Why is so hard to fall into a black hole?
What's the difference between a ballerina and a black hole?
Of course, a ballerina does not represent the death of a star.
The event horizon of a ballerina is really, really, really small, you wouldn't be able to
see it. It lies somewhere INSIDE the ballerina. Light can easily escape the ballerina. The
ballerina is not home to the strangest phenomena in the universe. It seems
they are quite different, huh? Well, apparently, according to physics,
they have at least one thing in common. When the ballerina starts spinning, she
experiences the same physical process as the black hole. It's a pretty simple concept of physics,
basically ubiquitous in the universe: the conservation of angular momentum.
This conservation law helps us explain why the ballerina spins faster if her arms are
close to her body. But it also explains why it is so hard to get sucked into a black hole.
Follow me in this video to get to know more about black holes...and ballerinas.
I'm sorry, my friend, but the truth is... falling into a black hole is no picnic!
It is actually way harder than you think! Science fiction movies and books always told
us black holes are huge sources of gravity, so strong that nothing can escape them,
not even light. They are thought to be the ultimate absorbers, the infamous singularities,
rives in the fabric of spacetime where laws of physics break down. Yet, it is way easier to
fall into a star, than into a black hole. Do you think I'm talking nonsense? Well, listen to this.
If you had a galaxy filled with black holes, and one filled with stars, which one do you think
would be the easiest to cross with your spaceship? I'm telling you, it's the one with black holes.
Because, even if they are the ultimate absorbers, their huge sucking effect is limited to a really
small radius. As long as you stay away from it, you won't fall, because black holes are
like pins in the space, and although they are able to distort it, the curvature is only really
peaked around the singularity. Anywhere else, you're safe, if you keep a certain distance.
For stars, the thing is much more complicated. The gravitational curvature produced by a star,
it being a sphere, is much more spread in terms of space. If you happened to cross a galaxy filled
with stars, you should be very careful not to enter their gravitational field. And this would
be harder to do because you will have “less free space to travel”. I think you got what I mean.
But there's so much more than this. These weird galactic holes are so hard to get inside. Einstein
himself believed you couldn't get inside them at all. We don't actually know if he was right
or wrong, but we do know physics does the best it can to avoid you falling into a black hole.
Sometimes we have also seen material being ejected from black holes. But we will come to this later.
I think everybody here is familiar with the idea of a black hole.
These objects have so much mass in a so small area, and therefore they're incredibly dense.
The space curvature becomes infinite. You can't escape from a black hole. Bolt
can't escape from the black hole. Maybe Elon Musk could come up with some clever ideas,
but the thing is... from what we know so far, not even light can.
Once you cross the event horizon of a black hole, you can say goodbye to the rest of the universe.
However, this doesn't necessarily mean you are going to fall into it! Before that happens,
you might probably start orbiting around the singularity for billions of years.
Of course, the closer you get to the black hole, the faster you spin, but this doesn't
make you fall into it. Just like a slide gets steeper and steeper the further along it you go,
so the black hole speeds you up, to relativistic speeds, the closer you get to the center. But
almost in all circumstances, you will not find yourself approaching the singularity.
And this is where the conservation of the momentum kicks in.
But to really understand this concept, we have to talk about the structure of a black
hole. It usually has an accretion disk surrounding the singularity. Scientists
know that because they have observed it. An accretion disk is a flattened, circular,
or elliptical structure that is formed when material falls towards a strong gravitational
force, such as a star or a black hole. Accretion disks are found surrounding a
variety of celestial bodies, from relatively small regions of a few thousand kilometers
around white dwarfs and neutron stars to protoplanetary disks around very young stars.
The biggest accretion disks, on the scale of The fact that accretion disks exist over
a huge range of scales and in apparently very different types of systems, suggests
some common physics behind their formation. Because angular momentum is always conserved,
small rotations in any in-falling material are amplified as it collapses
inwards towards the star or black hole. This is something like the motion of
spinning ice skaters, or ballerinas, who speed up as they bring their arms inwards.
Allow me to explain it better. Let's say you have a group of
objects. According to physics, the motion of those objects, also called “momentum”,
must always remain the same. This is what we mean when we say “the momentum must be conserved”.
