Notices
Results 1 to 28 of 28
Like Tree3Likes
  • 1 Post By Janus
  • 2 Post By Markus Hanke

Thread: A question on observed time in GR : the lightning example by Einstein

  1. #1 A question on observed time in GR : the lightning example by Einstein 
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    (The question is below)

    I first quote this paper by Einstein :

    Albert Einstein (1879–1955). Relativity: The Special and General Theory. 1920.
    IX. The Relativity of Simultaneity

    Up to now our considerations have been referred to a particular body of reference, which we have styled a “railway embankment.” We suppose a very long train travelling along the rails with the constant velocity v and in the direction indicated in Fig. 1. People travelling in this train will with advantage use the train as a rigid reference-body (co-ordinate system); they regard all events in reference to the train. Then every event which takes place along the line also takes place at a particular point of the train. Also the definition of simultaneity can be given relative to the train in exactly the same way as with respect to the embankment. As a natural consequence, however, the following question arises: 1
    Are two events (e.g. the two strokes of lightning A and B) which are simultaneous with reference to the railway embankment also simultaneous relatively to the train? We shall show directly that the answer must be in the negative.

    FIG. 1.
    2
    When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A —> B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the mid-point of the distance A —> B on the travelling train. Just when the flashes 1 of lightning occur, this point M' naturally coincides with the point M, but it moves towards the right in the diagram with the velocity v of the train. If an observer sitting in the position M’ in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. 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. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result: 3
    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-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event. 4
    Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, i.e. that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity; if we discard this assumption, then the conflict between the law of the propagation of light in vacuo and the principle of relativity (developed in Section VII) disappears. 5
    We were led to that conflict by the considerations of Section VI, which are now no longer tenable. In that section we concluded that the man in the carriage, who traverses the distancew per second relative to the carriage, traverses the same distance also with respect to the embankment in each second of time. But, according to the foregoing considerations, the time required by a particular occurrence with respect to the carriage must not be considered equal to the duration of the same occurrence as judged from the embankment (as reference-body). Hence it cannot be contended that the man in walking travels the distance w relative to the railway line in a time which is equal to one second as judged from the embankment. 6
    Moreover, the considerations of Section VI are based on yet a second assumption, which, in the light of a strict consideration, appears to be arbitrary, although it was always tacitly made even before the introduction of the theory of relativity. 7




    *Reading the first text in bold :

    (I understand the Lorenz contraction, time dilatation , the formula,
    what it means, the correlations between v and c etc.)

    He moved a short distance towards B , so he caught the light going from B to M earlier on his mirror, his mirror being at a slightly more advanced point M'.
    So he looks in that instant at his clock and notes an earlier time, that is as obvious as it can get.
    (And the observer on the embankment will see the beem from B just an instant later because he did not move towards B in that same amount of time travelled by the train, he stayed at rest, thus the beam took longer for him to get to his mirror than it did for the observer on the moving train.)


    *Then looking at the second text in bold :

    Why do we say that this observer on the train (in a different reference-body, co÷rdinate system) has his own particular time ?
    He simply looked at his clock earlier,because of the above reasons.


    > What am i missing here ?


    The strange thing is that i do not ask this because i would not believe in the whole case of time dilatation (Hafele-Keating) - i do -
    but reading the above puzzles me.


    Last edited by Noa Drake; January 5th, 2014 at 07:21 PM.
    Reply With Quote  
     

  2.  
     

  3. #2  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    The seconded highlighted section is the conclusion drawn from above. The important point is not just that observer on the train sees the light from B earlier but when each observer calculates the time of the two flashes (taking into account their distance from where the flashes happened and the speed of light), one will say they happened at the same and the other will say they did not happen at the same time. Therefore, in order to know when an event happened, we need to know which frame of reference is being talked about.


    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  4. #3  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    I see what you mean.But why is that particular time keeping ? He was closer to the source of emission B when reading his clock, why must that be regarded as different time keeping?


    My idea is that the Lorentz contraction of the clock, causes it to slow down by a molecular process yet to be defined and applicable to all types of clocks,
    which is why it can afterwards be shown to have run slow.
    So not by means of the relative motion, because he simply read his clock earlier in the course of time because he was now closer to B.

