Hello and welcome to another episode of Between
Two Nerds where once again we have invited
a small human being to sit between us and
have a little chat about science.
In this episode the small human being is…
Elyes and I'm 10 years old, nearly 11.
We had that discussion.
We think he is 23/24ths of his way to being
an 11 year old.
And you know the nerds.
I am Mo.
And I am Eugene.
Thank you for that.
Did I get a little jazz hands?
I like a little jazz hands.
Okay, so fire away Elyes, you’ve come prepared.
Oh yeah.
It's slightly intimidating for this one because
Elyes has brought three pages of notes.
Actually I think I brought four pages and
one half of the page.
I'm more intimidated by what's in your head
to be honest.
Let's see where it goes.
Okay, so first question?
What makes black holes so heavy?
I get that it accumulates matter and makes
it incredibly dense but what makes them so
heavy?
Correct.
Next question.
I think we should first start with what is
a black hole.
So if you talk about black holes there are
loads and loads of black holes.
Black holes is really interesting one from
a science perspective
It's a mystery
It’s a mystery but it's an unraveling mystery.
So if you take the last sort of 2, 3 decades,
thanks to a lot of the cool stuff that we've
been able to build and make and stick in space,
our understanding of black holes and frankly
our awareness of black holes has increased
massively over the last couple of decades
and will continue to do so.
But what is a black hole?
A black hole is a super super super heavy
object.
A very very dense object.
Huge amount of mass.
With lots and lots of gravity.
Loads of gravity.
There is a black hole in the center of our
galaxy
Supermassive one
Supermassive.
But there are way way bigger ones out there.
There is a big black hole in the center of
our galaxy in the same way that the sun has
a gravitational pull
Elyes: Is at the center of the solar system.
Absolutely, and the sun kind of holds our
solar system together, the black hole at the
center of our galaxy kind of does a very similar
job and that's the reason that our entire
galaxy kind of rotates and orbits this black
hole in the center.
If we think about gravity what makes a black
hole so different to say the sun or a planet.
So what gives something its gravitational
force.
Do you know?
How tightly the atoms are packed together?
And how many.
So it’s density and it’s mass.
So if you take planet Earth, it's a huge object
and has enormous mass.
But it's not very dense compared to a black
hole or the center…
Relatively speaking.
But compared to you, very heavy.
A great way to visualize this is if we had
a big trampoline in front of us, don't think
of it as the universe thing it is literally
a trampoline, one of those small ones that
you exercise on or a big one.
Whatever trampoline you like.
If I got an apple, which has mass and is kind
of planet-shaped, and I dropped it into the
middle of that trampoline what is going to
happen to that?
A tiny little indentation.
Absolutely.
You're going to see a little curvature of
that trampoline
The trampoline is spacetime?
Absolutely.
So it bends space and time?
Einstein called this, so here the trampoline
fabric, in our example, they call this the
fabric of spacetime.
They took space, the thing that we move in
and time put them together in something called
a continuum.
So these 4 dimensions now, 3 of space, 1 of
time and this is the fabric of our reality,
of our universe.
And it stretches…
It stretches so we've got the apple.
Now let's take the apple away and replace
it with…
A bowling ball.
A bowling ball.
An elephant.
Ok an elephant.
Now let’s take an elephant, squish the elephant
in something that fits on the trampoline in
a sort of spherical way.
Drop that in the trampoline.
What's going to happen to the trampoline?
The trampoline will either fall to the ground
For this one we're going to imagine that there
is a hole in the ground so the trampoline
can stretch as far as it needs to stretch.
It will sink extremely far.
Yeah it curves even more.
If you’re another object, so if you're a
planet or if I'm going to use the fruit.
If you've got the apple there and I'm a grape
traveling across that trampoline, that little
curvature that centers itself around the apple,
that's going to pull me in.
So I'm going to get attracted to the apple
depending on how fast I'm going.
With the apple, while we don't have a huge
amount of bend, a huge amount of curvature,
that grape has a reasonably good chance, if
its speed is high enough of rolling, it may
move slightly the way it rolls, and rolling
off and coming off that trampoline.
If we take our elephant we've got much more
curvature.
Something coming past is far more likely to
get stuck, trapped in that curvature and spin
down or fall down and join our squished elephant.
Now if we take a black hole, we take that
same thing you've got the same trampoline.
Now let's imagine we've taken the entire planet
Earth and squished it, squished it, and squished
it down something about the size of my head.
So that's going to be quite hard to lift.
But let's imagine we do lift it and we put
that in the middle of the trampoline.
So can you imagine now what's happening to
that trampoline?
Well it would probably get bent but in a very,
very thin way.
Yeah.
