Relativity of simultaneity and one-way speed of light

[Andromacus]

No. In fact you're confused about what observing the flashes means.

 

However, you are more or less correct that they must agree that the strikes struck, and where. (It's the when that's in question).

 

 

Again, way off. Events are events. Observing those events is something different.

 

For example, someone who isn't exactly between two lightening strikes could still determine if they were simultaneous - in their frame - by using the non-infinite speed of light and the distances from the strikes; it doesn't matter that they observed those strikes at different times. The events of the strikes and the observing of those strikes are different things.

 

 

In the train-embankment-lightening thought experiment, the setup is that both the embankment observer and train observer are exactly between the two strikes. Since both observers may consider themselves as at rest *, they are both entitled to know the strikes were simultaneous - in their frame - if they observe the strikes simultaneously. Since they can't both observe the strikes at the same time, they can't both consider the same two strikes to have been simultaneous.

 

 

(* there's no absolute to motion. The train observer can consider themselves as at rest, and the embankment (and its observer) are moving. Relativity tells us the rules of physics are the same for both observers.)

This seems mostly a discussion on semantics. I started by saying that the train passenger can infer, not know, that the events were not simultaneous, my point is that all that observers have is elements to make inference, because the whole point here, and I think we should agree on it, is that there is not an absolute simultaneity so nobody can "know" if the events(striking) are simultaneous or not, in fact by the relativity of simultaneity it makes no sense to even ask or declare any of the observers know if the events were "really" simultaneous.

My reply to md65536 was in the context of her apparently implying that distances were different from the fact that the train observer could infer that the events were not simultaneous. On rereading I can see how my emphasis to reject that idea might lead to some confusion.

So in fact the common misconception is deriving from the inference that the distances between lightning A and B are different for the two observers.

Edited September 2, 2015 by Andromacus

[pzkpfw]

This seems mostly a discussion on semantics. I started by saying that the train passenger can infer, not know, that the events were not simultaneous, my point is that all that observers have is elements to make inference, because the whole point here, and I think we should agree on it, is that there is not an absolute simultaneity so nobody can "know" if the events(striking) are simultaneous or not, in fact by the relativity of simultaneity it makes no sense to even ask or declare any of the observers know if the events were "really" simultaneous. ...

You are correct in saying "there is not an absolute simultaneity" *. But you take that too far.

 

Two events either were simultaneous in a frame, or not. Whether "known" or "inferred" by an observer in that frame is not the point.

 

What the point is, is that when observers are in relative motion, events that are simultaneous in the frame of one, won't be in the other. It's not about how they "know" or "infer" that fact.

 

(* Oddly, you contradict yourself. Earlier you seemed to demand that if the strikes were simultaneous in the embankment frame, that they must be simultaneous in the train frame, and that the train observer observing the strikes at different times is just an artifact of their observation. This kind of demand is requiring an absoluteness to simultaneity.)

Edited September 3, 2015 by pzkpfw

[david345]

If one chooses an epsilon other then 1/2 then they will get odd-ball results. c in one direction will be different then c in another direction. A ruler moving in one direction will be a different length then a ruler moving in another direction. In the train example you further screw things up by choosing 1/2 in one frame and other then 1/2 in the other frame. You claim this is necessary when it isn't. We can use 1/2 in both frames and describe the flashes as simultaneous in one frame and not simultaneous in the other These odd-ball results may add up in a odd-ball way but why use them? Malament claims you can only use 1/2. I have not read his paper. Using 1/2 and choosing the train's frame doesn't amount to an absolute time that must be the same for all. The time would be the same for all traveling the same velocity as the train.

Edited September 3, 2015 by david345

[swansont]

In d=vt you are assuming the galilean notion of simultaneity and galilean relativity, the Einstein's synchronization is equivalent except that there is a finite limit speed, but both Einstein's sync are isotropic, meaning a notion of simultaneity in wich the speed in one direction equals the speed in the opposite direction. The isotropy is an additional assumption Einstein made when requiring coordinates in wich the laws of Newton hold good.

 

d=vt only exists within a frame, so there is no simultaneity issue. There's no problem with it being Galilean, because there is only one frame, and it's light, which is unaffected by any relativistic corrections. c is invariant. d = vt is the right equation to use

 

In SR the relativity of simultaneity(equivalent to demanding synchronization to be convntional) is used in every transformation of frames, I wouldn't call that ad hoc.

 

I was calling your example ad-hoc. Can you show a general mathematical protocol using asymmetric speeds for light?

 

I'm speaking from personal experience here, I've taught relativity to mathematicians at the university in an Applied math program, and when commenting this issue with coleagues the usual reaction is something along the lines of "That's fine but SR is still the best thing we have", or "if it ain't broke don't fix it".

