Is it Possible to Escape a Black Hole?

[soothsayer]

I had a couple questions regarding the ability of objects and light to escape a black hole.

1) The even horizon is supposedly the boundary at which no object can escape from a Black Hole, as within this radius, the escape velocity is greater than c. This much I understand; If I throw myself into a black hole with a flashlight and shine that flashlight outward from the black hole after I cross the event horizon, that light will not escape from the black hole. However, if you enter into the black hole with a very high powered spaceship and cross the event horizon, I feel like all you would need to do would be to fire the ships rockets with enough force to slightly overcome the force of gravity from the black hole and you can slowly accelerate out of the black hole, a simple force diagram would suggest this. Is this not the case? And if so, isn't it a bit misleading to say NOTHING can escape from a black hole once it's past the event horizon? The escape velocity would be the minimum velocity an object must have to escape a gravity well given there is no added propulsion, correct?

2) I'm a bit confused as to what happens to light that is trapped in a black hole. GR tells us that light that is trying to escape from the surface of a massive object is subject to redshift, as gravity will apply a force to that light and thus, do work on it, lowering the energy of the photons. Take the above example, where I foolishly launch myself into a black hole with a flashlight. I cross the event horizon and turn the flashlight on and the photons can't move fast enough to escape the black hole. The red shifted frequency, relative to a far away observer, would approach zero as the beam approached the event horizon. What does this mean physically for the photons in the light beam, do they reach some point in the gravity well, slow to a stop and fall back down, like a ball thrown upward from Earth? Can the photons ever even be slowed below c in a vacuum? I don't understand what it means to say the photon energy and frequency is zero.

Any insight would be greatly appreciated! =)

 

[Nabeshin]

I had a couple questions regarding the ability of objects and light to escape a black hole.

1) The even horizon is supposedly the boundary at which no object can escape from a Black Hole, as within this radius, the escape velocity is greater than c. This much I understand; If I throw myself into a black hole with a flashlight and shine that flashlight outward from the black hole after I cross the event horizon, that light will not escape from the black hole. However, if you enter into the black hole with a very high powered spaceship and cross the event horizon, I feel like all you would need to do would be to fire the ships rockets with enough force to slightly overcome the force of gravity from the black hole and you can slowly accelerate out of the black hole, a simple force diagram would suggest this. Is this not the case? And if so, isn't it a bit misleading to say NOTHING can escape from a black hole once it's past the event horizon? The escape velocity would be the minimum velocity an object must have to escape a gravity well given there is no added propulsion, correct?

Even within the context of Newtonian physics this is incorrect. The whole point of escape velocity is that you would need to be going that fast to escape the system. So when escape velocity=c, you would need to be moving faster than c to escape from the system! We know this is impossible, you cannot accelerate a massive object to faster than c.

Within the context of GR I would explain it differently simply by saying that all timelike (and even null) world lines inside the event horizon terminate on the singularity. That is, no matter which path you take, all paths lead to the singularity. One way of thinking about it is that inside the event horizon, space becomes timelike. That is to say, whereas we are destined to move constantly forward in time, so too inside of the black hole are we destined to move constantly towards the singularity.

It's a funny fact that if you do try to fire a rocket while falling towards the singularity, not only will you fail to escape, but you actually shorten the amount of time it takes for you to fall to the singularity!

2) I'm a bit confused as to what happens to light that is trapped in a black hole. GR tells us that light that is trying to escape from the surface of a massive object is subject to redshift, as gravity will apply a force to that light and thus, do work on it, lowering the energy of the photons. Take the above example, where I foolishly launch myself into a black hole with a flashlight. I cross the event horizon and turn the flashlight on and the photons can't move fast enough to escape the black hole. The red shifted frequency, relative to a far away observer, would approach zero as the beam approached the event horizon. What does this mean physically for the photons in the light beam, do they reach some point in the gravity well, slow to a stop and fall back down, like a ball thrown upward from Earth? Can the photons ever even be slowed below c in a vacuum? I don't understand what it means to say the photon energy and frequency is zero.

Any insight would be greatly appreciated! =)

Photons always move at c. I wouldn't ever really say photon energy and frequency is zero. As you approach the EH, photons received by a distant observer tend towards this in the limit, but you will never "receive" a zero energy photon, whatever that means.

 

[Drakkith]

If all paths in spacetime lead back to the black hole, then it seems to me that it is the same for photons. Once they reach the point where they would be "zero" energy, they are drawn back into the center of the black hole.

 

[soothsayer]

Even within the context of Newtonian physics this is incorrect. The whole point of escape velocity is that you would need to be going that fast to escape the system. So when escape velocity=c, you would need to be moving faster than c to escape from the system! We know this is impossible, you cannot accelerate a massive object to faster than c.

Yes, if you just had some initial velocity lower than the escape velocity of a system, you won't escape the system, but that doesn't take any acceleration into account. If a throw a ball upward, it will fall back down, I can't give it enough initial speed to escape Earth's gravity, but if I have a rocket whose propulsion can overcome Earth's gravity, it wouldn't need to be going the escape velocity.

