The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (19 page)

Transferring vibrations

The idea that matter particles are discrete vibrations in fermionic fields helps explain features of the real world that would otherwise be puzzling, such as how particles can be created and destroyed. Back in the heady early days of quantum mechanics, people were struggling to understand the phenomenon of radioactivity. They could see how photons could be created from other particles, because those were just vibrations in the electromagnetic field. But what about radioactive processes, like the decay of the neutron? Inside a nucleus, huddled in close comradeship with a few protons, a neutron can last forever. When it is isolated by itself, however, a neutron will decay within a matter of minutes, transforming into a proton by emitting an electron and an antineutrino. The question is, where did that electron and antineutrino come from? People speculated that they had actually been hidden inside the neutron all along, but that didn’t seem quite right.

A beautiful answer was worked out in 1934 by Enrico Fermi, in the first real application of field theory to fermions—which was only appropriate, since those particles had been named after Fermi in the first place. Fermi suggested that you could think of each of these particles as vibrations in different quantum fields, and that each field exerted a tiny influence on the others, much like playing a piano in one room will cause the strings of a piano in a room next door to gently hum in sympathy. It’s not that new particles are magically created out of nothing; it’s that the vibrations in the neutron field are gradually transferred to the proton, electron, and antineutrino fields. Because it’s quantum mechanics, we can’t perceive the gradual transfer; we observe the neutron, and we either see it as a neutron, or we see that it’s decayed, with some probability that can be mathematically calculated.

Quantum field theory also helps understand how one particle can convert into others that it doesn’t even interact with directly. A classic example, and one that will be very important for us very soon, is a Higgs boson decaying into two photons. That sounds surprising, because we know that photons don’t couple directly to the Higgs. Photons couple to charged particles, and the Higgs couples to massive particles—and the Higgs is not charged, and photons are not massive.

The trick is the concept of virtual particles, which really should be thought of as virtual fields. A Higgs boson comes along, a vibrating wave in the Higgs field. That vibration can set up vibrations in the massive particles that the Higgs couples to. But maybe those vibrations don’t quite rise to the level of appearing as new particles; instead, they set up vibrations in yet another kind of field, in this case the electromagnetic field. That’s how a Higgs can turn into photons: First it turns into virtual charged, massive particles, and then they quickly convert into photons. It’s as if you had two pianos that were completely out of tune with respect to each other, and ordinarily wouldn’t resonate at all; but there’s a third instrument in the room, like a violin, that has enough flexibility to resonate with both of them.

Conservation laws

Because all particles arise from fields, even matter particles can appear and disappear in nature. But it’s not as if chaos has completely broken loose. Count up the electric charge before and after the neutron decays. Beforehand it’s zero, because you just have a chargeless neutron. Afterward it’s also zero; the proton has a positive charge, but the electron has a precisely balancing negative charge, and the antineutrino has no charge at all. It also seems that the number of quarks is the same before and after, since a single neutron produced a single proton. Finally, the number of leptons is exactly one before and after, if we introduce a trick of counting antimatter leptons as “minus one lepton” (and antiquarks as “minus one quark,” had there been any of those). Then the neutron is three quarks and zero leptons, while its decay products also add up to three quarks (the proton) and zero leptons (one for the electron and minus one for the antineutrino). That’s the reason we know an antineutrino rather than a neutrino is produced when neutrons decay.

These patterns are
conservation laws
—unbreakable rules that govern what particle interactions are allowed in nature. Along with the famous law of conservation of energy, we also have conservation of electric charge, of the number of quarks, and of the number of leptons. Some conservation laws are more inviolable than others; physicists suspect that quark and lepton numbers can sometimes change (very rarely, or under extreme conditions), but most believe that energy and electric charge are absolutely fixed.

With these rules in mind, we can understand which particles decay, and which ones last forever. The rule of thumb is that heavy particles like to decay into lighter ones, as long as the decay doesn’t violate any conservation laws. Electric charge is conserved, and electrons are the lightest charged particles, so they are completely stable. Quark number is conserved, and the proton is the lightest particle with nonzero quark number, so it is also stable (as far as we know). Neutrons are not stable, but they can form stable nuclei in the company of protons.

The Higgs boson, a very heavy particle with zero charge that is neither a quark nor a lepton, decays extremely quickly, so fast that we will never observe it directly in a particle detector. That’s one of the reasons it’s been so hard to find, and why our apparent success has been so sweet.

EIGHT

THROUGH A BROKEN MIRROR

In which we scrutinize the Higgs boson and the field from which it springs, showing how it breaks symmetries and gives the universe character.

I
n an otherwise empty seminar room at the California Institute of Technology, I was seated on one side of a table and local TV reporter Hal Eisner was seated across from me. In between us was a giant bowl of popcorn. Eisner seized a kernel of popcorn and waved it in front of my nose, asking me—begging, really—to use it to explain the Higgs boson. “If there were no Higgs boson, would this popcorn explode? It would explode, wouldn’t it?”

It was September 10, 2008, the day the first protons circulated around the LHC. For a previous generation of accelerators, startup was an understated affair, watched closely by a small band of interested physicists and ignored by the rest of the world. But the LHC is special, and the attention of people worldwide was focused on a handful of protons working up the strength to travel all the way around a seventeen-mile ring for the first time.

Hence, the reporters had come to Caltech, and other universities in other cities, to report on the excitement. 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.)

Still, the local news folks were very reasonably asking, what’s the big deal? We know this is important, but why, exactly? The search for the Higgs boson was always one of the first answers offered. Okay, so why is the Higgs so important? Something about mass, and breaking symmetries. Let’s get down to brass tacks:
Would the popcorn explode?

