Okay I was watching the Nova episode about blackholes and it brought an old question to mind.
My understanding of the event horizon is it’s the point where the escape velocity is greater then the speed of light, therefor it’s claimed nothing can escape because nothing can go faster then the speed of light.
However it doesn’t seem to me like you’d need to reach the escape velocity. You’d just need a buttload of fuel. Consider this thought experiment.
On earth you want to journey to the moon, but you’re old and frail and would never survive the acceleration needed to reach earth escape velocity. So you develop a fuel energetic enough to give a slow 1.01 G acceleration all the way to the moon. Thus you’ve escaped earth’s gravity without reaching escape velocity.
Assuming strong enough fuel, couldn’t a black hole’s event horizon be escape in a similar manor?
I’m thinking maybe my understanding of event horizons is off.
The problem is that you can’t reach c without an infinite amount of fuel. (Or, in other words, you can’t maintain 1.01g all the way up to c.) And since c is the escape velocity…
ETA: To clarify, you do need to reach escape velocity. Even in the geriatric-on-earth case, you’d still eventually get to escape velocity at 1.01g acceleration.
Remember, too, that a black hole’s gravity well is, from a distance, just like the gravity well of any other body. If you don’t get too close, you’re in no danger. You could even establish orbit around a black hole (although I wouldn’t advise it, given the consequences if your math is wrong). Despite what Disney told us, a black hole is not a cosmic Hooverizer.
Why would you? Once you’ve built up any amount of speed pointing away from the Earth, you no longer need to accelerate at all in order to escape. You just need a thrust that counters the acceleration of gravity. Your net acceleration after an initial lift could be precisely 0g and you could still end up on the moon, traveling just as slowly as you like.
Escape velocity from the surface is that velocity which is required in order to come to rest at an infinite distance from the Earth assuming you have zero thrust beyond that initial impulse. When you’re talking about anything that has its own source of thrust, “escape velocity” is no longer meaningful.
ETA: One major problem I can see with attempting this very close to a black hole is that the tidal forces would be ripping you and your spaceship apart, most likely resulting in the non-functioning of your thrust devices. :o
Yea but would you reach the moon before you hit escape velocity at 1.01g?
How about if instead of accelerating at 1.01g, you accelerated at 1.01 for a bit to build some intertia, say 20m/s, then cut your acceleration to 1g so your rocket compensated exactly for gravity? Seems like you’d just coast to the moon’s gravity well at 20m/s.
Because spacetime is changed, or at least in theory is changed so the distance between you and the event horizon increased to infinity. Some have theorized our universe is inside a black hole, instead of crunching, the stuff inside is falling ‘deeper’ into emptyness, spreading apart, redshifting as the gravity pulls things apart.
Instead of a planetary gravity well think of it as a funnel that just keeps going down.
The OP is right. A black hole has a defined and finite force at the event horizon. (We could do the math, but I’ll leave it to others). That force is probably pretty high (probably high enough to kill a human anyway) and it must have a horrific gradient, but if you can manage a higher acceleration and hold your ship together, you’ll eventually make it out.
In fact, as I understand it, things are swinging into and out of the event horizon all the time. It’s just that they always get pulled back in eventually. It’s like if I throw a baseball up into the air. I can’t throw hard enough to exceed escape velocity so it goes up and comes back down. A radioactive particle might decay just inside the event horizon, shooting particles out at high velocity. Those particles will get outside of the event horizon for a while before being pulled back in.
While this might be highly important to the things falling into the black hole, it’s a moot point for observers; if nothing can escape, we’ll never be able to detect it.
I have some technical details incorrect I am sure, but at the event horizon spacetime is so distorted that from your point of view the event horizon is receding from you at the speed of light. So you don’t even get the opportunity to approach the event horizon because you can never even reach it.
Right. Nevermind my post. I was thinking that the OP wanted to use up all that boatload of fuel while slowly building up a certain speed, and then coast out of the hole. In fact, he was talking about never running out of fuel. In this case, my post is useless.
The problem is that if you are at the event horizon you need an acceleration of C to not fall. Not falling on earth requires only that you sit there, Accelerating at 1.0000000001 g will move you upwards, slowly. You can accomplisht this with a rocket, by having a reaction mass ejected at an acceleration great er than 1 g. Accelerating at 1.00000001 C would work, except that it is not possible to eject your reaction mass at a velocity greater than C, so you can’t get any acceleration at all. So, you fall.
Engineering consideration for the hovercraft involve some difficulties as well.
Tris
PS I was going to say, this isn’t rocket science, but, well. . . it is.
Trisk, you can’t have “an acceleration of c”, any more than you can have a height of 150 pounds. They’re different units.
The problem with the OP’s reasoning is that that’s not why the event horizon is a point of no return. If you naively try to calculate the radius at which the Newtonian escape speed from a mass is equal to c, you’ll get an answer that has the same value as the Schwartzschild radius, but that’s just a coincidence, given that black holes are not Newtonian objects. You’re actually making multiple errors that all happen to cancel each other out.
The real reason that you can’t escape from within the event horizon is that there is no path from the inside of the horizon to the outside. Such paths simply do not exist. Once you’re inside the horizon, you can’t point the engines on your rocket to the center and propel yourself out, any more than you can point your engines towards tomorrow and propel yourself to last Thursday.
So the popular explanation the event horizon is where the escape velocity is C is incomplete. It’s where space is so stretched out you need to accelerate faster then light speed to move outward even an inch (which would also get you to last Thursday). That’s why it’s inescapable.
I’ll let someone else that is in the mood do the searching (the five-minute limit makes SDMB searching crippled and I can’t be bothered figuring out the ins and outs of finding it with Google) but quite a while back there was another question on the exact same topic. Why do I remember? Because I was the OP on the old one. Go back far enough on my posts and you should eventually find it.
Well, yes, but it no longer applies to you because you are moving under thrust. If you have continuous thrust instead of a single initial speed, you needn’t ever meet or exceed the escape velocity at any point with respect to the body in order to come to rest an infinite distance away from it, asymptotically speaking. You could simply adjust your thrust at every moment of the trip in order to keep your speed below the local escape velocity the whole time.
The speed of light has nothing to do with the ability to escape from a black hole. It just saying that the gravitational forces are so great that photons can not escape. If I travel away from a black hole I travel away from a black hole.
Escape velocity has nothing to do with the OPs question.
No. The (extremely simplified) theory is that particle pairs get created by quantum effects near the event horizon and one flies away while the other falls in. No information can leave the event horizon. Once something is inside, it effectively ceases to exist in this universe, which is why normal black holes can be described by only three variables – there is no other possible information we can obtain. Spacetime inside is so curved that a straight line “out” of the event horizon at C would really be a curved path around the inside of the event horizon.