However, I was talking to one of my professors and he explained the event horizon not only as a point where the escape velocity is c, it's a point where the escape ENERGY required is infinite, as shown by SR, at speed c, a massive object has infinite energy. So I do understand now why my original idea would not work.

Your GR explanation helped me out a little bit as well, I haven't taken a full-blown GR course, but it made sense to me.

Photons always move at c. I wouldn't ever really say photon energy and frequency is zero. As you approach the EH, photons received by a distant observer tend towards this in the limit, but you will never "receive" a zero energy photon, whatever that means.

Understandable, an outside observer could never observe a photon with no energy, as we observe a photon approach the BH, it will slowly become more and more redshifted, approaching an infinite limit of zero energy, that it will never reach. But what exactly occurs in the point of reference of the photons themselves?

 

[Drakkith]

Understandable, an outside observer could never observe a photon with no energy, as we observe a photon approach the BH, it will slowly become more and more redshifted, approaching an infinite limit of zero energy, that it will never reach. But what exactly occurs in the point of reference of the photons themselves?

My guess is that nothing actually happens to it, it just follows a curved path back to the black hole. Either the photons are not following a perfectly straight path away from the BH, or spacetime is warped enough so that even if they are, it simply curves back in on itself.

 

[sachinism]

My guess is that nothing actually happens to it, it just follows a curved path back to the black hole. Either the photons are not following a perfectly straight path away from the BH, or spacetime is warped enough so that even if they are, it simply curves back in on itself.

any backup for that thought ? as in what made you think that way ?

 

[Drakkith]

any backup for that thought ? as in what made you think that way ?

Nothing specific, just based on my understanding on how light works. It gets bent around gravitational sources, so seems to me it should be bent back to the black hole since space is so curved.

Imagine the old bowling ball on a trampoline example. If something is heavy enough, the dent in the trampoline, which represents space, is bent effectively vertical. Try to roll somthing up it and what happens? It curves back around and falls back down. Was thinking the same thing for light.

 

[Cbray]

Of what I've read, as I am an only a year 9 student.

From Einstein's theorem nothing can go faster than c (the speed of light) as gravity travels at the speed of light, your 'ship' could not 'fly' or what ever type of transportation subject of hovering or whatever cannot escape it. The only thing you would be doing is slowing down the time of your so call 'ship' will reach the singularity of the black hole.

Thanks,
Charles.

 

[Nabeshin]

Understandable, an outside observer could never observe a photon with no energy, as we observe a photon approach the BH, it will slowly become more and more redshifted, approaching an infinite limit of zero energy, that it will never reach. But what exactly occurs in the point of reference of the photons themselves?

Sorry, for some reason I was ignoring this thread when you responded... Glad the GR description made sense, my Newtonian explanation was a bit confusing I understand, sorry about that (I don't have the easiest time transitioning GR concepts back into Newtonian language -- probably isn't even possible all the time).

A big point we often try to drive home here is that photons do not have a reference frame. That is, you cannot within the context of relativity ask the question "What would X look like if I were a photon?" So the question doesn't really have an answer.

 

[TobyC]

Even within the context of Newtonian physics this is incorrect. The whole point of escape velocity is that you would need to be going that fast to escape the system. So when escape velocity=c, you would need to be moving faster than c to escape from the system! We know this is impossible, you cannot accelerate a massive object to faster than c.

I don't think this is strictly true is it? In Newtonian physics you don't have to achieve the escape velocity in order to escape an object's gravity if you have a force pushing you upwards as in this example. Escape velocity is just the speed you would need to escape the object's gravity if gravity was the only force acting on you.

Take an extreme example, if you had a jet pack which provided a constant force which was always exactly equal to your weight due to gravity but in the opposite direction, then ignoring air resistance you would behave like there were no forces acting on you and you could escape the Earth's gravity by traveling as slowly as you like, 1mph for example.

 

[russ_watters]

Take an extreme example, if you had a jet pack which provided a constant force which was always exactly equal to your weight due to gravity but in the opposite direction, then ignoring air resistance you would behave like there were no forces acting on you and you could escape the Earth's gravity by traveling as slowly as you like, 1mph for example.

My Relativity is a little thin, but I think just inside the event horizon, your weight is infinite, so the force would have to be infinite.

 

[Nabeshin]

I don't think this is strictly true is it? In Newtonian physics you don't have to achieve the escape velocity in order to escape an object's gravity if you have a force pushing you upwards as in this example. Escape velocity is just the speed you would need to escape the object's gravity if gravity was the only force acting on you.

Take an extreme example, if you had a jet pack which provided a constant force which was always exactly equal to your weight due to gravity but in the opposite direction, then ignoring air resistance you would behave like there were no forces acting on you and you could escape the Earth's gravity by traveling as slowly as you like, 1mph for example.

