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scottrwilson
Post  Post subject: Is Einstein’s thought experiment about simultaneity flawed?  |  Posted: Tue Dec 20, 2016 10:43 am

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It appears to me that Einstein’s thought experiment about simultaneity is flawed. Can anyone help resolve this?

Here is what he says, in: Relativity: The Special and General Theory. 1920.

To summarise:

A train is moving at high speed with a passenger at the precise centre. It passes a stationery observer. At the same moment that the passenger is alongside the observer, lightning strikes simultaneously at both ends of the train. The observer will notice the flashes to be simultaneous, because he is positioned equal distances from where the lightning struck and so the time taken for the flashes to reach him will be equal.
But the passenger, so Einstein argues, will not see the flashes simultaneously, because while the light travels to her, she is moving closer to one of the flashes and so will be slightly closer to one, but further from the other. He deduces the strikes cannot be considered simultaneous from the perspective of the passenger.

But, that appears to me to be nonsense, because we know that light takes time to travel, so we know that the passenger will not necessarily see them simultaneously, even if they were simultaneous.
The only conclusion the passenger could make is that, due to being in motion, they cannot know whether the lightning was simultaneous.
How does Einstein’s thought experiment show that simultaneity is relative? It only shows that our observation of simultaneity is relative.


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PhDemon
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 11:29 am

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Einstein's thought experiment is a simplification for general readers based on the maths of relativity. Unless you can spot a flaw in his maths you are on to a loser...

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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 12:48 pm

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PhDemon wrote:
Einstein's thought experiment is a simplification for general readers based on the maths of relativity. Unless you can spot a flaw in his maths you are on to a loser...


Point taken. Is that to say the theory can only be properly understood and explained using the maths, or does there exist a thought experiment or indeed any non-mathematical explanation which explains how simultaneity, space and time are relative.


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PhDemon
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 12:51 pm

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Although there are many thought experiments and explanations for non-mathematicians out there it is a deeply mathematical theory, any attempt to explain it with thought experiments or without the maths necessarily involves some simplification. I don't fully understand it but I've studied enough and spoken to enough specialists to know it is the current best explanation we have.

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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 1:01 pm

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PhDemon wrote:
Although there are many thought experiments and explanations for non-mathematicians out there it is a deeply mathematical theory, any attempt to explain it with thought experiments or without the maths necessarily involves some simplification. I don't fully understand it but I've studied enough and spoken to enough specialists to know it is the current best explanation we have.


Understood, but does simplification necessarily require all such explanations to be so obviously flawed. Indeed, what is the point of any explanation if it is so obviously flawed? Do you know any sensible explanations?

Your advice is appreciated.


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PhDemon
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 1:31 pm

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I don't think it is flawed (even though it is simplified), I think you are misunderstanding it. I'm not very good at explaining these things in short forum posts (Janus is the master there). Also I'm overly cynical and tend not to spend a lot of time on people questioning relativity as I've encountered a lot of crackpots! :lol:

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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 1:52 pm

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PhDemon wrote:
I don't think it is flawed (even though it is simplified), I think you are misunderstanding it. I'm not very good at explaining these things in short forum posts (Janus is the master there). Also I'm overly cynical and tend not to spend a lot of time on people questioning relativity as I've encountered a lot of crackpots! :lol:


I have a serious interest, but thanks anyway.


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geordief
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 5:08 pm

Joined: Wed Oct 01, 2014 10:45 pm
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scottrwilson wrote:
It appears to me that Einstein’s thought experiment about simultaneity is flawed. Can anyone help resolve this?

Here is what he says, in: Relativity: The Special and General Theory. 1920.

To summarise:

A train is moving at high speed with a passenger at the precise centre. It passes a stationery observer. At the same moment that the passenger is alongside the observer, lightning strikes simultaneously at both ends of the train. The observer will notice the flashes to be simultaneous, because he is positioned equal distances from where the lightning struck and so the time taken for the flashes to reach him will be equal.
But the passenger, so Einstein argues, will not see the flashes simultaneously, because while the light travels to her, she is moving closer to one of the flashes and so will be slightly closer to one, but further from the other. He deduces the strikes cannot be considered simultaneous from the perspective of the passenger.

But, that appears to me to be nonsense, because we know that light takes time to travel, so we know that the passenger will not necessarily see them simultaneously, even if they were simultaneous.
The only conclusion the passenger could make is that, due to being in motion, they cannot know whether the lightning was simultaneous.
How does Einstein’s thought experiment show that simultaneity is relative? It only shows that our observation of simultaneity is relative.


