Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time required to form a black hole

  1. May 31, 2010 #1
    How is it possible for a black hole to form within a finite time?
    When a star collapses - the more dense it becomes and the closer it gets to becoming a black hole the more time slows down.
    Shouldn't it take an infinite amount of time for it to become smaller then it's own schwarzschild radius?
  2. jcsd
  3. May 31, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You've forgotten the lesson of Special Relativity -- there is no absolute rate of time. A clock* ticks one second per second, no faster, no slower.

    A clock sitting on a collapsing star keeps ticking, always at a rate of one second per second. But once it crosses an event horizon, how long before the light from its face reaches an external observer?

    *: When functioning properly, of course.
  4. May 31, 2010 #3
    All the clocks (or time) when forming a black hole of inside a blackhole slows down for an observer who is outside the blackhole, but not for an observer inside the blackhole. ie.,clocks are running correctly in the blackhole but it seems that they are slowing down for an observer outside. So black hole has no objection in forming.
  5. May 31, 2010 #4


    User Avatar
    Science Advisor
    Gold Member

    The process of time inside an event horizon is debatable. Formation of an event horizon in a collapse event is, however, possible in a very short period of time as measured by external observers' clocks. A singularity is not required to create an event horizon. It is unclear if a true singularity ever actually forms based on external observers' clocks. The event horizon tells us nothing about the state of matter inside, aside from being sufficiently dense to form an event horizon.
  6. Jun 12, 2010 #5
    You are absolutely right. A black hole can would take an infinite amount of time as measured by the clock of an outside observer to fall through its own emerging event horizon and thus can never form. It is hardly surprising that the physics does not make sense inside a black hole. These things are physical impossibilities.

    To ilustrate the point better, consider two astronoughts falling into a black hole. Let's call them A and B. A is let's say a mile ahead of B.
    B would never see A fall through the event hoison. He would simply hover there for ever.
    But how can B see himself fall through the event horizon if A who is in front of him takes an infinate amount of time to fall through? B does not even have to be there. It makes no difference.

    Infinities do not exist in the real world. Even Einstein did not believe in the existence of these things!
  7. Jun 12, 2010 #6
    There are a few things you've said that are not precisely correct. Infinities do in fact, exist, such as "0". The universe could be infinite, and we might not know. You're making some very seeping generalizations without sources. Einstein, like those of his time believed that black holes were a function of the math of GR, but then, they didn't have the observational data we do now.
  8. Jun 16, 2010 #7
    As a general note to the OP, the infinite time for a black hole to form is only for an outside observer. Just because you don't see it happen, it still happens.

    This is a reason for why we don't know what's going on past an event horizon. We can't see inside it in the first place. It's like trying to say "here's a completely sealed box, you can't see inside it, now tell me what I keep in here." For this you'd just go by what you expect to be in there by logic, but you still can never know for sure what is actually in there.

    I'm no expert of course, so I'll rely on someone to correct me if I've said anything wrong. (:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook