Latest / Talking Science / Black Holes – What Happens at the Edge of Forever

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Black Holes – What Happens at the Edge of Forever · Talking Science

0:00
Imagine for a second that you cross a threshold
0:02
in space and suddenly turning around to go back
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the way you came is no longer a physical direction.
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Right. It just ceases to exist. Yeah, it becomes
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an impossibility. And not because your engines
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aren't strong enough or, you know, because you're
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caught in some invisible tractor beam. No, it's
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because the universe has contorted itself so
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violently that the direction of backwards has
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literally become a journey into the past. And
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since you can't travel back to yesterday, you
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just can't turn your ship around. It completely
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violates our biological intuitive mapping of
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reality. I mean, we're used to three dimensions
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of space where we have total freedom of movement.
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Right. Up, down, left, right. Exactly. And one
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dimension of time where we are dragged relentlessly
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forward. Yeah. But what you are describing is
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a regime where those fundamental architectures
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of reality actually swap roles. And that regime
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is exactly what we are dissecting today on this
0:57
deep dive. So welcome. It's great to be here.
0:59
Today, for you listening, we are pulling from
1:01
a massive stack of astrophysics research, papers
1:05
on general relativity, and some... a deeply unsettling
1:09
quantum mechanics theory. Very unsettling. Yeah.
1:12
And our mission today is to strip away all those
1:15
pop culture exaggerations. You know, the sci
1:18
-fi tropes of cosmic vacuum cleaners and glowing
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wormholes. We want to get away from the movie
1:23
monsters. Exactly. We are looking at black holes.
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But we're looking at them as profound, very real
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physics phenomena that fundamentally break our
1:32
current models of the universe. And I want to
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assure you, the listener, you do not need a...
1:36
science degree for this. We are going to take
1:39
our time and unpack this together. The breaking
1:41
of those models is actually what makes them so
1:43
incredibly valuable to physicists. Black holes
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aren't just destructive phenomena. They are the
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ultimate stress test for our equations. Because
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when you want to find out where a theory is flawed,
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you push it to its absolute limits. Right. And
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there is literally no environment in the universe
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more extreme than the interior of a black hole.
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okay let's unpack this because even the very
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name uh black hole is doing a lot of misleading
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work right out of the gate it's a terrible name
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honestly right it makes you think of an empty
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pit or like a literal hole in the ground but
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it's not a hole at all it's not even a traditional
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object like a planet or a star is it not at all
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To really grasp this deep dive, we first have
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to discard our everyday definitions of objects
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and places. A black hole is actually a region
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of space where matter has been extraordinarily
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compressed. A region of space. Okay. So to truly
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understand how weird this compressed region is,
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we have to look at the invisible line that defines
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it. The event horizon. Yes, the event horizon.
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We throw that term around a lot, but I really
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want to get into the mechanical physics of what
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it actually represents. Because what makes a
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black hole a black hole is that the gravity in
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this region is inescapable. That is the defining
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feature, yeah. And the common definition usually
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brings up escape velocity. So escape velocity
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is basically the speed you need to go to break
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free from something's gravity, right? Exactly.
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On Earth... you just need a big enough rocket.
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Right. You burn enough fuel, you hit about 11
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kilometers per second, and you can leave the
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Earth. But for a black hole, the escape velocity
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exceeds the speed of light. Which is the crucial
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detail. Because, as Einstein showed us, nothing
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in the universe can travel faster than light.
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Absolutely nothing. Nothing. Not matter, not
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light, not even information. So if the required
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speed to leave this region is faster than light,
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Absolutely nothing gets out. So the event horizon
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is just the boundary of this inescapable region.
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But I want to clarify what this boundary actually
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feels like, because sci -fi always makes it look
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like a shimmering force field or a solid wall
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of energy. Yeah, the movies always get that wrong.
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Right. But if you drifted across it, you wouldn't
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feel a bump. There's no wall. No wall at all.
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What's fascinating here is how space -time itself
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bends. Inside the horizon, space and time are
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so severely warped that... away from the center
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ceases to be a direction you can travel. Which
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brings us to this analogy from our source material
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that I absolutely love. It's the next Tuesday
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analogy. It's a perfect way to visualize it.
