The telescope that Galileo built in the late 1500s had the magnifying power of a pair of inexpensive binoculars available in any Wal-Mart, but it was enough to open up a new world. With this simple instrument, Galileo could see that Jupiter has four moons and that the sun has spots, which led him to the conclusion that the sun was rotating. Most spectacular, he found that the planet Venus had phases—powerful evidence that the Copernican view of our solar system, in which the sun, rather than the earth, is at the center, was correct. As people built better telescopes, knowledge of this complex, beautiful new world of the cosmos evolved. We became aware of a vast universe filled with bizarre objects—pulsars, quasars, black holes—and that we were inhabitants of an insignificant dot, part of a galaxy of billions of stars carrying their own solar systems.
With a few minor technical changes, the telescope was turned inward at the world of the small. The microscope revealed a vast, complex world of microbes so tiny that a thousand could fit comfortably on the period at the end of this sentence. This world eventually came to include genetics, microbiology, viruses and bizarre new worlds many hundreds of times smaller than microbes: atoms! To explain the behavior of atoms, scientists had to invent quantum theory, which led to semiconductors and other technologies that account for a huge portion of the 20th century's economic output.
Such is the power of a good instrument. Since much of nature involves things too small or too distant or too subtle to see, scientific advancement has always required the invention of better tools. Today, the scientific world is witnessing the completion of a new tool, the Large Hadron Collider (LHC). This is no pair of cheap binoculars. It is expected to advance the magnification of the properties of objects by the largest factor in the history of particle physics—by some reckoning, 500-fold beyond what can be achieved today. The LHC is a particle accelerator—a monster-size circular underground tunnel, 4.3 kilometers in radius, located at CERN, the European Organization for Nuclear Research, on the Swiss-French border near Geneva. In the tunnel, powerful superconducting magnets steer protons around a ring where huge voltages accelerate them until they pick up an amazing amount of energy—7 trillion electron volts at their peak. Over the next few months, as technicians bring the vast machinery online, high-energy protons will be made to collide with one another, causing them to break up into thousands of smaller particles, whose short, violent lives will be recorded by nearby detectors. Although the LHC isn't the first collider ever to be built, it attains the highest energy. What this means is that the collisions that take place inside it will be more violent, and that it has the ability to produce 100 times the number of collisions per second of any other collider.
Like Galileo's telescope, the LHC will give scientists new insight into a new world of the very small and, indirectly, of the very large. What will scientists see with the LHC? The machine's reach and sensitivity may well reveal a new world, a gift to the 21st century. What kind of world? Five centuries of hindsight make it possible to enumerate the implications of Galileo's telescope, but we have no such luxury with the LHC. How would a contemporary of Galileo's have been able to extrapolate from the telescope to the iPhone? In view of the understanding humans now have and the blending of the many worlds revealed by the many tools and instruments built since Galileo, will the LHC deliver surprises?
It had better. In this age of tight budgets, the LHC, a worldwide collaboration of thousands of scientists, engineers, students, has cost $8 billion, including significant pieces of national budgets. To appreciate what impact the LHC is likely to have in the coming decades, it's necessary to take a look at the fundamental questions it was built to answer. Only by venturing a few steps into the labyrinth of particle physics can we get a sense of how deeply this tool will look into the nature of the physical world.
At present, complexity is the bane of physicists. The closer we look, the more complicated and unwieldy the physical world seems to become. For the better part of a century, physicists have aspired to some theory of the universe that is simple and beautiful, but what we've found instead is a proliferation of particles and a morass of forces that don't seem to fit together in a coherent way. It's like having a separate remote for the television and another for the DVD player, and along comes the DVR with yet another. What you want is one simple universal remote—in physics, a theory of everything. Nobody believes that the LHC will magically provide one, but we are hoping that it will at least help us tidy things up a bit.
