what is the speed of gravity propagation?

lysdexia
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which -> whose propogates ->

which -> whose
propogates -> propagates
potention -> potential
neuby -> newby

How would it follow from flat space that there are no waves?

If you are going to lump time with space, then I'll one-up you and claim that gravity must live in 5D "space", not only with time and space but with mass.

"My other computer is a virus farm."

thinkingbear
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> Neither X-rays, nor

Message 4956 in response to message 4954

> Neither X-rays, nor anything else, can 'escape' a BH.

What about emission of Hawking Radiation? Or does that originate from our side of the event horizon as well? I am going to guess yes, but then how would it lead to the evaporation of a non-accreting black hole as the theory suggests?

Oliver
Joined: 24 Feb 05
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> > Neither X-rays, nor

Message 4957 in response to message 4956

> > Neither X-rays, nor anything else, can 'escape' a BH.
>
> What about emission of Hawking Radiation? Or does that originate from our
> side of the event horizon as well? I am going to guess yes, but then how
> would it lead to the evaporation of a non-accreting black hole as the theory
> suggests?
>
Let me try this one in English, but I can´t promise to do it well.

Hawking radiation originates exactly from the event horizon. In vacuum, virtual particles come into existence in pairs all the time: matter and antimatter. This is positive and negative energy. Normally this particles come into existence for a very short time, and annihilate each other in nearly no time. But exactly on the event horizon one of the partners may manage to escape, while the partner particle crosses the event horizon and will be absorbed by the black hole.
In this case, it looks like the particle which escaped comes right from the black hole. But it didn´t, it came from the event horizon. This is the Hawking radiation.
The particle which is absorbed by the black hole, consists of negative energy. If you put negative to positiv energy, it will annihilate what means that the black hole loses positive energy. This causes the evaporation of the black hole bit by bit. It takes a long time, though, until a black hole is gone. Very long. Depends on the size of the hole, but just to give you an idea: 2x10^67 years is the lifetime of a hole which equals the mass of the sun.

If you are interested in the maths:

Es gr

Mchl
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> neuby -> newby newby ->

Message 4958 in response to message 4955

> neuby -> newby

newby -> newbie

(Sorry everybody, but I just couldn't miss that one. Especially after googling for 'newby')

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Mr Gravity
Joined: 9 Feb 05
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Gravity waves are the

Gravity waves are the equivalent to sound waves in air. If a balloon pops in the far distance we can hear it due to the motion of the air rippling away from the point of burst. But only to a certain distance. Gravity waves work on the same principle. If a star goes supernova the dark energy within the surrounding space will carry the gravity ripply outwards. But to what distance? That is the question....

gravywavy
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> So the question is not in

Message 4960 in response to message 4952

> So the question is not in how the gravity projection into 3-d space escapes
> the black hole's event horizon but rather how the information about changes in
> black hole gravitational field manages to update itself beyond black hole -
> how does information about interior of event horizon leacks to outside world.
>

it doesn't and it can't.

Just as changes in an electric field cannot propagate faster than light, neither can changes in a gravitational field (*). This means that changes in a gravitational field cannot cross the event horizon.

Just before this black hole formed there was gravity from the stuff falling in.

Once it has fallen in, that gravitational field cannot change, as there is no way to get the information about any changes out of the black hole.

It turns out that a spherical shell of matter, just bigger than the event horizon has exactly the same gravitational field as the black hole will have once it is formed by that spherical shell contracting down through the event horizon, so there is no sudden change when that field gets 'stuck' at the limiting value.

It is a feature of how self-consisten GR is, that after crossing the event horizon the still collapsing matter always collapses so that the external field would not have to change anyway.

~~gravywavy

(*) strictly speaking there is no gravitational field in General Relativity, that term comes from Newton's approach to gravity. The argument is the same, but pedants may apply the following:

gravitational field -> matter induced distortion of spacetime

with thanks to lysdexia for the handy notation

~~gravywavy

debugas
Joined: 11 Nov 04
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> Just as changes in an

Message 4961 in response to message 4960

> Just as changes in an electric field cannot propagate faster than light,
> neither can changes in a gravitational field (*).

