Photons vs Gravitons

[TouchuvGrey]

TL;DR

Photons vs (hypothetical) gravitons

Hello Everyone:

New member Mike from Australia.

If i posted in the wrong forum please tell me.

Light is made up of photons. photons obviosly travel at c. photons are affected by gravity.
Gravity is (possibly) made up of gravitons.

Photon and the hypothetical gravitons both travel at c. They are the only things
we know of that travel at that speed.

Gravity affects light. Does light affect gravity ?

Is there any known or theoritical relationship between photons and gravitons ?

Mike
No degree or studies, just curious.

 

[PeterDonis]

If i posted in the wrong forum please tell me.

That depends. It seems like your question is not specifically about any quantum aspects of light or gravity, but just about this:

Gravity affects light. Does light affect gravity ?

The answer, in classical GR, to this question is yes: light has energy, and everything that has energy produces gravity. More specifically, everything that has energy produces spacetime curvature via the Einstein Field Equation of GR.

However, you use the terms "photon" and "graviton", which are not part of classical physics; they are quantum terms. If you're specifically interested in quantum aspects of light and/or gravity, please start a separate thread in the Quantum Physics forum. For the discussion here, instead of "photon" and "graviton", the simple terms "light" and "gravity" should be sufficient.

Also, welcome to PF!

 

[PeterDonis]

Is there any known or theoritical relationship between photons and gravitons ?

Classically, fhe field equations for light and gravity are similar, yes; both of them have the basic form of a differential equation whose LHS describes the field and its derivatives and the RHS describes the source--what produces the field. For light, the field is described by the EM field tensor and the source is the charge-current 4-vector. For gravity, the field is described by the Einstein tensor and the source is the stress-energy tensor.

Again, if you are asking specifically about similarities between the quantum aspects of light and gravity (which is what the terms "photon" and "graviton" refer to), please start a separate thread in the Quantum Physics forum.

 

[anuttarasammyak]

Hello.
Neutrinos (had been thought to) also travel at c. But recent study suggests almost c due to their not zero mass.
[edit] ( ) added. An example of experiments measuring neutriono speedhttps://arxiv.org/abs/1109.4897 Only mass zero particles including photon and hypothetical graviton travel at c by SR.

 

[PeterDonis]

Neutrinos also travel at c.

No, they don't:

But recent study suggests almost c due to their not zero mass.

Please don't post contradictory statements. Also, nonzero neutrino masses do not "suggest" that they do not travel at $c$; they require it.

 

[Matterwave]

To be fully technical, it is still possible that one of the neutrino mass eigenstates is 0. All we constrain with oscillation tests are mass splittings and not absolute mass. The other neutrino mass tests that I'm aware of simply give upper bounds to the mass eigenstates. I'm not aware of a publication that has ruled out 0 rest mass for the lightest mass eigenstate.

 

[PeterDonis]

To be fully technical, it is still possible that one of the neutrino mass eigenstates is 0.

Technically, yes, this is possible. My understanding is that it's considered extremely unlikely, because whatever mechanism makes the neutrino masses nonzero in the first place would be expected to apply to all of the mass eigenstates.

 

[Ibix]

Gravity affects light. Does light affect gravity ?

I think it's worth pointing out that light is analogous to gravitational waves, while gravity in the sense of the more-or-less static field holding us to the Earth is analogous to an electric and/or magnetic field.

But there is an asymmetry here. A gravitational field, including a gravitational wave, will deflect light and other gravitational waves (the latter is theoretical - I don't think lensing of gravitational waves has actually been observed yet). An electromagnetic field, including light, however, doesn't. What it does do is generate a gravitational field (again, theoretically - the energy needed to test this is absurdly large) that then deflects light and gravitational waves.

 

[jedishrfu]

Wheeler once posited the notion of a geon, a ring of light held together by the gravity of its own field energy. Geons have never been observed, and I believe there's some debate on whether they are stable, but theoretical physics entertains such notions in the search for quantum gravity and dark matter.

https://en.wikipedia.org/wiki/Geon_(physics)

 

[Roberto Pavani]

Regarding neutrino masses: it's worth noting that Neil Turok et al. (https://arxiv.org/abs/1803.08930) have proposed a cosmological model (CPT-symmetric universe) in which one neutrino mass eigenstate is exactly massless.
So $m_\nu = 0$ for at least one eigenstate is not just "not ruled out", it's an active theoretical proposal.

