They're one of the most mysterious things in the universe. Theorized from
Einstein's theory of relativity, they were thought to not even exist for much
of the 20th century. Today not only do we know that they exist, but in 2019 we were
able to see the first actual photograph of a black hole by a collection of
synchronized networks of telescopes called the Event Horizon Telescope. Black
holes are mysterious because they are utterly impenetrable. No one can ever
peek inside one of these things and come back to tell us what they saw.
Black holes are like the Las Vegas of celestial objects. what happens inside a
black hole stays inside a black hole. Yet they may hold the key to not only the
formation of our galaxy, but to our understanding of the true nature of the
universe itself. Although we can't see inside, we can calculate what's inside.
Let's take a trip there because what lies inside a black hole just might make
you rethink reality itself. What would you see as you fell in? Would everything
go dark? Or would you find yourself in - a new universe? The preposterous
fantastical trip inside a black hole is coming up right now...
first let's define what a black hole is. A black hole is not a celestial body
like a planet or a star. There's really no substance there, other than a severely
curved space-time. It's a region in space where matter is condensed to a
theoretical infinitely small point, so small in fact, that this point
effectively disappears from our universe. This is called the gravitational
singularity. This is much like a hole in space and is the final destination for
anything that enters the event horizon. The singularity is a region where none
of our equations, or notions of reality applies. So if there's nothing there what
does it mean when we talk about different sized black holes? When we talk
about the size of a black hole, we're really talking about the size of the
event horizon. This is like a sphere around the singularity. It's completely
dark because light and no other form of energy can escape beyond it. There is
something called Hawking radiation which is thought to escape from the event
horizon, but that's subject for another video. The event horizon is like the edge
of a waterfall beyond which space is actually falling in faster than the
speed of light. The radius of an event horizon is called a Schwarzschild radius.
And it's defined by this equation. Karl Schwarzschild calculated, using Einstein's
equations, that anything with a mass can theoretically become a black hole, if the
mass was compressed enough. If you compressed earth into the size of a
marble about one and a half centimeters in diameter, it would become a black hole.
Similarly, if you compress the Sun to a sphere about six kilometers in diameter,
it would become a black hole. What happens beyond the event horizon can be
ascertained to some degree by general relativity. but what happens at the
singularity, the hole in space-time, is anyone's guess. And there's at least one
competing theory Loop Quantum Gravity, that says that no singularity actually
exists. Something even more fantastical happens there. Let's explore what happens
there right now by taking a video journey to a black hole, and beyond. To do
this we have to choose the black hole very carefully. All black holes are not
the same. Ironically the more massive a black hole is the less its density and
the less dangerous it is from the perspective of its gravity ripping your
body apart. This is because the Schwarzschild radius doubles as the mass
doubles, according to the equation. But if the radius doubles, then proportionately
the volume increases eight times. So the density decreases quite a bit, as the
event horizon gets larger. Very large black holes like the one at the center
of our Milky Way, called Sagittarius A* has about the same density as that
of water. Ao if you want to go inside a black hole, we should choose this one or
bigger. If the black hole had only a mass a few times that of our Sun, you would get
torn apart or spaghettified long before you reached the event horizon, because
gravity would be so large that if you fell feet first, your feet would be
pulled much harder than your head. You could likely go deep inside Sagittarius
A*'s event horizon before tidal forces eventually tore you apart near
the singularity. We will use NASA's hypothetical IXS Enterprise to get to
Sagittarius A*. It will take us three years at a warp speed of 9,000 times the
speed of light.
When we arrive, we see that its event horizon is about 24 million kilometers
in diameter, which is about 17 times the diameter or Sun. For the sake of
simplicity, we will presume that Sagittarius A* is a non-rotating
uncharged black hole. This is also called a Schwarzschild black hole. A highly
curious member of our crew team, Adam has volunteered to enter the black hole,
while the rest of us watch from a safe distance away. Adam's spacesuit, by the way,
is made of a futuristic material that is super strong and immune to high-energy
radiation. As Adam gets closer to the event horizon, we noticed that he speeds
up then something strange happens, he slows down, and his spacesuit appears
to get redder and fainter, until he disappears from our view. He's still
there, but the light reflecting off of him is so red shifted that it is
invisible even to our infrared cameras. But if we had some futuristic technology
that allowed us to view Adam that close to the event horizon, he would appear to
be completely stationary. This is because at the event horizon from the
perspective of the ship, time stops completely. This is due to gravitational
time dilation as defined by Einstein's special relativity. Time slows down as
gravity gets stronger relative to someone not subject to that same gravity.
So Adam stays completely stationary for all of eternity from the perspective
of the enterprise. From Adams perspective, his time is running just fine. But he's
moving faster and faster close to the speed of light as he enters the event
horizon. We're also going to presume that nothing is falling behind him. The images
you're seeing now are not artists' impressions but actual computer
renderings based on calculations by dr. Andrew Hamilton of the University of
Colorado, who generously agreed to let us show them. They're some of the most
realistic available anywhere. The event horizon appears to get much larger and
far faster than he expected, as the curvature of space gets severe around the
event horizons. Space is so distorted that Adam sees multiple images of the
outside universe. Space-time curves more and more severely as he gets close to
the black hole. His lines of sight from the stars in the universe are bent
severely, but Adam sees nothing beyond the event horizon. It's completely dark.
As he peers back, all the light reaching him is being blue-shifted.
