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I A spaceship traveling close to the speed of light sending some data...

  1. Jan 6, 2017 #26
    How about "The Universe and Dr. Einstein" by Lincoln Barnett with a forward by Dr. Einstein himself? Serious enough for you? In Chapter 6, using the train-and-lightening thought experiment, he claims that the observer riding at the midpoint of the train will see the forward strike before seeing the rear strike. Not so. Since the train is an inertial reference frame it can be considered "at rest" while the countryside rushes past. SR says that the measured speed of light is the same for all observers. This includes the observer on the train. It is the trackside observer who sees the front strike reaching the train observer first, not the train observer himself.

    This isn't the example of the error I originally posed, but it is another false conclusion from a "serious" source.
  2. Jan 6, 2017 #27


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    No it isn't. It is you who are misinterpreting things. You need to be careful about specifying in what frame the lightning strikes are simultaneous. You cannot just say "simultaneous" without any qualifier that tells you what frame they are simultaneous in. If they are simultaneous in the ground's rest frame, the observer on the train will see the front strike first. This must be true in all reference frames and since the speed of light is the same in all directions, the strikes must therefore occur at different times in the train's rest frame.

    If the strikes are simultaneous in the train's rest frame, then the ground observer will see the back strike first.
  3. Jan 6, 2017 #28


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    The sequence with which an observer sees (with his eyes) a set of flashes is an invariant physical fact. All observers agree on it. If two strikes are simultaneous in the trackside observer's frame then it will be a physical fact of the matter that the midpoint observer on the train will see the front flash first.

    The two observers can and do account for that fact in different ways. The train observer will say that it is because the flashes were not simultaneous. The trackside observer will say that it is because the train observer moved toward one flash and away from the other.
  4. Jan 6, 2017 #29

    Mister T

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    But you've yet to establish your first claim that there was a false conclusion from a serious source.

    It could very well be that the the writer of the Discover article made an error in his explanation of time dilation, as you originally claimed. But we don't know.

    In your second claim about the relativity of simultaneity it is you who has made the error. It's possible for those two events be simultaneous in either frame, but not both. Because the two frames are equivalent.
  5. Jan 6, 2017 #30
    This is my last response. Obviously, you are simply bent on being argumentative. If you read my description you will see than I never used the term "simultaneous'. I made quite clear the difference between what the train observer sees and what the trackside observer thinks the train observer sees. In fact, if you read the book you will see that the author made exactly the same mistake you falsely accuse me of making. "A thunderstorm breaks, and two bolts of lightening strike the track simultaneously at different points."

    That's all I have to say. I think you just want to argue. I don't. Adios.
  6. Jan 6, 2017 #31


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    I am sorry you feel this way. But consider that essentially everyone in this thread knows relativity much better than you do. From the perspective of those who actually know the subject, it is you who are being argumentative and hell-bent on not accepting conclusions that have been scrutinised by 100 years worth of physicists and that agree very well with experiments.

    In popular literature, such assumptions will often be implicit. Since it is saying "strike the track simultaneously", the natural assumption would be to assume that this refers to the track's rest frame. You are making exactly the error of assuming that simultaneous in one frame means simultaneous in all frames. In fact, this is exactly what this thought experiment shows: Assuming that the light speed is the same in all directions in all inertial frames, the events cannot be simultaneous in the train's frame if they are simultaneous in the track's frame. This thought experiment by itself is not showing time dilation, it is showing relativity of simultaneity (although it is necessary to understand relative simultaneity if you want to understand why time dilation is symmetric).
  7. Jan 6, 2017 #32


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    Yes, but in this frame the light rays are emitted at different times, which is why they are received at different times by the observer at the center of the train. Same speed, same distance, different emission times = different reception times.

    But you implicitly assumed that the light rays were emitted at the same time in the train frame. They're not. See above.
  8. Jan 15, 2017 #33
    Hmm; have you ever wondered why these age-old thought experiments, the light from two 'simultaneous' lightning strikes or alternatively light from a single source in the middle of the train, continue to cause so much discussion and dissension?

    The basic premise is the simple one set out by Einstein in Chapter IX The Relativity of Simultaneity; observers at rest upon the embankment will observe the lights meeting at midpoint M proving that events A & B were simultaneous in their frame of reference. Observers at M' on the train '(considered with reference to the railway embankment)' are moving away from point M and therefore will not measure simultaneity.

