[♪ INTRO] It’s been over 50 years since black holes  were first detected, and we now know there may be 40 billion billion of  them in the observable universe.

They’re all over the place, including at  the center of almost every large galaxy. And yet they’re still among the most  mysterious objects in the universe.

Given that they’re completely invisible,  scientists have struggled to answer even relatively simple questions about  them, like how fast they spin, and whether or not they obey  fundamental laws of the universe. But now, a pioneering space telescope  is giving us a brand-new view of black holes… and a chance to  answer these questions at last. Back in 1915, Einstein published  his theory of general relativity, which predicted the existence of black holes.

It indicated that there were objects in  the universe so dense that they created a deep gravitational well  in the fabric of spacetime, trapping everything that got  too close, including light. And they squeezed all this matter  into an infinitely dense point. But the whole swallowing-light thing  made them hard to actually detect.

Black holes don’t emit or reflect any radiation. So, it wasn’t until 1964 that astronomers  first detected one indirectly, based on its gravitational  influence on another star. Since then, scientists have  indirectly observed many black holes.

Some are just a few times the mass of our Sun, while others are many billions  of times more massive. But beyond calculating their masses,  there’s a lot astronomers still don’t know about these bizarre, extreme objects. One of the biggest mysteries is about their spin.

Pretty much everything in the  universe spins like a top, including planets, stars, and galaxies. Black holes are no exception. And as an object contracts and gets  denser, it generally spins faster.

Just like how a figure skater  who tucks in their arms and legs will start spinning so fast they become a blur. It’s an example of a law of physics called  the conservation of angular momentum. And nothing in the universe is exempt.

So you’d think that black holes,  with their infinite density, would rotate so fast that their  spin approaches infinity too. But mathematical solutions to Einstein’s theory of general relativity suggest that’s not true. They indicate that there’s actually a  maximum speed that black holes can spin.

And any extra angular momentum will be  radiated away as gravitational waves. The problem is, the blackness of black  holes has made it tough to figure out whether or not this math holds up in practice. For the longest time, no one could  see how fast black holes really spin.

But in recent years, that’s changed. In 2012, NASA launched an  X-ray telescope called the Nuclear Spectroscopic Telescope  Array, or NuSTAR, into space. They actually launched it from underneath  a plane, which is pretty cool in itself.

But what followed was even cooler. Once the telescope was in low  Earth orbit, around 600 kilometers above Earth’s surface, it  unfolded a 10 meter-long mast and began to scan the sky for  the highest-energy X-rays. These X-rays are especially useful for  probing the universe because they can pass straight through the dust and gas  that obscure many objects in space.

So NuSTAR unveiled many phenomena  that had never been explored before. And one thing that it looked  at was gigantic black holes. Now, it’s not like NuSTAR could  see the black holes themselves, because as long as an object is invisible, it doesn’t matter how powerful your telescope is.

But, while black holes themselves are invisible, they’re often surrounded by a  huge, turbulent disk of material. As material in that disk swirls  in toward the black hole, it gets superheated and  radiates energy away as X-rays. So it glows in wavelengths that NuSTAR can detect.

In 2012, scientists focused the telescope  on the supermassive black hole at the center of a galaxy some  56 million light-years away. They were hoping to use the  information in the X-rays radiating from its disk to  measure the black hole’s spin. General relativity suggests that  the faster a black hole spins, the tighter its disk will hug  the horizon of the black hole.

And the closer it is, the more its radiation  will be affected by twisted spacetime. Previous observations by other telescopes  had looked at X-rays emitted by iron, which has a characteristic signature  that the telescopes could pick out. And they found that these X-rays  were warped and distorted.

But, unlike NuSTAR, these  older telescopes observed lower-energy X-rays that could  potentially be obscured by dust. So, astronomers weren’t sure  whether the distortion they saw was from the dust clouds or warped spacetime. Which meant they couldn’t draw any conclusions for sure about the black hole’s spin.

That’s where NuSTAR came in. NuSTAR detected this same iron signal,  but in higher-energy wavelengths that would cut through any dust. And it found that the signal was still distorted, proving that it wasn’t clouds causing the  distortion, but the black hole’s spin itself.

Now, scientists could precisely  measure the distortions in the iron signal to work out exactly  how fast the black hole was spinning. Through this, they found that  the black hole was rotating at 84 percent of the speed of light. That’s very close to the maximum possible  speed predicted by Einstein’s theory.

It’s the best evidence yet that black  holes do have a maximum spin speed, and that Einstein got this right decades  before we’d even detected a black hole. Beyond providing real-life evidence  supporting theoretical physics, measuring the spin of supermassive  black holes is important for understanding how they formed and  evolved throughout the early universe. NuSTAR has given us a rare look at  objects we have limited ways to explore.

And today, this telescope is still going strong, pulling back the curtain on these  giants and the universe as a whole. If you want to pull back the curtain  on how SciShow Space gets made, you can consider supporting the show on Patreon. Patrons can join our exclusive Discord, and get access to all kinds  of behind the scenes stuff.

To get started, check out  patreon.com/scishowspace. [♪ OUTRO]