0 Members and 1 Guest are viewing this topic.

In his 1905 paper Einstein effectively states that the simultaneity of events in the "stationary" frame must be assumed

If we consider the clock synchronisation thought experiment:the observer in the "stationary" frame is located at the mid-point between 2 clocks. A co-located emitter sends a light pulse to each clock (to start them ticking). The light pulses are reflected to the observer at the mid-point and arrive simultaneously. The observer concludes that their clocks are synchronised because they know the speed of light and the distance to the clocks, and because the light pulses returned simultaneously.While the observer in the "stationary" frame is performing this clock synchronisation, they observe a relatively moving observer perform the exact same synchronisation process. They are also located midway between 2 clocks.

The "moving" observer concludes that their clocks are synchronised. The "stationary" has observed that the clocks are not synchronised.

Here, in the original thought experiment, we are provided with a clear case of why the assumption of synchronisation/simultaneity is unjustified.

The oberver in the "stationary" frame observes the "moving" observer perform the exact same synchronisation procedure, with the light pulses simultaneously returning to the mid=point, yet, the clocks are not synchronised. This should, at the very least, cause the 'stationary" observer to question whether their clocks are in fact synchronised.

Imagine, on top of this, both observers are wearing body cameras and record footage of their counterparts synchronisation attempts. They then send the footage to each other - by light signal. Each observer will be presented with observational evidence that their clocks are not synchronised.

The reasonable conclusion in this scenario would be to accept that each was mistaken in their assumption about the simultaneity of the clock synchronisation events, give the observational evidence to the contrary.

Light Clock Thought ExperimentFor this, we need only consider the thought experiment involving each observer carrying a single light clock - a photon bouncing between mirrors. The "stationary" assumes thata their clock is ticking normally, while they observe the "moving" clock as ticking slowly, as the photon travels a longer, diagonal path between the 2 mirrors.Again, imagine each exchanging bodycam footage and being presented with evidence that their own clock is also ticking slowly. It makes sense to both observers.

What about the speed of light? If they measure the speed of light in the light clock, will they not measure it as having a slower speed, if they can only detect the vertical velocity component?

Any attempt to measure the speed of light will always yield the same value because their clock will be biased by the same factor.

Absolute time is indistinguishable from a timeless universe. Clocks provide units of comparison - they don't measure a background phenomenon called "time".

ConclusionThe above highlights the circular reasoning in the Einsteinian interpretation. It is the assumption of the simultaneity of events in the "stationary" frame which leads to the conclusion of the Reativity of Simultaneity.

I don't think Einstein would have referred to any frame as 'the stationary' frame in his 1905 paper. If your posts here are going to misrepresent the ideas of others, then your reasoning is fallacious.

Cutting in here. It is not specified, but it seems that you are envisioning the first two clocks not moving relative to the first observer, and these are two different clocks that are not moving relative to the second observer. The situation then is entirely symmetrical.

Two pairs of clocks, each pair synchronized in the frame in which the pair is stationary. If this is not the case, you need to say so.

More to the point, each observer concludes that the pair of clocks stationary relative to themselves are synchronized, and each observer concludes that the other pair of moving clocks are not. Again, entirely symmetrical.

The symmetry says it is entirely justified.

OK, so you are a relativity denier. That also wasn't clear at first. Maybe you just don't understand, but you are writing a paper somewhere, so you presumably think that you know this stuff.

They're not in his frame, and his clocks are not synchronized in the other observer's frame. There is no 'the stationary' frame. There's just this frame and that one.

That's fine since neither expects their own clocks to be synchronized in a different frame. This additional footage evidence is not required.

The simultaneity of all those events is frame dependent. That's what relativity of simultaneity means.

That it does. The theory again predicts this, so the neither observer needs bodycam evidence from the other to show what he already knows.

The vertical component is easily deduced with simple trigonometry. Light moving horizontal for instance has zero vertical velocity component.

