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#1 Tir21

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Posted 9 January 2017 - 02:15 PM

Simultaneity - Albert einstein and the theory of relativity. This video says that the person on the train will see lightning from front of the train first. But what if we take it that the train is stationary and the earth is moving below the train.

How could we tben explain the passenger on the train seeing the front bolt first
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#2 Lord Antares

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Posted 9 January 2017 - 02:31 PM

The speed of lights moves at a certain speed, however large. So of course it will be seen sooner by someone who is closer to the lightning bolt. Of course, this difference is imperceptable by humans and they would both agree to seeing it at the same time, but we are talking about technicalities, right?

Maybe I'm missing something because you haven't provided the video. There's not enough info to discuss more.


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#3 Tir21

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Posted 9 January 2017 - 02:39 PM

Thanks heres the video
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#4 swansont

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Posted 9 January 2017 - 02:40 PM

Simultaneity - Albert einstein and the theory of relativity. This video says that the person on the train will see lightning from front of the train first. But what if we take it that the train is stationary and the earth is moving below the train.

How could we tben explain the passenger on the train seeing the front bolt first

 

 

The conclusion from relativity is that strikes are not simultaneous in the train's frame if they are simultaneous in the earth's frame.


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#5 VandD

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Posted 11 January 2017 - 02:52 PM

 

 

The conclusion from relativity is that strikes are not simultaneous in the train's frame if they are simultaneous in the earth's frame.

Yest, but I think Tir21 wants to know how the thought experiment runs from the train passenger point of view. That's a different story.

I'll give it a try:

 

@Tir21

Very good question, Tir21.

Let's first start with Einstein's setup before jumping to your question. .

-The embankment observer sees both bolts simultaneously. For him, in his frame of reference, both lightning bolts happened simultaneously( lights from the bolts travel same length).

-The train passenger does not see both bolts simultaneously.

Why does this mean for him both bolts didn't happen simultaneously?

 

If one would state (as is often the case) it is because he moves toward the front bolt, we could throw in maybe also for the train observer both bolts happened simultaneously, but he only SEES the light from the events at a different moment in time because he moves towards the light coming fron the front bolt and thus sees that one first... No relativity of simultaneity in that scenario!

 

But in fact he does't see them simultanously because when he meets the embankment observer no lighting bolt occurs:

1. first the front bolt occurs (before the train passenger meets the embankment observer), 2. then both observers meet. No lighting bolt occurs.

3. Finally the rear bolt occurs, after both observers separated a split second ago.

(Note: in train frame both light will travel dame distance to train passenger)

 

Look at it this way: when train passenger and embankment passenger meet, both bolts happen simultaneously for the embankent observer. Now, IF both bolts would ALSO happen simultaneously for the train passener when both observers meet, then the train passenger HAS to SEE both bolts also simultaneously, because in his frame the lights travelling from the simultaneous events also travel same distance (not necessarily same distance as in embankment frame, but that is irrellevent because only the front and rear distance in the train frame is important for the train observer analysis!), and hence would reach him simultaneously. But.... because the train observer sees both bolts not simultaneously he concludes: "For me (in my frame), lighting bolts occuring simultaneously when I and embankment observer meet can not be correct".

We know the train passenger indeed does not see the light from both bolts simultaneously, because that's what's experimentally observed bij the embankment observer. Receiving light from one bolt after recieving light from the other bolt are two different events. And that cannot change for the train passenger.

 

 

Now we get to your question: a thought experiment from the train passenger's point of view for simultaneous lighting bolts in the embakment frame.

Both bolts occur simultaneously in the embankment frame. Lights from the bolts do not reach the train passenger simultaneously (=two events). That's what he will physically experience and observe. Your question now is: how could the train passenger EXPLAIN seeing the front bolt first?

Train passenger knows the set up: in embankment frame both bolts occur when both observers meet. And lights hit embankment observer simultaneously (=one event).

Now train passenger he reasons as follows. "IF they ALSO occur simultaneously for me when I meet embankment observer, then they are also at same distance from me, hence both lights from bolts HAVE TO reach me simultaneously. But the lights from the bolts do NOT reach me simultanously. It can only be because for me (in my frame), lighting bolts occuring simultaneously when I and embankment observer meet can not be correct"

 

Is there another option?