Basically, if a particle with momentum bumps into another particle that is standing still, and both
bounce away from each other, the amount of total momentum for the two particles must be equal to
the initial momentum of the first particle. This is the same principle that holds for
astronauts on the international space station when they move to keep themselves afloat, or what
happens when the ignition starts, the engines are fired and the rocket lifts off. Once you add up
the momentum imparted to the air by the propellant going down, and the amount of momentum given by
the rocket going up, then the upwards and downward momentum are equal, resulting in a net 0 momentum,
which by the way was the same you had when the rocket was standing still on the launch platform,
with no engines on. But what has this got to do with black holes and accretion disks?
Accretion disks are not actually a necessary part of black holes.
According to physics, simple black holes exist, they are just found there, alone and sad.
They're not complicated, and they just have a mass, a spin, and an electric charge.
However, some black holes are more interesting than others. For instance, if a star is found
near a black hole, it can start being torn apart by the black hole's gravitational pull.
Over the timescale of million years, material from the star starts spiraling around the black hole,
getting sucked and forming an accretion disk. This accretion disk can take something like 100 to 1000
million years to be completely absorbed by the black hole. Before that happens,
the matter coalesces into a flat ring around the black hole's event horizon.
And it does not fall in, due to the conservation of the momentum. In this case,
we talk about angular momentum, since the disk is rotating. We will use the example of a ballerina.
The ballerina has her arms outstretched and is spinning on a single point.
What will happen if the ballerina suddenly tucked her arms close to their body?
Intuitively, she will start spinning much faster. This is because the hands of the ballerina carry
angular momentum. When the arms are outstretched, the momentum in the hands is traveling at a
certain speed, and when the ballerina tucks her arms, the momentum is still trying to preserve
that speed, but now the hands are traveling along a much shorter path. Since it is doing so at the
same speed, the ballerina rotates faster. This is a classical book example of the
conservation of angular momentum. The beauty of physics is that results
are quite general, and they can be applied to the small scale as well as to the big cosmic scale.
The material surrounding a black hole is orbiting in the accretion disk, caught in the black hole's
gravity. The conservation of angular momentum is always taking place so, as soon as the material
gets closer to the center, it gets sped up, moving at the same speed on an ever smaller orbit,
just like in the ballerina example. This is what happens when material spirals toward the center
of a black hole. So you have to imagine this material stolen from another star,
getting sucked and spiraling into the black hole, as it speeds up to relativistic speeds.
The accretion disk material eventually reaches a point where it cannot go any
faster. It’s hit a roadblock, due to the limit imposed by the speed of light.
At this level, collisions and friction heat up the matter around the event horizon. Everything
becomes really hot. I don't suggest you be there: you would die in an oven! The temperature can
reach temperatures up to 10 million kelvin. When this happens, atoms collide and these
collisions cause them to give off more and more of the atoms' mass, transforming that into energy,
according to Einstein's famous law, E=mc^2. Scientists think between 10 to 40 % of an atom's
mass is given off this way in the form of energy! Due to this energy emission, the accretion disk is
observed to be really, really bright. But how bright can it get?
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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!
But what is even more interesting is that a small fraction of supermassive black holes—the
ginormous ones that lurk at the centers of galaxies, also called quasars,—fire off
light-speed jets of particles as they snack. Astronomers don't know what powers jets,
but they think they are powered by the coupling of a black hole with the accretion disk.
The most popular explanation of how jets form is that the fast-spinning accretion disk,
which contains charged particles, will produce a powerful magnetic field that is in contact
with the black hole. If the black hole is spinning, it drags on the field, winding it
into a tight cone at the rotational poles of the black hole. It is this twisted field that
accelerates particles away from the black hole as jets and, in the process, extracts energy from the
rotation of the black hole. So, to sum everything
up, scientists think it is possible – not easy though - to fall into a black hole. But that's
not the only way to go. It is also possible not to fall into them, too. There are a lot of
ways a black hole could destroy you. You would most likely die from the generated heat while
spiraling in the accretion disk; you could get spaghettified if you made it to the singularity.
Or maybe you could become part of a plasma jet injected into space at relativistic speeds.
I would recommend you not to get close to a black hole in any case.
This video ends here! What do you think about it? Let us know in the comments below! And if
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