Comment by thehappypm 10 months ago
I have a question about black holes, HN.
Let’s say you have a black hole. You fire a laser beam straight into it. Just by symmetry, shouldn’t it blueshift on the way in, gain some preposterous amount of energy — enough that it can escape?
From an outside observer, the black hole is finite in width though, though. and if the speed of light is constant, isnt that a contradiction? Or is a black hole somehow of infinite width?
It's not a contradiction because the path of the light enters the black hole event horizon but does not come back out.
All paths end at the ~center of the black hole. We don't really know what's there, at the so-called singularity at the center, it may be impossible to know. But it's probably something that light can hit and stop existing, or even if it can't it'll be stuck there in some way or another.
We know that the gravity (warping of spacetime) around a black hole is such that it cannot escape, even at light speed. What happens inside is less sure, but largely moot because none of it can ever affect anything outside of the event horizon ever again.
(Hawking Radiation or something else may make this technically a lie, but it's close enough to the truth to last a few trillion years)
> Just by symmetry, shouldn’t it blueshift on the way in, gain some preposterous amount of energy — enough that it can escape?
Escape requires a certain critical speed (escape velocity), and light always travels at the speed of light regardless of it's energy, so light's ability to escape a black hole is not energy dependent. At some radius (the event horizon), space itself falls into a black hole faster than the speed of light, and that's sets a hard limit on everything's ability to escape.
Past the event horizon physics get weird. Time becomes finite and ends for all observers at a point in the future called the singularity. Everything falls towards this point, gaining preposterous amounts of energy. "Preposterous" here means infinite energy, and infinite density, really just a whole lot of infinities. Those infinities are problematic, and mean there's probably something non-infinite and uniquely interesting going on in there, but since nothing can escape the event horizon we'll likely never know.
Here’s something else that I’m concerned with with about black holes. If a mass is one nanogram below that of a black hole, how similar is it to a black hole?
Yes, but only if the black hole is rotating. Fortunately, all known black holes do rotate.
There's a concept for using exactly this idea for interstellar propulsion. You fire a laser around a black hole, it gains energy, then hits you, imparting more kinetic energy than you spent firing the laser.
In essence, you're executing a gravity slingshot with photons.
You might enjoy the Halo Drive idea [0]. From my (very layman) understanding this uses this principle for propelling a spacecraft - you just need a moving black hole nearby :)
> shouldn’t it blueshift on the way in, gain some preposterous amount of energy — enough that it can escape
You're devilishly close to the Penrose process by which "energy can be extracted from a rotating black hole" [1]. (You can also make them explode [2].)
[1] https://en.wikipedia.org/wiki/Penrose_process
[2] https://en.wikipedia.org/wiki/Black_hole_bomb