Thanks to Endel for sponsoring today’s video.
We often think of black holes as destroyers. They suck everything within their reach into
them and give nothing back. They are the end, the final destruction of the universe.
And yet, what if I said to you that they might actually prove to be our salvation?
Black holes might provide the answer to travelling faster than the speed of light
and solving the energy crisis in ways that we couldn’t have even imagined until recently.
And, as by now I have come to expect, they do so by messing with the fabric of reality itself,
and by completely countering my expectations of physics. Perhaps we have been thinking about
black holes all wrong? I’m Alex McColgan, and you’re watching Astrum. Join me again for the
fifth video in my series about black holes, where once again, my mind has been blown by
the incredible potential and implications of these very real objects in our universe.
I’ve talked before about the formation of black holes in this series – including aspects about
their event horizons, how they are created, and how they might possibly end. But to understand
how a black hole ignores the usual limitations on faster than light travel – and does so in a way
that you can benefit from it without having to go inside a black hole’s event horizon – and how it
produces near-limitless energy at the same time, then we are going to have to understand more about
the features of black holes than we’ve covered so far. So, a quick recap – what is a black hole?
In its simplest form, a black hole is an object in space that is so massive and so dense,
that the gravity it creates is too powerful for anything to escape it. We are familiar with the
iconic black spherical zone that surrounds a black hole – this is the black hole’s event
horizon. This sphere is the demarcation point between escapable gravity, and inescapable
gravity. Because the gravitational pull increases the closer you get to a black hole, once you
go beyond the event horizon, nothing - not even light - can travel fast enough to get away again.
Beyond that, though, it’s actually quite difficult to say much about a black hole’s features at all.
Precisely because of the event horizon, we cannot see what the inside of a black hole looks like.
In fact, there are only three things we can say about black holes with any degree of certainty:
They have mass, they have charge, and they have angular momentum. You might wonder how we know
these things about black holes, given that no light can leave them to tell us about them.
The key to these three characteristics is that all three of them represent aspects of the black hole
that can be felt outside the black hole’s event horizon. Charge, for instance, works the same way
around a black hole as it does around any other charged object. That is to say, if a black hole
is charged, then it will attract objects that have a different charge to it and repel objects that
share its charge. Think of it like a giant magnet, pushing and pulling on the universe around it.
Scientists can track objects that approach a black hole, and by seeing how quickly certain objects
known to have a charge move towards it, scientists can predict the charge of the black hole itself.
Interplaying with this is mass. The mass of a black hole can also be felt outside the sphere
of its event horizon. In fact, it is the main creator of the event horizon in the first place.
This is because mass creates gravity and does so in a fairly linear fashion in accordance with the
same principles you might find in Gauss’ law – a theorem about electromagnetism - albeit with
a gravitational analogue. So, it’s possible too to calculate the mass of an object by seeing how far
away objects are before they start to accelerate towards it, and how quickly they accelerate.
Although obviously, you have to factor in charge at the same time, or your results might be skewed.
Finally, angular momentum, or spin. It is possible to detect the spin of a large-mass object, and we
are going to dive into the how in just a bit. For now, let’s just accept it as given, and recognise
that black holes are certainly very high mass objects. There are varying sizes of black holes
in existence. The smallest, known as micro black holes, have a mass that’s comparable to that of
our moon, or 7.35 × 1022 kilograms. They fit all this into a space that’s just 0.2mm in diameter,
which is incredible. It really gives you a sense of how dense a black hole can be – something
thinner in size than a human hair, packing the mass of the moon. And that’s just the smallest
ones. Stellar black holes have a mass equal to 10 times our Sun, and have a diameter equal to 60km.
Intermediate black holes are the mass of 1000 suns, and fit all of that into a diameter of
2000km, which is still smaller than the Earth. It is the largest black holes that really dwarf us,
with masses between 100,000 to 10,000,000,000 times the mass of the sun, and sizes ranging from
0.001 to 400 AU (an astronomical unit being the distance from the Earth to the Sun).
But other than those three features, there are in theory no other differences between them. If you
put two black holes in the same room and made sure they had the same mass, charge and spin,
it would be impossible to tell them apart. However, these three features
are enough to have some interesting effects on the area of space outside a black hole.
Travelling inside a black hole is impossible, space and time break down past the event horizon.
But we think we know a few things that must exist inside one. Beating the in heart of a black hole,
there is thought to lie the singularity. In truth, this actually is the black hole. When
we were discussing diameters earlier, that was just the diameter of the event horizon.
Again, we are not certain what a black hole actually looks like, because light can never
escape it. In a space that is infinitely small, there is a point where all the mass of the black
hole is packed so that it is infinitely dense. For the simplest models of black holes, the ones
that do not spin, this is a single point. In a rotating black hole, this is more like a little
spinning ring – otherwise, it would be difficult to define spin for a point that has no volume.
Our current physics get very strange around such a black hole. If ideal paths are travelled
around this point, it becomes mathematically possible to do some very strange things,
like meet up with your own past. This has some disturbing implications for causality
and gets into time travel paradoxes like the grandfather paradox,
so that probably only shows for certain that our ideas about singularities are not quite right yet.
