The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (5 page)

With all those disclaimers out of the way, let’s get back to emphasizing the singular role of the Higgs: It’s the final part of the Standard Model of particle physics. The Standard Model explains everything we experience in our everyday lives (other than gravity, which is easy enough to tack on). Quarks, neutrinos, and photons; heat, light, and radioactivity; tables, elevators, and airplanes; televisions, computers, and cell phones; bacteria, elephants, and people; asteroids, planets, and stars—all simply applications of the Standard Model in different circumstances. It’s the full theory of immediately discernible reality. And it all fits together beautifully, passing a bewildering variety of experimental tests, as long as there is the Higgs boson. Without the Higgs, or something even more bizarre to take its place, the Standard Model wouldn’t get off the ground.

Figuring out the trick

There’s something fishy about these claims that the Higgs boson is so important. After all, before we actually found it, how did we know it was important at all? What drove us to keep talking about the properties of a hypothetical particle nobody had ever observed?

Imagine you see a performance by a very talented magician, who performs an amazing card trick. The trick involves getting a playing card to mysteriously levitate in the air. You are puzzled by this trick, and you’re absolutely sure that the magician didn’t actually use mystical powers to make the card levitate. You’re also clever and persistent, and after quite a bit of thinking you come up with a way the magician could have done it, involving a thin thread secretly attached to the card. In fact you’re able to come up with other possible schemes involving blowing air and heat pumps, but the thread scenario is both simple and plausible. You even go so far as to reproduce the trick at home, convincing yourself that with the right kind of thread you’re able to do the trick just like the magician did.

But you go back to catch another performance of the magician’s act, where you are able to see the card levitate once again. His version looks just like the one you were able to put together at home—but try as you might, you can’t quite see the thread itself.

The Standard Model Higgs boson is like that thread. For a long time we hadn’t seen it directly, but we saw its effects. Or even better, we saw features of the world that make perfect sense if it’s there, and make no sense without it. Without the Higgs boson, particles such as the electron would have zero mass and move at the speed of light; but instead they do have mass and move more slowly. Without the Higgs boson, many elementary particles would appear identical to one another, but instead they are manifestly different, with a variety of masses and lifetimes. With the Higgs, all these features of particle physics make perfect sense.

In circumstances like these, whether we’re thinking about the levitating card or the Higgs boson, there are two options: Our theory is right, or an even more interesting and elaborate theory is right. The effects are there—the card floats, the particles have mass. There must be an explanation. If it’s the simple one we’ll congratulate ourselves on our cleverness; if it’s something more complicated, we’ll have learned something very interesting. Maybe the particle we found at the LHC does part of what the Higgs was proposed to do but not all of it; or maybe the job of the Higgs is played by multiple particles, of which we’ve only found one. We win no matter what, as long as we ultimately succeed in figuring out what’s going on.

Fermions and bosons

Let’s see if we can’t translate this inspirational metaphorical cheerleading about how important the Higgs boson is into a more specific explanation for what it actually is supposed to do.

Particles come in two types: the particles that make up matter, known as “fermions,” and the particles that carry forces, known as “bosons.” The difference between the two is that fermions take up space, while bosons can pile on top of one another. You can’t just take a pile of identical fermions and put them all at the same place; the laws of quantum mechanics won’t allow it. That’s why collections of fermions make up solid objects like tables and planets: The fermions can’t be squeezed on top of one another.

In particular, the
smaller
the mass of the particle, the more space it takes up. Atoms are made out of just three types of fermions—up quarks, down quarks, and electrons—held together by forces. The nucleus, made of protons and neutrons, which in turn are made of up and down quarks, is relatively heavy, and exists in a relatively tiny region of space. The electrons, meanwhile, are much lighter (about 1/2,000th the mass of a proton or neutron) and take up much more space. It’s really the electrons in atoms that give matter its solidity.

Bosons don’t take up any space at all. Two bosons, or two trillion bosons, can easily sit at exactly the same location, right on top of one another. That’s why bosons are force-carrying particles; they can combine to make a macroscopic force field, like the gravitational field that holds us to the earth or the magnetic field that deflects a compass needle.

Physicists tend to use the words “force,” “interaction,” and “coupling” in practically interchangeable ways. That reflects one of the deep truths uncovered by twentieth-century physics: Forces can be thought of as resulting from the exchange of particles. (As we’ll see, that’s equivalent to saying “as resulting from vibrations in fields.”) When the moon feels the gravitational pull of the earth, we can think of gravitons passing back and forth between the two bodies. When an electron is trapped by an atomic nucleus, it’s because photons are exchanged between them. But these forces are also responsible for other particle processes like annihilation and decay, not just pushing and pulling. When a radioactive nucleus decays, we can attribute that event to the strong or weak nuclear force at work, depending on what kind of decay occurs. Forces in particle physics are responsible for a wide variety of goings-on.

Aside from the Higgs, we know four kinds of forces, each with its own associated boson particles. There’s gravity, associated with a particle called the “graviton.” Admittedly, we haven’t actually observed individual gravitons, so the graviton is often not included in discussions of the Standard Model, although we detect the force of gravity every day when we don’t all float into space. But given that gravity is a force, the basic rules of quantum mechanics and relativity essentially guarantee that there are associated particles, so we use the word “graviton” to refer to those particles we haven’t yet seen on an individual basis. The way that gravity acts as a force on other particles is pretty simple: Every particle attracts every other particle (although very weakly).