If you think clocks are biased, how biased are they? For instance, how long does it really take for Earth to make one sidereal rotation (on average)? The biased clock in Greenwich says ~23:56:04. What is it really? If you can't answer it, then your bias argument falls apart.

OK, so maybe you don't like absolute time either. Clocks indeed provide units of comparison, as do meter sticks. That doesn't means I get no information from a clock. The timeless conclusion doesn't follow any more than a conclusion that my meter stick is dimensionless because its length is frame dependent.

Strawman fallacy. Einstein assumes no special 'the stationary' frame in special relativity.

Quote from: Halc on 08/06/2019 19:38:03I don't think Einstein would have referred to any frame as 'the stationary' frame in his 1905 paper. If your posts here are going to misrepresent the ideas of others, then your reasoning is fallacious.In order to render our presentation more precise and to distinguish this system of co-ordinates verbally from others which will be introduced hereafter, we call it the “stationary system.” (Einstein, 1905).

Apologies, I am assuming familiarity with the clock synchronisation thought experiment here

There is also a relatively moving observer with 2 clocks in the "moving system" - we are considering things only from one perspective, for the time being.

Quote from: HalcTwo pairs of clocks, each pair synchronized in the frame in which the pair is stationary. If this is not the case, you need to say so.This is the crux of the issue. The idea that they are synchronised is purely an assumption and, as is being argued, an unjustified/unjustifiable assumption.

We establish by definition that the “time” required by light to travel from A to B equals the “time” it requires to travel from B to A.(Einstein, 1905).

Quote from: HalcMore to the point, each observer concludes that the pair of clocks stationary relative to themselves are synchronized, and each observer concludes that the other pair of moving clocks are not. Again, entirely symmetrical.Again, this is the crux of the issue! They conclude that the "stationary clocks" are synchronised.

What is it that leads to this conclusion? There is no experiment that can determine the simultaneity of the clock synchronisation events, so they assume that the clocks are synchronised.

Why? Because they know the speed of light, the distance to each clock, and [crucially] the light reflected from the "stationary clocks" returns to them simultaneously.

Indeed, it is the symmetry of the situation which means that their assumption about the simultaneity of the clock events is unjustified. They observe their counterpart conduct the exact same synchronisation procedure, with the exact same observational evidence - distance to each clock, speed of light, and light pulses returning simultaneously. Yet, the "moving" clocks are not synchronised.

The symmetry of the situation should lead the "stationary" observer to at least question the assumption of simultaneity/synchronisation of their clocks.

Not a denier, just advocating for a different, more parsimmonious interpretation. It is similar to the Lorentz-Poincare interpretation except that it is a timeless interpretation.

The point is that the observer in the "stationary system" sees the relatively moving observer perform the exact same syncronisation procedure, with the exact same observational results but the moving clocks fail to synchronise.

How does the "stationary" observer know that the exat same issue isn't afflicting their own synchronisation procedure?

They don't because, in keeping with the Galilean Priniciple of Relativity, there is no experiment they can conduct to determine that their clocks are actually synchronised or that events are simultaneous, in their frame.

The notion of relativity of simultaneity follows from the unjustified assumption that the simultaneity of events can be determined in any frame. That is, it starts by assuming that events are simultaneous in a given frame, when no such determination can be made.

The point is that, as per the Galilean Principle of Relativity, the observer in the "stationary system" will always observe only the vertical velocity component of the photon and has no way off knowing if it has a horizontal component.

As per the thought experiment, if the oberver tries to measure the speed of light in their light clock they will have to use another light clock - the circularity should be apparent. This circularity is the "conspiracy"

Quote from: HalcOK, so maybe you don't like absolute time either. Without the assumpion of relativity of simultaneity we are left with absolute simultaneity and absolute time.

OK, so maybe you don't like absolute time either.

We can make time "disappear" simply by chllenging the assumption that "a clock measures [a background phenomenon called] time".

If we examine the processes of a clock, nowhere is this background phenomenon actually measured.

