# Escaping from a black hole

#### ACG

Hi!

What's wrong with the following method of seeing what life is like inside a black hole and escaping? I can't see anything obvious. We'll assume that time scales and tidal forces aren't an issue (it's obviously impractical -- but is it impossible)?

1. Get a spaceship which can travel at c (well, as close to c as you can imagine).
2. Fall into the black hole. Just below the event horizon, inside the hole, fire your rocket engine as fast as you can. This will slow your descent to an arbitrarily slow speed, though not stop it (and certainly not escape). In effect, you will be able to "hover" there as long as you want.
3. The black hole emits Hawking radiation and starts to lose mass. This causes the event horizon to retreat inward.
4. The retrating event horizon passes over your hovering ship. Your ship is now outside the event horizon.
5. You escape.

ACG

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#### mathman

Step 2 won't work - gravity will overcome your rocket. Meanwhile it will tear you and the rocket apart.
Step 3 takes essentially forever for anything other than microscopic black holes.

#### Ultrastar 1

Hi!

What's wrong with the following method of seeing what life is like inside a black hole and escaping? I can't see anything obvious. We'll assume that time scales and tidal forces aren't an issue (it's obviously impractical -- but is it impossible)?

1. Get a spaceship which can travel at c (well, as close to c as you can imagine).
2. Fall into the black hole. Just below the event horizon, inside the hole, fire your rocket engine as fast as you can. This will slow your descent to an arbitrarily slow speed, though not stop it (and certainly not escape). In effect, you will be able to "hover" there as long as you want.
3. The black hole emits Hawking radiation and starts to lose mass. This causes the event horizon to retreat inward.
4. The retrating event horizon passes over your hovering ship. Your ship is now outside the event horizon.
5. You escape.

ACG
First, lets assume for one insane minute that it is possible to find a black hole and get to it. Next, you enter the black hole, as you do, intense graitatinal force rips your
rocket apart. But lets say that you found a way around that part, and got inside the singuarty of the black hole. This is what will happen, you will continue to hover there untill the black hole destablilizes and closes in on you. (This will turn you into a very thin pankake.) That is why your method of escape has no chance of sucsess.

#### tiny-tim

Homework Helper
2. … Just below the event horizon, inside the hole, fire your rocket engine as fast as you can. This will slow your descent to an arbitrarily slow speed …
Hi ACG!

No it won't.

You can't hover inside an event horizon.

Once you're in, you inevitably reach the singularity, and there is a finite (and very short! ) maximum proper time in which you do so.

#### dmt740

Probably a meaningless question, but once you cross the event horizon, how fast would you accelerate toward the singularity? Would you ever reach anything close to c before encountering the singularity? Again, probably meaningless, but inquiring minds want to know.

#### tiny-tim

Homework Helper
In Schwarzschild coordinates, you cross the event horizon at the same speed as light, and once inside you go faster than light (in other words, inside an event horizon, the speed of light is a minimum instead of a maximum).

Staff Emeritus
Just to add to the problems, any black hole bigger than about a millimeter across accretes rather than evaporates.

#### Dmitry67

Well, event horizon is a relative thing, it is observer-dependent.

#### v2kkim

I have a different opinion. The falling astronaut can still observe, in principle, event horizon, like no light escape from event horizon. So event horizon exists independent of observers.

R

#### randeezy

Hi ACG!

No it won't.

You can't hover inside an event horizon.

Once you're in, you inevitably reach the singularity, and there is a finite (and very short! ) maximum proper time in which you do so.

I agree. The gravity would be so immense, it would be like a 350lb fisherman reeling in a 5oz fish. the fish (in this case the spaceship) would have no chance of escaping. Once you near the singularity, the "cone" shape of the "tail" of the hole would start to compress the ship, much like it compress light waves, or at this point, photon clusters.

It would be impossible to counter the pull of the gravity, unless you have and Star Wars/I] masivse "Gravity Repulsor Engine" You would inevitably be sucked inwith no escape.

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#### Dmitry67

I have a different opinion. The falling astronaut can still observe, in principle, event horizon, like no light escape from event horizon. So event horizon exists independent of observers.
No
Even just by accelerating you can create your own event horizon: http://en.wikipedia.org/wiki/Event_horizon#Event_horizon_of_an_accelerated_particle

And again, horizon location is a relative thing. From the same article:

Observers who fall into the hole are moving with respect to the distant observer, and so perceive the horizon as being in a different location, seeming to recede in front of them so that they never contact it.

#### klite

1. Get a spaceship which can travel at c (well, as close to c as you can imagine).
Even if you are just under c, you will still be unable to escape (the escape velocity of a black hole is c+).

2. Fall into the black hole. Just below the event horizon, inside the hole, fire your rocket engine as fast as you can. This will slow your descent to an arbitrarily slow speed, though not stop it (and certainly not escape). In effect, you will be able to "hover" there as long as you want.
Hover there as long as you want, but you will still slowly be hovering toward the black hole. Even if you slow down the time it takes to get to the singularity, you'd still reach it someday in your space ship.

You probably would be able to view the singularity for a short second before you are flattened (as was said before and assuming that you can get around the shredding and ripping). Before you are flattened yoou should engage the (warp drive if you have it) and escape

#### Dmitry67

No, in non-rotating BH singularity is always veiled by the apparent horizon.
In the rotating BH yes, you can see it.

#### Dmitry67

Hover there as long as you want, but you will still slowly be hovering toward the black hole. Even if you slow down the time it takes to get to the singularity, you'd still reach it someday in your space ship.
Some day? :)
In stellar-size BH you reach it in microseconds!

Also remember that in non-rotating BH the singularity is spacelike (in the future). And pseudoeuclidean space has a very contre-intuitive property, so, the harder you try to avoid the singularity the sooner you fall into it. The maximize your proper time before you hit it you should relax in a freefall trajectory.

#### Matterwave

Gold Member
Yea, I was gonna mention that inside the EH all geodesics lead toward the singularity, so no matter which way you accelerate, it's always toward the singularity...

(I could be wrong, this is what I've heard. I haven't studied GR myself)

#### klite

Some day? :)
In stellar-size BH you reach it in microseconds!

Also remember that in non-rotating BH the singularity is spacelike (in the future). And pseudoeuclidean space has a very contre-intuitive property, so, the harder you try to avoid the singularity the sooner you fall into it. The maximize your proper time before you hit it you should relax in a freefall trajectory.
I remember reading that somewhere, you're right. Sorry for misinforming

#### protonchain

Star Trek 2: Escape from BH-6542.D?

I should have guessed

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