What is the cross section of a graviton?

[KurtLudwig]

In particle physics there is usually a cross section for a particular particle . I came up with a cross section of 1.07 x 10^-42 m^2 for a graviton.

 

[phinds]

In particle physics there is usually a cross section for a particular particle . I came up with a cross section of 1.07 x 10^-42 m^2 for a graviton.

Amazing. How did you do that?

 

[Demystifier]

In particle physics there is usually a cross section for a particular particle .

No there isn't. There is a cross section for scattering of one particle on another.

 

[KurtLudwig]

Then what is the scattering cross section of a graviton on a proton? Does the scattering cross section have to do with the particles interacting?

 

[Demystifier]

Then what is the scattering cross section of a graviton on a proton?

Things like that have been calculated, but I don't know the result.

Does the scattering cross section have to do with the particles interacting?

Yes, exactly.

 

[Vanadium 50]

Things like that have been calculated, but I don't know the result.

It's infinite.

 

[KurtLudwig]

Do physicists have a tentative theory on how a gravitons interact with a protons, neutrons or maybe directly with quarks? I assume that gravitons travel through space similar to photons. Gravity only pulls baryons together, it never repels. Is this correct?

 

[king vitamin]

Do physicists have a tentative theory on how a gravitons interact with a protons, neutrons or maybe directly with quarks?

Sure, at accessible energies we can just use the effective field theory of gravitons to compute cross sections. There are lots of good arguments for why this effective theory should be true no matter what the full correct theory of quantum gravity is. The problem is that the result shows the interactions to be far too weak to detect in our experiments.

 

[KurtLudwig]

Please recommend a paper or textbook which states the effective field theory of gravitons and shows their calculations?
Can the interactions be detected by observing the velocities of two stars which are far away from any other stars and the center of our galaxy? Can a sensitive torsional pendulum be used to measure weak gravitational interactions?

 

[king vitamin]

Here is a review article on gravity as an EFT: https://arxiv.org/abs/1209.3511

Can the interactions be detected by observing the velocities of two stars which are far away from any other stars and the center of our galaxy? Can a sensitive torsional pendulum be used to measure weak gravitational interactions?

Once again, we currently do not have sensitive enough instruments (including torsional pendulums) to unambiguously measure these effects, they are far too small. As an example, the leading effect that quantum gravity has on the perihelion shift of Mercury is one part in $10^{90}$, which is far smaller than any precision we can obtain.

 

[Nugatory]

one part in $10^{90}$, which is far smaller than any precision we can obtain.

@Greg Bernhardt do we offer an award for super-awesome understatements? :)

 

[Demystifier]

It's infinite.

Unless, of course, you consider a wave packet with finite width.

 

[Keith_McClary]

8 × 10^-65 cm²

If we imagine the whole mass of the Earth to be used as a graviton detector, with the cross-section (20) per electron and the flux (23), the counting-rate is 2.4 × 10¹⁷ per second. If the experiment continues for the life-time of the sun, which is 5 billion years, the expected total number of gravitons detected will be 4. The experiment barely succeeds, but in principle it can detect gravitons.

https://publications.ias.edu/sites/default/files/poincare2012.pdf

 

[KurtLudwig]

My calculations were 24 orders of magnitude off.
Thank you for referring me to this incredible paper.
Freeman Dyson wrote "A second hypothesis is that the gravitational field is a statistical concept like entropy or temperature, only defined
for gravitational effects of matter in bulk and not for effects of individual elementary particles.
If the second hypothesis is true, then the gravitational field is not a local field like the
electromagnetic field. The second hypothesis implies that the gravitational field at a point
in space-time does not exist, either as a classical or as a quantum field."

 

[mitchell porter]

Do physicists have a tentative theory on how a gravitons interact with a protons, neutrons or maybe directly with quarks? I assume that gravitons travel through space similar to photons. Gravity only pulls baryons together, it never repels. Is this correct?