I stand corrected then. He is labeling the frames, for convenience, as 'stationary' and 'moving' then. So long as there is no assertion about the stationary one being in some way special (which would violate the first principle), this is acceptable.

Agree, and I didn't say that they were synchronized. I said each pair was synchronized in a specific frame, and not in the other. If you consider that an assumption, well, I suppose it could be, especially if we have a different interpretation of what it means for clocks to be synchronized.

that's pretty out of context. The definition of what? From reading the quote, it seems to be the definition of points in space (A and B), and points in space require a frame for their definition.

You described in your OP a simple experiment to do just that. Light from stationary equidistant clocks emitted at the two synchronization events are observed simultaniously. That's an empirical verification if I ever saw one.

Sorry, but it seems you contradict yourself. They know all these things, yet you say it cannot be known.

Nobody ever said they were.You confuse 'sychronized' with 'synchronized in frame X'. I notice you drop the frame references whenever it's convenient to your point. The only way two clocks can be synchronized (no frame reference) is if they follow the same worldline, essentially being the same clock.

They do question it, but all observers are right. Their clocks are synchronized in their frames, as they empirically verified.

OK, that's valid. If it makes no different predictions, what's the advantage of the interpretation? Does it simplify anything that's more complicated in the relative interpretation? You call it parsimonious, like it perhaps requires less effort in some way.

Again, the intentional drop of the frame reference. The moving clocks do very much synchronize in the second frame. He did not fail at all.

He is afflicted. Symmetry demands it. His clocks are similarly not synchronized in the other frame.

Galilean PoR doesn't say that. It says what can be done in one frame can be done in another, and since there is very much an empirical verification that can be performed (which you describe in your OP), both observers can verify that their clocks are actually synchronised or that events are simultaneous, in their frame.

But you show how to determine it. It isn't hard. I totally don't understand this assertion that given a frame, ordering of events cannot be determined. The assumption you speak of isn't made ever. It is a conclusion at best.

He does have a way. If it had a horizontal component, the light wouldn't come back to the detector that is stationary in that frame.

They're up front about that. A light clock by definition cannot be used to measure light speed, yes. It would take some other sort of clock. Light speed was initially measured using another reliable clock that could be moved closer and further away. It was an inertial clock (a steadily rotating thing like Earth), not based on light at all.

]No, relativity has no background standard. That was my point about building a device that cancels out its own bias. Relativity cares not about that background and doesn't assume it at all. It just cares that this duration is somehow comparable to that other duration over there, but never to a base rate.

As a notational convenience, yes he said that. It was not to make that frame in any way special/preferred.

The definition is [to paraphrase] that the journey time from A to B equals the journey time from B to A.

It has to be assumed bcos to actually measure it would require synchronised clocks - see the issue. It is the reason that the 1-way speed of light cannot be measured.

The returning light pulses are observed simultaneously. But the observer in the "stationary system" observes the light return simultaneously to the "moving" observer also, this is despite the fact that the "moving" clocks are not synchronised, from the perspective of the "stationary" observer.

This demonstrates to the "stationary" observer that the light would return [to him] simultaneously even if his clocks are not synchronised. He is simply assuming that they are synchronised in his frame.

If it is known, then it must be known that light pulses will return simultaneously whether clocks are synchronised or not

Given the symmetry, we only need to talk about the perspective from the "stationary" frame - so the frame can be assumed. Apologies, if that wasn't clear.

It's more parsimonious bcos it makes fewer assumptions

and it is simpler bcos it doesn't involve paradoxical scenarios where observers are both right and wrong,

and it eliminates the Relativity of Simultaneity.

In case the point still hasn't been clearly made:The "stationary" observer witnesses an alternative explanation for the simultaneous return of the light pulses. They see that the light pulses return simultaneously whether clocks are synchronised or not.

The bodycam footage offers empirical evidence that their clocks, in their localised region of space, are not synchronised.

This contradicts their assumption that the clocks are synchronised in their own frame.

His clocks reside in his own frame. He assumes that they are synchronised.

