The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (17 page)

I’m not sure if the Insane Clown Posse wants to hear it, but the importance of fields extends well beyond magnets. The world is really made out of fields. Sometimes the stuff of the universe looks like particles, due to the peculiarities of quantum mechanics, but deep down it’s really fields. Empty space isn’t as empty as it looks. At every point there is a rich collection of fields, each taking on some value or another—or more precisely, due to the uncertainty that accompanies quantum mechanics, a distribution of possible values we could potentially observe.

When we talk about particle physics, we don’t usually emphasize that we’re actually talking about field physics. But we are. The point of this chapter is to reorient our intuition, in order to appreciate how quantum fields are the ultimate building blocks of reality as we currently understand it.

The fields themselves aren’t “made of” anything—fields are what the world is made of. We don’t know of any lower level of reality. (Maybe string theory, but that’s still hypothetical.) Magnetism is carried by a field, as are gravity and the nuclear forces. Even what we call “matter”—particles like electrons and protons—is really just a set of vibrating fields. The particle we call the “Higgs boson” is important, but not so much for its own sake; what matters is the Higgs field from which it springs, which plays a central role in how our universe works. Astounding indeed.

In the first few chapters we gave a brief introduction to the particles of the Standard Model, and mentioned that all particles arise as vibrations in fields. In the past few chapters we looked at the accelerators and detectors that help us explore the subatomic world, including the LHC. In this chapter and the next we’re going to back up a bit, taking a closer look at the idea of a field, how particles arise from fields, how symmetries give rise to forces, and how the Higgs field can break a symmetry and give us the variety of particles we see. That will put us in perfect position to understand how experimentalists hunt for the Higgs, and what it means that we’ve found it.

The gravitational field

These days we recognize that fields are all around us, but it took a while for scientists to start thinking in terms of “field theory.” You might guess that the idea of a gravitational field is even more obvious than the idea of a magnetic field, and you’d be right. But it’s not
completely
obvious.

The most famous story about gravity involves Isaac Newton and an apple that supposedly fell on his head, inspiring him to concoct his theory of universal gravitation. (It’s mostly famous because Newton himself couldn’t stop telling it later in life, in an unnecessary attempt to add some extra juice to his reputation as a genius.) The simplest version of the anecdote says that the apple helped Newton “invent” or perhaps “discover” gravity, although a moment’s contemplation reveals that this doesn’t quite make sense. People knew about gravity before Newton came along—it’s not like nobody had noticed that apples fall down, not up.

What came to Newton was the connection between the fall of the apple and the motion of the planets. He didn’t invent gravity, but he realized that it was
universal
—the gravitational attraction that keeps the planets orbiting around the sun and the moon orbiting around the earth was the same force that pulls apples toward the ground. You might not think that even this is the kind of insight of which legends are made. After all, something keeps the planets from zooming off into interstellar space, and something pulls apples to the ground, so why shouldn’t they be the same thing?

If that’s what you’re thinking, it’s only because you live in a post-Newtonian world. Before Newton came along, we wouldn’t have blamed the earth’s pull for the fall of the apple—we would have blamed the apple itself. Aristotle, for example, thought that different kinds of matter all had natural states of being. The natural state of a massive body was to be on the ground. If it is lifted above the ground, it wants to fall.

This idea that falling is due to an object’s natural inclination rather than the earth pulling on it is actually quite intuitive. I once served as a science consultant on a big-budget Hollywood movie, for which the designers thought it would be cool to portray a thrilling fight scene on a planet that was shaped like a disk, rather than a sphere. And it would be cool, you can’t argue with that. But they planned to have the scene climax with the bad guys falling off the edge of the planet. Pulled by . . . what, exactly? If you think of falling as something that things naturally do, rather than as a consequence of some large object pulling on them due to gravity, it’s a natural mistake to make. (But we managed to keep it out of the movie.)