The LHC will bring us simplicity by taking us back to the beginning. It will give us a glimpse of the universe as it was at the moment of its birth. That is significant because things were much simpler back then. The only viable (so far) theory is that the universe was born 13.7 billion years ago in a cosmic explosion—the big bang—which created time and space. In this first instant, everything we see today—all the matter and energy that would ever exist—was compressed into an unimaginably small volume. At this moment, two vastly different domains—the inner space of particle physics, as revealed by the microscope tools (largely particle accelerators), and the outer space of cosmology and astrophysics, as revealed by data from earth-based telescopes and space-based telescopes such as the Hubble—were one and the same. As the infant universe expanded and began to cool, forming stars and galaxies, the realms of the small and the large diverged. Things began to get messy.
To figure out what principles undergird the universe, it's necessary to go back to the moment of the big bang and do some experiments. Unfortunately, that's about as easy as getting an interview with Isaac Newton or Alexander the Great. The next best thing is the LHC. It will enable us to replicate some of the conditions of the first few instants of the universe. Not all the conditions at once, of course, but enough to enable us to begin to understand the processes by which the primordial first particles collided and coalesced to form the nuclei and atoms that compose our sun and its planets. Theoretical physicists take insights from the colliders and weave a story of how the smallest components of matter conspire to make the more exotic objects in the sky—the black holes, pulsars, stellar explosions and so forth. By re-creating conditions in the universe moments after the big bang, the LHC will help us forge a coherent description of the universe.
It would be much easier to explain the current state of physics if we had a coherent view, but we don't. Instead, it's necessary to talk about the questions the LHC was built to answer—each question as one piece of a big puzzle. As we go through the questions, the outline of the puzzle will begin to take shape.
Why are there so many particles? So far, colliders such as Fermilab's Tevatron in Chicago or CERN's e+e- collider, both much smaller than the LHC, have given physicists evidence that atoms are not the smallest, most fundamental particles in the universe. That honor now goes to still smaller particles, called quarks and leptons. (It is now believed that all matter is composed of six types of quarks and six types of leptons.) And this is only the beginning—there are neutrinos and muons and Ws and Zs and so forth.
Why is it all so complex? Unquenchable optimism on the part of theoretical physicists leads them to a conviction that beneath this discouraging complexity is a beautiful simplicity. The theorists' hopes are strengthened by the role that the concept of symmetry seems to play when the theoretical ideas are examined mathematically. A kaleidoscope shows a bewilderingly complex pattern, but it can be explained by a simple pattern and system of mirrors. The LHC, physicists hope, will help them see the simple pattern emerging from the confusion of mirrors.
What holds the universe together? Gravity is the force that keeps my feet on the ground, but it's only one of four forces that exist in the universe. Another is electromagnetism, which is familiar to any schoolchild who has fashioned an electromagnet by twisting a wire around a nail and hooking both ends to a battery. Electromagnetism's crucial role is binding quarks and leptons together to make atoms and atoms together to make molecules. The atom's job is simplified by the existence of two other forces: the "strong" and the "weak," which operate in the domain of the nucleus of atoms. What drives physicists crazy is that all four forces don't mix: we've been able to devise theories linking all but gravity. We have a theory of the electromagnetic force, which makes very successful predictions. Similarly, we have a theory of the weak force and a satisfactory theory of the strong force. The crying need is for a theory that unifies all these three forces and that would also include gravity. (This would be the long-sought—but poorly named—theory of everything.) Although gravity seems like an obvious fact of life to the layperson, to the theoretical physicist it is deeply aggravating. Whereas the other three forces—strong, weak and electromagnetic—apparently have a common origin, gravity spoils everything.
To appreciate why gravity is such a bear requires going a bit further into the labyrinth. According to quantum theory, the force between two objects (either an attraction or a repulsion) requires an exchange of a "force-carrying" particle. Imagine two ice skaters playing catch. When Joe throws the baseball, he recoils from Moe. When Moe catches the ball, he recoils from Joe. It works in a similar way for attraction. Imagine our two skaters facing away from each other, toward separate walls. Joe throws a soft, bouncy ball hard and high toward his wall. It bounces over to Moe's wall and into Moe's glove. Look! They are now drawing together!