Simply ask the question: is it possible that the observer in the stationary frame is mistaken about the simultaneity of the clock synchronisation events? Given that they see a scenario where light returns simultaneously from non-synchronised clocks.

Both assume their clocks are synchronised

Quote from: HalcLight speed was initially measured using another reliable clock that could be moved closer and further away. It was an inertial clock (a steadily rotating thing like Earth), not based on light at all.An observer moving relative to the Earth cannot use the Earth.

Light speed was initially measured using another reliable clock that could be moved closer and further away. It was an inertial clock (a steadily rotating thing like Earth), not based on light at all.

The point is, if every clock is biased by the same factor, then it will all cancel out, as though the universe is "conspiring" to ensure the speed of light is always measured to have the same value.

clocks simply count units of measurement, like a tape measure counts metres.

That is a statement, and not one that defines anything. The statement is not true in general. A and B can be objects and have differing times for the light journeys due to the fact that objects are not obliged to stay put.That's why I had to look up the quote to see what definition was being referenced.

Wrong. Again, the earliest light speed measurement was done one way, and while it employed multiple clocks, they were not particularly synchronized.

The first observer sees that the moving clocks are not the same distance from the other observer when they're zeroed. That's why they're not synced in that frame.

They are verified in sync because he can measure that his clocks are equidistant from him, while the two moving-clock synchronization events are not equidistant from the moving observer, thus they must not be in sync.

Fine then. The moving clocks are not synchronized in that frame, and the all observers agree with that fact. There's no conflict.

There's no paradox to remove.Quoteand it eliminates the Relativity of Simultaneity.That it does. Seems to make everything more complicated to do so, but it does indeed eliminate that.

In this particular case yes. Not always, as I point out above.

What do you mean 'localized'? The clocks are separated, not local to each other. If they were local, they'd be synced in any frame.

How? You didn't say that they were shown to be not synchronized in that frame. No footage demonstrated that. I see no contradiction.

QuoteSimply ask the question: is it possible that the observer in the stationary frame is mistaken about the simultaneity of the clock synchronisation events? Given that they see a scenario where light returns simultaneously from non-synchronised clocks.The question lacks frame references, so is largely meaningless

No they don't. They're not absolutists, so they make no absolute assumptions like that. In all probability, none of the 4 clocks is synchronized in an absolute way since none of them attempted an absolute procedure to do it. Why should they? It serves no purpose.

QuoteQuote from: HalcLight speed was initially measured using another reliable clock that could be moved closer and further away. It was an inertial clock (a steadily rotating thing like Earth), not based on light at all.An observer moving relative to the Earth cannot use the Earth.The observer was on Earth. There was little choice in the matter at the time.

They're not all biased out by the same factor. They're all dilated to some extent for multiple reasons.

Agree, the units are arbitrary. No alien is going to come up with a meter or a second. That doesn't mean spacetime isn't fundamental. Just that the units into which we choose to slice it up are not....BTW, I don't think they're fundamental either. I just disagree with the validity of the argument you use to conclude that.

1) We can assume the reference frame to be that of "the stationary system". This is because, given the symmetry of the situation, both observers consider themselves to be in "the stationary system".

2) The contention here isn't that the Einsteinian interpretation isn't self-consistent. I [finally*] accept that it is self-consistent. You have made a few statements to the effect that there is no "conflict" or that "all observers agree**". The intention isn't to demonstrate a conflict or contradiction in the Einsteinian interpretation.

3) The point that is being attempted, is to demonstrate that there is an [unjustified] assumption being made in the Einsteinian interpretation. That assumption is that observers in "the stationary frame" assume that their clocks are synchronised (in their own frame of reference).

4) You have made repeated references to the frame dependence of simultaneity. The point being made is that, without the assumption of #3 above, one cannot arrive at a conclusion of frame dependent simultaneity.

5) Simultaneity of events in the "stationary system" is an assumption, while the bodycam footage represents observational evidence to the contrary.