    All very simple and straightforward. So how do all the difficult questions arise?

    Is there not something fundamental that is being overlooked here? - The change in perspective that Einstein introduced with his theories.
    A fundamental change introduced with the theory of relativity was from the Objective 'God-like' view of what was being measured to the Subjective view of specific observers. Not surprisingly different observers make different measurements. Science suddenly changed from observing a single overall view to taking multiple different perspectives of different observers while still trying to present a single reality.

    Nothing is fixed anymore it all depends on the relative movement of the observer and the observed.

    So perhaps it would be better to examine what happens objectively and then calculate how this would be seen subjectively from different frames of reference.

    Objectively: Light from two events A, B meet at event M. AM = BM so the light has travelled equal times at c from each source; therefore events A and B are simultaneous from an objective point of view. What do I mean by that? Well, they are simultaneous measured using the rods and clocks of the resting frame; i.e. measured by any observer at rest relative to events A, B and M.

    But, subjectively, every observer is at rest relative to Spacetime - as mapped by their frame of reference in which they are by default at the origin or null point.
    So for any observer present at M when events A and B occur will remain at M at the origin or null point of their frame of reference.

    Taking any particular view such as the embankment and giving that the status of being the only truth is making it the privileged view. Einstein avoided that when he added

  9. Jan 15, 2017 #34

    Mister T

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    Observers at rest on board the train can make the same claim. They can use rods and clocks of their resting frame to show that any observer at rest relative to events A, B, and M will see them as not being simultaneous. All you need to do is have the lightning strikes leave burn marks on both the train and the embankement. Likewise you can have an explosion occur at M that leaves the same type of burn marks.

    Lots of research has been done in an attempt to understand and improve student understanding of this and many other topics. But wondering why most people find it difficult to understand many of the topics of physics doesn't change the fact that they do.

    Relativity of simultaneity is not demonstrated experimentally by this thought experiment. It is demonstrated every minute of every day at hundreds of places across the globe by scientists, engineers, and technicians who deal with precision timing, fast-moving particles, or both. It is not the human intellect that demonstrates the validity of this or any other physics concept, rather it is Nature's behavior.
  10. Jan 15, 2017 #35


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    There is no such thing. Individual events do not define a state of motion.
  11. Jan 15, 2017 #36


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    This is not correct. SR still has things that are fixed; they just aren't the same things as in Newtonian mechanics.

    This is correct; in fact it is the key objective fact about the entire scenario. And furthermore, this objective fact does pick out one particular inertial frame among all the possible ones. But you are not correctly describing how that frame is picked out, or what the implications are.

    Having a frame in which you are at rest does not make you "at rest relative to spacetime"; the latter concept doesn't even make sense.
  12. Jan 16, 2017 #37
    Apologies for the use of hyperbole; for of course events are points fixed in space and time; but what do we mean by 'at rest'?
    Surely that can only be relative to a frame of reference - for when we refer to a point or body, or any Minkowski 'substantial point' it is has to be located within a frame of reference.
    Any Frame of Reference gives a fixed map relative to the event at its origin, or null point; a map in which everything in Spacetime is moving relative to that frame of reference.
    It is a somewhat tautological concept but every observer must be at rest relative to their frame of reference.

    Exactly! For then it is an observer from another frame for whom the train is moving. But the observer on the train, in his frame of reference is at rest and for him A, B and M' will be fixed points and AM' = M'B. For the train observer it is M and the embankment that is moving away.

    The train observer will measure simultaneity but for them it is the embankment observer who is moving away and therefore won't.
  13. Jan 16, 2017 #38


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    You are missing the entire point. Events are single points in space-time and cannot be assigned a state of motion. In order to assign a state of motion, you need to consider the world line of an observer, which is an extended one-dimensional curve in space-time.
  14. Jan 16, 2017 #39
    Maybe we can arrange these notions this way. I think that to be “at rest” means to introduce your own rest frame with Einstein synchronized clocks, as we do in Special Relativity. An observer cannot detect his absolute motion, but can subjectively assign himself this state. What actions he has to take, if he assigns himself state of motion? What his actions should be different from those, when observer assigns himself state of rest?

    1) He shouldn’t introduce his own reference frame with synchronized clocks, but has to use other guy’s one. For example, there is a reference frame K with Einstein – synchronized clocks A and B. In this reference frame moves clock C. Observer "in motion" possesses clock C and compares readings of this clock with clock A first and clock B then (successively).

