Problem with Relativity/Simultaneity (1 Viewer)

[andysoul]

Hey all,
I've got this serious problem with Einstein's 'Train thought Experiment'. you know the one where there is an observer standing in the middle of a train moving at high velocity, which passes an observer who is relitavely stationary on a platform, at the same moment they pass, lightning strikes both ends of the carriage (or the platform) simultaneously. the question then asks you to 'analyse the statements of both observers regarding the order of the lightning strike'.

Now, the explanation given in our textbook (physics context two) is highly unsatisfactory, as the diagrams don't agree with the writing. However the gist of it is clear, that the person in the train would see the forward strike first, because they have moved, TOWARD the light in the time it takes the light to travel to him or her. and the observer on the platform would see both strikes at the same time because they are not moving. HOWEVER, i disagree with this explanation.

here follows my explanation:
because both observers are in an inertial frame of reference, they would see the light approach them at 'c', this means that because the light has to travel the same distance from either end of the carriage, that they would BOTH see the light from each end simultaneously. I also will however concede that the observers will disagree with what each thought the other should have seen. the observer on the platform will think that the observer on the train should have seen the forward lightning strike first, and the observer on the train will think that the observer on the platform will have seen the rear lightning strike first.

Now, if you understood any of that, lol, i ask you to either pick holes in my argument or advise me not to disagree with what the Board of Studies thinks is correct.

thanks

 

[Tommy_Lamp]

This whole concept of Special Relativity is very complicated, and in my opinion, it is better to accept what you are taught and move on, rather then try and make a rational interpretation of it, all that's going to do is make it harder for you.

 

[JayWalker]

Hey all,
I've got this serious problem with Einstein's 'Train thought Experiment'. you know the one where there is an observer standing in the middle of a train moving at high velocity, which passes an observer who is relitavely stationary on a platform, at the same moment they pass, lightning strikes both ends of the carriage (or the platform) simultaneously. the question then asks you to 'analyse the statements of both observers regarding the order of the lightning strike'.

Now, the explanation given in our textbook (physics context two) is highly unsatisfactory, as the diagrams don't agree with the writing. However the gist of it is clear, that the person in the train would see the forward strike first, because they have moved, TOWARD the light in the time it takes the light to travel to him or her. and the observer on the platform would see both strikes at the same time because they are not moving. HOWEVER, i disagree with this explanation.

here follows my explanation:
because both observers are in an inertial frame of reference, they would see the light approach them at 'c', this means that because the light has to travel the same distance from either end of the carriage, that they would BOTH see the light from each end simultaneously. I also will however concede that the observers will disagree with what each thought the other should have seen. the observer on the platform will think that the observer on the train should have seen the forward lightning strike first, and the observer on the train will think that the observer on the platform will have seen the rear lightning strike first.

Now, if you understood any of that, lol, i ask you to either pick holes in my argument or advise me not to disagree with what the Board of Studies thinks is correct.

thanks

you state that the text book says this?
"that the person in the train would see the forward strike first, because they have moved, TOWARD the light in the time it takes the light to travel to him or her".

If that is true, then you are indeed correct, that is not true. They have got it back the front.

Because the person IN the train is in his own inertial frame of reference, He will see the light hit both doors at the same time.
The person on the PLATFORM, will see the front door open first because as the light is emitted, the train is moving to the right (or what ever direction you choose it to be, right is preferable due to positive direction).

Summary:
Observation of person on train: Light hit both doors same time
Observation of person on platform: Light hit front door and THEN second door

Hence the proposal that simultaneity (spelling?) is relative to the observer and their frame of reference..

Understand?

 

[zeropoint]

Hey all,
I've got this serious problem with Einstein's 'Train thought Experiment'. you know the one where there is an observer standing in the middle of a train moving at high velocity, which passes an observer who is relitavely stationary on a platform, at the same moment they pass, lightning strikes both ends of the carriage (or the platform) simultaneously. the question then asks you to 'analyse the statements of both observers regarding the order of the lightning strike'.

