{♫Intro♫} .

Back in 1915, Einstein published  his General Theory of Relativity.  And while he didn’t realize it, those equations  predicted the existence of extremely dense,   enormously massive objects with gravity so  intense that not even light could escape. You know, black holes.

But even Einstein, the guy who completely  reinvented our notion of space and time,   never actually thought such an extreme  object was physically possible. It took decades of both theoretical  and observational research   to show this phenomenon isn’t  just possible, it’s common. And this year, the 2020 Nobel Prize in  Physics was awarded to three scientists   who helped us understand black holes and  the massive role they play in our universe.

One half of the prize went to  the physicist Sir Roger Penrose,   whose theoretical work proved that black holes  were a natural consequence of Einstein’s theory. See, Einstein’s general relativity   united space and time in a single fabric  called spacetime that gets warped by mass. And what we know of as gravity is  just a manifestation of that warping.

Like, the Sun doesn’t pull  on the Earth—the Earth just   naturally travels along the curves that  the Sun’s gravity creates in spacetime. That was the basis of Einstein’s work. But soon, other physicists started realizing  his equations had some funky consequences.

Like, they implied that a dense enough object  would deform spacetime so much that anything   within a certain distance would have to travel  faster than the speed of light to get away. And since nothing can go faster than the speed  of light, that meant nothing could get back out.   In other words, the region would look like a  three-dimensional hole of blackness in space. Still, Einstein and other physicists figured  this extreme phenomenon could only happen in a   mathematically ideal situation,  where there was a perfectly smooth,   symmetrical sphere of matter.

So they didn’t  actually expect to find these things out in space. That’s where Penrose came in. In the mid-1960s,   he used a mathematical proof to show black holes  really could form in an imperfect universe.

To demonstrate this, he  invented a principle called a   trapped surface. It’s like a bubble in spacetime   where the fabric is so warped from gravity  that space and time essentially swap roles. As a result, moving backward in  space to escape the pull of gravity   becomes as impossible as it would be  for you or me to go backward in time.

Instead, everything that enters  the surface inevitably moves toward   one central spot inside of it. Eventually, all the matter and even the light  inside condense into a singular single point of   infinite density, where all the laws of  nature as we understand them break down. And Penrose found that you don’t need ideal  circumstances to get a trapped surface:  .

Once you have enough matter  trapped within a given volume,   if there’s not enough pressure to counteract  the gravity, it will inevitably collapse. In fact, this was the general  outcome of Einstein’s equations. So black holes were almost certainly out  there.

It was only a matter of finding them. And soon after, astronomers  started doing just that. The second part of the Nobel Prize was split  between the astronomers Andrea Ghez and Reinhard  .

Genzel, who led the teams that discovered the  black hole at the center of the Milky Way. The first clue came less than  a decade after Penrose’s proof,   when astronomers traced an odd  radio signal back to a tiny   region in the very center of our galaxy,  which they called Sagittarius A*. Astronomers had recently begun to suspect that   there might be a supermassive black  hole in the center of our galaxy,   and researchers thought this signal might  be coming from material falling into it.

But they had to wait a bit  before telescope technology   was advanced enough to study that area in detail. Finally, in the 1990s, Ghez and Genzel  formed separate teams to peer through 26,000   light-years of dust and gas between  us and the center of the Milky Way. Genzel’s group first used the New  Technology Telescope in Chile, but later moved to the more powerful Very  Large Telescope on a nearby mountain.

Meanwhile, Ghez and her team used  the Keck Observatory in Hawaiʻi. But neither one was looking directly at the  radio signal anymore. They were more interested   in a few dozen stars they could see whizzing  around it, which they studied in the infrared.

Unlike our Sun, which takes over 200 million years  to orbit the center of the galaxy, the motion of   these stars was noticeable on the scale of a  human lifetime—and that made them great targets. By charting the orbits of  those stars across decades,   the two teams of astronomers could infer the  mass of the mysterious object they were orbiting. And since certain stars got  super close to that radio source,   they could also estimate how small a  space that mass had to be packed into.

In the end, both teams got pretty similar results. They both inferred that Sagittarius A* had a mass   four million times the mass of our  Sun. And their measurements showed   that all that mass was contained within  a volume smaller than our solar system.

According to those equations of Einstein  that got this whole adventure rolling,   the only object that can do that is a black hole. So this year’s Nobel laureates helped  move black holes from the realm of   abstract math to a physical  reality. But as with most science,   it took many great minds across generations  of researchers to get to this point.

Thanks for watching this episode of SciShow  Space News! And if you want to hear the   whole story behind the discovery of the  black hole at the center of our galaxy,   you can check out our episode  on that right after this. {♫Outro♫} .