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Simultaneity & seeing the same event multiple times.

  1. Apr 14, 2010 #1
    I was thinking about the Andromeda paradox and the following occurred to me. Any thoughts on whether this would be possible?

    I'm driving my car in a remote, non-light polluted location and ahead of me, just above the horizon. I see a very-distant star (a supernova). Just then, I see the supernova explode in a flash of light, and then go dim. The whole process takes just a few seconds.

    I put my foot on the brakes and bring the car to a stop. In doing so, I am accelerating, ever so slightly, away from the supernova. This causes my frame of reference to shift, and thus my simultaneity plane shifts. But that slight shift in simultaneity is magnified over large distances. So as my car stops, I watch the explosion in reverse. By the time I've come to a stop, I see the supernova in its pre-explosion state. Standing still, I continue to watch it, and after some time passes, I see it explode again. Now I quickly turn my car around and accelerate in the opposite direction, away from the supernova again. In my rear view mirror, as I am accelerating, I see the explosion in reverse and the supernova again emerges in its pre-explosion state. Some time after I stop accelerating, I look in my rear view mirror and again watch the explosion in forward motion.

    All in all I've watched the explosion 5 times, 3 times forward, twice in reverse. Given a large enough distance between Earth and the supernova, is this really possible?
     
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  3. Apr 14, 2010 #2

    Matterwave

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    you'd have to drive your car faster than the speed of light...to get in front of the wave-fronts again.

    Just think this problem through in terms of photons emitted and the sphere they make. You should be able to see that no matter how far you are from the super nova you can't see it twice unless you drive faster than the speed of light.

    Also, reversal of the order of events is not allowed in SR...this is to preserve causality. If you could see the supernova "de-exploding" that would be a violation of causality since it would mean the explosion came before the mechanism that caused it to explode.

    The 2 arguments are very much intertwined. If you could go faster than the speed of light...you could violate causality.
     
  4. Apr 18, 2010 #3
    But if the supernova generated a sound during the explosion you could hear it many times this way. Then you could violate causality:smile:.
     
  5. Apr 18, 2010 #4

    Jonathan Scott

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    Seeing events is very different from calculating when they occurred. Simultaneity is about calculations of when something must have occurred from your own point of view, based on when you saw it in your own time line and how far away you think it is. If your velocity changes, that affects how far away you think it is and hence your calculation of when it happened. However, it does not affect the sequence of what you saw.
     
  6. Apr 18, 2010 #5

    Matterwave

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    Sounds is not treated so special like light in relativity. Sound waves do not travel at some constant speed in all inertial reference frames. Also, since sound waves always need a medium to propagate in, they always have a "preferred" (that of the medium) reference frame in which to view them.
     
  7. Apr 18, 2010 #6
    Let take a train has open carriages so that air is not moving with the train. Then the sound wave propagates through the air independently from the reference frame which is the train. Light is an information carrier of events in the relativistic theory, but sound wave is too an information carrier. Why should we treat them differently?
     
  8. Apr 18, 2010 #7

    Matterwave

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    The speed of sound as measured by people in the train would be different than the speed as measured by people outside the train who are at rest w.r.t. the air.

    Light is special.

    Recall the Lorentz factor: 1/sqrt(1-v^2/c^2) which determines time dilation and length contraction. The speed in there is the speed of light, not sound. So, the speed of light really does have something fundamentally to do with the fabric of the universe. The speed of sound depends completely on the medium it's moving in.

    IN THIS WAY, you can ALWAYS construct a preferred reference frame for sound (as I mentioned in the previous post), and say THAT is the reference frame from which you should be measuring things. There is no such frame for light.

    So, if the same event happen 2 different ways according to sound information, you know which reference frame you SHOULD HAVE gathered the data from and therefore the 2 different ways are NOT equally valid. Whereas, if the same event happened in 2 different ways according to light information, there IS NO reference frame from which you should have gathered data. And therefore, both ways are equally valid!
     
  9. Apr 18, 2010 #8
    Matter, for present purposes an observer, you or me, cannot travel faster than light but can travel faster than sound. So if we hear a sound, it is possible for us to travel at sufficient speed in the direction of the sound wavefront that has passed us, overtake it, and so hear it again. This is not possible with light due our speed being limited to less than the speed of light.

    Matheinste.
     
  10. Apr 18, 2010 #9

    Dale

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    FedEx is also an information carrier. The speed of FedEx and the speed of sound are not frame invariant, but the speed of light is frame invariant. We treat them differently because they are different.
     
