Thanks to Babbel for sponsoring today’s video.
When you’re walking on a beach, and you make a footprint in the sand, there is no
question in your mind that it is your foot that caused the footprint.
The order of causality is quite clear here, so much so that it seems laughable to even
need to assert it.
You made the footprint.
The footprint didn’t make you.
But what if it did?
What if I told you that on the cosmological scale, the fundamental relationship between
foot and footprint might be a little more blurred than you would intuitively think?
And shockingly, due to the nature of black holes and hawking radiation, there is some
evidence that this might just be the case.
I’m Alex McColgan, and you’re watching Astrum.
Join with me as we explore the strange relationship between mass and space curvature, and how
when it comes to black holes, something very strange is going on with reality.
But to begin with, we’re going to need to look at a principle called relativity.
But no, not that relativity.
Galilean relativity.
First described by Galileo Galilei in 1632, the idea of this form of relativity is that
there is no difference between being completely still and moving at a continuous speed.
Imagine there are two rooms, one on a ship and the other on land.
Both are soundproof and have no windows.
Imagine the sea is calm, so there’s no rocking at all.
The only difference between the two rooms is that one is moving, and the other is not.
Can you tell the difference between the two from the inside?
You might think that you’d be able to sense movement, but this is not the case.
For instance, right now you are careening through space at 110,000 km/h due to the Earth’s
movement around the Sun, and yet if you are sitting down at home while watching this,
it’s likely you would have said you weren’t moving at all.
In fact, Galileo realised that there was no test that could be done to tell the difference
between the two scenarios.
He even found that if you dropped a ball in the ship, from your perspective it would look
like it fell straight down, even if from the perspective of a person on land it would look
like it was falling diagonally.
Galileo realised that if you remove all frames of reference, say by being in space, there
is no way of telling if a planet is moving towards you, or you are moving towards a planet.
According to relativity, both are equally valid interpretations.
You might have noticed this yourself if you ever looked out the window on a train just
as another train suddenly passed by, quickly overtaking you.
Although both trains are going forwards, the other train is going faster than yours and
because you no longer have a frame of reference to compare your motion to, it might look as
if you are suddenly going backwards.
Einstein took this idea further with his equivalence principle.
Here he took the idea of two rooms again, but this time he was making an observation
about gravity.
If you were inside a windowless room floating in the vacuum of space and someone started
accelerating your room in the “up” direction – say by strapping a rocket to the bottom
of it – if the rocket accelerated at just the right speed, then it would feel identical
to if you were standing in a room on the surface of Earth.
In other words, there is no way to tell the difference between the acceleration caused
by gravity and the acceleration caused by a rocket – assuming you could stop the rocket
shaking you with all its rumbling, of course.
Both these principles rely on the idea of inertia – that objects do not like to move
if simply left on their own, and do not like to stop moving once they have started.
Any time you want a mass to do something different to what it is doing, a new force must be applied
to it.
Otherwise, it will remain inert.
But why would it feel to the man in the room with the rocket as if he were under the effects
of gravity?
Or perhaps a better question, why would it feel to us on Earth as if we were being accelerated
upwards by the effects of a rocket?
The Earth is not expanding in all directions at once, pushing us with it, surely?
While this is true, Einstein realised that the two felt similar because they both were
the same thing.
A form of acceleration.
However, there is another form of acceleration that better explains how gravity works than
simply applying a force to an object to push it like a rocket does.
Consider this spinning fairground ride.
If you have ever been on such a ride, [This: ], you will know the power of changing direction
as a form of acceleration.
When you stand against the wall of the ride, once it gets up to speed you feel a constant
force pressing you against the wall even when the ride spins at a constant speed.
This is because your mass is trying to move in a straight line at each point in the ride,
but the curvature of the ride is forcing you to alter your direction.
The battle between your inertia trying not to change what you’re doing and the wall
trying to alter your direction of travel manifests as the force you feel.
And as far as acceleration is concerned, there’s not much difference between the earth beneath
you accelerating you up, and you trying to accelerate down.
Einstein realised that this form of accelerating – acceleration caused by a curving path
– was the best explanation for gravity.
He came up with a theory that matter and energy cause a warping in the space around it, kind
of like how a ball might bend the surface of a taut rubber sheet it was placed on.
The larger the mass, the greater the curvature.
And once space was curved, any object trying to travel through it would be deflected by
that curve.
In the words of physicist John Wheeler, “Space tells matter how to move.
Matter tells space how to curve.”
For small masses, this curve in space would be very slight, but in dense masses this curvature
could get so great that it would be impossible for an object that got too close to it to
escape it.
These are the conditions we find near a black hole with its event horizon.
So, going back to our very first analogy of the footprint and the foot, if a black hole
is the foot, the curvature of space around it is the footprint.
It’s interesting to see all of this in action, and to understand how Einstein came to conclusions
which have been almost universally validated by scientists even a hundred years on.
But there’s nothing particularly weird about any of this so far.
Understanding the exact mechanisms behind it all doesn’t make it any stranger.
The black hole tells the space how to curve, and once curved, any object moving near it
is told how to move.