The train observer might think for him both bolts DID occur simultaneously, but both lighning bolts were not at the same distance from him... hence both lights do not reach him simultaneously...

But that's not a valid option. Because it would mean

the three events

1/ front lightning bolt

2/ both observers meet

3/ rear lightning bolt

occur simultaneously in train AND embankment frame, but the embankment observer considers both observers halfway between the two simultaneous lightning bolts, whereas the train observer considers both observer NOT halfway between simultaneous bolts...

You might think it's a valid option, but there is no theory to explain it.

 

-------

Your question might be: if we consider the train at rest end the embankment moving, why does the train passenger knows he will not NOT see both bolts simultaneously (lights from the bolts not reaching him simultaneously)?

Or better formulated, why HAS IT to be that way (not seeing them simultaneously)?

The only 'thought experiment' reasoning we can make is: he and the embankment observer know it happens that way, because in the embankment frame the bolts reaching the train observer are two different events and they can not melt together into one event in the other frame.

And the best way to find out is doing the experiment. Both lights from bolts will and do not reach the train observer. But as a thought experiment one has also to wonder why ;-).


Edited by VandD, 11 January 2017 - 02:58 PM.

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#6 swansont

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Posted 11 January 2017 - 03:15 PM

Yest, but I think Tir21 wants to know how the thought experiment runs from the train passenger point of view. That's a different story.

 

 

Perhaps a different story, but it had better end up with the same answer.


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#7 geordief

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Posted 11 January 2017 - 03:51 PM

My take on this is that simultaneity only occurs when two events are separated by zero space and zero time (a rare occurrence).

So, by my definition ,in this particular scenario neither observer can say that the 2 events are truly "simultaneous"

They can only claim they are simultaneous in so far as they respectively observe them as such,which is why they disagree.

If events are truly "simultaneous" according to my "higher" standard ,all observers will agree (perhaps trivially?)
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#8 swansont

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Posted 11 January 2017 - 04:15 PM

My take on this is that simultaneity only occurs when two events are separated by zero space and zero time (a rare occurrence).

So, by my definition ,in this particular scenario neither observer can say that the 2 events are truly "simultaneous"

They can only claim they are simultaneous in so far as they respectively observe them as such,which is why they disagree.

If events are truly "simultaneous" according to my "higher" standard ,all observers will agree (perhaps trivially?)

 

 

That's not the relativity take on it. Same place and same time are required for an unambiguous determination. Same time depends on your frame if the events are not co-located. Hence the relativity of simultaneity.


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#9 geordief

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Posted 11 January 2017 - 04:54 PM

That's not the relativity take on it. Same place and same time are required for an unambiguous determination. Same time depends on your frame if the events are not co-located. Hence the relativity of simultaneity.


Well my take was no doubt simplistic and not particularly useful or helpful here (just a special case I guess)

Edited by geordief, 11 January 2017 - 04:55 PM.

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#10 zztop

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Posted 11 January 2017 - 05:30 PM

Well my take was no doubt simplistic and not particularly useful or helpful here (just a special case I guess)

Two events are separated by the amount of time \Delta t and by the space \Delta x in frame F.

In frame F', the two events are separated by the time interval \Delta t'=\gamma(\Delta t -v \Delta x/c^2).

 

Now, if the events are simultaneous in F, it means that \Delta t=0.

 

This means that , in frame F', the events are separated by the time interval:

 

\Delta t'=-\gamma v \Delta x/c^2.

 

You can have \Delta t'=0 if \Delta x=0.

 

Another way of looking at it is : if \Delta x=0 then \Delta t'=\gamma\Delta t , so, in this PARTICULAR case,  simultaneity is preserved across frames of reference.


Edited by zztop, 11 January 2017 - 05:32 PM.

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#11 VandD

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Posted 12 January 2017 - 09:17 AM

My take on this is that simultaneity only occurs when two events are separated by zero space and zero time (a rare occurrence).


Two events separated by zero space and zero time are a contradiction in terms. Two events mean space separation and/or time separation. You cannot melt two events into one.
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#12 geordief

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Posted 12 January 2017 - 11:22 AM

Two events separated by zero space and zero time are a contradiction in terms. Two events mean space separation and/or time separation. You cannot melt two events into one.

