Thought Experiments

[christopherkirkreves]

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The Special Theory of Relativity

 

The Special Theory of Relativity is resting on three assumptions:

 

One. There is no absolute rest.

Two. The velocity of light is independent of the motion of the emanating source.

Three. The velocity of light is invariant for all observers.

 

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The Two Lightning Bolt Strikes Thought Experiment

 

 

 

 

A man is sitting still on an embankment alongside a set of train tracks. He has two mirrors fashioned together in a v shape so he can see down the length of the train tracks in both directions at the same time.

 

There is also man on a train. The train is moving down the tracks. He too has a v shaped set of mirrors that allow him to see down the length of the train tracks in both directions at the same time.

 

When the man on the moving train reaches the point where the man on the embankment is sitting two lightning bolts strike two points along the train tracks at the same time. The two points where the lightning bolts strike are equal distances from the two men at the time of the strikes.

 

 

After the lightning bolts strike the train tracks the man on the train continues to move. As the flashes of light move towards the two men the man on the train moves towards one flash of light and away from the other.

 

With the distance between the man on the train and the one flash of light getting shorter and with the distance between the man on the train and the other flash of light getting longer, this means the man on the embankment will see the flashes of light simultaneously while the man on the train will see the flash of light he is moving towards before he sees the flash of light he moving away from.

 

 

There is no known way to determine absolute rest. When two bodies are in motion relative to one another we cannot say which body is actually in motion and which body is actually at rest or if both are in motion and neither is at rest. And so it could be the train that is in motion or it could be the embankment that is in motion or both.

 

The Special Theory of Relativity tells us that the man on the train will consider himself to be at rest and the embankment to be in motion. And the Special Theory of Relativity tells us that the man on the train will consider the distances the two flashes of light have traveled to be the equal distances they have traveled relative to the train.

 

The Special Theory of Relativity also tells us that the man on the train will not see the one flash of light travel at a greater velocity towards him and the other flash of light travel at a lesser velocity towards him. He will see both flashes of light travel at the same velocity (300,000 km/sec).

 

So, if the two flashes of light have traveled equal distances at equal velocities but one arrives before the other, then, for the man on the train, the lightning bolt strike he is relatively moving towards must have occurred first and the lightning bolt strike he is relatively moving away from must have occurred later.

 

And so, Einstein concludes: “Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train ....”

 

 

The Special Theory of Relativity tells us that the velocity of light is independent of the motion of the emanating source.

 

And so, for the man on the train, when the two lightning bolts strike the two points along the moving tracks and create the two flashes of light, the sources of these flashes of light (the points of impact) being in motion does not affect the velocity at which these flashes of light travel towards him.

 

 

And so, in this thought experiment, given that the motion or lack of motion of the emanating source of the light is irrelevant, and given that it may be the man on the train who is at rest and embankment that is in motion, with the two flashes of light traveling at the same velocity over equal distances to the man on the train this means that the man on the train will see the two flashes of light at the same time.

 

To say that the man on the embankment will see the two flashes of light at the same time while the man on the train will see one flash of light before the other, is to say that the man on the embankment is the one actually at rest and the man on the train is the one actually in motion; which we cannot.

 

And to say that the man on the embankment will see the two flashes of light at the same time while the man on the train will see one flash of light before the other because “simultaneous events in one inertial frame of reference are not simultaneous in another inertial frame of reference” is a premise of this thought experiment and not just its conclusion is to engage in circular logic; which we should not.

 

The fact that the lightning bolt strikes occur outside the train and the man in the train is in the train is irrelevant. This thought experiment could just as easily have been proposed with a man sliding along the frictionless train tracks just by himself.

 

This thought experiment does not prove: “Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train ....”

 

The theoretical justification for the Special Theory of Relativity is the validity of this thought experiment.

 

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Alternative Thought Experiment

 

 

A different thought experiment was proposed by David Frost Comstock in 1910 (six years before Einstein proposed his two lightning bolt strikes thought experiment). Taking the premises of Einstein’s thought experiment and using them with Comstock’s thought experiment, it then does work.

 

In this thought experiment there is one light bulb in the middle of a moving car. It flashes. One flash of light goes to the front of the car. And another flash of light goes to the rear of the car.

 

From the perspective of the car at rest the man in the car will see the two flashes of light reach the front and the rear of the car simultaneously and from the perspective of the road at rest the man on the road will see the one flash of light reach the rear of the car first and then the other flash of light reach the front of the car later.

 

And so, given the premises, this proves: “Events which are simultaneous with reference to the embankment [car] are not simultaneous with respect to the train [road]....”

 

(FN: In his book Einstein takes some time to work out what he believes is the correct definition of “simultaneous” and he finds it in “If the observer perceives the two flashes of lightning at the same time, then they are simultaneous.” This alternative thought experiment does not prove that the flashes of light reaching the front and the back of the car are “simultaneous” for the man in the car in the way he defined this term. So, not only must the thought experiment be replaced with the alternative thought experiment but the definition of “simultaneous” must also be replaced. The definition he used does not apply here. In this thought experiment “simultaneous” cannot mean that identical clocks at the front and the rear of the car show the same time readings when the flashes of light reach them, because, if this theory is true, both the man in the car and the man on the road will say that the flashes of light reached the two clocks at identical time readings given the non-synchronization of clocks between inertial frames of reference. The man on the road has to measure the time the flashes of light reach the front and rear of the car based on synchronized clocks in his own inertial frame of reference. And so the man on the road would have to have two clocks: one at the point on the road corresponding to the rear of the car when the flash of light reaches it, and one at the point on the road corresponding to the front of the car when the other flash of light reaches it. The man in the car also has to have two clocks at the front and the rear of the car. And the definition of “simultaneous” is then identical time readings on these two clocks for each man in their own inertial frame of reference; which is the case for the man in the car and is not the case for the man on the road. And so in the revised statement “Events which are simultaneous with reference to the embankment [car] are not simultaneous with respect to the train [road]” the term “simultaneous” means something different than how Einstein defined it. It may be a trivial distinction but it should be noted.)

