She is among the world’s top theoretical physicists, influential among colleagues and well-known for popularizing the esoteric science of string theory, branes, and unseen dimensions. Now, though, Lisa Randall is focused on experiments underway beneath the floor of a European valley that may provide solid evidence to explain the fundamental nature of matter.
The Large Hadron Collider—LHC for short—is smashing protons at almost inconceivable speeds in its underground ring below the French-Swiss border. If the experiments work as expected, Randall says, they could advance human knowledge on a range of perplexing questions: Why do sub-atomic particles have mass? Why does gravity have such different strength than the other fundamental forces? And could there really be one or more dimensions that are unseen and undetected from our plane?
String theory still generates a lot of buzz, and the program for the AAAS Annual Meeting listed that as the subject of her talk. In fact, though, Randall was there to talk about nuts-and-bolts research. “What I really think is the most exciting thing on the horizon,” she said, “is the prospect of real experiments that might actually tell us what’s going on.”
As important as the immediate research questions, she suggested, is the broader scientific and social value of the LHC experiments. At a time of economic crisis, historic government budget deficits, and intense political conflict, the considerable cost remains justified, she said. Not only might the LHC work drive advances in computing, but research on these questions inspires some of the best young scientific minds of our time. And eventually, she said, it “could lead us to understand the universe better.”
During a 45-minute talk at the AAAS Annual Meeting in Washington, D.C., on Friday 18 February, Randall gave several hundred people in the packed hall a virtual tour of the collider, some of its main experiments, and the questions scientists are exploring there.
Much of Randall’s work has focused on theory and model-building, but also how they can be tested in experiments. While she’s not doing any of the LHC experiments, she’s consulted with colleagues who are. “I’m a theorist,” she explained. “[At the LHC,] I go around and say ‘gee whiz’ just like you would. What I do do is try to predict what they can find, how they should go about looking for it, and when they do find it, what it means.”
In the realm of theoretical physics, Randall is a rock star. She is the Frank B. Baird Jr. Professor of Science at Harvard University. She has published extensively in scholarly journals, and her work is frequently cited by other scholars. Her 2005 book, “Warped Passages: Unraveling the Mysteries of the Universe’s Hidden Dimensions” was named one of the New York Times’ 100 notable books of that year. She has co-curated an art exhibit exploring the theme of scale and has written the libretto for “Hypermusic: A Projective Opera in Seven Planes.” She has appeared on Charlie Rose’s PBS show and on “The Colbert Report.”
The LHC is a particle accelerator that rests in a tunnel not far from Geneva, Switzerland. The tunnel is 27 kilometers (about 16 miles) in circumference and at some points more than 150 meters (about 500 feet) underground. The accelerator fires beams of protons which circle the ring 11,000 times per second, and when the protons collide, instruments are able to capture data that allow scientists to see what kind of particles and energies were shed by the collision—and to recreate what was there just before.
The collider fired for the first time in September 2008, but within days there was an explosion and it had to be shut down. Now it’s operating again, at half the intended total energy, but according to Randall the operations are so smooth that a routine, planned shutdown has been delayed so that researchers can continue their work.
She spoke in superlatives of the instrument: It is the biggest experimental machine ever built. It achieves the highest energy and intensity. It is the coldest place in an extended location on Earth—1.9 degrees above absolute zero, even colder than space. It has the biggest vacuum over a large region, and the biggest magnets.
All of which allows researchers to explore unresolved questions in the Standard Model—the prevailing theory for how fundamental particles interact. (Classic mechanics would predict that electrons would collapse into the nucleus of an atom, but they don’t. Quantum mechanics, by focusing on particles and energy at the atomic and sub-atomic realm, can consider why laws at that small scale are different.)
As Randall explained it, the power of the LHC and its instruments to examine the aftermath of a high-speed proton collision allows research at smaller scales and shorter distances than ever before.
“Why do we want to do that?” Randall asked. “Well, the history of science, very abbreviated, you can think of as studying things in more and more detail, with more and more resolution at smaller and smaller distances, basically getting precision on smaller scales.”
Historically, she added, “no one understood anything for real until they actually went inside. That includes the human body, it includes blood circulation, that includes DNA, that includes all of biology.”
To illustrate what the LHC might find, Randall offered a short course in particle physics, and in the character and known behavior of some particles and some questions still unanswered.
A simulated event in one of detectors at the Large Hadron Collider features the appearance of the Higgs boson. | Image © CERN, the European Organization for Nuclear Research
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One particle of particular interest is the Higgs boson, a fundamental particle which remains hypothetical and unproven. Researchers have suggested that a Higgs field may be spread across space; perhaps it slows other particles as they move through it, and in that process gives them mass.
By extending the current run of the LHC through 2012, “we have a real hope of being able to say something very significant about the existence or non-existence of the Higgs boson,” Randall said. The Higgs boson, she added, “is the tip of the iceberg. It’s not everything we want to know, but it will be very interesting and important.”
Experiments at the LHC also may shed light on whether there are dimensions other than the three familiar to humans: up-down, left-right, forward-backward. Given human perceptions, that’s hard to imagine. But there is “no physical reason that we know that there have to be three dimensions of space,” Randall said. “There could be more. Of course, we as people experience three dimensions, but I think we all know that we are not all there is.”
She compared the human realm to a drop of water on a shower curtain: The drop only travels in the dimension established by the sheet of plastic. But that sheet can exist in a realm of higher dimensions. The particles we know of—and we ourselves—are lodged in a “brane” like the shower curtain. There may be other dimensions, but we don’t yet have awareness of them or means to detect them. Perhaps gravity is profoundly suppressed where we are, but stronger in another dimension.
To test for other dimensions, researchers at the LHC will be looking for another hypothetical particle: the Kaluza-Klein particle. If it exists, they will see what it decays into, just as physicists have discovered other heavy unstable particles here in three-dimensional space. But if the hypothesis is correct, the Kaluza-Klein particle it would have a “momentum associated with travel in an extra dimension.”
While there’s a strong following for such science in the public, to many people these issues reach the highest state of abstraction. And that creates political gravities for the researchers.
In an informal meeting with reporters after the lecture, Randall was asked: What would be more interesting, finding the Higgs boson or not finding it? That depends, she replied, on whether you’re talking physics or politics.
“The best argument to build a higher-energy machine is that we don’t find anything with this machine,” Randall said. “We know that something should be there, and we might just not have enough energy (in the LHC or other instruments). But it would be hard to make that argument to Congress—‘we didn’t find anything, so let’s build something else.’”
From a scientific point of view, though, Randall seems to be banking on dramatic new insights.
“Every time we’ve explored qualitatively different scales, we’ve found something qualitatively different,” Randall said in her lecture. “We have good reason to think that the scale the Large Hadron Collider is probing is very interesting… I think it’s safe to say that we are now entering a new era in physics.”
See full coverage from the 2011 AAAS Annual Meeting.