From: cary@carina.unm.edu (Ithlial the Archer)
Newsgroups: alt.alien.visitors,sci.skeptic
Subject: Re: Elapsed Time near c (was Re: Q&A on Extraterr
Date: 24 Jul 92 16:08:46 GMT
Organization: University of New Mexico, Albuquerque

In article <62706@cup.portal.com> thad@cup.portal.com (Thad P Floryan) writes:
>In article <a7gm+nj@lynx.unm.edu>
>cary@carina.unm.edu (Ithlial the Archer) writes:
>
>| [...]
>| The idea behind special relativity is that information cannot be conveyed
>| from place to place at a velocity exceeding the speed of light.  Since 
>| gravity betrays the presence of matter its effects cannot propogate 
>| faster than the speed of light.  Another way to look at it that gravity
>| like other forces is conveyed by bosons.  Bosons, depending on if they
>| are massive or not travel at the below or at the speed of light
>| respectively.
>| [...]
>
>Nicely explained; thanks!  (Ever think of becoming a science writer? :-)
>
>Now we have the problem of the Black Hole (I need to get a copy of Hawkings'
>book(s)): given that a BH is so massive it prevents photon emission, can one
>infer that gravity "outreaches" light?  I thought photons were massless, so
>what is the attractor?  Must the answer await the formulation of a Unified
>Field Theory?
>
The reason that information about mass (and charge and angular momnetum) is 
conveyed to the universe out side the evet horizon is that the gravitons
that mediate the gravitational field and the photons that mediate the
electromagnetic field are what we call 'vitrual' particles.  The aren't
real in anyway that we know it, their existence is implied only by the
force they mediate.  Also we do know the state of the star before it
becomes a black hole.  So with a couple of conservation laws and some
arithmetic we know what goes into the singularity, so we're not learning
anything that we didn't know about before.  The reason real photons
can't get out can be thought of a couple of ways.  One can assign the photon
a mass based on the classical formula p = mv, where v=c for a photon.
The momentum of a photon comes from the formual for total energy of 
a particle with no mass, E = pc.  Beleive it or not this works, even
though we're mixing relativistic and classical formulae.  (In a side
light the formula for calculating the event horizon can be done classically
and it works out correctly too)  So you use the mass that to calculate
the force, and newton's F = dp/dt and you see that you need any infinte
amount of momentum (ie energy) to escape from the event horizon and
probably an imaginary amount to get out from within the EH.

>Could a BH be detectable by OTHER than observed perturbations of other masses?
>In other words: does a BH radiate "anything", or is it the ABSENCE of 
>radiation
>and other energies that defines an instance or the presence of a BH?
>
The way that astronomers detect what are probably black holes is 
fairly straight forward.  We observe a binary system in which on of the
companions is emitting high energy radiation, like x-rays and gamma rays.
In the type of binary system these obervations are made one companion is
large, some type of giant star, red giants work best, while the other
is compact and pulls material off the larger star.  Red giants work
best since they are largest and their outermost material is only loosely
bound.  Since it is emitting this high energy radiation, we know that the 
material must be very hot (ie posses alot of kinetic energy).  The easiest
way to get kinetic energy is to fall down, stairs, cliffs or the potential
well of a massive object.  So since the marterial is so energetic it must
have fallen a long way, threfore the object that the stuff is falling onto
is pretty small, ie smaller than a small star, even a white dwarf.  
Another way to put upper limits on size is by measuring the time scale
over which the luminousity varies.  The time scale time the speed of
light is an absolute upperbound on the size of the region that's
doing the varying.  Anyhow if its smaller than a white dwarf it's a
neutron star or a blackhole, which depends on the mass.  Now since the 
stars are rotating about eachother at a rate we can measure we can
with the help of the law of gravitation determine their masses.  If
the mass is too high for the neutron degeneracy to keep it from collapsing
then it's a blackhole, otherwise it's a neutron star.



Ithlial


My opinions, mine, mine, mine!

To me, boxing is like ballet, except there's no music, no choreography,
and the dancers hit eachother.