[Fred]

Fred asked the Naked Scientists:
   
What is a graviton, how does it work and can it escape the pull of a black
hole?

What do you think?

[Soul Surfer]

The simple fact is that it doesn't have to for just the same reason that electrons and protons do not emit photons.  Quantum mechanics says that particles only EXCHANGE photons when they interact. now an atom containing an electron in an excited state can emit a photon as the electron moves from the excited level to a lower level.  That is how photons get emitted.

so coming back to black holes the gravitational field outside the event horizon is due to the mass that is inside the event horizon so when another mass passes by in its orbit (staying outside the event horizon)  it can exchange gravitons as the motion of the object curves it will also emit a tiny amount of gravitational radiation that can be detected at a distance.

Now let us assume that the energy of gravitons is like the energy of photons proportional to Planck's constant times the frequency.  extremely high frequency photons like light x and gamma rays have individually detectable photons but low frequencies like long wavelength radio waves have such tiny energies that they can only be detected in vast numbers now a long wavelength radio wave might have a frequency of around one hundred thousand cycles per second and is produced by making electrons accelerate and decelerate in wires around one kilometre in length.  That is the frequency of waves produced in a classical interaction is around the frequency of the movement.  now think about gravitational interactions close to a black hole with a mass of about ten times the mass of the sun.

A black hole with this mass is about 16 Km across and another body skimming close by in a comet like orbit at nearly the velocity of at 300,000 km per second will travel about 30km in half an orbit take about one ten thousandth of a second to do this.  so the radiated frequency of gravitational waves is about 10KHz which is only about one tenth of the frequency of long radio waves.  Now the interaction I have just described is about the highest single interaction energy that we are likely to see to the universe today (unless there are significant numbers of stable minimal black holes in the universe)  because the smallest sized black hole that can be made comes from the collapse of a high mass star it just takes too much intensity of energy to make a smaller one.  It therefore follows that individual graviton energies made nowadays in the universe are far too tiny to be detected and any individual gravitons detected must have been made in the very earliest phases of the big bang.

[Vern]

I still don't get it. If massive objects exchange gravitons to create an attractive force, and gravitons can't get out of a Black Hole, where do the gravitons that mediate the force of the Black Hole's gravity come from.

I'm sure it is clear to you; but I do not yet get the vision []

[lightarrow]

I still don't get it. If massive objects exchange gravitons to create an attractive force, and gravitons can't get out of a Black Hole, where do the gravitons that mediate the force of the Black Hole's gravity come from.

I'm sure it is clear to you; but I do not yet get the vision []

If you are located in a point of spacetime, then spacetime is warped there, you don't need to wait for particles arriving from another location.

[...lets split up...]

I'm trying really hard to follow you guys as i am interested in the answer, are you (lightarrow) saying that a black hole doesn't need to emit gravitons from itself to have a gravitational effect on an object?

[Vern]

I still don't get it. If massive objects exchange gravitons to create an attractive force, and gravitons can't get out of a Black Hole, where do the gravitons that mediate the force of the Black Hole's gravity come from.

I'm sure it is clear to you; but I do not yet get the vision []

If you are located in a point of spacetime, then spacetime is warped there, you don't need to wait for particles arriving from another location.

Warped space is GR. Graviton is QM. You can't mix and match can you ?

The way out of this dilemma is to leave the gravity phenomena to General Relativity as lightarrow has done. But then what use the graviton. It need not even exist IMHO.

[lightarrow]

I'm trying really hard to follow you guys as i am interested in the answer, are you (lightarrow) saying that a black hole doesn't need to emit gravitons from itself to have a gravitational effect on an object?

Exactly.

[lightarrow]

I still don't get it. If massive objects exchange gravitons to create an attractive force, and gravitons can't get out of a Black Hole, where do the gravitons that mediate the force of the Black Hole's gravity come from.

I'm sure it is clear to you; but I do not yet get the vision []

If you are located in a point of spacetime, then spacetime is warped there, you don't need to wait for particles arriving from another location.

Warped space is GR. Graviton is QM. You can't mix and match can you ?

The way out of this dilemma is to leave the gravity phenomena to General Relativity as lightarrow has done. But then what use the graviton. It need not even exist IMHO.

Infact. I tried to say it in a thread sometimes ago (this is one of those questions which comes out every Moon cycle  []) but then I loose will to always write the same thing...
Too many people like to think to gravitons as if they really were well established existing particles. Let them think it.

[Vern]

Too many people like to think to gravitons as if they really were well established existing particles. Let them think it.

Are we saying then that there is no QM solution for a Black Hole? [] It would please me if that is true, since I have a pet hypothesis for a universe which also does not have a solution for a BH.[]

[Soul Surfer]

It is the interaction of the two gravitational field that involves and produces the gravitons in the same way that it is the interaction  between the elctromagnetic fields of a proton and an electron that produces the photon when an electron has its course changed by interaction with a proton.  they do not need even to touch to do this.

Look up compron Scattering in Wikipedia this shoud help

[lightarrow]

Too many people like to think to gravitons as if they really were well established existing particles. Let them think it.

