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You might remember when scientists first detected gravitational waves back in 2015. It was a pretty huge deal.

They'd spent decades trying to prove this foundational piece of general relativity, and the discovery gave us a whole new window into the universe. But decades before all that excitement came two radio astronomers named Russell Hulse and Joe Taylor. They made their own smaller discovery that paved the way for one of the biggest achievements in observational astronomy.

Hulse was Taylor's PhD student in the early seventies, and he was using the Arecibo telescope in Puerto Rico to look for pulsars. Pulsars are neutron stars that spin really fast and emit jets of energy from their poles, so from Earth they look like blinking stars. The astronomer Jocelyn Bell-Burnell discovered the first one just a few years earlier, in 1967, after finding some odd-looking data while studying black holes.

And in the next few years, scientists discovered dozens more, but pulsars were still kind of mysterious. Hulse and Taylor wanted to learn more about these stars, and they had an idea: if they could find a pair of pulsars in a binary system, they could use information from their orbit to calculate some basic information, like their masses. So Hulse began an astronomical survey, which means he basically just pointed the telescope at the sky in the hopes of finding something exciting.

After two years and 32 discoveries of lone pulsars, in 1974 he found...well, something weird. It seemed to be a pulsar, but its flashing was uneven, which was totally out of character. See, usually pulsars flash like clockwork.

Literally! The timing of their pulses is more precise than an atomic clock. Bell-Burnell and her fellow researchers actually called the first pulsar LGM-1, because the timing was so reliable that it seemed like a signal from little green men.

But this pulsar's flashes varied by as much as 80 milliseconds. So, unless this was an entirely new astronomical object, something seriously odd was going on here. The weirdness of this object actually gave Hulse and Taylor some hope, though.

If the variation happened regularly, that would suggest that something was orbiting the pulsar, giving it a periodic tug. And once they broke down the signal, that's exactly what they found: the pulsar had a companion. Even though they couldn't actually see the second object, they could tell a lot about it based on the pulsar's signal.

The companion was orbiting the pulsar once every eight hours, in a very eccentric, or oval-shaped, orbit. And it exerted a serious gravitational pull on that pulsar. Enough of a pull that it was probably a compact object: either another neutron star or a small black hole.

So, success! They'd found what they were after. But when Hulse and Taylor published their results in 1975, the astronomy world lost its collective mind.

This system was like a ready-made laboratory for testing whether or not gravitational waves actually existed. 'Cause back then, gravitational waves were pure theory. General relativity predicted that they should exist and that they should ripple through spacetime itself. And, yeah, general relativity seemed like a pretty solid theory, but this was a really wild consequence, and we had absolutely no evidence of it.

Technically anything that has mass and moves, like the Earth or even your hand, emits gravitational waves. But the vast majority of the time, they're so incredibly weak that we have no way of detecting them. But two massive objects, like neutron stars, swinging around each other close to the speed of light?

That should make a gravitational splash. In a system like this, the gravitational waves should be big enough to carry away a significant amount of energy; enough for astronomers to measure the effects. Unfortunately, that's not the kind of setup scientists can just whip up in a lab.

And it's not that easy to find in space, either. In fact, before Hulse and Taylor came along, we had never seen such a system, like, out in the wild. So people were excited to see what would happen with the Hulse-Taylor pulsar.

If general relativity was right, some of the gravitational energy that was keeping the pulsars in orbit should radiate out of the system in waves. As the pulsars lost that gravitational energy, they should drop into a closer orbit. At the same time, as the orbit tightened, we should see the pulsars speed up to conserve momentum.

In 1978, Taylor did some follow-up observations on the pulsar's timing, and he found just that! In around four years, the orbit sped up by a fraction of a second, which was something scientists could actually measure. And the amount of orbital energy being lost to the gravitational waves lined up exactly with what general relativity predicted.

It was the first observational evidence that gravitational waves actually existed. And now that astronomers were as certain as they could be that these waves were a thing, they could focus their efforts on detecting them directly. Soon after these results were published, the first proposal for a large-scale, gravitational wave detector was submitted to the National Science Foundation.

It took about 20 years to conceive and build the sucker, but in 2002, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, went online. LIGO is sensitive enough to detect the expansion and contraction of space as much as 10,000 times smaller than a proton. But for the first eight years of operation, it picked up nothing but silence.

In 2010, LIGO shut down for some serious upgrades. The new, souped-up LIGO reopened in the fall of 2015. Days later, it made the first detection of gravitational waves, which astronomers were able to trace back to a pair of merging black holes nearly 1.3 billion light-years away.

All told, we've directly detected gravitational waves from 23 collisions, and two of them have been neutron star mergers, systems similar to the Hulse-Taylor binary. Until we found gravitational waves, we relied on light to tell us just about everything we know about the universe. But for the first time, we have an entirely new medium to probe what's out there.

And while lots of things get in the way of light, gravitational waves go through everything, because they travel through spacetime itself. That means these gravitational waves give us a whole new window into the universe, and they have a ton to show us. But before we get ahead of ourselves, we owe a lot to the chance discovery by a grad student nearly 50 years ago that led us here.

Thanks for watching this episode of SciShow Space! And if you want to learn more about pulsars, check out our episode on how astronomers stumbled across the first one, after mistaking it for an alien beacon. [♪ OUTRO].