By now, the world and his wife knows that the physicists at CERN have discovered the Higgs: the missing piece in the Standard Model, the Holy Grail, the particle at the end of the universe, unobtainium etc. Of course you’ve all read my explanation of what the Higgs is, but what the quantum hickery-jiggery is the Standard Model? To explain it all, let’s go back to the atom…
You all know that everything we can see in this world – you, me, my glass of wine, this rickety old computer – is constructed from atoms, don’t you? Of course you do. And almost all the mass of said atoms is concentrated in a small central nucleus (with positive electric charge), orbited by negatively-charged electrons (and “small” is an understatement – the nucleus forms such a small part of the atom that if you scaled it up to the size of a football and plonked it in Trafalgar Square, the electrons would be orbiting around the M25, twenty miles away). The nucleus itself is a jumble of two types of particles called protons and neutrons. Add to that the neutrino, a shy little particle produced in radioactive decay, and you have the complete family of elementary particles, which together form the building blocks of all matter as we know it. In the early Thirties, this was the whole picture – until 1936, when Carl Anderson found a new particle wandering through his experiment – the muon. This discovery was a great success for particle theory, having been predicted by Hideki Yukawa a few years earlier… until it turned out not to be the predicted particle at all, and was in fact some weird intruder into the peaceful particle homestead that had so far prevailed.
“Who ordered that?” – I I Rabi, upon learning of the muon’s true identity
The muon was just the start of the trouble. Soon there were all sorts of new particles turning up where they weren’t wanted – with names like pions, deltas, lambdas and rhos, physicists soon ran out Greek letters to use and fingers to count on. The multiplicity of this “particle zoo” was something of a headache. Electrons, neutrons and protons were meant to be elementary particles, the building blocks of all matter. Could all these different particles really be elementary, or might it be possible to construct a simpler model, where many of these particles were in fact composite, built from bricks even smaller than the proton and electron? By the mid-Seventies, the quark model had taken shape: protons and neutrons were replaced on the elementary particle podium by lighter particles called quarks. Protons consist of two up quarks and one down quark; neutrons consist of one up quark and two downs (note: this is a huge simplification). Various different configurations produce many of the other composite particles, with the addition of extra quarks, called strange and charm. The electron, on the other hand, retains its proud status as fundamental.
Full disclosure time: no-one’s ever seen a naked quark. They’re always found with a chaperone or two to protect their modesty, by which I mean they’re always found in combinations which hide a property they possess called colour (more on this later – to be clear, this property has very little to do with actual colour, since we can never see these particles). Quarks are either found dancing with an anti-quark (a combination called a meson), or with two other quarks, in a combination called a baryon, of which protons and neutrons are examples (the proliferation of bizarre names has been a feature of particle physics since its inception, and we’re not finished yet – it helps if you know your Greek). This inability to separate a quark from its associates is called confinement, and something that’s not properly understood to this day. So if we’ve never directly detected an isolated quark, how do we know they exist? Consider a bag of marbles that can’t be opened. We may not be able to pull a marble out to have a look at it, but we can be sure they’re there if we can feel them through the fabric and hear them clink around inside. Various experiments probing the interior of protons give us similar confidence in the existence of quarks.
So what of the muon, the earlier impostor? It became clear that this particle appeared to be all but identical to the electron, just two hundred times heavier. Along with it came a new neutrino, appropriately named the muon neutrino (all grouped together into a class of particles called leptons). This is an example of a highly salient but somewhat mysterious feature of the Standard Model of particle physics: up quarks, down quarks, electrons and electron neutrinos can be grouped together into a “generation” which seem to suffice for theoretical purposes. Nevertheless, they come accompanied by two other copies each, so that there are in total three generations of matter particles (or fermions, to give them their proper name). The full cast list, with all their silly names, is:
Up Down Electron Electron Neutrino
Charm Strange Muon Muon Neutrino
Top Bottom Tau lepton Tau neutrino
This is the roll call of matter particles thus far known to man. Are there more generations waiting around the corner, not yet found? The recent discovery of the Higgs would suggest (with various caveats) not.
So far I’ve left something pretty important out – how do these particles affect each other? A theory of non-interacting particles (a “free” theory) is not particularly interesting; interactions are necessary if this universe is to be more than a thin gas of solipsistic ghosts, oblivious to each other’s presence like strangers in a crowd. In the quantum world, “interacting” means that a particle can randomly transmogrify into two or more new particles – an electron can, for example, spontaneously turn into an electron and a photon (a photon being a particle of light). You can of course view this event as simply one electron emitting a photon. The reverse process also happens: an electron can absorb a photon. Put the two together and you can have one electron casting off a photon, which then gets absorbed by some other nearby electron. The photon carries energy and momentum away from the first electron and deposits it with the second. Imagine standing on an ice rink, throwing a heavy medicine ball to a friend. You will move backwards, away from your friend; your friend will in turn move away from you as soon as they are hit by the ball, absorbing its momentum. The transfer of momentum in this way is exactly what happens when one object exerts a force on another – particle exchange is the microscopic origin of force.
