Pulsars—fast-spinning, superdense neutron stars—are perhaps the most extraordinary physics laboratories in the universe. Research on these extreme and exotic objects already has produced two Nobel Prizes. Pulsar researchers now are poised to learn otherwise-unavailable details of nuclear physics, to test general relativity in conditions of extremely strong gravity, and to directly detect gravitational waves with a “telescope” nearly the size of our galaxy.
Neutron stars are the remnants of massive stars that exploded as supernovae. They pack more than the mass of the Sun into a sphere no larger than a medium-sized city, making them the densest objects in the universe—except for black holes, for which the concept of density is theoretically irrelevant. Pulsars are neutron stars that emit beams of radio waves outward from the poles of their magnetic fields. When their rotation spins a beam across the Earth, radio telescopes detect that as a “pulse” of radio waves.
By precisely measuring the timing of such pulses, astronomers can use pulsars for unique “experiments” at the frontiers of modern physics. Three scientists presented the results of such work, and the promise of future discoveries, at the American Association for the Advancement of Science meeting in Vancouver, British Columbia.
Pulsars are at the forefront of research on gravity. Albert Einstein published his general theory of relativity in 1916, and his description of the nature of gravity has, so far, withstood numerous experimental tests. However, there are competing theories.
“Many of these alternate theories do just as good a job as general relativity of predicting behavior within our solar system. One area where they differ, though, is in the extremely dense environment of a neutron star,” said Ingrid Stairs, of the University of British Columbia.
In some of the alternate theories, gravity’s behavior should vary based on the internal structure of the neutron star.
“By carefully timing pulsar pulses, we can precisely measure the properties of the neutron stars. Several sets of observations have shown that pulsars’ motions are not dependent on their structure, so general relativity is safe so far,” Stairs explained.
Another prediction of general relativity is that motions of masses in the universe should cause disturbances of space-time in the form of gravitational waves. Such waves have yet to be directly detected, but studies of pulsars in binary-star systems have given indirect evidence for their existence. That work won a Nobel Prize in 1993.