Fraser Cain, my buddy who runs the Universe Today website and co-admins the Bad Astronomy and Universe Today forum, wrote up a great blog entry about how to maximize your lifespan if you fall into a black hole. Don’t get me wrong– you’re still screwed, but you might get a femtosecond or two longer to live if you heed his advice.

all the math aside, which i can’t understand (and that’s more than i can count)…

BEFORE reaching event horizon :
– try to get away
AFTER passing event horizon :
– do NOT accelerate outward, instead
– start free falling
to get maximum subjective time…

i always thought, best place should be UNDER your spaceship, at L1…
like they did in “Dragon’s Egg”
granted, a _massive_ ship would be needed for any effect, but it’s pretty much academic anyway, isn’t it?

But could a ship orbit a singularity once inside an event horizon? If a free falling orbital trajectory were possible within a super massive black hole, could the ship hypothetically slowly descend?

“Nothing, not even light can escape the pull from a black hole once it passes into the event horizon.”

Okay, I’ve had this ridiculous thought experiment in mind for a while now. Could someone here point out where I’m going wrong?

We say that you can’t get out of a black hole because it would mean going faster than the speed of light, but why do we have to go faster than the speed of light to get away? The Earth’s escape velocity is about 11 km/sec, but things don’t have to go faster than 11 km/sec to get away from Earth. Let me explain:

Imagine that in the distant future, humanity builds a space elevator that reaches halfway to the orbit of the moon. Someone could now ‘escape’ from Earth by climbing to the top of the elevator, up to where Earth’s gravitational pull is weaker, and then zip off in a spacecraft at less then 11 km/sec. Because they were starting farther away from the Earth surface, the escape velocity was smaller.

To apply the analogy to a black hole, say I’ve got a 10^8 solar mass black hole (so tidal forces are negligible at the event horizon) and I’m standing on a spaceship that is orbiting just a few kilometers outside of the event horizon. I tether myself to the ship, jump out, and let myself fall past the event horizon until my tether runs out. Then, I start to climb back, hand over hand. I never actually need to reach the escape velocity of the speed of light for the same reason I never needed to reach 11 km/sec in our Earth-Space Elevator analogy, so I get back to my spaceship, where escape velocity is less then the speed of light, and fly away.

To anyone outside of the system, I would take an infinite amount of time for me to do this, so there would be no one left to report my discoveries to, but still, haven’t I managed to get inside and outside of a black hole alive?

Use the same strategy you’d use if you fell out of an airplane without a parachute. Shout “WATCH THIS!” and start flapping your arms really really fast. If it works, you survive and your friends are extremely impressed. If it doesn’t work, well then, it won’t really matter how silly you looked, will it?

“To anyone outside of the system, I would take an infinite amount of time for me to do this, so there would be no one left to report my discoveries to.”

Is there some other effect that I’m ignoring? Please don’t leave me in suspense!

Sorry, sometimes my humor goes over even my own head.

One problem is that black holes, even as seen from outside, evaporate within a finite amount of time. Before you get up to the event horizon, the whole thing will have exploded into a burst of Hawking radiation.

But could you explain how Hawking radiation makes a black hole ‘evaporate?’ I think I understand what Hawking radiation is (virtual particle/antiparticle pairs forming at the event horizon, one particle escaping, the other falling in) but I never really understood how that would cause a black hole to disappear.

Mass/energy still has to be conserved. If a particle escapes from the event horizon, even if it does so indirectly as the result of the creation of a virtual particle/antiparticle pair, the black hole itself has to lose mass to balance the books.

I’m no physicist, and I don’t pretend to understand how this really works. (I think the only way it can be accurately expressed is via the equations of quantum mechanics; all our attempts to describe it in English are just rough approximations of that.) And this is based on information from some years ago; I think the theories may have changed since then.

Greg Egan published a science fiction short story, “The Planck Dive”, about a dive into a black hole. The ship thrusted laterally throughout its fall (after crossing the horizon) and supposedly this increased proper time by some fraction. Was Egan just wrong? Or is there something I’m missing?

