How prone is a photon to interacting with uncharged structureless particles?

snorkack · Aug 31, 2023

How prone is a photon to interacting with uncharged structureless particles?

It must be fundamentally possible for a photon to interact with a particle that has no external charge and no internal charge either. Because electron and positron can and mostly do annihilate to two photons. Since electromagnetic interaction has CP (and indeed P) symmetry and is expected to therefore have T symmetry, a photon should be able to interact with another photon to produce a pair.

Given that γγ is expected to be possible, how strong are expected to be γ interactions (elastic or inelastic) with other structureless neutral particles:
γν (neutrinoes)
γZ⁰ (weak interaction boson)
γH⁰ (Higgs boson)
γg (gluon)
γG (graviton)?

mfb · Sep 1, 2023

At least γγ -> ee, μμ, WW have been observed, probably some more as well.

γν needs the electromagnetic and the weak interaction via loops so the cross sections will be extremely small unless the energy is very high. γG will be even worse.

γZ/γH cross sections I don't know, but Z and H don't live long enough to produce these collisions. H->γγ is a well-studied decay of course and H->Zγ is interesting as well.

γg should be negligible compared to γq and you can never have the former without the latter.

snorkack · Sep 4, 2023

mfb said:

γν needs the electromagnetic and the weak interaction via loops so the cross sections will be extremely small unless the energy is very high. γG will be even worse.

Generally the γG interaction is theoretically expected - photons carry mass and are subject to gravity - and also experimentally observed long before any of the other particles were even anticipated (the deflection of light by gravity, like Sun!)... but perhaps it´s poorly quantized? The observed gravitational waves tend to be low frequency, therefore low energy per quantum.
Gravitational waves near source should be significantly nonlinear. Are there any nonlinear effects when electromagnetic waves interact with strong gravitational waves? Like when the frequencies get close?

vanhees71 · Sep 4, 2023

Nothing whatsoever in Einstein's famous result about the gravitational deflection of light relies on photons, i.e., the quantization of the electromagnetic field but only on classical electrodynamics. Nowadays many, if not all, textbooks on relativity argue with a naive photon picture, which is however flawed. A true treatment of photons in GR, i.e., quantum field theory in a given "background spacetime" is very complicated and not needed here. What's called "photon" is rather the eikonal approximation for classical electromagnetic waves:

https://itp.uni-frankfurt.de/~hees/pf-faq/gr-edyn.pdf

Also photons don't carry mass but energy and momentum. The mass of the electromagnetic field is to high accuracy compatible with zero.

snorkack · Sep 4, 2023

vanhees71 said:

Nothing whatsoever in Einstein's famous result about the gravitational deflection of light relies on photons, i.e., the quantization of the electromagnetic field but only on classical electrodynamics. Nowadays many, if not all, textbooks on relativity argue with a naive photon picture, which is however flawed. A true treatment of photons in GR, i.e., quantum field theory in a given "background spacetime" is very complicated and not needed here. What's called "photon" is rather the eikonal approximation for classical electromagnetic waves:

https://itp.uni-frankfurt.de/~hees/pf-faq/gr-edyn.pdf

The standard handling of deflection of light by gravity and gravitational red/blueshift handles light as pure rays, without taking account of wave properties either.

vanhees71 · Sep 5, 2023

That's a bug, not a feature. That's why I wrote this little manuscript, because I wanted to understand classical physics within classical physics and not using some hand-waving wrong kind of "photon picture". Of course, at the end you calculate null geodesics, but these then have a clear classical-field-theoretical meaning as has "ray optics" as an approximation to "wave optics" in general.

snorkack · Sep 5, 2023

Do I get it correct that a Schwarzschild black hole absorbs some incident radiation and deflects the rest but that, in the far field, all the light that scatters is returned to its initial frequency in the frame where the hole is stationary?

nightvidcole · Dec 11, 2023

snorkack said:

How prone is a photon to interacting with uncharged structureless particles?

It must be fundamentally possible for a photon to interact with a particle that has no external charge and no internal charge either. Because electron and positron can and mostly do annihilate to two photons. Since electromagnetic interaction has CP (and indeed P) symmetry and is expected to therefore have T symmetry, a photon should be able to interact with another photon to produce a pair.