    This means the following : As he traveled a distance towards B at speed v, the clock ran indeed slow, say by 1 second.The clock should then read 2 seconds delay for him because he also read his clock 1 second earlier, but due to his own being having slowed down from contraction,as did the whole train, with the exact same pace during his displacement,he reads 1 second of delay on his clock instead of 2.
    After the event the clock would effectively show to have run slow by 1 second.


    Just trying to make sense of the observations.
    Last edited by Noa Drake; January 5th, 2014 at 07:49 PM.
    Reply With Quote  
     

  5. #4  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Noa Drake View Post
    I see what you mean.But why is that particular time keeping ? He was closer to the source of emission B when reading his clock, why must that be regarded as different time keeping?
    I think you are missing the point. That is only true from the point of view of Platform Guy.

    From the point of view of Train Guy, the two lighting strikes were equally distant from him. He considers himself stationary (by the principle of Galilean relativity) and so not moving towards or away from the lightning strikes. All he sees is that one flas gets to him first and therefore must have happened earlier.

    No "magic" changes to clocks are required (which wouldn't work anyway because these things are symmetrical: A sees B's clock run slow and B sees A's clock run slow. How would your Magic Molecular Process account for that. [That is not a question.])
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  6. #5  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,231
    Quote Originally Posted by Noa Drake View Post

    My idea is that the Lorentz contraction of the clock, causes it to slow down by a molecular process yet to be defined and applicable to all types of clocks,
    which is why it can afterwards be shown to have run slow.
    You are doing yourself a disservice by think of Time dilation being due to a "molecular process yet to be defined". Time dilation is simply the result that the measurement of time is frame dependent.

    The train experiment is a prime example of this.

    1. Two events occur such that in the frame of the embankment, they occur simultaneously. This is confirmed by an observer located halfway between the events.
    2. Another observer, on the train is next to the embankment observer when these events happen (according the the embankment observer).
    3. This puts the train observer halfway between the points at which the events occurred. (if the events are lightning strikes that leave char marks on both train and embankment, he will be halfway between the two char marks on the train)
    4. From the embankment perspective, the light from one event reaches the train observer before the light from the other event does.
    5. The train observer concurs that he sees the two events at different times. (He cannot do otherwise. He has to agree that the railway observer sees the two events at the same time, thus the light from one event must pass him on its way to the embankment observer and the light from the other event has to pass the embankment observer to reach him, thus the light from the two events reaches him at different times.)
    6. He is halfway between the char marks that the events left on the train. Since the speed of light is a constant, it would take an equal amount of time for the light from each event to travel the distance to him.
    7. Ergo, since he sees the two events occur at different times and he is an equal distance from the two events, the two events happened at different times.

    The same events that are simultaneous for the embankment observer are not simultaneous for him. The very concept of simultaneity is frame dependent. Note that this is not due to any physical alteration of the train or observer, but simply due to the fact the the two frames just do not measure time in the same way
    Markus Hanke likes this.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


    Edit/Delete Message
    Reply With Quote  
     

  7. #6  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    As Janus said. I should add that the phenomenon discussed here is relative time dilation, which is a coordinate effect, and happens due to there being relative motion between the two frames. This phenomenon in effect describes the relationship between two frames, but doesn't effect the proper time readings on clocks. Note that this is a physically distinct and very different phenomenon to gravitational time dilation, which is due to the presence of gravity and/or acceleration. A key difference is that relative time dilation is a symmetric effect ( A sees B dilated, whereas B sees A dilated as well by the same amount ), whereas the gravitational variety is not.
    Reply With Quote  
     

  8. #7  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    I've read the comments carefully, but i don't get wiser.When the lightning is sent from B to the observer on the train,this observer travels a very small distance towards B due to his speed v before the light with speed c reaches him.So obviously, as Einstein indicates, he receives the lightsignal from B slightly earlier than he receives that from A.Non-simultaneity is created here by the concept of this experiment.He looked at his clock earlier than the observer on the embankment.The experiment has put him in the spot M' , slightly more close to B, so he was not at M when both observers read there clocks.So why talk of 2 reference frames each with their own timekeeping ?
    Where is the relativity in that ? Is is absolute, as 1 frame, in my oppinion.

    Do you have explanations ?
    Reply With Quote  
     

  9. #8  
    Quagma SpeedFreek's Avatar
    Join Date
    Jan 2011
    Location
    London, UK
    Posts
    2,787
    Because the speed of light is always 299,792,458 m/s relative to whoever is observing it, this means light always propagates from a coordinate that is at rest in relation to the observer. So A and B always remain stationary relative to the observer.

