Physical scientist Nima Arkani-Hamed studied metals for a year in graduate school in the physics department at UC Berkeley. After a year, he switched to particles and strings. Soon he saw that the six compact extra dimensions of space required by string theory might be big enough to affect the particles created by the Large Hadron Collider (LHC), the largest and most complex machine ever built, located at Cern, the international physics laboratory near Geneva, Switzerland where the web was invented in the late 1980s.
Arkani-Hamed, now a Princeton physics professor, stars in the independent film Particle Fever about the recent discovery of the Higgs boson at the LHC. Before the discovery, he reflects on the possibility that the particle and the mechanism behind it, the Higgs mechanism, do not exist. The past two decades of his life, all his work on particles and strings, would be meaningless, he says.
It may be an understatement.
Take metals, the topic Arkani-Hamed dropped to pursue particles and strings. Why are they shiny? The late great physical scientist Richard P. Feynman gave a series of lectures on physics to undergraduates at Caltech in the early 1960s and he talks about the reflection of light by metals. He relates their luster to the plasma resonance, a collective oscillation of the conduction electrons in metal. Light excites the plasma resonance, Feynman explains, and the flow of conduction electrons generates the reflected light.
This is the Higgs mechanism! Peter Higgs refers to it in the paper proposing the particle we now call the Higgs boson. The particle realizes at high energy and short distance the same mechanism as the plasma oscillation in metals, Higgs writes. Without this mechanism, metals would have no luster, light would pass through the looking glass and we would have no reflection.
As meaningless as it may sound, such a world is the subject of intense study right now all around the world, even here in Los Angeles. While a graduate student at University of Illinois, Urbana-Champaign, Rahul Roy, now a physics and astronomy professor at UCLA, found that insulators come in two types: ordinary and topological. Ordinary insulators such as diamond and glass, conduct heat but not electricity. They are see-through. Diamonds may shine bright, but they are not shiny. Glass may glitter, but it lacks luster.
The topological insulators found so far are mainly alloys of heavy elements such as bismuth, antimony, selenium, and tellurium. Like ordinary insulators, they have a filled valence band and an empty conduction band. No conduction electrons means no plasma resonance and no reflection of light. The twist is at the surface.
At the interface between an ordinary and a topological insulator, the conduction band and valence band cross, creating massless electrons confined to the surface where the materials meet. Although massless, the electrons move slower than the speed of light. Following Roy’s discovery, a wide variety of experiments have turned up evidence for massless surface electrons in several candidate topological insulators.
Here we have a world where metal is not shiny. The massless surface electrons conduct electricity along the surface, yet light passes through without reflection. What if we could flip a switch between worlds, make ordinary metals clear and turn topological insulators shiny?
Last summer, Zahid Hasan, a colleague of Arkani-Hamed at Princeton, used the brightest source of coherent soft x-rays in the world, the Advanced Light Source at Lawrence Berkeley National Lab in the hills above UC Berkeley, to show that the magnetic insulator sodium iridate is topological. Xiao-Liang Qi at Stanford suggested five years ago that topological insulators with the kind of magnetism seen in sodium iridate (called anti-ferromagnetism for obscure historical reasons) respond to a magnetic field like a switch. Turning on the field couples light to magnetic fluctuations within the insulator via the surface states and switches the material from transparency to perfect reflection, provided the field is big enough and points in the right direction.
What about the other way: flipping a switch to make light shine through a mirror? Six years ago, Aaron Chou at Fermi National Lab in Batavia, Illinois tried just that. Using a giant magnet salvaged from the Tevatron collider, Chou applied a large magnetic field to ultra-short pulses of laser light in the hope of getting some of the light to switch into the axion, a nearly massless and weakly interacting particle. The axion would pass through the mirror then convert back into light on the other side. Chou’s results are consistent with no such switching at the magnetic field strength he used.
Rather than going to larger magnetic fields, Dmitry Budker at UC Berkeley proposed last spring to apply a static electric field to certain atoms and use precision magnetometry to measure the oscillating magnetic field created by the coupling of the nucleus of the atom to the axion. Xiao-Liang Qi made a similar proposal in the context of topological magnetic insulators five years ago. Remarkably, the axion is now believed to be a general property of string theory, following a recent survey of the topic by string theorists Petr Svrcek, formerly at Stanford, and Edward Witten, another colleague of Arkani-Hamed at Princeton. Again, there is the possibility that an experiment, perhaps the one proposed by Budker or the pair proposed by Qi, could show that the axion and the mechanism behind it do not exist. This would be bad news for string theory, including the case of large extra dimensions proposed by Arkani-Hamed. But again this understates things. The axion realizes the same switching mechanism as topological magnetic insulators.
“With one switch, everything changes.” This is the tag-line of the Higgs boson documentary Particle Fever. Indeed there is just one switch that changes metals from shiny to clear and magnetic insulators from clear to shiny. When we switch from studying metals to studying particles, as Arkani-Hamed and Higgs himself did, however, the Higgs mechanism does not change. The Higgs boson is no more, nor no less than the plasma resonance explained by Feynman to bright teenagers near Los Angeles over fifty years ago using math no more complex than high school algebra.