- Quote: The up quark’s antiparticle is the up anti-quark. Consequently, the up anti-quark’s antiparticle is the up quark. Similarly, the down quark’s antiparticle is the down anti-quark, and vice versa.
- Endnote: Notice that we do not say that “up quarks are particles and up anti-quarks are antiparticles”; that would be incorrect. They are each other’s antiparticles.
The semi-science-fictionalization of anti-matter has contributed to a huge amount of confusion concerning how anti-particles work. You’d think there was a obvious relation between the two, but actually it’s not straightforward.
I’ve mentioned in the book that the word “matter” is highly ambiguous. But in the context of anti-matter, the word matter means “anything made from atoms”… that is, any object made from electrons, protons and neutrons. “Anti-matter”, in that same context, means “anything made from anti-atoms”… those objects made from anti-electrons (i.e., positrons), anti-protons and anti-neutrons. Atoms are matter, anti-atoms are anti-matter. And light, meanwhile, is neither matter nor anti-matter
Because of this, scientists will sometimes go further refer to electrons, protons and neutrons as “matter”, and to positrons, anti-protons and anti-neutrons as anti-matter. [They may even extend this terminology to quarks as matter and anti-quarks as anti-matter, though often they are more careful and refer to these as “baryons” and “anti-baryons”, wisely leaving the word “matter” out of the discussion.]
But even when scientists do this, it doesn’t change the fact that it is completely wrong to say that “electrons are particles and positrons are anti-particles.” They are both particles, and are each other’s anti-particles. Here are six correct statements:
- electrons are particles
- positrons are particles
- photons are particles
- electrons are the anti-particles of positrons
- positrons are the anti-particles of electrons
- photons are their own anti-particles
The various quarks and anti-quarks are all particles, too, but up quarks are the anti-particles of up anti-quarks (and vice versa), top anti-quarks are the anti-particles of top quarks (and vice versa), and so forth. So it is with the particles known as W, Z and Higgs bosons; Z bosons are their own anti-particles, Higgs bosons are their own anti-particles, and positively charged W+ bosons are the anti-particles of negatively charged W- bosons (and vice versa). And so on.
(One of the big unanswered questions in particle physics concerns how this works for neutrinos; are they their own anti-particles, or are anti-neutrinos separate particles from neutrinos? See chapter 21, endnote 3.)
In short, a hydrogen anti-atom is an example of anti-matter, but no object is an anti-particle. A particle, seen as a member of a type, it can only be an anti-particle of some other type of particle.
Thus anti-matter is not made from anti-particles; it is made from particles, which just happen to be the anti-particles of those from which atoms are made.
The general rule, in any quantum field theory in a world with Einstein’s relativity and three spatial dimensions, is this:
- for every type of particle, there is another type of particle that is its anti-particle
- if two types of particles are each other’s anti-particles, then they must have exactly opposite charges (such as electric charge)
- if a type of particle has no charges of any kind, then it can potentially be its own anti-particle
The last is the case for photons, Z bosons and Higgs bosons, all of which are electrically neutral, and as I mentioned above, it might be true of neutrinos. But it’s not true of neutrons, because they carry a quark-number charge (also known as the “baryon number” I alluded to above): a neutron has three extra quarks, while an anti-neutron has three extra anti-quarks.
The nature of this rule is tied up with profound facts about relativity and quantum physics. I might extend this discussion further someday to explain more about where it comes from (the buzzword being “the CPT theorem” [or “CRT theorem”].)
