Einstein and Waves
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.
Albert Einstein (photo by Orren Jack Turner)
Yuan Shi, a graduate student in the Princeton Program in Plasma Physics. (Photo by Elle Starkman/ PPPL)
Russell A. Hulse (photo courtesy The Nobel Foundation)
Nobel Prize for Russell Hulse, formerly of PPPL, and Princeton’s Joseph Taylor
Discovery of the first binary pulsar earned the Nobel Prize in Physics for Russell Hulse, a former physicist at PPPL, and Joseph Taylor, a professor emeritus at Princeton University. Hulse joined the Laboratory in 1977 and retired in 2007.
The pair sighted the first binary pulsar when Hulse was a student of Taylor’s at the University of Massachusetts Amherst in 1974. They spotted the stellar object, a collapsed star that pulsed light while orbiting another star, from the Arecibo Observatory in Puerto Rico. The discovery received the Nobel Prize in 1993 and showed that the pulsar was slowly releasing energy in the form of gravitational waves — the first evidence of the waves predicted by Albert Einstein’s general theory of relativity, which followed his special theory.
Gravitational waves made headlines around the world in 2016 when a pair of U.S. observatories detected them in the form of faint ripples emitted during the violent collision of two massive black holes some 1.3 billion light years from Earth. “This is truly marvelous, exciting news,” Hulse said of the 2016 finding. “It opens a whole new window into observing the universe, as well as further confirming Einstein's general theory of relativity.”
