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Comment by omegant | Hacker News Reader

Complete ignorant question: there is always the discussion of "falling" in to a black hole, but could it be possible to "hang" something from outside the event horizont?(given an infinitely strong and quite long wire). If so, could it be possible to send any signal outside? Is it possible for a particle to vibrate in a radial axis away from the blackhole once inside the event horizont?. Does a particle follow in an orbit around the black hole once inside the event horizont? Or the acceleration towards the center is so big that is a vertical fall? I think that this last question is somehow replied on the article but I can't understand it. Amazing topic indeed!.

Edit: some corrections.

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It is not possible, and to see why it is likely easiest to take the perspective of an observer at a fixed distance from the black hole. ( The guy who holds on one end of the wire.) From his perspective, he sees that the wire is lowered towards the event horizon, but the tip of the wire is redshifted and moves slower and slower the closer it comes to the event horizon. This is because the light reflected by the tip of the wire needs more and more time to reach the observer. And in the end, the observer will never see the wire actually reaching the event horizon. And since signals in the wire can not move faster than the speed of light, the origin of any signal through the wire is further away from the event horizon than the perceived position of the tip of the wire.

About your other questions ( and being a bit sloppy with my definitions): Inside the black hole the radial coordinate becomes timelike, that means the singularity is in the future of every particle which has fallen into the black hole. And you can not orbit around the singularity any more than you can orbit around next Thursday.

No. You can imagine the fabric of space being dragged into the black hole faster than the speed of light. Since nothing within spacetime can move faster than that, no signals can ever leave the event horizon.

It's like a swimmer going over a waterfall. The water is like spacetime being dragged towards the black hole, and the swimmer is an object moving within spacetime. If the waterfall is falling faster than the person can possibly swim, the person can never get back up the waterfall.

For more info on this "river model" of spacetime, see:

http://jila.colorado.edu/~ajsh/insidebh/waterfall.html

http://arxiv.org/abs/gr-qc/0411060

Relatedly: Since spacetime can "flow" faster than the speed of light, you might think this allows FTL travel. Indeed, this idea is called the "Alcubierre drive".

Thank you I think this is the most graphic explanation. Then a mass suspended close to the event horizont would be in fact traveling very fast related to the space time inmediately arround and therefore increasing its mass acordingly. Is that right? Could it be possible to use this as a way to extract energy from the blackhole? Or more exacly from the space-time falling into the blackhole? Kind of river mill. (please forgive this naive questions!)

> Complete ignorant question: there is always the discussion of "falling" in to a black hole, but could it be possible to "hang" something from outside the event horizont?(given an infinitely strong and quite long wire).

It's a good question, because it exposes some subtle assumptions being made.

The string breaks, because you cannot have an infinitely strong wire in a universe that allows black holes. If we could have such a wire, black holes would not be formed, since the wire's material would be able to resist being crushed into a black hole. Therefore collapsing stars would compress into this material rather than into black holes, and there would be no black holes.

Once inside the event horizon, every path leads to the singularity. You cannot get further away from it, and you cannot orbit it (or otherwise keep distance constant).

All that the math suggests is that the escape velocity from inside the event horizon is beyond the speed of light, so it's physically impossible.

It doesn't seem impractical to be able to establish an orbit inside that radius, though, even a highly elliptical one, provided this orbit never slings you back out. It just seems if your orbit decays even slightly, you'll never be able to recover, leading to the inevitable conclusion of being pulled directly into the singularity.

Inside an event horizon, orbiting implies traveling faster than c from your reference frame. No can do.

(Note: this applies to an electrically neutral non-rotating black hole.)

Since the event horizon itself is supposed to be a fairly non-event to cross, why can't you just fly in a lazy circle around a large black-hole at a relatively "safe" distance and maintain an orbit that's decaying only very slowly?

Why is it necessarily the case that "stars would compress into this material rather than into black holes"?

For example, why can't it be the case that there are two kinds of matter, one that can be used to build infinitely strong wire, and one that can't, with stars being made of the latter? Or why can't it be the case that the crushing process doesn't take matter through the infinitely strong configuration on the way to black hole configuration?

> Why is it necessarily the case that "stars would compress into this material rather than into black holes"?

There's a good argument to be made that this isn't necessarily the case. I was thinking that all particles are interchangeable at sufficiently high temperatures (i.e. they can switch from one particle to another so long as energy is conserved). I overlooked the fact that stars don't get hot enough.

> Or why can't it be the case that the crushing process doesn't take matter through the infinitely strong configuration on the way to black hole configuration?

If the "infinitely strong" configuration got crushed into a black hole, it wasn't infinitely strong.

> For example, why can't it be the case that there are two kinds of matter, one that can be used to build infinitely strong wire, and one that can't, with stars being made of the latter?

How would the infinitely strong wire be held together? Electromagnetic force? Strong force? Weak force? Those forces (which are the only ones we know about apart from gravity) travel at c. If part of the wire is in, while part is out, we will necessarily break the bonds holding the material together if we attempt to pull the wire clear. If nothing else, this breaking will occur because those bonds rely on force carriers traveling at c, and so somewhere along the wire those force carriers will not see the items they are supposed to bind together.

Let us say we hold the wire so that a portion of it is past the event horizon. The bit of the wire at the event horizon, if held stationary, is traveling at c because light is also stationary there. Whatever bond existed at that point is no more; the force carriers can't make it to the particles they're supposed to bind together.

Instead of a string, how about a continuous stream of rocket fuel? You'd need a large enough black hole that:

  * the gravitational acceleration at the event horizon can be overcome by your rocket engines
  * the tidal forces at the event horizon don't kill you

Your reaction mass (rocket fuel) would be sacrificed to the black hole, but with outside help, could you escape the event horizon?

No. Inside the event horizon, all paths lead to the singularity. If you had gravitational sensors on your spacecraft, they would show the singularity all around you.

So which way do you actually fall is all directions are down? Or from your point of view you are motionless?

Presuming you're just letting gravity do its work and not, say, firing engines to resist...

You'd be in free fall, and that's a perfectly nice reference frame. Within that frame, your gravitational sensors would observe certain doom in every direction. This sphere of doom would get smaller until you were ripped apart from the tidal stress.

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