A very thin way.
Right down at the bottom of that curvature,
if we take the way that we're visualizing
that trampoline, the sides of that curve are
going to be pretty vertical.
Now anything that finds its way there isn't
going to be able to get out, it is trapped.
A black hole has that amount of mass that
it affects and impacts the curvature of the
trampoline which as you rightly said is the
curvature of spacetime to the extent that
its trap is huge.
So if anything gets too close to the way that's
curving there will come a point, which we
call the event horizon.
We call it the event horizon specifically
for light.
So light that travels pretty quick.
So the quicker you're going the less likely
you are to fall in.
Now nothing travels faster than light but
this has stretched and bent and curved spacetime
to such an extent and it’s got so much mass
that light travelling across that trampoline
that gets to the point where even the speed
that the lights is going, it cannot escape.
It’s now falling, falling, falling.
So that point where light cannot escape the
curvature of spacetime we call that the event
horizon.
And inside that, once you're inside the event
horizon frankly we know pretty much nothing.
And there's a very easy reason that we know
nothing.
Because no one’s been inside and come out.
Even if you send someone in there, they’re
never coming out.
And even if you send a probe that could tolerate
that gravitational crush, which is very unlikely.
Even if it could tolerate that crush, how
is that probe ever going to get a signal out
of the black hole?
Because nothing, including light is capable
of escaping.
So that's a black hole in a miniature nutshell.
Yeah.
So.
Honestly we didn't even know that these existed.
They were first theorized about 100 years
ago by William Herschel who called them dark
stars.
He thought they might exist.
It's only in the last 50, 60 years that we've
really found more…
Well we didn't even know if they existed and
even if they did exist how do you see a black
hole?
Why is it called a black hole?
Because you can't see it
Exactly right.
I can see you now because light is hitting
your face, bouncing off and coming into my
eyes.
But this if light hits this black hole well
it's not getting back out.
So that's why we call it black, because you
can see it
Will you just see right through it?
Well you do see something though.
This is Stephen Hawking.
You've heard of Stephen Hawking?
Yes.
I’ve read his book.
Brief history of time?
Uhhh, no.
Brief answers to big questions.
So there is this thing called Hawking radiation.
Even though a black hole doesn't give off
light, there are particles that come into
existence, in and out of existence inside
and around this black hole.
So it does radiate off these particles.
You call this Hawking radiation.
So there is something that you do see around
the black hole.
So it's like a ring of radiation around it.
But if you were to look at a black hole, so
they've taken a picture just this year.
Took a picture of it.
Imagine that’s your black hole.
It actually just doesn't look like a little
disk.
It's like a sphere and there's this ring around
it like the ring of Saturn, but of radiation.
But at the same time you're actually seeing
the front of the ring, facing towards it,
and you see the back of the ring at the same
time.
How is that possible?
Black holes are transparent?
Because the light escaping from the radiation,
remember space is warped here.
So the light traveling actually bends, it’s
called gravitational lensing.
This actually happens around the sun or star,
something with lots of gravity.
Light coming from something behind that star
can actually bend around the star and still
get to you.
So now you’re a star and there’s something
behind you there.
Light coming off of it.
If you have enough gravity can actually bend
around you and still get to me.
Because this has so much gravity that you're
seeing the back and the front of it at the
same time.
Which makes no sense because the light should
have been sucked in.
But then that light hadn't fallen into the
event horizon.
Yet.
It's outside the event horizon so it's fine.
It's gonna get past it will just get bent
as it goes around.
The fact that we can see it see that light
tells us that it did not cross the event horizon.
Otherwise it would just be black
Inside that you're not seeing anything at
all.
Now Elyes, So if we imagine back to Herschel
who was trying to figure all this out and
he's come up with his theory that these black
holes exist.
Has he just made that up?
Or does he have a reason?
He had a reason because, oh no that was the
dark matter explanation.
It’s very similar to the dark matter explanation.
Because something must have been holding other
things together, the stars together.
Absolutely.
We had an understanding or we had a growing
understanding of why stuff in space and stuff
anywhere, but if you to take space, why the
solar system behaves the way it does and what
impact gravity has.
What we were starting to observe was the sort
of response to black holes.
A bit like you've just mentioned, dark matter.
Dark matter we know nothing about.
But scientists theorize that must exist because
we see its impacts, we see its effect on everything
else.
Something is making stuff,. so with dark matter
something is making things expand away from
everything else.
Whereas black holes something as you rightly
just said is kind of holding all this stuff
together and the way the stars are moving
within our galaxy must mean that there is
something causing that.
That's where we start theorizing and we kind
of start hunting to prove that these black
holes exist, which is cool.
