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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 t

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?

karpierz 2y ago
Light travels in a straight line - black holes don't "pull" light in, they change what a straight line looks like in the space around it. The event horizon is the distance where all straight lines lead to the black hole.
JumpCrisscross 2y ago
> 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 https://en.wikipedia.org/wiki/Penrose_process [2] https://en.wikipedia.org/wiki/Black_hole_bomb https://en.wikipedia.org/wiki/Black_hole_bomb
kadoban 2y ago
It will gain energy. 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.
- thehappypm 2y ago
  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?
  - kadoban 2y ago
    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)
  - hollerith 2y ago
    It is finite in circumference, yes, but its geometry is not the usual geometry so diameter does not equal circumference divided by pi.
    - thehappypm 2y ago
      It must be infinite then?
      - kadoban 2y ago
        Maybe. Nobody really knows what happens at the center. General relativity I believe says "yes", but nobody knows how true it is in the case of ~infinitely dense crap in an infinitely small spot. Usually "infinite" means what really happens is not captured by the model, but nobody has a better answer yet as far as I know. 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.
      - hollerith 2y ago
        Yes.
trailynx 2y ago
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 :) [0] https://arxiv.org/abs/1903.03423 https://arxiv.org/abs/1903.03423
itishappy 2y ago
> 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.
- thehappypm 2y ago
  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?
mystified5016 2y ago
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.

karpierz 2y ago

Light travels in a straight line - black holes don't "pull" light in, they change what a straight line looks like in the space around it. The event horizon is the distance where all straight lines lead to the black hole.

JumpCrisscross 2y ago

> 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 https://en.wikipedia.org/wiki/Penrose_process [2] https://en.wikipedia.org/wiki/Black_hole_bomb https://en.wikipedia.org/wiki/Black_hole_bomb

kadoban 2y ago

It will gain energy. 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.

thehappypm 2y ago
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?
- kadoban 2y ago
  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)
- hollerith 2y ago
  It is finite in circumference, yes, but its geometry is not the usual geometry so diameter does not equal circumference divided by pi.
  - thehappypm 2y ago
    It must be infinite then?
    - kadoban 2y ago
      Maybe. Nobody really knows what happens at the center. General relativity I believe says "yes", but nobody knows how true it is in the case of ~infinitely dense crap in an infinitely small spot. Usually "infinite" means what really happens is not captured by the model, but nobody has a better answer yet as far as I know. 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.
    - hollerith 2y ago
      Yes.

trailynx 2y ago

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 :) [0] https://arxiv.org/abs/1903.03423 https://arxiv.org/abs/1903.03423

itishappy 2y ago

> 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.

thehappypm 2y ago
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?

mystified5016 2y ago

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.