Now, the explanation given in our textbook (physics context two) is highly unsatisfactory, as the diagrams don't agree with the writing. However the gist of it is clear, that the person in the train would see the forward strike first, because they have moved, TOWARD the light in the time it takes the light to travel to him or her. and the observer on the platform would see both strikes at the same time because they are not moving. HOWEVER, i disagree with this explanation.

here follows my explanation:
because both observers are in an inertial frame of reference, they would see the light approach them at 'c', this means that because the light has to travel the same distance from either end of the carriage, that they would BOTH see the light from each end simultaneously. I also will however concede that the observers will disagree with what each thought the other should have seen. the observer on the platform will think that the observer on the train should have seen the forward lightning strike first, and the observer on the train will think that the observer on the platform will have seen the rear lightning strike first.

Now, if you understood any of that, lol, i ask you to either pick holes in my argument or advise me not to disagree with what the Board of Studies thinks is correct.

thanks

It is easy to disprove your hypothesis by contradiction. Suppose that prior to this thought experiment both observers agree to measure the times of reception of both wavefronts using stopwatches. In the frame of reference of the outside observer O' it is evident that the wavefronts are emitted and received simultaneously. Therefore, O' will start his stopwatch when he receives both flashes but never stop it. You have purported that the inside observer O will observe O' to encounter the wavefronts at different times and hence start and stop his stopwatch in a finite time. Now, I either start my stopwatch and then stop it some finite time later or I start the stopwatch and leave it running. This is a fundamental truth. Thus your hypothesis is paradoxical.

 

[speersy]

here follows my explanation:
because both observers are in an inertial frame of reference, they would see the light approach them at 'c', this means that because the light has to travel the same distance from either end of the carriage, that they would BOTH see the light from each end simultaneously

Yes they would both see the light approach them at c but for your next statement you are thinking that the train stops at the split second the lightning strikes, however it is really travelling at relativistic speeds and thus he would appear to be travelling towards one strike and away from the other even in his frame of reference. NB. c still remains constant

 

[andysoul]

OK

i can't make head nor tails of any of your explanations (which perhaps is the reason why relativity is sooo annoying). however, in the meantime i've checked with a physics professor from sydney uni and he agrees with my hypothesis.

another thought has occured to me about this however, and that if the person in the train WAS to see the front lightning strike occur first, then the front lightning strike must have either occured first, or it must have appraoched him at > light speed, which is of course, impossible.

the only way i can find that the theory that he sees the front one first, is if the aether theory is true (which of course we were all taught was wrong ) and the person in the train is actually ABLE to move toward the wavefront.

for example, if you had two people, one moving at .75c left, relative to a stationary observer, and one moving at .75c right, relative to the same stationary observer. if one turns on a light, then according to the aether theory, the other moving observer would see the light come at him at 2.5 c (.75+.75. +1)
"this is of course, impossible"
the light would be severely blue-shifted, but would only travel at 'c' relative to ANY observer, hence the reason that some of you see my explanation as a paradox.


Einstein said that "light will travel at 'c' relative to any observer measuring it", and this holds true in my explanation, but not any of the other explanations offered which say that the train observer will see the front one first

 

[Jase]

So you're saying that if you stood still and i 'ran' a thousand miles away to another point in space (disregarding gravity, earth..). We get there at the same time?

If they BOTH see it simultaneously.. what is there to disagree about? By saying that one "thinks" the other should see it one way, you're delving into complex philosophical debate rather than simple HSC Physics. And plus, thought experiments and relativity are under scrutiny anyway, it's too hard to prove and too hard to disprove.

Wait and there's something about simultaneity and causality. .. The two observers see the lightning simultatneous in terms of a "time event", that is, to both, the lightning simultaneously strikes both ends and is separated only by time. Whereas on the space-plane, it is it not.. and..

okay im thouroughly confused now...