 

There's something to that. Relativity works, and is experimentally confirmed. That's an indication that there isn't a problem.

[md65536]

If one chooses an epsilon other then 1/2 then they will get odd-ball results. c in one direction will be different then c in another direction.

This is Reichenbach's epsilon notation, I don't think it's been mentioned in the thread.

 

A lot of "conventionality of simultaneity" work is done in the context of philosophy and I don't think it's up to par with scientific standards. Often, a writer will use an alternative definition of simultaneity, and implicitly redefine a bunch of quantities without making note of the redefinitions. So for example, if you have epsilon other than 1/2, the measure of "speed" that is different depending on direction is not exactly the same measurement of "speed" using Einstein's definition of simultaneity. Einstein did this first, and set the standard definitions for things like simultaneity and resulting definition of velocity, momentum, etc. Yet others will provide a different simultaneity definition, with a new measure of rate of motion, but they'll still call it "speed" even though it's not the same as the accepted definition in science. The quantity c is defined using the standard definition of speed; it makes no sense to me that it would change to fit some other new quantities that one might define. It is only some other measure, not the speed of light, that is different in different directions. The literature probably wouldn't agree with me on that, but I think they've generally been not nearly as careful as Einstein with their definitions and with hidden assumptions.

Edited September 3, 2015 by md65536

[david345]

The argument appears to go like this:

We can measure the two way speed of light but we can't measure the one way speed. Conventionalists argue that we should assume the one way speed can be anything as long as the round trip adds up. c can be faster in one direction and slower in the other direction. They argue that the speed of light can be infinite in one direction as long as the round trip time adds up. They reject that space is isotopic. A decent explanation can be found in 8-9 here.

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/significance_3/index.html

[studiot]

Wasn't Romer's measurement a one way method?

[swansont]

The argument appears to go like this:

We can measure the two way speed of light but we can't measure the one way speed. Conventionalists argue that we should assume the one way speed can be anything as long as the round trip adds up. c can be faster in one direction and slower in the other direction. They argue that the speed of light can be infinite in one direction as long as the round trip time adds up. They reject that space is isotopic. A decent explanation can be found in 8-9 here.

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/significance_3/index.html

 

The problem with the speeds being different is that energy and momentum aren't conserved, unless you come up with some new physics. And to say that you can't measure the one way speed is wrong, but certainly it's different than saying you can't test to see if the speeds are different, and the latter is all that's needed. Once you have established they are the same, the two-way speed measurement is a one-way speed measurement.

 

The Einstein synchronization protocol is not the only method of synchronization that complies with it. You can synchronize two co-located clocks and then use Eddington slow clock transport to move one clock away from the other.

 

As for the conservation of energy and momentum, I've done that experiment at some level. Two beams of light at the same frequency and intensity exert no net radiation pressure on atoms, so the photons have the same momentum, given by E/c. Ergo, absent new physics (which has not been presented), since E is the same, c is the same in both directions.

[Andromacus]

Wasn't Romer's measurement a one way method?

Well, it will depend on whether you are a conventionalist or a non-conventionalist what answer you give, because each ahas a different answer to the question if the one-way speed can be measured in principle.

The contradiction I'm highlighting is precisely that both views are compatible with SR(thus the endless and undecidable debate is still going on, although more quietly of late).

If the theory wasn't contradictory on this point an inmediate solution satisfying everyone would have been found.

[swansont]

If the theory wasn't contradictory on this point an inmediate solution satisfying everyone would have been found.

 

My experience with crackpots tells me that this is far from true.

[studiot]

 

Well, it will depend on whether you are a conventionalist or a non-conventionalist what answer you give, because each ahas a different answer to the question if the one-way speed can be measured in principle.

The contradiction I'm highlighting is precisely that both views are compatible with SR(thus the endless and undecidable debate is still going on, although more quietly of late).

If the theory wasn't contradictory on this point an inmediate solution satisfying everyone would have been found.

 

 

I'm sorry are you saying Romer did not measure the speed of light?

 

There is no question in my mind that it was a one way measurement, are you disputing this?

[md65536]

I'm sorry are you saying Romer did not measure the speed of light?

 

There is no question in my mind that it was a one way measurement, are you disputing this?

Romer's measurement assumed universal time. It also works with standard simultaneity.

 

Romer's measurement is based on changing delay of light as an observed object (Jupiter + Io) changes distance to the observer. This delay is not the same with standard simultaneity as it would be with other simultaneities. The one-way delay of light is not itself directly measured but is a consequence of standard simultaneity.