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Yeah, because once you cross that line, the center
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of the black hole is no longer a physical place
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in space, right? It mathematically becomes a
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moment in your future. Exactly. You can no more
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avoid moving toward the center than you can avoid
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next Tuesday arriving. Wow. Next Tuesday's coming
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no matter what you do. You could fire your rocket
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thrusters with all the fuel in the universe trying
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to fly away, but you're just moving toward next
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Tuesday. In fact, firing your engines might even
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get you to next Tuesday faster, depending on
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how you do it, because of how time dilation works.
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That is deeply unsettling. So if a black hole
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is just this region of severely bent space and
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time, how does the universe actually go about
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making one? Well, to build something that extreme...
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You have to look at the life cycle of massive
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stars. Okay, section two, birth, death, and cosmic
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scales. Let's talk about the stellar standoff.
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The stellar standoff, I like that. I mean, we
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look at stars and see these peaceful, glowing
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orbs, but break down the life of a star, and
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it's really a billion -year standoff between
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two titanic forces. Okay, what are the forces?
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On one side, you have gravity, the sheer mass
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of the star. is relentlessly trying to crush
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all of its matter inward. It's constantly pulling
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everything toward the center. Exactly. And on
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the other side, you have nuclear fusion. Deep
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in the core, atoms are smashing together, creating
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an unbelievable amount of energy that pushes
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outward. So gravity pushes in, fusion pushes
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out. Right. And as long as the star has fuel
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for fusion, those two forces balance perfectly.
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But the fuel isn't infinite. No, it's not. And
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when the fuel for fusion finally runs out, that
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outward push... just instantly vanishes. It just
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turns off. Yep. And gravity, which has been waiting
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for millions of years, definitively wins. The
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core of the star collapses in on itself at a
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significant fraction of the speed of light. Which
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leads to a supernova, right? I mean, for the
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listener, a supernova is when the outer layers
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of the star detonate outward in literally one
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of the brightest events in the universe. It's
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incredibly violent. The outside explodes while
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the inside crushes down. And if that collapsing
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core is heavy enough, it crushes all the way
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down into a black hole. Right. Though I should
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note, if the core is slightly lighter, It halts
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before becoming a black hole and becomes a neutron
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star, which is an equally strange, extremely
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dense leftover. Right. But for our deep dive
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today, we're talking about the cores that are
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too heavy to stop. They just keep collapsing.
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So this is how we get stellar mass black holes.
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Yes. They are formed from dying stars, and they
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usually weigh anywhere from five to maybe a few
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dozen times the mass of our sun. Okay, so that's
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one size. But our sources point out that black
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holes come in vastly different sizes. There are
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also supermassive black holes. Oh, yes. The scale
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difference is hard to even wrap your mind around.
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Supermassive black holes. Black holes can be
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millions to billions of solar masses, and they
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sit right at the centers of galaxies. Including
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our own, right? Our Milky Way galaxy has one.
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We do. It's called Sagittarius A star, and it
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weighs about four million times the mass of our
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sun. Four million suns just lurking in the center
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of our galaxy. But wait, if stellar mass black
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holes come from dead stars, how do we get a black
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hole that weighs four million suns? That is a
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genuine scientific mystery. It's actually an
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open question in astrophysics today. We know
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perfectly well how stellar -mass black holes
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form. But how supermassive ones got so big, we
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aren't entirely sure. Huh. So what are the theories?
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Well, they might have grown slowly over billions
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of years, just swallowing up gas and merging
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with other black holes. Okay, like building blocks.
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Right. But another theory suggests they might
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have formed huge right from the start, from the
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collapse of enormous pristine gas clouds. in
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the very early universe completely bypassing
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the star phase wow just directly collapsing into
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massive black hole yeah direct collapse it's
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a fascinating area of research it really is but
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you know hearing that a four million sun black
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hole is lurking in our galaxy usually triggers
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panic for people Which leads us to the biggest
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myth we need to bust today on this deep dive.
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I know exactly the myth you're talking about.