I want to try and clarify the point by means of the inner monologue of one of the observers. I think it will also be heplful to name our observers - good ol' Alice and Bob - because it will make it easier to keep track of reference frames.

Alice:The silly goose! His clocks are not synchronised,

Oh look, the light pulses returned to me simultaneously, that means my clocks must be synchronized.

We have not defined a common “time” for A and B, for the latter cannot be defined at all unless we establish by definition that the “time” required by light to travel from A to B equals the “time” it requires to travel from B to A. (Einstein, 1905).

It can't be any more explicit that what is being defined is the time required by light to travel from A to B equals the “time” it requires to travel from B to A, this is required to establish a "common time for A and B" i.e. to synchronise A and B.

Quote from: HalcAgain, the earliest light speed measurement was done one way, and while it employed multiple clocks, they were not particularly synchronized.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.

Again, the earliest light speed measurement was done one way, and while it employed multiple clocks, they were not particularly synchronized.

What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again.

Experiments that attempted to directly probe the one-way speed of light independent of synchronization have been proposed, but none has succeeded in doing so.

Quote from: HalcThe first observer sees that the moving clocks are not the same distance from the other observer when they're zeroed. That's why they're not synced in that frame.Again, this is the heart of the problem. The first observer, Alice, has no way of determining that the very same is not true for her own clocks - in her own reference frame.

You're conflating two different things here, the distance from the observer to each clock and the distance from the two "moving-clock synchronization events" to the moving observer.

You are somewhat mistaken though, bcos the moving observer, Bob, is equidistant from the clocks and therefore from the "moving-clock synchronization events".

I think what you mean is that the light pulse travels a shorter distance to one clock than the other.

The experimental support for relativity has lead us to accept such "spooky action"

My aplogies if I have caused confusion with my use of the term "localized". It might instead be more clear to talk in terms of two locations. We have the clocks "onboard the train" and the clocks "on the platform". In this sense relativity says, the clocks onboard the train are synchronised, in the frame of the train, but not from the perspective of an observer at rest on the platform.

Because reltivity says that this is more than just the case that the clocks appear unsynchronised from the platform

it says that both statementss are equally true. Therefore, the clocks onboard the train are both synchronised with each other and not-synchronised with each other, in their physical locations onboard the train.

Quote from: HalcThe question lacks frame references, so is largely meaninglessI have clarified that we can assume "in the frame of the 'stationary system" given the symmetry.

The question lacks frame references, so is largely meaningless

To rephrase: Simply ask the question: is it possible that the observer in the stationary frame is mistaken about the simultaneity of the clock synchronisation events, in the stationary frame?

Given that they see a scenario where light returns simultaneously from non-synchronised clocks.

Again, we can imply "in the frame of the "stationary system".

The point being contested is that simultaneity is frame dependent.

An unavoiable requirement of frame dependent simultaneity is that there must be one reference frame in which events are simultaneous (otherwise there is no basis for the relativity of simultaneity).

To delare that events are simultaneous in any frame simply isn't justified.

Quote from: HalcQuoteQuote from: HalcLight speed was initially measured using another reliable clock that could be moved closer and further away. It was an inertial clock (a steadily rotating thing like Earth), not based on light at all.An observer moving relative to the Earth cannot use the Earth.The observer was on Earth. There was little choice in the matter at the time.Maybe I've misunderstood your point. We were talking about the use of clocks to measure the speed of light, in the context of the thought experiment.

What clock are you suggesting that Bob use to measure the speed of light, where the effect of relative motion won't result in his clock offsettiing any discrepancy in the speed of light?

The metric system is neither fundamental nor emergent. "Time" is system of measurement like the metric system and so,

time attempts to express duration in commin units.

3D space is self-evident (not to say the holographic principle isn't correct). The "dimension of time" however, is not self-evident. We only ever observe things in the present instant i.e. the "now". Therefore, we cannot - not even in principle - oberve a temporal dimension; that is, we can never observe things extended in time.