Newton suggested that every object in the universe exerts a gravitational pull on every other object in the universe. Heavier objects exert a greater pull, and nearby objects are pulled more strongly than faraway ones. This idea fits the data beautifully, and represents a marvelous unification of what happens on earth and what happens in the sky.

But Newton’s theory of gravity bugged a lot of people. How does the moon, for example,
know
that the earth is exerting a gravitational pull on it? Earth is very far away, after all, and we’re used to forces being exerted when we bump into things, not when we’re elsewhere in the universe. This is the puzzle of “action at a distance,” and it disturbed Newton as well as his critics. At some point, however, when your theory does an amazingly good job at explaining a multitude of phenomena, you shrug your shoulders and admit that nature apparently just works that way. It’s pretty much the situation we’re in with quantum mechanics today: a theory that fits the data, but which we don’t think we understand as well as we should.

It wasn’t until the late 1700s that a French physicist, Pierre-Simon Laplace, showed that you didn’t have to think of Newtonian gravity in terms of magical action at a distance. Laplace realized that you could imagine a field filling all of space, later dubbed the “gravitational potential field.” The gravitational potential is distorted by massive bodies, just like the temperature of the air in a room is affected by a hot oven; the distortion is strong nearby, and fades as we get farther away. The force due to gravity arises because objects are pushed by the field itself: They feel a tug toward the direction in which the gravitational potential field is decreasing, much like a ball placed on an uneven surface will start rolling the direction in which the height of the surface is decreasing.

Mathematically, Laplace’s theory is identical to Newton’s. But conceptually, it fits in much better with our intuition that all physics, like politics, is local. It’s not that earth just reaches out and attracts the moon; earth affects the gravitational potential nearby, and that affects the potential right next door, and onward in a smooth sequence all the way to the moon (and beyond). The force of gravity isn’t a mysterious effect that leaps over infinite distances; it arises from the smooth variation of an invisible field that permeates all of space.

The electromagnetic field

It was in the study of electromagnetism where the idea of fields came into its own. There is an electric field, and also a magnetic field, but physicists just say “electromagnetism” as a single word to indicate that they are really two different manifestations of a single underlying field. The connection between the two wasn’t always so obvious.

Magnetism had been known since ancient times, of course; the Han dynasty in China had developed magnetic compasses more than two thousand years ago. And electricity had been recognized, both in the form of shocks you could receive from eels and the static electricity that collects on amber when it is rubbed with a cloth. There were even some hints that the phenomena were related; Benjamin Franklin, in between flying kites and fomenting rebellions, showed that it was possible to magnetize needles with electricity.

But the ideas didn’t truly come together until 1820, when a Danish physicist named Hans Christian Ørsted was giving a lecture on the nature of electricity and magnetism. Ørsted had thought of a clever way to demonstrate the hypothetical connection between the two: He would build an electrical circuit, and then run the current next to a magnet and see if its needle was deflected from true north by the running electricity. Unfortunately, an accident prevented Ørsted from actually carrying out the experiment before it was time for his lecture. He decided to simply do the experiment right there in front of the assembled crowd, convinced that it must work . . . and it did. He flipped a switch, electrical current flowed through a wire, and he saw a small but unmistakable jitter in the compass needle. According the Ørsted’s own account, the effect was quite small, and the audience went away unimpressed. But from that day forward, electricity and magnetism had merged into the subject of electromagnetism.

Through subsequent work by people such as Michael Faraday and James Clerk Maxwell, a sophisticated theory of the electromagnetic field was developed. Once this theory was in place, we could answer questions about the dynamics of that field. For example, what happens when you take an electric charge and shake it up and down? (The same question could have been asked about gravity, but the gravitational force is so weak it would be very hard to answer the question experimentally.)

What happens when you shake a charge is, quite naturally, that you create ripples in the electromagnetic field. And these ripples propagate outward as waves, much like waves on water when you drop a stone into it. There is a good name for these electromagnetic waves: light. When we turn on a light switch, what happens is that electrical current flows through the filament of the lightbulb, heating it up. That heating shakes up the atoms in the filament and their associated electrons, causing them to jiggle back and forth. That jiggling sets up waves in the electromagnetic field that travel to our eyes and are perceived as light.

