Reading for Sep 19

Hi everyone!

Next week on September 19th we will be talking about particle physics, a.k.a. subatomic physics. During our two-hour discussion, we will talk about the building blocks of subatomic particles together with the forces that act on them. The fancy physics name for all of this is the Standard Model of Particle Physics. It’s just like the Periodic Table for Chemistry, except that it applies to subatomic physics instead.

Please read the following before coming to class:-

1. The following “plain-language summary” of next week’s seminar material.
2. Chapter 1 of Greene. The most important parts are in pages 7-14.

Week 2 Overview

This week is all about particle physics. We will begin by discussing how physicists tell particles apart. This is important in the same way that being able to tell your kids apart is important! A clever idea is to pick invariants – properties of the subatomic particle that remain unchanged regardless of changes in perspective (like speeding up or rotating around). It’s like recognizing your friend Sam by the shape of his face: you can recognize him at all kinds of different angles and when he’s running or sitting still, but he’s still the same Sam.

The three types of invariant used to classify subatomic particles are: the mass m, the spin s, and the force-charges q. Mass can be anything from zero to large, but spin is quantized, meaning that it can only take on some very specific values: integers or half-integers, measured in units of a physical constant named h-bar. The values force-charges can take depends on the force (and we will not have time to delve into this detail). An example of a force-charge is the electric charge.

Nature makes an important distinction between bosons, which have spin 0, 1, 2, 3, … in units of h-bar, and fermions, which have spin 1/2, 3/2, 5/2, 7/2, … in units of h-bar. Up at high temperatures when particles race around with a lot of average energy of motion (kinetic energy), bosons and fermions behave pretty much the same. But when you’re down at extremely low temperature, it matters hugely whether you’re a boson or a fermion.

The Pauli Exclusion Principle (PEP) is a really important property of fermions: it says that “no two fermions can be in the same quantum state at the same time”. In plain language, this means that fermions have elbows. You simply cannot crowd them on top of one another: if you try, it never works. This fact helps explain (among other things) why atoms with more protons in their nucleus are physically larger. Bosons have no such requirement.

Incidentally, the name “fermion” is a salute to the awesome but dead Italian physicist named Enrico Fermi. Similarly, the name “boson” is a salute to the awesome but dead Indian physicist Satyendra Nath Bose.

Matter (a.k.a. stuff) in particle physics is composed of fermions, while the force-transmitter particles are bosons. It’s a bit mind-bending to imagine how exchanging a particle can represent a force, so in class we will discuss the Ice Skater Analogy to help visualize it. You can see more about this analogy in the online notes which I will post soon.

There are four different forces known in Nature – gravity, electromagnetic, strong nuclear and weak nuclear. Gravity holds you on Earth and Earth in orbit around the Sun. The electromagnetic force describes electricity and magnetism – which you can see are connected if you watch a compass needle move around during an electrical storm! As we discussed at the end of our Sep 12 seminar, the strong nuclear force holds the atomic nucleus together. You can see the power of the strong force when you set off a nuclear bomb. The weak force is responsible for the kind of radioactivity that powers our Sun. So even though it’s weaker between two subatomic particles than the strong force, it’s still mighty.

For now we’ll just mention that the electromagnetic and the two nuclear forces have spin one messengers, while gravity has a spin two messenger particle. The Higgs boson (responsible for mass of quarks, leptons and the weak bosons) has spin zero. All of these particles have been seen and studied in particle accelerator laboratories.

Another couple of big words used by particle physicists are hadrons and leptons. These come from the Greek and basically mean “heavy little bugger” and “light little bugger”. (Well, not quite! ) Hadrons is the name particle physicists use for baryons and mesons, which are colourless subatomic particles made only out of quarks and gluons. All hadrons feel the strong, or colour, force. Leptons, on the other hand, are unaffected by the strong force. The lepton group is composed of electrons and their heavier cousins the muons and taus, plus their associated neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino. While leptons don’t feel the strong force, they do feel the weak nuclear force. Every particle with an electric charge feels the electromagnetic force. Every particle with energy – i.e. every subatomic particle known in Nature – feels gravity.

We will close by giving a lightning tour of the Large Hadron Collider (LHC) based at CERN near Geneva, Switzerland. The LHC and the Higgs boson recently discovered there will be the subject of your first essay. I will provide a set of reliable sources about the LHC and the Higgs boson to help you get started.