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In 2008, there was a lot of excitement when
the Large Hadron Collider
(the LHC) was first turned on in Switzerland.
The particle accelerator was the largest ever
made,
representing decades of work by thousands
of people, and it promised
to unravel some of the deepest mysteries of
the universe.
But some people weren’t so excited.
In fact, they were afraid, because they worried
the LHC
would make a black hole and destroy the Earth.
I mean, that wasn’t true.
The fears were the result of some bad science
reporting,
and the LHC was never going to do that…
but there was actually
a kernel of truth to this idea.
After all, there are some theoretical physicists
who believed
the LHC could make a black hole.
It would just be a sub-microscopic one that
would
fizzle out instantly.
But while the accelerator hasn’t found any
of these micro black holes yet,
it’s still thought that maybe the next generation
of particle accelerators will be able to.
And if they can, that would provide evidence
for some
fascinating ideas in theoretical physics.
We’re talking about stuff like extra spatial
dimensions.
The foundation of this idea is that black
holes aren’t just things
out in space; they’re a natural consequence
of Einstein’s theory
of general relativity.
The theory tells us that matter warps the
fabric of spacetime,
and that the more matter there is in a region
of space,
the more it warps its surroundings and draws
nearby objects closer.
If there’s a lot of matter stuffed into
a very small volume,
then space becomes so warped that it becomes
impossible
to move away from the matter if you get too
close.
That is a black hole. And while they usually
form
when huge stars collapse, they can technically
form
any time there’s enough matter in a small
area —
so it’s possible to get some really tiny
ones.
Weirdly enough, though, making a tiny black
hole
doesn’t involve something really heavy,
but something really light.
And that’s where the Large Hadron Collider
comes in.
In many of its experiments, the LHC smashes
extremely light particles,
protons, together really fast.
Like, “just a hair short of the speed of
light” fast.
That allows us to learn all kinds of things
about what happens
when particles collide.
Like, we’ve seen some brand-new particles
emerge from these collisions.
But the important thing to know here is that
those super-fast protons
have a lot of energy for their size.
And that’s a big deal when it comes to black
holes.
See, we almost always talk about black holes
forming
because of some amount of mass.
But in the high-energy collisions of the LHC,
mass and energy
actually become interchangeable.
That’s the point of Einstein’s famous
“E equals m c-squared.”
It says that mass is proportional to energy.
So, theoretically, if you get some super-fast-moving
particles
really close together, then the energy of
all that motion
in one place can act like a lot of mass and
be enough
to form a black hole.
Just, y’know, a very tiny one.
The problem is… with our current understanding
of physics,
that cannot happen at the LHC.
To make the lightest possible black hole,
the particles
would each need about 10 quintillion electron
volts of energy.
That’s a one with 18 zeros after it.
And while the LHC is good — it can get particles
up to
14 trillion electron volts — it’s not
that good.
But! There’s a reason some physicists are
still thinking
about all of this.
And it’s because there’s a catch here:
These calculations all assume that there are
no problems
with general relativity.
And, well, we already know the theory has
problems.
I mean, it is very good and it makes lots
of good predictions,
but technically, it also predicts it should
be possible
for places to have infinite density.
And we know that’s just not a thing.
If infinities start popping up in your physics
results,
you know you probably pushed your theory past
its limits.
So, what is likely happening is that general
relativity
is only approximately true, and there’s
a more correct theory
we don’t know about.
That theory would produce different results
when you have
a lot of energy in a really small space, which
would solve
this “infinite density” problem.
Right now, scientists think this new theory
will probably be
some sort of quantum gravity — a combination
of quantum mechanics
and relativity. And there are already a few
candidates.
But because there is no direct evidence for
them yet,
scientists spend a lot of time probing the
mathematics
of these theories, looking for predictions
they make
that can be tested.
And one prediction some of these theories
make is that
the space we live in actually has more than
three spatial dimensions.
That would mean that in addition to being
able to go forwards, up,
and to the side, it’d be possible for things
to go in a direction
that’s at right angles to all of those directions
at once.
If you’re having a hard time visualizing
that… well, yes.
Because to our eyes, that’s clearly not
the world we live in.
There is no direction that we can see that’s
at right angles
to all three at once!
So if extra dimensions do exist, they have
to be extremely thin.
This is similar to how a piece of paper is
a 3D object,
but if you don’t look closely, it’s basically
2D.
Only when you zoom in can you see that very
thin extra dimension.
So, in some quantum gravity theories, the
extra dimensions
can be up to about a millimeter in size — but
in those cases,
only gravity can interact with them.
