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CERN: Expert Q&A

MIT particle physicist Peter Fisher answered questions about particle-smashing at the Large Hadron Collider and other subatomic matters on July 19, 2007.


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Q: We keep hearing about particle accelerators. But do particles in nature go as fast as they will go, artificially, in the LHC? What is the purpose of particle acceleration? Andre G., Guelph, Canada

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Peter Fisher: There are naturally occurring cosmic ray particles of very high energy, much higher than we could ever make on Earth. In the early days, particle physicists used detectors on mountaintops and balloons to study cosmic rays. But then Ernest O. Lawrence built the first accelerator, which could produce many more particles than you could count on from cosmic rays. Accelerators also make particles with known energies, so it is much easier to study what happens when they hit each other.

Q: I "get" that the LHC is designed to collide particles in order to shatter them and reveal their "pieces," but I don't get how you actually project one single particle through the LHC tunnel, keep it moving, and then directed so specifically that it can collide head-on with an opposing particle. I also don't get how you can do this repeatedly to have several collisions every second. Is there any way to explain that in a very simple and concise way? Mary Beth, Excelsior, Minnesota

Fisher: There is more than one particle in the accelerator; there are several thousand of bunches of particles (actually called a "bunch") moving in opposite directions. Most of the time, these bunches are in different channels, guided by magnetic fields. At each experiment, the channels merge and the bunches cross through each other. Each bunch has about a billion particles, so when one bunch passes through another, one or two particles smash into each other, making a spray of particles the experiment detects.

Q: While watching the program on the LHC at CERN, I heard that it would generate about 40 million megabytes of data per second! Where and how would they store that much information, and for how many days? This is leading-edge technology for sure! M. S. Keppler, Houston, Texas

Fisher: Data storage is a huge problem. The experiments do generate about 40 million megabytes of data per second, but the data is very quickly analyzed to see what parts can be left out, and then it is compressed. Then, all the data is stored in large disk farms at CERN and copies are sent to remote sites all over the world for analysis. All the data is kept permanently. The storage is cutting-edge, and the information technology group at CERN is always assessing the latest technologies in both data storage and transmission. However, they are very careful to only use products that have been verified as totally reliable. Everything is commercial; no components are home-built. However, the data-handling system is unique in that it is one of the largest and fastest in the world.

Q: Anticipating the world of knowledge that may be revealed when the LHC is in operation, what kinds of results do you think the LHC will provide (e.g., information on other dimensions or perhaps universes, the potential for new technologies, etc.)? Is what you expect to see different from what you hope to see? Christopher Boss, Battle Creek, Michigan

Fisher: Actually, I'm not sure what to expect to happen at the LHC. We could see evidence for new universes or new dimensions, or something we did not expect at all. As far as technology is concerned, just building the accelerator and big detectors have pushed magnet technology ahead a great deal, not to mention computing, microelectronics, and superconductors.

Q: I have a question, but first a prediction: I think that through the study of particle physics, we'll be able to see other dimensions and gain the ability to "experience" these dimensions. Do you think this will ever happen? J.C. Rivera, Puerto Rico

Fisher: At the LHC, we could experience other dimensions by seeing new particles pop out of them and through our input detector. As we learn more about new dimensions, we will be able to design better experiments to experience more of their properties. However, if there are new dimensions, they will most likely be very small, so I doubt we will be able to move about in them.

Q: As to why gravity is so weak compared to the other forces, I've heard something to the effect that it may result from interaction with a parallel universe. If true, this might be confirmed by observing the higher energy levels created at CERN. I've heard little about this line of reasoning. Can you shed light on this? Bob Whalen, Vista, California

Fisher: There are some ideas that allow for the interaction between two parallel universes or the three space dimensions and one time dimension we live in and other dimensions. The LHC could detect particles that could explain a parallel universe or extra dimensions, but it would only be the very first step toward experimentally studying such a theory.

Q: Do you think the studies at CERN or elsewhere will be able to tell us more about space and time, or potentially make time travel possible? Antonio Carlos Motta, Sao Paulo, Brazil

Fisher: Perhaps: Several of the theoretical ideas relate our theory of gravity, which incorporates our ideas of space and time, with particle physics. So if we see certain kinds of new particles, we could learn more about the structure of space and time. Regarding time travel, it is very hard to say. Most of the ideas of time travel rely on your getting very close to incredibly massive objects (called "cosmic strings"). While it could be that the LHC tells us something about cosmic strings, actually using them to travel in time would be very difficult from a practical standpoint.

Q: Will the results of the subatomic particles that come out of the proton collisions give any evidence of dark matter in space? What specific subatomic particles has mathematics theorized we could expect to discover? Jan DeMeerleer, Spokane, Washington

Fisher: There is a very well-developed theory called "supersymmetry" that predicts dark matter is massive particles. There is no experimental evidence for this theory yet, and one of the main goals of the LHC is to see if supersymmetry is the correct theory or not.