  11. Apr 19, 2010 #10
    This is only an assumption of the relativity theory. It comes out because light is the information carrier of the events. Speed of light also depends on the medium in which is propagated.
    If we place a clock on the train that ticks, then for the observer on the train the distance from the clock to his ear will be constant, but for an observer on the railway station the clock will tick slower, because as the train moves away, the tick sound has to travel longer distances as it is in the case of light. We could do the derivation of the Lorentz equations using sound wave, would we come up with a conclusion that nothing could travel faster than the speed of sound?
     
  12. Apr 19, 2010 #11

    Matterwave

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    Did you not read my post at all?

    You are free to come up with w/e weird physical laws you want to come up with. You can come up with speed-of-sound dependent transforms; however, we will quickly see that your theory doesn't agree with experiment while Einstein's theory does. =)

    The absoluteness of the speed of light is NOT an "assumption" of SR. SR only makes 2 assumptions, the frame-invariant nature of light, and the frame-invariant nature of physics.

    At the LHC or other particle accelerators, we can see the speed of light barrier VERY well. We keep adding more and more energy to these particles but they NEVER reach the speed of light...they just get arbitrarily close. Try telling a particle physicist working at Fermi-lab that "the speed of light is merely an assumption".
     
  13. Apr 19, 2010 #12

    Mentz114

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    I smell an agenda. In another thread tabarnard is arguing for FTL travel and now Tantalos adds his ha'penny worth ( which is overvaluing it a lot ).

    It's been correctly pointed out that SR has survived experimental tests at extreme velocities. So what gives ? The OP must have known that FTL travel is forbidden by SR - so why ask ?
     
  14. Apr 19, 2010 #13

    Dale

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    No, it comes out because c is frame invariant.
     
  15. Apr 19, 2010 #14
    I do not question the work of physicists. I am trying to understand the difficult relativistic theory. I am new to this topic and that is why I am asking silly questions.
    Next thing i do not completely understand is the twins paradox. One twin travels in a spaceship with a speed near the speed of light and his clock is ticking slower relative to his brother's clock, so he will become younger than his brother. But in order to check whether he is younger than his brother, he must stop and wait for his brother to arrive. While he waits and the brother is travelling, his clock will tick faster relative to the brother's clock. So when they finally meet, they will discover that they are still of the same age and their clocks show the same time. Is this correct or not?
     
  16. Apr 20, 2010 #15

    Mentz114

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    You are correct, if the journey of each twin is identical. It's clear from the symmetry of the diagram that the twins have equal proper time. If either spoils the symmetry, they will have different ages. It is also clear that it does not matter when the blue twin decides to leave, as long as the diagram is a parallelogram both trajectories have equal proper time.
     

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  17. Apr 20, 2010 #16

    Dale

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    Mentz114's answer is technically correct, but it is NOT what is usually meant by the twin's paradox. In the usual formulation of the twin's paradox one twin is inertial the whole time and the other is non-inertial. The non-inertial twin is always younger.

    So, for example, a traditional twin's paradox scenario like what Tantalos described might go something like this: In some inertial reference frame the inertial twin travels at .6c for 5 y, a total distance of 3 ly, meanwhile the non-inertial twin travels at .75c for 4 y and then stops for 1 y. If you work out the math you will find that the non-inertial twin is younger.
     
  18. Apr 20, 2010 #17

    Mentz114

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    I thought it was a completely correct answer to the question. He wasn't asking about the 'traditional' twin scenario.:smile:
     
    Last edited: Apr 20, 2010
  19. Apr 20, 2010 #18

    Dale

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    I read his question differently. I read it as asking about the traditional twin's scenario in the reference frame where the travelling twin is at rest on the final leg.
     
  20. Apr 21, 2010 #19
    But physical laws should be independent of reference frames. Then we should get the same result independent of speed of reference frame of the second twin. But I think we get diffrent results, because we did not take into account length contraction. The observer on earth will experience not only time dilatation but also length contraction of the paths the twins travel, Although to the earth observer they will appear at the same place after five years, they actually will not be at the same place due to length contraction. The twin that travels at 0.75c will be further away because his path was more contracted.
     
  21. Apr 21, 2010 #20

    Matterwave

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    Physical laws are independent of inertial reference frames. You can see physical laws taking different forms in accelerated reference frames (e.g. the Coriolis force arises due to rotation - there is no such force in inertial reference frames).
     
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