To attain this high energy, the proton beams are accelerated as they go around the tunnel and their paths are kept circular by dipole magnets. The bigger the tunnel, the less energy is required to keep the beams in the correct route. More acceleration is required when the circle is smaller.
The tunnel at CERN was already fixed in size since it had been used for a previous experiment—LEP (the Large Electron-Positron collider), for those who have followed particle physics developments over the years. The fixed tunnel size meant the LHC would require higher-field magnets than had ever been used on this scale before to allow for the high energy of the LHC. The decision was made to keep the energy down to only about 2 TeV for the first run to make sure everything functioned properly. Later the engineers planned to increase it to 10 TeV for the first actual data runs.
On September 12 a transformer broke, causing some small delays. In the meantime, scientists continued testing each of the tunnel’s eight sectors up to 5.5 TeV. Everything worked until the last sector. The crippling accident occurred when the energy was being raised from about 4 to 5.5 TeV, which required between 7,000 and 9,300 amps of current. This was the last moment for something to go wrong, and it did.
We are lucky that engineers and physicists are able to fix things before true operations begin. The accident, nonetheless, meant that the October 2008 celebration was premature. Although many CERN scientists were unhappy about the timing of the event, I saw the day more as a contemplation of this triumph of international cooperation. Many of the foreign partners were visiting for the first time. The person seated next to me during the ceremony worked for the European Union in Geneva but had never set foot inside CERN. Having seen it, he was hooked and plans a return visit with his colleagues.
A few of the speeches were truly encouraging and inspirational. The French prime minister, François Fillon, spoke of the importance of basic research and how the world financial crisis should not prevent scientific progress. The Swiss president, Pascal Couchepin, spoke of the merit of public service. Professor José Mariano Gago, Portugal’s minister for science, technology, and higher education, spoke about valuing science over bureaucracy and the importance of stability for creating important science projects.
One of the more interesting displays was located in the building where the magnets were tested; you could walk around and see the various pieces and how they fit together. The magnets (which are linked to a cryogenic system) are 15 meters long, which was in itself impressive to see. And there was a display with the piece called the bus bar, a superconducting cable that connects a dipole magnet that guides the beams around the ring to a quadrupole magnet that focuses the beams for a collision; splices that hold the cable together were the culprit in the LHC mishap.
Over the past year mechanisms have been put in place to detect similar problems before they can do any damage and to look for heat sources throughout. Fifty-three magnets (14 quadrupole and 39 dipole) have been replaced in the sector of the tunnel where the incident occurred. In addition, more than four kilometers of the vacuum beam tube have been cleaned, a new restraining system for 100 quadrupole magnets is being installed, 900 new helium pressure release ports are being added so that helium won’t do so much harm in the future, and 6,500 new detectors are being added to the magnet protection system. With these new systems to monitor and stabilize the LHC, the kind of pressure buildups that introduced all the damage should be avoided.
We don’t know how long it will take before we start getting answers from the LHC. Some discoveries may happen within a year or two; others could take a decade. It is a little anxiety-provoking, but the results will be mind-blowing, so the nail-biting should be worth it.
For those of you who were relieved by the delay because you thought LHC collisions would create black holes that would destroy the earth, let me assure you that your worries were misplaced. Black holes at the LHC are not even conceivable unless space and gravity are very different from what we thought. Gravity just isn’t powerful enough otherwise. Even if black holes could form, Stephen Hawking’s insight tells us that black holes radiate, and the minuscule ones suggested for the LHC would radiate away their energy immediately. Further, cosmic rays create particle collisions of comparable energy all the time, and if dangerous black holes could exist, they would have already destroyed all the structures we observe in the universe.
So the LHC won’t create dangerous objects. Rather, the particles that it ultimately creates should help answer deep and fundamental questions. We hope to learn about the origin of the mass of elementary particles and why those masses are what they are. Why isn’t everything whizzing around at the speed of light, which is what matter would do if it didn’t have mass? How is it that some force carriers are heavy and others, like the photon that communicates electromagnetism, have no mass? And why do the masses of all these particles have the values that they do? This question has to do with what is known as the Higgs sector. Searches for the particle called the Higgs boson will tell us whether our ideas about how elementary particle masses arise are correct. If current theory is correct, we know quite a lot about this particle’s interactions, but we don’t yet know its mass. So both of the large experiments at the LHC searching for the Higgs boson—CMS and ATLAS—have elaborate and well-defined search strategies in place.
We also hope to learn what underlies dark matter, the elusive stuff throughout the universe whose total weight is five times that of ordinary matter, but which remains invisible because it doesn’t emit or absorb light. Interestingly, stable particles that might be produced at the LHC should have about the right mass and interaction strength to match the inferred properties of dark matter. Exploring this energy scale should tell us which are the most likely candidates and maybe even expose the right one.
And we might learn about the nature of space itself. One theory that another physicist, Raman Sundrum, and I propose suggests there could be an extra dimension in the universe responsible for the weakness of gravity we feel here. Another universe separated from us in an extra dimension could be right next door—that is, separated by an infinitesimal distance—yet not seen. Because of the energy that will be achieved at the LHC, we hope to be able to explain the weakness of gravity and to find out whether an extra dimension of space is just an outlandish idea or an actual fact about the universe.
If our theory is correct, we would expect the LHC to be able to produce particles called Kaluza-Klein (KK) modes. These are particles with interactions similar to those of the particles we know but with heavier masses because they have additional momentum contained in an extra dimension. Only once the energy level is high enough can these particles be produced. The discovery of KK particles would provide an exciting insight into a greatly expanded world.
Another major search target is a supersymmetric theory. Supersymmetric models posit that every fundamental particle of the standard model (the particles that we know exist—electrons, quarks, and so on) has a partner—a particle with similar interactions but different quantum mechanical properties. If the world is supersymmetric, there should be many unknown particles that could soon be found.
Models are just suggestions for what might be out there. We don’t yet know what will be found. These models might correctly describe reality, but even if they do not, they suggest search strategies that will tell us the distinguishing features of as yet undiscovered matter.
The LHC presents a unique opportunity to create new understanding and new knowledge. Physicists are eagerly looking forward to what it will teach us. Will it be extra dimensions? Extra symmetries of space-time? Something completely unforeseen? We don’t know. But let’s look forward to discovering the answers. Nothing will ever replace solid experimental results.