Thanks to Brilliant for sponsoring today’s video. 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, don’t worry, it was for me too. One great way to
begin properly understanding this series would be to use a platform I’m particularly fond of,
Brilliant. It is an amazing tool for learning STEM interactively, and I’ve been enjoying
their content on scientific thinking. This course has built up my knowledge of the
universe in a fun way, and it explains the “whys” behind some of the things I see around me.
I mean, look at this little test about pressure. I know I’ve tried to do this with a real hose
without fully understanding what’s going on. But Brilliant explains it in a practical and visual
way. And it can give you the context necessary to truly understand some of the more difficult
topics I’ve covered in this series, about light and relativity. You can start for free, and learn
at your own pace, be it at home or on the go. To get started, visit brilliant.org/astrum or click
on the link in the description, and as a special deal for my viewers, the first 200 of you will get
20% off Brilliant's annual premium subscription. Thanks for watching! If you found value in this
content and want to support the production of future videos, please like and share the
video so that it can reach new audiences, and if you want to donate and have your name on
this list, you can become a patron or a member. Find the links to that in the description!
All the best, and see you next time.