 

 

The General Theory of Relativity is resting on the Special Theory of Relativity and the Special Theory of Relativity is resting on the two lightning bolt strikes thought experiment and the two lightning bolt strikes thought experiment is false. The foundation of this theoretical structure must be revised.

 

(see Comstock, D.F. (1910), “The Principle of Relativity”, Science 31 (803): 767–772)

 

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mod deletions

 

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Frankly, I don’t mean to be rude, but I don’t get it.

 

The point of the two flashes of light thought experiment is to show that “events which are simultaneous relative to the embankment are not simultaneous relative to the train.” I don’t understand what Physcisforums.com is saying about demonstrating assumptions not being justified.

 

?

 

Is PF saying that the two lightning bolt strikes thought experiment is about “disproving an assumption” rather than proving something?

 

Is PF saying that the thought experiment that the Special Theory of Relativity (and then the General Theory of Relativity) is resting on does not prove anything but rather “disproves an assumption”?

 

Is PF saying that this whole elaborate theoretical structure is resting on a “disproved assumption”?

 

No. The two lightning bolt strikes thought experiment is meant to prove that “events which are simultaneous in one inertial frame of reference are not simultaneous in another inertial frame of reference.” But it does not prove this.

 

However, Comstock’s thought experiment does.

 

Thank you.

 

- Christopher

 

(And, obviously, I should not have stepped away for two weeks.)

Edited September 3, 2014 by swansont
OT material removed

[Strange]

The Special Theory of Relativity is resting on three assumptions:

 

These are not the assumptions that special relativity is based on.

 

 

When the man on the moving train reaches the point where the man on the embankment is sitting two lightning bolts strike two points along the train tracks at the same time. The two points where the lightning bolts strike are equal distances from the two men at the time of the strikes.

 

We already know that times and distances are observer dependent and not necessarily the same for all observers. The thought experiment you are referring to (and making a bit of a hash of) is intended to demontrate that the concept of "at the same time" is also not the same for all observers. So if you say "at the same time" you need to specify which frame of reference this is in (I think that if you go back to Einstein's orginal description, you will find he does that).

 

Similarly, you need to specify which frame of reference you are referring to when you say where the lightning strikes happen (again, I think you will find Einstein does this).

 

 

So, if the two flashes of light have traveled equal distances at equal velocities but one arrives before the other, then, for the man on the train, the lightning bolt strike he is relatively moving towards must have occurred first and the lightning bolt strike he is relatively moving away from must have occurred later.

 

Correct.

 

And so, in this thought experiment, given that the motion or lack of motion of the emanating source of the light is irrelevant, and given that it may be the man on the train who is at rest and embankment that is in motion, with the two flashes of light traveling at the same velocity over equal distances to the man on the train this means that the man on the train will see the two flashes of light at the same time.

 

This contradicts what you previously said.

 

This has nothing to do with the movement (or otherwise) of the source. It is purely that the man on the train is moving towards (and away from) the place where the lightning strike happened. As you previously said.

 

 

The theoretical justification for the Special Theory of Relativity is the validity of this thought experiment.

 

Of course it isn't. This is a thought experiment, therefore it cannot prove or disprove anything. It is purely a teaching aid (failed in this case, apparently).

 

 

This is the private response to my essay from Physicsforums.com:

 

Then maybe you shouldn't have reproduced it.

 

I am not familiar with physicsforums but some forums don't allow people to present "personal theories". You might have had a better reception if you had presented your confusion as a question, rather than appearing to try and show relativity wrong.

 

 

Frankly, I don’t mean to be rude, but I don’t get it.

 

I don't mean to be rude, but that is obvious. I also don't get why you think your failure to understand something means that a very well-tested and practically useful theory should be wrong. I think you might need to reconsider where the problem is.

 

Your computer wouldn't work if special relativity were wrong.

 

 

Is PF saying that the two lightning bolt strikes thought experiment is about “disproving an assumption” rather than proving something?

 

It is an example of argumentum ad absurdum; you set up a scenario and show that you get contradictory results and therefore one or more of the assumptions must be wrong.

 

 

Is PF saying that this whole elaborate theoretical structure is resting on a “disproved assumption”?

 

This pedagogical example is not intended to prove anything, merely to explain it.

 

Also, science never proves things. It can only disprove them (if that).

 

Maybe you should start a new thread called, "Could someone help me understand relativity of simultaneity"

 

I find videos completely unhelpful, but I know some people like them so maybe this will help:

Edited September 3, 2014 by Strange

[swansont]

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Moderator Note

christopherkirkreves, references to your trials and tribulations at another site have been deleted. This is not the place to air your grievances. Please confine your discussion to science (which also means don't respond to modnotes)

[xyzt]

The General Theory of Relativity is resting on the Special Theory of Relativity

 

False, GR subsumes SR, it does not "rest on SR"

 

 

 

 

and the Special Theory of Relativity is resting on the two lightning bolt strikes thought experiment

 

False. SR rests on a set of axioms. Relativity of simultaneity is a CONSEQUENCE of those axioms.

