Millisecond pulsar, left foreground, is orbited by a hot white dwarf star, center, both of which are orbited by another, more-distant and cooler white dwarf, top right/
In studying the extremes of the universe, there are instruments you can’t just go out and build and install in some laboratory somewhere. Phenomena exist that require scales at the size of solar systems to study. One of them is gravity, the fundamental force that drives the never ceasing dance of celestial objects while keeping us pinned here on Earth along with an atmosphere we can breathe (to start). Strangely (it seems), gravity also happens to be the weakest of the fundamental forces by a longshot. Compared to, say, electromagnetism, gravity is an imperceptible breeze. But it holds together the universe.
So, we don’t really look for gravity over small distances. Well, we do, but it’s hard enough just to detect gravity between small objects, let alone answer questions about it. We look instead to space, and what gravity does to influence the behavior of very, very large things like planets and stars and galaxies. The National Radio Astronomy Observatory in West Virginia (so happens we made a documentary about it) is boasting the discovery of what might be the best ever stellar system to study gravity: two white dwarf stars hanging out in a very small space (smaller than Earth’s orbit around the Sun) with a millisecond pulsar. What’s governing the motion of this trio is only the purest high-grade gravity.
A West Virginia University grad student named Jason Boyles originally discovered the pulsar as part of a large-scale search for the bizarre stellar objects. A pulsar is actually a small neutron star that rotates very quickly as a sort of cosmic beacon or lighthouse; the pulsar in question, located about 4,200 light years from Earth, spins around 366 times every second. That high frequency spin allows astronomers to have a neat way of examining even the slightest gravitational effects between the three objects.
Basically, using a wide array of telescopes on Earth—including the NRAO's GBT, the Arecibo radio telescope in Puerto Rico, and the Westerbork Synthesis Radio Telescope in the Netherlands—we can chart the system with incredible precision while watching the pulsar for gravitational perturbations. As the three objects move around in accordance with gravity, the spinning pulsar’s radio wave beacon will change or shift just a little bit as the system changes shape.
Using this data, researchers have managed to chart the three-star system with perhaps the greatest precision ever made in astrophysics. Consider this: measurements made from 42,000 light years away with accuracy down to hundreds of meters. That’s nuts. Imagine spying on a virus from a telescope on Pluto.
The Strong Equivalence Principle
Having this system in the collective sights of Earth’s radio observatories means potentially answering one of the more interesting questions in physics. “While Einstein's Theory of General Relativity has so far been confirmed by every experiment, it is not compatible with quantum theory,” said the NRAO’s Scott Ransom in a press release. “Because of that, physicists expect that it will break down under extreme conditions." Testing the resilience of General Relativity involves testing what’s known as the Strong Equivalence Principle.
Strong Equivalence can be looked at as a variation of a very old idea about gravity. Recall Galileo’s drops from the Leaning Tower of two balls of different masses, demonstrating that objects will experience gravity in the same way regardless of mass. In the Strong version, the principle states that the internal structure and nature of an object doesn’t affect its gravitational pull. For example, a supernova collapsing into a neutron star shouldn’t change its total gravitational effect as some of the supernova’s mass is converted into gravitational binding energy (which keeps the newest incarnation of the star from dissipating into space) and that has pull too. If the principle were false, astronomers would find different gravitational effects from the two white dwarf stars and the pulsar.
“Finding a deviation from the Strong Equivalence Principle would indicate a breakdown of General Relativity and would point us toward a new, correct theory of gravity," said the University of British Columbia’s Ingrid Stairs. And, as always in science, finding things that are wrong is often the best or at least most exciting result, even if that wrong thing happens to be Einstein.