Q: How do the experiments at CERN and the LHC relate to string theory? Could they potentially prove or disprove the theory? Francisco Pedroso, Havana, Cuba

Fisher: String theory is very abstract and has not really connected with what we can measure in a strong way. To really probe most ideas of string theory, we would need much larger accelerators than we have now. However, string theory does make some predictions we may be able to test at the LHC.

Q: Good luck with the LHC. I hope you find out how heavy Higgs is doing and get him to lighten up so I can float to work on an antigravity device or fold the space of my closet and "appear" inside my cubicle. These things always seem to cause as many problems as they solve, however, so that's my question: What, if anything, do we feel we are headed towards and what are we trying to achieve? I know this is a broad question, but I'm assuming this is an infinite endeavor. Am I right? Chris, Austin, Texas

Fisher: Man's nature is to pursue things we do not understand; in that sense, the LHC is part of an infinite quest. More specifically, what we are really after is first a list of all the different kinds of particles there are and second, how they interact with each other. It may be possible that this is achieved in our lifetime.

Q: When the accelerator is completed, how soon after do you expect confirmation of particles like the Higgs boson? Seconds? Minutes? Years? Wes, Fitchburg, Massachusetts

Fisher: How long depends mainly on the mass of the Higgs boson. A very light Higgs could take a long time, several years. A more massive Higgs boson could be seen in a few weeks. But remember, these are very complex experiments that will be operating for the very first time, and we have to be very careful we do not fool ourselves. My best guess it that it will take at least one year and not more than three years to observe the results.

Q: If the Higgs particle does not exist, what is the next step to answering the mystery of mass? Rez, Houston, Texas

Fisher: There are several theories that do not involve Higgs particles. These theories make specific predictions that may be tested at the LHC, so if we do not see the Higgs, we will have to look for evidence that one of the alternate theories is correct. But if we do not find evidence for an alternate theory, theoretical physics will have to come up with something new.

Q: Enough about what the LHC will tell us—what won't the LHC be able to tell us about particle physics? Nicole Ackerman, Stanford, California

Fisher: Most likely, the LHC won't tell us much about neutrinos. Neutrinos are very light, and the effects of their mass will not show up well at the LHC. There are several very important experiments studying neutrinos. One, called EXO, will even tell us if the neutrino is its own antiparticle. (Particles and their corresponding antiparticles have the same mass, but their other properties are opposite.)

Q: Is there a limit to particle size, large or small? Cornelius Kleisma, Grand Rapids, Michigan

Fisher: The range is very large: neutrinos weigh less than a billionth of a proton and light particles, photons, weight nothing at all. On the other end, the heaviest particles people think about have a mass 10,000,000,000,000,000,000,000,000,000 times the proton. The heaviest particle we know about, the top quark, weighs about 200 times as much as a proton.

As far as size, electrons really seem to have no size at all. The largest fundamental particle we know about is the proton, and it is about one 1,000,000,000,000,000th of an inch across.

Q: How "elementary" do you think you can go in particle detection? Could a bigger machine tell you more and if so, how much bigger would it need to be? Why does bigger, in the case of the LHC, seem to mean better? John Casey, San Diego, California

Fisher: The higher the energy of the particles, the stronger the magnets you need to bend them within a circle (the LHC exists in a circular underground tunnel). Since we can only make magnets of a certain maximum strength, we have to make our circles bigger to reach higher energies. In the early 1990s, the United States started building a machine three times more powerful than the LHC, and it was 80 kilometers (50 miles) around. But the project was cancelled.

A new project is starting to build the next accelerator called the International Linear Collider (ILC). The ILC will not be circular, but rather two linear accelerators 15 miles long pointed at each other. But the ILC won't start for another 10 years or so.

Q: Should we continue to build larger accelerators to achieve more fundamental results, or do you expect this thing will answer all the questions nature allows us to answer? Thank you, and have fun in the rabbit hole! Trevor Heitlauf, Orlando, Florida

Fisher: We expect the LHC will give us a new set of questions to answer. Already, physicists are designing the ILC based on our best guesses about what will come out of the LHC. It takes a long time to design and build these machines, so we have to start the next one just when the current accelerator gets finished.

Q: Do groups of physicists reserve time at the LHC and perform what experiments interest them, or is there a general organization to what experiments are run and in what order? How does the process work? Sue, El Cerrito, California

Fisher: Each experiment has about 2,000 physicists who work together to make the experiment run. Everyone has to cooperate, or the experiments will not produce good results. The physicists decide as a group how the experiment will be configured and what data it will take. They then pour over the results and work together to agree on an interpretation. There are many, many different measurements that the experiments make, so there are teams who work on different parts. But before any result is made public, everyone has to agree.

Q: Who is funding the CERN project? Nancy Jones, Gary, North Carolina

Fisher: CERN is supported by the governments of its member states. Each government pays a fraction of its GNP (gross national product) to the operation of the lab and construction of the accelerator and experiments. In the United States, the National Science Foundation and Department of Energy support the U.S. physicists working at CERN.

Q: When the LHC goes online, hopefully, in 2008, can I be there to watch? Sarah Warren,, Tucson, Arizona

Fisher: CERN has many visitors' days and several very good displays on-site. They will certainly have screens giving the machine status all over the lab. Have a look at

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