 

 

 

 

and the two lightning bolt strikes thought experiment is false.

 

False. You simply misunderstand the thought experiment.

In addition, thought experiments cannot be used to disprove (falsify) a theory, only real experiments can do that.

As an aside, RoS is not testable. There is some debate about this issue in the philosophical (not physical circles) but the bottom line is that it isn't testable.

 

 

The foundation of this theoretical structure must be revised.

 

False. It is your understanding of physics that needs to be revised. This is why your post got deleted.

Edited September 3, 2014 by xyzt

[Delta1212]

 

False, GR subsumes SR, it does not "rest on SR"

 

 

 

 

 

False. SR rests on a set of axioms. Relativity of simultaneity is a CONSEQUENCE of those axioms.

 

 

 

 

 

False. You simply misunderstand the thought experiment.

In addition, thought experiments cannot be used to disprove (falsify) a theory, only real experiments can do that.

As an aside, RoS is not testable. There is some debate about this issue in the philosophical (not physical circles) but the bottom line is that it isn't testable.

 

 

 

False. It is your understanding of physics that needs to be revised. This is why your post got deleted.

I don't think that claiming RoS isn't testable is strictly true. We can certainly have measurements taken from two different frames that calculate the simultaneity of distant events differently. Perhaps it might be better to say that whether any two events separated in space can ever "really" be described as simultaneous or happening in a particular order isn't testable?

 

As a phenomenon of observation it's certainly testable, but as with most of physics, "what is really happening" behind those observations is not.

[xyzt]

I don't think that claiming RoS isn't testable is strictly true.

Actually, it is true.

[Delta1212]

Actually, it is true.

Ok, what makes it untestable?

[xyzt]

Ok, what makes it untestable?

Here is a very good paper from the Stanford library that explains why RoS is untestable.

[timo]

It would be soooo awesome for lazy people like me if you could be bothered to still answer the question instead of just linking to a non-further explained text ("very good paper" is not an explanation) that does not even contain any of the terms "testable", "untestable" or "test" in its body. Would also somehow serve the "discussion" aspect of this forum.

[xyzt]

It would be soooo awesome for lazy people like me if you could be bothered to still answer the question instead of just linking to a non-further explained text ("very good paper" is not an explanation) that does not even contain any of the terms "testable", "untestable" or "test" in its body. Would also somehow serve the "discussion" aspect of this forum.

Generally I post a lot of math backing up my claims, so I take exception to your dig..

Secondly, if you do not understand why RoS "conventionality" precludes testability, I could explain it to you. Just ask, don't throw digs.

Edited September 4, 2014 by xyzt

[Phi for All]

Generally I post a lot of math backing up my claims, so I take exception to your dig..

Secondly, if you do not understand why RoS "conventionality" precludes testability, I could explain it to you. Just ask, don't throw digs.

!

Moderator Note

Please calm down. Assuming timo's reply was a "dig" or personal attack against you seems premature. It seems more like a request for the relevant part of the paper to be cited, since timo's initial searches revealed no mention of "testable/untestable" in the paper you provided. This seems like a reasonable request for the future.

[timo]

If you do not understand why RoS "conventionality" precludes testability, I could explain it to you. Just ask.

In that case ...:

Ok, what makes it untestable?

[christopherkirkreves]

To the moderator:

 

Thank you for deleting my angry vent about the other forum deleting my post. It was bad form.

 

And thank you for allowing the rest of my post to remain on your forum. I will try to be more proper in the future.

 

(However, by then also leaving my response to my quote of them, it makes it sound like I’m saying “I don’t get it” to my own post.)

 

To those who have taken the time to read my essay and respond:

 

I understand that you think that I don‘t understand the Special and General Theories of Relativity. But, I don’t understand why.

 

I never said that the two lightning bolt strikes thought experiment proves anything in the physical world. The philosophy of science teaches us that a theory can never be proven true but it can be proven false. If my theory is that every time I flip my coin it will come up heads, and if I flip it a billion times and it comes up heads every time, I have not proven my theory true but rather I have confirmed it to a high degree of likelihood, because on the one billion and one flip it could come up tails (and if it does then I have proven my theory false). However, in the world of thought you can prove things. If, given certain assumptions (or premises) you reason you way to a conclusion, and if you logic is right, then you have proven that conclusion (in the world of thought).

 

My point is given the assumptions of the two lightning bolt strikes thought experiment (... and if I’m wrong about what the assumptions are then please specifically point out what the correct assumptions are ...) Einstein incorrectly then concludes that “events that are simultaneous in one inertial frame of reference are not simultaneous in another inertial frame of reference.” However, if you take those same assumptions and use them in Comstock’s thought experiment you can conclude “events that are simultaneous in one inertial frame of reference are not simultaneous in another inertial frame of reference.”

 

This must be established in order to then say “each inertial frame of reference has its own particular time” as Einstein does. And this must be established in order to justify the Lorentz Transformation, and this must be established in order to then go on and on showing such things as “clocks synchronized in one inertial frame of reference are not synchronized in another” and so on and so on including the idea “clocks experiencing centrifugal force run more slowly the greater the centrifugal force and therefore in all gravitational fields clocks run more slowly the greater the gravitational force” and so on and so on. The Theory of Relativity is a set of proofs based on proofs. And the initial proof is that “events which are simultaneous in one inertial frame of reference are not simultaneous in another.”