Given that γγ is expected to be possible, how strong are expected to be γ interactions (elastic or inelastic) with other structureless neutral particles:
γν (neutrinoes)
γZ⁰ (weak interaction boson)
γH⁰ (Higgs boson)
γg (gluon)
γG (graviton)?

Think about it in terms of the Feynman diagrams.

Photon and Neutrino: You can produce a W boson and corresponding charged lepton. This would be 1 EM vertex and 1 weak vertex.

Photon and Z0: You can produce a pair of charged leptons. Also 1 EM and 1 weak vertex. [A light quark pair is also possible, but the amplitude will be smaller because the quark charges are fractional while the leptons have a full charge.]

Photon and H0: You can produce a pair of charged particles that couple to the Higgs - could be leptons, quarks, or even W bosons. This would be 1 EM vertex and 1 Higgs interaction vertex. The coupling constant is substantial for many of the more massive particles such as the bottom and charm quarks, tau leptons, or of course the W boson.

Photon and gluon: Produce a pair of quarks - one EM and one strong vertex

Photon and graviton: In theory any pair of charged particles, one EM vertex and one gravitational interaction, but here you run into the added complication that gravity only interacts as a "tidal" force. At most, the coupling for the gravitational part is going to be suppressed by the ratio of the Planck mass to the particle mass. The photon and graviton would also need enough energy in their common center of mass frame to create the rest-mass of the pair. This one is in practice unobservable due to the extremely small effective coupling constant since achievable energies are so far below the Planck scale.

Note that for Z0 and H0, the massive particle can create the pair even without a photon being involved. In order to have most of these interactions involve a photon, you would need a photon beam of incredibly high intensity, colliding directly with Z0 or H0. Nearly impossible to do in a lab. Probably would need some sort of next-generation plasma wakefield accelerator that could reach energies of, at minimum, ~100 GeV (~200 GeV to produce a Higgs via Higgsstrahlung since you would need to produce other particles plus the Higgs). You would need an extremely long integration time and even then your signal/background ratio would be terrible since there would be a large amount of purely EM pair production happening at the same time...

ohwilleke · Dec 11, 2023

nightvidcole said:

Think about it in terms of the Feynman diagrams.

Photon and Neutrino: You can produce a W boson and corresponding charged lepton. This would be 1 EM vertex and 1 weak vertex.

Photon and Z0: You can produce a pair of charged leptons. Also 1 EM and 1 weak vertex. [A light quark pair is also possible, but the amplitude will be smaller because the quark charges are fractional while the leptons have a full charge.]

Photon and H0: You can produce a pair of charged particles that couple to the Higgs - could be leptons, quarks, or even W bosons. This would be 1 EM vertex and 1 Higgs interaction vertex. The coupling constant is substantial for many of the more massive particles such as the bottom and charm quarks, tau leptons, or of course the W boson.

Photon and gluon: Produce a pair of quarks - one EM and one strong vertex

Photon and graviton: In theory any pair of charged particles, one EM vertex and one gravitational interaction, but here you run into the added complication that gravity only interacts as a "tidal" force. At most, the coupling for the gravitational part is going to be suppressed by the ratio of the Planck mass to the particle mass. The photon and graviton would also need enough energy in their common center of mass frame to create the rest-mass of the pair. This one is in practice unobservable due to the extremely small effective coupling constant since achievable energies are so far below the Planck scale.

Note that for Z0 and H0, the massive particle can create the pair even without a photon being involved. In order to have most of these interactions involve a photon, you would need a photon beam of incredibly high intensity, colliding directly with Z0 or H0. Nearly impossible to do in a lab. Probably would need some sort of next-generation plasma wakefield accelerator that could reach energies of, at minimum, ~100 GeV (~200 GeV to produce a Higgs via Higgsstrahlung since you would need to produce other particles plus the Higgs). You would need an extremely long integration time and even then your signal/background ratio would be terrible since there would be a large amount of purely EM pair production happening at the same time...

One of these interactions is not like the other.

In all of the cases except the photon-graviton interaction, there is no interaction at tree level, but there could be an interaction at a higher level loop.

In the case of the photon-graviton interaction, while the interaction is indeed very weak, the graviton does have a coupling to the mass-energy of the photon at tree level.

How prone is a photon to interacting with uncharged structureless particles?

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