# Decay through gravitional interaction

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1. Aug 21, 2015

### Garlic

Hello,
I know that the gravitional force is the weakest of all forces (it is 10^-38 times weaker than the strong force), so it isn't possible a particle to decay through gravitional interaction. As far as I know, the strength of fundamental forces can change during different ages of the universe, because of the overall temperature of the universe etc. Let's say sometime the gravitional force (was/will be) strong enough that a particle can decay through gravitional interaction when given the right conditions, how would the decay equations look like?
When decaying through weak interaction, we see quark favours change themselves, during electromagnetical decay, photons can be emitted etc. Which attribute of a particle/an atom would change during gravitional decay? Would gravitons be emitted?

And are virtual gravitons predicted to be mediating force (for doing something) inside a nucleus?
Thank you

2. Aug 21, 2015

### Staff: Mentor

The weakness is not the fundamental reason. There is also no known gravitational coupling that would change the particle type. Black holes are an exception, but calling that a "particle decay" can be questionable.

Virtual gravitons can mediate a tiny attractive force in a nucleus that has no relevance because it is so tiny.

There is no indication that the strength would have changed in any way. The strength depends on the energy scale, but high-energetic processes were not limited to the past, they happen today as well.

3. Aug 21, 2015

### Avodyne

In principle, an atom or molecule can decay from an excited state to a lower-energy state by graviton emission, just like it can decay by photon emission. The rates are so small as to be completely negligible, but they are not zero in general. There are angular momentum selection rules that follow from the graviton having spin 2, just like there are angular momentum selection rules for photon emission that follow from the photon having spin 1.

4. Aug 21, 2015

Staff Emeritus
As Avodyne points out, there are decays (eta -> pi0 + G would be one) that can proceed gravitationally. The rates are so small that in the entire history of the universe there has probably never been such a decay. On the issue of the strength of the gravitational interaction changing over time, there's no evidence of that, but in any event, it can't be tested by a process that, even though technically non-zero, as a practical matter doesn't occur.

5. Aug 22, 2015

### snorkack

I have seen the estimate of a branching ratio between positronium decay to photons and to gravitons as 1040. But that estimate was 50 years old. For comparison, the branching ratio between positronium decay to photons and to neutrinos was in the region of 1020.
Any more modern estimates?

6. Aug 22, 2015

### Avodyne

The 10^40 should be about right. Roughly, the fine structure constant $\alpha$ gets replaced by $Gm^2$, where $G$ is Newton's constant and $m$ is the electron mass, and I am using natural units with $\hbar=c=1$. Roughly, $Gm^2/\alpha \sim 10^{-20}$. The positronium decay rate to photons is proportional to $\alpha^2$, so decay to gravitons should be smaller by $10^{-40}$. But there are $2\pi$'s to get right, and differing numerical factors from the different spins of the photon and graviton, etc.

7. Aug 23, 2015

### Garlic

How can graviton emission be predicted, even though gravitons are theoretical particles?

8. Aug 23, 2015

### Staff: Mentor

If gravity can be quantized we know some of its properties - it has to reproduce classical gravitational fields. We know the interaction strength, the spin of possible gravitons (2) and so on. Factors of 2 pi are up to the specific unknown theory but they won't change the result by many orders of magnitude.

9. Aug 23, 2015

Staff Emeritus
How could Higgs boson production be predicted back when it was a theoretical particle?