A massive neutron star and its lightweight companion provide a unique space laboratory to test general relativity.
You have to be really dense to prove Einstein right. And the pulsar J0348+0432 is really dense. Twice as massive as the Sun and roughly 20 kilometers wide, at its center this celestial lighthouse fits more than one billion tons into a sugar-cube-size volume. That gives it a surface gravity more than 300 billion times stronger than Earth’s.
So basically, forget breathing deeply.
J0348 also has a wimpy sidekick, a white dwarf less than one-tenth its mass. The white dwarf and neutron star whirl around each other in a fairly circular orbit every 2.46 hours, reaching velocities close to the speed of light.
These aspects of J0348 excite scientists who want to test Einstein’s theory of gravity in extreme environments. The binary isn’t the best all-around test of the famed physicist’s general theory of relativity, but it does probe a strong-gravity environment that hasn’t been accessible to observers before.
J0348 is one of only two neutron stars discovered with a mass of two solar masses. Before the discovery of the other object (J1614–2230) in 2010, astronomers had thought neutron stars didn’t grow above 1.5 solar masses, explains Thibault Damour (Institut des Hautes Etudes Scientifiques, France). “Now they find another one [at two solar masses], which is a big discovery in itself.”
General relativity’s no slouch when it comes to acing tests, but it’s possible that gravity might not follow those rules around massive, dense objects. Astronomers study such exotic objects to find out. They know that general relativity (the physics of big things) and quantum mechanics (the physics of the über-small) don’t meld well, so they suspect there’s a crack somewhere.
J0348 gives researchers a unique opportunity to test for those cracks. Other binary systems have similarly short periods or equally egregious disparities between the two members’ masses, but the combination of both allows observers to look for unique effects that shouldn’t be there if general relativity reigns.
The key is that the two dead stars don’t just orbit ad infinitum. They’re slowly spiraling in toward each other, radiating away energy in the form of spacetime ripples called gravitational waves. General relativity precisely predicts how much the orbit should decay with time. If extra effects are at work, other, unpredicted ripples could also appear.
With such a difference in the pulsar and white dwarf masses, this violation of Einstein’s gravity should be pretty obvious in J0348's orbit. But after careful study using radio observations to time the pulsar’s signal, John Antoniadis (Max Planck Institute for Radioastronomy, Germany) and his colleagues found no sign of extra physics. The orbital period shrinks by 8.6 microseconds each year (give or take 1.4), which is consistent with the predicted value.
J0348 complements another gravity-testing system, the double-pulsar pair J0737–3039, says study coauthor Michael Kramer (Max Planck Institute for Radioastronomy and University of Manchester, UK). J0737 has an orbital period only a couple of minutes different than J0348 — 2 hours 25 minutes versus 2 hours 28 minutes — but with two pulsars blinking steadily and a more elliptical orbit, J0737 reveals relativistic effects that the pulsar-dwarf pair doesn’t.
One really cool example is the Shapiro effect. This effect is a delayed arrival time for a signal coming from a massive object and is created because the beacon’s photons have to climb out of the ditch the object creates in the fabric of spacetime. (Even photons can’t leap over hills in a single bound.) Because J0737’s two pulsars follow elliptical orbits, they’re not always the same distance apart. That variation in turn changes the shape and depth of the gravitational well, which means that the delay also changes throughout the orbit.
J0737 will always be better in terms of the number and measurement precision of relativistic effects, Kramer says. But because its two pulsars have similar masses, effects that would only show up when the masses are different are difficult to test. That’s where J0348 comes in.
The new pulsar-white dwarf pair does rule out some non-Einstein effects (the extra spacetime ripples). But whether gravitational hiccups arise when two strongly interacting bodies are much closer together isn’t clear. Experiments dedicated to hunting for gravitational waves, such as Advanced LIGO, which is scheduled to begin observations by 2014, might reveal more. The growing Event Horizon Telescope network will also peer into our galaxy’s innermost sanctum to determine whether gravity behaves around the supermassive black hole as general relativity predicts.
Below, you can watch a video of a pulsar-white dwarf pair. The relative sizes of the two stars aren't to scale (the neutron star is a whole lot smaller in reality), but the animation does give you a sense of gravitational waves.
Reference: J. Antoniadis et al. “A Massive Pulsar in a Compact Relativistic Binary.” Science, 26 April 2013.