 

The two lightning bolt strikes thought experiment is not merely a pedagogical sideshow demonstration (and even if it was it still does not work). It is the initial proof that all of the other proofs are based on.

 

I don’t mind being wrong. I don’t like it, but I don’t mind it. I understand that you guys all think that I am wrong. That is fine. But can you please specifically show where my logic is wrong.

 

Thank you.

 

- Christopher

 

(PS: The discussion turned to how to test the Special Theory of Relativity. There are several tests of this theory: Michelson-Morley, Kennedy-Thorndike, Ives-Stilwell and so on. However, none of these test the assumption “the velocity of light is invariant for all observers.” I believe I have figured out how to do this. Again, thank you.)

 

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Test of an Initial Assumption of the Special Theory of Relativity

 

 

 

Send an electromagnetic wave to a satellite above Earth’s atmosphere and in the vacuum of space and not in synchronous motion with the rotation of the Earth. Then have that satellite relay the electromagnetic wave to another satellite in synchronous motion with the first satellite and also above Earth’s atmosphere and in the vacuum of space. And then have that second satellite return the electromagnetic wave back to Earth. And then do the same thing again but in the reverse direction.

 

From the perspective of the Earth at rest, the two satellites are in motion and the electromagnetic wave relayed from one to the other will travel two different distances in the two different cases. The Special Theory of Relativity predicts that from the perspective of Earth in one case it will take a longer time for the wave to be returned to Earth and in the other case it will take a shorter time for the wave to be returned to Earth.

 

 

As the satellites move they rotate relative to the Earth at rest. This rotation will make their linear motion increase and decrease. The linear motion of the satellite relaying the electromagnetic wave when it relays it and the linear motion of the other satellite when it receives the electromagnetic wave need to be the same for the flash of light to have moved between them in the same inertial frame of reference from the perspective of Earth. This can be done if the satellites are in the right positions.

 

 

And it will be different depending on which way the flash of light is being relayed.

 

(FN: Any bending of the wave as it is relayed from one satellite to the other due to gravity will need to be factored in.)

 

If the two electromagnetic waves take two different amounts of time to return to planet Earth then the Special Theory of Relativity will again be confirmed, and if the two electromagnetic waves take the same amount of time to return to planet Earth then the Special Theory of Relativity will have been shown to be false.

 

The variations in the atmosphere as the electromagnetic wave makes its way to and back from the satellites may make the results inconclusive. However, if this experiment were to be done over and over and over again a sufficiently large number of times then the individual skewed results would tend to balance out and the overall results would either tend towards the difference between the two cases as predicted by the Special Theory of Relativity or tend towards equal amounts of time in the two cases and so disprove the theory.

 

There are several tests of the Special Theory of Relativity: Michelson-Morley, Kennedy-Thorndike, Ives-Stilwell and so on. None of these directly test what happens when a wave of light moves across a vacuum in one inertial frame of reference and the observer is in another inertial frame of reference. This is a direct test of the Special Theory of Relativity assumption of “the speed of light is invariable for all observers.”

 

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Synchronized Clocks

 

 

The above proposed test requires synchronized clocks. One method is to synchronize two clocks next to one another then slowly transport one of them to the other location very slowly. Due to time dilation this will cause them to become unsynchronized but only to a small degree.

 

There is another way to synchronize two clocks at a distance.

 

 

Start with four clocks; place one at one of the two locations and place three at the other location. Synchronize two of the nearby clocks. Then transport (quickly or slowly) one of them to the other stationary clock. Then synchronize those two clocks with the slightly out of sync time with the first clock.

 

Then return the moved clock back to the first clock. Synchronize the fourth clock with the out of sync clock. Then return the clock that was already moved back to the other clock.

 

The velocity at which the moved clock is moved each time must be the same and the three accelerations must be the same and the three decelerations must be the same.

 

 

The time discrepancy between the first and the second clock is at much or less than the time discrepancy between the first and the fourth clock. And the time discrepancy between the second and the fourth clock is as much or less than the time discrepancy between the second and the third clock.

If this is done carefully enough these should be equal amounts. The two processes are perfectly inversely symmetrical.

 

If the second clock was behind the first clock 3/4 of the time difference between the first and fourth clocks then the fourth clock would be behind the second clock 3/4 of the time difference between the second and third clocks. But this would mean the time difference between the second and third clock would be greater than the time difference between the first and the fourth clocks. And given that the processes are perfectly inversely symmetrical this logically cannot be. For the time difference between the first and the fourth clock to be the same as the time difference between the second and third clocks the time difference between the first and the second clocks must be 1/2 the time difference between the first and the fourth clocks.

 

And so, after doing this reset the second clock forward 1/2 of the time difference between the first and the fourth clocks and to a logical certainty (although maybe not to a practical one given how precise the moving can be done) they are synchronized.

 

This actually could be done with fast moving clocks. The amount of discrepancy is unimportant. What is important is that the two processes are as perfectly inversely symmetrical as possible.

 

(FN: This could be done repeatedly over and over again with any number of clocks to get an overall amount and reduce any biases in the actual physical process.)

[xyzt]

 

 

There are several tests of the Special Theory of Relativity: Michelson-Morley, Kennedy-Thorndike, Ives-Stilwell and so on. None of these directly test what happens when a wave of light moves across a vacuum in one inertial frame of reference and the observer is in another inertial frame of reference. This is a direct test of the Special Theory of Relativity assumption of “the speed of light is invariable for all observers.”