Is there a problem with your analysis?

You say "It only shows that our observation of simultaneity is relative."

That sounds fair.Different observers in different moving frames of reference will always disagree as to whether 2 events are simultaneous.

Perhaps you feel that the 2 events are "really" simultaneous ,but only one of the observers can see it? It would be a mistake to think that ,in my opinion.

The two events ,as described are neither "really" simultaneous nor nonsimultaneous. It is all in the observation.

Is that any help?


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janus
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Tue Dec 20, 2016 7:01 pm

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scottrwilson wrote:
It appears to me that Einstein’s thought experiment about simultaneity is flawed. Can anyone help resolve this?

Here is what he says, in: Relativity: The Special and General Theory. 1920.

To summarise:

A train is moving at high speed with a passenger at the precise centre. It passes a stationery observer. At the same moment that the passenger is alongside the observer, lightning strikes simultaneously at both ends of the train. The observer will notice the flashes to be simultaneous, because he is positioned equal distances from where the lightning struck and so the time taken for the flashes to reach him will be equal.
But the passenger, so Einstein argues, will not see the flashes simultaneously, because while the light travels to her, she is moving closer to one of the flashes and so will be slightly closer to one, but further from the other. He deduces the strikes cannot be considered simultaneous from the perspective of the passenger.

But, that appears to me to be nonsense, because we know that light takes time to travel, so we know that the passenger will not necessarily see them simultaneously, even if they were simultaneous.
The only conclusion the passenger could make is that, due to being in motion, they cannot know whether the lightning was simultaneous.
How does Einstein’s thought experiment show that simultaneity is relative? It only shows that our observation of simultaneity is relative.


A quick post to point out what you are missing. The strikes are simultaneous for the embankment observer because he is equidistant from each strike and he knows that the light, traveling at c, took equal times to reach him. Thus seeing the strikes at the same time means they originated at the same time.

We also know that the train observer will be next to different points of the tracks when the light from each strike reaches him. This is something that the train observer must agree on, and since he can't be next to two different points of the tracks at the same time, he must see the light from the strikes at different times.

The strikes hit the ends of the train, which are an equal distance from his position of the Train.

Now here's the critical bit that you are missing; According to the postulates of Relativity, the speed of light is invariant. And what this means is that everyone, regardless of their relative speed to each other or to the light source, measures the same value for the speed of light relative to themselves. Thus according to the train observer, the light from both strikes come at him at c. He is an equal distance from the pats of the train where the strikes occurred, so the time spent by the light traveling from each end must be the same. Thus since he sees the strikes at different times, the strikes must have occurred at different times.

The point is that the train experiment illustrates what we would expect to happen if the postulates of Relativity hold. Your perceived flaw is a result of your not adhering to these postulates. You are a assuming a different model for the behavior of light than Einstein does.

Now, in of itself, the train experiment does not prove the relativity of simultaneity as its results depend on the validity of the postulates. However, the postulates also lead to other testable conclusions that we can and have verified. And from this we can conclude that the train experiment is a valid conclusion. For example, we can verify time dilation. But time dilation can only be a part of a self-consistent reality that also includes length contraction and the relativity of simultaneity. So by extension, if time dilation happens, so does the relativity of simultaneity.


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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 10:26 am

Joined: Tue Dec 20, 2016 10:33 am
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janus wrote:
scottrwilson wrote:
It appears to me that Einstein’s thought experiment about simultaneity is flawed. Can anyone help resolve this?

Here is what he says, in: Relativity: The Special and General Theory. 1920.

To summarise:

A train is moving at high speed with a passenger at the precise centre. It passes a stationery observer. At the same moment that the passenger is alongside the observer, lightning strikes simultaneously at both ends of the train. The observer will notice the flashes to be simultaneous, because he is positioned equal distances from where the lightning struck and so the time taken for the flashes to reach him will be equal.
But the passenger, so Einstein argues, will not see the flashes simultaneously, because while the light travels to her, she is moving closer to one of the flashes and so will be slightly closer to one, but further from the other. He deduces the strikes cannot be considered simultaneous from the perspective of the passenger.