Max, you can’t orbit “just outside” the event horizon without using nearly infinite amounts of thrust. Even at the speed of light a zero-thrust orbit must not pass below 150% the height of the event horizon or it’ll be lost.

For a supermassive black hole with low tidal forces per meter, either your tether gets to be long enough for tidal forces to win anyway, or your ship has to accelerate so hard away form the event horizon just to maintain height that the tension is just as great.

Once you’re at the event horizon, one way or another you have to generate infinite acceleration to maintain height, which would be both impossible and extraordinarily lethal due to the infinitely blue-shifted particles hitting you on their way down. Below the horizon, even infinite acceleration doesn’t cut it.

The teather idea is a bit like hoisting yourself on your own petard. Unless I’m mistaken(and despite evidence to the contrary, I don’t ever believe I am), your weight inside the event horizon would make you simply pull yourself closer and closer to the ship, while pulling the ship, a teeny bit at a time, nearer the event horizon. End result, everyone is doomed.

Wow. I had actually considerd this before that you would be better off going with the forces, rather than against. But if you have a spacecraft with some sort of warp engine and all sorts of other stuff we don’t totally understand or know is possible now, then all bets are off, right? Because maybe you could be protected i warpbubble or subspace somethingorother.

And as you get closer, you say “nanoseconds” but won’t time start to slow down and eventually begin getting all fubared? like stopping or going in the wrong way or something?

what i always wonder about with the whole ‘space elevator’ ideas is the question, where to get the lateral velocity…

if you go ‘up’ in an elevator on a rotating planet, say, earth for example, you not only would have accelerate upwards but also east! you have to account for the fact, that you are moving far slower standing on the ground then when your up a few thousands of km. in everyday life the difference between groundfloor and rooftop is negligible, but for e.g. ground to geosynchronous orbit it IS significant. if you don’t accelerate east you’de be pushed against the western wall of the cabin. are there material that could sustain this stress in addition to thirtysix thousand kms of its own weight?

anyway, what i wanted to say: you don’t fire a rocket to 11km/s in one big blast (not if anything goes according to plan) but in a rather more slow and ‘gentle’ way… but you would reach this speed to escape earth!

“anyway, what i wanted to say: you donâ€™t fire a rocket to 11km/s in one big blast (not if anything goes according to plan) but in a rather more slow and â€˜gentleâ€™ wayâ€¦ but you would reach this speed to escape earth!”

Not if you start from a higher altitude. I see now where the idea went wrong, but I’m still confident that escape velocity gets smaller with altitude. Getting away from Earth if you’re already at the distance of the moon doesn’t require accelerating past 11 km/sec.

If you take an elevator to geo-sync orbit, it’s true your velocity need never approach 11 km/sec however,,,heh,here it comes,,,your potential energy and kinetic energy will be identical at 24,000 km and a rotational velocity of about 1500 km/hr as it would have been if you had been given an instantenous velocity at earths surface of 11 km/sec. It’s really not about velocity, though that’s what’s usually used when describing how rockets/space drives/etc get us off the planet. It’s just easier to talk about the instaneous velocity rather than K.E.= 1/2 MV^2 and Potential energy of Mass x Acceleration x Height.

It’s really all about the total energy of the craft. That ties into the energy required to escape a black hole, where the potential energy at the event horizen approachs infinity, so the kinetic enery to escape must be greater than that, and unless you’re into transfinite numbers, that ain’t possible,,,in this space /time, anyway,,,unless you’re a tachyon,,,

Can I heartily recomend Gregory Benford’s ‘Galactic Center Saga’?

High jinks in an overpopulated galaxy where the human empire has colapsed, the dominant AI species regard us as an annoyance, and the supermassive black hole in the centre of the galaxy has entered a ‘feeding’ phase.