 

[JayWalker]

So you're saying that if you stood still and i 'ran' a thousand miles away to another point in space (disregarding gravity, earth..). We get there at the same time?

If they BOTH see it simultaneously.. what is there to disagree about? By saying that one "thinks" the other should see it one way, you're delving into complex philosophical debate rather than simple HSC Physics. And plus, thought experiments and relativity are under scrutiny anyway, it's too hard to prove and too hard to disprove.

Wait and there's something about simultaneity and causality. .. The two observers see the lightning simultatneous in terms of a "time event", that is, to both, the lightning simultaneously strikes both ends and is separated only by time. Whereas on the space-plane, it is it not.. and..

okay im thouroughly confused now...

You're complicating an easy principle!!!
Just learn what the book says, and recite it...

 

[andysoul]

You're complicating an easy principle!!!
Just learn what the book says, and recite it...

uh huh, but what the book says, does not make one single bit of sense, apart from being garbled, it is contradictory

 

[JayWalker]

uh huh, but what the book says, does not make one single bit of sense, apart from being garbled, it is contradictory

Your argument makes no sense what so ever, i have given you an explination to your question about your text book,

Disregarding the aether theory (einsteins relativity was after aether was disproved). Why the hell would the person IN the train see the lightning strike first?! The lightning strike experiment is a thought experiment to prove relativity and length contraction is you take it far enough, hence it doesnt matter if the person saw the front one first because that explains the relativity of simultaneity, which is exactly what it is trying to prove!!!

 

[zeropoint]

Jaywalker.

Sorry, but your explanation is wrong --- on two counts. Firstly, you are referring to the wrong thought experiment. The gedanken we are discussing involves a relativistic train struck by two distinct lightning strikes that induce two spherical wavefronts of light, propagating isotropically from the origin of the lightning strikes. Second, you have confused the order of events in _your_ hypothetical thought experiment. Supposing the train is moving in the positive x direction (right) in the outside frame of reference, then the left-hand (rear) door opens prior to the right-hand (front) door as the left-hand door is approaching the origin of the light source whereas the right-hand door is receding from the origin of the light source.

andysoul.

If the professor agrees with your hypothesis, then he/she either fails to truly understand special relativity; or she misunderstands your reasoning (probably the latter). In any case, you should understand why your hypothesis is a contradiction as I have already offered a method to disprove it.

 

[speersy]

I dont see why the person in the train would see the lightning strike he is travelling towards first as he is moving towards it and hence away from the other. I think u are getting mixed up with the light flashes on the train and the ones off the train.

 

[zeropoint]

I dont see why the person in the train would see the lightning strike he is travelling towards first as he is moving towards it and hence away from the other. I think u are getting mixed up with the light flashes on the train and the ones off the train.

Can you see that in the frame of the outside observer, the distance between the inside observer and the front wavefront is decreasing at a rate of v + c and the distance from the back wavefront is decreasing at a rate of c - v? Considering that the inside observer is initially equidistant from either end of the train, it should be clear that he will see the front strike first, followed by the back strike. This all assumes however that the outside observer witnessed both flashes simultaneously in her frame of reference.

 

[d_elmo]

i thought it was the carriage person who saw the light at the same time, but its been hacked into me that its the other way round, like the textbook says.

the person on the platform will see both lightning strikes hitting each end of the carriage at the same time and the person in the carriage will see one hit first because they are moving towards it, and then the second they are moving away from.

i know its confusing and the whole topics stupid but the way the textbook put it is the correct way.

 

[Dash]

To make a long story short... Take Tommy's advice...

This whole concept of Special Relativity is very complicated, and in my opinion, it is better to accept what you are taught and move on, rather then try and make a rational interpretation of it, all that's going to do is make it harder for you.

Don't waste too much time on something so trivial. If the books says it so, do it!

