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Black holes, white holes, and time

  1. May 20, 2009 #1
    Please forgive me if I am posting in the wrong place, and also if this has been discussed before.

    Gravity is so strong in a black hole not even light can escape, Can the light be accelerated past c once inside of the event horizon? I know the universal answer that nothing can exceed c, but don't the laws of physics start to break down in a singularity? If (theoretically) the light did exceed c, it's my understanding that it could go back in time. Having read the discussions I understand that there is no point of reference for a photon, but some suggest that time for the photon doesn't exist, which leads me to wonder if the photons are accelerated past the speed of light they would then travel backward through time and return to their point of origin, and this return is seen by us as quasars(white holes?) and even the big bang(the biggest white hole ever?). The number one problem I have with this is the chicken and the egg.

    I hope this is clear enough to illustrate my thoughts.
  2. jcsd
  3. May 20, 2009 #2


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    Welcome to PF!

    Hi ocman! Welcome to PF! :smile:
    From a distance, light may seem to change speed (for example, light going very near the Sun)

    But measured locally, light always has a speed of c.

    (and, of course, the inside of an event horizon isn't a singularity … the singularity is only at the exact centre :wink:)
    No, there's nothing in the maths that suggests that going faster than c would send anything backwards in time … that's just a Star Trek thing. :wink:
  4. May 20, 2009 #3
    Hmm. I've always believed that traviling faster than c (which is impossible anyway) would send you back in time.
  5. May 20, 2009 #4
    Vorbis is right. Tiny Tim, I'm sorry. :smile:

    Take tachyons for example. They have imaginary relativistic mass. How? They exceed speed C. According to Einstein's formula,

    E = mc^2
    ___(1- v^2/c^2)^ 1/2

    Tachyons go backwards in time because they exceed speed c.

    If I run fast, I age more slowly. If I run near the speed of light, I age extremely slowly. If I run at speed c (ignore the impossibilities) I will not age at all. If I outrun this, I start to get younger, or I age backwards in time.

    But, it doesn't matter. If a photon would exceed speed C, it would attribute imaginary relativistic mass. This is most likely not possible.
  6. May 21, 2009 #5


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    Hi Vorbis! Hi benk99nenm312! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    Nope … time dilation is √(1- v2/c2), which of course -> zero as v -> c …

    but for v > c, √(1- v2/c2) is imaginary, not negative …

    as I said …
  7. May 21, 2009 #6
    √(1- v2/c2) has two solutions, just like every other square root. It would be possible to get a negative solution.

    As I stated, tachyons have some odd properties. One, they go backwards in time. Two, they accelerate when they lose energy. How would this exotic property be possible without going backwards in time?
  8. May 21, 2009 #7


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    Yes … so slower-than-light particles can travel either forward or backward in time.

    But tachyons can't … their time dilation is imaginary (and minus imaginary is still imaginary :wink:)
    No they don't …

    If we see a tachyon going from A to B, another observer may see it go from B to A … that's all!
    Nooo … energy = m0/√(1 - v2/c2), so either the energy of tachyons is also imaginary, or their rest-mass is.
  9. May 21, 2009 #8
  10. May 21, 2009 #9


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    Hi benk99nenm312! :smile:
    Not following you … we must observe things going forward in time, mustn't we? :confused:
    Sorry, not following any of this :redface: … and I'm definitely not saying it.
    The tachyon appears to one observer to go, say, from the torch to the screen, but appears to another observer to go from the screen to the torch.

    Both observers see it going forward in time (but the second observer thinks it looks weird :rolleyes:)
  11. May 21, 2009 #10

    Exactly. The second observer sees the process happen backwards, or reversed. That is what I was trying to say. But even so, from the particle's view, it is doing something sensible. If I were to throw an apple at a velocity of 1o meters per second, it continue forward. If I were to somehow give it energy.. enough to travel past the speed of light (somehow), it would gain velocity, and then, after it passes speed c, something happens. The apple, From its reference frame, would continue forward, accelerating as it does so. But I would start to see the apple slow down, and reverse its direction of motion. :biggrin:

    It is that difference in apparent motion through space and time that I am trying to explain. Its prosseses are reverse from the observer's point of view. That is what I'm getting at. The apple itself never travels backwards in time, but we see its motion reversed. It, in a way, acts like an apple traveling backwards in time, because the motion in space is reversed. However, you are right when you say it is not actually going backwards in time.

    I hope that clears it up. :smile:
  12. May 22, 2009 #11


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    Nooo … if you keep your velocity the same, you will simply see the apple go faster and faster (and faster than light) in the same direction

    that's what speed is!

    No observer will see anything reverse its direction of motion.
  13. May 22, 2009 #12
    But you have said it yourself.
    "The tachyon appears to one observer to go, say, from the torch to the screen, but appears to another observer to go from the screen to the torch."

    I'm getting confused. I need some decent sleep. :smile:

    Also, now that I think of it, we couldn't see it pass the speed of light. Light wouldn't be able to keep up with it.
  14. Aug 12, 2009 #13
    so there might be stuff travelling faster than the speed of light but we just can't detect it?
    Last edited: Aug 12, 2009
  15. Aug 12, 2009 #14
    Not quite what I meant haha.

    You wouldn't see it from behind, because light would have trouble catching up. Although, photons are still bouncing off the front face of the apple, at least, I think. In the last example, the front face is on the opposite side, so it would make it rather difficult to see I believe. This is a hazy subject for most haha, since it is speculative to begin with.
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