At 08:11 PM 12/28/2012, Jeff Berkowitz wrote:
I think a lot of the reasoning about photons, above, is wrong. The
red shift has nothing to do with gravity, only the relative velocity
of the photon source relative to the observer.
Eek. Apparently not.
If an event just outside the event horizon of a black hole emits a
photon, an observer at rest relative to the black hole will observe
no red shift regardless the strength of the black hole's gravitational field.
Apprently this is not so, and it directly contradicts many sources
that might be expected to get it right. The red shift is not a
motion-related doppler shift, it is a gravitational shift, purely.
If the observer then accelerates away from the black hole, similar
photons emitted from the same source will appear to be red shifted.
It's entirely an observational effect. There is no loss of energy
from the photon and no need to store anything anywhere.
This topic is a continual temptation to me to stick my foot in my
mouth. What I'm getting is that there is a lot I don't understand
about black holes and particularly about the event horizon.
Essentially, I've felt that I have a decent understanding of special
relativity, but general relativity is another animal, and
gravitational effects on light are an aspect of general relativity.
The event horizon, it is being said, is the point at which no path
exists for the photon to escape, to travel away from the singularity.
This is caused by the intensity of the gravitational field, which is
a fixed value at the event horizon. That's the value that allows no
escape. Just outide the event horizon, the photon may escape, but
does not escape unscathed. It loses energy climbing the gravitational
potential field. It red-shifts as it loses energy. (That energy is
being converted to potential energy, just as with any object with
momentum away from a gravity source loses momentum, trading it for
potential energy.)
The puzzle to me here is the statement made that an object travelling
toward the black hole will not only be seen through a red shift, but
will also appear to slow, such that it never passes the event
horizon, it just gets closer, but more and more slowly, until it is
red-shifted out of observability. It is alleged that this takes forever.
And I don't understand that.
To resolve this, part of what I'll need to look at are the equations
for gravitational red shift, or the effect of gravity on light.
Then I can look at what would happen with light emitted outside the
event horizon (which I presume will fall out of the gravitational
equations), and can construct a thought-experiment for an object
approaching the event horizon, which was the original problem here.
It *looks* to me like some material that is popularly stated about
black holes and event horizons might be incorrect, but I certainly
don't know enough to claim that with any clarity.
I *do* imagine that I know enough to deny that the red shift being
talked about here is the ordinary doppler shift, i.e., due to the
relative velocity between the source and the reference frame.
