason why RoS is not measurable.

I hope we are having Healthy Discussion over here!!!

 

[Neddy Bate]

Yes, and in the embankment frame, the proper time of the left lighting strike is $$t$$, and the proper time of the right lighting strike is also $$t$$, making the lighting strike events simultaneous in that frame.

And in the train frame, the proper time of the left lighting strike is $$t'_L = \gamma(t - \frac{vx_L}{c^2})$$, and the proper time of the right lighting strike is $$t'_R = \gamma(t - \frac{vx_R}{c^2})$$, making the lighting strike events NON-simultaneous in that frame.

Now all you have to do is let the embankment frame measure the times shown on the moving clocks $$t'_L = \gamma(t - \frac{vx_L}{c^2})$$, and $$t'_R = \gamma(t - \frac{vx_R}{c^2})$$, even though those are not proper times for the embankment, and you have measured RoS. Agreed?

No. Because The lorentz Transformation are not for "proper" time but "are" for "coordinate" time. That's where you are mistaken. Now have you got the clue what might be RoS about?

Here is a link to the Wikipedia article on Coordinate time:

http://en.wikipedia.org/wiki/Coordinate_time

"In the special case of an inertial observer in special relativity, by convention the coordinate time at an event is the same as the proper time measured by a clock that is at the same location as the event, that is stationary relative to the observer and that has been synchronised to the observer's clock using the Einstein synchronisation convention."​

I hope you are aware that the train and embankment are both inertial. So the coordinate times I calculated are the same as the proper times. I have no choice but to conclude that you don't know what you are talking about. Why don't you just tell me whether you think Einstein's lightning strikes would be measured to be simultaneous by the train frame. Let's start with that. Please give me a simple "Yes" or "No".

 

[OnlyMe]

Unfortunately, OnlyMe's misunderstandings about RoS are not uncommon. Below is a link to a study called, "Student understanding of time in special relativity: simultaneity and reference frames."

http://arxiv.org/abs/physics/0207109

From the conclusion on page 19:

"This investigation has identified widespread difficulties that students have with the definition of the time of an event and the role of intelligent observers. After instruction, more than 2/3 of physics undergraduates and 1/3 of graduate students in physics are unable to apply the construct of a reference frame in determining whether or not two events are simultaneous. Many students interpret the phrase “relativity of simultaneity” as implying that the simultaneity of events is determined by an observer on the basis of the reception of light signals. They often attribute the relativity of simultaneity to the difference in signal travel time for different observers. In this way, they reconcile statements of the relativity of simultaneity with a belief in absolute simultaneity and fail to confront the startling ideas of special relativity."​

And this quote, from pages 11 and 12, seems to describe OnlyMe's behavior exactly:

"During the interview, many students seemed to resist thinking about simultaneity in terms of emission, rather than reception, of the signals. As the interviews progressed, we realized that part of the difficulty was that they believed strongly that, in every reference frame, the two events occurred at the same instant. Many seemed to treat the non-simultaneity of the reception of the signals as a way of reconciling this belief with what they thought they had learned about the relativity of simultaneity."​

Neddy, I did finally get through the paper linked above. Personally I believe it says more about the instructor/ teachers than the students. It would have been nice to have some presentation of how they were teaching RoS, that led to so much confussion.... There are some issues they raised that I will have to think about. But they were presenting a one sided examination of the results... Only what they saw as faults. So it is hard to clearly identify any potential solution, without more information about the teaching methods that led to the students confusion.

I also see within the context of their results that my original intent in going deeper into Einstein's 1920 version of the hypotheical was doomed from the start. What I initially thought would be achieved was, a list of what components an analog of the hypotheical would require to demonstrate or measure (depending on definition) RoS. That became side tracked very quickly.

The problem seems to be that I was attempting to go through the hypotheical from a naive prespective, one step at a time.., while that process, it seemed to me was being questioned from an end point perspective...

Einstein did not provide all of the needed tools in just those two Sections 8 and 9, to arrive at any realistic analog. He never assigned locations A' and B' in the train frame until the Section 10, and it was not until after he introduced the Lorentz transformations that he began to deal with a real comparison of time and distance, between inertial frames...

In retrospect most of the confusion and disagreement, at least from my part, was that "we" were talking about two different things. I was trying to examine the sequential construction of his hypothetical, ultimately with the intent of isolating which points or issue were important, while most of the criticism, seems to have been judging that examination of the step by step construction as representing some end point statement about the definition of RoS.

In hindsight now it is clear that even by the end of Section 9 Einstein had not fully defined RoS. He had presented no tools or means of measurement of distance or time. We never got past the point of simutaneity being the result of when the flashes were observed..., and to any judgment of when the strikes actually occurred, in the moving frame.

