I am interested in answering some of the most fundamental questions about the universe we live in through precise observations and laboratory measurements. I am presently engaged in projects that explore the nature of dark matter and dark energy as well as the relationship between gravity and quantum mechanics. In order to understand what our measurements mean, we must understand the shortcomings of our measurement procedures and analysis techniques. Consequently, my work focuses not only on observation and experiment, but also on how the experimental or observational procedure itself affects interpretation of the measurements.
If Lorentz symmetry is broken (see below), then the universe is full of a dynamical fluid that may have significant cosmological consequences, according to Harvard theorists. The fluid is composed of the Nambu-Goldstone bosons resulting from the broken symmetry, which couple only derivatively and are therefore a condensate of ghost particles. This ghost ether is capable of driving the accelerated expansion rate of the universe, and can, under the right circumstances, mimic dark matter. Conveniently, the ghost ether is also expected to interact with ordinary matter, opening the possiblilty of detecting it in the laboratory. In particular, it couples particles with non-zero intrinsic spin together, so they feel an unique force that depends on the relative orientation of their spins and their velocity with respect to the ether.
At the Center for Experimental Nuclear Physics and Astrophysics at the University of Washington, we searched for the interaction between the ghosts and electrons using a sensitive torsion balance containing a large net spin dipole and spin sources placed outside the torsion balance apparatus. The spin sources were made from SmCo magnets and magnet iron, making use of the same premise as the spin pendulum pucks described below. We found no evidence for the ghost condensate, and our experiment places a stringent new upper limit on anomalous forces between electron spins. Our experiment also places limits on axion-like particles, other exotic particles and unparticles, and the role of spin in gravity. Atomic experiments should soon place comparable bounds on similar forces between proton and neutron spins.
Theory references:"Ghost Condensation and a Consistent Infrared Modification of Gravity"
N. Arkani-Hamed, et al., JHEP 0405 (2004) 074
"Universal Dynamics of Spontaneous Lorentz Violation and a New Spin-Dependent Inverse-Square Law Force"
N. Arkani-Hamed, et al., JHEP 0507 (2005) 029
Experimental references: coming soon . . .
Precision frequency standards, in addition to being useful, have traditionally led to wide-ranging scientific advancements. A good example is the surprising discovery that the earth's rotation period can vary by up to 4 microseconds on a daily basis. This led to a veritable revolution in geophysical theory when the the first atomic clocks were turned on close to fifty years ago. In intervening years, atomic clocks have been put on chips, become an integral part of GPS, been flown around in commercial airplanes to make the first direct tests of special relativity, and compared to one another to test the time-dependence of fundamental constants. It is this last application which provides the primary scientific motivation to strive for higher precision in frequency standards.
Narrow optical transitions are appealing candidates for precision frequency standards because they have an intrinsically high quality factor. They have an additional advantage for measurements of the time-variation of the fine structure constant because ratios of optical transition frequencies typically depend only on powers of the fine structure constant and not other parameters that might also be varying in time. Consequently, optical clock comparison tests would be a valuable contribution to the vast but difficult to interpret body of existing measurements. The Fortson group at the University of Washington has chosen to explore neutral Yb as a candidate for a new precision frequency standard.
The Yb atom is a convenient and rich system to work with. It has two cooling and trapping transitions within the reach of modern lasers, and a doubly forbidden transition suitable for a clock with mHz precision (see figure). The ultimate goal is to trap up to a million Yb atoms in an optical lattice, combining the Doppler-eliminating Lamb-Dicke confinement of ion traps with the large ensemble of atoms available from neutral atom trapping techniques. The optical lattice would operate at a "magic" wagelength at which the light shift of the ground and excited states of the clock transition are the same, leaving the transition frequency itself unaffected. This particular clock transition is weakly allowed in the five odd Yb isotopes, allowing us to make one of the first measurements of its frequency by direct laser excitation in the more abundant 171 and 173 isotopes. Although the same transition is inaccessible by an electric dipole transition in the even isotopes, a four-level EIT scheme would also allow an extremely precise frequency determination. During my time in the Fortson lab, I participated in the direct measurement of the clock transition and a calculation to explore the possibility of an Yb EIT clock.
An accessible reference about atomic clocks:"Splitting the second: the story of atomic time"
Tony Jones, IOP Pub., 2000
Measurements of the Yb clock transition:
"Observation of the 1S0-3P0 transition in atomic ytterbium for optical clocks and qubit arrays"
T. Hong, C. Cramer, E. Cook, W. Nagourney, E.N. Fortson, Optics Letters, 30, 19, p.2644-6
"Observation and absolute frequency measurements of the 1S0-3P0 optical clock transition in neutral ytterbium"
C.W. Hoyt et al., Physical Review Letters, 95, 083003
EIT in Yb:
"Optical clocks based on ultranarrow three-photon resonances in alkaline Earth atoms"
T. Hong, C. Cramer, W. Nagourney, E.N. Fortson, Physical Review Letters, 94, 050801
Cavitation, the process by which bubbles form in a liquid, is both of great interest in a number of practical situations and a poorly understood physical process. Bubbles form easily when a liquid is agitated, as occurs when water rushes through hydroelectric power plants and boat propellers. These bubbles are capable of releasing a tremendous amount of energy as they collapse, damaging nearby surfaces and are thus a serious design concern. On the other hand, this cavitation erosion is convenient and useful as a mechanism for ultrasonic cleaning. Despite its ubiquity and simplicity, it is difficult to study cavitation in a controlled manner because bubbles form readily on any impurities present in a liquid and surfaces of the vessel containing it. To truly understand the physical process at work, one must carefully control impurities in the liquid and on container walls, which is difficult to accomplish under ordinary conditions.
At Brown University, Humphrey Maris has made a systematic study of cavitation in liquid helium. Helium is an ideal liquid in which to study cavitation because it remains in its liquid state down to absolute zero, allowing unwanted impurities to be filtered out by cooling the liquid to mK temperatures. Bubbles spontaneously appear in a cell filled with cold liquid helium when the pressure is sufficiently low. By focusing acoustic waves with a hemispherical piezoelectric transducer, we were able to create a well-defined region of low pressure in which we monitored bubble formation by observing laser light scattered by nucleating bubbles. We introduced impurities systematically by placing a radioactive beta-source in the helium cell which emitted electrons into the liquid. My undergraduate thesis work was the first measurement of bubble nucleation on electron impurities in liquid helium-3, laying the groundwork for a comparison of a viscous fermi liquid to superfluid helium-4.
An accessible reference:"Negative Pressures and Cavitation in Liquid Helium"
Humphrey Maris and Sebastien Balibar, Physics Today, Feb 2000, p.29-34
For more on comparing helium-3 and helium-4:
"Quantum Cavitation: A Comparison Between Superfluid Helium-4 and Normal Liquid Helium-3"
S. Balibar et al., Journal of Low Temperature Physics, 114, 3-4, Nov. 1998, p.459-71