    If you watch a moving object and it emits light in all directions, the light propagates from the place the moving object was when the light was emitted - the emission point does not move with the object - the light comes from a place that is stationary in relation to you whilst the object moves through the propagating sphere of light. But if you are moving with the object that emits the light, the light propagates from the object itself - the object remains centred on the propagating sphere of light - the light comes from a place that is stationary in relation to you.

    In Einstein's example, each observer sees the light come from places that are both stationary and equidistant. So if the light reaches the observer at the same time, then the events of the lightning strikes occurred at the same time, but if the light reaches the observer at different times then the events of the lightning strikes occurred at different times.
    "Ok, brain let's get things straight. You don't like me, and I don't like you, so let's do this so I can go back to killing you with beer." - Homer
    Reply With Quote  
     

  10. #9  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Noa Drake View Post
    I've read the comments carefully, but i don't get wiser.When the lightning is sent from B to the observer on the train,this observer travels a very small distance towards B due to his speed v before the light with speed c reaches him.
    That is only as perceived by Platform Guy it is not an absolute statement. You are assuming that is some sort of True or absolute point of view.

    From Train Guy's frame of reference, he has NOT got any closer to the source of the flash (the end of the train). It is, and always was, the same distance away as the other flash. But he sees one flash before the other. They both happened the same distance away. He is stationary with respect to the flashes (he is not moving relative to the ends of the train where the flashes occurred). He knows the speed of light is the same for both flashes. Therefore he knows that one of the flashes occurred before the other.

    Train Guy sees the point where flash A occurred (the end of the train) is moving towards Platform Guy. He will deduce that the light from A has to travel less distance to get to Platform Guy and therefore Platform Guy will see the two flashes at the same time.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  11. #10  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,231
    Quote Originally Posted by Noa Drake View Post
    I've read the comments carefully, but i don't get wiser.When the lightning is sent from B to the observer on the train,this observer travels a very small distance towards B due to his speed v before the light with speed c reaches him.So obviously, as Einstein indicates, he receives the lightsignal from B slightly earlier than he receives that from A.Non-simultaneity is created here by the concept of this experiment.He looked at his clock earlier than the observer on the embankment.The experiment has put him in the spot M' , slightly more close to B, so he was not at M when both observers read there clocks.So why talk of 2 reference frames each with their own timekeeping ?
    Where is the relativity in that ? Is is absolute, as 1 frame, in my oppinion.
    As already pointed out, the train observer is not closer to one event than the other when he sees the lights. The char marks on the embankment may be different distances from him, but they are not the events. From his frame, the lightning strikes both train and embankment leaving char marks. The char marks on the embankment then move away from the points of strikes. the char marks on the train do not. They do not move relative to the points of the strikes occurred as far as anyone in the train is concerned nor do they move relative to anyone on the train.

    Thus Lightning strikes occurred where char marks on train are.
    Char marks on train are equal distance from observer.
    Light takes an equal time to travel equal distances.
    Ergo, if observer on train does not see the flashes of light from the strikes at the same time, the strikes that created those char marks did not happen at the same time.

    The embankment observer is also exactly between the two events, he however sees them at the same time. again, equal distance traveled by the light means equal times traveled by the light, thus the lightning that create the flashes that he sees occured at the same time.

    We are not concerned with that fact that the train observer reads a different time on his watch when he sees the light than the embankment observer does, we are concerned with the fact that he has too make two different readings on his watch, one for each light flash, while the embankment observer only makes 1reading for both flashes. Then when each observer backtracks the lights to their origins, the embankment observer concludes that each flash originated at the same time by his watch and the the train observer concludes that the two flashes originated at two different times from each other.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


    Edit/Delete Message
    Reply With Quote  
     

  12. #11  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Why would train guy deliberately choose the train as frame of reference, when he knows, had we explained him the experiment in advance, that this referencepoint would lead him to false conclusions ? Just a way to say that the point of view of the platform guy is the only one that counts, other conclusions follow from it.
    What is then the importance of installing such a reference frame that does not give you correct information on the follow up of events ? I would still prefer then to stick with parameters that are real, being the Lorenz contraction and one frame (" in this example" should be added here, as pointed out by Strange).