A related thought: if neutrinos are massive, it should in principle be possible to decelerate them to arbitrarily low speeds. To my knowledge, no experiment has ever succeeded in "slowing down" to arbitrary slow speed a neutrino.
This is an interesting asymmetry with other massive particles.

 

[Matterwave]

A related thought: if neutrinos are massive, it should in principle be possible to decelerate them to arbitrarily low speeds.

It's difficult to do this since basically any amount of energy will kick the neutrino to relativistic speeds ($E\gtrsim m\lt 0.5\text{eV}$). Further, neutrinos only interact via the weak force (with a very small weak cross section) and gravity so we'd have to be very clever indeed.

Relic neutrinos from the cosmic neutrino background should be slow and non-relativisitic though. But it's hard to detect them.

 

[Roberto Pavani]

neutrinos only interact via the weak force (with a very small weak cross section)

That's an interesting point.
Note also that for neutrons the cross section increases as they slow down ($\sigma \propto 1/v$), which is why thermal neutrons are so useful for nuclear physics.
If neutrinos were truly massive, one might expect a similar behavior, slower neutrinos having a larger weak cross section.
But this enhancement has never been observed (nor have slow neutrinos themselves).
That's why I consider this an interesting asymmetry.

 

[Matterwave]

If neutrinos were truly massive, one might expect a similar behavior, slower neutrinos having a larger weak cross section.

No, one might not expect this behavior. The interaction for the strong nuclear force is very different than the weak interaction which is mediated by massive W and Z bosons.

I believe low energy (relative to W/Z boson mass) neutrino cross sections scale as $G_F^2 E_\nu^2$. But it's been a while since I did neutrino physics. So you might want to check me on that.

You also have to consider how we actually detect neutrinos. For example, one channel through which we detect them is via neutrino electron elastic scattering producing cherenkov radiation in ultra pure water (e.g. at Super-K). In that scenario you want a high energy neutrino for obvious reasons.

 

[Roberto Pavani]

You're right, my fault. And from your formula $\sigma \propto G_F^2 E_\nu^2$, slow neutrinos would be even harder to detect, making the asymmetry even sharper.

 

[Matterwave]

They earn their name "ghost particle" :)

 

[oracle99]

The answer, in classical GR, to this question is yes: light has energy, and everything that has energy produces gravity. More specifically, everything that has energy produces spacetime curvature via the Einstein Field Equation of GR.

I thought about it in these terms: If you converted the entire mass of the earth into energy, (by E=mc2) that vast amount of energy would still only exert 1G of gravity.

 

[Ibix]

I thought about it in these terms: If you converted the entire mass of the earth into energy, (by E=mc2) that vast amount of energy would still only exert 1G of gravity.

But only briefly. "Converting to energy" actually means "converting to radiation", which would immediately spread out. In a second it would occupy a sphere of radius one light second, with a surface gravity of less than 0.0005g. Note that the radiation flux at this distance from a totally converted Earth is something like $5\times 10^{23}\mathrm{Jm^{-2}}$, although the gravitational effect is near negligible.

 

[oracle99]

But only briefly. "Converting to energy" actually means "converting to radiation", which would immediately spread out. In a second it would occupy a sphere of radius one light second, with a surface gravity of less than 0.0005g. Note that the radiation flux at this distance from a totally converted Earth is something like $5\times 10^{23}\mathrm{Jm^{-2}}$, although the gravitational effect is near negligible.

There is an old joke "you can put a billion dollars worth of plutonium into a suit case, but only briefly.."

 

[Herman Trivilino]

If you converted the entire mass of the earth into energy, (by E=mc2) that vast amount of energy would still only exert 1G of gravity.

But only briefly. "Converting to energy" actually means "converting to radiation", which would immediately spread out.

It would be an explosion. The amount of gravity produced would be the least concern.

 

[anuttarasammyak]

You also have to consider how we actually detect neutrinos. For example, one channel through which we detect them is via neutrino electron elastic scattering producing cherenkov radiation in ultra pure water (e.g. at Super-K). In that scenario you want a high energy neutrino for obvious reasons.

