The story of black holes begins more than 100 years ago, in 1915,
during the First World War. Imagine Russian shells raining down on German trenches. A middle-aged
lieutenant huddled by a faint electric lamp in a dugout, is reading a bundle of papers sent to
him from the Prussian Academy of Sciences.The gunfire grows louder around him, but
he is lost in the theory of General Relativity, written by someone he’d known for over a decade,
Albert Einstein. With just pen and paper in hand, this volunteer lieutenant escapes the
horrors of war by dreaming of planets and stars drifting through the vast emptiness of space.
Using mathematics as his tool, Karl Schwarzschild sat there in a muddy trench and crafted the first
exact solutions to the equations that describe how spacetime is warped by massive objects. Einstein
himself was surprised at the solution. When he saw Schwarzschild's calculations, he said, “I had
not expected that one could formulate the exact solution of the problem in such a simple way.”
Over the next few decades, the ramifications of Schwarzschild’s calculations led to a startling
conclusion: as gravity increased, there must come a point where even light could not escape
the warping of spacetime around a massive enough object. At first, astrophysicists called these
objects frozen stars and then later dark stars. In the 1960’s American physicist,
John Wheeler, coined the term “black holes” which has stuck ever since.
Black holes are something that not only scientists find fascinating,
but so does the general public. They seem to be in the news and movies all the time. In this video,
I’m going to look at some of the most mind-bending aspects of these extreme
oddities of nature that just might blow your mind. That’s coming up, right now!
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In my last video, I talked about how stars are in a kind of celestial tug of war,
a titanic struggle between the force of gravity which tries to collapse the star inward,
and the energy of fusion in its core which creates a tremendous outward pressure, which
counteracts the force of gravity. This puts the star into what’s called a hydrostatic equilibrium.
But what happens when large stars run out of fuel and the outward pressure decreases?
Gravity can then overwhelm the star and cause it to collapse. A series of events then occurs.
And if the star is massive enough, the most common result is colossal explosion called a
supernova. And what then remains in the center of the leftover debris of this explosion is a
black hole, if the collapsed core has a mass of between 2 to 3 times the mass of our sun.
If you want to know more about what the processes are that lead to a supernova
explosion and the physics that causes the collapsed core to
become a neutron star or a black hole, please check out my prior video up here.
When the core collapses to a black hole, the tug of war is over. Gravity has won!
Gravity has reached a point where nothing can resist its pull. Not even light can escape.
But what even is a black hole?
When gravity completely overwhelms a celestial body against all other opposing forces, according
to the equations of General relativity, all its mass can be collapsed into an infinitesimally
small volume, called a singularity. A singularity theoretically has all its mass in zero volume of
space, thus it would have infinite density. You should know that a singularity may
not be actually be a real thing, but just the limit of our mathematical equations. Infinity is an
abstract concept, not a tangible thing. It’s more likely to be a red flag telling us that our theory
is incomplete. What we do know is that there is something there with a very high mass in a very
small volume where light does not escape, and the equations of general relativity do not work.
Black holes are interesting to physicists because it’s where quantum mechanics, the theory of the
very small meets gravity, the theory of the very large, because now we have a presumably
very small object, perhaps a quantum object with an enormous amount of gravity. Understanding these black
hole may lead to a quantum theory of gravity, which is currently the holy grail of physics.
In a black hole, the singularity is completely enclosed by a boundary known as the event horizon.
This boundary is not made of anything. It is just an intangible sphere within
which the curvature of spacetime is so strong that light cannot escape.
The radius of this sphere is called the Schwarzschild radius because as you might guess,
it comes from the solution to Einstein’s equations that our volunteer lieutenant from the first World
War, Karl Schwarzschild calculated. Since no light can escape from within this event horizon,
any objects inside, including the singularity itself, can’t be directly observed. Anything
that crosses into this horizon is swallowed forever. For this reason, black holes are
considered a kind of edge of space. It’s a one-way exit from our universe. As far
as we know, nothing inside a black hole can ever communicate with our universe, even in principle.
The size of a black hole is defined by its event horizon, and is actually quite small. If the sun
was compacted to the size of black hole, it’s event horizon would be a sphere only about 6km
or 4 miles wide, smaller than many asteroids. And if the earth was compacted to black hole,
it would only be about two centimeters or an inch wide, about the size of a ping pong ball.
But size is not the strangest aspect of a black hole. As you get closer to the event horizon,
the flow of time slows down, compared to flow of time from a point far away from it.
The gravity of any massive body produces this time effect, including the Earth. But Earth's
gravity is so weak that the slowing of time is not noticeable. For example,
at sea level you age one-billionth of a second less every year than you would if you lived on
top of Mt. Everest because of the slightly lower gravity on top. Near a black hole, the slowing of
time is extreme. From the viewpoint of an observer outside the black hole, time stops completely at
the event horizon. So an astronaut falling into the hole would appear frozen in time at its edge
from the perspective of an observer far away. But from the perspective of the astronaut, time
would flow normally, and he would just continue to fall right on through into the event horizon.
Inside of the event horizon is where the real mind blowing stuff lies. Einstein's theory of General
Relativity still works inside the event horizon, just not at the singularity. And according to
Relativity, time and space trade places inside the black hole. So the singularity which we view
as being at the center of the black hole, is not a location once inside, but a moment in time. So
the flow of time itself draws everything into the singularity of the black hole. The future
of everything inside would be there. There is no place to go except towards the singularity.
It would be located in every direction within the black hole. No force in the universe could
stop you from going towards the singularity, any more than we can stop the flow of time.
At the singularity, from our perspective, at the very center of the black hole,
is where our understanding completely breaks down. Einstein's theory of gravity seems to
predict that time itself is destroyed at the center of the hole: time comes to an
abrupt end there. Because of this, a black hole is sometimes described as the "reverse of creation."