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The cosmic vacuum cleaner. Yes, the cosmic vacuum
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cleaner. People think black holes roam around
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space just aggressively sucking things up. They
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absolutely do not. So what does this all mean
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for us? Let's use that thought experiment from
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the source material. I think it perfectly illustrates
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the point. What if we magically replaced our
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sun right now with a black hole of the exact
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same mass? It's a great thought experiment. If
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we swapped our one solar mass sun for a one solar
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mass black hole, Earth's orbit would not change
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one bit. Wait, not one bit. We wouldn't get sucked
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in? Nope. We wouldn't get sucked in. Now, to
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be fair, we would quickly freeze to death because
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we wouldn't have the sun's light and heat anymore.
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Right. The lights would go out. That would be
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bad. Very bad. But gravitationally, we would
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confidently keep orbiting on the exact same path.
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Because the rule of gravity behaves exactly the
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same way for a black hole as it does for anything
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else. Exactly. Gravity depends only on two things,
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mass and distance. And since neither the mass
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of the center object nor our distance from it
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changed in our thought experiment, the gravitational
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pull doesn't change at all. Here's where it gets
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really interesting to me, though. If a black
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hole doesn't pull any harder than a star, what
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makes them dangerous? Because they are dangerous,
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right? Oh, absolutely. But what makes them dangerous
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isn't that their gravity reaches out further.
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It's that their mass is incredibly concentrated.
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Concentrated, right. Because they're so small
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and dense, you can get much, much closer to their
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center than you ever could to a normal star.
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Because with a star, you just bump into the surface.
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Exactly. but a black hole has no solid surface
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to bump into. You can get insanely close to the
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center of mass, and that is when gravity gets
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dangerously strong. Okay, so what actually happens
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if you make the mistake of flying directly at
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one and getting too close to that concentrated
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mass? Let's take the journey inward. The journey
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inward. This is where things get a bit gruesome.
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Right. We need to talk about tidal forces because
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gravity gets weaker the further away you are
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from an object. Right. And it gets stronger the
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closer you get. So to define a tidal force for
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you listening, if you fall feet first toward
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a stellar mass black hole, your feet are meaningfully
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closer to the center of the black hole than your
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head is. Yes. By about, you know, five or six
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feet, depending on how tall you are. Right. And
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because you are so incredibly close to the concentrated
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mass, the pull on your feet is overwhelmingly
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stronger than the pull on your head. Massively
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stronger. And the result of that is spaghettification.
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Spaghettification. It's a real technical term.
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You would be stretched lengthways and squeezed
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sideways into a very long, very thin stream of
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matter. It sounds incredibly undignified and
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fatal. Highly fatal. Yeah. But crucially. For
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a small, stellar -mass black hole, this violent,
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stretching death happens before you even reach
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the event horizon. Wait, really? Before you even
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cross the line? Yeah. The tidal forces outside
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a small black hole are so severe that you're
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torn apart in normal space. You never even see
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the inside. That is wild. Okay, so a small black
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hole kills you outside the front door. But wait,
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what does a supermassive black hole do? Because
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that must be a billion times worse, right? This
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is the counterintuitive twist. A supermassive
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black hole is actually much safer to cross. Safer?
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How is a giant black hole safer? Because it is
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so massive, its event horizon is pushed enormously
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far out from its center. We're talking millions
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or billions of miles away from the singularity.
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Oh, I see. So when you cross that boundary, you
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were still incredibly far away from the center.
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The difference in gravity between your head and
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your feet at that massive boundary is quite mild.
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So no spaghettification at the event horizon.
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None at all. You could sail right across the
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event horizon in perfect comfort. No alarms would
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go off. There'd be no sign you crossed it. Just
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smooth sailing. Smooth sailing. But, and this
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brings us back to section one, even though it
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was comfortable, your fate is entirely sealed.
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Because the center is now your future. Exactly.
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You just have a finite amount of time drifting
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inside before the tidal forces deeper in finally
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catch up and finish you off. That is profound
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dread. Right there, just comfortably waiting
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for next Tuesday. So that's what you would experience
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if you fell in. But what if a friend was watching
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you fall in from a safe distance? Ah, now we
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get to the observer, the clock, and relativity.
