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I’d been looking forward to Sean Carroll’s new book for months, and now it’s over. No matter, because I’m reading it again. It’s that good.
Carroll’s written a book that hits me pretty much exactly where I am in my understanding of particle physics and the Higgs boson. I feel like I have a much deeper understanding today than I did a week ago. He also has some great lines that make the whole pursuit of the Higgs not just an intellectual adventure but a deeply human story. Here Carroll describes the moment when the LHC’s beam was switched on for the first time.
“It was early morning Geneva time, but California is nine hours behind, so it was late the previous night for us. Computer monitors were set up for everyone to follow along, although the strain on CERN’s servers soon broke the Internet feed. Pizza was ordered and passed around, helping the assembled scientists settle into a comfort zone. (A substantial fraction of the atoms in the body of a typical physicist were once in the form of pizza.)”
I’m going to give Carroll’s description of the Higgs, or at any rate my understanding of his description, in outline form. Maybe it’ll help you understand, too. Probably it will just make things clearer for me. If you’re intrigued, read the book. It’s outstanding.
1) Everything is a field. The force fields are the easiest to understand, and of these I like to think of electromagnetism. Imagine holding a magnet in your palm. All around that magnet you can picture little numbers. Those numbers tell you the strength of the magnetic field at that particular point. You could do the same thing with a charged rod, except now it would be the electric field strength. Everywhere in the universe that magnet (or that charged rod) cause the field to have a number – very far away the number is indistinguishable from zero, while close up the number takes on a value that shows both how strong the source is and how near the source the point lied.
A little harder to picture is the idea that particles themselves are just disturbances in a field. To make it easy, thing of a “person field.” Everywhere there’s a person, the value of the field is 1. Everywhere there’s not a person, the person field is 0. This is not particularly useful for people, because we’re all different. For elementary particles, though, this idea is quite useful, because every electron is identical to every other electron. The electron field, then, shows where electrons are and are not.
Now the Higgs field. Not the Higgs particle; I’m actually not even going to talk about that in this entry. The Higgs field is what matters, as Carroll emphasizes again and again. The Higgs field is more like the electric or magnetic field than the electron field. It’s a value everywhere in space. (One difference is that electric and magnetic fields have a direction, too, but the Higgs field does not. For this reason, sometimes it’s called a scalar field.)
The biggest difference, though, is that while electric and magnetic fields are generally zero unless there’s something nearby, the Higgs field always has a value (in fact, the same value) everywhere. It’s like being surrounded by air, or water, your whole life and never realizing it. We can remove air or water if we know how, but as far as we know we can’t remove the Higgs. Wherever we go, there the Higgs field is, with its non-zero value permeating space.
2) Fields can interact. Carroll’s fine example of this is beta decay. In this process a neutron fires of an electron and an antineutrino, becoming a proton in the process. In fact we now know that one down quark in the neutron turns into one up quark. No matter. Picture the fields. Where there was a “1” in the neutron field (or down quark field, if you prefer), there is a gradual interaction that causes the neutron (down quark) field to drop to zero and the electron, antineutrino and proton (up quark) fields to all gain what the neutron field lost.
What about the Higgs field? Particles interact with the Higgs field, but they don’t change their identities. Instead, the Higgs field changes how the particles behave. Why?
3) The non-zero value of the Higgs field in empty space is the key.
“If the Higgs were like other fields, resting at zero in empty space, its interaction strength with other particles would simply measure how likely it would be for the Higgs (particle) to interact with that particle if they happened to pass by each other . . . But because the (field) is not zero, it’s like the other particles are interacting with it constantly – and it’s those persistent, inevitable interactions with the background that create the mass of the particle.” (p 127)
It’s the constant interaction between the non-zero Higgs field (the ocean in which we live) and the particles of matter from which we’re made that give our particles (electrons and quarks, at least) mass. (The protons and neutrons that make up most of our mass actually get theirs from the energy of the strong interaction, but that’s a story for another time.)
There’s much, much more to the story than this, of course, but what I like so much about Carroll’s book is he lays out the basic argument in simple and accessible language, and then adds on the complexity bit by bit to take the reader deeper and deeper along the journey. It’s a powerful approach that forges a new path between the purely journalistic approach that superficially covers the science and the overly technical approach that loses me (and others, I suspect) in jargon and detail. Enjoy the book the first time, as I will the second.