QuoteQuote from: HalcThe first observer sees that the moving clocks are not the same distance from the other observer when they're zeroed. That's why they're not synced in that frame.Again, this is the heart of the problem. The first observer, Alice, has no way of determining that the very same is not true for her own clocks - in her own reference frame.Nonsense. She has a tape measure. That's how its done. She can't use it for Bob's clocks since they're moving, but both Alice and Bob know that Bob's clocks are not synced in Alice's frame.I tire of this endless repeating of the same points. Are you going to say anything new?

Can we drop the whole bodycam thing then? The purpose of that seemed to be to prove a case to the other observer over some point about which they disagreed, but they agree on all points. There is no conflict as you say.

I'm not sure why you're having such difficulty with the idea of "the stationary system" aka "the stationary frame". In his 1905 paper, Einstein starts by talking about an observer in "the stationary system" and builds up from there.

I'm simply following the convention used by Einstein in that paper.

We're looking at things solely from Alice's perspective, in Alice's frame, on the platform and making deductions based on her observations.

You keep retorting with "frame dependence" but that is simply using the conclusion to justify the assumption - I'm challenging the assumption, so "frame dependence" isn't a given.

=========Conflation=========Here is the issue, here is the assumption I have been trying to demonstrate. You are conflating the distance that Alice measures, [with her measuring tape] from the emitter to each clock, in her frame of reference, with the distance that the light pulses travel from the emitter to each clock, in her frame of reference.

Herein lies the assumption.

Remember that [from her reference frame] Alice sees Bob measure the distance from his emitter to each of his clocks, on the train, with his measuring tape.

She sees that he measures the distance from emitter to each clock as being equidistant on the train, with his measuring tape.

What she then sees is the light pulse from Bob's emitter, travel a shorter distance to the clock at the rear of the train - because that clock is moving towards the light pulse - and travel a longer distance to the clock at the front of the train - because that clock is moving away from the light pulse. She sees all of this from her vantage point, in her frame on the platform.

The opposite is true for each light pulse on the return leg, so the effects cancel out and the light pulses return simultaneously to Bob.

He clearly designates one arbitrary frame as such for notational convenience. I notice you didn't include that part of the quote despite its importance.In post 6 (point 1) you redefined the words to mean one's own frame, which makes the term ambiguous in absence of identification of whose frame we're talking about. I like that much better since it emphasizes the symmetry. Anyway, I framed all my replies for post 6 with this new definition of 'the stationary frame' which you give in the first point of that post.

Then your point 1 contradicted that convention. Why can't we say Alices's frame instead? Alice is designated the stationary one if we're to go by Einstein's convention. The first observer is arbitrarily designated the stationary one.

QuoteWe're looking at things solely from Alice's perspective, in Alice's frame, on the platform and making deductions based on her observations.Then don't quote statements made about different frames.

Why is it a bad assumption to make? All clocks, observer, tape measures, etc are all stationary in this frame and remain so for the duration between the relevant events.

QuoteRemember that [from her reference frame] Alice sees Bob measure the distance from his emitter to each of his clocks, on the train, with his measuring tape.Bob is moving, so she doesn't see this happen in her reference frame. He's measuring things that are not standing still in Alice's frame, so his measurements are invalid in that frame. How can Bob know where the clocks will be (in Alice's frame) when they emit their sync pulse? They haven't gone off yet, or they did a while ago, and either way, they're not at that spot when he takes his distance measurement.

QuoteShe sees that he measures the distance from emitter to each clock as being equidistant on the train, with his measuring tape.She sees no such thing. Bob is using completely invalid methods for making that measurement in Alice's frame, like trying to measure the height of a hyperactive child that won't stop jumping up and down.

I agree that the one pulse goes the longer distance in that frame, and thus takes longer to get between the clocks than does the other pulse. This is easily worked out from the premise of constant light speed. The gif Janus showed you (does he make those?) makes that pretty clear.

