(mellow music)
- When LIGO announced that they had detected
gravitational waves earlier this year,
we were inundated with artistic renderings
of gravitational waves.
Fantastic images like this one produced by the LIGO team
themselves that help us to visualize
this weird phenomenon.
But in all the graphics that I've seen
there's a kind of disconnect between the objects
and the waves that they produce.
There's no way to see how one causes the other.
And I think we can fix that with an actual,
physical demonstration of gravitational waves.
My plan is to take a classic demonstration
of the warping of space-time and just push it a bit further.
See if I can get it to demonstrate something
it's never demonstrated before.
I'll run through the standard demonstration first.
So here, we've got a sheet of Lycra.
Imagine that this is our universe,
or at least a two dimensional version of our universe.
And just like the real universe when you introduce
a massive object, it warps space time.
And just like in the real universe
when you warp space time the paths of other objects
bend around like that.
So there you go.
We have maybe a moon orbiting around a planet.
That's pretty cool.
Actually, I'm quite pleased with this
because this ball here looks a lot like Mars.
And the reason it looks like Mars is because
it's rusty, it's covered in iron oxide
and Mars is that reddish brown color for the same reason.
It's covered in iron oxide.
But anyway, you can also simulate a massive object
in this universe by pressing your finger
into it like this.
So there you go, I'm warping space
just in the same way that a massive object would
and if I want to simulate a more massive object
I just push harder.
That will become important later.
This analogy for the real universe is okay.
It's not perfect.
In fact, I touch on some of the imperfections
in the previous video about LIGO that I made with my wife
where we discuss gravitational waves.
But this universe does have something in common
with the actual universe that makes it really good
for demonstrating gravitational waves
and that's a speed limit.
So our universe has a speed limit
of about 300 million meters per second.
Nothing can travel faster than that.
Most things travel slower than that.
Some things travel at the speed limit.
So light for example, travels at the speed limit
of the universe, which is why we call the speed limit
of the universe, the speed of light.
And so you probably know this, the sun is very far away
so it takes light about eight minutes to reach us
meaning that if the sun were to suddenly go out
we wouldn't be aware of it for another eight minutes.
And it's the same with gravity.
Gravity travels at the speed of light.
So if the sun were to suddenly disappear
we wouldn't be aware of it, we wouldn't feel
the gravitational effect of that for another eight minutes.
And I can demonstrate that in this universe here.
So if I suddenly introduce a massive object
into this universe.
Let's watch that in slow motion.
So you can see this region of space
isn't yet aware of this new massive object
that's been introduced into the universe
because that warping of space-time is traveling outwards
from the source at the speed of light.
And there you go, only now does it feel
the effects of that massive objects.
Of course, in the real universe objects don't
suddenly disappear or appear, but they do move.
So, you know, if I have a massive object here
and I move it over there, then objects in the distance
over there won't feel that change in the warping
of space time until it's propagated over that distance.
And this is how we can demonstrate gravitational waves.
So what we want is two black holes orbiting each other.
This is what LIGO detected.
So I can have maybe my two fingers pressing
into the fabric and I can orbit them around each other.
I actually get a little bit stuck
because I can't pass my arms through each other.
So I've built something.
I built this device here so I can spin this around
in my universe.
And you'll notice we're not seeing any gravitational waves.
And that's because I'm not spinning this thing fast enough.
At the point that LIGO was able to detect
gravitational waves in those orbiting black holes,
those orbiting black holes were traveling close to
the speed of light.
So, I need to spin this thing close to the speed of light.
Fortunately for me not the speed of light of the universe
but the speed of light of this universe.
And I can calculate the speed of light of this universe
if I look at this slow motion footage here
I can eyeball the distance.
So I can calculate that it takes about 32 frames
of footage to travel about 20 centimeters.
And we're looking at 1,200 frames per second.
So the speed limit in this universe
or the speed of light in this universe if you like,
is about eight meters per second.
So I need my black holes to be moving
at about eight meters per second.
So how many turns is that per second?
Well, the radius of the orbit is not 0.07 meters
meaning the total circumference
of the orbit is about 0.44 meters.
So I divide eight meters by 0.44.
That gives me about 18 turns per second.
I need to spin this 18 times every second
which I'm not gonna be able to do with my fingers
but that's okay because I brought a drill.
But that's okay because I brought I drill.
(drill rumbles)
Okay, here we go.
(drill whirs)
Well, that looked quite promising.
Let's have a look at that in slow motion.
(mellow music)
How cool is that, an actual physical demonstration
of gravitational waves?
A couple of extra things.
Firstly, one thing that this universe doesn't have
in common with the actual universe is that it has an edge.
And so far as we know the actual universe
doesn't have an edge.
And if it does, then it's so far away
that it isn't interesting in terms of this discussion here.
But when you have an edge like that in physics
it's called a boundary condition.
And if the boundary conditions are right
waves get reflected and you can see that in this video here
you can see the waves being reflected.
Einstein's equations of gravity allow for all sorts
of things that we don't observe
in the universe like wormholes, for example.
They're not explicitly disallowed by Einstein's equations
of gravity so maybe we'll find them one day.
And I wonder if boundary conditions are allowed
in Einstein's equations of gravity as well.
I don't actually know.
So if you've got any thoughts on that, please let me know.
The other thing I wanna talk about was
how I captured the slow motion footage
of the gravitational waves, because I didn't use
a slow motion camera.
Instead, I utilized the stroboscopic effect.
And the way the stroboscopic effect works
is if you've got something that repeats
again and again and again, then you line up the frame rates
of your camera with the speed of repetition
of the thing you're trying to film.
So, because of the gravitational waves were repeating
it was, you know, the same thing again and again and again,
I was able to use this stroboscopic effect.
So imagine this, imagine you've got this thing
and you set your camera to one frame per second
and you also rotate this thing once per second.
So it spins all the way around and you take a frame
it spins all the way around again, you take another frame.
You spin it around again, you take another frame.
There, you take another frame, again.
There and so on.
You keep going every time.
So if you play all those frames together
it just looks at that frame one,
frame two, frame three, frame four, frame five.
It looks like it's stationary.
So if you then speed the thing up
so it's actually going slightly faster
than one turn per second, then you know,
you take a frame, it spins around
and then you'd take another frame there.
And then you take another frame there,
and then you take another frame there.
So when you go through the frames, now
it's like, there, there, there, there, there, there, there.
So the thing is slowly moving around.
(drill whirs)
So my camera was running at 25 frames per second.
So in the UK, you have a choice of 25 or 50,
whereas in America it's 30 or 60.
I'm not gonna get into why because people
have already made great videos.
In fact, Tom Scott has made a great video link here
about why there's a difference in the two countries.
But suffice to say,
I needed to spin this thing at about 18.
So either faster or slower than
the frame rate of the camera.
But I also needed to spin it close to the speed of light
in the universe.
So the speed of light in the universe was 18,
I need to spend at 18 times per second
to match the speed of light whereas I needed 25 frames
per second for the camera.
So I actually had to speed up the speed of light
in the universe to, to capture those frames.
So after I did that bit about how fast I had to spin it
I then tightened the fabric on the frame
to speed up the speed of light
in my universe to get it to the speed I needed it to be.
Anyway, I hope you enjoyed this video.
If you did, please hit subscribe
and I will see you next time.
(beaker clanks)