LONDON: The fact that scientists can see gravity perform the same in our solar system as it does in a distant star system helps confirm that the gravitational constant truly is universal.
Gravity, one of the four fundamental forces of nature, appears reassuringly constant across the universe, according to a decades-long study of a distant pulsar. This research helps to answer a long-standing question in cosmology: Is the force of gravity the same everywhere and at all times? The answer, so far, appears to be yes.
Astronomers using the National Science Foundation’s Green Bank Telescope in West Virginia and its Arecibo Observatory in Puerto Rico conducted a 21-year study to precisely measure the steady “tick-tick-tick” of a pulsar known as PSR J1713+0747. This painstaking research produced the best constraint ever of the gravitational constant measured outside our solar system.
Pulsars are the rapidly spinning, superdense remains of massive stars that detonated as supernovae. They are detected from Earth by the beams of radio waves that emanate from their magnetic poles and sweep across space as the pulsar rotates. Since they are phenomenally dense and massive, yet comparatively small — a mere 12–15 miles (20–25 kilometers) across — some pulsars are able to maintain their rate of spin with a consistency that rivals the best atomic clocks on Earth. This makes pulsars exceptional cosmic laboratories to study the fundamental nature of space, time, and gravity.
This particular pulsar is approximately 3,750 light-years from Earth. It orbits a companion white dwarf star and is one of the brightest, most stable pulsars known. Previous studies show that it takes about 68 days for the pulsar to orbit its white dwarf companion, meaning they share an uncommonly wide orbit. This separation is essential for the study of gravity because the effect of gravitational radiation — the steady conversion of orbital velocity to gravitational waves as predicted by Albert Einstein — is incredibly small and would have negligible impact on the orbit of the pulsar. A more pronounced orbital change would confound the accuracy of the pulsar timing experiment.
“The uncanny consistency of this stellar remnant offers intriguing evidence that the fundamental force of gravity — the big ‘G’ of physics — remains rock-solid throughout space,” said Weiwei Zhu, an astronomer formerly with the University of British Columbia in Canada and lead author of the study. “This is an observation that has important implications in cosmology and some of the fundamental forces of physics.”
