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I never bought that humans could travel through an event horizon[1]. Say that your leg is inside the black hole, but the rest of you isn't. How can the atoms j

I never bought that humans could travel through an event horizon[1]. Say that your leg is inside the black hole, but the rest of you isn't. How can the atoms just outside the horizon detect the quantum forces from the atoms just inside? Electromagnetic forces are transmitted by photons, which are forbidden to cross this barrier. Thus, if you pulled back away from the horizon, you would have an event-horizon-curvature-shaped hole where your lower leg had just been. Similarly, for any matter traveling through the black hole, unless two atoms and all their electrons passed through the horizon simultaneously, no chemical bond could survive (since from the point of view of one atom, the other "blinks" out of existence.) Thus, anything that crosses an event horizon must be fundamental particle mush, no? Assuming gluons were also forbidden to cross back, even protons and neutrons would be torn apart. [1] I mean one around a very massive black hole - so that spaghettification is yet to occur. P.S. - This argument takes a very literal interpretation of fundamental particles; indeed, it would be interesting to test the quantum-ness of matter to see how this worked - i.e. whether "stuff" really is particles or waves - since waves could, potentially, exist on both sides of the barrier at once.

splat 13y ago
There's a difference between hovering above an event horizon and sticking your leg through versus free-falling through an event horizon. The former is not an inertial reference frame, so you would feel enormous forces due to the fact that you were (somehow) generating enough thrust to counteract the gravitational force of the black hole. No human could survive that. But in the latter you feel no forces except for tidal forces because you are simply following the curvature of the local spacetime. If you were falling into a supermassive black hole you would not feel anything as you passed through the event horizon. You would still retain full control over your limbs until a few milliseconds (I think, I might be a few orders of magnitude off) before you hit the singularity.
- tehwalrus 13y ago
  OK, orbiting rather than hovering. Also, whatever speed you cross the horizon there would still be some delta_t > h_bar where leg atom A can't see leg atom B. Similarly, the drop in blood pressure for the parts of blood vessels just outside the horizon (although, given the rest of the prediction, blood pressure is the least of your worries at this point.)
  - Sir_Cmpwn 13y ago
    The problem is that you don't understand how an event horizon works. It's not a definite border where suddenly there's a lot more gravity. It's the point at which gravity becomes strong enough (note that gravitational forces are still very strong right next to the event horizon) that not even light could avoid being pulled into the singularity eventually. You wouldn't have to travel directly into the signularity, if that helps clarify things. You could orbit around it for a while and your orbit would decay in a spiral pattern. This excerpt from Wikipedia might be helpful: https://mediacru.sh/rd-9YJEtJRGo https://mediacru.sh/rd-9YJEtJRGo I would expect that you'd be torn apart by tidal forces before you reached the event horizon anyway.
    - splat 13y ago
      You would be torn apart by tidal forces before reaching the event horizon of a stellar mass black hole, but you wouldn't be torn apart by tidal forces before reaching the event horizon of a supermassive black hole.
  - splat 13y ago
    You cannot orbit a black hole immediately outside the event horizon. The smallest distance at which a circular orbit is possible is called the photon sphere because you must be moving at the speed of light to maintain a circular orbit. The photon sphere is at 1.5 Schwarzschild radii.
    - tehwalrus 13y ago
      Ah, apologies, so this would be impossible with a living thing. If we substitute for a machine? Could you, perhaps, build a strong enough spaceship to dip in close enough to dangle something passed the horizon, and then accelerate away again? I don't suppose you'd be able to measure anything from the resulting object, even if you could get it back... Even if you couldn't, I still find it implausible that even large objects would travel across the even horizon effectively instantaneously.
      - 13y ago
hrjet 13y ago
IIRC: from the point of view of the human, it takes infinite time to reach the event horizon, since their speed will approach the speed of light. So the actual experience of passing through the event horizon never occurs!
- tehwalrus 13y ago
  That's approaching the singularity, not the event horizon, no? EDIT: OK, maybe it is, but from which observation point? An external observer may never see someone cross the horizon, but their local time would keep on ticking. Time always seems normal to you, remember, just other peoples' goes crazy in highly relativistic situations.
- pdonis 13y ago
  from the point of view of the human, it takes infinite time to reach the event horizon No, it doesn't. The proper time (time by the infalling observer's clock) that it takes to fall to the horizon is finite. their speed will approach the speed of light This isn't correct either. The infaller will see the horizon pass him at the speed of light, but that's because the horizon is a lightlike surface--it is moving outward at the speed of light.
Florin_Andrei 13y ago
You forget that in relativity, the frame of reference is everything. In your frame of reference, you never pass through the event horizon. As you get closer, the event horizon seems to recede. A false event horizon seems to arise around and behind you. You will eventually reach the center, in a finite time - in your frame of reference. Of course, in my frame of reference, something very different happens. As you approach what I think is the event horizon, you slow down more, and more, and more, until you get frozen forever in molasses of space-time.
pdonis 13y ago
Say that your leg is inside the black hole, but the rest of you isn't. This can only be true for a very short time; if your leg is inside the horizon, you must be falling through it (if you're not, your body is being torn apart). So your leg can send a signal to your head just fine; by the time it reaches your head, your head will have fallen through the horizon. Similar remarks apply to atoms, chemical bonds, etc.

