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

by Sean Carroll

  As you might guess, a lot of hard thought and spirited disagreement go into deciding which events to keep and which to toss. It’s natural to worry that some real gems are being thrown away in all that discarded data, so the physicists at CMS and ATLAS are constantly working to refine their triggers in response to both improved experimental know-how and novel ideas from the theorists.

  Sharing data

  Even after running everything through the trigger, we’re still left with a hundred events per second, each characterized by about a megabyte of data. Now we have to analyze it. And by “we,” I mean “the thousands of members of the ATLAS and CMS experiments, working at institutions all over the world” (which don’t actually include me). For the physicists to analyze the data, they need access to it, which means a challenge in information transmission. Fortunately, this issue was anticipated for years, and physicists and computer scientists have worked hard to construct a Worldwide LHC Computing Grid that connects computing centers in thirty-five different countries, using a combination of the public Internet and private optical cables. In 2003, a new land speed record for data was set when more than a terabyte of information traveled more than five thousand miles from CERN to Caltech in under thirty minutes. That’s like downloading a full-length feature film in seven seconds.

  This kind of crazy speed is necessary; in 2010, the four main experiments at the LHC produced more than thirteen petabytes of data. The Grid, as it is affectionately known, takes this data and parcels it out to different computing centers around the world, arranged in a series of tiers. Tier 0 is CERN itself. There are eleven Tier 1 sites, which play an important role in sifting through and classifying the data, and 140 Tier 2 sites, where specific analysis tasks are performed. This way every physicist in the world who wants to analyze LHC data doesn’t have to connect directly to CERN, running the risk of breaking the Internet for good.

  Necessity is the mother of invention. It should come as no surprise that the unique data challenges presented by particle physics have led to unique solutions. One of those solutions, from many years ago now, has changed the way we all live: the World Wide Web. The Web originated in a 1989 proposal from Tim Berners-Lee, who at the time was working at CERN, and is currently director of the World Wide Web Consortium. Berners-Lee thought it would be useful for physicists at the lab to have access to different kinds of information, stored on distributed computers, through a hypertext system based on Web documents and links between them. The WWW is this system of interlinked files, built on top of the data-sharing network we call the Internet (for which we can’t give CERN any credit). The Web as we currently know it, and all the effects it has had on our lives, are spinoffs from basic research in particle physics.

  Fabiola Gianotti, the Italian physicist who is the current leader of ATLAS, told me that the most pleasant surprise when the LHC first turned on wasn’t the performance of her experiment, although that was quite impressive—it was that the data transmission system functioned flawlessly right from the start. Not that the process has been entirely without challenges. In September 2008, soon after the first particles had circulated in the LHC, the computing system at CMS was hacked by a group billing itself as the “Greek Security Team.” They did no real damage, and in fact claimed to be performing a public service, as they replaced a Web page with a warning in Greek that said, “We’re pulling your pants down because we don’t want to see you running around naked looking to hide yourselves when the panic comes.” Order was quickly restored, and the disturbance didn’t delay the experiment in any way—although it maybe prompted a closer look at Internet security throughout CERN.

  With the LHC itself humming along, CMS and ATLAS running at the peak of their capabilities, and data being rapidly shared and analyzed around the globe, all the pieces are in place for a full-on assault on the important questions in particle physics. One new particle is in the bag, and we’re looking for more.

  SEVEN

  PARTICLES IN THE WAVES

  In which we suggest that everything in the universe is made out of fields: force fields that push and pull, and matter fields whose vibrations are particles.

  The Insane Clown Posse, a hip-hop duo known for their provocative lyrics and scary clown makeup, caused a stir in 2010 with their single “Miracles.” At this point in their career, Violent J and Shaggy 2 Dope (not the names they were born with) were no strangers to controversy. They had engaged in a feud with Eminem, explored an unsuccessful stint as professional wrestlers, and once gave a brief concert to a bewildered audience only to find out that they were in the wrong building. Their songs tell stories of necrophilia and cannibalism, and in one case said mean things about Santa Claus. Also, Violent J was arrested after a show for hitting an audience member thirty times with his microphone.

  But the “Miracles” controversy was something different. The lads weren’t aiming to shock but to share their wonder at the world around us. It came out like this:

  Stop and look around, it’s all astounding

  Water, fire, air and dirt

  F***ing magnets,

  How do they work?

  Through the magic of the Internet, this little snippet gained quite a bit of notoriety, especially from scientifically minded types who were eager to point out that we actually have a pretty good idea of how magnets work.

  I would like to stand up just a tiny bit for Insane Clown Posse. Yes, we’ve understood magnetism for quite some time, and scientific investigation generally enhances our appreciation of natural phenomena rather than draining the magic out of everything. However, they have put their fingers on an important fact we may be too quick to overlook: Magnets are actually pretty astounding.

  What’s amazing about magnets is not that they stick to metal—lots of things stick to lots of other things, from geckos to pieces of chewing gum. What’s amazing is that, when you bring a magnet close to a piece of metal, you can feel it being attracted before they’re actually touching. Magnets aren’t like adhesive tape or glue, which must be in contact with something before sticking. Magnets reach out, across apparently empty space, to pull things toward them. Kind of freaky, when you think about it.

  Physicists call this type of thing “action at a distance,” and it used to bother the world’s greatest minds as much as it bothers Violent J and Shaggy 2 Dope. These days we are less bothered, because we’ve figured out that the space across which the magnet is apparently reaching isn’t really “empty” at all. It’s filled with a magnetic field—invisible lines of force that reach out from the magnet—ready to grab ahold of any susceptible object that might come their way. We can make these lines of force seem more tangible by putting the magnet in the presence of some small iron filings, which line up with the magnetic field in beautiful patterns.

  The important point is that the magnetic field is there whether or not it’s grabbing on to anything. If there is a magnet, there is a magnetic field that surrounds it, even though we can’t see it. The field is strong when we’re close to the magnet, and weaker far away. In fact, there’s a magnetic field at absolutely every point in space, regardless of whether there are any magnets nearby. The field might be quite small—or even precisely zero—but at every point there is some answer to the question “What’s the value of the magnetic field here?” (It really is “the” magnetic field, not a separate one for every magnet; put two magnets near each other and their fields just add together.)

  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 part
icle 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.