Can gravitons bend space-time?

[glueball8]

Can gravitons bend space-time? How can it be shown experimentally? Ideas?

 

[dmatador]

What does your intuition tell you concerning this question?
Space-time is bent by mass, and gravity (and thus, i suppose, the hypothetical gravitons) exists through this sloping of space-time. Gravitons are said to have no mass (if they exist), so intuitively, how do you suppose they could affect space-time? Secondly, no one is sure if gravitons exist, so maybe the question should initially be "how can we find out if gravitons exist?" and then ask what do they do and how do they behave.

 

[Spinnor]

What does your intuition tell you concerning this question?
Space-time is bent by mass, and gravity (and thus, i suppose, the hypothetical gravitons) exists through this sloping of space-time. Gravitons are said to have no mass (if they exist), so intuitively, how do you suppose they could affect space-time? Secondly, no one is sure if gravitons exist, so maybe the question should initially be "how can we find out if gravitons exist?" and then ask what do they do and how do they behave.

I thought the curvature of spacetime was curved by both mass and energy (along with other stuff)? As gravitons have energy doesn't it follow they would scatter off each other? The crossection must be pretty small? Would the simplest Feynman diagram of the scattering of two real gravitons include two, not one, virtual gravitons?

In the attempts to formulate a quantum theory of gravity is there a "quantum" of spacetime change that moves at the speed of light?

Thanks for any clarification!

 

[glueball8]

What does your intuition tell you concerning this question?
Space-time is bent by mass, and gravity (and thus, i suppose, the hypothetical gravitons) exists through this sloping of space-time. Gravitons are said to have no mass (if they exist), so intuitively, how do you suppose they could affect space-time? Secondly, no one is sure if gravitons exist, so maybe the question should initially be "how can we find out if gravitons exist?" and then ask what do they do and how do they behave.

Of course I mean assuming gravitons exist. Ya, doesn't energy also bend space-time?

Ya I know it's far sketched, but I'm still curious on the implications. I don't know, but somehow I have a feeling that self interaction will relate to some fundamental symmetry.

What type of mathematics is needed to analyze gravitons if it can bend space-time? (Is there even any?)

 

[jtbell]

As I understand it, a quantum theory of gravity that includes gravitons is supposed to supersede general relativity and its "classical" picture of space-time curvature, analogously to the way quantum electrodynamics and its photons supersede classical electrodynamics and its electric and magnetic fields.

In this view, a graviton would be a quantum of spacetime curvature, similarly to the way a photon is a quantum of the electromagnetic field. To put it another way, gravitons would not "cause" spacetime curvature, but rather, they would "be" spacetime curvature in some sense.

 

[Haelfix]

Gravitons represent a perturbation of spacetime, the quantum of the gravitational waves from GR.

Its easier to think of what a gravitational wave is. They represent a sort of propagating change of the geometry, a jiggling of the matter/energy if you will: http://en.wikipedia.org/wiki/Gravitational_wave

In much the same way, a graviton is basically just that, but at the quantum level. The subtlety is that they are basically a single term in an infinite expansion that represents this effect. The full classical gravitational wave is then a superposition of many of these gravitons which are all in what's called a coherent state.

 

[Naty1]

Can gravitons bend space-time?
theoretically yes since they are packets of energy.
anything with mass or energy exhibits gravitational effects. Black holes exhibit residual gravitational effects and affect spacetime from certain frames.

How can it be shown experimentally? Ideas?

We can't even detect gravitions yet.

I'm not positive we have absolutely unambiguous "proof" that any massless "particle" exists, including the photon...
this gets back to quantum theory and the likelyhood there is no clear distinction between wave and particle.

 

[cragar]

A graviton must posses a self-energy . And doesn't a magnetic field create a gravitational field .

 

[Chronos]

LIGO is still the gold standard experimentally. 2010 may be the breakthough year for this project.

 

[glueball8]

A graviton must posses a self-energy . And doesn't a magnetic field create a gravitational field .

But a EM field doesn't create a EM field.

Also strong force has the property of self creation.

