Every human being that has ever lived could fit inside 1 cubic inch of space at the center of a newly found neutron star, called PSR J0348+0432, located about 7,000 light years from Earth.
But extreme density is not its most unusual feature. The star, which packs about twice the mass of the sun into an object less than 13 miles in diameter, spins around 25 times a second, emitting a steady and detectable radio pulse. Plus, it has a companion close by, a dying star known as a white dwarf, which circles around every 144 minutes.
Assembling the pieces of the system took some time, but when astronomers realized what they had found an idea took shape: Would the pulsar's extreme gravity cause the pair to move closer together at the rate predicted by physicist Albert Einstein's long-standing general relativity theory? Or, was this a situation better explained by other models, such as those that tiptoe into the realm of quantum mechanics where the rules of gravity break down?
ANALYSIS: Death By Black Hole Firewall Incineration It Shall Be
"There are many theories about what happens to matter under such extreme conditions," John Antoniadis, with the Max Planck Institute for Radio Astronomy in Bonn, Germany, told Discovery News.
Making the measurements required patience and extreme precision, but in the end the gravitational impact predicted by Einstein's theory proved correct. In this case, the loss of energy due to gravity waves from the system escaping into space slowed the pair's orbital period by eight-millionths of a second per year.
"It is essential to know the masses of the pulsar and white dwarf to high accuracy because these are the actual inputs that General Relativity or other theories use to predict the orbital decay," said astronomer Ryan Lynch, with McGill University in Montreal, Canada.
ANALYSIS: Kepler Watches White Dwarf Warp Spacetime
Astronomers also needed a way to precisely measure the pair's orbital period. The pulsating neutron star served as their clock.
"These things came together to make J0348 a power tool," Lynch wrote in an email to Discovery News.
Scientists are continuing to study the system in hopes of learning more about how it formed. They believe it has been in its present state for about 2 billion years.
Astronomers also remain on the hunt for even more extreme conditions to test Einstein's theory. In particular, scientists would like to find a pulsar orbiting a black hole, which is an object even denser than a neutron star that does not even allow photons of light to escape its gravitational grip.
NEWS: Rocky, Low-Mass World Discovered via Microlensing
"This would allow us to test the properties of black holes in great detail and see if they follow Einstein's predictions," Antoniadis said.
Gravity at the surface of J0348, the heaviest neutron star found so far, is 300 billion times stronger than Earth's gravity. At its center, 1 billion tons of matter can fit into an area the size of a sugar cube.
"General Relativity keeps passing the tests we put it to, so we have to keep searching for the most extreme conditions in our universe if we are going to find the point where it breaks down and start to test new physics," Lynch said.
The research is published in this week's Science.