Since the big news from CERN last Wednesday, a few people have asked me what this whole Higgs business is really about. They’ve heard that the Higgs field (or is it boson?) gives everything mass, and they’ve heard that mass is needed to slow us down, because otherwise we’d all be whizzing about at the speed of light – but a somewhat more thorough explanation is in order.
Quite a few people have also seen explanations on the net, including this one from Ian Sample at the Guardian. It’s a nice exposition of Higgs theory and is readily accessible, but I fear it perpetuates a couple of misconceptions that could do with clearing up…
Mass is not drag
In 1993 William Waldegrave, then a science minister in John Major’s government, set a challenge to come up with the best pedagogical explanation for the workings of the Higgs. David Miller, of UCL, came up with the winning suggestion, along roughly these lines:
Imagine a room full of people, milling about, talking to each other. Margaret Thatcher enters the room [this was 1993, remember]. As she tries to traverse the crowd, she finds an enormous number of lackeys and lickspittles scrambling around her, impeding her progress and making it difficult for her to stop or change direction. A junior minister enters and has a similar, but much smaller effect. In this elaborate analogy, the crowd is the Higgs field and the former Prime Minister is a particle whose coupling to the field is represented by her celebrity: the more famous you are, the greater the crowd you draw in and the larger the momentum you develop. In this way, interaction with the Higgs field confers mass.
It is a fantastic explanation, intuitively understandable. From it we get many variations on the same theme, not to mention deviations, all of which imagine the Higgs field as some kind of viscous soup, molasses or (as in Dr Sample’s example) a tray of sugar, which exerts a drag effect on everything which flows through it, spoiling their dreams of ultimate light speed travel. There’s just one problem: mass is not drag. There is no equation in particle physics that looks like a drag equation and nor should there be; drag is explicitly a phenomenon of motion. A particle at rest feels no drag, no matter how sticky the gloop in which it rests, but it certainly does have mass. This concept of mass needs some adjustment…
So what is mass?
The key to understanding this comes from what is unquestionably the most (possibly only) famous equation in the world:
What this equation tells us is that mass (m) is a form of energy (E) – the c2 is just a factor that we include to give us the right units. A particle can have energy for various reasons, be it through motion or through its interaction with some field. What Einstein discovered is that there is another form of energy that is intrinsically associated with any particle and is even present when the particle is not moving. It is this energy that we call “mass” (or “rest energy”). The implication of this is that anything that does not have mass can never be brought to rest. Thus it is for photons, the particles that make up a beam of light: they are massless and condemned forever to roam this world, like the Flying Dutchman, at the speed of light, never coming to rest.
Why does the Higgs field give things mass? What is the Higgs field anyway?
There are various fields that exist in nature, from gravitational fields to magnetic ones. The Higgs field is similar to these (though not quite the same, for reasons I won’t explore). The chief difference that should interest us is that, while magnetic fields are caused by magnets (and gravitational fields by heavy objects), the Higgs needs no source and crucially it is in fact constant throughout space (gravity is only appreciable near massive objects; it diminishes with distance). In the case of electric fields, it is only electrically charged objects which feel them; electrically neutral objects do not. Similarly, only certain particles are aware of the Higgs field – they carry a kind of “Higgs charge”. It takes energy to put an electrically charged object in an electric field and for similar reasons there is an energy associated with a particle which finds itself in a non-zero Higgs field. Now we understand how this gives a particle mass! Because the Higgs field is constant and non-zero everywhere, every particle which feels the field will always have an intrinsic energy associated with it, regardless of where it is or how fast it is moving. In other words, it has mass.
So that’s the Higgs field – what’s the “boson”?
In Ian Sample’s video explanation, he uses a tray of sugar to represent the Higgs field and claims that the bosons are represented by the individual sugar grains, as if the Higgs field comprises a vast cloud of individual bosons. This really isn’t right and in fact the truth is a great deal stranger – one that almost every current exposition of this topic neatly avoids.
Most educated people know that this world is made of atoms, and atoms of electrons and a nucleus, and the nucleus of protons and neutrons, and protons and neutrons of quarks (and beyond, maybe it’s turtles all the way down…). So ultimately, this world is constructed, at its most basic level, from particles, right? Well, not in the modern field theory view. In this view, the fundamental ontological category is the field. We’ve already encountered that in the form of electric, magnetic, Higgs fields and more, but on exactly the same footing are things we normally consider to have pretty solid particle properties – electrons, protons etc. Fundamentally, they are fields (and from this descend their quantum wave-like behaviour). What appear to us to be particles are in fact ripples in that field. For example, when we say that photons are particles of light, we mean that they are ripples in an underlying electromagnetic field (I have elided a very important detail that shouldn’t concern us here). An electron is a ripple in an underlying “electron field”. So what is a Higgs boson? It is a ripple in the Higgs field (the word “boson” indicates that it is more like the force fields – electric, gravitational etc – than electrons and protons, which are collectively called “fermions”).
Hopefully that’s given a slightly better picture of what’s going on in particle theory. Probably it hasn’t, for which I can only apologise. Giving the right explanation is not always compatible with being easily understood, but I think it is important. At some point, I’d like to come back and add a little more, explaining just why the Higgs is interesting, or even necessary.