4 ms·

Yes anything falling in approaches the speed of light as it approaches the event horizon. So to external observers they slow down. However keep in mind that a

by sliken 3y ago

Yes anything falling in approaches the speed of light as it approaches the event horizon. So to external observers they slow down. However keep in mind that all black holes are growing (and will continue until the cosmic backround radiation gets lower than the rate of hawking radiation. So I think of black holes as onions, everything approaches the event horizon and then (to external observers) get frozen at the edge, until the black hole gets bigger. Also keep in mind that from the perspective of someone falling in, there's nothing particularly noticeable as you cross the event horizon.

pdonis 3y ago
> anything falling in approaches the speed of light as it approaches the event horizon Only relative to static observers, i.e., observers who are hovering at a constant altitude above the horizon. But there is no such observer at the horizon, so no observer ever sees the infalling object going past them at the speed of light. > So to external observers they slow down. No, that isn't the reason they appear to slow down to external observers. This is curved spacetime and you can't use your intuitions from Special Relativity, since that is only valid in flat spacetime. The reason the infalling object appears to slow down to an external observer is that the light it emits takes longer and longer to get back out to the external observer, because of the way spacetime is curved. > I think of black holes as onions, everything approaches the event horizon and then (to external observers) get frozen at the edge, until the black hole gets bigger. This view is wrong. Things fall into the hole. They don't pile up at the horizon. They appear to slow down as they approach the horizon, but that is an optical illusion created by the curvature of spacetime.
- sliken 3y ago
  Objects falling towards the event horizon accelerate ever closer to light speed, which causes time dilation. So it's not just that light takes longer to escape, right? The dilation gets pretty extreme, you could watch suns be born and die as you fall in.
floxy 3y ago
>Also keep in mind that from the perspective of someone falling in, there's nothing particularly noticeable as you cross the event horizon. So I've heard that before, but let's say you are slowly spiraling into a supermassive black hole with very low tidal forces near the event horizon. You get close to the event horizon. Light can't escape. You stick your hand into / past the event horizon. Seems like a pretty definite discontinuity in "noticeability", if one second you can see your hand and move it around, and the next you can't see it and can't yank it back.
- sliken 3y ago
  Nothing anywhere close to event horizon can "slowly spiral in". There are no stable orbits within 1.5x the diameter of the event horizon. To get anywhere close without falling in would require a very powerful rocket, one that could accelerate to near light speed just to stay there. Now if you are falling in your hand never disappears, it's just that your body would accelerate into the black hole a little faster than your hand is accelerating because you are pulling back on your hand. But your hand never could have a negative velocity (moving away from the blackhole) no matter what you do.
- naasking 3y ago
  > You stick your hand into / past the event horizon. That wouldn't really happen. When people say that light cannot escape, they don't mean that a beam of light can't pass the horizon heading outwards, they mean that the curvature of spacetime is such that the photons will always curve back around and fall back into the hole. Locally, the horizon will look more or less the same to you, except at high curvature you'll start to see Doppler shift of light from outside vs. inside. At extreme curvature, an arm pointed towards the singularity will look red shifted and an arm pointed away from the singularity will look blue shifted. Although at this level of curvature, you'd already be ripped apart so you wouldn't really notice anything at all.

by sliken 3y ago

pdonis 3y ago

> anything falling in approaches the speed of light as it approaches the event horizon Only relative to static observers, i.e., observers who are hovering at a constant altitude above the horizon. But there is no such observer at the horizon, so no observer ever sees the infalling object going past them at the speed of light. > So to external observers they slow down. No, that isn't the reason they appear to slow down to external observers. This is curved spacetime and you can't use your intuitions from Special Relativity, since that is only valid in flat spacetime. The reason the infalling object appears to slow down to an external observer is that the light it emits takes longer and longer to get back out to the external observer, because of the way spacetime is curved. > I think of black holes as onions, everything approaches the event horizon and then (to external observers) get frozen at the edge, until the black hole gets bigger. This view is wrong. Things fall into the hole. They don't pile up at the horizon. They appear to slow down as they approach the horizon, but that is an optical illusion created by the curvature of spacetime.

sliken 3y ago
Objects falling towards the event horizon accelerate ever closer to light speed, which causes time dilation. So it's not just that light takes longer to escape, right? The dilation gets pretty extreme, you could watch suns be born and die as you fall in.

floxy 3y ago

>Also keep in mind that from the perspective of someone falling in, there's nothing particularly noticeable as you cross the event horizon. So I've heard that before, but let's say you are slowly spiraling into a supermassive black hole with very low tidal forces near the event horizon. You get close to the event horizon. Light can't escape. You stick your hand into / past the event horizon. Seems like a pretty definite discontinuity in "noticeability", if one second you can see your hand and move it around, and the next you can't see it and can't yank it back.

sliken 3y ago
Nothing anywhere close to event horizon can "slowly spiral in". There are no stable orbits within 1.5x the diameter of the event horizon. To get anywhere close without falling in would require a very powerful rocket, one that could accelerate to near light speed just to stay there. Now if you are falling in your hand never disappears, it's just that your body would accelerate into the black hole a little faster than your hand is accelerating because you are pulling back on your hand. But your hand never could have a negative velocity (moving away from the blackhole) no matter what you do.
naasking 3y ago
> You stick your hand into / past the event horizon. That wouldn't really happen. When people say that light cannot escape, they don't mean that a beam of light can't pass the horizon heading outwards, they mean that the curvature of spacetime is such that the photons will always curve back around and fall back into the hole. Locally, the horizon will look more or less the same to you, except at high curvature you'll start to see Doppler shift of light from outside vs. inside. At extreme curvature, an arm pointed towards the singularity will look red shifted and an arm pointed away from the singularity will look blue shifted. Although at this level of curvature, you'd already be ripped apart so you wouldn't really notice anything at all.