Combining quantum mechanics and Einstein’s theory of special relativity to solve puzzles in plasma physics

Among the intriguing issues in plasma physics are those surrounding X-ray pulsars — collapsed stars that orbit around a cosmic companion in a binary system and beam light at regular intervals, like lighthouses in the sky. Physicists want to know the strength of the magnetic field and density of the plasma that surrounds these binary pulsars, which can be millions of times greater than the density of plasma in stars like the sun.

Researchers at PPPL have developed a theory of plasma waves that can infer these properties in greater detail than standard approaches. The new method analyzes the plasma surrounding the pulsar by coupling Einstein’s theory of relativity with quantum mechanics, which describes the motion of subatomic particles such as the atomic nuclei — or ions — and electrons in plasma.

The key insight comes from quantum field theory, which describes charged particles that are relativistic, meaning that they travel at near the speed of light. “Quantum theory can describe certain details of the propagation of waves in plasma,” said Yuan Shi, a graduate student in the Princeton Program in Plasma Physics and lead author of a paper published in the journal Physical Review A. Understanding the interactions behind the propagation can then reveal the composition of the plasma.

Shi developed the paper with assistance from co-authors Nat Fisch, director of the Program in Plasma Physics and professor and associate chair of astrophysical sciences at Princeton University, and Hong Qin, a physicist at PPPL and executive dean of the School of Nuclear Science and Technology at the University of Science and Technology of China. “When I worked out the mathematics they showed me how to apply it,” said Shi.

The analysis combines the techniques of high-energy physics, condensed matter physics, and plasma physics. Putting these disciplines together “gives tremendous power to explain things that we couldn’t understand before,” Shi says.