 

Yes it's a one-way measure of the speed of light, but it is generally accepted that you can't measure a one-way speed of light without a notion of simultaneity.

 

All of the arguments that standard simultaneity is not conventional, are based on things like "symmetry", not on experimental evidence.

Edited September 3, 2015 by md65536

[Andromacus]

 

 

 

The problem with the speeds being different is that energy and momentum aren't conserved, unless you come up with some new physics. And to say that you can't measure the one way speed is wrong, but certainly it's different than saying you can't test to see if the speeds are different, and the latter is all that's needed. Once you have established they are the same, the two-way speed measurement is a one-way speed measurement.

Most physicist will tell you that you can't measure the one way speed because you would have to measure it leaving and arriving at two different distant places. How do you stablish the synchronization of the clocks at those 2 distant places in a consistent way without first demanding the distance between the places to be absolute and the time to be global so that both clocks can be sync before hand?

 

 

The Einstein synchronization protocol is not the only method of synchronization that complies with it. You can synchronize two co-located clocks and then use Eddington slow clock transport to move one clock away from the other.

Slow clock transportation is well known to be equivalent to Einstein's sync.

 

 

As for the conservation of energy and momentum, I've done that experiment at some level. Two beams of light at the same frequency and intensity exert no net radiation pressure on atoms, so the photons have the same momentum, given by E/C. Ergo, absent new physics (which has not been presented), since E is the same, c is the same in both directions.

 

Already the Michelson and Morley experiments were supposed to be prove of this 125 years ago.

[studiot]

I didn't discuss simultaneity, just commented that the first known measurement of the speed of light was one way and not a bad estimate for 1697 at 3.5 x 10⁸ metres per second.

 

There were two significant claims in the first post and this one was incorrect.

 

The second one is a very good question and I am still thinking about it.

Edited September 3, 2015 by studiot

[Andromacus]

 

My experience with crackpots tells me that this is far from true.

Can you elaborate this into a more physical argument about the specific issue discussed here? I've got the feeling many of your arguments are appeals to authority rather than physics

I didn't discuss simultaneity, just commented that the first known measurement of the speed of light was one way and not a bad estimate for 1697 at 3.5 x 10⁸ metres per second.

And it would help if you explained in wich sense is a one-way speed, because the only option I see is that you are using absolute time and distance, and yet I'm sure you realize newtonian(galilean) and SR concepts of time are different.

[studiot]

 

And it would help if you explained in wich sense is a one-way speed, because the only option I see is that you are using absolute time and distance, and yet I'm sure you realize newtonian(galilean) and SR concepts of time are different.

 

 

Since you made the claim, it is up to you to offer substantive proof that your claim is correct.

 

 

As to the difference between Newtonian and /Einstinian speed, or correction from one to the other, I don't see that the relative speeds of the objects make sufficient difference to outweigh other experimental errors of the time.

Even today, over 300 years later, we do not have the technology to make a two way determination to Jupiter.

[md65536]

Since you made the claim, it is up to you to offer substantive proof that your claim is correct.

Is this about the one-way speed of light? Since when do members have to offer up substantive proof of accepted mainstream science?

 

There is no known or accepted theoretical way to measure the one-way speed of light independent of some coordination of times at two locations.

 

 

 

Unfortunately there is too much argument in this thread between people who don't understand conventionality of simultaneity, and one who doesn't understand special relativity. I don't see any common ground understanding.

[swansont]

Most physicist will tell you that you can't measure the one way speed because you would have to measure it leaving and arriving at two different distant places. How do you stablish the synchronization of the clocks at those 2 distant places in a consistent way without first demanding the distance between the places to be absolute and the time to be global so that both clocks can be sync before hand?

Slow clock transportation is well known to be equivalent to Einstein's sync.

Umm, asked and answered? Slow clock transport gives the same result — that's how you do it. How does it suffer from the one-way vs two-way speed of light issue? There's no light involved.

 

The demand that distance to the place be absolute and time be global seems to be a new hurdle/ moved goalpost.

 

Already the Michelson and Morley experiments were supposed to be prove of this 125 years ago.

 

How did the interference show conservation of energy and/or momentum?

Can you elaborate this into a more physical argument about the specific issue discussed here?

I've seen a number of instances where people continue to deny something is true, in the face of clear and overwhelming evidence. Not everybody responds to facts and logic in the same way. Some have an emotional attachment to their ideas. So the notion that "everybody would be satisfied" by a certain argument has been shown to be false in the past.

I've got the feeling many of your arguments are appeals to authority rather than physics

Such as?

[md65536]

Umm, asked and answered? Slow clock transport gives the same result — that's how you do it. How does it suffer from the one-way vs two-way speed of light issue? There's no light involved.