It has been years but I think that ‘Furious Gulf’ is the one where they enter the event horizon (the black hole being so big that the tidal effects are acceptable)

A typical rocket works more like your tether than an escape velocity condition. That is because the rocket produces thrust over time, while the “escape velocity” assumes the velocity is instantaneously applied and then no more thrust is added. If you assume constant thrust over time greater than the downward pull, then the escape velocity is irrelevant.

However, the black hole situation is a bit different. Acceleration requires thrust, which is an expenditure of energy. Dangling on a tether over the event horizon would require a substantial amount of energy to thrust you back over the horizon. I believe the requirement is on the order of infinity. It would also require an exceptionally strong tether. I suspect as soon as you crossed the event horizon your tether would sever due to internal stresses (assuming it lasted that long). If somehow the tether were infinitely strong (and stronger than the tension), then your space ship would have a very difficult time maintaining stationkeeping while being pulled in by your infinite mass.

Or maybe there’d be a rift in the space-time continuum and you’d pop back safely into “real space” some billion light years away in another quadrant and have to proceed back home.

The problem relating to “spagettification”, (mentioned in BA’s link), where the closer part of your body is actually accellerated faster, because it is marginally closer to the singularity, can be partially solved by laying “flat”, ie. – as if floating as sky-divers do.

Similarly, the spacecraft would present its flattest surface (probably upside-down so the center-of -interest, the Black Hole itself, could be seen thru the Observation windows), and this would cause less stretch for those happy little nano-seconds of extra time again.

Every little bit helps. (Anyone for bungy-jumping?)

no one is ever going to know unless we go to one. this is all speculation and theory. no equation will give us the answer or prove anything. what we think we know is all a guess from what little man knows about the universe. mankind will never truly know what happens when he ventures to close to a black hole because it will never happen. i really dont think we would see the back of our own heads if we were that lucky. i dont buy that theory or many others

all the math aside, which i can’t understand (and that’s more than i can count)…

BEFORE reaching event horizon :

– try to get away

AFTER passing event horizon :

– do NOT accelerate outward, instead

– start free falling

to get maximum subjective time…

i always thought, best place should be UNDER your spaceship, at L1…

like they did in “Dragon’s Egg”

granted, a _massive_ ship would be needed for any effect, but it’s pretty much academic anyway, isn’t it?

heng

—

REALLY bad astronomy:

http://img503.imageshack.us/img503/9171/tubedn0.jpg

But could a ship orbit a singularity once inside an event horizon? If a free falling orbital trajectory were possible within a super massive black hole, could the ship hypothetically slowly descend?

I thought the trick was to find a crack in the event horizon and bombard it with warp particles.

Quote:

“Nothing, not even light can escape the pull from a black hole once it passes into the event horizon.”

Okay, I’ve had this ridiculous thought experiment in mind for a while now. Could someone here point out where I’m going wrong?

We say that you can’t get out of a black hole because it would mean going faster than the speed of light, but why do we have to go faster than the speed of light to get away? The Earth’s escape velocity is about 11 km/sec, but things don’t have to go faster than 11 km/sec to get away from Earth. Let me explain:

Imagine that in the distant future, humanity builds a space elevator that reaches halfway to the orbit of the moon. Someone could now ‘escape’ from Earth by climbing to the top of the elevator, up to where Earth’s gravitational pull is weaker, and then zip off in a spacecraft at less then 11 km/sec. Because they were starting farther away from the Earth surface, the escape velocity was smaller.

To apply the analogy to a black hole, say I’ve got a 10^8 solar mass black hole (so tidal forces are negligible at the event horizon) and I’m standing on a spaceship that is orbiting just a few kilometers outside of the event horizon. I tether myself to the ship, jump out, and let myself fall past the event horizon until my tether runs out. Then, I start to climb back, hand over hand. I never actually need to reach the escape velocity of the speed of light for the same reason I never needed to reach 11 km/sec in our Earth-Space Elevator analogy, so I get back to my spaceship, where escape velocity is less then the speed of light, and fly away.

To anyone outside of the system, I would take an infinite amount of time for me to do this, so there would be no one left to report my discoveries to, but still, haven’t I managed to get inside and outside of a black hole alive?