 

This is, like all your other claims, false. There are several tests that falsify your above claims. You only need to learn, instead of making wild and incorrect claims.

[Phi for All]

!

Moderator Note

Since the thread has changed direction from clarifying questions about relativity to defense of an alternative explanation, we need to move this to Speculations so students looking for mainstream answers don't get confused. Please take the time to read the special rules that govern this section.

 

As usual, responding to modnotes in thread will get your parking space revoked.

[Strange]

But can you please specifically show where my logic is wrong.

1. Thinking that you can prove or disprove anything with a thought experiment.

 

2. Thinking that your misunderstanding of a thought experiment has any significance.

 

3. Thinking that the theory is based on relativity of simultaneity

 

4. Thinking that your misunderstandings of a popular science book trump experimental evidence.

 

5. Oh, I give up. Like all the rest, you will just ignore all attempts at explanation and assume you know best. Good luck.

[christopherkirkreves]

Okay. I tried. I give up.

 

You believe that I am simply unreasonable.

 

However, while many of you told me that I was wrong, not a single one of you showed me that I was wrong.

 

And what I mean by that is if the two lightning bolt strikes thought experiment is so obviously right, then walk me through it. Show me why the man on the embankment will see the two flashes of light at the same time and the man on the train will see the one flash of light before the other. I don't think you can do it. And I don't think you can do it not because I'm someone who refuses to see reason, but because the logic is not there.

 

So, okay, it's off to the Speculations folder. I lost.

 

And I apologize to the moderator for addressing him directly, but it seemed appropriate. And I do thank you for just moving me to Speculations and not straight up deleting me.

 

Thank you.

 

- Christopher

[xyzt]

 

However, while many of you told me that I was wrong, not a single one of you showed me that I was wrong.

 

Classical.

[christopherkirkreves]

If by "classical" you mean that "the classic response of someone in denial is to say that you told me I was wrong while not showing me that I was wrong when in fact that person had been shown that he was wrong" then please show me where in this thread you showed that "the man on the embankment will see the two flashes of light at the same time while the man in the train will see the one flash of light before the other." Maybe I'm in such denial that that that logic is here in this thread but because I'm in such denial my eyes can't see it.

 

Please show it to me.

 

To say that I'm "classically" in denial is again to "tell" me that I'm wrong while not "showing" me that I'm wrong.

 

Thank you.

 

- Christopher

[Phi for All]

However, while many of you told me that I was wrong, not a single one of you showed me that I was wrong.

 

!

Moderator Note

On the contrary, every single person who responded showed you that you were wrong, and exactly where you were wrong. For whatever reasons, you didn't understand what they showed you. You spend more time defending your reasoning than trying to take the replies on board and work with them.

 

Re-read the thread before you give up. Try to understand their answers before defending your own. That may help.

 

You still have to park on the street for the rest of the month for responding to a modnote, though.

[Janus]

Okay. I tried. I give up.

 

You believe that I am simply unreasonable.

 

However, while many of you told me that I was wrong, not a single one of you showed me that I was wrong.

 

And what I mean by that is if the two lightning bolt strikes thought experiment is so obviously right, then walk me through it. Show me why the man on the embankment will see the two flashes of light at the same time and the man on the train will see the one flash of light before the other. I don't think you can do it. And I don't think you can do it not because I'm someone who refuses to see reason, but because the logic is not there.

Okay, I'll take up the challenge.

 

First off, we start with the fact that we start in the embankment frame, not because we claim anything special about it or that it is "stationary", but because we have to start somewhere. Then have two events(the lightning strikes) in this frame that are determined to be simultaneous in this frame. We then determine that the light from these strikes meet at a point halfway between them, where we have placed an observer.

 

Again none of the above assumes anything special about the embankment frame. We have just chosen to start the set up of the problem this way.

 

We also include a train with its own observer. This observer happens to be adjacent to the embankment observer, at the moment in the embankment frame that the strikes occur. The train along with the observer, in the direction of one of the strikes. while the light from the strikes travel towards the embankment observer, the train observer and embankment observer separate. We end up with a situation where the light from one strike has to pass the train observer to get to the embankment observer, and the light from the other flash has to pass the embankment observer before it can reach the train observer. The light from one flash must reach the train observer before the other.

 

In addition, since the train observer and embankment have a relative motion with respect to each other, the train observer will be in a different spot relative to the embankment when one flash reaches him than he is when the other flash reaches him. This is a fact that both frames will agree on. For example, we could have the train observer carry a camera that takes a picture of the embankment when the light from either strike reaches it. As seen in the embankment frame the camera takes two different pictures of two different points of the embankment.

 

Now if we switch to the train frame, we have to stay true to these facts: The light from each strike reaches the train observer at different times and when he is next to different points of the embankment, and his camera will have recorded two pictures confirming this. From there the observer can note that since he was halfway between the two points where the flashes occurred relative to the train, and light the light had to take an equal time to travel from each point, the strikes the created them occurred at different times.

 

Not only did he determine this for himself, but if he looks back at the embankment observer, he will see him being lit up by the light from each flash at the same time.

 

While there are events that are simultaneous for the train observer and not so for the embankment observer, they will not be the same events we started this scenario with. Both frames cannot equally claim that the same lightning strikes are simultaneous for them.