The identification of light with waves in the electromagnetic field represents another great triumph of unification in physics. It went further when we realized that what we call visible light is only particular wavelengths of radiation—those that can be seen by the human eye. Shorter wavelengths include X-rays and ultraviolet light, while longer wavelengths include infrared light, microwaves, and radio waves. The work of Faraday and Maxwell received spectacular confirmation in 1888, when the German physicist Heinrich Hertz was able to produce and detect radio waves for the first time.

When you use a remote control to turn on your TV, it looks like action at a distance, but it’s really not. You push the button and an electrical current starts to jiggle back and forth inside a circuit in the remote, creating a radio wave that propagates through the electromagnetic field to the TV and is absorbed by a similar gizmo. In the modern world, the electromagnetic field around us is made to do an enormous amount of work—illuminating our environment, sending signals to our cell phones and wireless computers, and microwaving our food. In every case it’s set up by moving charges that send ripples out through the field. All of which, by the way, was completely unanticipated by Hertz. When he was asked what his radio-wave-detecting device would ultimately be good for, he replied, “It’s of no use whatsoever.” Prodded to suggest some practical application, he replied, “Nothing, I guess.” Something to keep in mind as we contemplate the eventual applications of basic research.

Waves of gravity

It wasn’t until after physicists understood the relationship between electromagnetism and light that they began to wonder whether a similar phenomenon should happen with gravity. It might seem like an academic question, since you need an object the size of a planet or moon to create a gravitational field big enough to measure. We’re not going to pick up the earth and shake it back and forth to make waves. But to the universe, that’s no problem at all. Our galaxy is full of binary stars—systems where two stars orbit around each other—presumably shaking the gravitational field as they go. Does that lead to rippling waves spreading in every direction?

Interestingly, gravity as Newton or Laplace described it would
not
predict radiation of any kind. When a planet or star moves, the theory says that its gravitational pull changes instantaneously all across the universe. It’s not a propagating wave but an instant transformation everywhere.

That’s just one way in which Newtonian gravity doesn’t seem to fit well with the changing framework of physics that developed over the course of the nineteenth century. Electromagnetism, and especially the central role played by the speed of light, was instrumental in inspiring Albert Einstein and others to develop the theory of special relativity in 1905. According to that theory, nothing can travel faster than light—not even hypothetical changes in the gravitational field. Something would have to give. After ten years of hard work, Einstein was able to construct a brand-new theory of gravity, known as “general relativity,” that replaced Newton’s entirely.

Just like Laplace’s version of Newtonian gravity, Einstein’s general relativity describes gravity in terms of a field that is defined at every point in space. But Einstein’s field is a much more mathematically complicated and intimidating field than Laplace’s; rather than the gravitational potential, which is just a single number at each point, Einstein used something called the “metric tensor,” which can be thought of as a collection of ten independent numbers at every point. This mathematical complexity helps general relativity accrue its reputation as a very difficult theory to understand. But the basic idea is simple, if profound: The metric describes the curvature of spacetime itself. According to Einstein, gravity is a manifestation of the bending and stretching of the very fabric of space, the way we measure distances and times in the universe. When we say, “The gravitational field is zero,” we mean that spacetime is flat, and the Euclidean geometry we learned in high school is valid.

One happy consequence of general relativity is that, just like with electromagnetism, ripples in the field describe waves traveling at the speed of light. And we have detected them, although not directly. In 1974, Russell Hulse and Joseph Taylor discovered a binary system in which both objects are neutron stars, rapidly spinning in a very tight orbit. General relativity predicts that such a system should lose energy by giving off gravitational waves, causing the orbital period to gradually decrease as the stars draw closer together. Hulse and Taylor were able to measure this change in the period, exactly as predicted by Einstein’s theory; in 1993, they were awarded the Nobel Prize in Physics for their efforts.