Nothing here is outside our expectations based on day-to-day observations.
But when we start to look at Hawking radiation, something very strange happens.
We covered Hawking radiation in my last video on black holes, and if you want a more in-depth
take on the theory now would be a good time to go ahead and check out that video.
But as a quick refresher, Hawking radiation is a currently undetectable radiation that
physicist Stephen Hawking theorised is emitted by black holes due to principles of quantum
uncertainty.
This radiation is thermal, and is almost always extremely long wavelengths.
But the most important thing to bear in mind about it for the purposes of our current video
is that it is non-local.
This means that it does not appear from the black hole itself, but appears from the area
of space around it.
To be clear, I do not mean beyond the singularity of the black hole but still within the black
sphere.
That’s hard to define anyway, space as we know it does not exist there.
Remember, the black ball you see here is simply the demarcation point between inescapable
curvature and escapable curvature – the event horizon.
I do not even mean right up against the event horizon, although that is sometimes how this
theory is portrayed.
People sometimes speak of two particles popping into existence right up against the event
horizon, with the anti-matter particle just inside it so it falls in, while the normal
particle is just outside and so escapes.
This is not what is happening.
Instead, the region of space this radiation can pop into existence is several times the
size of the event horizon – a distance up to billions of km away.
And when the largest black holes we have can comfortably fit multiple solar systems side
by side inside of their event horizon, the idea that a photon of radiation can pop into
existence this distance again outside the event horizon is crazy.
It happens even in a place where there is literally nothing there.
In short, it is not so much that hawking radiation is coming from the black hole directly.
Instead, it is coming into existence from the curvature of space that the black hole
is creating, and can happen quite far away from the black hole itself.
But if that’s true, then things work completely opposite to what we might expect, as you’ll
see in a moment.
Consider what happens in this order.
As energy leaves the curvature of space, the curvature lessens because of something known
as the conservation of energy.
And as this reduction of the curvature happens, the black hole then shrinks.
This is crazy.
This is like the footprint getting smaller, and so the foot shrinks accordingly.
It feels very wrong.
Things can’t possibly work that way.
And yet, Einstein hinted that such a thing might indeed be possible.
In one of his equations, he stated that the curvature of space-time was proportional to
the mass-energy of an object.
But proportional is not causational.
There’s no presupposition that one causes the other in this relationship.
We are comfortable with the idea of changing mass and so changing curvature, but it works
just as well if you go the other way and change the curvature to change the mass.
If this is true, then it hints at a universe where mass is simply a projection caused by
space curvature.
When you shine a light at an object - say, your hand - and it makes a shadow on the wall,
the shadow is a projection caused by the existence of your hand interacting with light.
Normally, in this analogy, you might be forgiven for believing that we are the hand.
It is our mass that creates the curvature of space around us.
And yet, do we really know that it doesn’t work the other way around?
Are we simply the projections, shadows on the wall of the universe being brought into
life by something much more fundamental going on in the curvature of spacetime, and yet
we’re going around thinking that we’re the thing that’s real?
We don’t truly know.
Given that all you know is the reality you experience, it would be difficult for you
to be able to tell the difference between the two scenarios.
But if relativity has taught us anything, it’s that if there’s no way of telling
the difference between two situations, then we can’t completely dismiss that we’re
in one and not in the other.
Either that, or the two might be more linked than we thought.
Of course, obviously this is all just a theory.
There is no hard proof that Hawking Radiation is even a real thing, although there have
been some experiments that hint that it might be.
But this is just something interesting to think about.
And even if it does prove to be the case that reality is a projection, it’s not going
to affect your day very much.
You will still think and feel, and that’s more than enough reason for you to go about
doing what you’re currently doing.
But it is an example of how when we start to examine the very fundamental building blocks
of reality by exploring the weird warping effects of black holes, it can cause us to
challenge assumptions about our very nature.
After all, when you’re asking the question “Am I real,” and the answer is “It’s
not certain,” that’s more than a little concerning.
Today's video is sponsored by Babbel.
Have you ever looked around you, like I have, and watched on with envy as people switch
between languages with ease?
I’ve actually just moved back to the UK from Switzerland, where people often know
2 to 3 languages pretty fluently.
So, I was excited when Babbel approached me as I thought this would be an excellent tool
for me to be able to improve my communication with my Swiss wife in German.
The app started out by finding out my level, and then gave me a lesson based on what it
felt I needed to work on.
I really like it so far, I can do it on my PC or my phone, and the courses integrate
vocabulary and grammar with lessons about the language’s culture, people and history,
which helps with the memorisation and gives context so I can know why I should use certain
words in different situations.
If you have a language you want to learn from scratch, or a language you want to brush up
on, why not use 2022 as a year of language self improvement.
If you use my link in the description below, you can get 65% off you subscription, plus
there’s a 20-day money-back guarantee, so definitely at least check it out!
Thanks for watching, if you want more about black holes, check out this playlist here.
A big thanks to my patrons and members for supporting the channel, especially this video
to Andrew M Gross for donating $50, I really appreciate it.
If you found value in this video and want to support too, check the links in the description
below.
All the best and see you next time.