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General relativity. Because Einstein taught us
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that gravity slows time. He did. And this is
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a very real effect. And to prove this isn't just
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theoretical math for you listening, I want to
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bring up everyday life. We use GPS satellites
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all the time, right? Every single day. And because
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those satellites are up in orbit, they are further
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away from Earth's gravity than we are on the
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ground. Right. The gravity is slightly weaker
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up there. Which means their clocks actually tick
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at a slightly different rate than our clocks
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down here. Exactly. Time runs a tiny bit faster
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for the satellites. And if our software didn't
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mathematically correct for this time difference,
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our satellite navigation would drift by kilometers
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every single day. your phone's map would just
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become useless within hours. Totally useless.
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So time dilation is a proven fact. Okay, so scale
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that up. If a planet's gravity alters time a
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tiny bit, what happens to time at the edge of
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a black hole? Near a black hole, this time -slowing
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effect becomes absolutely enormous. So let's
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go back to the friend watching me fall in. I'm
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falling toward a supermassive black hole. What
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does my friend see? If your friend is watching
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you from a safe distance, they're going to see
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your movements start to drag out. you'll seem
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to slow down. Like slow motion. Yeah, extreme
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slow motion. And the light bouncing off your
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ship actually stretches out to the red end of
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the light spectrum. It gets redder and dimmer.
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Yes. To your friend, you appear to completely
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freeze at the boundary of the event horizon.
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You freeze, you turn red, and you slowly fade
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away. From their perspective, you never actually
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cross into the hole. I just get stuck there forever.
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But from my perspective... From your perspective...
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Looking at your own watch, you fall in and cross
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the boundary in a perfectly ordinary amount of
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time. You don't feel slow at all. Wait, so my
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friend sees me frozen outside forever, but I
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experience falling inside. Who is right? If we
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connect this to the bigger picture of physics,
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both perspectives are 100 % correct. How can
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they both be correct? That doesn't make sense.
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It's because there is no universal clock that
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arbitrates between the two of you. Reality in
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physics is entirely dependent on the observer.
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Wow. Spending time, freezing reality, stretching
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light. I mean, this all sounds like completely
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abstract mathematics. And actually, for decades,
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even Albert Einstein doubted that black holes
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existed in nature. He did. He thought they were
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just a mathematical quirk in his equations that
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the real universe wouldn't actually allow to
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form. So how do we know black holes are real?
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How do we get the receipts? Well, this is how
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science builds confidence. We look for completely
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independent lines of evidence all agreeing with
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one another. And we have three major pillars
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of evidence now. Let's go through the three pillars.
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Evidence number one, invisible orbits. Right.
15:17
So astronomers spent years looking at the very
15:20
center of our galaxy, and they track individual
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stars whipping around an apparently empty point
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in space. Just orbiting nothing. Orbiting what
15:28
looked like nothing. But based on the incredible
15:30
speed of those star orbits, the math proved there
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had to be 4 million suns worth of mass crammed
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into a region smaller than our solar system.
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And only a black hole fits that math. You can't
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put 4 million normal stars in a space that small.
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They'd just collide. Exactly. And that specific
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discovery was so ironclad it won a Nobel Prize.
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Incredible. Okay, evidence number two. Gravitational
15:52
waves. This happened back in 2015. Yes. Detectors
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on Earth, these massive, highly sensitive instruments,
15:59
picked up actual ripples in the fabric of space
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-time itself. Ripples in space? Yes. Two black
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holes had spiraled into each other and merged
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over a billion light years away, and that collision
16:11
sent shockwaves through the universe. When those
16:14
waves washed over Earth, our detectors felt the
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stretch and squeeze of space, and the signal
16:19
matched our theoretical predictions perfectly.
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Which is... Just mind -blowing. We literally
16:23
felt them crash. But human beings are visual
16:25
creatures. Which brings us to evidence number
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three, direct images. Ah, 2019, the Event Horizon
16:31
Telescope. Right. They captured actual images
16:33
of the supermassive black holes in the galaxy
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M87 and our very own Sagittarius A star. But
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I have to ask an obvious question on behalf of
16:42
a listener here. Go ahead. Wait, if absolutely
16:44
no light can escape a black hole, how do you
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take a picture of it? It's a great question.