 

[cragar]

so then I am guessing that a gravitational field cannot create another gravitational field ?

 

[elven]

so then I am guessing that a gravitational field cannot create another gravitational field ?

In what sense? Assuming that gravitons exist, a gravitational field would be an area in which the effects of a graviton would be exerted, therefore, to 'create' another gravitational field, one of two things might happen:

• The gravitons replicate (?)
• Some gravitons 'leave' the initial field (is it impossible? gravitons are particles, therefore they are also affected by gravity)

 

[cragar]

interesting but , can a gravitational field keep creating another field and that field creates another field and so on , is that possible.

 

[Naty1]

Gravity IS a self interacting phenomena..that's why it took Einstein so long to develop general relativity...via Tensors, Riemann manifolds,etc. The interaction is described by the Einstein tensor.

In rereading my post #7, I don't like it much now...because as in other posts here we are mixing two theories...GR says gravity is a warping of space, a geometrical description ...gravitons result from quantum mechanical (particle) considerations rather than geometrical ones...so describing either particle interactions or geometric explanations depends on which theory you are using...GR and QM don't mesh/mix all that well so far...that would be the result of grand unification combining all forces.

I posted once before, but have forgotten the exact example, of the magntitude of graviton exchange...as derived from QM...But the number of gravitons exchanged in QM theory is really,really tiny...like,for example, a pair of electrons might exchange a graviton once a year...don't quote me on that example but you get the idea...really really tiny...

 

[Naty1]

Here is a discussion of possible interest regarding whether gravitons are required in quantizing GR...

https://www.physicsforums.com/showthread.php?t=368390&highlight=graviton+exchange

 

[haael]

n rereading my post #7, I don't like it much now...because as in other posts here we are mixing two theories...GR says gravity is a warping of space, a geometrical description ...gravitons result from quantum mechanical (particle) considerations rather than geometrical ones...so describing either particle interactions or geometric explanations depends on which theory you are using...GR and QM don't mesh/mix all that well so far...that would be the result of grand unification combining all forces.

However, there is a perfect analogy between space warping and exchanging virtual particles. These are two mathematical ways to describe the same thing. So GR and QFT are not as far away of each other as it seems.

 

[tom.stoer]

We had this discussion here for a coupleof times; I used to ask "what is a graviton?"

 

[Naty1]

However, there is a perfect analogy between space warping and exchanging virtual particles. These are two mathematical ways to describe the same thing. So GR and QFT are not as far away of each other as it seems.

The conclusion does not follow from the statement.

 

[tom.stoer]

However, there is a perfect analogy between space warping and exchanging virtual particles. These are two mathematical ways to describe the same thing. So GR and QFT are not as far away of each other as it seems.

I have never seen a theory which describes exchange of virtual particles bending spacetime via a back-reaction. So I don't see this analogy. Where shall it come from?

 

[haael]

I don't see this analogy. Where shall it come from?

Both theories describe interacting bodies.

First we have flat spacetime or free particles. We can do various mathematical operations, including differentiation.

Then we turn on interactions. Our differential operator changes. In GR we have:
$$D_{\mu} \phi_{\nu} = d_{\mu} \phi_{\nu} - \Gamma^{\alpha}_{\mu \nu} \phi_{\alpha}$$
while in QFT:
$$D_{\mu} \phi(q) = d_{\mu} \phi(q) - i A_{\mu} q \phi(q)$$
.
These formulas are pretty similar. Traditionaly we say that the first describes spacetime warping and the second that there are virtual quanta of $$A_{\mu}$$ field, but it is essentialy the same thing.
The difference is that GR warps spacetime itself and QFT bends some abstract space of internal symmetry. Alternative description: in QFT we have quanta of the gauge field while in GR bodies interact with exchanging virtual quanta of spacetime deformations - the gravitons.

So GR and QFT are not quite that different. The last thing is to find interpretations of gauge fields in GR and spacetime bending in QFT. Kaluza-Klein-like theories are most promising IMO.