I think you mean that light going from Bob to the front and back to Bob travels the same net distance as the one going from Bob to rear and back. Yes, that's obviously true since the departure and arrival of both signals is simultaneous (one event 1 and 3). But since in Alice's frame the one going forward takes so much longer on the first leg, the rear mirror reflection event (that starts the clock there) obviously occurs first in that frame, so the clocks are very obviously not in frame as far as Alice is concerned. This is exactly what she expected and Bob agrees with that assessment.

Quote from: HalcAll clocks, observer, tape measures, etc are all stationary in this frame and remain so for the duration between the relevant events.Be careful here with your frame dependent terminology. They all remain stationary, with respect to what?

All clocks, observer, tape measures, etc are all stationary in this frame and remain so for the duration between the relevant events.

Bare in mind that one cannot be stationary with respect to a set of imaginary, mathematical co-ordinates.

To describe the physical scenario we should more accurtely say that Alice is at rest relative to the emitter, and both are at rest relative to the clocks. This is her synchronisation set up.

But, Alice can say the exact same thing about Bob's set up. He is at rest relative to the emitter, and both are at rest relative to the clocks. This is his synchronisation set up, as observed by Alice. Both are in motion relative to each other.You say that "all clocks, observer, tape measures, etc are all stationary in this frame and remain so

" but you are leaving out the central actor in the whole synchronisation process, the light pulses. They do not remain "stationary". So, how is the distance they travel in Alice's frame determined. She assumes that the distance traveled by the light pulse to clock A is the same as the distance traveled by the light pulse to clock B and that both these distances are the same as the distance she measured from the emitter to each clock.

She observes Bob making similar measurements on the train (from her perspective). He too assumes that the distance traveled by the light pulse to clock A is the same as the distance traveled by the light pulse to clock B

Observing - what she considers to be - Bob's failed synchronisation attempt, offers an alternative explanation for her own synchronisation procedure. It is possible that, in her frame, clock A was moving towards the light pulse while clock B was moving away from it.

Everything would still appear the same to her, everything would appear stationary in her frame and the light pulses would return simultaneosuly just the same. As per the Galilean Principle of Relativity, there is no experiment she can do to verify or dismiss this.

Quote from: HalcBob is moving, so she doesn't see this happen in her reference frame. He's measuring things that are not standing still in Alice's frame, so his measurements are invalid in that frame. How can Bob know where the clocks will be (in Alice's frame) when they emit their sync pulse? They haven't gone off yet, or they did a while ago, and either way, they're not at that spot when he takes his distance measurement.Be careful with your use of the term "moving" here because your confusing two different things.

Bob is moving, so she doesn't see this happen in her reference frame. He's measuring things that are not standing still in Alice's frame, so his measurements are invalid in that frame. How can Bob know where the clocks will be (in Alice's frame) when they emit their sync pulse? They haven't gone off yet, or they did a while ago, and either way, they're not at that spot when he takes his distance measurement.

At this point in the discussion, we're not talking about Bob's observations of Alice, we're talking about Bob setting up his synchronisation procedure, on the train.

Quote from: HalcBob is using completely invalid methods for making that measurement in Alice's frame, like trying to measure the height of a hyperactive child that won't stop jumping up and down.You do realise that this is contradictory to what Einsteinian relativity actually says, right?

Bob is using completely invalid methods for making that measurement in Alice's frame, like trying to measure the height of a hyperactive child that won't stop jumping up and down.

The whole point is that Bob's measurements of Alice's experiments are just as valid as Alice's measurements of her own experiments.

This where the Relativity of Simultaneity comes from. Bob measures events to be non-simultaneous in Alice's frame (that she assumes are simultaneous in her frame).

Alice makes the above observation of Bob's light pulses. She assumes that the same is not true for her because she employs the Einsteinian convention that the time from A to B equals the time from B to A.

Her observations of Bob's synchronisation procedure offer an alternative explanation for her own

The point being that Alice has no way of knowing that the same has not happened in her own synchronisation procedure.

