Albert Einstein could only imagine the effects of a gravitational field on a high-precision clock. But now researchers have turned his thought experiment into a reality, and the new observations vindicate Einstein's theory.
The high-precision clock that made the new study possible is a rapidly spinning neutron star called a millisecond pulsar that's orbited by a compact white dwarf star. The pulsar, known as PSR J0437-4715, is about 450 light-years away.
A research team led by astrophysicist Willem van Straten of the Swinburne University of Technology in Hawthorn, Australia, monitored the arrival times of the brief radio beeps emitted by the pulsar, allowing the team to figure out the shape and orientation of the orbit very precisely. They managed to measure the position of the pulsar in the sky to within one-hundred-thousandth of an arc second--the angle subtended by a pixel on your computer monitor as seen from a distance of some 8000 kilometers.
But the most important result, says van Straten, is their measurement of the so-called Shapiro delay. When the white dwarf is on the near side of its orbit, the pulsar's radio signals travel through the white dwarf's gravitational field. According to general relativity, space in a strong gravitational field is curved, so the radio pulses have to travel a slightly longer distance than you would expect, resulting in a delay in their arrival time of about one-ten-millionth of a second. The team reports this delay in the 12 July issue of Nature.
Earlier studies just showed that Einstein's general relativity theory is self-consistent, says Frank Verbunt, a theoretical astrophysicist of Utrecht University in the Netherlands. "But in this case, it's a real independent test of the validity of general relativity," says Verbunt.
--GOVERT SCHILLING
Related site
A primer on general relativity