Well ,we can as a limit and from an observer's frame of reference can't we?

 

But I have agreed it is of little consequence.


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#13 Phi for All

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Posted 1 February 2017 - 04:41 PM

!

Moderator Note

Hijack and subsequent replies split off to their own thread in Speculations.


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#14 swansont

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Posted 2 February 2017 - 02:37 PM

!

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Posts asking about the split have been moved to support, because really?


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#15 madmac

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Posted 4 February 2017 - 01:21 AM

Tir21.

I see that no-one has answered your question (the first post in the thread).

Einstein in his 1905 article on relativity doesn't mention a train thought experiment, but he does analyse the same situation using imaginary rays of light & clocks & co-ordinate systems (with no paid observer as such), and somewhat later says .....

 

-- "It is clear that the same results hold good of bodies at rest in the stationary system, viewed from a system in uniform motion" --

 

although here he is talking moreso about his next idea down the line, & not forgetting that this is only an English translation of his German. 

But i have never seen a detailed analysis confirming the same results when viewed as u say by the passenger in a carriage at rest & with the platform & embankment moving past.


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#16 VandD

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Posted 4 February 2017 - 01:22 PM

Tir21.

I see that no-one has answered your question (the first post in the thread).

You probably missed (or didn't fully understand) my post #5


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#17 madmac

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Posted 4 February 2017 - 09:30 PM

VanD.

Yes i had read your post, sorry i forgot.

I think that the video showing a short bullet-train with observer on platform approx. L/2 from train & with a non-understandable explanation of events, indeed a misleading explanation, is impossible to analyse. Properly explained it would be possible, albeit having to use non-zero Y co-ordinate info (messy).

 

But if u move the platform-observer closer in so that he is kissing the window u now have Einstein's original thought experiment. Now its easy. U simply look at the whole thing in mirror-image, ie looking from the train-observer's point of view. In her frame she sees the 2 flashes simultaneously, & the platform-observer moving past, etc etc etc. Too easy.


Edited by madmac, 4 February 2017 - 09:32 PM.

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#18 pzkpfw

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Posted 4 February 2017 - 10:34 PM

My underline ...

 



VanD.

Yes i had read your post, sorry i forgot.

I think that the video showing a short bullet-train with observer on platform approx. L/2 from train & with a non-understandable explanation of events, indeed a misleading explanation, is impossible to analyse. Properly explained it would be possible, albeit having to use non-zero Y co-ordinate info (messy).

 

But if u move the platform-observer closer in so that he is kissing the window u now have Einstein's original thought experiment. Now its easy. U simply look at the whole thing in mirror-image, ie looking from the train-observer's point of view. In her frame she sees the 2 flashes simultaneously, & the platform-observer moving past, etc etc etc. Too easy.

 

No, she doesn't. Her experience (of whether the flashes reach her at the same time) can't contradict the platform observers experience (of whether the flashes reach her at the same time). Given the setup, the events were not simultaneous, for her.

 

Based on http://www.bartleby.com/173/9.html , it boils down to:

(warning : in the past I've found the bartleby website to do "dodgy" things; found a better source once but lost it).

 

1. Both observers are entitled to consider themselves as at rest (and the other observer moving).

2. Both observers, given the initial setup, are entitled to consider themselves as located between the two events.

3. Either observer, seeing the flashes at the same time, will then consider the events to be simultaneous.

4. The flashes are seen at the same time by the embankment observer, so the events happen to be simultaneous for them.

5. As shown from the embankment observers point of view, the flashes don't reach the train observer at the same time.

6. The flashes can't reach the train observer at the same time and also not reach the train observer at the same time.

7. So, the train observers experience can't contradict the embankment observers experience.

8. So the events were not simultaneous for the train observer.

9. Vice versa, two different events, which happen to be simultaneous for the train observer, won't be for the embankment observer.

 

Notes on the above 

 

1. In the original thought experiment Einstein is clear to say "Then every event which takes place along the line also takes place at a particular point of the train."  Note the lines for A and B drawn across both the track and the embankment. People sometimes think events happen in some frame of reference. That's not right, events happen in all frames. The event of an ice cream hitting the footpath occurs whether I'm standing still next to the person who dropped it, or I'm riding by on my motorcycle. However, Einstein tries to remove that confusion here.

