脉冲星：宇宙的物理学“礼物”

Pulsars（脉冲星） , 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（超新星） . They pack more than the mass of the Sun into a sphere（范围，球体） 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 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 theory of General 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.

Recent research on pulsars in binary-star systems with other neutron stars, and, in one case, with another pulsar, offer the best tests yet of General Relativity in very strong gravity. The precision of such measurements is expected to get even better in the future, Stairs said.

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 study of pulsars in binary-star systems have given indirect evidence for their existence. That work won a Nobel Prize in 1993.

Now, astronomers are using pulsars throughout our Milky Way Galaxy as a giant scientific instrument to directly detect gravitational waves.

"Pulsars are such extremely precise timepieces that we can use them to detect gravitational waves in a frequency range to which no other experiment will be sensitive," said Benjamin Stappers, of the University of Manchester in the UK.