The ball—the force-carrying particle—is called a boson, and there's a different one for each type of force. Experimenters working with small colliders have revealed the presence of the bosons that carry the strong force between quarks, electrically charged particles and the weak force. However, the force carrier of gravity—a particle called a graviton—is completely different. Particle accelerators are useless here because the gravitational force is fantastically weak. Do this test: Drop a paper clip. It falls, being attracted by the entire planet. Now, hold the paper clip using the magnet that keeps your shopping list onto the fridge. The pull of the entire Earth fails; the little magnet wins. When the force of gravity is tested carefully against the electrical force, it is found to be weaker by a factor of one followed by 40 zeros.
Is the LHC powerful enough to produce a graviton? No. That would require a much bigger collider. Nevertheless, we do have some hope that the LHC will add to our understanding of Einstein's gravity. It will have to be done indirectly, however. We'll have to look at many other phenomena and make inferences about gravity.
What is the God particle? One of the paths to seeing into the cosmic kaleidoscope would be to observe a particular type of boson called the Higgs. A boson, remember, is a particle associated with a force. The Higgs boson accounts for the mass of other particles. Think of the Higgs as a field of mud. When you walk through it, you move more slowly, as if you had put on weight. By the same token, the presence of a Higgs boson would make a particle heavier. For reasons that are too elaborate to go into here, the Higgs is within the realm of the LHC—it is absolutely possible that we will discover it soon. And finding it would unlock many mysteries. That's why some people like to call it the God particle.
Ask a physicist why it was necessary to build the LHC, and the answer is unvarying: "Higgs!" Talk about the Higgs has been around for decades. Its power to turn the heads of experimentalists, as well as leaders from the United States, Europe and Japan, is impressive. It heads the list of motivations for building expensive particle accelerators.
Here's why: the Higgs could be the cause of the complexity of our array of particles and forces. Call it the Higgs field (think mud), and let's say it pervades all of space. Without a Higgs field, quarks and leptons and all other particles would all have zero mass. The four forces would simplify to one, the array of quarks and leptons would coalesce, and theoretical physicists would turn to the employment ads. With a Higgs field in the picture, particles tramp through mud: the electrons acquire a bit of mass, the muons get more, the beauty quark gets really heavy and the top quark gets obese. Particles called Ws and Zs acquire large masses, whereas the photon simply ignores the Higgs field. But now the mathematics becomes complex, the four forces re-emerge—and there is full employment for theorists.
The Higgs is key to unraveling the "mirror system" in our kaleidoscope example. Higgs also cures some mathematical pathology. Vigorous but fruitless searches for the Higgs were made at the major accelerators, but definitive evidence for Higgs phenomena will almost surely emerge from early LHC research.
How does a collider help solve these mysteries? The LHC is built to cause collisions between particles, and then give physicists a view of the resulting debris. Particles, as we know from quantum theory, are associated with force fields, which means if you find a particle (like the Higgs boson) you've also found the associated force field (the Higgs field). If the energy level of the collider is high enough—and the LHC is the highest yet—its collisions will produce more massive particles. That raises the odds that out of the millions of collisions produced each second, the LHC will produce a Higgs boson. These will then be picked up by detectors hooked to powerful computers, to thunderous applause in the control room. If this happens 10 or 20 times, it will quickly lead to worldwide rejoicing! This is how all the particles we know about—the quarks and leptons and bosons—were discovered in older particle accelerators. What makes the LHC a big deal is that its energy is high enough to produce the Higgs boson.
The quest for unification, however, doesn't end with the Higgs. One of the telltale signs of a unifying "theory of everything" would be something called supersymmetry. This is a mathematical theory suggesting that all the known particles—quarks and leptons—must have twins. None of these twins have been discovered (but that hasn't stopped us from giving them poetic names: squarks, sleptons). The LHC, however, could change that.
