Astronomers hope to use pulsars scattered around the galaxy as a giant gravitational wave detector. But why do we need them, and how do they work?
Gravitational waves, or ripples in the fabric of space-time, from all sorts of sources constantly slosh throughout the universe. Right now, you are being slightly stretched and squeezed as wave after wave passes through you. Those waves come from merging black holes, the explosions of giant stars and even the earliest moments of the Big Bang.
On Earth, we've developed incredibly sensitive gravitational wave detectors that have been able to sense brief-but-loud events, such as black hole mergers, which last only a few seconds but generate such enormous signals that we can detect them. ("Enormous” is a relative term here; the distortion resulting from the passing wave is less than the width of an atomic nucleus.)
But ground-based detectors have a much harder time finding low-frequency gravitational waves, since those take weeks, months or even years to pass through Earth. Those kinds of low-frequency waves come from mergers of giant black holes, which take a lot longer to merge than their smaller cousins do. Our detectors simply don't have the sensitivity to measure those small differences over such long time spans. For that, we need a much, much larger detector.
So, instead of using instruments on the ground, we can use distant pulsars to help us measure gravitational waves. This is the idea behind so-called pulsar timing arrays.
Powering up the pulsars
Pulsars are already fantastic objects, and that's especially true for the kinds of pulsars used as gravitational wave detectors.
Pulsars are the leftover cores of giant stars and are among the most exotic objects ever known to inhabit the cosmos. They are ultradense balls made almost purely of neutrons, with some electrons and protons thrown in for good measure. Those spinning charges power up incredibly strong magnetic fields — in some cases, the most powerful magnetic fields in the universe.
Those intense magnetic fields also whip up strong electric fields. Together, they power beams of radiation (if you're getting Death Star vibes here, you're not far off) that blast out from the magnetic poles in each direction. Those magnetic poles don't always line up with the rotational axis of the pulsar, in much the same way Earth's North and South magnetic poles don't line up with our planet's rotational axis.
This forces the beams of radiation to sweep out circles in the sky. When those beams cross over Earth, we see them as periodic flashes of radio emission, putting the "pulse" in "pulsar."
Pulsars are incredibly regular. They are so heavy, and spin so quickly, that we can use their flashes as extremely precise clocks. But most pulsars are susceptible to random starquakes (when the star's contents shift around, disturbing the pulsar's rotation), glitches and slowdowns that change their regularity. That means most pulsars aren't good for studying gravitational waves.
So instead, timing arrays rely on a subset of pulsars known as millisecond pulsars, which, as the name suggests, have rotational periods of a few milliseconds. Astronomers think millisecond pulsars are "revived" pulsars, spun up to incredible speeds after infalling material from a companion star accelerates them like a grown-up pushing a kid on a schoolyard merry-go-round.
Because of their ludicrous speed, millisecond pulsars can maintain fantastic precision over very long timescales. For example, one pulsar, PSR B1937+21, has a rotational period of 1.5578064688197945 +/- 0.0000000000000004 seconds. That's the same level of precision as our best atomic clocks.
And those millisecond pulsars are perfect gravitational wave detectors.
Timing the array
Here's how it works. First, astronomers observe the rotational periods of as many millisecond pulsars as possible. If a gravitational wave passes over Earth, over a pulsar or even between us, then as it passes, it will change the distance between Earth and the pulsar. As the wave moves, the pulsar will appear slightly closer, then slightly farther, then slightly closer, and so on until the wave has moved on.
That change in distance will appear to us as changes in the rotational period. One flash from the pulsar may arrive a bit too soon; then another may arrive a little too late. For a typical gravitational wave, the shift in the timings is incredibly tiny — a change of just 10 or 20 nanoseconds every few months. But the measurements of the millisecond pulsars are sensitive enough that those changes can be detected — at least in principle.
The "array" part of "pulsar timing array" comes from studying many pulsars at once and looking for correlated movements: If a gravitational wave passes over one region of space, then all the timings from the pulsars in that direction will shift in unison.
Many collaborations across the world have used radio telescopes to study pulsar timing arrays for decades. So far, they've had limited success, finding shifts in timings from various pulsars but no hints of correlations. But every year, the techniques get better, and the hope is that soon, these arrays will unlock a huge part of the gravitational wave universe.
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