Black Holes

John K Clark (
Mon, 13 Apr 1998 22:09:39 -0700 (PDT)


On Sun, 12 Apr 1998 Reilly Jones <> Wrote:

>Certainly, an observer outside the event horizon of a black hole
>observing the speed of light inside the event horizon would [...]

Stop right there, if you're outside the event horizon then you can't see
anything inside it.

>How does this change as the observer approaches the event horizon at
>a tangent?

If I was far away and watching you I would see you and your watch slow down
as you approached the event horizon, and when you reached it you and your
watch would stop completely. From your own point of view you'd see nothing
special when you crossed the event horizon, it just marks the point of no
return for you. Well ok, you might notice the tidal forces trying to crush
you, but if the black hole was millions of times as massive as the sun they
wouldn't be large enough to be lethal even at the event horizon, and wouldn't
be until you got a lot closer to the central singularity. And once you
crossed the event horizon there is no way to stop from getting closer to the

>If light is travelling lengthwise between two incredibly long,
>incredibly dense parallel plates, which screen out the bulk of the
>density of space, would not the speed of light increase over that
>outside of this system?

Interesting example, although the plates wouldn't have to be incredibly long
or dense. If you place two flat mirrors very close together then there can
not be virtual photons of every wavelength in the vacuum between the mirrors,
as there is outside, because some will interfere destructively. There are
more virtual particles in the vacuum outside the mirrors pushing them
together than between the mirrors pushing them apart. This force is
extremely weak, but recently it has actually been measured in the lab.
2 mirrors really do want to come together.

If you define a vacuum far from any object as having zero pressure and if the
vacuum can still push the plates together then the space between the plates
must have negative pressure. According to Einstein, mass is not the only
thing that can warp space and time, that is, create gravity, pressure can too,
although this only becomes important when the pressure is VERY high, like
inside a Neutron star. New theory says negative pressure should do
interesting things to time and space also, just like positive pressure, but
this part has not been confirmed experimentally because the effect is so tiny.

An observer's clock between the plates will (probably) run a little fast and
for an observer outside the plates distances would (probably) expand a little.
Since speed is distance divided by time nobody sees a change in the speed of
light. This negative pressure idea is a little similar to the old cosmological
constant idea of Einstein, he later said that was the worst mistake of his
life, but maybe not, it may play an important part in the evolution of the

>As to contradictory evidence for the Big Bang dogma, review the
>scientific literature over the past oh, three or four years

The question was not whether the Big Bang is true or not but weather it's a
scientific theory or not, you said it was not but the only reason given is
that you just don't like it. If you have other reasons I'd like to hear them.

>Even at background radiation levels, black holes are slowly melting
>at the event horizon

According to Hawking a black hole left to itself would slowly melt away. In
the vacuum virtual particles and antiparticles are constantly being created
but they collide after a very short time and are destroyed. Things are
different near a black hole, tidal effects at the event horizon would
sometimes be strong enough to pull these virtual particles apart, one could
fall into the black hole leaving the other one free to become a real particle
and radiate away into the outside world. Thus Black holes are not really
black, they give off radiation just like any object above absolute zero.
The smaller the black hole the larger the tidal effect at the event horizon,
so there is an inverse relationship between the mass of a black hole and
its temperature. Large black holes are very cold, small ones are hot, at the
last moments of their life they are so small and so hot, many trillions of
degrees, that they explode.

A black hole of 2 solar masses is pretty cold, it radiates energy at
3 * 10^-8 degrees Kelvin, that energy comes at the expense of the black hole's
mass but would take 10^67 years to evaporate. Actually it would take even
longer, the cosmic background radiation is at 2.7 degrees so such black holes
are actually getting larger and will keep doing so unless the universe cools
to 3 * 10^-8 degrees or less. If that ever happens it will take a long time.

John K Clark

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