Einstein's theory of general relativity gets most extreme test yet

The most massive neutron star known and its tightly orbiting companion, a wimp of a white-dwarf, have provided one of the most extreme tests yet of Einstein's theory of general relativity.

The theory has again passed with flying colors – for now.

Although the theory has cleared test after test over the past century, researchers keep trying to find its limits. They don't think it's wrong, just incomplete.

The other basic forces of nature – the strong force, which binds particles in an atom's nucleus, the weak force, which governs radioactive decay, and electromagnetism – have found explanations in quantum physics. Gravity is the only force that so far has resisted assimilation.

Many physicists are convinced that resistance is futile and that at some point gravity will yield to a quantum-physics explanation. But that breakdown may only become apparent under the most extreme conditions – conditions no human technology can establish.

So researchers turn to the cosmos for their extremes. And in the binary pair identified as PSR J0348+0432, they've found perhaps the most extreme conditions yet.

The pair is located some 7,000 light-years from Earth. The neutron star is all that remains of a star at least 10 times more massive than the sun that ended its luminous run in an explosion known as a supernova. Astronomers estimate that the neutron star is about 12 miles across. But it is so dense that a thimble full of the matter the explosion left behind would weigh about 1 billion tons.

It's white dwarf companion is the slowly cooling end state of a star like the sun.

White dwarfs are dense as well, typically packing roughly half of the sun's mass into an object slightly larger than Earth. This one, however is a lightweight, tipping the scales at about 17 percent of the sun's mass into an object roughly seven times larger than Earth.

Follow-up observations at radio and visible wavelengths revealed a duo that orbits its combined center of mass once every 2.46 hours. Considering the two objects are about a 500,000 miles apart, that's a mighty brisk pace.

"What we were looking for were changes in the orbital period," Dr. Lynch explains, referring to the time it takes for the two objects to orbit each other.

Those changes arise because the act of orbiting dissipates energy. That energy leaves in the form of gravity waves – ripples in space-time, the very fabric of the cosmos. These ripples travel through space almost as though some interstellar housekeeper was shaking out the sheets.

This loss of energy shortens the time it takes to complete an orbit, signaling that the two objects are slowing and inching closer to one another. Different theories of gravity offer up different predictions for the rate at which the orbits of objects as close and as massive as these decay.

The key issue: "Can we measure that number precisely enough that we can say this agrees with general relativity or disagrees?" Lynch says. After careful measurements using the Arecibo Radio Telescope in Puerto Rico to track the pulsar, and the Very Large Telescope in Chile to track the white dwarf, the answer is: Yes we can, and it agrees with general relativity.

Beyond the test of Einstein's theory of general relativity, the system also poses a challenge to ideas about how binary systems form, Lynch adds.

The neutron star was discovered in 2009 as researchers at the National Radio Astronomy Observatory's facility at Green Bank, W. Va., combed through data gathered two years earlier during a hunt for rapidly spinning neutron stars, dubbed pulsars.

Pulsars earned their name because they emit radio waves as they spin, acting like beacons in the cosmos. Researchers were able to detect this neutron star because it, too, is a pulsar, spinning once every 39 milliseconds.

The team, led by John Antoniadis, with the Max Planck Institute for Radio Astronomy in Bonn, Germany, also combed through data gathered by the Sloan Digital Sky Survey to see if anything showed up in the pulsar's vicinity at visible wavelengths. That's when they found the pulsar's companion.

Astronomers have found other pulsars that spin as fast as the pulsar in the PSR J0348+0432 system, he says. But when such pulsars appear in binary systems, their companions tend to have more mass.

It's the combination of a pulsar with a relatively long spin period in a tight orbit with a relatively low-mass white dwarf "that makes this a little strange," he says, adding that the combination suggests that the system had a unique evolutionary history,

So how fast is the orbital period decreasing? The pace is slowing by about 2.7 ten-trillionths of a second per second. At that rate, some 400 million years from now, the binary system will become an ultra-compact binary system with X-rays for a beacon, the team suggests.

If the neutron star ends up near the high end of the mass scale for such objects as it draws matter from its partner, an eventual merger with the white dwarf could lead to a catastrophic collapse into a black hole – an object whose gravity is so strong that not even light, traveling at 186,000 miles a second, can escape. If the neutron star star ends up with a more middling mass, the white dwarf in essence would be considered a planet once it cools sufficiently.

A formal report of this test of Einstein's theory of general relativity was published Thursday in the journal Science.