At 06:16 PM 12/26/2012, David Roberson wrote:
I am hoping to establish that there exists a boundary from which an
object becomes invisible to us once it is crossed.
There must. An "event horizon" is a "boundary in spacetime beyond
which events cannot affect an outside observer." (WP).
The article notes that, from a perspective of an observer who is
"behind" the object (i.e., the object is along a line between the
observer and the black hole center), the object never appears to
reach the event horizon, the image being increasingly red-shifted as
the object approaches the horizon.
That puzzled me. It didn't seem to be correct. But I was misreading it.
Light would be red-shifted as the object emitting it approaches the
event horizon, yes. The event horizon is the bundary where escape
velocity reaches the speed of light. Light doesn't slow down, though,
it shifts frequency or wavelength, and the wavelength as the object
approaches the event horizon would approach infinity. Aother way of
saying that would be that the photon energy approaches zero.
Old Black Hole Exploring Spaceships Never Die, They Just Fade Away.
But the WP article indicates that the object would never "appear to
reach the event horizon," which could be read to imply that it slows
down. No, it would not slow down, it would be, unless under some
other accelerating force, accelerating toward the black hole, and
that could be seen. As it approachs the Event Horizon, the light, or
any other signal, would be red-shifted until no energy reaches the
observer as it reaches the Event Horizon.
The signals do still travel at the speed of light.
David, you didn't *exactly* state it correctly. The object becomes
less and less visible as it approaches the Event Horizon, not "once
it is crossed."