But light gets affected by gravity - it slows down near gravity sources
Why should we expect that gravitons slow down in gravitational field ?
Why gravitons can't escape black hole, maybe they easily do ?

> Just before this black hole formed there was gravity from the stuff falling
> in.
> Once it has fallen in, that gravitational field cannot change, as there is
> no way to get the information about any changes out of the black hole.

It sounds innatural to me. Why matter inside black hole cannot affect its gravitational field ?

> (*) strictly speaking there is no gravitational field in General Relativity,
> gravitational field -> matter induced distortion of spacetime
Would not it be more natural to look at gravity as more and less dence regions of gravity in a flat space-time ? Mathematical model of curved space-time can be looked as density variations on a flat space time

gravywavy
Joined: 22 Jan 05
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> But light gets affected by

Message 4962 in response to message 4961

> But light gets affected by gravity - it slows down near gravity sources

Not exactly. If you have an observer at the place where the light is, that observer still sees it traveling at the same c as any other observer.

From a prudent vantage point a long way away, we see that intrepid observer slowed down and we might at first think that the light has slowed down too, but when we do the appropriate corrections we find it hasn't slowed down for the intrepid observer.

> Why should we expect that gravitons slow down in gravitational field ?

Because in General Relativity everything has to behave the same way in a gravitational field (this comes from the so called principle of equivalence which was one of the inspirations for GR. Change that and you no longer have GR.

One of the exciting things about gravity waves, once we have actually seen them passing the earth, is that some time down the line we might be able to measure their speed directly. Later still, we might repeat the Michelson-Morley experiment somehow but with gravity waves, though it is hard at present to see how. What a surprise if it turns out you are right, and gravity waves are not affected in the way light waves are! It would mean throwing out the whole of GR and finding a new place to build on.

> Why gravitons can't escape black hole, maybe they easily do ?

because light can't, and gravity has to have the same effect on everything

> It sounds innatural to me.

Every part of relativity sounds unnatural. We can't easily get our minds rtound the fact that time runs at different speeds for different observers, because there is nothing in our every day lives where the effect is big enough to notice. It all contradicts common sense, it is all counter-inuitive. If it sounds natural you haven't grasped what is going on!

>Why matter inside black hole cannot affect its
> gravitational field ?

There are two answers. The first answer is that it could not get the information out in order to tell the outside field how to change.

The second is that despite the fact that you have some control over how fast you approach the singularity, once you pass the event horizon you can only exert that control by throwing stuff around (like how a rocket pushes forwards by throwing stuff out the back). When you do all the sums, the total movement of all the matter within your reach always and inevitably adds up to the fact that there would not be any change in the outside field anyway. (That is what I meant by the rather cryptic comment that GR is self-consistent).

> Would not it be more natural to look at gravity as more and less dence regions
> of gravity in a flat space-time ? Mathematical model of curved space-time can
> be looked as density variations on a flat space time
>

That is building a theory on a completely different foundation to the one Einstein chose. Doesn't mean it is wrong, but it does mean you are starting right at the beginning with an empty piece of paper and the need to learn a load of new maths.

If you came up with a theory that worked, starting from your foundation, there would be features of that theory that also felt unnatural. This is becasue any theory that deals with fast moving objects has to explain away the facts that different people (moving at different speeds relative to each other) still see the speed of light to be constant and have different observations about how fast clocks are running.

Those observations are bizarre, unnatural, yet can be demonstrated with reasonably simple apparatus. But if you can predict those effects, but make a different prediction about how gravitons move, we'd have what Physicists really love, two theories that fit current data and a way to test the differences. If you can build your theory it makes looking for gravity waves absolutely vital!

~~gravywavy