splat 13y ago

There's a difference between hovering above an event horizon and sticking your leg through versus free-falling through an event horizon. The former is not an inertial reference frame, so you would feel enormous forces due to the fact that you were (somehow) generating enough thrust to counteract the gravitational force of the black hole. No human could survive that. But in the latter you feel no forces except for tidal forces because you are simply following the curvature of the local spacetime. If you were falling into a supermassive black hole you would not feel anything as you passed through the event horizon. You would still retain full control over your limbs until a few milliseconds (I think, I might be a few orders of magnitude off) before you hit the singularity.

tehwalrus 13y ago
OK, orbiting rather than hovering. Also, whatever speed you cross the horizon there would still be some delta_t > h_bar where leg atom A can't see leg atom B. Similarly, the drop in blood pressure for the parts of blood vessels just outside the horizon (although, given the rest of the prediction, blood pressure is the least of your worries at this point.)
- Sir_Cmpwn 13y ago
  The problem is that you don't understand how an event horizon works. It's not a definite border where suddenly there's a lot more gravity. It's the point at which gravity becomes strong enough (note that gravitational forces are still very strong right next to the event horizon) that not even light could avoid being pulled into the singularity eventually. You wouldn't have to travel directly into the signularity, if that helps clarify things. You could orbit around it for a while and your orbit would decay in a spiral pattern. This excerpt from Wikipedia might be helpful: https://mediacru.sh/rd-9YJEtJRGo https://mediacru.sh/rd-9YJEtJRGo I would expect that you'd be torn apart by tidal forces before you reached the event horizon anyway.
  - splat 13y ago
    You would be torn apart by tidal forces before reaching the event horizon of a stellar mass black hole, but you wouldn't be torn apart by tidal forces before reaching the event horizon of a supermassive black hole.
- splat 13y ago
  You cannot orbit a black hole immediately outside the event horizon. The smallest distance at which a circular orbit is possible is called the photon sphere because you must be moving at the speed of light to maintain a circular orbit. The photon sphere is at 1.5 Schwarzschild radii.
  - tehwalrus 13y ago
    Ah, apologies, so this would be impossible with a living thing. If we substitute for a machine? Could you, perhaps, build a strong enough spaceship to dip in close enough to dangle something passed the horizon, and then accelerate away again? I don't suppose you'd be able to measure anything from the resulting object, even if you could get it back... Even if you couldn't, I still find it implausible that even large objects would travel across the even horizon effectively instantaneously.
    - 13y ago

hrjet 13y ago

IIRC: from the point of view of the human, it takes infinite time to reach the event horizon, since their speed will approach the speed of light. So the actual experience of passing through the event horizon never occurs!

tehwalrus 13y ago
That's approaching the singularity, not the event horizon, no? EDIT: OK, maybe it is, but from which observation point? An external observer may never see someone cross the horizon, but their local time would keep on ticking. Time always seems normal to you, remember, just other peoples' goes crazy in highly relativistic situations.
pdonis 13y ago
from the point of view of the human, it takes infinite time to reach the event horizon No, it doesn't. The proper time (time by the infalling observer's clock) that it takes to fall to the horizon is finite. their speed will approach the speed of light This isn't correct either. The infaller will see the horizon pass him at the speed of light, but that's because the horizon is a lightlike surface--it is moving outward at the speed of light.

Florin_Andrei 13y ago

You forget that in relativity, the frame of reference is everything. In your frame of reference, you never pass through the event horizon. As you get closer, the event horizon seems to recede. A false event horizon seems to arise around and behind you. You will eventually reach the center, in a finite time - in your frame of reference. Of course, in my frame of reference, something very different happens. As you approach what I think is the event horizon, you slow down more, and more, and more, until you get frozen forever in molasses of space-time.

pdonis 13y ago

Say that your leg is inside the black hole, but the rest of you isn't. This can only be true for a very short time; if your leg is inside the horizon, you must be falling through it (if you're not, your body is being torn apart). So your leg can send a signal to your head just fine; by the time it reaches your head, your head will have fallen through the horizon. Similar remarks apply to atoms, chemical bonds, etc.