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Falling into a black hole is a lot harder than it sounds. You might expect it to be relatively easy.
After all, aren’t these the ultimate absorbers, quite literally the largest sources of gravity
out there? Shouldn’t it be easier to fall into them than any other thing in the universe?
You might have thought so but, paradoxically, your intuition is wrong. These galactic maws
are one of the hardest places in the universe to actually get inside,
so much so that during his lifetime, Einstein believed you couldn’t get inside them at
all. And not only that, but black holes might even eject you away from them at
speeds close to the speed of light. Want to know why? Well, you’re about to find out.
I’m Alex McColgan, and you’re watching Astrum. Join me today as we explore this concept that
turns common sense on its head, and uncover the incredible effects that this has on space
and matter. There is quite literally nothing like it in the rest of the universe.
By now, if you have been watching the other videos in this series, you are familiar with the idea of
black holes. They are incredibly dense objects that have so much mass in so small an area,
that space curvature has become infinite. This means that no object can escape a black hole
once it passes the demarcation zone known as the event horizon. Not even light can
move fast enough to outpace the acceleration caused by these objects’ incredible gravity.
So shouldn’t it be that these objects would be incredibly easy to get into? Like a slide that
gets steeper and steeper the further along it you go, you might expect to speed up more and more
the closer you get to the black hole’s centre. However, while this is right, it is also wrong.
You do speed up – so much so, that your speed will begin to approach that of light. However,
in almost all circumstances, you will not find yourself approaching the centre of the black hole.
And this isn’t me talking about some strange quirk of time or relativity, but something
that will be observable from whatever frame of reference you’re watching from. Confused? Don’t
worry. Allow me to explain, through the real-world example of something called an accretion disk.
Black holes are, at their heart, very simple. In something known as the “no-hair theorem”, black
holes are said to be devoid of almost any feature, just like a head with, well, nothing on it. The
features of a black hole are usually fairly plain too. They have charge, mass, and spin,
and that’s about it. We discussed most of them in my last video, which you can check out here.
As such, accretion disks are not actually a necessary part of black holes. Black
holes can exist just fine without them, sitting there, dark and unobservable in space. However,
when mass such as an unlucky star strays too close to the black hole’s gravitational pull,
it can be torn apart by the vast forces at work and sucked towards the black hole’s centre.
Strangely enough, though, this matter does not all immediately fall into the black hole’s event
horizon. Instead, the matter usually coalesces into a sort of flat ring that orbits around the
black hole outside the event horizon. While eventually it does all enter, this process
can take a long time. Some accretion disks take 100-1000 million years to be completely absorbed.
So, what is going on here? Why does the matter not simply enter the black hole? The answer is
that it comes up against a surprising principle of physics known as the conservation of momentum.
First described by mathematician John Wallis in 1670 and then pioneered by his contemporary
Newton a decade or so later, the idea goes like this: If you have a group of objects, the motion
of those objects, aka their momentum, collectively must always remain the same. If one particle with
momentum bumps into a particle that is standing still, and both bounce away from each other,
the amount of total motion for the two particles must equal the amount of the first particle on
its own. No momentum can be lost. If you have a rocket on a launch pad with zero momentum,
it can only give itself momentum by firing propellant in the opposite direction.
Once you add up the amount of momentum imparted to the air by the propellant going down,
and the amount of momentum given by the rocket going up, then the upward momentum
and the downward momentum are equal, resulting in the same net 0 momentum you had to start with.
This falls a little outside our expectations. After all, we as humans often stop and start
walking around, seemingly without obeying this law. However, if you evaluate all the particles
involved, this law is always kept. You would struggle to move anywhere without a floor to
push against. Momentum imparted to the floor must equal the amount of momentum imparted to you,
but in the opposite direction – you just don’t notice it because the floor is so much bigger
than you, the amount of momentum you give it does not move it in any noticeable way.
But what has this got to do with falling into a black hole?
Well, consider this next example, this time to do with angular momentum. Imagine a ballerina,
who has their arms outstretched and is spinning on a single point. The particles
in their hands have momentum. They are moving a certain distance in a certain amount of time.
However, then they tuck their arms close to their body. What happens? Well, they suddenly
start spinning much faster. This is a classic example of momentum trying to be conserved.
You see, the momentum in the hands is still trying to travel at the same speed it was previously
travelling at. However, suddenly because it’s closer to the body, it’s now travelling a much
smaller distance, but is doing so at the same speed. Effectively, it has much less distance
to travel to complete one revolution, and as a result, completes that revolution much faster.