 

[tom.stoer]

The problem is that the concept of virtual particles is limited to perturbative regimes and is known to fail in many cases where non-perturbative methods are required; this is true even in standard QFT. There is chance to see color confinement based on virtual gluons. In GR you can calculate tree-level and one-loop level corrections, but you know that you construct this theory on flat (or at least non-dynamic) spacetime. And you will never get fully dynamics of spacetime based on static spacetime plus virtual particles.

I agree that GR and QFT are mathematically rather similar (one can even describe GR as a gauge theory; in that case it shares many similarities like local gauge symmetry in tangent space, Gauss law constraint, etc.), but that does not mean that the concept of virtual particles really makes sense.

What I am wondering is that so many people know about the similarities between QFT and GR and at the same time try to use concepts which are known to fail even in many areas of QFT

 

[haael]

What I am wondering is that so many people know about the similarities between QFT and GR and at the same time try to use concepts which are known to fail even in many areas of QFT

I don't say that QFT-like approach to GR will lead us to Truth. I just say, that both GR and QFT describe... something and that "something" is similar in these theories both physically and mathematically. We can call it spacetime warping from GR standpoint or virtual particles from other side, but it is the same. What I say: GR and QFT are not that incompatible as most people think. That is, we can "mix" GR and QFT to some extent, say introduce gravitons.

 

[tom.stoer]

I don't say that QFT-like approach to GR will lead us to Truth. I just say, that both GR and QFT describe... something and that "something" is similar in these theories both physically and mathematically. We can call it spacetime warping from GR standpoint or virtual particles from other side, but it is the same. What I say: GR and QFT are not that incompatible as most people think. That is, we can "mix" GR and QFT to some extent, say introduce gravitons.

I agree with everything except for the "virtual particles from other side". They are a very limited concept and I don't think that they will help to understand QG.

 

[Naty1]

So GR and QFT are not quite that different. The last thing is to find interpretations of gauge fields in GR and spacetime bending in QFT. Kaluza-Klein-like theories are most promising IMO.

(1) both have infinities of their own;Smolin lists combining Gr and quantum theory one of the five great problems in theoretical physics, (2) rather unlikely, I think, but stranger things have happened, (3) Well multi dimensional theories have no far proven to be the panacea anticipated...again we'll have to see...seems more likely to me we need a new approach, maybe entropic gravity origins, Verlinde like...

 

[tom.stoer]

The big difference is that ART/GQ is diff.-inv.

 

[cragar]

If gravitons bend space-time , then would this mean the graviton has a gravitational field and therefore the graviton's field could emit its own particles and for conservation of energy to hold would the graviton have to go down in frequency to conserve energy.

 

[cragar]

Gravity IS a self interacting phenomena..

What do you mean by this , Could gravitons be similar to photons in their behavior ,
like the photon is the excitation of the EM field , but the photon does not have it's own EM field , So could the graviton be the excitation of the Gravitational field , but not necessarily have its own gravitational field , And if a photon is a self sustaining EM field like it has an E and B components the E field creates a B field and so on , can a gravitational field create another gravitational field or could it have 2 components like the photon .
I realize I am kind of mixing classical physics and QM .

 

[IRobot]

In GR we have:
$$D_{\mu} \phi_{\nu} = d_{\mu} \phi_{\nu} - \Gamma^{\alpha}_{\mu \nu} \phi_{\alpha}$$
while in QFT:
$$D_{\mu} \phi(q) = d_{\mu} \phi(q) - i A_{\mu} q \phi(q)$$

Put together, it's shocking how much those formulas look similar!

 

[tom.stoer]

In ordinary QFT (temporal gauge) you have
Gauss|phys> = 0
(H-E)|E> = 0

In GR (gauge formulation) you have
Gauss|phys> = 0
Diff|phys> = 0
H|phys> = 0

So the theories look similar with respect to the gauge symmetry, but especially due to the diffeomorphsim invariance they are nevertheless different.

Graviton and photon are rather different as photons do not couple to photons (they do not carry charge). Already in QCD (which is more similar to GR) one observes that the perturbative formulation based on "plane wave gluons" is not the hole story an is insufficient in the strong coupling regime.