16:48
The trick is you don't actually photograph the
16:51
hole itself. You photograph the furiously glowing
16:55
gas swirling right around it. Oh, the stuff waiting
16:57
to fall in. Exactly. The accretion disk. It's
17:00
superheated and glowing brightly. So what you
17:02
see in the image is this bright ring of light
17:05
and the black hole is the dark circular void,
17:08
the shadow. right in the very center. The silhouette
17:11
of the black hole against the glowing gas. Exactly.
17:14
Okay. So we have the receipts. We know they're
17:17
out there and we know they trap absolutely everything
17:19
that falls in. But does a black hole last forever?
17:23
Now we are getting to the really cutting edge
17:25
stuff. Section seven. Hawking radiation and the
17:28
ultimate paradox. Right. So in the 1970s, Stephen
17:31
Hawking proved that black holes aren't perfectly
17:33
black. They leak. They leak. Because of extremely
17:37
complex quantum effects happening right at the
17:39
microscopic edge of the event horizon, black
17:42
holes very, very slowly leak energy back into
17:45
space. And this is what we call Hawking radiation.
17:47
Yes. And the consequence of this radiation is
17:50
that black holes evaporate. They evaporate like
17:53
a puddle of water. Basically. It happens incredibly
17:56
slowly. I mean, a stellar mass black hole takes
17:58
vastly longer than the current age of the universe
18:01
to disappear. Yeah. But in principle, given enough
18:04
time, they eventually vanish completely. OK,
18:07
so this gives me a huge aha moment. If the black
18:10
hole eventually evaporates and vanishes, what
18:14
happens to all the stuff that fell in? And there
18:16
it is. You just hit on the information paradox.
18:19
The information paradox. It is arguably one of
18:21
the deepest unresolved conflicts in modern physics
18:24
today. Because it creates a clash of our best
18:26
theories. A fatal clash. So you have quantum
18:28
mechanics, which is the physics of the very small,
18:31
and quantum mechanics insists that information
18:34
about the universe, the exact quantum makeup
18:36
of particles, can never, ever be destroyed. It's
18:39
a fundamental rule. Information must be preserved.
18:41
Always. But then you have general relativity,
18:44
the physics of the very large, which describes
18:46
black holes. And relativity seems to say that
18:50
once stuff falls into a black hole and then that
18:53
black hole evaporates via random Hawking radiation,
18:55
that information is gone forever. Erased from
18:59
the universe. Erased. Completely destroyed. So
19:01
quantum mechanics says it can't be destroyed
19:03
and general relativity says it absolutely is
19:05
destroyed. That is the fault line. Black holes
19:08
are the exact place where our two best, most
19:11
successful scientific theories fatally contradict
19:14
each other. They cannot both be right. They can't.
19:17
And that is why black holes are so important.
19:19
They are the ultimate laboratory where the next
19:22
grand theory of physics, whatever replaces or
19:25
unites these two theories, will have to prove
19:27
itself. Wow. We have really covered some ground
19:31
today. I mean, we've gone from the death of massive
19:33
stars to regions of space where time literally
19:36
freezes. We dispelled the myth of the cosmic
19:39
vacuum cleaner. Right. Earth would be fine orbiting
19:41
a black hole sun. Cold, but fine. And we've arrived
19:44
at the very limits of human understanding with
19:46
the information paradox. For you listening, it's
19:49
incredible to think that black holes, these things
19:51
defined entirely by what they trap, are ironically
19:54
our greatest teachers about how the universe
19:56
actually works. They force us to confront what
19:59
we don't know. They really do. And before we
20:01
go, I want to leave you, the listener, with a
20:04
final provocative thought drawn from this paradox.
20:07
Think about that contradiction at the very edge
20:10
of a dying, evaporating black hole. The moment
20:14
of truth? Exactly. When the universe finally
20:17
forces our hand and humanity manages to resolve
20:20
the information paradox, which of our sacred
20:23
laws will we have to rewrite? A profound question.
20:25
Will we have to completely change our understanding
20:28
of gravity and space? Or will we have to shatter
20:31
the rules of quantum mechanics and accept that
20:33
the universe deletes its own history? One of
20:35
them has to give. Something I have to break.
20:37
Thank you so much for taking this deep dive with
20:39
us today.