The right answer is “yes, if the Higgs boson (or more carefully, the Higgs field in which the boson is a propagating wave) were to suddenly disappear, ordinary matter would no longer hold together, and objects like kernels of popcorn would immediately explode.” But it is misleading to think of the Higgs as some kind of force that binds atoms together. The Higgs is a field that permeates space, giving heft to particles like electrons, allowing them to form atoms, which bind into molecules. Without the Higgs, there wouldn’t be atoms, there would just be a bunch of particles zooming separately through the universe.

It’s a common problem when translating deep concepts from modern physics into the language of everyday life. You want to say things that are completely correct (of course), but you also want to give people the right
impression
, which isn’t the same thing—it does no good to say correct things if nobody has a clue what you’re talking about, and they might even start thinking something wrong on the basis of your explanation.

Fortunately for us, it’s not
that
hard to really understand what’s going on. The Higgs field is like the air, or the water for fish in the sea; we don’t usually notice it, but it’s all around us, and without it life would be impossible. And it is literally “all around us”; unlike all the other fields of nature, the Higgs is nonzero even in empty space. As we move through the world, we are embedded in a background Higgs field, and it’s the influence of that field on our particles that gives them their unique properties.

The Higgs boson is not any old particle. When the Tevatron at Fermilab discovered the top quark in 1995, it was an amazing triumph of effort and ingenuity. But we were already familiar with quarks and weren’t really expecting to discover something completely surprising. The Higgs is more than that; we haven’t found any other particles like it. Its field fills space, breaks symmetries, gives mass and individuality to the other particles of the Standard Model. If the top and bottom quarks didn’t exist, our lives would go on essentially unchanged. If the Higgs boson didn’t exist, the universe would be an utterly different place.

A prizewinning analogy

In 1993, the LHC was still an idea on a drawing board, and it was far from certain that it would make the journey to reality. A group of physicists from CERN were pitching the massive project to William Waldegrave, the science minister of the United Kingdom at the time. Waldegrave was interested in the idea, but he couldn’t quite grasp the central selling point: the idea of the Higgs boson. “He didn’t understand a word of what was said,” recalled physicist David Miller of University College London.

But Waldegrave didn’t simply give up; he challenged the scientists to provide him with an understandable explanation of the role of the Higgs boson, one that would fit on a single piece of paper. He offered a bottle of vintage champagne to whoever came up with the best explanation. Miller and four colleagues managed to cook up an engaging metaphor that was deemed suitable by the science minister. All five got bottles of champagne, and of course the United Kingdom supported the LHC.

Here’s an updated version of Miller’s analogy. Imagine that Angelina Jolie and I both walk across an empty room. (The original explanation used Margaret Thatcher rather than a movie star, for obvious political reasons, but all that matters is that we consider someone famous.) For purposes of the thought experiment, let’s assume that the speeds at which we naturally walk are the same. In that case, we will cross the room in the same amount of time. There is a symmetry: It doesn’t matter whether it’s Angelina or me who is walking across the room; the elapsed time will be equal.

Now imagine that there is a party going on in the room, and it’s filled with revelers chatting away. I walk across the room, maybe a bit more slowly than I did when it was empty: I have to briefly pause and adjust my path to weave through all the partygoers, but for the most part I pass through unnoticed. When Angelina walks across the same room, it’s a completely different story. As she passes by, all sorts of people stop her to get autographs or take pictures or just make small talk. Effectively, her “mass” is larger: It takes more effort for her to get moving and cross the room than it would for me. (I am not saying that Angelina Jolie is fat; it’s just a metaphor.) The symmetry that we used to have is broken by the presence of other people in the room.

A physicist would say that Angelina Jolie “interacts more strongly” with the party guests than I do. That strength of interaction is a reflection of her greater celebrity; nobody thinks to stop me and get an autograph, but a famous actress undergoes frequent interactions with the background crowd.

Now replace me with an up quark, Angelina with a top quark, and the partygoers with the Higgs field. If there is no Higgs field filling space, there is a perfect symmetry between an up quark and a top, and they behave in the same way, just as Angelina and I walk across an empty room at the same speed. But a top quark interacts more strongly with the Higgs than an up quark does. If the Higgs field is “turned on,” the top gets a greater mass, and it takes more effort to get it moving, just like it takes Angelina more effort to push through a crowd of partygoers than it takes me.

As with any analogy, this one is not perfect. Like a crowd of partygoers, the Higgs field fills space, affecting anything that moves through it. But unlike a crowd of people, or anything else we are familiar with, I can’t measure my velocity with respect to this background field; it looks exactly the same no matter how I am moving. It takes more effort to get a particle moving in the presence of the Higgs field, but once it gets moving it stays moving, just as Galileo or Newton or Einstein would have expected. The Higgs field doesn’t drag you down to its velocity, because it doesn’t have a velocity. There’s really no analogy for that in everyday life, but it’s how the world appears to work.

Before Einstein and relativity came along, many physicists thought that the electromagnetic waves were vibrations in a medium called the “aether.” They even tried to detect the aether by looking for changes in the speed of light depending on the motion of the earth; if the light was traveling in the same direction as the aether it should move faster, and against the aether it should move slower. But they found no difference. It was the genius of Einstein to realize that the whole idea of aether was unnecessary, and the speed of light is absolutely constant through empty space. You don’t need an aether field to support the electromagnetic field; the electromagnetic field can just exist.