    I do thank you for giving eleborate comments on the issue.
    Last edited by Noa Drake; January 7th, 2014 at 07:19 AM.
    Reply With Quote  
     

  13. #12  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Noa Drake View Post
    I've read the comments carefully, but i don't get wiser.When the lightning is sent from B to the observer on the train,this observer travels a very small distance towards B due to his speed v before the light with speed c reaches him.
    Not from his point of view., He is not travelling towards the source of the flash. The source of the flash is the end of the train. It is not moving relative to him. After observing the two flashes, he can have a coffee and then measure the distance to where the flashes occurred. He will find they both happened the same distance away from his seat.

    By thinking that he is moving towards the flash, your are privileging one particular point of view as being "real". The statement that Train Guy is moving towards B is arbitrary and, from his point of view, untrue.

    So obviously, as Einstein indicates, he receives the lightsignal from B slightly earlier than he receives that from A
    Only as seen by Platform Guy. Not as seen by himself.

    Is is absolute, as 1 frame, in my oppinion.
    That is not consistent with the speed of light being the same for all observers. It also implies that this train station in the German countryside is the single stationary point in the entire universe!
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  14. #13  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    @Strange

    I did not mean that there is only one frame generally, only within this experiment i suggested that we should only regard the frame of the stationary embankment to explain things.

    On the speed of light and the observer :

    Say i had a spaceship that could go at the speed of light (regardless of wether that would be possible, just hypothetically speaking).
    Say a ray of light would be emitted through space, next to me in the spaceship.
    Say i could travel instantly at the speed of light along side of the emitted ray from the moment it was emitted.
    Then i would observe the head of the ray as 'stationary' next to me.
    But i would also know that that is not the reality of things, i know it goes at the speed of light,
    while i perceive it as not going faster or slower than me in the spaceship.


    Basically it boils down to this : What was wrong with simple 3D reality and time tick away at one rate, along with the Lorentz contraction doing its job ?

    The aswer from GR would be the observation that time seems to go slower with regard to relative motion.
    Whereas i defend the idea that the Lorentz contraction is responsible for these time changing events.

    The calculations for contraction, time slowing and mass increase use the same factor, in the same span of the event of relative motion.
    So how can we with certainty conclude which is the cause of which during 'the relative motion' ?

    To find out the correct causalities , one should first assess why exactly the c factor is present in the contraction equasion and not some other factor in relation to v.
    What does material contraction (or dilatation or mass increase)
    have to do with the speed of light , or of EMR all together, fysically speaking, as in what goes on out there precisely ?
    Same remark for E=mc▓ for that matter, not challenging the correctness of it of course - it is - but finding the causalities that clarify the presence of the speed of light in the formula.


    It is in my oppinion reccommendable to find that out first,
    then we can start saying meaningfull things on what caused what, and what the whole picture should or could look like.



    A suggested answer in the direction of some clarification is that :
    The speed of gravitational waves, in the general theory of relativity, is the speed of light in vacuum : c.

    Agreed, but then WHY en HOW ? > The key to the whole picture lies precisely here
    Last edited by Noa Drake; January 7th, 2014 at 07:43 AM.
    Reply With Quote  
     

  15. #14  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Noa Drake View Post
    I did not mean that there is only one frame generally, only within this experiment i suggested that we should only regard the frame of the stationary embankment to explain things.
    Why should we only use that frame? How is that different from saying there is only one frame?

    Basically it boils down to this : What was wrong with simple 3D reality and time tick away at one rate, along with the Lorentz contraction doing its job ?
    It just doesn't work. Try it and you get results that contradict reality.

    Whereas i defend the idea that the Lorentz contraction is responsible for these time changing events.
    They are, in general, complementary. What is seen as length contraction in one frame of reference can be explained by time dilation in another.

    What does material contraction (or dilatation or mass increase) have to do with the speed of light , or of EMR ?
    That is just the way the geometry of the universe we live in works. Light (or more generally, massless particles) must travel at the speed of light. Nothing else can.