It still assumes that the transported clock's time is the same as the stationary clock's.

[swansont]

It still assumes that the transported clock's time is the same as the stationary clock's.

Um, yes. How does that assumption rely on the speed of light?

[md65536]

Um, yes. How does that assumption rely on the speed of light?

As you say, "Slow clock transport gives the same result". This assumption holds only if the other does.

 

Enough of this farce. Can you back up your crackpot claims with any citations at all? I'll start: https://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.'

 

You refuse to say whether you accept that standard simultaneity is a convention, but you argue as if it is an empirical fact, seemingly not understanding the difference.

[swansont]

As you say, "Slow clock transport gives the same result". This assumption holds only if the other does.

It's something that can be tested. Synchronize the co-located clocks. Move one. Test Einstein synchronization. Move the clock back. Check the synchronization. Repeat for arbitrary directions.

 

It works if both methods work, but it seems to me one does not explicitly rely on c being invariant. i.e. they are independent methods.

 

Enough of this farce. Can you back up your crackpot claims with any citations at all?

Excuse me? Crackpot claims?

[md65536]

It's something that can be tested. Synchronize the co-located clocks. Move one. Test Einstein synchronization. Move the clock back. Check the synchronization. Repeat for arbitrary directions.

 

It works if both methods work, but it seems to me one does not explicitly rely on c being invariant. i.e. they are independent methods.

As Andromacus mentioned, equivalent synchronization is achieved with the two methods.

 

See https://en.wikipedia.org/wiki/Einstein_synchronisation:

Einstein synchronisation (or PoincaréEinstein synchronisation) is a convention for synchronising clocks at different places by means of signal exchanges.

[...]

The same synchronisation is achieved by "slowly" transporting a third clock from clock 1 to clock 2, in the limit of vanishing transport velocity.

[...]

Most attempts to negate the conventionality of this synchronisation are considered refuted, with the notable exception of Malament's argument, that it can be derived from demanding a symmetrical relation of causal connectibility. Whether this settles the issue is disputed.

 

Excuse me? Crackpot claims?

Yes. No citations. No statement on whether or not you accept that Einstein's definition of simultaneity is conventional (are you avoiding it because you don't know what that means?). Only "um"s and sarcasm and unrelenting deep-seated misunderstanding.

 

 

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

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.

Do you refute this? Do you understand "all experimentally verifiable predictions of this theory do not depend on that convention"? As I understand it, that means that the experimentally verifiable predictions do not prove that Einstein's simultaneity definition is non-conventional, because if it was then the predictions of SR would depend on it. Edited September 3, 2015 by md65536

[swansont]

Yes. No citations. No statement on whether or not you accept that Einstein's definition of simultaneity is conventional (are you avoiding it because you don't know what that means?). Only "um"s and sarcasm and unrelenting deep-seated misunderstanding.

 

I see no citation for that definition of crackpottery. (Yes, more sarcasm)

 

I haven't been discussing Einstein's definition of simultaneity (or at least I've been trying to talk about something else), and I'm not at your beck and call to answer questions outside of what I was wanting to discuss. Especially if you're going to be a dick about it. (conventional meaning whether it's part of a convention one adopts, i.e. we can choose it, or if it's forced upon us by the physics, something I talked about in post #2 of this thread)

 

I'll go back to not engaging with you, thank you very much. Lunchtime is over. I have to go back to tending to my atomic clocks.

[studiot]

Is this about the one-way speed of light? Since when do members have to offer up substantive proof of accepted mainstream science?

 

There is no known or accepted theoretical way to measure the one-way speed of light independent of some coordination of times at two locations.

 

 

 

Unfortunately there is too much argument in this thread between people who don't understand conventionality of simultaneity, and one who doesn't understand special relativity. I don't see any common ground understanding.

 

Why are you being so rude about this?

 

I may be missing something vital, in which case I would be only to pleased to have it pointed out in a technical manner.

 

I did my Geodesy in an era before GPS although we thought we had it easy (and we did compared to Everest for instance).

 

We effected a considerable amount of distance measurement using various forms of EDM, which relies on the speed of light.

It was always good practice to calibrate equipment against a known premeasured distance (or a distance measured by other means) and I have seen instances where serious errors occurred when this was not done.

In effect this calibration is a one way measurement of the speed of light.

Oh, and if you are thinking that this requires time measurement and synchronisation, think again.

For some types of EDM this would indeed be true.

But not for a Tellurometer, which does not measure time, but phase.

 

Further as swansont has already indicated, you should not be measuring time, but time difference.

Edited September 3, 2015 by studiot