Would gravitons theoretically act like photons?

[Josiah]

Hi, would gravitons theoretically act like photons? They're both particles and waves at the same time. Is there anything we can use from what we know about photons and use them to understand gravitons?
Thanks
Josiah

 

[Drakkith]

Yes, they would behave like photons in the sense of being a particle and a wave at the same time. Like the photon, gravitons are believed to be massless and are bosons, though of a different spin type, photons being spin 1 and gravitons being spin 2.

 

[PeterDonis]

Hi, would gravitons theoretically act like photons?

Not exactly. As @Drakkith points out, they are spin 2, not spin 1. That makes their behavior different in some ways.

They are both massless bosons, so anything that can be inferred from being a massless boson would apply to both photons and gravitons.

 

[Philip Koeck]

Not my field at all, but I have a question too.

I always thought that the gravitational field and the electrostatic field are both vector fields, but now I read that gravitons build (carry, mediate?) tensor fields.
How can the gravitational force be due to a tensor field?

 

[weirdoguy]

How can the gravitational force be due to a tensor field?

Because General Relativity Gravity is a vector field in Newtionian gravitation, which we know is only an approximation.

 

[Demystifier]

How can the gravitational force be due to a tensor field?

It's related to the fact that the gravitational force is fictitious, very much like the inertial forces such as the centrifugal force. Roughly speaking, Einstein theory of gravity is what you get when you combine (i) inertial forces of Newtonian mechanics, (ii) Newtonian gravity and (iii) special relativity.

 

[PeterDonis]

How can the gravitational force be due to a tensor field?

See Misner, Thorne, & Wheeler, Exercises 7.1, 7.2, and 7.3, for a good discussion of why scalar and vector fields cannot describe gravity, and a tensor field is required.

 

[OmniGooch]

 

[Josiah]

Yes, they would behave like photons in the sense of being a particle and a wave at the same time. Like the photon, gravitons are believed to be massless and are bosons, though of a different spin type, photons being spin 1 and gravitons being spin 2.

If gravitons are like photons, then wouldn't gravitons get absorbed by electrons the way photons can be absorbed by electrons?

 

[PeroK]

If gravitons are like photons, then wouldn't gravitons get absorbed by electrons the way photons can be absorbed by electrons?

Why do photons interact with electrons?

 

[Drakkith]

If gravitons are like photons, then wouldn't gravitons get absorbed by electrons the way photons can be absorbed by electrons?

Both should be 'absorbed' by photons, but through different mechanisms and with different results. Keep in mind we are vastly oversimplifying things here.

 

[PeroK]

Both should be 'absorbed' by photons, but through different mechanisms and with different results. Keep in mind we are vastly oversimplifying things here.

I'm not sure what you mean by this?

 

[Drakkith]

I'm not sure what you mean by this?

That's alright, I'm not sure I understand what I mean by this either. Don't type up posts right before you go to bed about something you're only vaguely familiar with.

 

[snorkack]

The interaction potential of an electron with proton in a protium atom due to gravity is 2.3*10³⁹ times smaller than the interaction potential of the same due to electrostatic force.
How would the partial half-lives of an excited hydrogen atom by photon emission and by graviton emission compare?
Note that spin 2 means graviton emission, and graviton absorption, should follow different selection rules than interaction with spin 1 photon, so probably direct comparison of partial half-lives for same orbital pairs would be less instructive.

 

[SergejMaterov]

I think it is too early to talk about "graviton" at all for obvious reasons:
1. quantum gravity has not been built
2. "graviton" has not been discovered
Quantum field theory predicts a lot, but creates more problems.

 

[SergejMaterov]

You can of course be skeptical about my previous post, but:
If it does exist, then formally the "graviton" should be very similar to the photon, and many techniques (Feynman rules, soft theorems, double copy) can be transferred. They share the same quantum‑wave duality, but that’s where most of the similarity ends.
Typical photon decay of the hydrogen 2P→1S state has a partial lifetime $τ_γ∼10^{−9} s$ (electric‑dipole transition, Δℓ=±1).
Gravitational analogy of the fine structure constant:
$$\alpha_g = \frac{\hbar c}{G m_e m_p} \sim 3 \times 10^{-42}, \text { versus } α_{EM}≈1/137$$
Multipole: quadrupole emission rates scale like (ka)⁴ relative to dipole (with k the wave number, a the Bohr radius). Rough back‑of‑envelope for the rate ratio $\frac{\Gamma_g}{\Gamma_\gamma}$ is of order $$\left( \frac{\alpha_{\mathrm{EM}}}{\alpha_g} \right)^2 \sim 10^{-79} \;\Rightarrow\; \tau_g \sim \tau_\gamma \times 10^{79} \sim 10^{70}\ \text{s} \sim 10^{62}\ \text{yr}$$
Due to the super-weak force of gravitational interaction, no atomic transitions to the graviton will ever be observed: a huge gap of $10^{60} - 10^{80}$ years.

 

[PeterDonis]

If it does exist, then formally the "graviton" should be very similar to the photon, and many techniques (Feynman rules, soft theorems, double copy) can be transferred

We already know what such a QFT looks like: it's the QFT of a massless spin-2 field that was developed by Feynman, Desert and others in the 1960s and early 1970s.

 

[PeterDonis]

Gravitational analogy of the fine structure constant:

This doesn't really work because the gravitational constant is not dimensionless. You can compute a ratio for a particular interaction, as you did, but there's no general way to give a dimensionless "strength of gravity" relative to other interactions.

 

[SergejMaterov]

Gravity’s coupling G is dimensionful, so you only get a meaningful gravitational fine‑structure constant once you specify masses or energies.
In hydrogen: $\alpha_g=\frac{Gm_em_p}{\hbar c}\sim3\times10^{-42}$
In particle scattering at energy E: $\alpha_g(E)\sim\frac{GE^2}{\hbar c}$
This lets you compare to α and explains why atomic graviton emission (quadrupole, $∝α_g^2$) is ∼10⁻⁷⁹ times slower than photon emission. This makes the process physically irrelevant.