As you would know, the standard theory of gravity is Einstein's general relativity. That is a geometric theory in which "spacetime tells matter how to move; matter tells spacetime how to curve" (John Wheeler).

But it can also be represented as a field theory (like the Maxwell theory of electromagnetism), in which the field is the metric tensor (a 4x4 matrix that encodes angles and distances). For field theories in "flat" space-time (no curvature), quantum mechanics has mathematical methods which allow you to construct the corresponding theory of particles. So Feynman said, suppose we rewrite the equations of general relativity, so that we have a new field which equals the difference between the curved-space metric in the geometric theory, and the flat-space metric tensor; and then we apply the mathematical procedures of quantum field theory, to this flat-space gravitational field theory.

This gives you a definite theory of how gravitons interact with everything, and it is the standard theory of how gravitons behave. The paper in comment #10 talks a little about it, starting on page 11.

I should emphasize that these gravitons are quantum "particles". They exhibit all the peculiar quantum properties like "superposition", and "entanglement". Ultimately they come from applying the uncertainty principle to Einstein's metric tensor, so such a graviton is really a fuzzy little packet of space warp - and I apologize for that phrase, I'm just trying to emphasize that it is not like the particles in the old "corpuscular" theories of gravity from Newton's time.

While this is the standard quantum theory of gravity, it also breaks down at short distances, which is why it is used just as an "effective" theory, and why people expect that reality is described by something a little different, like string theory or "asymptotic safety".

You asked about gravitons interacting directly with quarks. This is definitely what the theory says - the gravitons interact directly with all fundamental degrees of freedom, i.e. all elementary particles. And here I want to mention that somehow, this aspect of gravitational theory has allegedly found a use in nuclear physics, in characterizing the internal structure of nucleons. No one is claiming to observe individual gravitons scattering from quarks or anything like that. But somehow the theoretical calculation is supposedly useful in describing forces inside the proton. I don't understand it and I am actually deeply skeptical, but I mention it since you asked.

 

[KurtLudwig]

Thank you for your detailed explanation.
I have printed out Freeman Dyson's paper and will read it carefully.
Should I be looking for a title of "effective quantum gravity theory" on the internet?

 

[mitchell porter]

You might also want to read about "effective field theory" in general, although it is a somewhat technical concept. In perturbative quantum field theory, i.e. QFT in which calculations are refined by considering the contributions from processes with more and more particles, there is a distinction between a "renormalizable" field theory, which makes sense up to arbitrary energies, and an "effective" field theory which is only valid up to a specific energy scale. The quantum field theory of gravity which Feynman and others constructed, is actually "non-renormalizable", it doesn't make sense as a renormalizable theory, but it does produce a theory that is "effective" in the technical sense, and therefore valid over a certain range of energies.

 

[KurtLudwig]

My original question was:
In particle physics there is usually a cross section for a particular particle . I came up with a cross section of 1.07 x 10^-42 m^2 for a graviton.

After re-reading Freeman Dyson's incredible paper "Is a Graviton Detectable" the cross-section of a graviton is
8 × 10−65𝑐𝑚2. Keith McClary was right.


“The cross-section for absorption of a graviton by any kind of particle will be the same magnitude 4𝜋2𝐿𝑝 2 = 4𝜋2𝐺ℏ𝑐−3 = 8 × 10−65𝑐𝑚2
spread over a range of graviton energies extending from the binding-energy of the particle to e few times the binding energy. For any macroscopic detector composed of ordinary matter, the absorption cross-section will be of the order of 10-41 square centimeters per gram.”

 

[DrChinese]

As you would know, the standard theory of gravity is Einstein's general relativity.

Not stated in this thread is that gravitons are hypothetical at this point. Regardless of the pros and cons of them being quantum entities, or of GR breaking down in certain realms. GR has passed all experimental tests asked of it so far.

Obviously: this is a very exciting time for cosmology with the search on for things like dark matter and dark energy, which would presumably require adjustments to the Standard Model to account for these. A lot of candidate ideas are out there currently, but there are more questions than answers at this point.