Because the singularity is so small, it’ll take the successful merging of quantum theory
and general relativity theory to properly explain what is going on inside a black hole,
and we have not yet managed to do this. It may one day turn out that singularities do
not exist in the hearts of black holes at all. But this is the extent of our knowledge so far.
Well, whatever it is that lies inside a black hole, it powers our faster than light engine,
because like most objects in the universe, it spins. And oh, does it spin.
As we travel out from the centre of the black hole, we pass through the event horizon with
little fanfare. The event horizon actually cannot be detected locally – although a person outside
the black hole might watch you slow down to a complete stop as you travel through it,
from your perspective it actually might seem like time is flowing normally.
Normally, that is, until the universe outside the black hole runs its course in an instant because
time outside the black hole is travelling so fast compared to you. This is the essence of
relativity, and we talk more about it in another of my videos, which you can look at here.
In fact, the only evidence you might have that you’ve passed the event horizon at all
is because of something that exists just outside it – the photon sphere.
In a zone just outside the event horizon, there exists a point in space where if a
photon enters it at just the right angle, it will enter a perfect orbit around the black hole
in much the same way the moon perfectly orbits the earth. This infinitesimally thin zone is
known as the photon sphere, and given the number of photons that have flown past black holes in
all the millions of years they have existed, it is probably filled with photons. It is
quite possible that you would be instantly fried as you passed through this point.
However, it is just outside here that we find the zone that interests us.
The Ergosphere. This is the zone around a black hole where we can most easily detect its spin. And
this is because, in this zone, it is impossible for us to not move. You see, mass affects space.
We see this in the curving effect of gravity on the travel of objects through that region
of space. However, it might be more accurate to say that mass “drags” on the space around it.
As it moves through space, it brings a little bit of that space along with it for the ride.
And when an object as massive as a black hole spins, there is an effect known as frame-dragging.
To put it simply, reality around the black hole begins to spin in a whirlpool that cannot be
fought against. Much like a real whirlpool, anything caught within the ergosphere is spun
around the black hole, because the frame of reference it sits in is being pulled.
Sort of like how a person moves because they are standing on a moving walkway. The greater
the spin of the massive object, the faster this happens. And in the ergosphere, this can occur
at a speed so fast that by the event horizon space is moving faster than the speed of light.
You would need to travel faster than the speed of light in the opposite direction just to stay at a
relative standstill from the point of view of an outside observer. Which of course, you cannot do.
But isn’t this against the laws of physics? Doesn’t Einstein say
that nothing can travel faster than the speed of light? The answer to that is yes,
but black holes have found an interesting loophole. You see, this rule only applies locally.
Right where you are, in your frame of reference, nothing can go faster than the speed of light.
But thanks to relativity, it is possible for frames of reference to move away from each other
so fast that objects in them appear to be breaking this light barrier from your point of view.
But if you moved next to them and entered their frame of reference,
they would seem to slow down, and would start obeying the laws of physics again.
It's a really weird effect, but frame-dragging is an actual thing.
It is by measuring frame-dragging that scientists can learn the spin of a black hole.
However, according to a man called Roger Penrose, there may even be a way of exploiting it. If you
were to send a rocket into this section of the ergosphere, the rocket would speed up due to being
caught in the whirlpool of reality. Once it had gained enough speed, it could then fire propellant
in such a direction that it pushed itself out of the whirlpool again, but now travelling at a much
faster speed. This method – named the Penrose Process – could hypothetically net you energy
equal to about 20% of the mass of your rocket. Now that might not sound like much, but remember,
according to Einstein’s e=mc2, your 20% mass would produce energy equal to itself times by 299
792 458. Squared. That’s a lot of energy. So, to harness this colossal kinetic energy, all you
would need to do is travel to the nearest black hole, which is roughly 3000 light-years from us,
and enter its ergosphere with a rocket capable of surviving the intense gravitational forces there.
Ideally, you would need to find one that was not surrounded by an accretion disk,
because those get up to temperatures of millions of degrees, as they are swung around at near-light
speeds and melt from solids down to gas and plasma. But you get the idea. Easy!
Ok, maybe this is a little impractical for us. But the implications for faster than light travel that
black holes demonstrate through frame-dragging might just offer us the key to one day beat the
light barrier for real. Not by going faster than light ourselves, but somehow convincing the frame
of reference we are in to travel at those faster speeds, just like they do around a black hole.
Of course, if this requires the energy of a black hole to accomplish,
we might be out of luck for now. But it’s an incredible glimpse into what is possible,
and scientists are already looking into the power of frame-dragging for future travel.
But maybe that’s a topic for another video. Either way, this all just highlights once again
how our universe really is very different from what we might have ever imagined.
If this series about black holes and how they warp our universe in ways you may have thought
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Thanks for watching! This is the second from last episode in the series.
If you want to catch up on any episodes you missed, check out the playlist here. If you found
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