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Scientists Detected 2 Black Hole Mergers Just 21 Mins Apart, But It's Not What We Hoped

MIKE MCRAE
31 AUG 2019

Last Wednesday, a gravitational wave detection gave astronomers quite the surprise. As researchers were going about their work at the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of gravitational waves rolled in just minutes apart.

 

The first, labelled S190828j, was picked up by all three of LIGO's gravitational wave detectors at 06:34 am, coordinated universal time. The second, S190828l, was measured at 06:55 – a mere 21 minutes later.

Both seemed to be the run-of-the-mill dying screams of black holes as they squish together. But here's why it's so surprising: astronomers wouldn't expect to see a pair of signals in such quick succession.

In fact, this is only the second time two detections have rolled in on the same day. What's more, at first glance they also seemed to echo from more or less the same patch of sky.

"This is a genuine "Uh, wait, what?; We've never seen that before..." moment in gravitational wave astronomy," astrophysicist Robert Routledge from McGill University later tweeted, after openly speculating that it mightn't be a mere coincidence.

Nobody can blame Routledge for getting excited. Unexpected events like this are what discoveries are made of, after all. As he said, this is science in real time.

One possibility briefly kicked around was that S190828j and S190828l were actually the same wave, divided by some sort of distortion in space before being roughly thrown together again. This would have been huge.

 

Gravitational lensing – the warping effect an intervening mass has on space, as described by general relativity – can divide and duplicate the rays of light from far-off objects. It has become a useful tool for astronomers in the measurement of distances.

If this had indeed been a two-for-one deal, it would be the first time a gravitational wave had been observed through a gravitational lens.

Alas, it's now looking pretty unlikely. As the hours passed, new details emerged indicating the two signals don't overlap enough to be originating from the same source.

So close, and yet so far. Right now, this twin event is looking more like a coincidence.

To look on the bright side, we now live in an age where the detection of the crash-boom of galactic giants isn't a rare event, but rather an endless peel of thunder we can record and measure with an insane level of accuracy. It's hard to believe the first collision was detected only a few years ago.

 

Scientists face a problem in the wake of freaky events like this one. On the one hand, wild speculations have a habit of taking on a life of their own when discussed so frankly in a public space, transforming into an established fact while barely half baked.

But time can be of the essence when we're scanning a near-infinite amount of sky for clues, too. By throwing ideas out broadly, different groups of researchers can turn their attention to a phenomenon and collect data while it's still hot.

This is what scientists do best – stumble across odd events, throw out ideas, and debate which ones deserve to be inspected and which should be abandoned.

If there's more to S190828j and S190828l than meets the eye, we'll let you know. For now, we can be disappointed that there was no Earth-shaking discovery, while still being amazed that we have the technology to discover it at all.

We really ought to celebrate the 'disappointments' a little more often.