NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/astro/8276

Internet Message Format | 1992-07-23 | 3.6 KB

Path: sparky!uunet!usc!zaphod.mps.ohio-state.edu!mips!darwin.sura.net!jvnc.net!netnews.upenn.edu!sagi.wistar.upenn.edu
From: [e-mail] (Matthew P Wiener)
Newsgroups: sci.astro
Subject: Re: On the formation of black holes
Message-ID: <[e-mail]>
Date: 23 Jul 92 19:36:09 GMT
References: <[e-mail]> <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail] (Matthew P Wiener)
Organization: The Wistar Institute of Anatomy and Biology
Lines: 65
Nntp-Posting-Host: sagi.wistar.upenn.edu
In-reply-to: [e-mail] (Warren G. Anderson)

In article <[e-mail]>, anderson@fermi (Warren G. Anderson) writes:
>Look, it's simple. If a stationary external observer observes a radially
>infalling test particle emitting classical radiation she will see emissions
>from that particle for an inifinite proper time.

This is simply and utterly false.  The external observer sees the black
hole form in finite proper time, and for purely geometric reasons.

>> This is a separate issue.  The geometry of collapse alone suffices to
>> establish a cone of last influence.

>You seem confused as to what the cone of last influence represensts. It is
>not the last point at which the infalling particle has an influence on an
>external observer, but rather locus of points at which an external observer
>can last influence the infalling particle.

Those are two different notions.  I am speaking about the former.  I'm
willing to believe that people use the term "cone of last influence" for
either, but I'm only using it for the former.

>> Actually, geometry alone would say
>> that you could see that instantaneous moment of crossing, but I don't
>> think such notions are well-defined physically.

>No it doesn't.

Sure it does.

>            Despite your protests, the Penrose diagram does indeed
>contain all the necessary information to understand the causal relationships
>between different test particles in a spherically symmetric space-time.

Of course it does and I've never said or implied otherwise.

>It is clear from the Schwarzschild diagram that emissions will reach the
>stationary observer for an infinite proper time.

Wrong.  The infalling object hits the event horizon at some event, and
from there is a cone (at 45 degree for c=1) reaching out from that.  Where
it crosses the observer's worldline, that's it: no more influence from the
object falling in (except via mirrors, of course).

>> Uh, I kept saying over and over again that S-time was just a coordinate,
>> and not to be taken seriously without careful interpretation.  If it gets
>> through on the fifth try, great!

>Either you are not reading my messages very carefully or you are being
>belligerent. I never brought S-time into this at all, you did. All I'm
>trying to figure out is why?

The reason people thought for so long that a black hole never quite forms
in an external observer's frame, but is some kind of frozen star hovering
just outside the event horizon, is by misinterpreting S-time as frame-time.
I was explaining to someone else to *not* make that now well-known mistake.

>There is no geometrical explanation. The infalling test particle (if it
>emits classical fields) can influence a stationary observer for an infinite
>observer proper time. There is no geometric barrier to this. I think maybe
>*you* had better have another look at MTW Ch. 33.

There is a geometric barrier, pure and simple.  Look at MTW Fig 33.1 and the
cone heading out.  That's what it boils down to.  The quantum effects merely
accelerate this process.
--
-Matthew P Wiener ([e-mail])