Do anti-photons exist?

Question

I know what anti-matter is and how when it collides with matter both are annihilated. However, what about anti-photons? Are there such things as anti-photons?

I initially thought the idea preposterous. However I am curious because, if anti-photons don't exist, then anti-matter could theoretically transfer its energy to normal matter - through the mechanism of light. Is it right?

score 49 · Accepted Answer · answered Aug 17 '11 at 07:05

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Well, they do and don't. Depends on your point of view. Here's the story.

Quantum field theory requires for consistency reasons that every charged particle has its antiparticle. It also tells you what properties will the anti-particle have: it will have the same characteristic from the point of view of space-time (i.e. Poincaré group) which means equal mass and spin. And it will have all charges of opposite sign than a matter particle.

If the particle is not charged then QFT doesn't impose any other constraint and so you don't need antiparticles for photons (since they are not charged). But you can still consider the same operation of keeping mass and spin and swapping charges and since this does nothing to photon, you can decide to identify it with an antiphoton. Your call.

answered Aug 17 '11 at 07:05

Marek

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There are uncharged Antineutrons – Helder Velez Aug 17 '11 at 10:21
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I would say that QFT requires that every particle has an antiparticle, but that some uncharged particles (like photons) can be their own antiparticles. – Peter Shor Aug 17 '11 at 11:06
@Peter: I wouldn't. The need for antiparticles only arises since the vacuum needs to be charge-neutral and so every time a charged particle is created one also needs to create minus charged antiparticle. So no charges, no antiparticles required. You would only be giving new (anti)names to all old objects... – Marek Aug 17 '11 at 11:17
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Doesn't the need for antiparticles also come from the CPT symmetry? I guess maybe that's the same thing you're saying, but from a mathematician's point of view. – Peter Shor Aug 17 '11 at 11:25
@Peter: I don't follow. What has CPT to do with any of this? – Marek Aug 17 '11 at 12:11
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The CPT symmetry turns particles into antiparticles. This is equivalent to saying that, in a Feynman diagram, a particle going forward is the same thing as an antiparticle going backward, which is close to what you said. – Peter Shor Aug 17 '11 at 12:13
@Peter: uh, no. What I said is derived purely from charge-invariance of the theory (either annihilation of the vacuum by the charge operators or invariance of the Hamiltonian w.r.t. associated symmetry). P and T have nothing to do with this. Even if CPT were not the symmetry of any QFT, antiparticles would still be required for charged particles. And the reason this contains non-trivial information is because the theory predicts new quantum numbers that you didn't start with (contrary to the chargeless case). But you can surely call photons antiphotons if it makes you happy :) – Marek Aug 17 '11 at 12:44
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@Helder: Antineutrons have baryon charge -1 (corresponding to a $U(1)$ baryon symmetry). One should understand charges here more generally than just electrical ones. After all, properties of arbitrary QFT (such as Yang-Mills SU(N)) surely can't depend on electromagnetism but will depend on the corresponding charges of the theory (for Y-M the color charges). – Marek Aug 17 '11 at 20:02
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More simply: Neutrons are made of quarks. An antineutron is made of antiquarks, which do have the opposite charge. – Aug 17 '11 at 20:26
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@Marek I'm so sure that you will have the same opinion about EM capabilities after reading the Douglas Pinnow book 'Our Resonant Universe' . I posted glimpse about it, here. – Helder Velez Aug 17 '11 at 21:56
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If a photon is its own anti-particle, why doesn't it annihilate with itself (or another photon) and if it did, what would that produce? – Michael Feb 27 '14 at 15:55
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@Michael: because photons are their own antiparticle photon number is not conserved. Photons can just transfer their energy and momentum to something else and disappear. Compare this to leptons and quarks where lepton number and baryon number are conserved. – John Rennie Jun 10 '14 at 19:30
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@Michael: photons do annihilate themselves. Reactions like $\gamma + \gamma \to e^+ + e^-$ exists. Now it's true that you won't find a single vertex with 2 photons because a process involving a photon must be related to a flux (current in QFT jargon) of electric charges. With 2 (neutral) photons, such flux is impossible. – Paganini Jan 18 '15 at 17:46
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Hmmm ... I'm thinking that the loop level contributions (which are leading order for reactions like those @Paganini discusses) would be different if it made sense to identify the anti-photon as in any way distinct (because you would need extra diagrams to account for loops involving anti- rather than plain photons). – dmckee --- ex-moderator kitten Sep 11 '17 at 17:38
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If youre going to just say “charged particle” in a answer in a physics forum and dont mean electrical charge, that should be said explicitly. Because 99.9% of the incidences of “charged particle” mean electrical charge; the phrase appears on countless electromagnetism problems. And for centuries has meant only that. Especially after people display that very confusion in the comments and you say it explicitly (only in the comments not the answer). People running around saying anything without an electric charge has no antiparticle. Two pointed me here. – Al Brown Jul 28 '21 at 17:07

