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When people talk about black holes, there’s one thing that pretty much always comes up:
Black holes get their name because the infinitely tiny, infinitely dense point in the center
has a gravitational pull so strong that even light can’t escape.
The thing is, that might not always be true.
For the past half century — basically as long as we’ve known black holes are a thing
— astrophysicists have been debating the existence of something that should be a black
hole, except it’s neither black nor a hole.
They’re called naked singularities, and if they exist, they’ll rewrite physics as
we know it.
When a star dies, it undergoes a gravitational implosion and starts to collapse in on itself.
If the star is massive enough, nothing can stop the collapse, and all that matter turns
into a single point in space.
We call that point a singularity, and it has zero volume and basically infinite density.
Like with basically everything involving infinity, it’s hard to even imagine what that means.
But that’s astrophysics for you — things get weird.
And you might want to fasten your seatbelt, because they’re about to get a whole lot weirder.
A singularity isn’t the same thing as a black hole, but it is what causes the black hole.
The term “black hole” refers to everything inside the event horizon, the point where
the singularity’s gravitational pull becomes so strong that light can’t escape.
It’s impossible to see anything inside it from the outside.
And if you decided to go inside the event horizon to check out what’s going on, you’d
never get out again.
So, sure, for a moment you’d be the only person in the universe to actually know what’s
happening down there, but you’d never be able to tell anyone and you’d be stuck until you died.
In 1965, an astrophysicist named Roger Penrose demonstrated that all black holes must have
singularities within them.
Makes sense.
But he couldn’t prove that all singularities need to have a point-of-no-return event horizon,
and therefore a black hole, surrounding them.
In other words, he couldn’t prove that it was impossible for a singularity to be naked.
He was pretty sure naked singularities couldn’t exist though, even if he couldn’t mathematically prove it.
Four years later, he coined what’s known as the conjecture of cosmic censorship, which
basically just says that it’s impossible for a singularity to exist without a black
hole around it.
Again, he couldn’t prove it — it was just a conjecture.
But it was really hard to imagine how an infinitely dense point could exist without a black hole
around it, and all these decades later, many astrophysicists still subscribe to cosmic censorship.
But not all of them.
We’ve obviously never observed a naked singularity, but that doesn’t mean it’s impossible
for them to exist.
This long-running debate actually led to one of many wagers Stephen Hawking has publicly
made about astronomical discoveries.
In the early 1990s, he bet Caltech’s Kip Thorne and John Preskill that naked singularities
can’t exist.
The loser had to, quote “reward the winner with clothing to cover the winner’s nakedness,”
which was definitely on-theme.
Months later, Hawking actually found mathematical evidence — though not definitive proof — that
when a black hole finishes evaporating, it might leave behind a naked singularity.
If the idea of a black hole evaporating sounds super strange … well, it is.
But it’s one of the many quirks of quantum mechanics, which predicts that a pair of particles
can spontaneously pop into existence with one on either side of the event horizon.
If the one outside has the right trajectory, it’ll escape off into the universe, leaving
the black hole with a teeny tiny little bit less mass.
But! Quirks of quantum mechanics didn’t fall within the confines of the bet, so Hawking
technically hadn’t lost.
He had to concede in 1997, though, when computer models found a special case for fine-tuned
parameters that would produce a naked singularity from an imploding star.
Basically, it’s like trying to balance a sharpened pencil on the pointy end.
Highly improbable, but not impossible.
Hawking made the most of his loss, though — he gave Thorne and Preskill T-shirts featuring
a woman in nothing but a towel, along with the words “Nature Abhors a Naked Singularity.”
So, simulations are able to suggest naked singularities might form if conditions are
just right, but what about more general cases?
Well, researchers have found that if our universe had a different number of dimensions, or was
shaped differently than it is, then yes — it could form naked singularities.
But all this could mean that naked singularities only work on paper, not in practice.
We’re kinda stuck with the universe we’ve got.
If by chance we actually learn of a real naked singularity floating around out there in the
cosmos, though, it could change our understanding of the universe.
Mainly because we’d be able to study something that’s governed by both quantum mechanics
— the science of the very small — and general relativity, the science of the very massive.
As it stands, these two theories work almost perfectly when you’re using each of them
on their own, but they don’t play well together.
When you try to apply them both at the same time, like when something is both super duper
massive and super duper tiny — basically, a singularity — they spit out nonsense answers.
But being able to directly observe a singularity would give us the data to either unite them,
or scrap them for a different theory entirely.
A unified theory of the universe would do more than just reveal the secrets hiding in
black holes.
Right now, anything that happened before 10 to the negative 43rd seconds after the Big
Bang is a big mystery because both quantum mechanics and general relativity would apply to it.
That’s such a tiny fraction of time that you might think it wouldn’t really matter
anyway, but, like, those were the very first moments of our universe.
In other words, for being infinitely tiny, naked singularities are a pretty big deal.
If you’re interested in diving further into singularities, not literally, SciShow’s
sponsor, Brilliant has a lesson in their Astronomy unit about black holes that walks you through
the escape speed, the Schwarzschild radius, and how we can detect black holes using gamma
ray radiation and accretion disks.
Let’s see how I do….
Well, hello!
And welcome to just outside the event horizon of our awesome black hole here.
We’re going to dive into the Brilliant.org lesson about black holes.
And like with all Brilliant.org lessons it starts with some basic information about black
holes in general, how they differ from other cosmic situations, and then we dive into some
more detailed information.
For example this first question asks us about gravitational potential energy of an object
on the surface.
Whether that would be near the surface of a black hole or, as this questions starts
us very simply, is the surface of the earth.
So then we get our basic equation and then it asks us a clear question.
If the radius of Earth were smaller and the earth was denser, what would happen to potential energy?
And so my guess, don’t judge me if I get it wrong, would be that your potential energy
would be lower.
So I can click on that answer, and I nailed it!
And we just continue on from here learning all sorts of awesome stuff.
And just so you know the first 200 people that sign up at brilliant.org/scishowspace
will get 20% off their annual subscription AND support SciShow Space - so thanks!
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