November 3, 2009
AMHERST, Mass. – Larry Hunter, professor of physics at AmherstCollege, has received a three-year, $359,733 grant from the National Science Foundation to test for violations of two major physical theories: Local Lorentz Invariance (LLI) and time-reversal invariance. The findings of his experiments may one day have profound implications on particle theory and could drastically change scientists’ thinking about what Hunter describes as “the fundamental underlying symmetries of nature.”
Hunter’s first test will explore LLI, a long-held scientific principle that the laws of physics are identical for all observers, independent of each observer’s speed, direction of motion or orientation in space. (The Web site Ars Technica has an excellent explanation of the theory in this piece.) The study, which will focus on the nucleus of a mercury atom, will measure the energy of the particle’s nucleus as a function of the alignment of its spin in relation to the fixed stars. It will involve postdoctoral researcher Steven Peck and several Amherst undergraduate assistants.
Hunter explained that every particle in a nucleus (every neutron and proton, to be specific) has the intrinsic atomic property of “spin.” Spin can be thought of as an arrow—or “a pseudo-vector,” he said—that points in a particular direction. He and his team will try to ascertain whether the energy of the nucleus depends on the direction that the spin points in space. “For example, it might be that the nucleus has a higher energy when the spin points towards the constellation Virgo than when it points away from it,” he said. “Such an observation would indicate a ‘preferred direction’ in space and would violate LLI.” Because a violation of LLI has never been observed, verifying a violation would mean a major rethinking of most physical theories, including relativity.
The experiment builds on a test Hunter did about 15 years ago and will be performed using two key pieces of equipment in his Amherst lab: one of the world’s most sensitive nuclear magnetometers using mercury atoms and one of the world’s most sensitive electron magnetometers using cesium atoms. “We hold the size of the magnetic field of our apparatus constant using the electron magnetometer and then see if the nuclear magnetometer changes as we point our magnetic field in different directions,” he explained of the test. “This allows for an exquisitely sensitive measurement of the energy associated with the nuclear spin.”
Hunter’s second experiment—which is being conducted in collaboration with Joel Gordon, Amherst’s Stone Professor of Natural Science, Emeritus, and a different team of Amherst undergraduates—will test another fundamental symmetry of nature, time-reversal invariance. This test involves seeking out the permanent electric dipole moment (EDM) of the electron (a subatomic particle with a negative electric charge) using a solid called gadolinium iron garnet (GdIG).
According to Hunter, an EDM is a charge distribution characterized by the distance that separates positive and negative charges. In a charged particle such as the electron, it can also be thought of as the distance between the center of mass and the center of charge. Hunter and his group previously attempted to measure the EDM of an electron by applying a magnetic field to a GdIG sample and looking for the predicted voltage across it. This was an important first step, but the precision achieved was inadequate to improve the bounds on the electron EDM. Now, armed with a refined apparatus and greater knowledge of the phenomenon, they hope to improve their sensitivity to the electron EDM by an order of magnitude.
As with the LLI experiment, Hunter explained that the existence of a permanent EDM of any fundamental particle violates a major physical principle—in this instance, time-reversal invariance. (Everything2.com has its own definition of the theory here.) While the scientific community has been able to demonstrate that two exotic particles (the K and B mesons) violate time-reversal symmetry, a permanent electron EDM has yet to have been observed. Whatever the outcome of the experiment, Hunter’s findings could have profound implications on various theories of fundamental particles, especially supersymmetry.
“The National Science Foundation has now supported our research for more than two decades,” he said of his grant. “This new support will allow us to offer Amherst undergraduates unique opportunities to participate in fundamental physics research. I am enormously grateful that this enterprise will be allowed to continue and to grow.”
A member of the Amherst faculty since 1983, Hunter has long engaged in a wide variety of research activities with undergraduates. Thirty-five of his students have completed honors thesis projects under his guidance, and most of them have co-authored publications that resulted from their thesis work.
His longest-running experiment has been his search for an electron EDM. He received the 1990 American Physical Society (APS) Award for Research at an Undergraduate Institution “for his outstanding research in atomic physics, particularly his search for the electric dipole moment of the electron, and for his enthusiastic inclusion of undergraduate students at AmherstCollege in his research program.” In 1994, he served as chair of the APS Precision Measurement and Fundamental Constants topical group.
Hunter received a B.A from ColumbiaUniversity and M.A. and Ph.D. degrees from the University of California, Berkeley. His project is titled “Searching for Preferred Directions in Space and Time.” To read more, go to www.nsf.gov/awardsearch/showAward.do?AwardNumber=0855465 .