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?
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?
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].)
The event horizon is the distance where all straight lines lead to the black hole.
There _is_ no amount of energy it could gain to escape though. It goes no faster, gets no closer to escape. It just splats into the black hole and we'll never see it again no matter how much energy it started with or gained.
A better answer would be something like a grand unified theory of gravity and the other forces, including quantum effects, which doesn't exist. It's like the holy grail of physics, you'd be the next Einstein if you figure it 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)
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.
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.