Judging from the results in the paper you linked, I am not sure that beginning with Einstein's hypothetical as presented in that book, any real success would be easily found.

Again, it would also have been interesting to me, had the author's of the paper included not just a brief description of how they define RoS, but some description of how they taught it... But then it was really intended for purposes of their own course development, not as a teaching tool in itself.

Ultimately it seems the confussion is so deeply entrenched in the current discussion, that there is no constructive reason to continue. Not within the context of that version of the hypothetical.

 

[PhysBang]

Einstein did not provide all of the needed tools in just those two Sections 8 and 9, to arrive at any realistic analog. He never assigned locations A' and B' in the train frame until the Section 10,

This is entirely irrelevant to the question of the relativity of simultaneity. Einstein assigns locations, in the frame co-moving with the train, to the events A and B that are precise enough to establish that these events are not simultaneous in that frame.

In retrospect most of the confusion and disagreement, at least from my part, was that "we" were talking about two different things. I was trying to examine the sequential construction of his hypothetical, ultimately with the intent of isolating which points or issue were important, while most of the criticism, seems to have been judging that examination of the step by step construction as representing some end point statement about the definition of RoS.

But the facts are you have been horribly mangling the step-by-step by insisting that the lightning strikes were simultaneous in the train frame when Einstein begins Chapter 9 with the open question about whether or not this is the case.

In hindsight now it is clear that even by the end of Section 9 Einstein had not fully defined RoS. He had presented no tools or means of measurement of distance or time. We never got past the point of simutaneity being the result of when the flashes were observed..., and to any judgment of when the strikes actually occurred, in the moving frame.

He gave enough of a definition in Chapter 8 to be usable. It seems to be the same one he uses in the original 1905 paper. That you do not like it doesn't mean that it's not there.

Ultimately it seems the confussion is so deeply entrenched in the current discussion, that there is no constructive reason to continue. Not within the context of that version of the hypothetical.

Sure, blame everyone else for you mistake.

 

[OnlyMe]

Sure, blame everyone else for you mistake.

PhyBang, I was not blaming anyone. I was acknowledging that the discussion had become too confused. That confusion goes both ways... And that it, that particular version of the hypothetical, was a poor choice for the purposes of my initial intent, of exploring the possibility of an analog test. At the time it seemed that because it did not involve relativistic velocity, it would be a simpler case to explore. I was obviously wrong in that.

 

[ash64449]

No. Because The lorentz Transformation are not for "proper" time but "are" for "coordinate" time. That's where you are mistaken. Now have you got the clue what might be RoS about?

Here is a link to the Wikipedia article on Coordinate time:

http://en.wikipedia.org/wiki/Coordinate_time

"In the special case of an inertial observer in special relativity, by convention the coordinate time at an event is the same as the proper time measured by a clock that is at the same location as the event, that is stationary relative to the observer and that has been synchronised to the observer's clock using the Einstein synchronisation convention."​

I would like to highlight this one too:

"In the special case of an inertial observer in special relativity, by convention the coordinate time at an event is the same as the proper time measured by a clock that is at the same location as the event, that is stationary relative to the observer and that has been synchronised to the observer's clock using the Einstein synchronisation convention."​

Event taking Placeat the same location as that of clock's position can only have proper time same as coordinate time. Can't you understand that?

Aren't the two lightning strikes spatially separated?? Your quoted article itself gives me the proof. It said only for same location and not for spatially separated location. Lightning strikes are spatially separated. So proper time may not be equal to coordinate time?


Are you claiming that proper time is not invariant?? No use. because Proper Time by definition is invariant. It is described as the invariant interval between two events and it is a space-time interval.

Your claiming that proper time changes is completely wrong. Coordinate time is the one changes and that's what Lorentz Transformation gives.

And Note that your quoted article is of spacial case only. Only sometimes proper time same as coordinate time.

So Lorentz Transformation is all about coordinates. I have no doubt about that for sure.

 

[ash64449]

I hope you are aware that the train and embankment are both inertial. So the coordinate times I calculated are the same as the proper times.

No.Because events are spatially separated. from your previous post i found that you are claiming a change in proper time of two events in difference reference frame by showing LORENTZ TRANSFORMATION. But Proper time is invariant. So It is the first clear evidence that what you said that Lorentz transformation is MAINLY for calculating proper time is wrong. Lorentz Transformation for measuring coordinate time is correct.

I have no choice but to conclude that you don't know what you are talking about. Why don't you just tell me whether you think Einstein's lightning strikes would be measured to be simultaneous by the train frame. Let's start with that. Please give me a simple "Yes" or "No".

No. What is the use of this? Lightning strikes are simultaneous in the embankment and not for the train frame. No use of this to prove that Lorentz transformation is equation for measuring proper time.