Follow that lighthouse Mark Garlick/SPL

WANT to get to the bottom of one of the biggest mysteries in science? The best way might be to catch sight of a fast-spinning stellar corpse.

General relativity, which describes massive objects like black holes, and quantum mechanics, which governs subatomic particles, are tremendously successful in their own realms. But no one has yet come up with a way to unite them.

A theory of quantum gravity is one of the most sought after in physics (see “The string-loop theory that might finally untangle the universe“). Several candidates exist, but current Earth-based experiments can’t test them directly. Now, Michael Kavic at Long Island University in New York and his colleagues have devised a cosmic test. Their apparatus: a binary system made up of a black hole and a pulsar.


Only tens of kilometres across, a pulsar forms when a star at least eight times the mass of the sun runs out of nuclear fuel and explodes as a supernova. What remains is a rotating object that also emits beams of radio waves from its magnetic poles.

Those poles seldom coincide with its rotational axis, meaning a suitably placed observer will see the radio signal “flashing” past with near-perfect regularity, like a lighthouse beam. This eerie repetition meant that when pulsars were discovered in the 1960s, they were thought to be alien beacons. That regularity also makes them good quantum gravity probes, says Kavic.

“If they do observe something, that would be big. It would be a whole new field of study”

Some theories, like one proposed by Steven Giddings at the University of California, Santa Barbara, in 2014, predict that the black hole’s internal state can be linked to quantum fields outside, in the black hole’s “atmosphere”. This coupling would show up as fluctuations in the space-time around the black hole.

If a pulsar is orbiting it, its radio signal will look normal whenever the pulsar passes in front of the black hole. But when the black hole eclipses the pulsar, the radio beam will reach us via a region of space-time that is steeply curved by the immense gravity.

General relativity predicts that as a result, the signal will arrive early or late at our radio telescopes, with the discrepancy altering smoothly as the pulsar orbits. Quantum gravity, however, says the fluctuating space-time will alter the signal in irregular ways – such that a graph of the arrival times will look “fuzzy”.

Studying a fuzzy pulsar could confirm Giddings’s version of quantum gravity. Kavic and his colleagues propose searching for pulsar-black hole pairs using planned instruments such as the Square Kilometre Array and the Event Horizon Telescope (arxiv.org/abs/1607.00018v3).

Crucially, this type of measurement has been done before: astronomers have examined pulsars in binary systems with neutron stars, which are stellar corpses that don’t emit a lighthouse-like radio beam. “We know how to do this,” Kavic says.

Those observations failed to detect any departures from general relativity. But black holes are more massive than neutron stars, so warp space-time more dramatically and could show a measurable effect.

Some theorists are sceptical. Samir Mathur at Ohio State University in Columbus says the test might just not work. The quantum effects would need to extend far enough outside the event horizon – the surface inside of which matter can’t escape the black hole – to affect those pulsar beams that skirt the black hole. Even Giddings says there’s some luck involved in finding a binary that fits the bill.

That said, Mathur feels the idea is a good one. “If they do observe something, that would be big,” he says. “It would be a whole new field of study.”

This article appeared in print under the headline “Fuzzy pulsars could help unmask quantum gravity”