Where am I going wrong here?

Where am I going wrong here?Relativity.

The details are left as an exercise.

Use the same strategy you’d use if you fell out of an airplane without a parachute. Shout “WATCH THIS!” and start flapping your arms really really fast. If it works, you survive and your friends are

extremelyimpressed. If it doesn’t work, well then, it won’t really matter how silly you looked, will it?Oh, I thought I accounted for relativity:

“To anyone outside of the system, I would take an infinite amount of time for me to do this, so there would be no one left to report my discoveries to.”

Is there some other effect that I’m ignoring? Please don’t leave me in suspense!

Sorry, sometimes my humor goes over even my own head.

One problem is that black holes, even as seen from outside, evaporate within a finite amount of time. Before you get up to the event horizon, the whole thing will have exploded into a burst of Hawking radiation.

Hmmm. I guess that makes sense.

But could you explain how Hawking radiation makes a black hole ‘evaporate?’ I think I understand what Hawking radiation is (virtual particle/antiparticle pairs forming at the event horizon, one particle escaping, the other falling in) but I never really understood how that would cause a black hole to disappear.

Mass/energy still has to be conserved. If a particle escapes from the event horizon, even if it does so indirectly as the result of the creation of a virtual particle/antiparticle pair, the black hole itself has to lose mass to balance the books.

I’m no physicist, and I don’t pretend to understand how this really works. (I think the only way it can be accurately expressed is via the equations of quantum mechanics; all our attempts to describe it in English are just rough approximations of that.) And this is based on information from some years ago; I think the theories may have changed since then.

Maybe Phil can explain it better than I can.

Greg Egan published a science fiction short story, “The Planck Dive”, about a dive into a black hole. The ship thrusted laterally throughout its fall (after crossing the horizon) and supposedly this increased proper time by some fraction. Was Egan just wrong? Or is there something I’m missing?

The Planck Dive is at http://gregegan.customer.netspace.net.au/PLANCK/Complete/Planck.html

(hoping the blog software doesn’t think I’m a spammer for including a link …)

Max, you can’t orbit “just outside” the event horizon without using nearly infinite amounts of thrust. Even at the speed of light a zero-thrust orbit must not pass below 150% the height of the event horizon or it’ll be lost.

For a supermassive black hole with low tidal forces per meter, either your tether gets to be long enough for tidal forces to win anyway, or your ship has to accelerate so hard away form the event horizon just to maintain height that the tension is just as great.

Once you’re at the event horizon, one way or another you have to generate infinite acceleration to maintain height, which would be both impossible and extraordinarily lethal due to the infinitely blue-shifted particles hitting you on their way down. Below the horizon, even infinite acceleration doesn’t cut it.

The teather idea is a bit like hoisting yourself on your own petard. Unless I’m mistaken(and despite evidence to the contrary, I don’t ever believe I am), your weight inside the event horizon would make you simply pull yourself closer and closer to the ship, while pulling the ship, a teeny bit at a time, nearer the event horizon. End result, everyone is doomed.

Wow. I had actually considerd this before that you would be better off going with the forces, rather than against. But if you have a spacecraft with some sort of warp engine and all sorts of other stuff we don’t totally understand or know is possible now, then all bets are off, right? Because maybe you could be protected i warpbubble or subspace somethingorother.

And as you get closer, you say “nanoseconds” but won’t time start to slow down and eventually begin getting all fubared? like stopping or going in the wrong way or something?

Admittedly unrelated, but I think it’s worth a blog entry unto itself:

http://www.youtube.com/watch?v=9AWwn0ctAM4

Excellent news! Now I know what to do if the particle accelerator up in NJ goes loopy.

what i always wonder about with the whole ‘space elevator’ ideas is the question, where to get the lateral velocity…

if you go ‘up’ in an elevator on a rotating planet, say, earth for example, you not only would have accelerate upwards but also east! you have to account for the fact, that you are moving far slower standing on the ground then when your up a few thousands of km. in everyday life the difference between groundfloor and rooftop is negligible, but for e.g. ground to geosynchronous orbit it IS significant. if you don’t accelerate east you’de be pushed against the western wall of the cabin. are there material that could sustain this stress in addition to thirtysix thousand kms of its own weight?

anyway, what i wanted to say: you don’t fire a rocket to 11km/s in one big blast (not if anything goes according to plan) but in a rather more slow and ‘gentle’ way… but you would reach this speed to escape earth!

