If I understood well, then nothing can escape from a black hole because the escape velocity is greater then the speed of light and it is not possible to travel faster then with that speed. An exception is Hawking radiation which can escape due to quantum mechanics; I compare this with the "tunneling" effect, in semi-conductors, but maybe this is not quit a valid comparison.
My question: what is it that gives me information about this black hole as nothing can escape from it? If for example the black hole changes because it sucks up more matter then I will get this information because I will accelerate faster towards it. Is it the Hawking radiation that gives me this information or is there some other explanation?

Thomas

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Hum,
Your local space-time gives you `information`.
(As in gravity can `escape` a black hole - though in reality, it stretches out from the black hole)

Thanks for your answer.

With what speed does gravity escape? Is this an instantaneous process or is there a time delay?

Is there energy exchange involved because there is some form of information exchange between me and the black hole?

And a last question, if there is a time delay and energy transfer, then is that what causes gravity waves?

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First of all, gravity doesn't "escape" from a black hole, The gravitational field simply exists.
Likewise, since a black hole can have an electric charge, it also has an electromagnetic field that simply exists.
Although we cannot "see" a black hole, because photons cannot excape, we can detect one because of the effects it has on its surroundings.

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Gravity can't escape anything to my understanding. The point of gravity is that you're being pulled toward an object, not pushed away from it. So in a sense, by stating that there is gravity emanating from a black hole you're only solidifying the point that nothing escapes it.

And actually Kaptain K, I thought that there was one kind of radiation which was capable of escaping a black hole, on the lower end of the spectrum if I remember correctly. I may be completely wrong but it's something I heard once. So I guess if that's genuine, that you could "detect" the existence of a black hole by identifying that as the one and only kind of radiation coming from a single source.

Also Thomas, gravity itself does not have a "speed" because it is a force, not an object or kind of matter.

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The idea behind hawking radiation(which causes blackholes to evaporate), is that two "vitural" particles can be created (in the quantum froth) one as matter and one as anti-matter they borrow energy from the universe and anailiate each other almost instaintly. The idea with a black hole is that if one of these falls into a black hole the other particle becomes "real" the mass/energy is subtracted from the universe via the blackhole and a particle/energy appears to be emmited.
Why do we need this radiation? I don't know, I recently heard that hawking himself said that the theory's flawed- did anyone else here about this...? It ties quantium mechanics with realtivity so it's kinda nice....

We can detect black holes if they are in a binary via X-ray's their accreation disks give off....

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I guess that by 'lower end of the spectrum' you mean radio waves. They´re nothing but photons, and equally unable to leave the black hole.

The electromagnetic force propagates at the speed of light. It is thought that gravity waves also propagate at that speed.

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I'm no expert, but from my understanding, for the most part, a black hole is detected from the radiation of the stuff swirling around it at tremendous speeds - stuff that hasn't yet crossed the event horizon (after which nothing escapes). The accretion disk can also have a magnetic field, and the outer part of the field cannot keep up with the ultra-fast inner part, so the field "slips" around the BH, becoming a hugely powerful dynamo. This causes jets or beams of particles shooting out of the magnetic poles at close to the speed of light. Beyond the apparently massive gravity, there are lots of "emanations" from near the BH that allow detection and therefore inference of the BH.

I think the amount of Hawking radiation of comparatively insignificant. I don't think the tunneling comparison is valid because Hawking radiation comes from just outside the event horizon. The tunneling idea is interesting in itself, though, and I know of no reason the occasional particle couldn't tunnel out of a BH on its own based solely on Heisenberg uncertainty.

But as I said, I'm no BH expert.

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That´s an interesting idea, indeed, and I (surprisingly) found out that it´s been proposed. Come to think of it, tunneling would be kind of a natural extension to the Hawking radiation reasoning.

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Hum,
Yeah, it is a natural extension to the Hawking radiation.
In that idea called quantum `entanglement` is needed to get rid of the theory's flaws, (an inconsistency - `information` is trapped inside the BH and lost to the universe)...
A simple idea to solve that, but the mathematics is daunting.

>>The electromagnetic force propagates at the speed of light. It is thought that gravity waves also propagate at that speed.

Yeah,
A good visualisation is to regard gravity like a carpet on the floor.
The carpet stretches from one wall to another wall, instantaneously.
But, bumps/waves in the carpet, or things on the carpet, can only move below a speed limit , (c).

There are (at least) two ways of looking at this. One way is to remember that, from the perspective of someone far from the black hole, nothing ever crosses the event horizon. The gravitational effects of that mass are still propagating outward from just outside the event horizon, and will continue to do so. In some sense, it's as though the gravitational field is a frozen remnant of what once was there.

