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for massless particles is therefore the direct route – there is simply no clock associated with any other route. This is exactly what we would expect: it means that we can use the hopping rule for massless particles when the particle is massless. However, for particles with non-zero mass, kinks are allowed, although if the particle is very light then the shrinking factor imposes a severe penalty on paths with many kinks. The most likely paths are therefore those with very few kinks. Conversely, heavy particles do not get penalized much when they kink, and so they tend to be described by paths with lots of zig-zagging. This seems to suggest that heavy particles really ought to be thought of as massless particles that zigzag their way from A to B. The amount of zig-zagging is what we identify as ‘mass’.

This is all rather nice, for we have a new way to think about massive particles. Figure 11.5 illustrates the propagation from A to B of three different particles of increasing mass. In each case, the rule associated with each ‘zig’ or ‘zag’ of the path is the same as that for a massless particle, and for every kink we are to pay a ‘the clock must be shrunk’ penalty. We should not get overly excited yet because we have not really explained anything fundamental. All we have done is to replace the word ‘mass’ with the words ‘tendency to zig-zag’. We are allowed to do this because they are mathematically equivalent descriptions of the propagation of a massive particle. But even so, it feels like an interesting thing and, as we shall now discover, it may turn out to be rather more than just a mathematical curiosity.

We are now going to move into the realm of speculation – although by the time you read this book the theory we are about to outline may have been verified. The LHC is currently busy colliding protons together with a combined energy of 7 TeV. ‘TeV’ stands for Tera electron volts, which corresponds to the amount of energy an electron would have if it were accelerated through a potential difference of 7 million million volts. To get a sense of how much energy this is, it’s roughly the energy that subatomic particles would have had about a trillionth of a second after the Big Bang and it is enough energy to conjure out of thin air a mass equal to 7,000 protons (via Einstein’s E = mc2). And this is only half the design energy; if needed, the LHC has more gas in the tank.

Figure 11.5. Particles of increasing mass propagating from A to B. The more massive a particle is the more it zig-zags.

One of the primary reasons that eighty-five countries around the world have come together to build and operate this vast, audacious experiment is to hunt for the mechanism that is responsible for generating the masses of the fundamental particles. The most widely accepted theory for the origin of mass works by providing an explanation for the zig-zagging: it posits a new fundamental particle that the other particles ‘bump into’ on their way through the Universe.

That particle is the Higgs boson. According to the Standard Model, without a Higgs the fundamental particles would hop from place to place without any zig-zagging and the Universe would be a very different place. But if we fill empty space with Higgs particles then they can act to deflect particles, making them zig-zag and, as we have just learnt, that leads to the emergence of ‘mass’. It is rather like trying to walk through a crowded pub – one gets buffeted from side-to-side and ends up taking a zig-zag path towards the bar.

The Higgs mechanism is named after Edinburgh theorist Peter Higgs and it was introduced into particle physics in 1964. The idea was obviously very ripe because several people came up with the idea at the same time – Higgs of course, and also Robert Brout and François Englert working in Brussels and Gerald Guralnik, Carl Hagan and Tom Kibble in London. Their work was itself built on the earlier efforts of many others, including Heisenberg, Yoichiro Nambu, Jeffrey Goldstone, Philip Anderson and Weinberg. The full realization of the idea, for which Sheldon