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B Black hole time reversal?

  1. May 18, 2016 #1
    hi PF,
    "TIME is reversed inside the even horizon" what does it means?.
    My sort of opinion is that for an observor outside the event horizon,the time becomes to move slow.but inside the event horizon it does not.Is that my opinion is correct or not about time reversal??
    And "the black hole evaporates that light is able to escape"in these line they had mentioned that the black hole evaporates,since the blackhole is not composed of any matter then how evaporation may occurs?
  2. jcsd
  3. May 18, 2016 #2
    Time dilation depends on the observer's frame of reference.
    An outside observer would see see a watch owned by somebody falling in to be slowing down.
    As far as the person falling in is concerned, nothing has changed, time passes at the usual rate.
    I don't know about time reversal, where did that idea come from?

    The radiation proposed by Stephan Hawking does not come from inside the the black hole, but is produced at or just outside the event horizon.
    It is a quantum mechanics effect hard to describe other than mathematically, but roughly speaking the radiation created outside the horizon corresponds to loss of mass inside the horizon, yet nothing actually moves from the inside to the outside.
    Last edited: May 18, 2016
  4. May 18, 2016 #3
    thanks for your reply rootone.you had told that "nothing actually moves from inside to outside" not even light moves away from inside to outside.
  5. May 18, 2016 #4


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    Look at the Penrose diagram of the full Kruskal solution (inside and outside of the Schwarzshchild black hole). What happens to the space and time coordinates as you cross the event horizon?
  6. May 18, 2016 #5


    Staff: Mentor

    It means someone is trying to use imprecise ordinary language to describe physics, instead of using precise math. There are coordinates in which the ordinary language quoted above can be a valid interpretation of the math; but there are other coordinates (such as the ones bapowell mentioned) where it isn't. The best way to resolve these kinds of issues is to stop looking at coordinate-dependent concepts and start looking at invariants. There are no invariants that correspond to "time being reversed" inside the event horizon.

    There is no invariant way to compare the "rate of time flow" of an observer inside the horizon with that of an observer outside the horizon. So this statement isn't correct.

    The hole has energy; that's where the energy contained in the Hawking radiation comes from.
  7. May 18, 2016 #6

    George Jones

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    Statements of this sort are sometimes made in good physics books, but I think that they are highly misleading.

    Other people have answered, but I, too, will give it a go.

    Humans use human-given names to label places and times: 1) sometimes these names are appropriately suggestive; 2) sometimes these names are misleading; 3) sometimes, confusingly, the same names are given to different places. For example, several years ago, a couple booked a vacation flight to Sydney. They didn't realize how many cities have the same name, "Sydney", and they ended up in Sydney, Nova Scotia, Canada instead of Sydney Australia. More information than just "Sydney" was needed to pin down which Sydney.. I recently was reminded of an inappropriate name when, for the first time in many years, I heard the Johnny Cash song "A Boy Named Sue".

    For historical reasons, humans, unfortunately, have given the same names, coordinates (t,r), to events in two different regions of a black hole spacetime (extended Schwarzschild), i.e., an example of 3) above. Just as in the Sydney example, the ambiguity is lifted if more information is given, e.g., (t,r) in region I (outside the event horizon), or (r,t) in region II (inside the event horizon). In region I, the labels t and r are appropriately suggestive, i.e., t is a time coordinate and r is a space coordinate. In region II, the labels t and r are misleading, i.e., t is a space coordinate and r is a time coordinate.

    So, it is not that time reverses, it is that humans have used the same human-given name t outside and inside the event horizon, with t representing time outside and t representing space inside. It would have been better to use completely different labels for regions I and II, but these names were given about a century ago, and they have stuck.
  8. May 18, 2016 #7

    Thank u peterdonis. now i understood clearly.
  9. May 18, 2016 #8
    Thank u george jones.i had grasped your points
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