My glamorous class mate says this is unbelievable.
[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]
Wouldn’t it be cool to have a little black hole in your
office? You know, maybe as a trash bin. Or to move around the furniture. Or
just as a kind of nerdy gimmick. Why can we not make black holes? Or can we? If
we could, what could we do with them? And what’s a black hole laser? That’s
what we’ll talk about today.
Everything has a gravitational pull, the sun and earth but also you and I and every
single molecule. You might think that it’s the mass of the object that
determines how strong the gravitational pull is, but this isn’t quite correct.
If you remember Newton’s gravitational law, then, sure, a
higher mass means a higher gravitational pull. But a smaller radius also means
a higher gravitational pull. So, if you hold the mass fixed and compress an
object into a smaller and smaller radius, then the gravitational pull gets
stronger. Eventually, it becomes so strong that not even light can escape.
You’ve made a black hole.
This happens when the mass is compressed inside a radius known as the
Schwarzschild-radius. Every object has a Schwarzschild radius, and you can
calculate it from the mass. For the things around us the Schwarzschild-radius
is much much smaller than the actual radius.
For example, the actual radius of earth is about 6000
kilometers, but the Schwarzschild-radius is only about 9 millimeters. Your
actual radius is maybe something like a meter, but your Schwarzschild radius is
about 10 to the minus 24 meters, that’s about a billion times smaller than a
And the Schwarzschild radius of an atom is about 10 to the
minus 53 meters, that’s even smaller than the Planck length which is widely
regarded to be the smallest possible length, though I personally think this is
nonsense, but that’s a different story.
So the reason we can’t just make a black hole is that the Schwarzschild radius
of stuff we can handle is tiny, and it would take a lot of energy to compress
matter sufficiently. It happens out there in the universe because if you have
really huge amounts of matter with little internal pressure, like burned out stars,
then gravity compressed it for you. But we can’t do this ourselves down here on
earth. It’s basically the same problem like making nuclear fusion work, just
many orders of magnitude more difficult.
But wait. Einstein said that mass is really a type of energy, and energy also
has a gravitational pull. Yes, that guy again. Doesn’t this mean, if we want to
create a black hole, we can just speed Up particles to really high velocity, so
that they have a high energy, and then bang them into each other. For example,
hmm, with a really big collider.
Indeed, we could do this. But even the biggest collider we have built so far,
which is currently the Large Hadron Collider at CERN, is nowhere near reaching
the required energy to make a black hole. Let’s just put in the numbers.
In the collisions at the LHC we can reach energies about 10
TeV, that corresponds to a Schwarzschild radius of about 10 to the minus 50
meters. But the region in which the LHC compresses this energy is more like 10
to the minus 19 meters. We’re far far away from making a black hole.
So why were people so worried 10 years ago that the LHC might create a black
hole? This is only possible if gravity doesn’t work the way Einstein said. If
gravity for whatever reason would be much stronger on short distances than
Einstein’s theory predicts, then it’d become much easier to make black holes.
And 10 years ago the idea that gravity could indeed get stronger on very short
distances was popular for a while. But there’s no reason to think this is
actually correct and, as you’ve noticed, the LHC didn’t produce any black
Alright, so far it doesn’t sound like you’ll get your black hole trash can. But
what if we built a much bigger collider? Yes, well, with current technology
it’d have to have a diameter about the size of the milky-way. It’s not going to
happen. Something else we can do?
We could try to focus a lot of lasers on a point. If we used
the world’s currently most powerful lasers and focused them on an area about 1
nanometer wide, we’d need about 10 to the 37 of those lasers. It’s not strictly
speaking impossible, but clearly it’s not going to happen any time soon.
Ok, good, but what if we could make a black hole? What could we do with it? Well,
surprise, there’s a couple of problems. Black holes have a reputation for
sucking stuff in, but actually if they’re small, the problem is the opposite.
They throw stuff out. That stuff is Hawking radiation.
Stephen Hawking discovered in the early 1970s that all black
holes emit radiation due to quantum effects, so they lose mass and evaporate. The
smaller the black holes, the hotter, and the faster they evaporate. A black
hole with a mass of about 100 kilograms would entirely evaporate in less than a
Now “Evaporation” sounds rather innocent and might make you
think of a puddle turning into water vapor. But for the black hole it’s far
from innocent. And if the black hole’s temperature is high, the radiation is
composed of all elementary particles, photons, electrons, quarks, and so on.