    Why? Is that the only possibility? Could c have a different value? Who knows. That is closer to philosophy than science. (Until we have a deeper theory which might explain some of these things - but will just raise other unanswered questions.)
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  16. #15  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    What was wrong with simple 3D reality and time tick away at one rate, along with the Lorentz contraction doing its job ?
    Lorentz transformations preserve space-time intervals between given events only in Minkowski space-time, i.e. on a (3+1) dimensional manifold. As Strange has correctly pointed out, if you try to apply Lorentz transform to Euclidean 3-space, you get unphysical results. See also here : Lorentz transformation - Wikipedia, the free encyclopedia
    Reply With Quote  
     

  17. #16  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    Same remark for E=mc▓ for that matter, not challenging the correctness of it of course - it is - but finding the causalities that clarify the presence of the speed of light in the formula.
    It's the magnitude of the energy-momentum vector for a particle at rest. Again, the general version of this works only in (3+1)-dimensional space-time.
    Reply With Quote  
     

  18. #17  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    The speed of gravitational waves, in the general theory of relativity, is the speed of light in vacuum : c.

    Agreed, but then WHY en HOW ? > The key to the whole picture lies precisely here
    Because gravitational waves are solutions to the relativistic wave equation



    It is the presence of the d'Alembert operator here that implies a propagation velocity of exactly c in these free wave equations. If you want to you can look at it in another way too - gravitational waves can be associated with gravitons, which are massless spin-2 particles; since they are massless, they can propagate only along null geodesics, which implies again a local propagation velocity of exactly c.
    Reply With Quote  
     

  19. #18  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,231
    Quote Originally Posted by Noa Drake View Post
    Why would train guy deliberately choose the train as frame of reference, when he knows, had we explained him the experiment in advance, that this referencepoint would lead him to false conclusions ? Just a way to say that the point of view of the platform guy is the only one that counts, other conclusions follow from it.
    What is then the importance of installing such a reference frame that does not give you correct information on the follow up of events ? I would still prefer then to stick with parameters that are real, being the Lorenz contraction and one frame (" in this example" should be added here, as pointed out by Strange).

    I do thank you for giving eleborate comments on the issue.
    There is no correct or unique frame that gives the real results.

    Both frames are equally valid and give equally valid results. The conclusions made by the train frame are no less correct than those made in embankment frame. Your attempt to give the embankment frame priority is misguided.

    You are trying so hard to preserve absolute simultaneity that you are ignoring the fact that the universe says it doesn't exist.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


    Edit/Delete Message
    Reply With Quote  
     

  20. #19  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,231
    Quote Originally Posted by Noa Drake View Post

    Say i had a spaceship that could go at the speed of light (regardless of wether that would be possible, just hypothetically speaking).
    Say a ray of light would be emitted through space, next to me in the spaceship.
    Say i could travel instantly at the speed of light along side of the emitted ray from the moment it was emitted.
    Then i would observe the head of the ray as 'stationary' next to me.
    But i would also know that that is not the reality of things, i know it goes at the speed of light,
    while i perceive it as not going faster or slower than me in the spaceship.
    But this this is not what you would see.

    Even Hypothetically speaking, a rocket traveling at c is not a proper frame of reference.

    it's like dividing by a smaller and smaller number " 1/100, 1/1000, 1/1000...0
    As long as the number you are dividing by is larger than 0, you can get an answer, but at 1/0, you can't. 1/0 has no answer, it is undefined. In the same way, traveling at lightspeed is undefined as a frame.


    However, we can have our ship travel at 0.999...9c. And then, if it measures the speed of the light relative to itself, it will get an answer of c (not c - 0.999...9c)

    we could also have a light flash meeting it from the other direction, and the ship would also measure it as moving at c relative to them. If our occupants measured how long each pulse took to traverse the length of the ship, they would get the same answer for both directions; tail to nose and nose to tail.

    For the person to which the ship is traveling at 0.999...9c, the light traveling from tail to nose takes longer to traverse the ship than the passing from nose to tail. This leads back to the same conclusion as the train example; events that are simultaneous in one frame are not simultaneous in a frame with relative motion to the first.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


    Edit/Delete Message
    Reply With Quote  
     

  21. #20  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Noa Drake View Post
    I've read the comments carefully, but i don't get wiser.When the lightning is sent from B to the observer on the train,this observer travels a very small distance towards B due to his speed v before the light with speed c reaches him.
    That is only as perceived by Platform Guy it is not an absolute statement. You are assuming that is some sort of True or absolute point of view.

    From Train Guy's frame of reference, he has NOT got any closer to the source of the flash (the end of the train). It is, and always was, the same distance away as the other flash. But he sees one flash before the other. They both happened the same distance away. He is stationary with respect to the flashes (he is not moving relative to the ends of the train where the flashes occurred). He knows the speed of light is the same for both flashes. Therefore he knows that one of the flashes occurred before the other.