She assumes that the distance the signals travel to each clock is the same - as per the Einsteinian convention

- however, it is just that, an assumption. If she was mistaken in her assumption, she would have no way off knowing bcos the signals would travel the same net distance and arrive back simultaneously.

All of Alice's components are stationary in her frame. Bob and his clocks and tape measure are obviously not stationary in Alice's frame.

Let's clear all components out of the spaceship, including Alice, so we are left with just the spaceship. Is the spaceship stationary?

Relative to what?It certainly is relative to the spaceship! However, Bob might disagree.

ou can also set up a frame which moves with the spaceship and the spaceship is at rest relative to that.

ou seem to have problems with the different viewpoints here, but there is no surprise that Alice and Bob see different things and that these differences are symmetrical, but this does not affect whether they can say their own clocks in their own frames are synchronised. The same sort differences of viewpoint occur in Galilean relativity. Also, if you follow through the differences of measurements performed in their respective coordinates (coordinate transforms) you will see that these differences are to be expected and easily understood.It makes no difference that Alice is intelligent and can work out what’s happening, she takes measurements from her frame. In Galilean relativity we can see that Bob on a railway carriage sees a ball bouncing up and down, in order to shoot it down his gun need only track up and down, Alice however sees the ball following a sine wave, a very different prediction.We are also used to everyday experiences of frame dependant views, we all know that the passenger in a car following a curve continues in a straight line, but that doesn’t make the centrifugal force any less real to the passenger.I would recommend you look at the coordinate transforms for Alice’s viewpoint in order to understand this situation as you are getting lost in misunderstanding what the bodycam footage really shows - the view from one frame only, which is not relevant to what the other frame views.Alice doesn’t need to question whether her clocks are synchronised, she’s a bright girl and can work out that they are, but that synchronisation only applies to her frame.

it's a simple foundational question from which we can expand. So, is the spaceship at rest - you can add any qualifiers you wish.

Are you familiar with the empirically equivalent Lorentz-Poincare interpretation, which is based on absolute simultaneity?

The simple answer is that we don’t know.

The qualifier is that we don’t care, because it is at rest relative to it’s own reference frame and that’s all we need to know.

Yes I am. Again, we don’t care, because we have all we need to work out the relative viewpoints and measurements.

Quote from: HalcAll of Alice's components are stationary in her frame. Bob and his clocks and tape measure are obviously not stationary in Alice's frame.OK, let's try to break things right down. Let's take a common formulation of the thought experiment. Let's say Alice and Bob are in spaceships.All components are stationary, in her frame.

It certainly is [stationary] relative to the spaceship! However, Bob might disagree.

Quote from: Colin2BQuoteSo, is the spaceship at rest?The simple answer is that we don’t know. Precisely!

QuoteSo, is the spaceship at rest?The simple answer is that we don’t know.

So, is the spaceship at rest?

Alice has no way of knowing which situation is the true situation - as per the Galilean Principle of Relativity.

Objects in the physical world can only be in motion or at rest relative to other physical objects.

I know you are committing the fallacy of reification with respect to the mathematical co-ordinates and saying that the spaceship is at rest relative to them;

Interesting that "we" don't care. Both interpretations make different ontological claims about the underlying physical structure of the universe. You might think that this would be of concern to a field of research that concerns itself with the nature and functioning of the physical world.

That is beside the point tho. The point is that there is an alternative interpretation of Relativity which doesn't involve the Relativity of Simultaneity.

I'm probably making things a bit difficult for myself here, so to try and simplify:Einstein's clock synchronization convention unequivocally states that the synchrony of clocks must be assumed. This is done by establishing - as a matter of definition - that the time for light to travel from A to B equals the time from B to A.

This applies in the Set-up with Alice and Bob where the time from emitter to A is established by definition to be equal to the time from emitter to B.

Bob's observational evidence demonstrates that Alice's assumption is incorrect.

To try and refer to frame dependence - to disqualify Bob's observational evidence of Alice's synchronization procedure - is simply to assume the conclusion of frame dependence.