Let's go back to that point in time 13.7 billion years ago, when all of space and its contents occupied almost zero volume. Ever since then, the universe has been expanding; all galaxies are moving away from one another. But gravity is attractive, pulling the galaxies closer together, which should slow down the expansion. In 1998, two groups of experimenters tried to measure the rate at which the expansion of the universe was slowing, but their answer came as a shock: the expansion was not slowing; it was accelerating! Something, therefore, must be acting mysteriously to push all matter outward. We call this dark energy. Dark energy is probably one of the most baffling but most important discoveries about our universe. When we calculate the amount of energy required to push all galaxies away from each other, the figure is massive: it accounts for about 75 percent of all the energy in the universe.
How can the LHC help find dark energy? Dark energy has a greater virtue than merely baffling theoretical astrophysicists. Dark energy adds to the total amount of energy in the universe, which compensates for the curvature caused by matter. (Einstein told us that matter causes space to curve, but if energy and matter are balanced, there's no curvature—and the universe is flat.) But like some kinds of energy, dark energy may possibly have a particle associated with it—a "dark energy particle." Because the LHC is designed to look for particles, it could conceivably find a dark energy particle (if it exists).
There's another unsolved mystery about the motion of the galaxies and stars within galaxies. When astronomers calculate how galaxies and stars should move according to the laws of gravity, they find that the equations give them a wrong answer. Observations show that stars and galaxies behave as though there is much more matter in the galaxy than can be deduced by counting stars (estimating by experience the mass of each star and adding them all up). To understand the stability of the stars in a galaxy, one must assume over ten times more matter than is observed. What could be causing this discrepancy? The conclusion is that galaxies are surrounded by clouds of matter that exert gravitational forces but which do not shine—so we call it dark matter.
What is this dark matter made of? All kinds of candidates have been considered. Could it be small dead stars that don't emit any light? Black holes? Huge planets? Today the origin of dark matter is unknown. Nevertheless, if not for dark matter, galaxies would not have formed and we would not exist. The theory of supersymmetry, if true, predicts the existence of a huge number of new particles, twins to the quarks, leptons, and bosons we have met, including some excellent candidates for dark matter. These would show up in the analysis of LHC collisions.
Where did all the antimatter go? In the 1930s, theorists predicted that every charged particle has an antimatter twin—an electron has a positron; a proton has an antiproton. However, in our galaxies and as far as our wonderful tools can reach, all we can see is matter. Its absence is attributed to a tiny asymmetry: When the universe was created, slightly more matter was made than antimatter. When matter and antimatter collide, they annihilate one another, producing photons—particles of light. Thus, all the antimatter is "consumed" by the matter and what's left over is pure matter and light. The nature of the asymmetry is poorly understood, but the LHC may help us understand it.
Are there more than just the four dimensions (three of space and one of time)? One of the more exciting prospects of the use of the LHC is in finding what are called extra spatial dimensions beyond the familiar breadth, depth and height. Extra dimensions are a feature of string theory, one of the most exciting speculations in science, and one approach to figuring out how to unify gravity and the other forces. It proposes that the world is made of tiny vibrating strings. These vibrations are the particles we have discussed. (If you don't understand that, don't worry: few people can get their heads around string theory.) The existence of extra dimensions is one of string theory's most awesome predictions. Most of these dimensions are curled up so tightly that they are unobservable. In some versions of the theory, gravitational energy leaks out of our three-dimensional system into these hidden dimensions, providing a mechanism for the weakness of the gravity force. Using the LHC, we might discover such hidden extra dimensions by studying reactions where energy seems to disappear (the energy moves along dimensions we can't see). Such a discovery would greatly encourage string theorists. (Oh well, not all discoveries are beneficial.)
This brief summary doesn't come close to naming all the expectations of the LHC in solving the puzzle of the universe. Although the machine is now just starting to do its work, the true sharpness of the LHC "telescope" won't become apparent for the next several years, and its magic will truly unfold through 2020. Certainly we will have answers to questions we know how to ask and, if history is any guide, we will also find answers to questions we have never dreamed of asking. Just as we did with Galileo's telescope.