 

The other factor that comes into play here if you really want a fuller picture is length contraction.

 

Consider the following animation:

 

 

It shows events according to the embankment frame. When the end of the train and the red dots align, flashes of light expand outward and meet at the embankment observer. But what hasn't been mentioned yet is that since the train is moving in this frame, it is also length contracted. The train that fits just between the red dots in the embankment frame is a length contracted train.

 

If we switch to the frame of the train, it will be its normal proper length and the embankment (including the distance between the red dots) will be contracted. The train no longer fits between the two red dots and one end of the train aligns with its red dot before the other does. Since the flashes occur when the ends of the train and the red dots align, the flashes occur at different times as shown here.

 

 

Note that the flashes still meet at the embankment observer, and that the each flash reaches the train observer when he is next to the same point of the embankment as he was in the first animation.

 

The point is that the events that both frames must agree on are those that are co-located. Each flash must leave its red dot when one end of the train is next to it or one point of the train must be next to a given point of the embankment when the light from one strike arrives, Etc. Everything else follows from that. Once you establish how these events occur in one frame, the other frame follows suit.

 

And just because we start in one frame does not mean that it is special in any way. We could have done the same animations above where the red dot's align with the ends the train in the second animation and not the first so that the flashes originate simultaneously in the train frame, but they wouldn't be the same red dots on the same points spots of the embankment and they wouldn't be the same flashes of light, so you have just set up a different scenario rather than a different view of the same scenario.

Edited September 6, 2014 by Janus

[DimaMazin]

If by "classical" you mean that "the classic response of someone in denial is to say that you told me I was wrong while not showing me that I was wrong when in fact that person had been shown that he was wrong" then please show me where in this thread you showed that "the man on the embankment will see the two flashes of light at the same time while the man in the train will see the one flash of light before the other." Maybe I'm in such denial that that that logic is here in this thread but because I'm in such denial my eyes can't see it.

 

Please show it to me.

 

To say that I'm "classically" in denial is again to "tell" me that I'm wrong while not "showing" me that I'm wrong.

 

Thank you.

 

- Christopher

Still you didn't make math of simultaneity . I already made exact math of simultaneity(otherwise it can't be), but it contradicts experiments which prove length contraction of moving object. I have no right to "advertise" it , therefore I can show link of my topic,if you need, in personal message.

Edited September 7, 2014 by DimaMazin

[xyzt]

If by "classical" you mean that "the classic response of someone in denial is to say that you told me I was wrong while not showing me that I was wrong when in fact that person had been shown that he was wrong" then please show me where in this thread you showed that "the man on the embankment will see the two flashes of light at the same time while the man in the train will see the one flash of light before the other." Maybe I'm in such denial that that that logic is here in this thread but because I'm in such denial my eyes can't see it.

 

Please show it to me.

 

To say that I'm "classically" in denial is again to "tell" me that I'm wrong while not "showing" me that I'm wrong.

 

Thank you.

 

- Christopher

Here

[christopherkirkreves]

I have reread the entire thread.

 

Strange said:

 

1.

 

“We already know that times and distances are observer dependent and not necessarily the same for all observers. The thought experiment you are referring to (and making a bit of a hash of) is intended to demontrate that the concept of "at the same time" is also not the same for all observers. So if you say "at the same time" you need to specify which frame of reference this is in (I think that if you go back to Einstein's orginal description, you will find he does that).

 

Similarly, you need to specify which frame of reference you are referring to when you say where the lightning strikes happen (again, I think you will find Einstein does this).”

 

 

I disagree. The two lightning bolt strike flashes of light thought experiment is not merely meant to “demonstrate something already known” but rather to “demonstrate this is true.”

 

How did we already know that times and distances are observer dependent before it was demonstrated?

 

It had to, at some point, be first demonstrated. Michelson-Morley does not demonstrate this.

 

2.

“This contradicts what you previously said.

This has nothing to do with the movement (or otherwise) of the source. It is purely that the man on the train is moving towards (and away from) the place where the lightning strike happened. As you previously said.”

 

Yes. I set up the thought experiment as Einstein did and then walked to the conclusion that Einstein did. And then I went back and showed why this was false.

 

Yes. I am saying the way Einstein laid out the logic is incorrect. So, yes, I contradict my summarization of his logic, because it needs to be.

 

-

 

And, no, the man on the train is not moving towards the two flashes of light. If so, then this is a test of absolute rest. If there is no absolute rest then the man on the train can just as easily be considered as being at rest. And it is the flashes of light that are moving towards him.

 

Einstein states that the air above the embankment has been removed. And so once the lightning bolt strikes occur the are no more connected to the inertial frame of reference of the embankment as they are to the inertial frame of reference of the train.

 

From the perspective of the man on the train at rest (which Einstein says he is justified in believing) the two flashes of light come from two moving sources (the points of impact) from equal distances (as Einstein sets up this thought experiment to include) and at equal velocities.

 

And so, the man on the train will see the two flashes of light simultaneously.

 

3.

 

“Of course it isn't. This is a thought experiment, therefore it cannot prove or disprove anything. It is purely a teaching aid (failed in this case, apparently).”

One of the assumptions of the Lorentz transformations is that each inertial frame of reference has its own “time.”

 

This is counterintuitive.

 

And, so, how is Einstein justified is in making this counterintuitive assumption?

