How does the Higgs mechanism create mass?
We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs boson. Read the previous posts: “What is the Higgs boson?”, “Why is the Higgs boson called the ‘god particle’?”, and “Is the particle recently discovered at CERN’s LHC the Higgs boson?”
By Jim Baggott
Through thousands of years of speculative philosophy and hundreds of years of hard empirical science, we have tended to think of mass as an innate property (a ‘primary quality’) of material substance. We figured that, whatever they might be, the basic building blocks of matter would surely consist of microscopic lumps of some kind of ‘stuff’.
But this is not quite how it has worked out. There was a clue in the title of one of Albert Einstein’s most famous research papers, published in 1905: ‘Does the inertia of a body depend on its energy content?’ This was the paper in which Einstein suggested that there was a deep connection between mass and energy, through what would subsequently become the world’s most famous equation, E = mc2.
We experience the mass of an object as inertia (the object’s resistance to acceleration) and Einstein was suggesting that the latter is determined not by mass as a primary quality, but rather by the energy that the object contains.
So, when an otherwise massless particle travelling at the speed of light interacts with the Higgs field, it is slowed down. The field ‘drags’ on it, as though the particle were moving through molasses. In other words, the energy of the interaction is manifested as a resistance to acceleration. The particle acquires inertia, and we think of this inertia in terms of the particle’s ‘mass’.
In the Higgs mechanism, mass loses its status as a primary quality. It becomes secondary — the result of massless particles interacting with the Higgs field.
So, does the Higgs mechanism explain all mass? Including the mass of me, you, and all the objects in the visible universe? No, it doesn’t. To see why, let’s just take a quick look at the origin of the mass of the heavy paperweight that sits on my desk in front of me.
The paperweight is made of glass. It has a complex molecular structure consisting primarily of a network of silicon and oxygen atoms bonded together. Obviously, we can trace its mass to the protons and neutrons which account for 99% of the mass of every silicon and oxygen atom in this structure.
According to the standard model, protons and neutrons are made of quarks. So, we might be tempted to conclude that the mass of the paperweight resides in the masses of the quarks from which the protons and neutrons are composed. But we’d be wrong again. Although it’s quite difficult to determine precisely the masses of the quarks, they are substantially smaller and lighter than the protons and neutrons that they comprise. We would estimate that the masses of the quarks, derived through their interaction with the Higgs field, account for only about 1% of the mass of a proton, for example.
But if 99% of the mass of a proton is not to be found in its constituent quarks, then where is it? The answer is that the rest of the proton’s mass resides in the energy of the massless gluons — the carriers of the strong nuclear force — that pass between the quarks and bind them together inside the proton.
What the standard model of particle physics tells us is quite bizarre. There appear to be ultimate building blocks which do have characteristic physical properties, but mass isn’t really one of them. Instead of mass we have interactions between elementary particles that would otherwise be massless and the Higgs field. These interactions slow the particles down, giving rise to inertia which we interpret as mass. As these elementary particles combine, the energy of the massless force particles passing between them builds, adding greatly to the impression of solidity and substance.
Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog posts.
On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs. Read the previous posts: “What is the Higgs boson?”, “Why is the Higgs boson called the ‘god particle’?”, and “Is the particle recently discovered at CERN’s LHC the Higgs boson?”