Since time began at the Big Bang, the singularity may be like its opposite, where time stops. But
no one knows how or why time could come to an abrupt end, any more than we know how time was
created in the first place. Einstein's theory of gravity no longer applies here, and new laws
of physics have to be found that describe what happens at the center of a black hole.
But here’s the astonishing fact about everything I just described about the interior of a black hole:
Whatever is inside the event horizon is causally disconnected from us. It remains
forever in the future. Whatever is inside hasn't happened yet from our perspective.
it’s important to study black holes because it could lead to new theories perhaps allowing
us to finally figure out a quantum theory of gravity. But we can only study it from the outside,
since no information or evidence can ever escape black holes. They are truly black,
since not even light can escape. We can only “see”
black holes indirectly because of the way their gravity affects stars and pulls matter
into orbit . As gas flows around some black holes, it heats up, paradoxically making these
invisible objects into some of the brightest things in the entire universe. So for example,
some of the brightest and most distant objects in the universe are quasars. They remained a
mystery for a long time, but scientists have determined that they are actually supermassive
black holes surrounded by a gaseous accretion disc. The gases heat up due to friction and
release enormous amounts of energy. These black holes are so immense their accretion disks can
outshine the galaxies in which they reside. This is not a star. It’s the M87 elliptical
galaxy and it contains trillions of stars! It’s much larger than the Milky Way and yet
look at that jet! Although this jet is roughly 5,000 light years in length, astronomers have
measured pressure waves in the surrounding gas. These shockwaves extend over 85,000 light-years!
All of this mass is coming from the poles of a supermassive black hole at the heart
of the galaxy. So Black holes are not like celestial vacuum cleaners. They don’t suck
everything up. On the contrary, they may throw out as much material as they consume.
This sounds counterintuitive, but because of the forces at play around black holes,
a large percentage of the matter falling in toward a black hole gets thrown out along
its poles in jets of gas shooting out at close to the speed of light.
But most black holes don’t have such accretion discs, or have much fainter discs, so they are
not so easily seen. So, other methods have to be used to find them. The supermassive black hole
near the center of our Milky way galaxy called Saggitarious A* was deduced because of the tight
orbits of stars we COULD see orbiting it. It has even been imaged with the Event horizon telescope.
Now here is a fact you might find surprising. Black holes are fairly common. Scientists estimate
that a new black hole is formed somewhere in our universe, every second. In our Milky Way
galaxy alone, there are an estimated 100 million black holes floating around among the stars. So
for about every 1000 stars that are visible, there is a black hole among them that you can’t see.
Most of these black holes are, well, black. They don’t have an active accretion disk like the one
in M87 or Gargantua in the movie Interstellar, so we can’t see them directly. The nearest black
hole from earth is estimated to be less than 80 light years from us, but it’s too dark to see.
It might surprise you to learn that black holes have stable orbits that would allow planets to
orbit around them. We haven’t seen any of these dark worlds as they’re far too small and too dim
to observe with our telescopes, but there is no reason for them not to exist. When we
look at the math, not only can black holes have planets orbiting them. They could have far more
planets than a regular star! The gravitational gradient around a black hole is so steep that it
allows for MILLIONS of potential orbits, whereas a regular star can only support a fraction of that.
The maximum theoretical number of Earth-like planets that could exist around our Sun in the
habitable zone is six. Replace our Sun with a black hole a million times its mass but roughly
the same physical size and 550 Earth-size planets could orbit in the same region without bumping
into each other or disrupting one another. This might sound strange, but it is all about
how gravity dominates spacetime around a black hole. When you consider a star like our sun,
planets like Mars, Earth, Jupiter and Saturn shift into what’s known as orbital resonance
where the slight tug they make on each other as they go around the sun keeps them stable. They
naturally move into orbits that are quite distant from each other. Around a black hole, though,
all of that changes. The black hole’s gravity is so dominant it negates how planets disrupt
each other and that allows for far more stable orbits to exist. So now hundreds of potential
planets quite close to each other could each be in stable orbits around the black hole.
Now, in practice, there probably aren’t too many planets around black holes because the supernova
that forms the black hole in the first place would have blown them away, but in theory, the black
hole could capture rogue planets that may have formed elsewhere, even on other solar systems that
get a little too close. Could these planets harbor life? It is not out of the question. The source of
energy could come from the accretion disc like in the movie Interstellar, or even captured stars orbiting the black hole.
My friend Peter Cawdron who is a science fiction author, has just released a new novel
that expands on this idea. It’s called The Tempest based on Shakespeare’s famous
play—only his version of the story is set on a dark world orbiting a black hole! If
you enjoy reading plausible science fiction, check out the link in the description below.
How long do black holes last? Our sun has a lifespan of about 10 billion years. Dwarf
stars last much longer. Even though the universe is only 13.8 billion years old,
there are red dwarf stars that will live on for a hundred billion years,
but that’s nothing compared to black holes. Supermassive black holes are estimated to
remain stable for a septenvigintillion years. What’s a septenvigintillion? Well, a million
has six zeros after it. A billion has nine zeros. A septenvigintillion has 84 zeros after
it! Eventually, they too dissipate due to Hawking radiation, but not until 10^84 years have passed.
It’s a length of time that boggles the mind. To give you an idea, if every second represented
13.8 billion years of time passing, which is the current age of the universe,
then a septenvigentillion years would take much longer than 13.8 billion years to pass.
So Black holes are going to be around for a long time, long after all the
stars have formed and died out, and the universe goes dark. Black holes
will continue to roam in the darkness of the universe. They are the future of our universe.
My next video will be posted right up here. So be sure to
check it out. I’ll see you there my friend.