    Train Guy sees the point where flash A occurred (the end of the train) is moving towards Platform Guy. He will deduce that the light from A has to travel less distance to get to Platform Guy and therefore Platform Guy will see the two flashes at the same time.

    Speedfreak indicates :

    "Because the speed of light is always 299,792,458 m/s relative to whoever is observing it, this means light always propagates from a coordinate that is at rest in relation to the observer. So A and B always remain stationary relative to the observer."


    And also you say :
    "From Train Guy's frame of reference, he has NOT got any closer to the source of the flash (the end of the train). It is, and always was, the same distance away as the other flash. "

    I understand now , i think, what the issue is.

    According to GR : When as an observer i move together with a light source that is ahead of me, emitting towards me,
    then the light source will always be at rest relative to me. Is that correct ?

    So after the instance of the lighting striking, the light is emitted from B to the train guy, but train guy moves along with it,
    so receives both signals at the same instance anyway, according to his frame of reference.

    I have two remarks to that :

    -Wouldn't that imply that just after the lightning struck, the emitting source at B would have to continu emitting for a tiny instance of moments ?
    I mean if the interval of 'light striking' took say 1 nanosecond, then '1 nanoseconds of event' are conveyed by the emission at B, yet the train guy receives emission after that also while the source is moving to the right side. That would be impossible, because the lightning strike would have had to take longer than it did.


    -Secondly : Sorry if some might be offended by my suggestion of a medium for gravity (and EMR), but i connot make this point without it.
    (In another thread i'm working on a hypothesis that tries to overcome the old problems of 'the aether', a word to be avoided.., and works completely different, i am making steady progress on having it respect the observed object behaviours of GR as well as Newton's laws, doing what spacetime does, a discrete spacetime, continuous in apparent behaviour, but discrete on Plancksize level)

    >If the lightwaves move through a locally stationary medium that is itself responsable for maintaining the speed of light c for the light, then the observer (trainguy) would still not move relatively to the emitting source (B), but after the lightning flash strikes, he would get closer to the initially broadcasted 'interval of emitted light' from B that represents the lightning strike.
    Last edited by Noa Drake; January 9th, 2014 at 05:34 AM.
    Reply With Quote  
     

  22. #21  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    According to GR : When as an observer i move together with a light source that is ahead of me, emitting towards me,
    then the light source will always be at rest relative to me. Is that correct ?
    If your relative velocity is zero, then yes, otherwise no.

    If the lightwaves move through a locally stationary medium
    Stationary with respect to what ?
    Reply With Quote  
     

  23. #22  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    Quote Originally Posted by Noa Drake View Post
    -Wouldn't that imply that just after the lightning struck, the emitting source at B would have to continu emitting for a tiny instance of moments ?
    I mean if the interval of 'light striking' took say 1 nanosecond, then '1 nanoseconds of event' are conveyed by the emission at B, yet the train guy receives emission after that also while the source is moving to the right side. That would be impossible, because the lightning strike would have had to take longer than it did.
    I don't really understand what you are asking. But it did make me think of something: if your view that we should only consider the platform point of view was correct, then that would imply that the guy on the train would see a light at the front of the train blue-shifted by the Doppler effect (because in your world view, he is moving towards it). That doesn't happen.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  24. #23  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Noa Drake View Post
    -Wouldn't that imply that just after the lightning struck, the emitting source at B would have to continu emitting for a tiny instance of moments ?
    I mean if the interval of 'light striking' took say 1 nanosecond, then '1 nanoseconds of event' are conveyed by the emission at B, yet the train guy receives emission after that also while the source is moving to the right side. That would be impossible, because the lightning strike would have had to take longer than it did.
    I don't really understand what you are asking. But it did make me think of something: if your view that we should only consider the platform point of view was correct, then that would imply that the guy on the train would see a light at the front of the train blue-shifted by the Doppler effect (because in your world view, he is moving towards it). That doesn't happen.

    I would agree with that.
    But his length contraction due to that short travel at speed v would then compensate the blue shift with exactly the same amount, no ?