FYI of recent progress
"XENONnT now extends this legacy, lowering the solar-neutrino energy threshold to 17 keV." https://xenonexperiment.org/xenonnt...rgy-solar-neutrinos-scattering-off-electrons/

 

[pervect]

Energy, momenum, and stress all contribute to gravity in GR. Usually only the first two (energy and momentum) would be used used to model light beams.

Stress terms in GR are usually negligible anyway, except for pressure, which in the interior of massive astronomical bodies can be gravitationally significant.

As others have pointed out, it takes a lot of energy to produce any significant gravitational effects.

The idea that mass causes gravity is a Newtonian concept - in General Relativity, it's energy, momentum, and stress, combined into a mathematical entity called "The Stress Energy Tensor" that is the source of gravity. I like to briefly summarize Einstein's field equations by saying that the thing on the left you don't understand (without specialized study, at least), called the Einstein curvature tensor $G_{\mu\nu}$ is proportional to the thing on the right you don't understand, the Stress Energy Tensor $T_{\mu\nu}$ and can be regarded as the source term for gravity in GR.

I do have a quirky sense of humor, but I hope the brief overview is helpful. Wiki has an entry on the stress-energy tensor, but I suspect the Wiki article would be unhelpful without a significant amount of advanced study.

A rather interesting analogy, called "GEM", short for "gravitoelectromagnetism", makes an anolgoy between the electrostatic coulomb force of a stationary charge, and the gravitational force of a stationary mass. A moving charge generates a current, which is the source of an additional magnetic field that is different from the electrostatic coulomb field. Something simlar happens in a light beam, in the GEM anaology the momentum of the light beam is analogous to the electric current.

 

[PeterDonis]

Usually only the first two (energy and momentum) would be used used to model light beams.

This isn't quite true. Light beams have pressure as well. Indeed, their stress-energy tensor, in the standard treatment, is traceless, which can't be the case if only energy and momentum are present, but no pressure.

 

[flippiefanus]

To my knowledge, no experiment has ever succeeded in "slowing down" to arbitrary slow speed a neutrino.

One needs to have some force that can affect neutrinos to slow them down. (Gravity also affects them but that is not a force.) The only force that affects them is the weak force, which we don't know how to use yet.

 

[anuttarasammyak]

One needs to have some force that can affect neutrinos to slow them down. (Gravity also affects them but that is not a force.) The only force that affects them is the weak force, which we don't know how to use yet.

As for neutrinos emitted from the Sun, apparatus on the spaceship leaving the Sun with near light speed would be able to observe slow neutrinos.
In this same sense “red-shift” of neutrinos from distant stars should take place.

 

[Matterwave]

At least two mass eigenstates of the cosmological remnant neutrinos should already be non relativistic due to the expansion of the universe. There likely exist a cosmic neutrino background of "slow" neutrinos. We just can't directly detect them. I'm not up to date on the indirect detection path.

 

[pervect]

This isn't quite true. Light beams have pressure as well. Indeed, their stress-energy tensor, in the standard treatment, is traceless, which can't be the case if only energy and momentum are present, but no pressure.

The treatment of light beams I've seen in the past (I think it was and ancient book by Tollman) used null dust to model light beams. I think I've seen other authors do the same, though I don't recall the exact sources. The null dust approximation omits the pressure terms, which are small.

A full treatment using actual electromagnetic fields instead would have pressure terms, I believe, but would be unnecessarily complicated. I don't think I've ever seen one discussed, of course I have hardly seen everything. The null dust solution would follow traditional geometric optics - the full exact treatment would involve diffraction.

Probably one of the more accessible facts about light and gravity is that parallel light beams don't attract each other gravitationally. Using the GEM model, this is because the magnetic-like forces cancel out the electric-like forces in the GEM anology. Anti-parallel beams attract strongly - I think it was 4x as much as a simple model based on E=mc^2 would predict. I don't recall the source on this, though.

 

[PeterDonis]

The null dust approximation omits the pressure terms, which are small.

Actually, no, it doesn't, although the term "dust" might suggest that it does. The pressure of radiation is not small; it's equal to its momentum flux and energy density. See, for example, here:

https://en.wikipedia.org/wiki/Null_dust_solution#Einstein_tensor