 

[andysoul]

zeropoint

Can you see that in the frame of the outside observer, the distance between the inside observer and the front wavefront is decreasing at a rate of v + c and the distance from the back wavefront is decreasing at a rate of c - v? Considering that the inside observer is initially equidistant from either end of the train, it should be clear that he will see the front strike first, followed by the back strike. This all assumes however that the outside observer witnessed both flashes simultaneously in her frame of reference.

just a small thing. um, you will agree with me that light cannot travel faster than 'c', and that in two identical mediums light will travel at ONLY 'c', regardless of the state of motion of the observer (in an inertial frame of reference). yes? now i see your point in the above statement, in fact, i even agree with the first part of it. however, if you imagine yourself to be inside the train, it appears that the outside observer is moving towards the rear of the train, at the same velocity that he/she observes you moving to the right (pos. x direction). now, both observer viewpoints are equally valid, because they both are in inertial frames of references. therefore, from the observer in the trains P.O.V light will move at 'c', and as the light has to move the same distance from either end of the carriage, he will see the simultaneously. he will in fact also observe the person outside the train as seeing the REAR lightning strike first, because that is how it appears to him, as the rearward sphere of light has moved toward the outside observer (or the observer has moved closer to the sphere of light, its all relative )

the error in your reasoning is that the spheres of light expanding will both appear indentical from each observers P.O.V, this assumption, although it seems logical, is incorrect, because both observers see light moving at 'c' RELATIVE TO THEMSELVES. even though they will disagree about what the other should have seen, their viewpoints are both equally valid.

your discussion is merely of the viewpoint of the observer standing outside the train. the question asks to analyse the statements of BOTH observers, so you have to, so to speak, put yourself in both observers shoes, and analyse each one seperately, forgetting completely about what you have said the other will observe, but the two viewpoints may be contradictory, and that is the main lesson of simultaniety.
you simply can't use the outside observers P.O.V to put words in the other one's mouth, you have to analyse the situation from the train observer's P.O.V

 

[andysoul]

i also just found this site
it probably explains it better than i can
http://homepage.mac.com/ardeshir/Einstein'sTrain.html
it explains it quite well, but seems to find a way to contadict the theory of relativity, which i can't quite grasp.
however, i think i must now concede that i/we are not really qualified to completely solve such a problem, and the necessary understanding of the concept of relativity is simply not there, therefore, i'm just gonna argue my way through if the question comes up, using all the logic i can
thanks for your input guys

PS i'm going to TRY and find someone really qualified to give me an answer, and when i do (or even if) i'll post it here.

PPS i wish i had a time machine so i could go back and ask einstein

 

[zeropoint]

zeropoint
just a small thing. um, you will agree with me that light cannot travel faster than 'c', and that in two identical mediums light will travel at ONLY 'c', regardless of the state of motion of the observer (in an inertial frame of reference). yes? now i see your point in the above statement, in fact, i even agree with the first part of it. however, if you imagine yourself to be inside the train, it appears that the outside observer is moving towards the rear of the train, at the same velocity that he/she observes you moving to the right (pos. x direction).

They both have the same speed with respect to each other, however, the velocities are reversed in sign.

now, both observer viewpoints are equally valid, because they both are in inertial frames of references. therefore, from the observer in the trains P.O.V light will move at 'c', and as the light has to move the same distance from either end of the carriage, he will see the simultaneously.