Yeah that's how they discovered the one, So
we haven't actually seen the black hole in
the center of our galaxy, Cygnus or Sagittarius?
It’s one of those moments where, whatever
you say, I am just going to nod.
So the supermassive black hole in the center
of our Milky Way galaxy.
We don't actually see it.
We don't see any Hawking radiation.
We're just not close enough I guess.
But what we're seeing is that stars around
or close to it are behaving in a way that
suggests there's an incredible amount of gravity
in the center.
So we're seeing the effects and that's how
we know that it's there.
And judging by how they're moving we can also
know how much gravity is there.
Because we know how much gravity is there
then we know how heavy that black hole is,
how massive it is.
That's a supermassive one.
Any ideas?
What do you think would happen if you were…
Let's imagine we're floating around in space
and we're right next to this cool big black
hole.
I’m getting in.
I’m getting in.
Who’s first?
It’s fine once you’re in!
I give you a little push.
What do you think might happen?
Zzzzzz, spaghettified.
So you’ve heard of spaghettification.
So what's happening here, now because this
bending of spacetime is so steep, Let's say
we fell in feet first.
The difference between the amount of gravity
at my feet and my head is huge.
Right now it's also different because my feet
are closer to the Earth and my head is.
But the difference is so small it has absolutely
no effect on us.
But in a black hole, once you're inside that
then horizon, even as you're falling in to
the event horizon, there's so much gravity,
that your feet will start to get stretched
out, they’re falling in faster than your
head is.
So you'd be spaghettified, that’s the scientific
term, into this infinitely long human spaghetti.
Sounds great.
It doesn’t.
Then as you're falling in.
Remember that radiation around the black hole,
do you remember the Hawking radiation?
So basically a black hole, you can think of
as like you're falling into like this wall
of fire.
So once you’re falling in you're gonna get
hit by all of this radiation and not be very
alive, I’m guessing.
To be honest, there is very little fun to
be had.
Yeah.
Another part of this is, if I gave you a little
push and watching you fall, actually though,
when I'm looking at you falling in, at some
point it would look like you were just frozen
there at the event horizon, not moving.
Forever.
Forever.
Because you're going so fast because time
starts to slow down and the time slows down
so much that one instant stretches on for
infinity.
So it's just like you're frozen in time.
Even for you just that one instant would be
all of eternity.
So total rubbish.
Yeah.
Let's not go.
So who’s getting in first?
You?
I did notice that in every example you gave,
you'd given Elyes a little push.
I’d push him first.
It would be cool, if we could just send something
in there and get some way of sending that
information back out.
It would be just incredible because…
It’s really dark in here, and I’m a little
bit squashed.
Because all of our theories of physics, science,
break down at that instant.
So there's no way even for us to predict what
would happen.
No one’s been near a black hole but we're
already making all these predictions of what
would happen as we fell in.
But then at some point once you pass that
event horizon all of our science just breaks
down.
It just doesn't work at that level because
now you're in this tiny, tiny region of space
with this incredible amount of gravity and
the two theories that govern these two domains,
so you've got Einstein's general relativity
for the gravity part.
And you've got quantum mechanics for the tiny,
tiny, tiny space that you're in right now.
Those two don't really work well with each
other or they give us answers, when you try
to put them together, they give us answers
that just don't make sense.
So that's one of the biggest things, areas
of study that scientists are trying to work
on right now.
Trying to put these two laws, these two theories
together and get them to work.
Or it should be.
Quantum gravity is what it's called.
So that's a very important part of our area
of study for scientists at the moment.
Anything we don't understand is a cool thing
to study.
Are you going to be a scientist?
Probably.
Good.
You’ve got the look.
That's half the battle.
I never had the right look for science.
Why not?
I haven’t got the hair.
All good scientists have got hair.
Scientific fact.
You know what...
I can’t.
I can think of some famous swimmers.
Kip Thorne who won the Nobel Prize for…
I think he doesn’t have hair.
Sorry Kip Thorne, if you do.
Or congratulations Kip Thorne if you don’t.
Did you enjoy that Elyes?
Yes.
Very much.
Good.
So are you going to jump into that black hole
the minute you get a chance?
No.
Are you going to push Mo?
Who’s going to answer all your questions?
Maybe.
I’d wing it.
Alright, well it was great having you on,
Elyes.
Great questions.
We will definitely have you back on to answer
the others.
Absolutely.
I don’t think we got through your however
many pages of notes, we probably got through
two of the sentences.
We may have done 5% of those question and
those weren't all the questions you had to
begin with.
Well that was awesome.
Thank you.
Give yourself a little round of applause.
Should we wave bye?
Bye guys thanks for listening and watching.
Bye.