This causes the ballerina to spin faster when they tuck their hands in,
and slower when they stretch their hands out. Now imagine this on a cosmic scale. In most scenarios,
matter does not fall in a perfectly straight line towards a black hole. Almost always it
will miss it slightly and will start spiralling in towards its centre as it’s caught in the black
hole’s gravity. It now has angular momentum. As it gets closer towards the centre of the black hole,
it starts speeding up, moving at the same speed on a smaller and smaller orbit, gaining
more and more angular spin the further down the gravity well it falls, just like the ballerina.
You want to fall a little further in? You just have to spin a little faster.
However, unlike the ballerina, this matter has the speed of light to contend with. Nothing in
the universe can travel faster than the speed of light. This is a law discovered by Einstein. So,
what happens to our spinning matter as it falls further and further into the black hole? Due to
the massive forces and curvature involved, it eventually reaches a point where it cannot go
any faster. It’s hit a roadblock. And because it cannot spin faster, it cannot fall further into
the black hole. This has several effects. To begin with, as you can imagine, that creates friction.
All of this matter, spinning at such blistering speeds around the edge of the event horizon,
starts bumping into each other. And when this is taking place at near light speeds, things get
very hot. Matter in a black hole’s accretion disk can reach temperatures up to 10 million kelvin.
This is enough to melt anything down to a hot plasma. All these constant collisions pummel
the atoms involved, causing them to give off more and more of this energy, like squeezing a lemon.
This reduces their mass. Between 10 and 40% of an atom’s mass is given off
this way in the form of energy, which then radiates out across the universe.
For point of comparison, nuclear fusion – the process taking place in the sun – converts only
about 0.7% of mass into energy. Let that sink in for a moment. Consider how bright the sun is,
at 0.7%. How bright can a black hole’s accretion disk get?
The brightest such disks are known as quasars, and they can reach brightnesses that exceed 1000
times the total brightness of every star in the Milky Way Galaxy combined! The good news is that,
additionally, some of that momentum starts to be shed with the departing energy.
More gets shed by imparting it to matter further up out of the accretion disk,
as faster moving particles knock into slower particles moving just above them,
giving them an extra push and slowing down the lower particles. In this way,
matter starts to lose its angular momentum and begins to finally fall into the black hole itself.
More momentum can get shed through one of the most striking features of quasars and black
holes – their jets. We don’t understand everything about these jets – how they form and what they are
comprised of – and only a small fraction of black holes with accretion disks have them. But current
theories suggest that they are caused by magnetic forces that are created by the spinning accretion
disk, or even the rotational power of the black hole itself, which draws up material from the
accretion disk and fires them out into space. It's likely that as the accretion disk spins, magnetic
fields form in keeping with Ampere’s law, due to all those moving electrically charged particles.
The power and shape of these fields are such that there is only a narrow channel at the
north and south poles of the black hole for particles to escape. These magnetic fields may
work in a way similar to the rifling on a gun – channelling particles down a narrow barrel.
Particles moving at near relativistic speeds have only one direction they can go, even though
we don’t quite know yet why they go. Perhaps they are like the steam of a kettle, fired out
through the only gap that exists in the face of this incredible gravitational and heat pressure.
And when they go, they GO. Relativistic jets travel further than the galaxies that they
originate from, and are often millions if not billions of light-years long. One jet with the
catchy name of PSO J352.4034-15.3373 (PJ352-15 for short) has its x-rays reaching Earth from 12.7
billion light-years away, albeit faintly. This is because the radiation produced by such jets is
very focused in one direction. In an effect known as relativistic beaming, or the lighthouse effect,
when the beam is pointed away from us, it is much harder to see. Take for example the now famous M87
galaxy. Here, very clearly, a relativistic jet is detected by Hubble. This is the one coming towards
us. There is very likely another jet, but we can’t see it because it’s going in the other direction.
It’s worth noting that this energy does not come from the black hole directly – remember,
nothing can escape from a black hole. Instead, the matter and radiation come from the accretion disk
surrounding the black hole. And again, a lot about these jets is still theoretical. We can see them,
even observe them moving over time, but we don’t fully understand them, or what causes them.
Our understanding of accretion disks does not even fully explain how conservation of momentum
is kept – there is still some mystery about where all the momentum goes. But the sheer power at play
is undeniable. Einstein may have been wrong – it evidently is possible to fall into a black hole.
But when some black holes are firing material away from them at near relativistic speeds
for distances spanning galaxies, well… it’s evidently possible to not fall into them too.
And once you factor in the force of matter that is millions of degrees hot, pushing out at you as
they attempt to shed their own momentum… perhaps you wouldn’t want to get too close to one anyway.
If the concepts and science behind this series have been a little hard to wrap your head around,
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