4 ms·
Once a black hole -- a collapsar -- forms, and you want to toss something into it, it is best to imagine two timelines. From the point of view of the object be
by at_a_remove 4y ago
Once a black hole -- a collapsar -- forms, and you want to toss something into it, it is best to imagine two timelines.
From the point of view of the object being hurled into the black hole, things proceed more or less normally: you accelerate as you fall, but light behind you becomes curiously more blue and brighter. You eventually reach the event horizon, which ought to be called EH-sub-0, because it is only the first event horizon. It might be helpful to think of a black hole's interior as infinitely layered event horizons, event horizons "all the way down." You'd note a restriction of movement -- "up" (away from the black hole) is no longer a possibility; every direction is some variation of down, perhaps down and to the left, down and east, whatever, but always down. Eventually tidal forces take over depending on the size of the thing -- you might notice them before or after the outmost event horizon, and "spaghettification" occurs even as you are pummeled with X-rays and gamma radiation from behind (millennia of impacting photons blueshifted and jammed into a smaller timeline).
From the outside, however, your astronaut or thrown Cylume lightstick becomes more and more red, and dim, slowly approaching that event horizon but you'll never see it get there as it now emits infrared and not much of that. You switch on your FLIR and you can see it, for a while, but it grows dimmer and eventually disappears off of that. Eventually it emits very weak radio waves, and you lose track of the thing, but even if you spent a million years building longer and longer antennae, you'd never see it hit.
The effects you see of a black hole are 1) gravitational lensing (photons bending in their trajectory around the exterior of a black hole, just above the outermost event horizon), 2) the formation of an accretion disc (as matter swirls into it, growing hot from friction and compression), 3) absolute blackness if you managed to get a "transit" of something particularly large across a light path, 4) other knock-on gravitational effects, like disturbed orbits.
- wizzwizz4 4y ago> You'd note a restriction of movement -- "up" (away from the black hole) is no longer a possibility; every direction is some variation of down, perhaps down and to the left, down and east, whatever, but always down. You'd notice if you were doing calculations as you fell. Locally, nothing particularly special happens at the event horizon, as far as I understand; just, past that point, all future directions lead to the centre of the black hole. (Plenty of past directions still point outwards, so you wouldn't even see anything particularly special unless you were paying close attention to, say, the apparent shape of the black hole.)
- kulahan 4y agoThis is a little confusing to me. Could the gravity not be the parabolic reflector? The actual black hole itself is creating the "dent" in spacetime and the gravitational lensing is "pushing" the forces in a parabolic manner. Since that gravitational lensing keeps things focused for a very huge distance, it could keep the beam collimated, no? Sorry, I love this topic, but I don't know much about physics. This might be a really dumb question.
- at_a_remove 4y agoParabolas have a nicely defined axis. Non-rotating, uncharged black holes are spherically symmetrical, so no parabola. The lensing happens in all directions.
- kulahan 4y agoI understand now, thank you for explaining
- mr_toad 4y agoWhat happens to the matter at the edge of the event horizon when the event horizon grows? And wouldn’t the density of in-falling matter quickly exceed what is physically possible?
- at_a_remove 4y ago1) From the outside, you wouldn't see anything, because that matter is already emitting maybe one enormous radio wave-sized photon per year. But from the view of someone falling in, you might get an imperceptible jump like you had gotten slightly closer. Remember, if you dropped something Earth-massed into a black hole (say on the other side), the black hole wouldn't grow even a full centimeter in radius. This would be very difficult to notice in practice. "Kind of a "stutter" as you approach, maybe. 2) No. What's physically possible is the ridiculousness of a neutron star's core. If you surrounded an event horizon with something of that density with the snap of your fingers (say one hemisphere), well, just as suddenly you would have a local amount of mass that would be quite formidable and the event horizon would seamlessly expand in that direction. You wouldn't see it, just watch your half-neutron-star shell wink out as you have "too great" a concentration of mass in a particular volume. See #1.