In other words, when you bring two magnets close together, the force you feel (be it attractive or repulsive) is the result of all the electrons in one magnet exchanging a stream of photons with the electrons in the other magnet. This may raise a question – if photons are particles of light, why do we not see a lightstorm firing between the magnets? The reason we see nothing is that the photons are not in fact real particles, but ones called virtual photons. In my first discussion of the Higgs, I noted that particles should be considered to be ripples in an underlying field from which descend all the particle’s properties. A virtual particle is one that can never actually be measured, because it’s not really a proper particle at all. It is an irregular disturbance that crashes and dissipates before it can be observed, breaking up into or colliding with new (real) particles. With this in mind, you should take the previous paragraph’s talk of “particle exchange” with a pinch of salt; it’s more pedagogical than physical.
Virtual particles generally violate some pretty sacred laws of physics, like conservation of energy, so it’s a good thing we can never observe them. It’s not even a problem if we *know* such transgressions occur behind our back; quantum theory revels in allowing the impossible to occur while our eyes are closed. A quantum prisoner could escape from any prison just so long as we don’t try to watch him pick the lock or scale the walls.
That, in a nutshell, is how electromagnetism works at the particle level (actually the real kernel of quantum electrodynamics is the beautiful mathematical elegance of gauge theory, but I’ll have to save that for those of a stronger constitution). It is a fantastically well-understood theory, one that underlies almost all the phenomena and interactions of everyday physics (with gravity taking care of the rest). It’s not the whole story though: investigations in the early twentieth century revealed two new forces that lacked an explanation. The strong nuclear force provides an attractive force between protons and neutrons which keeps nuclei bound together – until they decay (or at least, the unstable ones do), for which we need another interaction, the weak nuclear force. Having had such success with electromagnetism, physicists were keen to apply similar principles to understand the nuclear forces, despite their obvious differences – electromagnetic fields have infinite range, while nuclear forces do not extend beyond the nucleus itself.
Eventually, these efforts would be met with enormous success and the short range of the forces explained in rather different ways. By the mid-seventies, a theory known as quantum chromodynamics became accepted as the best model of strong interactions. Only the quarks experience the interaction, as they have a property called colour charge (mentioned earlier). This is exactly analogous to electric charge, except that there are three different colours: red, blue and green (and the analogue of the photon is called the gluon). Protons and neutrons have three quarks inside them, each with a different colour – the combination of all three colours actually has no colour at all. The same goes for mesons, which consist of a quark and an anti-quark (again, colourless as a whole). Confinement, as mentioned above, is the phenomenon that coloured objects are never observed. The attraction between protons and neutrons is sometimes known as the residual strong interaction, as it can be thought of coming from the strong force being smuggled out of protons in colourless particles called pions (these are the particles originally predicted by Yukawa in 1935 and eventually discovered in 1947, earning Yukawa the Nobel prize). The reason this force doesn’t escape the nucleus is that pions have mass (unlike the photon, and indeed the gluon) – the consequence of this is that they only have limited time and space to pull the quantum sleight-of-hand needed for virtual particles to do their undercover work.
The approach to the weak force, on the other hand, gives mass directly to the force particles themselves, known as W and Z bosons. So there we go: weak interactions are just like electromagnetic interactions, except that the force-carrier particles are massive, which leads to a small range for the force, right? Unfortunately not, because it turns out this is strictly verboten in quantum theory. W and Z bosons (as well as photons and gluons) belong to a class of particles called vector bosons. Give them mass and all kinds of shit goes wrong – quantum theory screams, gives up and drops to the floor in a huff. This was obviously considered something of a problem for a while, until some physicists came up with a solution – everybody’s favourite, the Higgs!
And that’s it. The force particles needed to fill out the Standard Model: the photon, gluon, W and Z bosons. The Higgs completes the set; predicted almost fifty years ago and finally making a coy, teasing appearance in experiments.
The Standard Model was completed in the Seventies and has met every test and probing we have devised, earning numerous Nobel prizes for its inventors and discoverers along the way. We’ve kicked it and pushed it, yet it refuses to yield. It’s bloody brilliant. And frankly, a little boring. If we’re honest, the Standard Model has had it all its own way for too long. Now, however, we have a new weapon: the Large Hadron Collider. It’s a big, beautiful, physics-busting behemoth of a machine and it’s our best hope in the quest to kick the Standard Model in the knackers. Certainly, we know the theory is not invincible. As it stands, it doesn’t quite make sense in all its details, and it fails to explain some pretty big questions currently bugging fundamental physics. I’ll outline the main challenges in an upcoming post, along with the various ways we might hope to solve them.