 

6. The most common way to show this, is with the train observer holding a bomb that goes off if two detectors facing forward and back detect the flashes at the same time. The bomb will either explode or not. From the embankment observers point of view it's clear the flashes can't reach the bomb at the same time, so it won't explode. There's no alternate Universe where the flashes from the same two events reach the bomb at the same time and it does explode.

 

Another way is for the train observer to be holding two more mirrors, to reflect both flashes to the embankment observer. Consider whether the embankment observer would see those reflected flashes at the same time or not.


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#19 madmac

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Posted 5 February 2017 - 12:15 AM

pzkpfw.

I don't follow.

If one takes Einstein's exact description of his train thought experiment, then u or i should be able to describe the events from the train-passengers viewpoint, simply by changing Einstein's wording, eg replacing embankment with train, & train with embankment, etc etc, & u reach the same conclusions. 

 

In this mirror-image sort of analysis the main player becomes the train-passenger, & she (its a she in the video) sees the flashes of any simultaneous lightning strikes simultaneously if she is midway (which she is). And of course here it doesn't make any difference whether the strikes hit the embankment (Einstein) or the train (video)(not that anyone said that it did, but just saying).

 

The bomb paradox sort of thing never comes up in my mind, it only comes up if one believes that Einstein's time dilation & length dilation are real, which they are not.  

 

I almost remember the words of a Phd who is a solid Einsteinian & who believes that the M&M result was null etc etc (yes, the whole catastrophe), an expert at math, but  wrote one of the most profound things about SR that i have seen, saying --

 SR is merely a code for deciphering non-local information.

These are my words, i don't remember the exact words & i cant find the article.

The tricks & contortions in Einstein's code are there to give correct information, not to describe reality.


Edited by madmac, 5 February 2017 - 12:27 AM.

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#20 pzkpfw

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Posted 5 February 2017 - 12:32 AM

pzkpfw.
I don't follow.
If one takes Einstein's exact description of his train thought experiment, then u or i should be able to describe the events from the train-passengers viewpoint, simply by changing Einstein's wording, eg replacing embankment with train, & train with embankment, etc etc, & u reach the same conclusions.


For two different flashes, sure. He makes that clear. That's what he means by the "vice versa" at the end of that section.

In the experiment, it's stipulated that the two events were simultaneous for the embankment observer, then asks if they were also simultaneous for the train observer.

You could equally stipulate two events that are simultaneous for the train observer and ask if they are also simultaneous for the embankment observer.

Either way ... it's shown that the same two events can't be simultaneous for both observers.
 

In this mirror-image sort of analysis the main player becomes the train-passenger, & she (its a she in the video) sees the flashes of any simultaneous lightning strikes simultaneously if she is midway (which she is).


You are making the mistake of assuming that if the two events simultaneous for anyone, that they are simultaneous for everyone. That'd be a common (pre-relativity) assumption, but it's exactly what the experiment disproves. That two events are simultaneous of not depends on the point of view of the observer. Simultaneity is relative.

Two different events may be simultaneous for the train observer - but they won't be the same two events that were simultaneous for the embankment observer.

The point is: simultaneity is relative.
Not: simultaneity is absolute, but people have different views.

And of course here it doesn't make any difference whether the strikes hit the embankment (Einstein) or the train (video)(not that anyone said that it did, but just saying).


Good, then you're not confused by something that misleads a lot of people.

The bomb paradox sort of thing never comes up in my mind, it only comes up if one believes that Einstein's time dilation & length dilation are real, which they are not.


What?? Time dilation & length dilation are irrelevant here.

It simply shows that if, according to the embankment observer, the flashes reach the train observer at different times, then also according to the train observer the flashes reach them (the train observer) at different times.
 

I remember the words of a Phd who is a solid Einsteinian & who believes that the M&M result was null etc etc, & he is expert at math, yet he wrote one of the most profound things about SR that i have seen, he said that SR was merely a code for deciphering non-local information.
The tricks & contortions in the code are there to give the correct information, not to describe reality.


That's an incorrect interpretation. The effects of relativity are as real as anything.

No one can say that the view of the embankment observer or train observer is more correct or more real. Both of their views of the Universe are "real" - even though they get different answers for whether two events happen to be simultaenious or not.
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