But, that appears to me to be nonsense, because we know that light takes time to travel, so we know that the passenger will not necessarily see them simultaneously, even if they were simultaneous.
The only conclusion the passenger could make is that, due to being in motion, they cannot know whether the lightning was simultaneous.
How does Einstein’s thought experiment show that simultaneity is relative? It only shows that our observation of simultaneity is relative.


A quick post to point out what you are missing. The strikes are simultaneous for the embankment observer because he is equidistant from each strike and he knows that the light, traveling at c, took equal times to reach him. Thus seeing the strikes at the same time means they originated at the same time.

We also know that the train observer will be next to different points of the tracks when the light from each strike reaches him. This is something that the train observer must agree on, and since he can't be next to two different points of the tracks at the same time, he must see the light from the strikes at different times.

The strikes hit the ends of the train, which are an equal distance from his position of the Train.

Now here's the critical bit that you are missing; According to the postulates of Relativity, the speed of light is invariant. And what this means is that everyone, regardless of their relative speed to each other or to the light source, measures the same value for the speed of light relative to themselves. Thus according to the train observer, the light from both strikes come at him at c. He is an equal distance from the pats of the train where the strikes occurred, so the time spent by the light traveling from each end must be the same. Thus since he sees the strikes at different times, the strikes must have occurred at different times.

The point is that the train experiment illustrates what we would expect to happen if the postulates of Relativity hold. Your perceived flaw is a result of your not adhering to these postulates. You are a assuming a different model for the behavior of light than Einstein does.

Now, in of itself, the train experiment does not prove the relativity of simultaneity as its results depend on the validity of the postulates. However, the postulates also lead to other testable conclusions that we can and have verified. And from this we can conclude that the train experiment is a valid conclusion. For example, we can verify time dilation. But time dilation can only be a part of a self-consistent reality that also includes length contraction and the relativity of simultaneity. So by extension, if time dilation happens, so does the relativity of simultaneity.


I think the crucial misunderstanding here might be regarding what is assumed by Einstein as part of the thought experiment. My understanding is that it is supposed to serve as a non-mathematical description of how Einstein reaches his postulates. You (I think) are saying it’s the other way around, that we must assume the postulates hold before we can agree with the thought experiment.

Having read the article very carefully, however, it does appear quite clear that the former is true. He describes “the law of the propagation of light in vacuo” (as being constant) as one of the very few assumptions, prior to the thought experiment. He does not appear to mention his postulates as prior assumptions to the experiment.


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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 10:54 am

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geordief wrote:
scottrwilson wrote:
It appears to me that Einstein’s thought experiment about simultaneity is flawed. Can anyone help resolve this?

Here is what he says, in: Relativity: The Special and General Theory. 1920.

To summarise:

A train is moving at high speed with a passenger at the precise centre. It passes a stationery observer. At the same moment that the passenger is alongside the observer, lightning strikes simultaneously at both ends of the train. The observer will notice the flashes to be simultaneous, because he is positioned equal distances from where the lightning struck and so the time taken for the flashes to reach him will be equal.
But the passenger, so Einstein argues, will not see the flashes simultaneously, because while the light travels to her, she is moving closer to one of the flashes and so will be slightly closer to one, but further from the other. He deduces the strikes cannot be considered simultaneous from the perspective of the passenger.

But, that appears to me to be nonsense, because we know that light takes time to travel, so we know that the passenger will not necessarily see them simultaneously, even if they were simultaneous.
The only conclusion the passenger could make is that, due to being in motion, they cannot know whether the lightning was simultaneous.
How does Einstein’s thought experiment show that simultaneity is relative? It only shows that our observation of simultaneity is relative.


Is there a problem with your analysis?

You say "It only shows that our observation of simultaneity is relative."

That sounds fair.Different observers in different moving frames of reference will always disagree as to whether 2 events are simultaneous.

Perhaps you feel that the 2 events are "really" simultaneous ,but only one of the observers can see it? It would be a mistake to think that ,in my opinion.

The two events ,as described are neither "really" simultaneous nor nonsimultaneous. It is all in the observation.

Is that any help?

Is that to suggest that reality can only be assumed to be exactly what it appears to be? For example, when I see a flash of light, I should assume the source of the flash actually emitted the light at the same time that I saw it, i.e. that no time elapsed in between. Then that contradicts the finite speed of light, which is explicitly assumed within Einstein’s thought experiment.


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geordief
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 11:38 am

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scottrwilson wrote:
Is that to suggest that reality can only be assumed to be exactly what it appears to be? For example, when I see a flash of light, I should assume the source of the flash actually emitted the light at the same time that I saw it, i.e. that no time elapsed in between. Then that contradicts the finite speed of light, which is explicitly assumed within Einstein’s thought experiment.

