Pulsar Proves Gravitational Constant is 'Rock-Solid'

Through extremely high precision measurements of a pulsar orbiting a white dwarf star, astronomers have found that the gravitational constant, which dictates the force of gravity, is "reassuringly constant" throughout the universe.

Just as the speed of light in a vacuum (c) and Planck's constant (h) well-known universal constants, the gravitational constant (or simply "G") has been long assumed to be constant everywhere throughout the cosmos. But how can we be sure?

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In the past, scientists have bounced lasers off the moon to measure the Earth-moon distance, thereby arriving at a precise measure of G. But now, astronomers using the Green Bank Telescope (GBT) in West Virginia and the Arecibo Observatory in Puerto Rico have taken a long look beyond the solar system and recorded the steady flashes of radiation produced by a spinning neutron star, or pulsar, thousands of light-years away.

Pulsars are the cosmic clocks of our universe. They are ancient remnants of larger stars that have run out of fuel and gone supernova and now consist of extremely dense, degenerate matter less than 20 miles in diameter. Pulsars also possess powerful magnetic fields that can generate extremely collimated beams of radio emissions. Each time a pulsar spins, the polar beams may sweep in the direction of Earth, registering as a pulse — much like a lighthouse appears to flash in the distance.

By timing these pulses on the finest of scales, astronomers have come to see these objects as the most precise timekeepers in the universe, rivaling even the most advanced atomic clocks we have on Earth.

Now, through the study of one special pulsar called PSR J1713+0747, astronomers have arrived at the best, and most precise, measure of G outside the solar system.

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"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 astronomer Weiwei Zhu, formerly with the University of British Columbia in Canada, in a NRAO press release. "This is an observation that has important implications in cosmology and some of the fundamental forces of physics."

Zhu is lead author of a new study accepted for publication in the Astrophysical Journal.

PSR J1713+0747 is an ideal "laboratory" to study some of the most fundamental quantities of space, time and relativity. For starters, it has a uniquely wide orbit around the white dwarf, taking 68 days for the pulsar to complete one orbit. It is also extremely bright, one of the brightest pulsars known. As pair orbit one another, an extremely tiny amount of energy is lost from the system, via gravitational waves — a phenomenon predicted by Einstein's general theory of relativity.

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Their wide and stable orbit means that this energy loss is extremely small and has a negligible impact on the orbit of the pulsar, making it a prime target for any gravitational observations. (A more compact orbit would cause more energy to be carried away from the system via gravitational waves, introducing errors in the measurements of the pulsar's orbital characteristics.)

So, we can now precisely measure the gravitational nature of this star system — why does it matter?

The pulsar-white dwarf binary are located 3,750 light-years from Earth and the value of G derived over 21 years of radio observations almost exactly match the most precise measurements of G we've carried out in our solar system. Therefore, it appears (in this test at least) that G is constant throughout the known universe.

"Gravity is the force that binds stars, planets, and galaxies together," said astronomer and co-author Scott Ransom, with the National Radio Astronomy Observatory (NRAO) in Charlottesville, Va. "Though it appears on Earth to be constant and universal, there are some theories in cosmology that suggest gravity may change over time or may be different in different corners of the Universe."

"These results — new and old — allow us to rule out with good confidence that there could be 'special' times or locations with different gravitational behavior," said astronomer and co-author Ingrid Stairs, also from the University of British Columbia in Canada. "Theories of gravity that are different from general relativity often make such predictions, and we have put new restrictions on the parameters that describe these theories."

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"The gravitational constant is a fundamental constant of physics, so it is important to test this basic assumption using objects at different places, times, and gravitational conditions," added Zhu. "The fact that we see gravity perform the same in our solar system as it does in a distant star system helps to confirm that the gravitational constant truly is universal."

Interestingly, we're soon going to have another "general relativity laboratory" when the worldwide Event Horizon Telescope (EHT) starts producing high-precision data, possibly by the end of this year.

The EHT is a global interferometer of distributed radio antennae that is recording data from the supermassive black hole at the center of our galaxy, known as Sagittarius A* (or Sgr A*). Astronomers are getting ready, for the first time, to look into a strong gravity laboratory, exposing the most extreme gravitational environment known, potentially exposing physics beyond general relativity.

It will be interesting to see if the value for G holds steady even at the edge of an event horizon…

For more about the Event Horizon Telescope, read "Event Horizon Telescope Will Probe Spacetime's Mysteries."

Source: NRAO

This article was provided by Discovery News.