# B Why do photons only interact with charged particles?

#### jeremyfiennes

Summary
Why do photons only interact with charged particles?
Why do photons only interact with charged particles?

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#### Dr.AbeNikIanEdL

I would say it is the other way around, we call those particles charged that the photon interacts with (with the magnitude of the charge giving the strength of the interaction).

#### jeremyfiennes

Not with you. Atoms consist of protons, neutrons and electrons. The charges of the first are balanced by the last. Charge arises in the atom. Photons are external to the atom. And interact with electrons (and also apparently protons), but not with neutrons. Why?

#### PeterDonis

Mentor
Why do photons only interact with charged particles?
Because the definition of a charged particle is a particle that photons interact with.

Photons are external to the atom. And interact with electrons (and also apparently protons), but not with neutrons. Why?
The question you are asking here is not "why do photons only interact with charged particles?". The question you are asking here is "why aren't neutrons charged particles?"

And the answer is, because the particular bound state of three quarks that we call a "neutron" has the charges of all the quarks canceling, for a net charge of zero, whereas the bound state of three quarks that we call a "proton" does not.

(Btw, I don't understand why you say "also apparently protons"--why "apparently"? Do you think there is somehow an open question about whether protons are charged?)

#### PeterDonis

Mentor
And interact with electrons (and also apparently protons), but not with neutrons.
Actually, it's not true to say that photons don't interact at all with neutrons, because neutrons have a nonzero magnetic moment, which means they do have magnetic interactions (which are interactions with photons). They just don't have electrostatic interactions.

#### Dr.AbeNikIanEdL

Charge arises in the atom.

What do you mean by that?

I don’t think there is a real answer to why a particular particle has charge 0 or not. Of course proton and neutron are themselves bound states of quarks, so their charge is give by the charge of the quarks they contain. But that is not really an answer since it just shifts to why quarks have a particular charge.

#### jeremyfiennes

All this is interesting, but isn't answering my question. A photon is a chargeless energy packet. So why should it interact only with charged particles, and not with others?

#### Dr.AbeNikIanEdL

A photon is a chargeless energy packet.
Ok, I guess this is where the problem is. Photons are the particles that mediate the electromagnetic interaction, they appear after quantizing the em field. There are many particles, all have Energy and may or may not be charged. But coupling to charged particles is really what makes a photon a photon.

#### jeremyfiennes

So photons only interact with charged particles because they only interact with charged particles?

#### Dr.AbeNikIanEdL

Basically yes (as @PeterDonis pointed out, a bound state of charged particles can still interact even if its net charge is zero). There is really no notion of a photon required here. What would your answer be to “Why does the electromagnetic field only influence charged particles?”

#### jeremyfiennes

Because a charged particle is by definition one influenced by an electromagnetic field. But a photon is NOT defined as one that interacts only with charged particles. That is a photon characteristic. A photon is by definition a carrier of electromagnetic energy.

#### s00mb

Hi, I've been reading this and I was wondering is this talking about virtual photons or just general interactions of photons with charged particles? Reason I am wondering was because of the comment about "mediating the electromagnetic" interaction. I'm thinking about particle physics and particle decays and probably confusing these two but this seems a great learning opportunity for me =) Thx

#### Dr.AbeNikIanEdL

Where are you getting your definition of what a photon is from?

If we start with quantum theory, there is a quantum field for the electromagnetic force, and its excitation are photons. Charged particles are (by definition of the word charged) the particles this quantum field couples to, and hence its excitation interact with those particles (and essentially dont know about other particles that are around). In the classical limit this gives classical electromagnetism, where we have classical fields that interact with the charged particles.

So I guess you are thinking about this problem kind of backwards. Saying “only charged particles couple to photons” is really the quantum version of saying “only charged particles are influenced by em fields”.

A photon is by definition a carrier of electromagnetic energy.
What exactly do you think this means?

#### PeterDonis

Mentor
a photon is NOT defined as one that interacts only with charged particles. That is a photon characteristic. A photon is by definition a carrier of electromagnetic energy.
You are confused. "A carrier of electromagnetic energy" is the same thing as "interacts only with charged particles". Both are equivalent ways of defining what a photon is.

#### PeterDonis

Mentor
is this talking about virtual photons
This is a "B" level thread so I think all of the complications involved in virtual particles are out of scope; those would require at least an "I" level and quite possibly an "A" level discussion.

#### vanhees71

Gold Member
All this is interesting, but isn't answering my question. A photon is a chargeless energy packet. So why should it interact only with charged particles, and not with others?
The fact that the electric charge (everywhere in this thread were it's said "charge" what's implicitly meant is "electric charge") of the photon is 0 is only telling you that there is no direct electromagnetic interaction between photons, though quantum fluctuations lead in fact to an interaction; the lowest order in perturbation theory is $\alpha_{\text{em}}^4$, i.e., it's a very small cross section; this "light-by-light scattering" has just quite recently been observed in ultrapheripheral lead-lead collions by the ATLAS collaboration a the LHC.

Also physics cannot answer such "why questions". As any natural science the aim of physics is to observe nature and find generally valid "fundamental laws of nature". One finding is that matter, among other fundamental characteristics like mass, has a property described by "electric charge". Whenever a body carries electric charge there's an electromagnetic field around it which leads to a force acting other electric charges. This is the picture of the most simple case of electrostatics, and that's how electricity has been discovered by the ancient Greeks.

Nowadays we know that electric charge is a fundamental property of the elementary particles making up matter (quarks and leptons), which are described by spin-1/2 fields, and that the electromagnetic interaction is mediated by the electromagnetic field, described as a massless spin-1 Abelian gauge field which itself carries no charge.

That, of course, doesn't answer your "why question". It's only the most condensed summary of observations about nature concerning the electromagnetic interaction. That's all you can expect from the natural sciences.

#### jeremyfiennes

The fact that the electric charge (
This seems as far as we can get. Thanks.

#### PrashantGokaraju

It is because of how gauge theory is understood as a part of quantum mechanics. The conserved current

jμ = i(φ*(∂μφ) - φ(∂μφ*))

is conjugate to Aμ. The interaction between the charge current and photons is jμAμ

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#### PrashantGokaraju

The current jμ obeys the conservation law

d*j = 0

#### vanhees71

Gold Member
It is because of how gauge theory is understood as a part of quantum mechanics. The conserved current

jμ = i(φ*(∂μφ) - φ(∂μφ*))

is conjugate to Aμ. The interaction between the charge current and photons is jμAμ
Well, usually you use minimal substitution, i.e., you make $\partial_{\mu} \rightarrow \partial_{\mu} + \mathrm{i} q A_{\mu}$. In the case of scalar relativistic QED or non-relativistic QM you get an additional term, leading to a two-particle-two-photon vertex.

#### Khashishi

Photons are quanta of electromagnetic waves, so they interact with matter the same way as oscillating electric and magnetic fields.

"Why do photons only interact with charged particles?"

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