Sometimes, we don’t get to decide what our show’s about. So many threads come together at the same time driving the decision for us. This is one of those situations. We’ve gotten so many questions from listeners in just the last week about antimatter that our show had just been chosen for it. You command, we obey. Let’s talk about antimatter.

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Anti-neutrons? Should have mentioned that even though neutrons have no charge, there still exists anti-neutrons.

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Keeper of the Jabberwock

Well technically neutrons aren't fundamental particles anyway since they're made up of anti-quarks. That said, a particle doesn't have to have an electric charge to have anti-particle, a particle/anti-particle pair have opposite quantum numbers. A neutrino is neutral yet still has an anti-particle, in this case the quantum numbers are the lepton number and parity. Neutrinos have a +1 lepton number (and are all "left handed"), anti-neutrinos -1 lepton number (and are "right handed").

A photon however is its own anti-particle!

Another interesting point about anti-matter was that it was theorized before it was discovered. It just naturally comes out of combining Special Relativity with Quantum Mechanics, antimatter *must* exist for both theories to be correct.

Neutrinos were also predicted before being discovered. In that case, there needed to be a particle to conserve and carry away the lepton number in weak interactions.

BTW I just discovered the AstronomyCast, so I haven't had a chance to listen to them all yet, have they mentioned Supersymmetry yet? That's even cooler. Each fermion/boson would have a boson/fermion superpartner. Photoninos, winos, sleptons, neutralinos, all sorts of crazy names!

I would like to hear more about anti-neutrons in a followup show. Is it possible to sequester anti-neutrons in a deuterium atom? Or are the processes that hold atoms stable specific to one flavor of matter?

Oh, and while I'm thinking about it, would it be possible to have a particle with 1 1/3rd charge by combining quarks and anti-quarks?

And while we are on the subject of atomic nuclei, what effects do we expect to see of neutron stars as an environment where the strong and weak forces dominate over the electromagnetic force?

Kirk Job Sluder, Bloomington, IN

That's a good one, I'd never thought about that. I'm pretty sure that the quarks and anti-quarks in the proton and anti-neutron would find a way to annihilate. A proton is two ups and a down and an anti-neutron is two anti-downs and an anti-up, so what's left would be one up and one anti-down which would be some sort of meson--a pion if I remember my college physics correctly--and a bunch of energy. Mesons are particles that only consist of two quarks, a quark and an anti-quark pair.

I'm not sure about the strong force (i.e. the residual color force between quarks) between a proton and anti-neutron. Obviously mesons can form so there is an attraction, but I don't know how the residual strong force between a hadron and anti-hadron would work out.

Penta-quarks have been theorized but there's no evidence they actually exist at the moment.

They've been described as just really big atomic nuclei consisting entirely of neutrons (not quite true as they'd probably have a solid iron "atmosphere" and probably some kind of weird quark matter at the very center). One consequence of neutrons bound by the strong force is that they don't decay. Free neutrons actually have a half-life of 12 minutes before one of the down quarks decides to decay into an up via the weak force.

Hi Rick

I'm hoping you'll be around later in the year for the LHC... I've got a feeling we might have a few questions for you!

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

Or to put it another way, most of the popular science stuff I've read hasn't explained to me how we know that white dwarfs and neutron stars have the properties attributed to them, in contrast to black holes where there is often abundant explanation about density and the emission effects of infalling matter. So a show that talks about the continuum of density and how we infer that neutron stars are made of super-condensed neutrons would be very interesting to me. Neutron stars appear to suffer from the "middle child" syndrome of science writing.

Pretty much the only technicial explanation I've read was in the big old Gravitation book by Wheeler Thorne and Misner, in fact, I gave a presentation on it in grad school. There are these equations of state that describe how the density and pressure vary as a function of the radial distance from the center of the star. A white dwarf is modelled as a degenerate gas of electrons. Degenerate meaning that the electrons are squeezed as close together as they can possibly can, occupyng all the lowest energy states. This "Fermi pressure" is what keeps the star from collapsing any further. This state is fairly incompressible up to the point where reverse electron capture starts occuring and the electrons start interacting with the quarks in the nucleus via the weak nuclear force. An electron and an up quark get changed into a neutrino and down quark via a W particle exhange. Which basically turns all the protons into neutrons. At that point there's a different equation of state used for the neutron gas. I believe they're still a little unsure as to the exact state of matter in the core though.