2023-02-12 22:06:26 UTC
to go slower and slower as it approaches. And that eventually it appears
to stop at the event horizon. Perhaps my understanding of this is wrong.
But if that's what happens, how does anything get into a black hole? They
increase in mass over time, yes?
[[Mod. note --
Let's start with the simplest case: a non-rotating (Schwarzschild) black
hole (BH), and an object falling radially inwards towards the BH (so that
the object has no angular momentum about the BH). And let's equip the
falling object with a light/radio transmitter so observers can monitor
Based on the light/radio signals, a distant observer will "see" the
object's infall appear to slow down and eventually "freeze" just outside
the BH's horizon. The light/radio signals will also get more and more
But this "freezing" is an optical/radio illusion: the object actually
continues to accelerate inwards, and falls in through the BH's horizon
in a finite time. The "freezing" is caused by the large gravitational
redshift of light/radio signals emitted by the object just outside the
horizon, taking a very long time to propagate outward to the distant
observer. (More precisely, that propagation time approaches infinity
as the emission point gets closer and closer to the horizon.)
That is, if we imagine the infalling object emitting period light/radio
flashes, as the infalling object gets close to the horizon the flashes
take longer and longer to propagate out to a distant observer, and are
more and more redshifted in the process. Once the object passes through
the horizon, its light/radio signals don't get out to the distant observer;
the distant observer sees only those signals emitted before the object's
For the more general case where the BH is spinning, everything above is
still true, but the mathematics is more complicated (the infalling object's
path won't stay radial unless it's falling in along the BH spin axis).
If the infalling object has angular momentum about the BH, then (depending
on the details) it may orbit the BH and not actually fall in.
There's a nice discussion of falling-into-a-BH in the physics FAQ at