That means that if you measured anything except
gravity
at those scales, like, say, the strength of
a magnet,
it would behave as it does in a 3D world.
But if you measured the strength of gravity
at those scales,
it would behave as it does in a higher-dimensional
world,
and that behavior would be different.
Why this “gravity only” rule?
Well, if anything else interacted with this
extra dimension,
we would already know about it!
On that kind of tiny scale, we’ve seen every
other force of nature
work as-predicted, but we have not been able
to test gravity —
that’s just because it’s so weak compared
to the other forces,
not because of anything to do with relativity.
So we’re not sure what gravity is like in
those situations
— and it could very well act differently.
Now, the nice thing is, if gravity were interacting
with other dimensions like this, it’d be
pretty easy to tell.
See, in the 3D world we live in, gravity follows
what’s called the inverse square law: If
you decrease the distance
between two objects by half, the gravitational
force
between them increases by a factor of four.
But in a world with more dimensions, it’s
no longer
the inverse square law.
In, say, a 9D world, cutting the distance
in half
might mean increasing the force by a factor
of 256.
So if there are extra dimensions smaller than
one millimeter,
then below that scale, the gravity between
colliding objects
gets a lot stronger a lot more quickly.
And that means good things for black holes
in the LHC.
It can smash particles together with really
high energy,
which means they get pushed really close together.
And if the pull of gravity were stronger on
tiny scales,
it would be easier to push lots of energy
really close together,
which is what you need to make a black hole.
In fact, depending on which theory you use,
it’s thought that
if enough extra dimensions exist, you may
“only” need
to smash protons together at around 10 trillion
electron volts
to get a black hole.
And that’s an energy that’s reachable
with the LHC!
So far, though, the search hasn’t been promising.
One paper from 2016 reviewed LHC data with
collisions at up to
13 trillion electron volts, and found no evidence
that black holes were being made that way.
Then, building on that, a 2018 paper found
that, at those levels,
gravity wasn’t behaving differently due
to
extra spatial dimensions, either.
But there is a lot we don’t know about these
theories.
It could be that the energy we need is just
a bit higher than what we
have now, meaning the LHC or a future replacement
could reach it.
That could give us the first experimental
evidence
for quantum gravity!
And, as a very nice bonus, it could also teach
us something
about black holes themselves.
See, in the 1970s, Stephen Hawking predicted
that all black holes —
from big ones to tiny ones you’d see in
the LHC —
should release energy in what’s called Hawking
radiation.
And while most physicists are completely confident
that this is true,
no one has been able to observe it.
That’s because more massive black holes,
like the ones you’d study
in space, should emit radiation that’s way
too faint
to see with telescopes.
But according to Hawking, less massive black
holes
should release hotter radiation that’s easier
to detect.
So, if you had an itty-bitty black hole pop
up
in your particle accelerator… you would
be able to measure it.
If the LHC made a black hole, it would be
so small
that it would radiate away all its energy
in about
an octillionth of a nanosecond.
But there would still be enough radiation
to detect
with our instruments — and thus, provide
the first
definitive proof that Hawking radiation is
real.
So, even if the evidence isn’t promising
so far,
there’s a reason people are looking into
this.
Because if there’s any chance that the LHC
could make a black hole,
we would probably want to try to make it.
Unfortunately, though, the evidence keeps
piling up
that it might not be possible — even with
those extra spatial dimensions.
And some of that evidence doesn’t come from
our big,
fancy particle accelerator: It comes from
nature.
All the time, high-energy particles from deep
space,
called cosmic rays, are hitting our atmosphere.
And lots of them have energies even higher
than
the particles in the LHC.
So if these black holes can be made, they
should be forming
in the upper atmosphere.
But we don’t see them there.
Or from collisions anywhere else in space.
Based on these observations and some modeling
assumptions,
one 2019 paper predicted that extra dimensions
can be ruled out
up to the exa-electronvolt range.
That’s almost a million times higher than
the energies in the LHC,
which means the prospects of seeing these
micro black holes
do not look so good.
But other people have used different assumptions
to say that
a near-future LHC replacement could maybe
see micro black holes.
So while things aren’t looking great, that
doesn’t mean
the case is closed.
There are a ton of quantum gravity models
out there, so there may be
a way to test our hypotheses and theories
down the road.
And even if it doesn’t work out in the end…
Well, black holes or not,
when the LHC is turned back on in 2021 after
some maintenance
and upgrades, people are hoping that it will
discover
all sorts of new things, so there are still
plenty of reasons
to get excited.
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