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Loss of simultaneity explained by classical physics

  1. May 20, 2008 #1
    if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity?

    in other words, it is not necessary to go outside of classical physics to explain loss of simultaneity.
     
  2. jcsd
  3. May 20, 2008 #2
    Yes, that's what Lorentz showed, which is why so many concepts in special relativity are named after him.
     
  4. May 21, 2008 #3

    Garth

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    No - SR results in simultaneity being a frame dependent concept, it is not simply an allowance for the time taken for light-speed communications to reach the observer.

    Consider a star 1000 light-yrs away, you are seeing it as it was 1000 years ago.

    You may ask, "What is the star doing now, might it have gone supernova?"

    To answer this question you take the state of the star as you see it (as it was 1000 years ago) and project forward 1000 years into its future, you conclude that it was unstable and that now it has gone supernova, and now it no longer exists as it was.

    Another observer happens to be passing close by you, traveling towards the star at high speed. They see the star in the same state as you do. The photons from the star reach you and them simultaneously as you both pass each other close by.

    However, they measure the star's distance as, say, only 800 light years and conclude that they are seeing it as it was 800 years ago.

    They ask the same question and project the state of the star 800 years into its future and conclude that it was unstable and that now it is about to go supernova, but not just yet.

    Both observers have a clear idea of when now is, but one thinks that is when the star is about to go supernova and the other just after it has exploded.

    The concept of now, or simultaneity, is frame dependent.

    I hope this helps.

    Garth
     
    Last edited: May 21, 2008
  5. May 21, 2008 #4
    you didnt even touch my question at all. you answered something else entirely.
     
  6. May 21, 2008 #5

    Garth

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    That depends on what you mean by "Classical Physics".

    In classical physics both observers would think the star is at the same distance.

    If you introduce Lorentz contraction and an invariable speed of light then you are back in SR.

    Garth
     
    Last edited: May 21, 2008
  7. May 21, 2008 #6
    what does a star 1000 light years away have to do with my q?
     
  8. May 21, 2008 #7

    Garth

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    One problem I have is that you have not clearly stated a question!

    The title of your thread is "loss of simultaneity explained by classical physics" and you posted:
    You seem to be stating that the loss of simultaneity is caused by a finite speed of light that was present in classical physics. Am I right in thinking this?

    If so then the answer is no- the classical physics understanding of the finite speed of light does not lead to a loss of simultaneity.

    Both classical and relativistic physics have a clear idea of what is 'here and now' for the two observers passing close by in my example above, where the two physics diverge is over what 'over there and now'.

    The two mutually moving observers at the moment they pass close observe the rest of the universe at a distance from them.

    Their light cones coincide, they observe the same events simultaneously and realise that they are seeing these events as they were some time in the past.

    However, they differ on what they think is happening now, i.e. when they project forward in time an event, observed in the present as it was in the past, to their individual concept of what is happening over there now.

    That is what that supernova star 1000 light years away has to do with your question, if I have understood your question properly.

    Garth
     
  9. May 21, 2008 #8
    if an object is made of particles that are interacting via electric and magnetic fields that propagate at the speed of light then does it not follow that a moving object will experience a loss of simultaneity within its own body?
     
  10. May 21, 2008 #9

    Garth

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    What do you mean by: "experience a loss of simultaneity within its own body"?

    Garth
     
  11. May 21, 2008 #10
    exactly that.
     
  12. May 21, 2008 #11

    Garth

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    I have explained the SR concept of simultaneity, that two events, separated by some distance, that are thought of as being simultaneous by one observer (e.g. the moment 'now') are not thought of as simultaneous by another observer moving relative to the first observer.

    It's importance is that we divide time up into 'immutable past', 'present' and 'changeable future', yet moving observers disagree as to which events, distant from them, are in the past, present and future!

    Your statement of "a loss of simultaneity within its own body" makes no sense to me.

    Garth
     
  13. May 21, 2008 #12
    how would be decide whether 2 events, one occurring at the front and one at the back of his ship (which is moving near light speed), were simultaneous?
     