score 18 · Answer 2 · answered Apr 07 '15 at 11:48

The short answer to "are there anti-photons" is "yes", but the disappointment here is that anti-photons and photons are the same particles. Some particles are their own antiparticles, notably the force carriers like photons, the Z boson, and gluons, which mediate the electromagnetic force, the weak nuclear force, and the strong force, respectively. Particles that are their own antiparticles must be electrically neutral, because an aniparticle has the opposite electrical charge as its partner particle. Other things must also be zero, like the number of quarks. A neutron cannot be its own antiparticle because it is made up of quarks and an antineutron is made up of antiquarks. A $\pi_0$ is made up of a quark and an antiquark and is in fact its own antiparticle also.

You can find lots out about particles at the particle adventure(http://particleadventure.orghttp://pdg.lbl.gov), part of the Particle Data Group's web site( ).

what about the force carrier W bosons, they are not neutral. So the one (W-) is antiparticle of other (W+) ? — 23rduser, Sep 13 '19 at 16:59
This is by far the best of the three answers. It’s too bad Dale picked the other confusing answer. — Al Brown, Jul 28 '21 at 17:02

score 0 · Answer 3 · answered Jun 28 '23 at 01:57

Photons are simply their own antiparticles, but aren't called antphotons.

A subatomic particle that is its own antiparticle is called a "Truly neutral particle". It remains itself under the charge conjugation. All charges: electric, magnetic, color, flavor etc, must be zero.

Positronium, the bound state of an electron and a positron (e−, e+) is also truly neutral, so two photons colliding (two photon physics) can produce a positron and an electron. Or other composite particles that are truly neutral, like a proton antiproton, which requires a lot of energy. (I believe particle accelerators can't provide, but it happens in cosmic rays.)

Remember though that photons do not really collide (at least this is extremely rare). Pair production is a little more complicated than that and happens near a nucleus.

Although the photons behave almost as if real, they are technically virtual.

score 0 · Answer 4 · answered Jun 28 '23 at 02:28

Physics question from University Illinois. Is that anti-photons and photons are the same particles. Some particles are their own antiparticles, notably the force carriers like photons, the Z boson, and gluons. As they have electrically neutral charge. Where as other characteristics like spin and such...

https://van.physics.illinois.edu/ask/listing/1153

score -13 · Answer 5 · answered Feb 13 '15 at 19:31

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It is my understanding that antiparticles are the opposites of each other such as electron positron pair -- which means the positron is an electron flipped 180 degrees on its axis --same angular velocity, same size but PHOTONS are not particles, they are electromagnetic waves which rotate 360 deg for every wavelength so in effect are "positive" 50% of time and "negative" 50% of the time as they move forward so each "photon" impulse is its own anti"impulse" half the time

answered Feb 13 '15 at 19:31

Dew1

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I don't see anything right here. An antiparticle is a particle with the same mass, but different charge as a particle (hence the $e^-$ and $e^+$). Photons also don't have a charge (they're neutral), so it can't be charged as you suggest. – Kyle Kanos Feb 13 '15 at 20:04
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Your claim about spin is wrong. Whether the particle is matter or antimatter does not affect its spin "direction." Also, photons are particles - they're the particle equivalent of electromagnetic waves. And elementary particles are point particles, so there's no point in comparing sizes. – HDE 226868 Feb 13 '15 at 20:14
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"photons are not particles"... Don't tell that to the Standard Model... – Sean Apr 07 '15 at 11:54

Do anti-photons exist?

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