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Back in 1915, Einstein published\h his General Theory of Relativity.\h

And while he didn’t realize it, those equations\h predicted the existence of extremely dense,\h\h

enormously massive objects with gravity so\h intense that not even light could escape.

You know, black holes.

But even Einstein, the guy who completely\h reinvented our notion of space and time,\h\h

never actually thought such an extreme\h object was physically possible.

It took decades of both theoretical\h and observational research\h\h

to show this phenomenon isn’t\h just possible, it’s common.

And this year, the 2020 Nobel Prize in\h Physics was awarded to three scientists\h\h

who helped us understand black holes and\h the massive role they play in our universe.

One half of the prize went to\h the physicist Sir Roger Penrose,\h\h

whose theoretical work proved that black holes\h were a natural consequence of Einstein’s theory.

See, Einstein’s general relativity\h\h

united space and time in a single fabric\h called spacetime that gets warped by mass.

And what we know of as gravity is\h just a manifestation of that warping.

Like, the Sun doesn’t pull\h on the Earth—the Earth just\h\h

naturally travels along the curves that\h the Sun’s gravity creates in spacetime.

That was the basis of Einstein’s work.

But soon, other physicists started realizing\h his equations had some funky consequences.

Like, they implied that a dense enough object\h would deform spacetime so much that anything\h\h

within a certain distance would have to travel\h faster than the speed of light to get away.

And since nothing can go faster than the speed\h of light, that meant nothing could get back out.\h\h

In other words, the region would look like a\h three-dimensional hole of blackness in space.

Still, Einstein and other physicists figured\h this extreme phenomenon could only happen in a\h\h

mathematically ideal situation,\h where there was a perfectly smooth,\h\h

symmetrical sphere of matter. So they didn’t\h actually expect to find these things out in space.

That’s where Penrose came in. In the mid-1960s,\h\h

he used a mathematical proof to show black holes\h really could form in an imperfect universe.

To demonstrate this, he\h invented a principle called a\h\h

trapped surface. It’s like a bubble in spacetime\h\h

where the fabric is so warped from gravity\h that space and time essentially swap roles.

As a result, moving backward in\h space to escape the pull of gravity\h\h

becomes as impossible as it would be\h for you or me to go backward in time.

Instead, everything that enters\h the surface inevitably moves toward\h\h

one central spot inside of it.

Eventually, all the matter and even the light\h inside condense into a singular single point of\h\h

infinite density, where all the laws of\h nature as we understand them break down.

And Penrose found that you don’t need ideal\h circumstances to get a trapped surface:\h\h

Once you have enough matter\h trapped within a given volume,\h\h

if there’s not enough pressure to counteract\h the gravity, it will inevitably collapse.

In fact, this was the general\h outcome of Einstein’s equations.

So black holes were almost certainly out\h there. It was only a matter of finding them.

And soon after, astronomers\h started doing just that.

The second part of the Nobel Prize was split\h between the astronomers Andrea Ghez and Reinhard\h\h

Genzel, who led the teams that discovered the\h black hole at the center of the Milky Way.

The first clue came less than\h a decade after Penrose’s proof,\h\h

when astronomers traced an odd\h radio signal back to a tiny\h\h

region in the very center of our galaxy,\h which they called Sagittarius A*.

Astronomers had recently begun to suspect that\h\h

there might be a supermassive black\h hole in the center of our galaxy,\h\h

and researchers thought this signal might\h be coming from material falling into it.

But they had to wait a bit\h before telescope technology\h\h

was advanced enough to study that area in detail.

Finally, in the 1990s, Ghez and Genzel\h formed separate teams to peer through 26,000\h\h

light-years of dust and gas between\h us and the center of the Milky Way.

Genzel’s group first used the New\h Technology Telescope in Chile,

but later moved to the more powerful Very\h Large Telescope on a nearby mountain.

Meanwhile, Ghez and her team used\h the Keck Observatory in Hawaiʻi.

But neither one was looking directly at the\h radio signal anymore. They were more interested\h\h

in a few dozen stars they could see whizzing\h around it, which they studied in the infrared.

Unlike our Sun, which takes over 200 million years\h to orbit the center of the galaxy, the motion of\h\h

these stars was noticeable on the scale of a\h human lifetime—and that made them great targets.

By charting the orbits of\h those stars across decades,\h\h

the two teams of astronomers could infer the\h mass of the mysterious object they were orbiting.

And since certain stars got\h super close to that radio source,\h\h

they could also estimate how small a\h 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\h\h

four million times the mass of our\h Sun. And their measurements showed\h\h

that all that mass was contained within\h a volume smaller than our solar system.

According to those equations of Einstein\h that got this whole adventure rolling,\h\h

the only object that can do that is a black hole.

So this year’s Nobel laureates helped\h move black holes from the realm of\h\h

abstract math to a physical\h reality. But as with most science,\h\h

it took many great minds across generations\h of researchers to get to this point.

Thanks for watching this episode of SciShow\h Space News! And if you want to hear the\h\h

whole story behind the discovery of the\h black hole at the center of our galaxy,\h\h

you can check out our episode\h on that right after this.

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