It’s really unhealthy. And a small black hole converts energy into a lot of
those particles very quickly. This means a small black hole is black basically
a bomb. So it wouldn’t quite work out the way it looks in the Simpson’s clip.
Rather than eating Up the city it’d blast it apart.
But if you’d manage to make a black hole with masses about a million tons,
those would live a few years, so that’d make more sense. Hawking suggested to
surround them with mirrors and use them to generate power. It’d be very climate
friendly, too. Louis Crane suggested to put such a medium sized black hole in
the focus of a half mirror and use its radiation to propel a spaceship.
Slight problem with this is that you can’t touch black
holes, so there’s nothing to hold them with. A black hole isn’t really
anything, it’s just strongly curved space. They can be electrically charged but
since they radiate they’ll shed their electric charge quickly, and then they
are neutral again and electric fields won’t hold them. So some engineering
challenges that remain to be solved.
What if we don’t make a black hole but just use one that’s out there? Are those
good for anything? The astrophysical black holes which we know exist are very
heavy. This means their Hawking temperature is very small, so small indeed that
we can’t measure it, as I just explained in a recent video. But if we could
reach such a black hole it might be useful for something else.
Roger Penrose already pointed out in the early 1970s that it’s possible to
extract energy from a big, spinning black hole by throwing an object just past
it. This slows down the black hole by a tiny bit, but speeds Up the object
you’ve thrown. So energy is conserved in total, but you get something out of
it. It’s a little like a swing-by that’s used in space-flight to speed Up space
missions by using a path that goes by near a planet.
And that too can be used to build a bomb… This was pointed out in 1972 in a
letter to Nature by Press and Teukolsky. They said, look, we’ll take the black
hole, surround it with mirrors, and then we send in a laser beam, just past the
black hole. That gets bend around and comes back with a somewhat higher energy,
like Penrose said. But then it bounces off the mirror, goes around the black
hole again, gains a little more energy, and so on. This exponentially increases
the energy in the laser light until the whole thing blasts apart.
Ok, so now that we’ve talked about blowing things Up with
bombs that we can’t actually build, let us talk about something that we can
actually build, which is called an analogue black hole. The word “analogue”
refers to “analogy” and not to the opposite of digital. Analogue black holes
are simulations of black holes in fluids or solids where you can “trap” some
kind of radiation.
In some cases, what you trap are sound waves in a fluid, rather than light. I
should add here that “sound waves” in physics don’t necessarily have something
to do with what you can hear. They are just periodic density changes, like the
sound you can hear, but not necessarily something your ears can detect.
You can trap sounds waves in a similar way to how a black
hole traps light. This can happen if a fluid flows faster than the sound speed
in that fluid. You see, in this case there’s some region from within which the
sound waves can’t escape.
Those fluids aren’t really black holes of course, they don’t
actually trap light. But they affect sound very much like real black holes
affect light. If you want to observe Hawking radiation in such fluids, they
need to have quantum properties, so in practice one uses superfluids. Another
way to create a black hole analogue it is with solids in which the speed of
light changes from one place to another.
And those analogue black holes can be
used to amplify radiation too. It works a little differently than the
amplifications we already discussed because one needs two horizons, but the
outcome is pretty much the same: you send in radiation with some energy, and
get out radiation with more energy. Of course the total energy is conserved,
you take that from the background field which is the analogy for the black
hole. This radiation which you amplify isn’t necessarily light, as I said it
could be sound waves, but it’s an “amplified stimulated emission”, which is why
this is called a black hole laser.
Black hole lasers aren’t just a theoretical speculation. It’s reasonably well
confirmed that analogue black holes actually act much like real black holes and
do indeed emit Hawking radiation. And there have been claims that black hole lasing
has been observed as well. It has remained somewhat controversial exactly
what the experiment measured, but either way it shows that black hole lasers
are within experimental reach. They’re basically a new method to amplify
radiation. This isn’t going to result in new technology in the near future, but
it serves to show that speculations about what we could do with black holes aren’t
as far removed from reality as you may have thought.
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