 

Because before the lays out the Lorentz Transformations in Chapter 11 he spends the first 10 chapters and first 33 pages of his book in laboriously laying out this thought experiment and proving that “events which are simultaneous in one inertial frame of reference are not simultaneous in another and therefore every inertial frame of reference has its own particular time.”

 

To quote him directly from pages 30 and 31:

“We thus arrive at the important result: Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train and vice versa (relativity of simultaneity). Every reference-bot (co-ordinate system) has its own particular time …”

 

If he’s not “proving” this “result” then why is it important? He is “proving” or “demonstrating” or “showing” or whatever word you want to use that this is true.

But he is wrong. The two flashes of light thought experiment does not “demonstrate” that events that are simultaneous in one inertial frame of reference are not simultaneous in another. Both the man on the embankment and the man on the train will see the two flashes of light at the same time. The two flashes of light thought experiment does not “show” that events that are simultaneous in one inertial frame of reference are not simultaneous in another. Both the man on the embankment and the man on the train will see the two flashes of light at the same time.

When I use the word “prove” when talking about this thought experiment I am not saying that this “proves” that this is how the physical world actually works. I’m saying that thought experiments are equivalent to empirical physical evidence.

 

But, yes, you can “prove” (or “demonstrate” or “show”) things are true with thought experiments … in the world of thought experiments. And that is what Einstein is doing. He is “demonstrating” or “showing” that “events that are simultaneous in one inertial frame of reference are not simultaneous in another” based on the two lightning bolt strikes thought experiment. He is not proving this is empirically physically true, but rather is it showing that it is true based on the premises of this thought experiment in the world of thought experiments.

But he is wrong. Both the man on the embankment and the man on the train will both see the two flashes of light at the same time.

 

4.

 

“I also don't get why you think your failure to understand something means that a very well-tested and practically useful theory should be wrong.”

 

Whether or not the physical world works according the ideas laid out in the Special and General Theories of Relativity is totally separate from whether or not the two lightning bolt strikes thought experiment works.

Someone can be wrong, but still get it right.

 

I am not questioning whether or not there are many, many tests that confirm the predictions of the Special and General Theories of Relativity. What I am point out is that the two flashes of light thought experiment does not work.

 

5.

 

“This pedagogical example is not intended to prove anything, merely to explain it.”

 

I totally disagree. For him to lay out the Lorentz Transformations with the implicit assumption that “every inertial frame of reference has its own particular time” is to lay out a counterintuitive assumption. And I suppose there is no rule against counterintuitive premises, but one should not. However, if one can “demonstrate” that a counterintuitive assumption is actually correct, then all the better. And, again, he takes 10 Chapters and 33 pages to do this.

 

But, again, his logic is flawed in this first 10 chapters and first 33 pages. Both men will see the two flashes of light at the same time.

 

---

 

xyzt said:

 

1.

 

“False, GR subsumes SR, it does not "rest on SR"”

 

“Subsumes” or “rests” I don’t care. The point is that you don’t get to the theory of General Relativity without the theory of Special Relativity.

One of the foundations of General Relativity is the thought experiment showing that acceleration and gravitation are equivalent (and later modified so say just so on a “local” level). And this is separate and independent of Special Relativity.

 

But the General Theory of Relativity also rests on the idea that because in a spinning disk the outer edges move faster than the inner portions to another body at rest to the spinning disk and so, given the conclusions of the Special Theory of Relativity, the clocks at the edge of this disk will run more slowly than clocks at the center of the disk and (quoting from page 90): “Thus o our circular dis, or, to make the case more general, in every gravitational field, a clock will go more quickly or les quickly, according to the position in which the clock is situated (at rest).”

 

You need the Special Theory of Relativity to get to the General Theory of Relativity, and, before all that, you need first show that “events which are simultaneous in one inertial frame of reference are not simultaneous in another inertial frame of reference” so be able to make the counterintuitive claim that “all inertial frames of reference have their own particular time” without simply asserting something counterintuitive as true.

 

2.

 

“False. SR rests on a set of axioms. Relativity of simultaneity is a CONSEQUENCE of those axioms.”

 

Yes. I agree. I misspoke.

 

I was summarizing.

 

I am sorry.

 

However, what this simply means is that after accepting the axioms of the Special Theory of Relativity the first part of that theory reasoned out (the two flashes of light thought experiment leading to the conclusion “events which are simultaneous in one inertial frame of reference are not simultaneous in another inertial frame of reference”) is incorrect.

 

And, so, it is not what the Special Theory of Relativity is actually resting on that needs to be replaced with Comstock’s thought experiment, but rather the first part of the Special Theory of Relativity, that then the rest of the Lorentz Transformations and the rest of the Special Theory of relativity is resting on, that needs to be replaced.

 

I misspoke.

 

I will try to be more precise in the futurre.

 

3.

 

 

 

“False. You simply misunderstand the thought experiment.

In addition, thought experiments cannot be used to disprove (falsify) a theory, only real experiments can do that.

As an aside, RoS is not testable. There is some debate about this issue in the philosophical (not physical circles) but the bottom line is that it isn't testable.”

Please show me why the man on the embankment will see the two flashes of light simultaneously while the man on the train will see one before the other. Please show why I misunderstand this thought experiment.

And, again, I’m not talking about the actual physical world. I am talking about whether or not this thought experiment works as a thought experiment in “showing” what it purports to show: that events which are simultaneous with respect to the embankment are not simultaneous with respect to the train.

 

----

 

Strange said:

 

“1. Thinking that you can prove or disprove anything with a thought experiment.