No,of course it is not true that "reality can only be assumed to be exactly what it appears to be" (your quote).

It is not what I meant when I said,perhaps sloppily "It is all in the observation." (my quote)

The mechanics of observation seem very pertinent though to this scenario,particularly when two observers** disagree as to two events being simultaneous.

Disclaimer: I am very far from authoritative in this subject. Like you ,no doubt and others I am interested in and generally defer to what those with a firmer ,surer and deeper grasp and experience of the subject have to say.

** in 2 different and relatively moving frames of reference


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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 11:58 am

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geordief wrote:
scottrwilson wrote:
Is that to suggest that reality can only be assumed to be exactly what it appears to be? For example, when I see a flash of light, I should assume the source of the flash actually emitted the light at the same time that I saw it, i.e. that no time elapsed in between. Then that contradicts the finite speed of light, which is explicitly assumed within Einstein’s thought experiment.

No,of course it is not true that "reality can only be assumed to be exactly what it appears to be" (your quote).

It is not what I meant when I said,perhaps sloppily "It is all in the observation." (my quote)

The mechanics of observation seem very pertinent though to this scenario,particularly when two observers** disagree as to two events being simultaneous.

Disclaimer: I am very far from authoritative in this subject. Like you ,no doubt and others I am interested in and generally defer to what those with a firmer ,surer and deeper grasp and experience of the subject have to say.

** in 2 different and relatively moving frames of reference


That's the problem with Einstein's experiment. He does appear to be taking what the moving observer observes as an explanation of why simultaneity is relative. But if such an observer knows they are moving towards a lightning strike, or even if they don't know, it would be wrong of them to deduce that the lightning strikes were not simultaneous, simply from the fact that they see flashes which are not simultaneous, as Einstein asserts.


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geordief
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 12:27 pm

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scottrwilson wrote:

That's the problem with Einstein's experiment. He does appear to be taking what the moving observer observes as an explanation of why simultaneity is relative. But if such an observer knows they are moving towards a lightning strike, or even if they don't know, it would be wrong of them to deduce that the lightning strikes were not simultaneous, simply from the fact that they see flashes which are not simultaneous, as Einstein asserts.

As I said I prefer to defer to others more capable so as not to misspeak and mislead but I feel that Special Relativity has proved its grounding by the amount of ongoing predictions it has made.

I generally feel that when a particular part of the theory is hard to get my head around it is down to my obtuseness and my inability to juggle with more than one idea at a time.

Even when I have learned a lesson to my satisfaction ,I will likely forget it and so it can be a case of one step forward and two steps back at times.

What is "simultaneity" anyway? How would do you define it starting from basic principles?


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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 1:13 pm

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geordief wrote:
What is "simultaneity" anyway? How would do you define it starting from basic principles?


A good question, but Einstein's article defines it as a person who has measured himself to be equal distances from 2 points, then who sees lightning strike those 2 points and sees them simultaneously. I think that is straight forward and satisfactory. He knows they are simultaneous because he knows the light travels the same distance to him at the same speed from each strike.

http://www.bartleby.com/173/

Chapter 8


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iNow
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 1:17 pm
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Invariance of the speed of light is from Maxwell, not Einstein.

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scottrwilson
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 1:24 pm

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iNow wrote:
Invariance of the speed of light is from Maxwell, not Einstein.


I know. Einstein uses it as one of his assumptions.


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geordief
Post  Post subject: Re: Is Einstein’s thought experiment about simultaneity flaw  |  Posted: Wed Dec 21, 2016 2:14 pm

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scottrwilson wrote:
geordief wrote:
What is "simultaneity" anyway? How would do you define it starting from basic principles?


A good question, but Einstein's article defines it as a person who has measured himself to be equal distances from 2 points, then who sees lightning strike those 2 points and sees them simultaneously. I think that is straight forward and satisfactory. He knows they are simultaneous because he knows the light travels the same distance to him at the same speed from each strike.

http://www.bartleby.com/173/

Chapter 8

And this definition ,it seems only applies within the same frame of reference.Is there a definition that "spans" * all the different inertially moving frames of reference ?(apparently not)

All observers can agree on some things** but not on this .

* the definition does "span" but gives different outcomes.

**eg spacetime intervals....


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