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Is the relativity of simultaneity relative?

  1. Nov 17, 2006 #1
    Is the relativity of simultaneity relative???

    I've been thinking about this for a while and this is really troublesome...

    The short question: Why can’t we draw the conclusion that simultaneity is relative only when events involving light/electromagnetic radiation are considered?

    The explanation or the long question:

    In order to show that two events are relative, Einstein used the example of two lightning bolts occurring in the front, respectively in the back of a train. Alternatively, one can arrive at the same result by imagining that from a source placed in the centre of a train two light signals emerge towards the end of the train. In the reference frame of the train, the two light signals reach their destinations simultaneously, but from the reference frame of an external observer that watches the train passing by, the two events are not simultaneous (the light signal sent in the direction opposite to that in which the train is moving will arrive its destination first).

    However, the theory of relativity claims that all events simultaneous in one frame are not simultaneous in other frames, which move relatively to that frame. Thus, the fact that simultaneity of events is relative is not confined to events involving light, but – according to the special relativity – the simultaneity of all events, regardless of their nature, is relative. My question is what legitimates one to draw such a conclusion, since the relativity of simultaneity explicitly happens only to those events involving light (or other form of electromagnetic radiation). If, instead of light signals one chooses other kinds of signals (whose speed is not invariant), then one does not violate Galilean invariance, and hence, one does not obtain deviations from simultaneity, no matter how big the relative velocity of the considered frame. Why can’t we draw the conclusion that simultaneity is relative only when events involving light/electromagnetic radiation are considered?

    By no means do I question the validity of special relativity (I’m not that stupid), but I am stupid enough not to see why simultaneity is relative for all simultaneous events (like mechanical events, for example), not just for those involving light.
  2. jcsd
  3. Nov 17, 2006 #2


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    In a nutshell, I think the answer is that all the known fundamental laws of nature--not just electromagnetic laws--are invariant under the Lorentz transformation, meaning that they will obey the same equations when translated into any one of the different coordinate systems given by the Lorentz transformation. If a violation of this rule was ever found, it might provide a basis for considering one frame to be "preferred" and thus for its definition of simultaneity to be preferred as well, although it seems unlikely that it should just be a coincidence that all the different fundamental laws found so far have obeyed this rule.
  4. Nov 18, 2006 #3
    Einstein himself once commented that the critics of SR argue that too much emphasis is unjustly attributed to the velocity of light - Einstein didn't really have too good an answer to this criticism - Jesse's post is probably as good as any. It works whenever its been put to the test. Einstein was very intuitive - there were aspects of SR that were not provable in 1905. His concept of universal light speed and its determination of simultaneous events was one of them. We would of course have to modify this dependence upon the velocity of light if some information transmitting fields are ever shown to travel faster than light.
  5. Nov 18, 2006 #4
    The whole reason for relativity theory is that, philosophically, the laws of physics should not be subjective. And it seems the only way to say "Maxwell's equations are true for everyone" is by also saying that time runs differently for different people, or as you put it, "simultaneity (even of events that don't involve electromagnetism) is relative".
  6. Nov 18, 2006 #5
    The key is to realize that it is only the " Time Readings " that determine whether or not two events are considered to have occurred simultaneously or not.

    We exist within the dimensions known as Space-Time. What direction we travel while within that Space Time, also determines how much our measurement instruments extend across not only Space, but across Time as well.

    If the Train is moving at a high velocity across Space, then the Train has also rotated across Time. As it rotates, its Spatial length contracts, and that which is lost in Spatial length, is now gained in Time length.

    Hence, if synchronized clocks were positioned at both ends of the Train while the Train was at rest in Space, once the rotation occurs, thanks to then moving at a high velocity, the clocks are no longer equal in readings. As the train continues to move at this new high velocity, the clocks are still ticking at the same rate as each other, although now both ticking slowly in comparison to their normal ticking rate which occurs while they are at rest in Space.

    The velocity of light is constant !

    And so, if light was released from both ends of the train, and it is done so when precisely 12:00 AM is indicated at the clock where it is released from, then the light will be released from one end not exactly at the same time as it is from the other end, for the clocks are not in agreement, and one clocks registering of 12:00 AM is not the same as it is for the other clock.

    But for an observer who is positioned at the center of the Train, he is not aware of the fact that these clocks are no longer synchronized in readings. And so when light from both ends of the Train reaches him at the center at the same time, and let's say that the light itself is the readings of the clocks which have LED digital displays, all seems well to him and both clocks lit up at precisely 12:00 AM. From his point of view, the speed of light is still the speed of light even though he and his Train are moving at a high velocity across Space.

    Form his point of view, the release of light at both ends of the train, were synchronized events.

    And so it is to be noted that the rotation of the Train across Space-Time, is proportional with the addition in Velocity of the Train across Space. The change of the Time readings on the clocks therefore proportionally compensate for Spatial Velocity of the Train relative to the Universal Spatial Velocity of Light.

    He may then stop his train. The clocks become re-synchronized. Once again at the next 12:00 AM for instance, the clocks may light up again, and again the man in the middle sees both clocks light up at the same time. In the train mans world, this synchronicity remains no matter what.
    Last edited: Nov 18, 2006
  7. Nov 18, 2006 #6
    Are there any experiments which confirm special relativity? I know of the Kennedy–Thorndike experiment, but it involves light. I am curious whether a different kind of experiments showing relativistic effects like time dilation or length contraction have been performed (I'm thinking of experiments involving mechanical devices like mechanic clocks, rather than light clocks).
  8. Nov 18, 2006 #7


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    consult the other thread you started.
  9. Nov 18, 2006 #8


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    A short list:

    They've flown atomic clocks on jets to confirm that the clocks show time dilation.

    The fact that muons formed in the upper atmosphere have too short of a half-life to reach the surface, yet they do because time dilation slows their decay rate.

    Every day, in particle acclerators around the world, particles are created which live longer due to time dilation. These same particle accelerators have to take Relativistic effects into effect in their very design to operate properly.

    The entire GPS system has to be adjusted to account for Relativistic effects in order to remain accurate.
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