“anyway, what i wanted to say: you donâ€™t fire a rocket to 11km/s in one big blast (not if anything goes according to plan) but in a rather more slow and â€˜gentleâ€™ wayâ€¦ but you would reach this speed to escape earth!”

Not if you start from a higher altitude. I see now where the idea went wrong, but I’m still confident that escape velocity gets smaller with altitude. Getting away from Earth if you’re already at the distance of the moon doesn’t require accelerating past 11 km/sec.

If you take an elevator to geo-sync orbit, it’s true your velocity need never approach 11 km/sec however,,,heh,here it comes,,,your potential energy and kinetic energy will be identical at 24,000 km and a rotational velocity of about 1500 km/hr as it would have been if you had been given an instantenous velocity at earths surface of 11 km/sec. It’s really not about velocity, though that’s what’s usually used when describing how rockets/space drives/etc get us off the planet. It’s just easier to talk about the instaneous velocity rather than K.E.= 1/2 MV^2 and Potential energy of Mass x Acceleration x Height.

It’s really all about the total energy of the craft. That ties into the energy required to escape a black hole, where the potential energy at the event horizen approachs infinity, so the kinetic enery to escape must be greater than that, and unless you’re into transfinite numbers, that ain’t possible,,,in this space /time, anyway,,,unless you’re a tachyon,,,

Gary 7

Can I heartily recomend Gregory Benford’s ‘Galactic Center Saga’?

High jinks in an overpopulated galaxy where the human empire has colapsed, the dominant AI species regard us as an annoyance, and the supermassive black hole in the centre of the galaxy has entered a ‘feeding’ phase.

It has been years but I think that ‘Furious Gulf’ is the one where they enter the event horizon (the black hole being so big that the tidal effects are acceptable)

A typical rocket works more like your tether than an escape velocity condition. That is because the rocket produces thrust over time, while the “escape velocity” assumes the velocity is instantaneously applied and then no more thrust is added. If you assume constant thrust over time greater than the downward pull, then the escape velocity is irrelevant.

However, the black hole situation is a bit different. Acceleration requires thrust, which is an expenditure of energy. Dangling on a tether over the event horizon would require a substantial amount of energy to thrust you back over the horizon. I believe the requirement is on the order of infinity. It would also require an exceptionally strong tether. I suspect as soon as you crossed the event horizon your tether would sever due to internal stresses (assuming it lasted that long). If somehow the tether were infinitely strong (and stronger than the tension), then your space ship would have a very difficult time maintaining stationkeeping while being pulled in by your infinite mass.

Or maybe there’d be a rift in the space-time continuum and you’d pop back safely into “real space” some billion light years away in another quadrant and have to proceed back home.

The problem relating to “spagettification”, (mentioned in BA’s link), where the closer part of your body is actually accellerated faster, because it is marginally closer to the singularity, can be partially solved by laying “flat”, ie. – as if floating as sky-divers do.

Similarly, the spacecraft would present its flattest surface (probably upside-down so the center-of -interest, the Black Hole itself, could be seen thru the Observation windows), and this would cause less stretch for those happy little nano-seconds of extra time again.

Every little bit helps. (Anyone for bungy-jumping?)

Ivan.

no one is ever going to know unless we go to one. this is all speculation and theory. no equation will give us the answer or prove anything. what we think we know is all a guess from what little man knows about the universe. mankind will never truly know what happens when he ventures to close to a black hole because it will never happen. i really dont think we would see the back of our own heads if we were that lucky. i dont buy that theory or many others