When I originally typed up my "Newtonian" argument it was worded rather poorly, what I meant to say is the following: To escape from the black hole (to infinity, in the Newtonian context) you must travel at c. No matter what your initial velocity, or how fast you accelerate, you can never obtain this velocity, so it stands to reason that you will never escape to infinity.

It's a silly argument to make anyways, since black holes are decidedly non-Newtonian, and we should probably just stick to the physics which actually describes them -- GR. I regret trying to go that route in explaining it now...

 

[soothsayer]

My Relativity is a little thin, but I think just inside the event horizon, your weight is infinite, so the force would have to be infinite.

Thanks, that makes a lot more sense. I guess Newtonian analogs just don't work for Black Holes.

 

[soothsayer]

Sorry, for some reason I was ignoring this thread when you responded... Glad the GR description made sense, my Newtonian explanation was a bit confusing I understand, sorry about that (I don't have the easiest time transitioning GR concepts back into Newtonian language -- probably isn't even possible all the time).

A big point we often try to drive home here is that photons do not have a reference frame. That is, you cannot within the context of relativity ask the question "What would X look like if I were a photon?" So the question doesn't really have an answer.

You're right about the photon reference frame. I understood this idea, too, I just didn't think about it. It's always difficult to come to terms with, it seems like there should be some way or some frame which can detect what the photon is actually doing.

Thanks for the insight, PF!

 

[surajt88]

Even within the context of Newtonian physics this is incorrect. The whole point of escape velocity is that you would need to be going that fast to escape the system. So when escape velocity=c, you would need to be moving faster than c to escape from the system! We know this is impossible, you cannot accelerate a massive object to faster than c.

Escaping from the Earth is analogous to escaping from a black hole. Are all rockets fired from Earth have a velocity of 11.2 km/s? Will it not be possible to maybe use a jet pack that moves up at say 1 km/s? I can understand that the density of air reduces as we go higher and the force required to move at 1 km/s will be higher. If I had a huge amount of fuel, can't i just escape Earth at just 1 km/s? Where does this escape velocity come in then? Will I be violating the rule that an object has to be moving at 11.2 km/s to escape earth?

 

[Sourabh N]

Escaping from the Earth is analogous to escaping from a black hole. Are all rockets fired from Earth have a velocity of 11.2 km/s? Will it not be possible to maybe use a jet pack that moves up at say 1 km/s? I can understand that the density of air reduces as we go higher and the force required to move at 1 km/s will be higher. If I had a huge amount of fuel, can't i just escape Earth at just 1 km/s? Where does this escape velocity come in then? Will I be violating the rule that an object has to be moving at 11.2 km/s to escape earth?

Remember that 11.2 km/s is the escape velocity for an object (under the influence of Earth's gravity and no other force) from the surface of Earth. Yes, you can start with a speed of 1 km/s, travel for a few kms, and then achieve the escape velocity at that distance from Earth to "escape".

 

[Drakkith]

Escape velocity is often misunderstood. If you had a rocket that only traveled at 100 KM/H, as long as it could maintain that speed then you could escape the Earth's gravity simply by keeping the engines on. But as we don't have unlimited fuel eventually we have to turn the engines off, and usually this is pretty early in a flight. (sub orbital for the main boosters and engines) Also, to go into orbit DOES require a certain velocity.

If i were to fire a cannonball straight up into the air, it would have to move at 11.2 km/s initially to escape Earth's gravity since it has no engine providing thrust to keep it moving against gravity.

 

[surajt88]

Remember that 11.2 km/s is the escape velocity for an object (under the influence of Earth's gravity and no other force) from the surface of Earth. Yes, you can start with a speed of 1 km/s, travel for a few kms, and then achieve the escape velocity at that distance from Earth to "escape".

So is it not possible to go all the way up at 1 km/s even if I had all the fuel I need at my disposal?

 

[Sourabh N]

So is it not possible to go all the way up at 1 km/s even if I had all the fuel I need at my disposal?

Ah, it is. I realize after Drakithh's post that my post doesn't make it clear. What I meant was, escape velocity is not a necessity for escaping. If you start with some velocity and maintain it, you should be able to escape Earth.

 

[Masleyko]

What about teleportation out of BH ? but guess fails at same point as particeles can't escape from there .. but in combination of BH evaporation and BH info paradox .. and statement that information has to be preserved .. wouldn't that way work ? :) just curious

 

[Janus]

Ah, it is. I realize after Drakithh's post that my post doesn't make it clear. What I meant was, escape velocity is not a necessity for escaping. If you start with some velocity and maintain it, you should be able to escape Earth.

Of course in doing so, you will eventually reach a distance from the Earth where the escape velocity is less than your velocity.

 

[JaredJames]

What about teleportation out of BH ? but guess fails at same point as particeles can't escape from there .. but in combination of BH evaporation and BH info paradox .. and statement that information has to be preserved .. wouldn't that way work ? :) just curious

Discussing black holes in this regard is rather speculative as it is, to drag teleportation into it is just non-sense. Let's stick to what we know for know.