If you solve Einstein's field equations, you can predict that black holes should exist. But you also predict another thing that exists called a white hole. Now, white holes are similar to black holes. There is a different solution to the field equations and they have some different properties. One of these properties is that instead of nothing being able to escape the event horizon like a black hole for a white hole, what it means is that nothing can enter the event horizon. So nothing can go into a white hole, whereas nothing can come out of a black hole. So a white hole and a black hole are very similar. A white hole is just a tiny reverse black hole in relativity. It's important to choose your coordinate system wisely because time and space depend on gravity and speed. Now, the easiest way to understand the difference between a white hole and a black hole is to talk about a coordinate system in which you're free falling. In this coordinate system, a black hole appears as though it is sucking in space-time. So space-time is ever-flowing inward towards the center of the black hole. But for a white hole, space-time is always flowing outward away from the center of the white hole. So remember that in a free-falling coordinate system, black holes are sucking in space-time and white holes are spewing out space-time. Now here's where it gets interesting. So for a black hole, it's always sucking in space-time. And the closer you get to the center of the black hole, the faster it's sucking in space-time. So, for example, picture a drain with water flowing as you're further away.
The water is not flowing very fast, but as it gets closer and
closer to the center of the drain, as speeds up faster and faster until it gets
extremely fast right at the center of the drain.
Now, at the very center of the black hole is a
singularity. And at that point, space-time is getting sucked in at an infinite rate.
But that's not the interesting part of the interesting part is a little bit
away from the singularity called the event horizon at the event horizon. That
is the point at which space-time is flowing inward at the speed of light. So
that means exactly at the event horizon, light cannot escape because space-time
itself is being sucked inward at the speed of light. And so photons can't go
any faster than that. So they're forever held at the event horizon. Now, inside
the event horizon of a black hole, time is flowing faster than the speed of
light. And so that's why nothing can ever escape from the black hole, because
inside the event horizon, Space-Time itself is moving faster than light and
nothing can move faster than light, so it can never escape it. Now, this still
doesn't violate Einstein's law that says nothing can move faster than the speed
of light because that law is talking about objects in space-time about each
other. They can't move faster than the speed of light, but it's not talking
about space-time directly. Space-time itself, while being sucked into the black
hole can move faster than the speed of light.
So that's a black hole. But what about a white hole? Well, a
white hole also has a singularity at the center. It's made of mass, just like a
black hole. So that means it also has gravity. So objects are also attracted to
it and get pulled towards it. But here's where it gets interesting. When an
object gets pulled towards a white hole, the object never goes into the white
hole, even though it's attracted to it. Well, remember I said the white holes,
instead of sucking in space-time, they're spewing it out. It's flowing outward
from the white hole. And so that means as an object gets closer and closer to
the white hole space-time is pushing out. So let's say you are moving at the
speed of light, going towards a white hole while Space-Time itself is moving
against you this time.
So that means that you would just be completely
stopped at the event horizon of the white hole. And since nothing can ever move
faster than the speed of light, that means that nothing can get into the white
hole, because at the event horizon, space-time is moving at the speed of light
and nothing can go faster than it. So nothing can get inside of it. Now, inside
the event horizon, spacetime is moving faster than the speed of light going in the
opposite direction.
So you can see that a black hole and a white hole are similar.
The only difference between them is the direction of the flow of space-time.
Now, previously, I made a video about white holes, and in this video, I showed
you a real-life experiment. Scientists have used to make predictions about
white holes. It's a pretty simple experiment. All you need to do is turn on
your kitchen sink.
Now, you may have overlooked the pattern. This pattern shows
up whenever you have a stream of water hitting a flat surface below.
And it's kind of this wall of water that forms
so you can define this inner circular area where I've taken a cross-section of
it here, you can call the jump radius so you can show that anything within this
jump radius, anything within these walls of water or this hydraulic jump area,
you can show that the water is moving faster than the speed of wave
propagation. So what that means is that if you were to make a little splash in
the water, make a wave here, the waves could propagate out. But if you made a
little splash in the water here, the speed of this water is faster than the
speed of wave propagation in the water. And so the wave itself could never get
past this hydraulic jump here. So now here's where the comparison comes in.
Anything that's inside of this jump radius is the equivalent to anything that's
within a white hole. And the hydraulic jump here is similar to the event
horizon of a white hole. So what that means is that once something or some
information leaves this hydraulic jump here, it can never come back in.