Are we saying then that there is no QM solution for a Black Hole? [] It would please me if that is true, since I have a pet hypothesis for a universe which also does not have a solution for a BH.[]

No, I'm saying that a quantum theory of gravity doesn't exist yet, even if a lot of intuitions makes particle physicists think that gravitons should exist.

[Vern]

they do not need even to touch to do this.

Yes; I know about Compton scattering.

It seems that I remember that one of the main attractions of QM theory was that it eliminated that spooky action-at-a-distance thing that Newton rejected straight out.

Are we now saying that its okay?  I know about virtual photons and Feynman diagrams.

[Vern]

No, I'm saying that a quantum theory of gravity doesn't exist yet, even if a lot of intuitions makes particle physicists think that gravitons should exist.

Shucks [] I thought for a minute there that I might have found a co-conspirator.

[yor_on]

A graviton is the answer to the photon in Quantum field theory.
It shares a lot of the properties of a photon.

The graviton must be massless as gravity is limitless, and it is charge less, it follows 'c', and must have a spin of 2.
That idea of spin seems to come both from Quantum field theory and String theory?

It seems that it is string theory that defines the idea of spin (2)
http://motls.blogspot.com/2007/05/why-are-there-gravitons-in-string.html

--

A photon has a spin of 1
http://en.wikipedia.org/wiki/Photon#Recent_research
--

"And the idea is also sprung out of the success with relating other (Standard Model) forces of nature with similar particles: electromagnetism with the photon, the strong interaction with the gluons, and the weak interaction with the W and Z bosons."

That said I'm not entirely sure that we need it?
We do need something explaining the propagation at 'c'.
But it is with that as with 'charge'.

It may as much be what is 'missing' as what is there that decides what we call the propagation.
And that as I see it, would then be more of a geometric function of spacetime than actual 'free flowing' 'particles' creating the propagation.

'pressure Waves' in a jello sort of.

----------

Then again, as string theory seems to start from 'one dimensional strings' that will be very near to how I see spacetime.
On the other tentacle I have such difficulties wrapping my mind around that concept?

I won't bore you with that, just say that I find it easier to see space and matter as a symmetry, not needing or even capable, to define what 'it' should be defined to its smallest constituents. It's easier to understand to me that way, and after all, it's my limitations that creates my world:)

That said any 'definition' works as long as it answers to all phenomena we can observe.

But to me it is like defining what 'energy' might be.
(It's a white Bunny with violet eyes curling up on your knees, and yes, your knees is everywhere)

------------

Compton scattering is what happens when photons lose energy due to its interactions with 'matter'.
"Inverse Compton scattering also exists, where the photon gains energy (decreasing in wavelength) upon interaction with matter.
The amount the wavelength changes by is called the Compton shift."

What that have to do with gravitons?
I'm not sure, it would be nice to see an explanation of that one Soul Surfer?
Is it a relationship you see?

I found this paper though at http://arxiv.org/pdf/gr-qc/0601043
In it, it states that "Gravitons cannot be detected by Compton scattering of gravitons of neutral particles"?

---
When I write I miss words, sorry about that.
The more I want to say something the less understandable it seems to be?

Perhaps those who spells worst are the ones one should give some time?
But I do?

Awh: English.

(now, that's better:)

[Vern]

That said I'm not entirely sure that we need it?
We do need something explaining the propagation at 'c'.
But it is with that as with 'charge'.

Exactly; there is no reason to associate the speed of the graviton with that of the photon since we know the photon's speed comes from its constituent fields as Maxwell described them.

But if we can make gravity out of photon fields then .... []

Then again, as string theory seems to start from 'one dimensional strings' that will be very near to how I see spacetime.
On the other tentacle I have such difficulties wrapping my mind around that concept?

I've been following string theory since it was first postulated years ago. I was really hoping it would go away, but it is very persistent. I don't think it represents reality. Too many dimensions.

[yor_on]

Yep, string theory is touching the 'envelope'.
I know that if I could understand the math behind it I would see its symmetry.

But when looking at what i can see I have problems accepting that one dimensional strings define it?
That doesn't mean that it is wrong, just that my mind fails when comprehending it.

[Vern]

I can't fit string theory into any visualization of a universe that I can imagine. That doesn't detract from the theory; it just means that I won't be a contributor to its evolution.

[lightarrow]

A graviton is the answer to the photon in Quantum field theory.
It shares a lot of the properties of a photon.

The graviton must be massless as gravity is limitless, and it is charge less, it follows 'c', and must have a spin of 2.
That idea of spin seems to come both from Quantum field theory and String theory?

A quantum theory of gravity doesn't exist yet, so the graviton and its properties are just speculations, as well as string theory's predictions.

[...lets split up...]

Ah, i see said the blind man to his deaf daughter sitting at the edge of the round table, a black hole cannot exert gravity on particles not proven to exist  []

[lightarrow]

Ah, i see said the blind man to his deaf daughter sitting at the edge of the round table, a black hole cannot exert gravity on particles not proven to exist  []

If you had read just a few posts before, you would have found it's written that there is no need of gravitons for the grav. interaction to exist.