The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (6 page)

This is where the “quantum” part of quantum field theory comes in. There’s a lot to say about quantum mechanics, perhaps the most mysterious idea ever to be contemplated by human beings, but all we need is one simple (but hard to accept) fact: How the world appears when we look at it is very different from how it really is.

The physicist John Wheeler once proposed a challenge: How can you best explain quantum mechanics in five words or fewer? In the modern world, it’s easy to get suggestions for any short-answer question: Simply ask Twitter, the microblogging service that limits posts to 140 characters. When I posed the question about quantum mechanics, the best answer was given by Aatish Bhatia (@aatishb): “Don’t look: waves. Look: particles.” That’s quantum mechanics in a nutshell.

Every particle we talk about in the Standard Model is, deep down, a vibrating wave in a particular field. The photons that carry electromagnetism are vibrations in the electromagnetic field that stretches through space. Gravitons are vibrations in the gravitational field, gluons are vibrations in the gluon field, and so on. Even the fermions—the matter particles—are vibrations in an underlying field. There is an electron field, an up quark field, and a field for every other kind of particle. Just like sound waves propagate through the air, vibrations propagate through quantum fields, and we observe them as particles.

Just a bit ago we mentioned that particles with a small mass take up more space than ones with a larger mass. That’s because the particles aren’t really little balls with a uniform density; they’re quantum waves. Every wave has a wavelength, which gives us a rough idea of its size. The wavelength also fixes its energy: It requires more energy to have a short wavelength, since the wave needs to change more quickly from one point to another. And mass, as Einstein taught us long ago, is just a form of energy. So lower masses mean less energy mean longer wavelengths mean larger sizes; higher masses mean more energy mean shorter wavelengths mean smaller sizes. It all makes sense once you unpack it.

Stuck away from zero

Fields have a value at every point in space, and when space is completely empty those values are typically zero. By “empty” we mean “as empty as can be,” or, more specifically, “with as little energy as it is possible to have.” According to that definition, fields like the gravitational field or the electromagnetic field sit quietly at zero when space is truly empty. When they’re at some other value, they carry energy, and therefore space isn’t empty. All fields have tiny vibrations because of the intrinsic fuzziness of quantum mechanics, but those are vibrations around some average value, which is typically zero.

The Higgs is different. It’s a field, just like the others, and it can be zero or some other value. But it doesn’t
want
to be zero; it wants to sit at some constant number everywhere in the universe. The Higgs field has less energy when it’s nonzero than when it’s zero.

As a result, empty space is full of the Higgs field. Not a complicated set of vibrations that would represent a collection of individual Higgs bosons; just a constant field, sitting quietly in the background. It’s that ever-present field at every point in the universe that makes the weak interactions what they are and gives masses to elementary fermions. The Higgs boson—the particle discovered at the LHC—is a vibration in that field around its average value.

Because the Higgs particle is a boson, it gives rise to a force of nature. Two massive particles can pass by each other and interact by exchanging Higgs bosons, just like two charged particles can interact by exchanging photons. But this Higgs force is
not
what gives particles mass, and it’s generally not what all the fuss is about. What gives particles mass is this Higgs field sitting quietly in the background, providing a medium through which other particles move, affecting their properties along the way.

One major difference between the Higgs field and other fields is that the resting value of the Higgs is away from zero. All fields undergo tiny vibrations due to the intrinsic uncertainties of quantum mechanics. A larger vibration appears to us as a particle, in this case the Higgs boson.

As we travel through space, we’re surrounded by the Higgs field and moving within it. Like the proverbial fish in water, we don’t usually notice it, but that field is what brings all the weirdness to the Standard Model.

Executive summary

There is a great deal of profound and challenging physics associated with the idea of the Higgs boson. But for right now let’s just give the overall summary of how the Higgs field works and why it’s important. Without further ado:

• The world is made of
  fields
  —substances spread through all of space that we notice through their vibrations, which appear to us as particles. The electric field and the gravitational field might seem familiar, but according to quantum field theory even particles like electrons and quarks are really vibrations in certain kinds of fields.
  
• The Higgs boson is a vibration in the Higgs field, just as a photon of light is a vibration in the electromagnetic field.
  
• The four famous forces of nature arise from
  symmetries
  —changes we can make to a situation without changing anything important about what happens. (Yes, it makes no immediate sense that “a change that doesn’t make a difference” leads directly to “a force of nature” . . . but that was one of the startling insights of twentieth-century physics.)
  
• Symmetries are sometimes
  hidden
  and therefore invisible to us. Physicists often say that hidden symmetries are “broken,” but they’re still there in the underlying laws of physics—they’re simply disguised in the immediately observable world.
  
• The weak nuclear force, in particular, is based on a certain kind of symmetry. If that symmetry were unbroken, it would be impossible for elementary particles to have
  mass
  . They would all zip around at the speed of light.
  
• But most elementary particles do have mass, and they don’t zip around at the speed of light. Therefore, the symmetry of the weak interactions must be broken.
  
• When space is completely empty, most fields are turned off, set to zero. If a field is not zero in empty space, it can break a symmetry. In the case of the weak interactions, that’s the job of the Higgs field. Without it, the universe would be an utterly different place.
  

Got all that? It’s a bit much to swallow, admittedly. It will make more sense when we complete our journey through the rest of the chapters. Trust me.

