[Bob P]

Bob P  asked the Naked Scientists:

Hi,

This is a Q. that one of my other podcasts (an astronomy one ) has raised, but not answered.

How does gravity work with respect to black holes? How do gravitons (if they exist) "get out" to interact with other bodies to make gravity work?

Cheers,

Bob

(aka. Redlace Selene, 2nd life)

What do you think?

[Soul Surfer]

You are thinking about gravitons in quite the wrong way.  Many people tend to think that a body with a gravitational field emits gravitons all the time.  This is untrue.  That's just like saying that a body with an electrical charge emits photons all the time.  It is probably a bit easier to see that this is not true.  A body with an electrical field only exchanges photons with another charged body when they interact and the presence of the charges causes a change in the motion of both of the bodies.  it is exactly the same with gravitons  they are only exchanged when two gravitating bodies interact.  under those circumstances it is possible for a small amount of the interaction energy to be lost by radiation in the same way that  interacting charged bodies can emit radiation. A charged or gravitating body does nothing when it is inactive.

[LeeE]

Heh heh - SoulSurfer and I tend to disagree about a few things, from time to time, and this might be another example.

What SoulSurfer says is absolutely correct...   if QM is correct.  However, QM hasn't been completely/entirely proved just yet - it's only about 99% proved - hence the need for the LHC, which will raise the degree of proof to around 99.9%

If the relativistic view of the world is a sixteen piece infant's jigsaw puzzle, made out of chunky plywood pieces and showing a cartoon face, the QM jigsaw is 2048 pieces and forms a photorealistic image of the entire landscape. The only problem though, is that the QM jigsaw has been put together upside down, so you can't actually see the image, and some of the pieces only fit if they're sticking out side-ways.  Oh yeah - and although you've got the right number of pieces, there isn't the right number of spaces to place them...   or is it the other way around?

[DoctorBeaver]

Whereas in string theory you've got 5 jigsaws all mixed up together in same box  []

[lightarrow]

Bob P: we still don't know if gravitons exists because we still don't have a quantistic theory of gravitation. Anyway, even if they existed and were emitted continuously, there is no need for them to come from the inside of the black hole, they can come from the very place where you are: a black hole determines a spacetime curvature here and so you could measure the gravitons coming from the curvature here.

About gravitons, I haven't understood how objects theorized from the quantization of gravitational field, could match with the fact that, in GR...the gravitational field doesn't exist at all!   [] 

[DoctorBeaver]

About gravitons, I haven't understood how objects theorized from the quantization of gravitational field, could match with the fact that, in GR...the gravitational field doesn't exist at all!   [] 

Alberto - although the gravitational field is not explicit in GR, it is not precluded. GR says that matter warps spacetime and spacetime tells matter how to move. It does not say how matter warps spacetime. That is where gravitons and the gravitational field could come into it.

[lightarrow]

Alberto - although the gravitational field is not explicit in GR, it is not precluded. GR says that matter warps spacetime and spacetime tells matter how to move. It does not say how matter warps spacetime. That is where gravitons and the gravitational field could come into it.

You mean that gravitons are the means through which matter warps spacetime?

[DoctorBeaver]

That is indeed what I mean. Can you think of a reason it could not be correct?

[lightarrow]

That is indeed what I mean. Can you think of a reason it could not be correct?

The fact is that I don't know anything about gravitons and how they operate inside the theories that contemplate them.

[DoctorBeaver]

That is indeed what I mean. Can you think of a reason it could not be correct?

The fact is that I don't know anything about gravitons and how they operate inside the theories that contemplate them.

Ditto

[yor_on]

The graviton is a boson, with zero mass, a spin of 2, that travels at the speed of light.
In a way similar to the photon, but with gravity acting as the 'force' acting between body's instead of electromagnetism.

They are thought to be unconstrained by any branes in M theory (strings) and are consisting of closed strings which allows them to move freely between branes, as compared to 'open' strings where at least one of their endpoints will be attached to a brane restricting its movement.

As for how they can be freed from a Black Hole.
You have virtual particles helping us out there:)

" The black hole event horizon is where normal matter (and forces) must exceed the speed of light in order to escape, and thus are trapped. The horizon is meaningless to a virtual particle with enough speed. In particular, a charged black hole is a source of virtual photons that can then do their usual virtual business with the rest of the universe. Once again, we don't know for sure that quantum gravity will have a description in terms of gravitons, but if it does, the same loophole will apply---gravitational attraction will be mediated by virtual gravitons, which are free to ignore a black hole event horizon."

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980601a.html

[DoctorBeaver]

yor_on - Thank you. I am familiar with what you said in the 1st 2 paragraphs.

I am intrigued, though, as to why the event horizon is meaningless to a virtual particle. Or do you just mean a massless virtual particle?

[lightarrow]

yor_on - Thank you. I am familiar with what you said in the 1st 2 paragraphs.

I am intrigued, though, as to why the event horizon is meaningless to a virtual particle. Or do you just mean a massless virtual particle?

Maybe it refers to the fact that a virtual particle is "out of shell", that is E² = (cp)² + (mc²)² doesn't hold for them.

[yor_on]

lightarrow:)
would you like to explain that one further.
It sounds intriguing and it's new to me.

As i understand it it has to do with virtual particles only existing for such a short moment that they, even though interacting with our spacetime, won't be touched by Heisenberg's uncertainty principe. That allows them all kind of strange things, amongst them traveling faster than light according to Nasa. And doing that they can escape the event horizon. Also there are another idea considering 'Pair production' that might be used to under certain circumstances creating 'real particles' freed from the black hole by using virtual.

"Virtual particles are often popularly described as coming in pairs, a particle and antiparticle, which can be of any kind. These pairs exist for an extremely short time, and mutually annihilate in short order. In some cases, however, it is possible to boost the pair apart using external energy so that they avoid annihilation and become real particles.

This may occur in one of two ways. In an accelerating frame of reference, the virtual particles may appear to be real to the accelerating observer; this is known as the Unruh effect. In short, the vacuum of a stationary frame appears, to the accelerated observer, to be a warm gas of real particles in thermodynamic equilibrium. The Unruh effect is a toy model for understanding Hawking radiation, the process by which black holes evaporate."

http://en.wikipedia.org/wiki/Vacuum_fluctuation

[LeeE]

Whereas in string theory you've got 5 jigsaws all mixed up together in same box  []

And when you've completed them, they turn out to be a woolly jumper.

[lightarrow]

lightarrow:)
would you like to explain that one further.
It sounds intriguing and it's new to me.

As i understand it it has to do with virtual particles only existing for such a short moment that they, even though interacting with our spacetime, won't be touched by Heisenberg's uncertainty principe. That allows them all kind of strange things, amongst them traveling faster than light according to Nasa.

Exactly, the equation I wrote is not satisfied for this possible reason.

[yor_on]

much appreciated LA