University of Washington
Adam Richie-Halford was gung-ho for the Air Force when he joined the ROTC program at Embry-Riddle Aeronautical University. As the son of an airline aviator, he dreamt of becoming a test pilot.
Once he entered the engineering physics curriculum, however, his priorities shifted, Richie-Halford says. “I realized I’m more of a nerd than I thought.”
While in the Air Force, Richie-Halford earned a physics master’s degree and became fascinated with quantum mechanics – the rules that govern subatomic interactions. In the quantum world, energy and matter interact as both particles and waves, and two particles can influence each other over great distances.
“Usually when you think about quantum physics, you’re talking about microscopic things you can’t see,” says Richie-Halford, a Department of Energy Computational Science Graduate Fellowship (DOE CSGF) recipient. “But there are some phenomena that would not exist without quantum effects, like superconductivity and superfluidity, that you can see macroscopically.”
With University of Washington Physics Professor Aurel Bulgac, Richie-Halford uses high-performance computing (HPC) to model quantum neutron interactions under extreme conditions, such as inside neutron stars. These dead star remnants are extraordinarily dense – a half-gallon carton of neutron star matter would weigh as much as Seattle-area landmark Mount Rainier – and can have powerful magnetic fields. Neutron stars often reveal themselves as pulsars, sending out jets of particles as they rotate and appearing to radio astronomers as regularly spaced bursts of radiation, like the sweep of a lighthouse.
Scientists theorize that neutrons inside these stars become superfluid – flowing with no loss of kinetic energy. They also may develop quantum vortices – superfluid quantum tornados with empty cores large enough to engulf atomic nuclei. The vortices may become pinned on density irregularities in the star’s crystal crust, and their pinning and depinning is believed to cause pulsar glitches in which the stars occasionally spin faster than usual.
Physicists disagree on the mechanisms behind pulsar glitches and vortex pinning. Computer simulations provide clues, but it’s a massive many-body problem, with each neutron strongly interacting with others in a complex dance.
Bulgac and his colleagues model superfluid vortices with a technique based on density functional theory (DFT), which dramatically reduces a problem’s dimensions. The simulations, however, start with neutrons at a low-energy initial state – a “snapshot at time zero,” Richie-Halford says. “It was only so long before I said, well, how do we get the initial state?”
That brings in a second mathematical technique: quantum Monte Carlo simulation. Monte Carlo methods randomly sample the myriad possible parameter values to arrive at the most likely low-energy state.
To understand these methods, it helps to think of neutron matter’s energy as a landscape with hills and valleys. Richie-Halford wants to find the true ground state, in the lowest valley. He starts with an arbitrary initial state and then perturbs it slightly, calculating the resulting energy differences. In essence, the algorithm takes a random walk across the hills until it reaches the valley – the low-energy ground state.
It may not yield the precise quantity for the system’s minimum energy, but the method provides “an upper bound in our approach.” Comparisons with other techniques show it’s usually near the true minimum.
Richie-Halford’s research goes a step further to explore quasiparticle properties, portraying the strongly interacting neutrons as a system of weakly interacting particles, like a gas. They’re called quasiparticles “because they’re not real, but we can pretend they’re real” to simplify the calculations.
Richie-Halford calculates what properties a quasiparticle formulation must have – what its mass should be, for example – to accurately capture real system interactions. “Then, hopefully, people can use that quasiparticle picture” to learn about strongly interacting systems without huge simulations. He’s found that the effective mass of the strongly interacting neutron system differs little from the noninteracting version. That’s surprising because they behave so differently.
Richie-Halford’s research has run on Titan, a Cray XK7 at Oak Ridge National Laboratory; Stampede, a Dell at the University of Texas; Hyak, UW’s HPC cluster; and other systems.
After leaving the Air Force, Richie-Halford was determined to pursue a science career. He approached Bulgac about joining his group but delayed entry so he and his wife, Zoë True, could serve two years in Morocco with the Peace Corps.
Read the entire article in DEIXIS, the DOE CSGF annual. [PDF, pages 13-15]
Image caption: Quantized vortex connection and separation in the Unitary Fermi Gas. As two vortices approach each other, they combine, exchange vortex material and separate. This is the underlying mechanism of quantum turbulence. Credit: A. Bulgac, Y.-L. Luo, P. Magierski, K.J. Roche, and Y. Yu, Real-Time Dynamics of Quantized Vortices in a Unitary Fermi Superfluid, Science, 332, 1288 (2011).