Black holes are the most powerful and extreme things in the universe and they are wildly
weird and complicated. What would happen if you fell inside one and what are they really?
First we need to talk about space and time: Space and time are the grand stage where the
play of the universe unfolds. But space isn’t a fixed stage and time doesn’t tick the
same for everyone everywhere. In short, they are relative. Matter bends space and bent
space tells matter how to move. Put some stars and planets on the stage, and it sags underneath
them. That misshapen stage, with all its little warps and dips, gives us gravity. Black holes
do not just bend the stage, they are like trap doors. Places with so much mass that
the universe formed a ‘no-go’ zone where the rules change.
Most black holes form when very massive stars die. We explained this process in detail in
our neutron star video – all you need to know, that in the final moments of really
massive stars, their insides implode, at nearly a quarter of the speed of light. This packs
so much mass, so close together that it creates something so dense that it sort of breaks
the stage of the universe. A black hole with ten times the mass of the sun would be barely
60 kilometers across.
If you look directly at a black hole it looks like... nothing. The space under their control
is blocked by an invisible, one-way border called the event horizon. The event horizon
forms a shell around a region of space that, once entered, is shielded from the rest of
the universe forever. Because the black hole trap door deforms space so much, not even
light can escape it. And with nothing escaping to transfer information from the inside, it’s
impossible to tell what it really looks like.
We still can observe black holes because of their effect on matter. Things can orbit black
holes just as they can orbit the sun or a planet. Many black holes have discs of matter
orbiting outside the event horizon. This matter can become incredibly hot as close orbits
can speed this matter up to half the speed of light, and tiny amounts of friction and
collisions between particles heat them to a billion degrees, making the space around
these black holes, ironically, incredibly bright.
What would happen if you would try to get close, or even inside a black hole? First
of all, you would see the strangest funhouse mirror in the universe. Matter isn’t the
only thing orbiting a black hole: gravity is so strong near them that light can orbit
too. If you hovered just outside the event horizon, at the photon sphere, in any direction
you’d just see… yourself! Straight ahead would be the back of your own head, as light
from your back travels around the black hole to your eyes. Gravity also alters the passage
of time itself. The stronger the gravity, the slower time passes. While you watch the
universe above you speed up, those far away will watch you in slow motion. If you chose
to fly away from the black hole, you might find that eons have passed for the rest of
the universe, a freakish one-way time-travel trip to the future where your loved ones are
long dead.
But getting close to a black hole can be incredibly dangerous. A painful death by ‘spaghettification’
awaits you. Because your feet are closer to the black hole than your head they feel a
stronger pull of gravity, enough to pull you apart. As you descend it gets worse, the pulling
gets stronger, your body squeezed thinner and straighter until you’ve been reduced
to a thin stream of hot plasma, gobbled up in one final slurp, never to be seen again.
Spaghettification is only a risk with smaller black holes, since they have much smaller
radii. If you go to the center of a galaxy and find a supermassive black hole, you might
be able to experience crossing the event horizon.
As you approach the event horizon, a distant observer would think they never saw you enter
it, seeing you stop and fade. The last light you emit trickling up and out, away from the
event horizon. Meanwhile, from your perspective the void of the black hole rises up to meet
you, as light from fewer directions can reach you. The blackness envelopes you until your
only view of the universe you left is a tiny spot of light.
Here, inside the event horizon, space and time are so horribly broken that real time
travel is possible, so it’s probably a good thing that nothing gets out. If anything could
escape it could create all sorts of time travel paradoxes and issues that break the universe.
As scary as the event horizon is, it keeps us safe from that drama.
Whether you have survived this long doesn’t really matter, as now there is only the certainty
of crushing death in your near future. Inside the event horizon space time itself is so
bent and warped, that whatever direction you move here, every “forward” you go, leads
only towards the center of the black hole. Trying to go any direction only brings you
to the center faster. To survive the longest, you must do: nothing.
In the center of the black hole, we find the singularity. A single point with all the matter
that has ever crossed the event horizon, all crushed to a point infinitely small. There
is no memory of the things that made it as stuff disappears down the black hole trapdoor
forever. The singularity makes all things equal. This actually breaks the universe in
really cool ways. We made a whole video about this problem if you want to learn more. But
in a nutshell, everything that comes too close becomes black hole matter, concentrated at
the singularity.