    Meaning that the shortened wavelengths of the approaching lightwave would be allowed to 'lengthen back to normal' due to the contraction of the train guy. With exactly a null resulting compensating saldo, since both incidents (length contraction train guy and frequency increase or wavelength shortening) are happening due to relative motion here, over exactly the same time interval and distance travelled.
    > Hence train guy would observe that in his point of view the lightwave did not shift.
    Reply With Quote  
     

  25. #24  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Noa Drake View Post
    According to GR : When as an observer i move together with a light source that is ahead of me, emitting towards me,
    then the light source will always be at rest relative to me. Is that correct ?
    If your relative velocity is zero, then yes, otherwise no.

    If the lightwaves move through a locally stationary medium
    Stationary with respect to what ?
    Stationary to the earth, the rails, the embankment.

    The train and train guy not stationary to it.
    Reply With Quote  
     

  26. #25  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    Stationary to the earth, the rails, the embankment.
    And what about if you conduct the same experiment on Mars ? Now think carefully - if you reply "stationary wrt the surface of Mars", then that means it cannot be stationary on Earth. And vice versa, since these two bodies are in relative motion. Yet light behaves the exact same way everywhere.
    So I ask again - wrt what is the medium stationary ?
    Reply With Quote  
     

  27. #26  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Noa Drake View Post
    Stationary to the earth, the rails, the embankment.


    And what about if you conduct the same experiment on Mars ? Now think carefully - if you reply "stationary wrt the surface of Mars", then that means it cannot be stationary on Earth. And vice versa, since these two bodies are in relative motion. Yet light behaves the exact same way everywhere.
    So I ask again - wrt what is the medium stationary ?

    Marse would create its own gravitational 3D particle field that is stationary to it.
    Such a field would rotate with a fade out towards the outskirts, with the spinning planet in the middle.

    No place here to elaborate on that i think, i'll keep that for my other thread.
    Reply With Quote  
     

  28. #27  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Back to the issues of Train Guy and the lightning : Imagine a simple visual to make it more clear :

    Lightning strikes at B and A


    Now say that at point B (and A) we have a kanon shooting a kanonball
    that represents conveying to Train Guy the total amount of light.


    GR says Train Guy receives the ball (a painfull affair no doubt… ) , in his frame,
    at exactly the same time as ball A.


    But in the mean time he and the train AND the kanon at B moved to the right side a bit.


    How could the trajectory or the speed or anything of the ball B, be changed by moving the kanon
    after the ball is fired ?


    This is my point , considering a medium for emr-transport and gravity that is stationary in the whole rotating planetary frame here,
    stationary that is to the embankment and the planet, nonstationary to the kanonball,the kanon, the train and train guy.




    Following GR the ball would have to hover stationary a small while in the air before going on at speed c towards train guy,
    or it would have to go backwards for a while even before going to train guy, in order to arrive in the same instant as kanonball A.
    Obviously that would be nonsense.


    So what is the answer then ?
    Reply With Quote  
     

  29. #28  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    So what is the answer then ?
    The answer is that neither the canon nor the guy change at all in their own frames. What does change here is the relationship in space-time between emitter and receiver; that is why the notion of simultaneity is frame dependent. SR ( not GR - GR is needed only to deal with gravity ) is a theory about relationships between reference frames in flat space-time, not about the frames themselves. Do you see the difference ?

    To make this even clearer, consider this : you place a pretzel on a table, then kneel down and look at it side-on. All you are going to see is a boring old piece of baked dough. Now stand up and look at the pretzel top-down - you are now seeing a much more complicated shape with three holes in it. You have not changed, nor has the pretzel, but was has changed is the geometric relation ( =perspective ) between your eyes and the pretzel. SR is very much the same - relative motion changes the geometric relationship between frames of reference in space-time, and the various things like time dilation, length contraction etc are manifestations of this. It's like changing perspective, in a way.
    Howard Roark and Noa Drake like this.
    Reply With Quote  
     

Similar Threads

  1. Time warp and Einstein : Trillion Frames Per Second
    By sciencestudy in forum Physics
    Replies: 4
    Last Post: December 1st, 2013, 02:42 AM
  2. Energy, Time, and Einstein
    By The Stick in forum Physics
    Replies: 4
    Last Post: June 26th, 2011, 11:52 AM
  3. Einstein on Time Travel
    By Unofre in forum Astronomy & Cosmology
    Replies: 11
    Last Post: July 11th, 2008, 04:29 PM
  4. einstein' time theory in 1905
    By almirza in forum Physics
    Replies: 1
    Last Post: December 4th, 2005, 03:39 PM
  5. Replies: 2
    Last Post: August 25th, 2005, 11:07 AM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •