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Comment by chton | Hacker News Reader

There is more to being antimatter than just having the opposite charge. The spin of the particle also matters. For a particle to be its own antiparticle, it would have to have spin 1/2. All elementary fermions have that property, but not much else.

Of the 2 classes, fermions and bosons, only fermions can be their own antiparticles. Bosons are defined with having an integer spin, so they can never have spin 1/2. Of the fermions, none are known with neutral charge except for neutrinos, and we're not sure if those are Majorana particles or not.

Photons, as you mention, are bosons, with spin 1, so they can't be their own antiparticle.

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There is no rule requiring antiparticles to also have spin 1/2, nor that they in general be n+1/2 spin particles (fermions).

All lepton/quarks observed have half half-integer spin so there are no examples there. The SUSY sleptons/squarks would have anti-particles but integer spin if they exist, though. The W+/W- have spin 1 and are eachother's antiparticle. Z0 and the photon are their own anti particles, also spin 1. Gluons (spin 1) have anti-particles that are all another types of gluon. For composite particles, anti-deuterium and anti-helium both have integer spin.

For what anti-particles actually are, I suggest looking up both C and CP conjugation.

> For a particle to be its own antiparticle, it would have to have spin 1/2

This is wrong. Being anti-particle has nothing to do with a particular spin. Photon is anti-photon (which by the way has spin 1).

Photons are generally considered to be their own antiparticles: http://van.physics.illinois.edu/qa/listing.php?id=27107

Photons are something of a special case because they are massless. Gravitons, too. As bosons, their interactions are not limited by the Pauli exclusion principle, so they can not annihilate each other. They interact through different means (electromagnetic). They're both because they only have the common properties of particles and their antiparticles.

It's basically like saying "the number 0 is its own negative number". It's correct according to some definitions, but not useful.

Physicist here. You are confusing things. Having rest mass or Pauli-exclusion principle has nothing to do with qualification of being an anti-particle.

Z boson, for instance, does have mass and is its own anti-particle.

> It's basically like saying "the number 0 is its own negative number". It's correct according to some definitions, but not useful.

Photons have zero charge; an anti-particle has negative of the particle's charge (and at the same time, same rest mass and spin).

Aha, thanks for the correction, I must have picked up some bad info somewhere. I'll do my research better next time.

I'd say that zero being it's own negative number is an important thing not to forget. :)

Neutrons are fermions with neutral charge.

Exactly, and they also have spin 1/2, but we're unsure of their other properties that might make them Majorana particles.

They are composite particles and we know exactly how they behave: like fermions. They are not Majorana fermions, and nothing about them is mystery.

I explained how you're totally confusing things somewhere else in the thread.

My brain added an 'I' in the middle of "neutron" in the parent comment. Probably because I was in the middle of reading something else about neutrinos :) I definitely agree that neutrons are familiar territory.

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