    2) If an observer ascribes himself state of rest, he introduces his own reference frame and adds another clock D into another spatial position. He synchronizes clocks C and D by Einstein. Clock A (and then clock B) now moves in his reference frame. Then he compares readings of clock A with clock C first and clock B then. Obviously, clock A dilates. So, if we describe motion and use just one reference frame, we need 3 (three) clocks. If there are two reference frames and each is "at rest", we need 4 (four) clocks.

    3) Let’s observer ascribes himself state of rest. Then another observer or observable object (source of light, for example) moves at parallel line to axis X in observer’s frame. In this case the observer, who assigns himself state of rest has to accept beams of light that were released, when this observer and source WERE at points of closest approach. If he has a telescope, he keeps his telescope along Y axis straight up.

    4) If observer ascribes himself state of motion in other guy’s reference frame, he accepts beams of light, when he and observable object ARE at the points of closest approach. In this case he keeps his telescope at oblique angle to direction of his motion “into front”. The source appears to be in the front of him, though actually is straight “under” him at points of closest approach. He thinks that he keeps his telescope at oblique angle in order to take into account aberration of light, as astronomers do observing distant stars.

    It should be noted, that if two observes move relatively to each other, they cannot ascribe themselves equal states simultaneously. Of one assigns himself state of rest, another has to assign himself state of motion. For example, if one observer releases beam of light straight up along y axis, another one, who moves in his frame, has to tilt his telescope at oblique angle to direction of his motion “into front”. They can calculate these angles using aberration of light formula.

    Or vice versa.

    Otherwise he will not see the beam of laser light.
    Last edited: Jan 16, 2017
  15. Jan 17, 2017 #40
    I'm sorry I don't understand what you mean here. '... In order to assign a state of motion ...' - in order to assign a state of motion to what? An Event? But as you have just stated viz. ' Events are single points in space-time and cannot be assigned a state of motion. ' ...???

    And I don't understand why do you say
    in response to me saying
  16. Jan 17, 2017 #41
    Einstein was quite specific in chapter IX viz. As observed from the embankment:
    - M is stationary mid way between A and B and therefore the lights will meet at point M.
    - M' is moving toward light B and away from light A and will therefore see light B first.

    I believe everyone accepts this, But do the observations from M' on the train lead to the same conclusion?

    Let us take an allegory.
    Let the Earth take the place of the train then a star will take the place of point B when the Earth is moving towards it and point A, 6 months later when the Earth is moving away from it.
    Does the light from the star take less time to reach the Earth when the Earth is moving towards it and more time when the Earth is moving away from it?
    i.e. does the speed of light vary according to the relative velocity of the Earth and the star?
    For the observer on the Embankment the speed of light is constant it is the train's speed that affects the time of the lights arriving at M'.
    For the observer on the train there is only the speed of light for they cannot be moving relative to the star for that would be the equivalent of the speed of the light from the star depending on the relative movement of the light source that is the star.
  17. Jan 17, 2017 #42


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    Here, you are considering a non-inertial frame of reference, even leaving aside the complications of GR and pretending the solar system is actually an orrery. The coordinate speed of light is not constant or invariant in such frames, and there is not even necessarily a clear winner for a definition of spatial distance. Thus we can't generalise our SR-in-inertial-frames intuitions to such frames.

    I expect the results to be consistent if the relevant calculations and definitions are handled carefully. All you are doing is changing from simple coordinates to complicated ones.
  18. Jan 17, 2017 #43

    Mister T

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    It has to be located in every frame of reference.

    You miss the point. When the two light rays meet at M they trigger an explosion that leaves a burn mark on both the platform and the train. An observer on the platform will have a burn mark that's at rest relative to him. He can use rods at rest relative to him, and clocks at rest relative to him, to conclude that the two lightning strikes were not simultaneous.

    It is entirely tautological because you are defining a person's frame as their rest frame. Note that there is no need to do such a thing, you just refer to it as their rest frame.
  19. Jan 19, 2017 #44
    Really? But surely the observer on the platform concludes that they were simultaneous?
  20. Jan 19, 2017 #45
    OK then, lin simple terms.

    Events A and B are fixed in space and time in each and every frame of reference.

    The train is the rest frame of the observer at M'.

    In the train frame A,B and M' are each fixed points.

    For the lightning strikes to not be simultaneous M' must be moving relative to A and B.