 

2. Thinking that your misunderstanding of a thought experiment has any significance.

 

3. Thinking that the theory is based on relativity of simultaneity

 

4. Thinking that your misunderstandings of a popular science book trump experimental evidence.

 

5. Oh, I give up. Like all the rest, you will just ignore all attempts at explanation and assume you know best. Good luck.

”

 

You and I disagree. I think that in the world of thought experiments you can prove things with thought experiments. Why did Einstein take 33 pages to “show” “demonstrate” that “events which are simultaneous with respect to the embankment are not simultaneous with respect to the train” and call this an “important result” (page 30) if he was just “illustrating” something and not “proving” something in the world of thought experiments?

 

I do believe that this theory is based on the relativity of simultaneity. If this was not “established” (“proved”) first then the premises of the Lorentz transformations that “every inertial frame of reference has its own particular time” would simply be counterintuitive without anything to back it up and therefore suspect.

 

Again, I am talking about Einstein’s argument about the two flashes of light in his book and not about experimental evidence or anything to do with the actual facts in the real world.

I hope I don’t appear to be ignoring you now. But maybe so. It may appear that I’m just typing in a bunch of words while ignoring the merits of you critique of me. I hope not. All I can do is try. I cannot control your impression of me.

 

-----

 

And so …

 

Before the point in the thread where I was ready to give up, and I said no one had shown me that I was wrong but only told me that I was wrong, I still do not see where anyone had in fact shown me that I was wrong about Einstein’s thought experiment being wrong. I do not see here anyone showing me that “the man on embankment will see the two flashes of light at the same time while the man on the train will see the one flash of light before the other.”

 

----

 

To Janus:

 

Thank you for taking up the challenge.

 

“Now if we switch to the train frame, we have to stay true to these facts: The light from each strike reaches the train observer at different times and when he is next to different points of the embankment, and his camera will have recorded two pictures confirming this.”

 

It sound to me like you are saying the same thing that Einstein said.

 

He wrote (page 30): “Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A.”

 

I think this is where he makes his mistake. He is thinking of the beams of light somehow being “in” or “more connected” to the inertial frame of reference of the embankment and “separate” and “independent” and “not connected, or less connected” to the inertial frame of reference of the train.

 

I don’t think you can say that the one flash of light will reach the man on the train before the other.

 

In fact, the embankment is totally unnecessary. If this thought experiment works then it could be two light bulbs rest in free space relative to one another man in free space. Then then there is a another lone man, from the perspective of the other three bodies, moving towards one light bulb and away from the other. But, from the perspective of the lone man, it is the other man and the two light bulbs that are in motion. And if it is stipulated that when the two light bulbs are an equal distance from the lone man and they flash at the same time (as it is in Einstein’s thought experiment) then the motion of the two light bulbs is irrelevant to the velocity at which the flashes of light travel to the man and so since they travel over equal distances the lone man will see the two flashes of light at the same time.

 

The embankment is irrelevant.

 

And if it is said that the lone man will see the one flash of light before the other because simultaneous events in one inertial frame of reference are not simultaneous in another, then we are back to circular logic as this thought experiment is meant to “demonstrate” or “show” or “prove” this conclusion (in the world of thought experiments).

 

If you think I misunderstood the point you were making, please let me know.

 

(The idea you bring up about each flash of light must pass the one man before the other man is very interesting. I’ve never thought about that before. But I don’t think that gets you to “events which are simultaneous in one inertial frame of reference are not simultaneous in another,” but rather perhaps to “events which are simultaneous in one inertial frame of reference are simultaneous in another, but from the perspective of both inertial frames of reference these same events are not simultaneous in the other inertial frame of reference” which is not what Einstein was going for. And, again, if we simply switch over to Comstock’s thought experiment we get to where Einstein was attempting to go.)

 

I’m going to skip over your thought on length contraction for now. Length contraction comes after the Lorentz Transformations. Before the Lorentz Transformations Einstein simply asserts: “A priori it is by no means certain that this last measurement will supply us with the same result as the first. Thus the length of the train as measured from the embankment may be different from that obtained by measuring in the train itself.” (page 33).

 

I have no problem with the Lorentz transformation and their conclusions once “events which are simultaneous in one inertial frame of reference are not simultaneous in another and therefore every inertial frame of reference has its own time” has been established, which Comstock’s thought experiment does and Einstein’s does not ... to my logic.

 

Again, thank you for taking the challenge, and please let me know if I overlooked an important point in your response.

 

Thank you.

 

- Christopher

Edited September 8, 2014 by christopherkirkreves

[christopherkirkreves]

(I only have two hours of internet time at the public library, and I made a lot of typos rushing through that long response yesterday.)

 

And one big one was: “I’m saying that thought experiments are equivalent to empirical physical evidence.” I meant to say the opposite: “I’m [not] saying that thought experiments are equivalent to empirical physical evidence.”

 

To DimaMazin:

 

Thank you for offering to send me your math, but I am totally unqualified to analyze it. I understand the concepts of the Special and General Theories of Relativity, but not the math.

 

And I know that some people say that the Theory of Relativity is all math based, and therefore I am opening myself up to the comment that I shouldn’t be saying anything about this theory or he concepts contained within it if I don’t understand the math.

 

But, oh well. Such a comment still would not show that “the man on the embankment will see the two flashes of light at the same time while the man on the train will see the one flash of light before the other.”

 

Thank you for your offer.

 

Cheers!

 

- Christopher