Only if you assume that the wavefronts departed from either end at the _same time_ in the frame of reference of the train. It turns out, however, that given the current circumstances, this is an invalid assumption.

he will in fact also observe the person outside the train as seeing the REAR lightning strike first, because that is how it appears to him,

``because that is how it appears to him'' is not a useful criterion for mounting an argument. Your claim that different observers will disagree on the _existence_ of different events is a logical impossiblity, far more fundamental than relativity theory.

as the rearward sphere of light has moved toward the outside observer (or the observer has moved closer to the sphere of light, its all relative )

This is true, however you have overlooked the fact that the front wavefront departs earlier than the back wavefront, so both wavefronts arrive simultaneously at the outside observer in both frames of reference.

the error in your reasoning is that the spheres of light expanding will both appear indentical from each observers P.O.V,

Please point out where I mentioned any of the this.

you simply can't use the outside observers P.O.V to put words in the other one's mouth, you have to analyse the situation from the train observer's P.O.V

It is straightforward to examine the situation from either frame of reference, however, given that your hypothesis has already been proven false, this all seems rather redundant. If the outside observer witnesses both events simultaneously in her frame of reference, then, in the _outside frame_, the inside observer will encounter the front wavefront earlier, as I proved earlier in the thread. I assume we are in total agreement on this point. So I only need to convince you that the order of events of _reception_ are identical in both frames of reference?

It is known that in the outside frame, the outside observer receives both wavefronts simultaneously, so it is reasonable to say that the time interval between reception of the front and back wavefronts for the outside observer in the outside frame of reference is zero, agreed? If we perform this experiment, the outside observer will activate her stopwatch at the instant she receives both wavefronts and will leave it running, awaiting a second wavefront which will never arrive. Let's say your hypothesis is correct, and that the train-based observer will watch the outside observer receive the back wavefront, followed by the front wavefront. You must therefore believe that the train-based observer will observe the outside observer to activate her stopwatch when she receives the back wavefront and then stop timing when she receives the front wavefront. When we bring the experimenters together, the outside observer will still be holding her running stopwatch, whereas the train-based observer, in a state of disbelief, will claim that the outside observer stopped timing during the course of the experiment. Clearly this is a paradox, so your hypothesis must be false.

The only way to resolve the paradox is to assume that the inside observer watches the outside observer encounter both wavefronts at the same time. Since in the train frame, the outside observer is moving in the negative x-direction, she must be closer to the centre of the back wavefront at the instant she receives the light from both wavefronts in the train frame. However, since the speed of light is constant, this suggests that the front wavefront departed before the back wavefront, to arrive at the outside observer at the same time as the back wavefront. Therefore, the inside observer must encounter the front wavefront, followed by the back wavefront.

 

[andysoul]

ok
based upon your reasoning, the opposite is indeed also possible, that the person on the train (who is in an inertial frame of reference) viewed both flashes as simultaneous. all you need to do is reverse positions. because both flashes had the same distance to travel (end of train to the middle), at the same speed, 'c', from either end of the train. therefore the observer in the train will only start their stopwatch and never stop it? you see? and they will have, as you said, observed the person on the ground recieve two seperate flashes.

both yours, and my arguments are both perfectly logical, and therein lies the paradox, no matter which way you look at it.

i think, however, that both of them, upon meeting would have still running stopwatches, as both viewed the lightning as simultaneous. the theory of relativity throws up seemingly illogical paradox's in many of it's applications. However, as has been stated in many places, both observers are correct in their observations, even though they are contradictory, due to the nature of light.

i think this is one of the few cases where it is in fact CORRECT to agree to disagree, as both are correct.

 

[Shuter]

Just read his notes.

I just did, he explains it all very well.

http://www.bartleby.com/173/9.html
and
http://www.bartleby.com/173/11.html


The major point is that the distance and TIME taken for the light to travel are not infact the same from both viewpoints. e.g. observer outside the train will perceive that the train is actually a different length than what it is. Similarly, the person inside the train will perceive the time taken for each lightning strike to reach him as being different as what the person outside the train perceives.


The othersite and you also seem to assume this flawed principle:

"The space-interval (distance) between two points of a rigid body is independent of the condition of motion of the body of reference".

Which is not true as shown by the theory of time dilation, legnth contraction and relativity.


Therefore the person in the train will infact perceive the first lightning strike as taking place before the one that occurred at the back of the train.



Case closed.