It’s my 50th blog so it’s only fitting I devote it all to an aspect of Physics I find particularly engrossing – elementary particles and quantum field theory. Maybe it’ll get you freshers geared up for the first year of your undergraduate degree!
I am currently attempting to get to grips with my fourth year project this month and am fully immersed in PhD research. A lot of what I have been reading has been covered in the ‘Elementary particles’ course I took last year, led by Professor Tim Hinton. The course covers topics ranging from the elementary particles themselves to beyond the Standard Model. I thought I would give you a brief introduction to the subject to give you a taste of what a third year course is like.
Anyone taking the course would have completed an earlier compulsory module in ‘Relativity and particles’ so would be familiar with the groupings of particles and would understand terms like bosons, mesons and leptons. Let’s pretend we forgot absolutely everything and are sitting in the lecture hall looking a little nonplussed as Professor Hinton begins to outline the course material. Hang on, what’s a meson again?? And where does the proton, neutron and electron fit in all this?
GCSE chemistry will have taught you the constituents of the atom: the nucleus -containing both protons and neutrons- and the orbiting electrons. These particles all have mass (even if the electron mass is absolutely miniscule) and are all relatively stable – at this point the lifetime of a proton is thought to be around 1033 years! Along with these massive particles you know of at least one other with no mass at all – the good old photon. Perhaps you have also heard of the neutrino, which also has no mass. Those of you really on the ball will also know about quarks. I’ll get to those later.
Watching the news and reading your science magazines you will know this list of elementary particles has grown considerably via particle accelerators. Scientists can also study cosmic rays which reach energies 100 times that of the LHC! Under these conditions particles with extremely short lifetimes can be produced. These particles fall into categories, which is where those mesons, leptons and baryons come into it.
Let’s start with the hadrons. These contain quarks and both mesons and baryons are grouped in this category. Mesons contain one quark and something called an anti-quark. Baryons on the other hand contain three quarks and two examples are the proton and the neutron. There are a large number of hadrons, with names like ‘lambda’, ‘sigma’ and ‘eta’ – noticing a theme?
Another type of particle is a lepton. Leptons are sometimes called ‘light particles’ and do not contain quarks. The six leptons are: the electron, the electron neutrino, the muon, the muon neutrino and the tau along with the tau neutrino. Leptons are classed as fermions, along with the individual quarks. This is down to a property known as spin, which is the intrinsic angular momentum of particles. Fermions are known as spin-1/2 particles since their spins are all 1/2, 3/2, 5/2 and so on. Baryons are also fermionic, however mesons are known as bosonic since they have integer spins of 1,2,3 etc.
The most fascinating part of the theory lies with the interactions of all these particles. Of the four fundamental forces you will be very familiar with two; gravity and electromagnetism. These two forces have a long range so we can experience them in our macroscopic world. The two other forces are the weak force and the strong force. The strong force has an absolutely tiny range of approximately 10-13cm, 100 000 times smaller than an atom! Therefore is very important in the binding of tiny atomic nucleus. The weak force is over 100 times smaller than even the strong force and only plays a role in processes that cannot occur due to any of the other forces. Hadrons participate in strong interactions while leptons do not.
The forces are an interesting area of study; however the really good bit comes in the form of quantum field theory. I am currently reading a book called “Grand Unification Theories” by Graham G. Ross which explains the idea behind quantum fields quite nicely. Imagine a particle packed in at all sides by neighbouring particles, Ross states:
“If a particle is suddenly pushed this cannot produce an instantaneous change in the forces acting on a neighbouring particle, because no signal can travel faster than the speed of light. To maintain conservation of energy and momentum at every instant, it is suggested that the pushed particle produced a field carrying energy and momentum, some of which is eventually given to the neighbouring particle.” Chapter 1, page 4.
A field of energy and momentum. Now those of you studying A-level Physics may be a little troubled by this. You will have studied Einstein’s Photoelectric Effect along with the Bohr atom and you will know energy actually occurs in small packets in accordance with quantum mechanics. And that’s where this theory gets really interesting. Each force is carried by yet more particles called exchange particles.
These force-carrier particles are all bosons (hence they have the integer spin of 1,2,3…). The strong force is mediated by the gluon while the electromagnetic force is mediated by the photon and the gravitational force is exchanged via the graviton. The greater the mass of the exchange particle the smaller the range of the force hence the force-carrier particles for the weak force are massive since the force has such a tiny range. The weak force exchange particles are the W and Z bosons.
It is quantum field theory that allows us to predict if a process occurs in nature, since each interaction is limited to occurring only when certain conditions are met, known as symmetries of a local gauge theory. The simplicity of the theory is inviting and many scientists, including Einstein, have worked on creating a ‘unified’ field theory, where each force is a different facet of the same field. The electromagnetic and the weak interactions have already been united into a single unified ‘electroweak’ theory.
A question that puzzled the scientific community is the fact the W and Z bosons in the electroweak theory have a mass while the photon does not. The basic equations behind the theory indicate all the exchange particles of the electroweak force should be mass-less. So what gives the bosons their mass? The answer came in 1964 in the form of the Higgs field. As with all the fundamental fields we’ve talked about, the Higgs field has an exchange particle called the Higgs boson, something I’m sure you have heard off. Particles that interact with this field obtain a mass, only not all particles do. Click here for an award-winning explanation of this process.
Now I am going to stop here before this blog becomes a small essay! Though I think that particular boat may have already sailed…
Though I will say this (someone stop me typing): you will notice gravity does not appear to play a big part in quantum field theory. That is because it has not been resolved at a quantum level, and the theory that may hold the answer- you guessed it – String Theory (and that’s a whole other blog).