Now, let's say this food coloring represents information or
some object coming out of the white hole. You can see that something can come
out of the white hull. So I start within the event horizon and I let it come
out and it comes out quite easily.
You come out, but you can see that once it's out, it can
never come back in. It tries to come back in, but it can never get back in. If
I try to get anything in it, try-hard. It can never get back inside of it.
So what would a real white hole look like? What we now know
what a real black hole looks like. So first, let's see a real black hole, and
then I'll show you what happens. If that black hole were white, how it would
look different.
So here's a rendering of the black hole at the
center of our Milky Way galaxy.
The first thing that you'll notice is that it's not very
black at all. It's extremely bright. Now, the reason it's so bright is that
it's constantly spewing out radiation, Hawking radiation, and also because of
the accretion disks around it. Now, the accretion disks happen because as
things approached the black hole, they approach the speed of light and as they
approach the speed of light, their time relative to ours slows down.
And so basically, it's as though they become frozen in time.
Eventually, they become redshifted out of view and we never actually see them
enter the black hole. And so what you get around the black hole is this frozen
ring of light. And you'll notice that everything is lensed because of the black
holes, gravity bends, space-time. And so it's bending the light from the stars
behind it. And so it appears as though it's this giant lens. So here's what it
looks like approaching the black hole at the center of our galaxy. Now, the
part of the black hole that you can't see anything. That is the event horizon
in which no light can escape at all.
So you'll notice that as you gaze into the black hole, you
can't see anything inside of it because nothing can escape it. So now here's
the interesting thing. Remember that I told you a black hole and a white hole
are different solutions to the same problem. So what we can do in the
simulation is just flip the black hole to a white hole. And so let me do that
right now, OK, I'm going to turn this giant black hole into a white hole.
And done so, the first thing you'll notice is that the
accretion disk disappears from the white hole. That's because space-time itself
is pushing things outward. And so in a free-falling coordinate system, you
can't ever stay around it. It's always being pushed outward.
You'll notice that it still has the iron jets
coming out of it, but now let's fly in towards the center of the white hall and
see what happens. So you'll notice as we get closer, we still get the
gravitational lensing effect. But you'll notice that there's no dark center you
can see inside of the white hole.
That's because any light that is coming from the center of
the singularity, the white hole is still exiting. So we can still see inside of
it.
So the inside of the white hole could be another universe
altogether or a different part of our universe. You can see this a little bit
clearer when I put on this grid pattern and then look at the white hole so
you'll notice the eye going towards the center of the white hole. Now you can
see that there's almost like an entire universe inside the white hole.
So now what would happen if a black hole and a
white hole were now to combine?
What happens when a black hole and a white hole combine?
Well, the black hole
just swallows up the white hole. OK, now let's see what it would look like in
real life.
It's easy to believe that it would end up still being a black
hole, meaning that space-time is being sucked into it.
But what about if the white hole had a larger mass than the
black hole would end up being a white hole?
Or what if a black hole in a white hole had the
same mass and they collided together would end up being? Well, at the heart of
that question is the relationship between black holes and white holes, black
holes and white holes might seem very different, but they're just two sides of
the same coin. If you're going just based on gravitational effects, there's
absolutely no way you can tell the difference between a black hole in a white
hole. They both act the same way. Now, there are a lot of theories surrounding
black holes and why holes. Some of those theories are that black holes can turn
into white holes and white holes actually can turn into black holes. And at the
creation of a singularity, a black hole and a white hole are actually in a
quantum superposition of both of them. So it's a black-white hole until
eventually one of them emerges. And the reason you have to take into
consideration quantum effects is that at the center of a black hole and a white
hole is a singularity. So something extremely small, smaller than an atom, all
of that mass is contained at that single point.
And so quantum effects dominate. And so if you had a black
hole in a white hole of the same mass that came together, I don't know whether
it would be a black hole or a white hole at the end of it. White holes are purely
theoretical right now. There are a few problems with the existence of white
holes. The first is that they don't obey the second law of thermodynamics. Now,
Einstein's theory of general relativity doesn't take into account
thermodynamics. And so we can have solutions to these equations that don't
necessarily have to obey the laws of thermodynamics and white holes are one of
those solutions. Another reason why Whitehall's probably don't exist is that
they're very unstable. So you can show the white holes will extremely fast turn
right into a black hole, even if there's some way that they can be formed in
the universe. But the mathematics of Einstein's field equations led us to
understand black holes before we even prove they existed.
This could be the case for white holes, but we won't know
until further research is done.
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