  14. May 21, 2008 #13

    Garth

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    As we are talking about events within the ship its speed (relative to what?) is irrelevant.

    Set off a flash bulb at both events, if an observer elsewhere in the ship observes the flashes, allows for the light-speed delay for the flashes to reach her, and concludes they happened at the same time, then they were simultaneous in her frame of reference.

    Garth
     
  15. May 21, 2008 #14
    the ability to ignore the speed of the ship is part of relativity. we are assuming a purely classical universe for this thought experiment.

    ok, lets look at this. how would she know that the 2 flashes were simultaneous? she knows the distance between them. she knows when they arrived at some predetermined point. so the q is how does she know the speed of light? in a classical universe the speed of light would not be constant.
     
    Last edited: May 21, 2008
  16. May 21, 2008 #15
    I think granpa was initially talking about the components which make up our bodies which are all in motion and bonded by electrochemical and nuclear forces (which propagate at the speed of light).

    If you were able to take a moment in time and freeze it and then analyse it, you could list a huge number of events associated with my body. Say one for each molecule, using some arbitrary standard for assigning a sufficiently precise location for these molecules. If I were to be moving relative to another observer, I myself would not be simultaneous relative to that observer. That is to say, while all my molecules are simultaneous for me (inaccurately speaking, since they are moving around), they are not simultaneous for the observer relative to whom I am moving (as a whole).

    It is like me being a rod, lying down parallel with the direction of relative motion. My head will not be simultaneous with my feet according to the observer not at rest relative to me. I am just taking it to molecular level, but the principle remains the same.

    Talking about my body not being simultaneous with itself would rely on an arbitrary, highly precise reference frame. Say I pick a molecule in my body at random, and consider a very short period of time in which that molecule is inertial, I could say the frame in which this molecule is at rest is my "reference frame". Most of me is not at rest relative to this frame. The problem with trying to say I am not simultaneous with myself (ie relative to my arbitrary "reference frame") is that none of my other molecules are at rest relative to the majority of my body either. To have any sort of meaningful and internally consistent simultaneity I would need to have a subtantial proportion of my composite components at rest with each other. Then I could say that relative to other parts of me, that substantial proportion of me is not simultaneous.

    But it is long searched.

    cheers,

    neopolitan
     
  17. May 21, 2008 #16

    Garth

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    Velocities are relative in classical physics, ever since Galileo and Newton.

    Within the ship, the bow the centre and the stern are all travelling at the same velocity, relative to some external reference point.

    Classical physicists such as Hippolyte Fizeau and Léon Foucault in 1849 measured the speed of light.

    The bow (B) and stern (S) are stationary in the observer's frame of reference, dividing their distance from her by the classical speed of light she works out the two time delays; subtracting these from the two times the flashes were observed she can tell whether B happened before A, after A, or was simultaneous with A.

    What's the problem?

    Garth
     
  18. May 21, 2008 #17
    the ship would not perceive light to be moving at c. she cant measure the one way speed of light becaues she has no synchronized clocks (thats what we are trying to create). but she can measure the two way speed of light.

    no, neopolitan, you missed my point.
     
    Last edited: May 21, 2008
  19. May 21, 2008 #18
    it seems to me now that perhaps i asked the wrong q. if she can determine the velocity of light relative to her ship then she can indeed determine the absolute time of events. but the real issue of simultaneity is whether those events would transpire on board the ship moving near the speed of light the same way they would if the ship was stationary.
     
  20. May 21, 2008 #19
    Okay, granpa.

    Fortunately it was entertaining to think it all through, so no harm done.

    cheers,

    neopolitan
     
  21. May 21, 2008 #20
    we need to create some kind of system of INTERACTING objects within the ship then accelerate the ship to near the speed of light and see what happens. the only system i can thingk of would be an antenna in the front and back each of which sends a pulse to the other one whenever it receives a pulse. pretty weak i know but its all i can think of.

    obviously they would drift out of synch as the ship moves faster. if this happened to every part of the ship and affected everything that happened on board the ship then that would be a loss of simultaneity.
     
    Last edited: May 21, 2008
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