An alternate view is to remember that in quantum theory, forces are mediated by the exchange of virtual particles. So the gravitational interactions with the black hole are mediated by virtual gravitons, and electromagnetic interactions with a charged black hole are mediated by virtual photons. Virtual particles do not have all of the limitations that real particles have. The weirdest one is that they can have negative energy, but they also need not travel below the speed of light. Virtual particles can escape an event horizon to mediate forces.

I dont even think gravity can "move."
That's like asking where the color moved to. Color isnt an object that can be moved its a description. Gravity is a warp in space-time so it would be the space-time that's moving, not gravity.

Thanks again for all the interesting replies.

Gravity may not "move", but it changes. An EM field changes too and with that it can transport energy/information. from one place to an other.
Electrons "form" EM fields, EM fields move electons. Mass "forms" space-time, space-time moves mass. I do know about EM waves but little about gravity waves. Maybe my question is, to what extend can we compare these two, because they come from the same kind of differential equations.

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The fallowing is just a statement of General Relativity and how it goes with the discussion.

From what I have learned, Gravity goes at right near the speed of light, true that gravity does not extend, but fallows the curvature of space time. But if what is creating the gravity disappears, the gravity would cause a wave of space time to bounce,(like drooping a rock in a lake) this wave emits in all directions at exactly the speed of light and anything that would have been effected by the gravity simply lets go. So for example Earth and the Sun, if the sun instantly vanished, the gravity (or curvature) that holds Earth in orbit would disappear in 8.3Min (or the time it takes light to reach earth) sending Earth out of orbit and we would just go floating around space! Really neat if you ask me So gravity does really have a speed, just not in the way we are used to it.

(Let me know if I have any of this wrong)

Edit: Elyk is right, space time is what is moving, not gravity, I forgot to include this, sorry.

They actually did an experiment using Jupiter to measure the "speed of gravity" - results were good (around c), but came under fire - report here.

Not So Much Floating, More Of a ZOOM!!!

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Well, I was saying in relation to other objects, but yes, it is certainly a ZOOM!!

Question, if we were to loose the gravity from the Sun, and were "ZOOMING" out into space at (I think it is like 26,000mph, but I could be way off) Would the earths warping of space time slow it down? I mean, does the fabric of space time create drag on large bodies in space?

Probably Not ...

But, The Moon Might ...

Riight Now, It's Actually, More Gravitationally Bound, To The SUN!

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Well, I guess the question is does an object actually touch the "fabric", or does it more like hover above it, if it came into contact I bet it would create a kind of drag, but if it just simply floats above it, I guess it would have no effect, but it seems like an extraordinary question to me, and I would love for someone who would know to answer, if anyone knows that is...

Well there is a thing called frame dragging....
where a roating object pulls and twists space-time.
It was measured around earth early this year...

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Thanks mickal, thats very interesting..glad to know I thought of something that was only recently discovered instead of it being known for a long time.

Actually, it has been known since 1918 and is a prediction of GR. it's also called the Lense-Thirring effect or Gravitomagnetism. That it was discovered only recently around Earth, is due to the extremely small amount of drag around the earth. Gravity Probe B was desinged to detect this effect, but it was also found by the LAGEOS AND LAGEOS2 satellites late last year.

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Since we are [and were "ZOOMING" out into space at (I think it is like 26,000mph, but I could be way off) Would the earths warping of space time]

Cause a "Wake" behind our direction of motion, in Space Time?

Tensor;

Dragon Star wrote;
[So for example Earth and the Sun, if the sun instantly vanished, the gravity (or curvature) that holds Earth in orbit would disappear in 8.3Min (or the time it takes light to reach earth) sending Earth out of orbit and we would just go floating around space!] Zooming!

I have seen this "if the sun instantly vanished" thing before.

I think that the Earth would "Instantly" start moving in the straight line it is headed in.

Not because Gravity (or the information) would travel faster than light, but just because, if the mass of the sun was instantly gone, the masses attraction would be instantly gone.

Is this correct???


Thanks
RussT

Nope, Dragon star is correct. It would take the 8.3 minutes for the effect to reach the earth, with the effect spreading out from the sun at c. Under Newtonian gravity, it would act instantly. The incompatibility between Newtonian gravity (where the effect was figured to travel at an infinite speed) and SR (where c is the limiting speed), was one of the main reasons Einstein developed GR, so that gravitational theory and SR would be compatible.

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Tensor
Senior Member

Thanks!

So if a dense object whizzed by 186,000 miles at the point of closest approach at half the speed of light, then I'd experience max acceleration towards that object at the point of closest approach, and not a second after point of closest approach?

Then my force vector would not point directly towards the object, where it appears to be to me. I would see it at point of closest approach, but the vector would appear to be leading it by 45 degrees at this point.

Weird!

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If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Weirder ...

Due to Relativistic Effects, The Angle Would Actually Be, a Little More That 8.5% Steeper, At 48.86 Degrees, instead!!!!

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don't we expect black holes to be electrically neutral?

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