Then there is electromagnetism—in the 1800s, physicists figured out that the phenomena of “electricity” and “magnetism” were two different versions of the same underlying force. The particles associated with electromagnetism are called “photons,” which we see directly all the time. Particles that do interact via electromagnetism are “charged,” while those that don’t are “neutral.” And just to keep you on your toes, electrical charges can be positive or negative, with like charges pushing each other apart and opposite charges attracting. The ability of like charges to repel each other is absolutely crucial to how the universe works. If electromagnetism were universally attractive, every particle would simply attract every other particle, and all the matter in the universe would do its best to collapse into one giant black hole. Fortunately we have electromagnetic repulsion as well as attraction, which keeps life interesting.

Nuclear forces

Then we have the two “nuclear” forces, so called because (unlike gravity and electromagnetism) they only extend over a very short distance, comparable in size to the nucleus of an atom or less. There is the strong nuclear force, which holds quarks together inside protons and neutrons; its particles are charmingly named “gluons.” The strong nuclear force is (unsurprisingly) very strong, and interacts with quarks but not with electrons. Gluons are massless, just like photons and gravitons. When a force is carried by massless particles, we expect its influence to stretch over a very long range, but the strong force is actually very short ranged.

In 1973, David Gross, David Politzer, and Frank Wilczek showed that the strong force has an amazing property: The attraction between two quarks actually grows in strength as the quarks are moved apart. As a consequence, pulling two quarks apart requires more and more energy, so much so that you eventually just create more quarks. It’s like pulling on a strip of rubber, with each end representing a quark. You can pull the two ends, but you never get one end all by itself. Instead you create two new ends when the rubber snaps. As a result, you will never see an individual quark alone in the wild; they (and the gluons) are confined inside heavier particles. These composite particles made of quarks and gluons are known as “hadrons,” from which the LHC gets its middle name. Gross, Politzer, and Wilczek shared the Nobel Prize in 2004 for this discovery.

Then there is the weak nuclear force, which lives up to its name. Although it doesn’t play much of a role in our immediate environment here on earth, the weak force is nevertheless important to the existence of life: It helps the sun shine. Solar energy arises from conversion of protons into helium, which requires turning some of those protons into neutrons, which proceeds by the weak interaction. But down here on earth, unless you’re a particle or nuclear physicist, you don’t see too much of the weak force in action.

Three different kinds of bosons carry the weak force. There is the Z boson, which is electrically neutral, and there are two different W bosons, one with a positive electric charge and one with a negative electric charge, dubbed W
⁺
and
W
^-
for short. The
W
and
Z
bosons are quite massive by elementary-particle standards (about as heavy as an atom of zirconium, if that’s any help), which means that they are hard to produce and decay away fairly quickly, all of which contributes to why the weak interactions are so weak.

In casual speech we use the word “force” to refer to all kinds of things. The force of friction when something is sliding, the force of impact when you smash into a wall, the force of air resistance as a feather falls to the ground. You will have noticed that none of these forces made our list of the four forces of nature, nor do any of them have bosons associated with them. That’s the difference between elementary-particle physics and colloquial usage. All of the macroscopic “forces” that we experience as part of our daily routine, from the acceleration when we depress a car’s gas pedal to the tug on a leash when a dog suddenly sees a squirrel and takes off, ultimately arise as complicated side effects of the fundamental forces. In fact, with the notable exception of gravity (which is pretty straightforward, pulling everything down), all of those everyday phenomena are just manifestations of electromagnetism and its interactions with atoms. This is the triumph of modern science: to boil the marvelous variety of the world around us down to just a few simple ingredients.

Fields pervade the universe

Of these four forces, one has long stood out as weird: the weak force. Notice that gravity has gravitons, electromagnetism has photons, and the strong force has gluons; one kind of boson for each force. The weak force comes with three different bosons, the neutral Z and the two charged Ws. And these bosons are responsible for strange behaviors, as well. By emitting a W boson one kind of fermion can change into another kind: a down quark can spit out a W
^-
and change into an up quark. Neutrons, which are made of two downs and an up, decay when they’re by themselves outside a nucleus—one of their down quarks emits a W
^-
, and the neutron converts into a proton, which has two ups and a down. None of the other forces change the identity of the particles they interact with.

The weak interactions, basically, are a mess. And the reason is simple: the Higgs.

The Higgs is fundamentally different from all the other bosons. The others, as we’ll see in Chapter Eight, all arise because of some symmetry of nature connecting what happens at different points in space. Once you believe in these symmetries, the bosons are practically inevitable. But the Higgs isn’t like that at all. There is no deep principle that requires its existence, but it exists anyway.

After the LHC announced the Higgs discovery on July 4, hundreds of attempts were made at explaining what it was supposed to mean. The biggest reason why this task is such a challenge is that it’s not really the Higgs boson itself that is all that interesting; what matters is the Higgs
field
from which the boson arises. It’s a fact of physics that all the different particles really arise out of fields—that’s quantum field theory, the underlying framework for everything that particle physicists do. But quantum field theory isn’t something we teach kids in high school. It’s not even something we often discuss in popular physics books; we talk about particles and quantum mechanics and relativity, but we rarely dig into the wonders of quantum field theory underlying it all. When it comes to the Higgs boson, however, it’s no longer adequate to skirt around the ultimate field-ness of it all.

When we talk about a “field,” we are talking about “something that has some value at every point in space.” The temperature of the earth’s atmosphere is a field; at every point on the earth’s surface (or at any elevation above the surface) the air has a certain temperature. The density and humidity of the atmosphere are likewise fields. But these aren’t
fundamental
fields—they are just properties of the air itself. The electromagnetic field or the gravitational field are, in contrast, believed to be fundamental. They’re not made of anything else—they are what the world is made of. According to quantum field theory, absolutely everything is made of a field or a combination of fields. What we call “particles” are tiny vibrations in these fields.