This lack of a memory of its past, means that a black hole has only three properties: their
mass, spin, and electric charge. Everything else is lost. They’re a lot like fundamental
particles in that respect. This actually means that every single black hole in the universe
is the same. Sure, their mass is different and some spin faster than others. But If we
were to put all the singularities into a magical physics museum, they would be identical, like
electrons.
But just like fundamental particles, the properties of singularities are the best ways we can
describe them on paper, rather than an accurate representation of reality. Our current theories
about the Universe, namely general relativity, are just not able to describe or explain them.
The curvature of space becomes infinite, density becomes infinite, and our rules just don’t
make sense. The singularity has no surface or size, something like a divide-by-zero error
in the universe. So singularities might not even exist or be completely different things.
But this is all we know right now, from the best prediction we have, from our best current
theory of spacetime.
Also, basically everything you’ve ever heard about black holes, even in this video, is
about theoretical black holes that aren’t spinning, because their math is so much easier.
But since black holes were born from dying stars that were spinning extremely quickly
in their last moments, as far as we know, all black holes in the universe should be
spinning right now. At incredible speeds too, up to 90 % the speed of light. This means
in reality, black holes are even more screwed up than they usually get credit for.
The singularities of rotating black holes are even wilder. The rotation causes them
to swell outwards into a sort of ringularity. This rotation is so powerful, that space itself
is dragged along. This creates another region around spinning black holes, called the Ergosphere
where it’s impossible to stay still, no matter how hard you try. Like a rushing whirlpool
of spacetime, the tide is irresistible and black hole makes you orbit it whether you
want to or not.
Ok. So what will happen with black holes as the universe ages and dies around them? Again,
we don’t know but have some ideas based on our current understanding of physics: Hawking
radiation. In quantum field theory the vacuum of space is boiling with quantum fluctuations.
These fluctuations are creating matter and antimatter pairs of particles from nothing
which only exist for a very short time before annihilating. When this happens near the event
horizon of a black hole, one of these particles can fall in, stopping them from annihilating.
The escaping particle is Hawking radiation. Ultimately, the mass of this particle must
come from the black hole, so over eons black holes will shrink and radiate away. Hawking
radiation is not the stuff that fell into the black hole, it’s new stuff, stealing
mass from it. As the black hole shrinks the Hawking radiation gets stronger, faster and
faster, until what’s left eventually evaporates in a flash of high energy radiation like a
nuclear bomb. And then, nothing.
But that won’t happen for a long long time. A black hole with the mass of our sun has
a lifetime of 10^67 years. Which means that it would take 10,000 billion, billion, billion,
billion,billion, billion years, to lose 0.0000001% of its mass. But most black holes are way
more massive than our sun. The most massive supermassive black holes in the centers of
galaxies have lifetimes of 10^100 years. How long is that? Imagine an hourglass, filled
with one grain of sand for every single particle in the universe. Every ten billion years one
single grain of sand falls to the bottom. If we waited for the entire sand to fall down,
not even a percent of the lifetimes of these black holes would have passed. There is no
good concept for our brains to grasp these time scales.
Will we ever truly understand black holes? Really know what’s going on inside them?
No one knows. We can only see their outsides, and the theories we have probably get their
insides wrong. But it’s okay to not know everything. It just means there is still work
to be done. It means there are still mysteries to solve and big ideas to think about, which
is why humans do science. In the end we at least can be sure that we still have plenty
of time left to think about them before the last one melts away.
If you want to continue thinking about black holes a tad more, you can do so by getting
one of the many black hole related things the Kurzgesagt team made with love. Like our
Black Hole Infographic Poster that teaches you all the Black Hole basics. Or our Choose
Your Own Black Hole Adventure Poster that takes you on a spectacular ride and might
leave you in need of a cuddle. In which case we made a Black Hole & White Hole Plushie
for that. And if that’s still not enough we also just released super comfy Space Socks,
very shiny Space Pins, a new Tote Bag & Pouch Set and have many other great space related
products that are all designed with love and care by us here at kurzgesagt.
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