Re: information loss in black holes
From: John Baez (baez_at_galaxy.ucr.edu)
Date: 08/13/04
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Date: 13 Aug 2004 08:36:29 -0400
In article <20040630011102.7605542CC9@mail.netspace.net.au>,
Greg Egan <gregegan@netspace.zebra.net.au> wrote:
>In article <37d84b42.0406290104.3402b89f@posting.google.com>,
>daniel@elit.net (Daniel Elander) wrote:
>> Someone wrote:
>> > When people talk about information loss in a black hole, they are
>> > referring to the fact that someone outside the event horizon would
>> > have no way of knowing what is written on the piece of paper.
Maybe, but this isn't what had physicists like Hawking worried.
>> But for the outside observer it takes an infinite amount of time for
>> the paper to fall into the black hole. So it would seem the paradox
>> never happens.
There isn't really a "paradox", by the way - just some puzzles.
>>Or am I wrong?
>The statement "for the outside observer it takes an infinite amount of
>time" for something to fall into a black hole is true in two senses, but
>you have to look very carefully at what those statements are to see what
>they tell you about information accessibility.
One certainly must get this stuff straight to appreciate what's
going on, so it's good you explained that stuff!
but there's also other stuff one must get straight, which nobody
completely understands yet - and *that's* why this problem is famous.
The stuff you explained is all about classical general relativity,
not taking quantum theory into account. Stuff falls into black holes;
we can argue about whether it ever gets completely in, but we can
straighten out the semantics of this issue and then there's no problem.
The main thing is that classically, if I drop a rock into a black hole,
it never comes back out.
So, we lose information about the rock, or at least lose *access* to it -
a semantic fine point not worth worrying about. But, this is no big deal.
The rock fell into a black hole, so what do you expect? OF COURSE you
you lose information about it. If you want, you can say the information
about it is stuck inside the black hole. But there's no paradox, and
not even much of a puzzle.
The "information loss problem" starts when we try to take quantum
theory into account. Now it seems that black holes radiate, lose
energy, shrink, and eventually disappear. This leads to some
interesting puzzles.
For example:
If we drop a rock in, does it actually fall in before the hole evaporates?
Where, if anywhere, does the information about this rock go after
the black is gone? Is it lost, or does it come out in the radiation
somehow?
Now Hawking says it comes out in the radiation.
I guess some of you have already read Paul Ginsparg's article
about this:
http://www.arxiv.org/abs/physics/0408033
If you haven't, you should. He's not convinced.
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