In fact, as he gets closer to the event horizon, light that was infrared, not
visible to him before, is now in the visible spectrum. And light that was
visible before, is blue shifted to x-rays and even gamma rays. If it weren't
for his indestructible suit, he would have been incinerated from this
radiation. Since Adam is experiencing severe time dilation, near the event
horizon, would he be able to see the entire future history of the universe,
just before he enters the event horizon? Theoretically yes, but in reality no,
unless he had a very powerful rocket attached to his suit and he could hover
over the event horizon. Then he could view this, but it would only be viewable
as a small dot of light, directly overhead. And it would be so blue shifted
and bright that he would not be able to make out much of anything. You might ask
how is it possible the people on the ship see Adam as not moving, but Adam is
moving and doing just fine from his perspective. This is one of the crazy
things about reality that Einstein's equations and black holes reveal. Both
perspectives are correct. There is no timeline in our universe where we would
see Adam crossing the event horizon. Yet for Adam,
he's moving just fine and about to go inside. How can this be, that two realities
are equal? This is because the laws of quantum mechanics require that Adam
remains on the outside of the event horizon because otherwise it would
violate one of the fundamental conservation laws that information can
never be lost. On the other hand, Einstein's theory of relativity requires
that relative to Adam, nothing is different. Time for him ticks normally as
he goes straight on through the event horizon. So does this mean that Adam
enters a different universe when he crosses over? This is speculation, but it
is entirely possible, because the event horizon serves as a kind of cosmic
firewall that keeps two different timelines, and perhaps two different
universes, apart. Let's get back to Adam because he's now inside the event
horizon. But not much changes him. The view of the outside universe remains
visible. And he can also see some parts of the universe that have fallen behind
him as well. but he can't see light that fell before him. And he
can never actually see the singularity, because all light is headed towards it.
No light is headed away from it, towards his eyes. He falls with the
faster-than-light flow of space-time but time for Adam is ticking just like it
was outside the event horizon. He experiences a very weird sensation
though. He still has a sense of upness and downness, inside the black hole, but
every direction feels downward. Even looking up, feels like looking down.
Space-time is bent so drastically towards the singularity, that there's
literally nowhere to go but towards the singularity. If Adam
tries to fire his rockets to go back up towards the event horizon, where he
entered, he will actually be accelerating towards the singularity faster. Time
becomes space inside the black hole. Like time is always moving in our universe,
space is always moving inside the event horizon. The same way that all
possibilities move toward the future for us, all of space moves towards the
singularity inside the event horizon. And what happens once Adam reaches the
singularity? He'll be ripped to shreds. He will be a kind of soup made of all the
fundamental particles that will get crushed. And what was once Adam, will
become one with the singularity. At least that's what most physicists think will
happen to Adam. Is there any way that Adam can escape this grim fate? Believe
it or not, there are two theoretical scenarios in which he may actually
survive, if of course his space suit was strong enough. In the first case, if this
black hole is a charged black hole called a Reisner-Nordstrom black hole,
this math would be similar to a spinning black hole, as most black holes are
believed to be. In this case, the singularity would take the shape of a
one-dimensional ring. And this spinning singularity creates such a high
centrifugal force, that space near it becomes gravitationally repulsive
instead of attractive. And the spinning singularity creates an inner horizon
that is a wormhole, or an Einstein-Rosen bridge. Once you hit this inner horizon,
you would see an infinitely bright point of light which is an image of the
outside universe reflected by the repulsive singularity, then you'd be
catapulted out and through a white hole. What's a White Hole? It's like a black
hole reversed in time. Nothing goes in. Everything
comes out. this would mean that if Adam's suit was strong enough, he would be
catapulted to another universe. And in the second case, if the theory of Loop
Quantum Gravity is correct, which is a competing theory to string theory, then
there would be no singularity in the black hole at all. This case would be
very similar to the rotating black hole, except there would be no infinitely
bright light. Adam would be transported to another universe through a wormhole.
See my video and loop quantum gravity if you want to know more about what that
theory is. Now all the scenarios I highlighted here are possible but are
they realistic, because these simulations don't take you to account the tremendous
amount of matter and light that is likely falling into the event horizon?
What would a trip inside a black hole look like that had matter and light? Well
we're in luck because Dr. Hamilton also created what is considered the most
realistic trip inside a black hole that takes all those things into account. And
here is what the computer simulations say if you or Adam were to fall into
Sagittarius A*, the four million solar mass black hole at the center of
our galaxy. Here's what it would look like as you approach the event horizon.
Notice that nothing special happens as we cross the outer event horizon, the
point of no return. The tidal forces here would not be enough to rip you apart. And
you could make it through here just fine. then you are about to enter the inner
event horizon. Notice how the outside universe goes brighter and more blue
shifted. As you go inside the inner event horizon there isn't a wormhole
here unlike the Reisner nordstrom black hole, because something called
"mass inflation instability" instantly rips any wormhole apart. This instability is
caused by the tremendous energy and apparent gravity caused by incoming
positive energy and outgoing negative energy, due to all the matter and light
that has fallen inside the black hole. This energy and apparent gravity is so
intense in fact that it would instantly vaporize you the moment that you entered
The inner event horizon. And as in the first case, you get crushed and you
become one with the singularity. This happens pretty quick, so your demise will
be pretty painless. How long does it take? Well, in all scenarios it will take only
about 16 seconds, so not much time for sight seeing! A black hole is where
quantum mechanics and relativity collide. Gravity
becomes a dominant force at the quantum skill at the singularity. Now I've been
describing the singularity as a point or a ring. That's hypothetical because it is
really a three dimensional boundary where relativity dies, and all our
notions of reality are gone. And some kind of new physics like quantum gravity
takes over. What quantum gravity is is probably the biggest question in physics.
Just about anything can happen as far as we know. The great secret that black
holes may reveal to us is that there is no objective reality. Reality depends on
whom you ask. It seems to be observer dependent. Ultimately, Einstein's
equations may lead us to an understanding that not only is time
relative, but reality itself may be relative.
Arvin Ash here. If you like our videos, then consider subscribing. And ring the bell so that you can be informed
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