The rest of the book will be a back-and-forth journey through the ideas behind the Higgs mechanism and the experimental quest to discover the boson. We’ll start with a quick overview of how the particles and forces of the Standard Model fit together, then explore the astonishing ways in which physicists use technology and gumption to discover new particles. After that it’s back to theory, as we think about fields and symmetries and how the Higgs can hide symmetries from our view. Finally we can show how the Higgs was discovered, how the news was spread, who will get the credit, and what it means for the future.

It should be clear why Leon Lederman thought that the God Particle was an appropriate name for the Higgs boson. That boson is the hidden piece of equipment that explains the magic trick the universe is pulling on us, giving particles different masses and thereby making particle physics interesting. Without the Higgs, the intricate variety of the Standard Model would collapse to a featureless collection of pretty much identical particles, and all of the fermions would be essentially massless. There would be no atoms, no chemistry, no life as we know it. The Higgs boson, in a very real sense, is what brings the universe to life. If there were one particle that deserved such a lofty title, there’s no question it would be the Higgs.

THREE

ATOMS AND PARTICLES

In which we tear apart matter to reveal its ultimate constituents, the quarks and the leptons.

I
n the early 1800s, German physician Samuel Hahnemann founded the practice of homeopathy. Dismayed by the ineffectiveness of the medicine of his time, Hahnemann developed a new approach based on the principle of “like cures like”—a disease can be treated by precisely the same substance that causes it in the first place, as long as that substance is properly manipulated. The way to manipulate it is known as “potentization,” which consists of diluting the substance repeatedly in water, shaking vigorously each time. A typical method of dilution might mix one part of substance and ninety-nine parts water. You prepare a homeopathic remedy by diluting, shaking, diluting again, shaking again, as many as two hundred times.

More recently, Crispian Jago, a professional software consultant and recreational skeptic, wanted to demonstrate that he doesn’t believe homeopathy is a valid approach to medicine. So he decided to apply the method of serial dilution to an easily obtained substance: his own urine. Which he then proceeded to drink. Because he was a bit impatient, he only diluted the urine thirty times. Except that he didn’t call it “urine,” he called it “piss,” and then proclaimed that he was developing a cure for being pissed, which translates either as angry (for those in the U.S.) or inebriated (for those in the U.K.). The results, naturally enough, were presented to the world in the form of a boisterous YouTube video.

Jago had good reason for being undisturbed by the prospect of drinking urine that had been diluted in a 1:99 concentration thirty times over: By the time he got to the final glass, there was none of the original stuff left. Not just “a minuscule amount” but really none at all, if his dilutions were sufficiently careful.

That’s because everything in our everyday world—urine, diamonds, french fries, really everything—is made of atoms, usually combined into molecules. Those molecules are the smallest unit of a substance that can still be thought of as that substance. Separately, two hydrogen atoms and one oxygen atom are just atoms; combined, they become water.

Because the world is made of atoms and molecules, you can’t dilute things forever and have them maintain their identity. A teaspoonful of urine might contain approximately 10
²⁴
molecules. If we dilute once by mixing one part urine with ninety-nine parts water, we’re left with 10
²²
urine molecules. Dilute twice and we have 10
²⁰
molecules. By the time we’ve diluted twelve times, on average there’s only one molecule of the original substance remaining. After that, it’s all window dressing—we’re just mixing water into more water. With about forty dilutions we could dilute away every molecule in the known universe.

So when Jago finished the procedure and took his final triumphant swig, the water he was drinking was as pure as any that would ordinarily come out of the tap. Advocates of homeopathy know this, of course. They believe that the water molecules retain a “memory” of whatever herb or chemical was used in the original dilution, and indeed that the final solution is more potent than the substance was to start. This violates everything we know about physics and chemistry, and clinical trials rate homeopathic remedies no better than placebos at combating disease. But everyone is entitled to their own opinion.

We are not, as the saying goes, entitled to our own facts. And the fact that matter is made of atoms and molecules is a striking one. Really there are two critical facts: first, that we can take matter and break it up into little chunks that represent the smallest possible unit of that kind of thing; and second, that it only takes a few fundamental building blocks combined in different ways to account for all the variety of the observable world.

At first glance the particle zoo can seem complex and intimidating, but there are only twelve matter particles, which fall neatly into two groups of six: quarks, which feel the strong nuclear force, and leptons, which do not. It’s an amazing story, put together over the course of a century, from the discovery of the electron in 1897 to the detection of the last elementary fermion (the tau neutrino) in 2000. Here we’ll take a whirlwind tour, saving the quantitative details for Appendix Two. When the smoke clears we will have a relatively manageable collection of particles from which everything else is made.

Pictures of atoms

Everyone has seen cartoon images of atoms. They are usually portrayed as tiny solar systems, with a central nucleus surrounded by orbiting electrons. It’s an iconic image, which serves, for example, as the logo of the U.S. Atomic Energy Commission. It’s also misleading in a subtle way.

The cartoon atom represents the Bohr model, named after Danish physicist Niels Bohr, who applied insights from the early days of quantum mechanics to the model of atoms that had been previously developed by New Zealand–born British physicist Ernest Rutherford. In Rutherford’s version of the atom, electrons orbit the nucleus at any distance you might imagine, just like planets in the real solar system (except they are attracted to the center by electromagnetism, not by gravity). Bohr modified that idea by insisting that the electrons can travel only on certain particular orbits, which was a great step forward in fitting the data from radiation emitted by atoms. These days we know that the electrons don’t really “orbit” at all, because they don’t really have a “position” or “velocity.” Quantum mechanics says that the electrons persist in clouds of probability known as “wave functions,” which tell us where we might find the particle if we were to look for it.