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Experiences of particles moving at relativistic speeds in non-vacuum

  1. Aug 7, 2005 #1

    ACG

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    I've probably got something wrong here, but bear with me.

    An electron is moving through a material faster than the speed of light in that material. I know particles tend to do this from time to time -- we have Cerenkov radiation.

    If a TV camera were attached to this electron in a way which would not slow it down (you know what I'm talking about), would it witness "tachyonic" behavior such as things going back in time and stuff like that? Granted, the object is moving slower than c, but if the light in the medium the electron is moving is moving slower still strange things can happen.

    Which aspects of relativity would be violated / seem to become irrational in situations like this? You obviously wouldn't get "negative lengths" from length contraction exceeding c.

    Basically, if all of the rules involve nothing exceeding the speed of light but the speed of light is itself slowed because of the medium, how would the particles in the scenario know that the speed of light they are experiencing is not c?
     
  2. jcsd
  3. Aug 7, 2005 #2
    This operation of slowing down the recorded motion must experience some kind of phase transition at recorded motion with c.

    But this is something that makes one think...

    Best Regards,

    DaTario
     
  4. Aug 7, 2005 #3

    ACG

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    Hmm...I never thought of that. But the phase transition part...I'm now starting to wonder what happens if the electron (or neutron so you don't need to worry about charges and stuff) starts decelerating as it interacts with the medium (or because of the increased mass of attaching the TV camera :smile: ) and eventually reaches the speed of light in that medium.

    I just had a thought of what would happen if we've got a whole bunch of neutrons doing this...if time goes backwards from their perspective or it stands still, does a neutron recorded on TV never decay? And if the other particles were perceived as going backwards in time by the "tachyonic" neutron with the camera, would we suddenly see electrons and protons suddenly "reverse decaying" into neutrons? There's GOT to be something wrong here...

    ACG
     
  5. Aug 7, 2005 #4

    EnumaElish

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    They don't, and needn't. To my knowledge, laws of physics do not work like consecutive speed limits on a state road. E.g. between two cities the speed limit is 65mph but within a city (a denser medium) it's 35mph.

    Lightspeed is more like, how fast can an automobile drive, technically? Let's say the fastest a car can go is 200 mph; faster than that, and it would take off into the air (become a tachyon, as it were). Then regardless of the "local" speed limit, one can drive a car up to 200 mph and not become a tachyon.

    In short, rgardless of the medium, the universal speed limit is c (as opposed to the speed of light in the medium). You are confusing c* (inside medium) with c (in vacuum). The physical speed limit is c independent of the medium.
     
  6. Aug 7, 2005 #5

    ACG

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    The ultimate speed limit is in fact c, I agree, but that brings back the first question: if it's possible for a rocketship to travel in excess of c* but by necessity less than c, what would the astronauts see when they looked out the window?

    Come to think of it, could a 0.9999c-max ship escape from a black hole they're about to fall into by radioing out an SOS just outside the event horizon which says, "Enterpise, we're about to fall into a black hole here and we're in vacuum. We obviously can't get out -- yet. However you've got a convenient star there consisting entirely of gas where c* is 0.98c, and we've got our powerful engines which can reduce our fall right to a nonzero but very slow rate. We'll buy time before we hit the singularity -- enough time for you to tractor beam that 0.98c gas over to the hole and drop it in right on top of us...: That's GOT to be illegal, somehow...maybe the time domains are screwed up in the black hole example?
     
  7. Aug 7, 2005 #6

    JesseM

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    I think it would have to be true that the electron would see certain things running backwards if it was going faster than the speed of light in the medium (although that's not the same as saying the events run backwards in the electron's own inertial rest frame). Imagine the electron moving away from a clock in the medium which sends out a signal every second--if the electron is moving faster than the signals, then it should keep passing successive signals from earlier and earlier times as it moves away from the clock.
    Nothing would really be violated, because it's understood that the "c" in relativitistic equations is the speed of light in a vacuum.
     
    Last edited: Aug 7, 2005
  8. Aug 7, 2005 #7

    EnumaElish

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    I guess I agree with that characterization. The faster-than-light particle should be able to catch up with light propagated from earlier events.
     
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