    The Observer at M' must be at rest in their rest frame! They can only be moving measured from another frame with which they have a relative velocity; or
    the light sources at A and B would have to be moving in the train frame and we would have to abandon the 2nd postulate! viz.
    We can say the observer at M' is moving toward B and away from A in the embankment frame as the speed of light is c relative to M and M' is moving in that frame, but in the frame of M' that cannot be and M' cannot be moving relative to A and B.

    What could Einstein mean in chapter IX, The Relativity of Simultaneity when he wrote:
    [my highlighting]
    by vice versa? Other than 'Events which are simultaneous with reference to the train are not simultaneous with respect to the embankment'?
    i.e. that simultaneity is relative depending on the observer's frame of reference.

    I am not trying to change anything here, all I am doing is applying the laws of relativity and reading exactly what Einstein himself wrote.
  21. Jan 19, 2017 #46


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    No. If A and B are events they only exist at a particular time. You cannot say that an event is a fixed point in space - it is a point in space at a given point in time. There is no notion of an event "moving" because in order for something to move it must exist at different times. You keep repeating the same basic mistake - events cannot be assigned a state of motion and all your reasoning is built on the assumption that it can.

    Edit: The following statements are therefore meaningless:
    With regard to:
    No, you are not doing what Einstein wrote. You have a fatal misunderstanding of what an event is. Note that the concept of an event is not particular to special relativity - the same assumptions that you do would be fallacies also in classical Newtonian mechanics. Regardless, despite what many laymen seem to think, Einstein's writing is not the definite authoritative go-to text on relativity and definitely not the most accessible. The understanding of relativity has developed significantly since 1905.
  22. Jan 19, 2017 #47


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    M' is not an event, I think. It seems to be the worldline of the observer on the train. A and B are events. To make fixed points from them you need to draw a worldline through them, and you have freedom to draw that worldline in any timelike direction. You've chosen to do that such that the worldlines are parallel to M'. Fine. But you need to be aware when you decide that "A and B are fixed points" then you have added structure that isn't inherent in the experiment.

    The thing is that the observer on the embankment can also draw a pair of worldlines through A and B that are parallel to his worldline, M. So he can also consider the strikes to have occurred at fixed points by adding extra structure in the same way that the train observer did. Again, there's nothing wrong with this, but you need to be aware that you've done it.

    "M' is a worldline representing motion towards the worldline the embankment observer chose to draw through B" is the correct way to put that.
  23. Jan 19, 2017 #48
    Events have a FIXED spacetime interval between them to ANY AND ALL observers.
    Last edited: Jan 19, 2017
  24. Jan 19, 2017 #49
    It seems @Grimble understands
    Yes, everything it is very simple. We don't need any world lines.

    There is an Embankment. Observer E is in the center of the Embankment (in the origin). Points A and B are at equal distances from E to the left and right. Two flashes flash simultaneously in E reference frame. Two beams of light approach E at the same time. Distance from E to A and B is the same, he makes conclusion, that flashes flashed simultaneously.

    A train moves relatively to the Embankment to the right (in positive X direction) . Observer T1 is in the center of the train. When flashes flashed, an observer T1 was just in the front of E. While light beams reached E, T1 moved to another spatial position X1 and light beams reached him not simultaneously, first the RIGHT (B) and then the LEFT (A).

    But the observer T1 thinks like that. It is not I, who moved. I was at rest and moved nowhere. Distance to A and B is the same. It is the guy E moved to the left. But beams of light arrived not simultaneously, though distance was the same. I ACTUALLY saw flash B first and flash A then. Velocity of light is c thus flashes flashed at different moments.

    The passenger T2 was going in another carriage of the train. HE IS a burn mark on the train. He appeared just in the front of E (in the origin), when light beams reached E. But T2 knows, that he is not in the center of the train, but he sees two light beams approaching him at the same time too since he is just in the front of E.

    T2 thinks like that. I have always been at rest since my birthday. It is E approached me from the left. I moved nowhere, but light beams arrived simultaneously. I am not in the center and distances to A and B are different, thus B released flash earlier than A. Though light beams arrived simultaneously, they were released at different moments.

    The same reflections (just opposite), if the flashes were released simultaneously in train’s frame.
  25. Jan 19, 2017 #50
    Remember ALL frames are equivalently valid and correct.
    A will "see" V occurring at W
    B will "see" X occurring at Y

    The only things that will be universally agreed upon are the speed of light and the spacetime interval between W and Y
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