On Fri, Jul 24, 2009 at 10:21 PM, Henry Baker <hbaker1@pipeline.com> wrote:
My original question regarded "classical" GR, where interchanging "positive" and "negative" time doesn't change any laws. I.e., if you take a GR movie and simply run it backwards, it still agrees with all the GR laws.
Yes, but the object in the center is not a black hole; it's the time-reversal of a black hole. A black hole is like an accelerating falling object. The time reversal of this, a decelerating rising object, is consistent with the laws of physics. That doesn't mean that a falling object can spontaneously start rising. In the same way, a black hole can't spontaneously become a time-reversed black hole and start emitting instead of absorbing.
So, you don't need any notion of "white holes" -- I'm still talking about regular old "black" holes.
You're talking about time-reversed black holes. To quote http://en.wikipedia.org/wiki/White_hole: In astrophysics <http://en.wikipedia.org/wiki/Astrophysics>, a *white hole*is the theoretical time reversal <http://en.wikipedia.org/wiki/T-symmetry> of a black hole<http://en.wikipedia.org/wiki/Black_hole>. While a black hole acts as a vacuum, drawing in any matter<http://en.wikipedia.org/wiki/Matter>that crosses the event horizon <http://en.wikipedia.org/wiki/Event_horizon>, a white hole acts as a source that ejects matter from its event horizon. The sign of the acceleration is invariant under time reversal, so both black and white holes attract matter. The only potential difference between them is in the behavior at the horizon. So the thing you're talking about is exactly what is called a white hole. Andy
So, nothing in these laws keeps such a black hole from burping an object out into space.
We can ask what initial conditions would enable such a burp & may complain that they are highly unlikely, but they aren't ruled out by the laws themselves.
Now I'm curious about what this implies for the "classical" energy, momentum, etc., of the object being burped.
At 03:40 PM 7/24/2009, Dan Asimov wrote:
Re time reversal:
The fact that in our neighborhood in spacetime we see stars radiating only in positive time -- and all the entropic consequences of this -- is usually attributed to initial conditions, not to any actual "law" of physics.
(The Ehrenfest urn model is usually considered a good microcosm of thermodynamics, and shows time reversal symmetry.)
It's just a guess, but it seems likely to me that we are aware of only a small or infinitesimal fraction of the universe, and that some other time and place have stars radiating in negative time.
(Or is it possible that entropically opposite parts of the universe somehow always end up pinching off from each other by black holes?)
--Dan
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