0:00 Okay, get this.0:01 If you could somehow crush our Sun down until it became a black hole, it would be less than three kilometers across.0:09 Wait, what?0:10 Three kilometers?0:11 That’s like...0:12 the size of a small airport.0:15 The whole SUN?0:16 The WHOLE sun!0:17 And you know, at its very center would be a singularity—a point of infinite density.0:23 It's almost the same problem we talked about last week with the Big Bang, just...0:29 inverted!0:30 Okay, so you’ve got my attention.0:32 What even IS a black hole?0:34 Beyond, you know, a scary thing in sci-fi movies.0:38 Right!0:38 It’s actually so much cooler than that.0:41 A black hole is a place in space where gravity is so strong that nothing—and I mean NOTHING, not even light—can escape.0:50 It’s the ultimate one-way street.0:52 So that’s why they’re black.0:55 Because light can’t get out.0:57 Exactly!0:57 And that boundary, that line you cross where escape becomes impossible?1:03 That’s called the event horizon.1:05 It’s the point of no return.1:07 So it's like a cosmic vacuum cleaner, just sucking everything in?1:12 If one came near our solar system, we'd be toast?1:16 Ah, I am SO glad you said that.1:18 That is the biggest misconception!1:20 They are absolutely NOT cosmic vacuum cleaners.1:24 Seriously.1:25 Gravity outside the event horizon works just like it does for any other object.1:31 You could orbit a black hole just like we orbit the Sun.1:35 Really?1:35 You could fly by one?1:37 Totally!1:38 As long as you stay outside that event horizon, you're fine.1:42 But the second you cross it...1:44 you can never come back.1:46 Okay, so how do you even make one of these things?1:50 You said crushing the Sun, but I assume that’s not the usual method.1:55 No, not quite.1:56 The most common way is just...1:58 the death of a truly enormous star.2:01 We’re talking a star at least twenty times more massive than our Sun.2:06 Twenty.2:06 Okay.2:07 So, its whole life, the star is in this perfect balance between gravity trying to crush it and the nuclear fusion in its core pushing out.2:17 But when it runs out of fuel...2:19 gravity wins.2:20 And it wins BIG.2:22 The core just collapses.2:23 It collapses catastrophically.2:26 The outer layers get blasted away in a supernova explosion, but if the core left behind is heavy enough—more than about three times our sun’s mass—nothing can stop the collapse.2:39 It just keeps crushing itself down, and down, and down...2:43 until it forms that singularity.2:46 A point of basically infinite density.2:48 And the event horizon forms around it.2:51 How big is that?2:52 You said the Sun's would be tiny.2:55 It's all about mass!2:56 The rule is roughly three kilometers of radius for every solar mass.3:02 So a black hole with the mass of ten suns would have an event horizon about thirty kilometers across.3:09 The size of a city, basically.3:11 A city-sized object with the mass of TEN suns.3:15 That is...3:16 that's a lot to wrap your head around.3:18 And that's just a baby one!3:20 The supermassive black hole at the center of our own Milky Way galaxy, Sagittarius A, is about four MILLION times the mass of the sun.3:30 Okay, never mind.3:32 My brain just blue-screened.3:34 But here's the part that REALLY gets me.3:37 You want to know what happens if you fall in?3:40 I'm almost afraid to ask.3:42 Is it just...3:43 instant darkness?3:44 A quick end?3:45 Well, the tidal forces would be insane.3:48 Gravity would pull on your feet so much more strongly than your head that you'd be stretched out like spaghetti.3:56 Spaghettification.3:58 I've heard that.3:59 It sounds...4:00 unpleasant.4:01 Oh, it would be!4:02 But here is the absolute wildest part.4:05 As you actually cross the event horizon—the point of no return itself—you wouldn't feel a thing.4:12 Hold on.4:13 What?4:13 No bump.4:13 No sign.4:14 No "Welcome to the Black Hole" banner.4:17 To you, falling in, it would just be another point in spacetime.4:22 You'd float right across it without even knowing you’d just passed the point where escape became physically impossible.4:31 That is...4:31 that's terrifying.4:33 The idea that the point of no return is completely unnoticeable.4:37 Isn't it just the most mind-bending thing?4:41 Your fate is sealed, the laws of physics say you can ONLY move toward the singularity, but you have no idea.4:49 So we’ve got these stellar-mass ones, and then the supermassive ones like the one in our galaxy.4:56 And we've even seen a picture of one, right?4:59 The one from 2019?5:01 Yes!5:01 The Event Horizon Telescope!5:03 They took a picture of the monster at the center of the M87 galaxy—a black hole with the mass of 6.5 BILLION suns.5:11 And it looked exactly like the theories predicted: a black shadow surrounded by a glowing ring of superheated gas.5:20 It was one of the most beautiful moments in modern science.5:24 So we know they're there.5:26 We know how they work, mostly.5:29 But what about the very center?5:31 That singularity.5:32 Ahhh, yes.5:33 The big question.5:34 You said it’s a point of infinite density, but...5:38 that can't be real, can it?5:40 Physics isn't supposed to have infinities.5:43 You are asking the EXACT question that keeps physicists up at night.5:48 The singularity is where our best theory of gravity, Einstein’s general relativity, just breaks.5:55 It gives us an impossible answer.5:58 Most physicists think it’s a sign that we need a better theory—a theory of quantum gravity—to explain what’s really going on in there.6:08 So the biggest, most powerful objects in the universe...6:12 end in a question mark we can't answer yet.6:15 Exactly.6:16 They are the ultimate cosmic mystery.6:18 And that’s why I love them.
