DR WEINFURTNER: So we are in the Black Hole Laboratory.
What you're looking at
that's an analogue rotating black hole.
As a first step,
we fill up the space in here
in the back.
We put water in there
and then we also put
what's called fluorescent dye.
-makes your fingers dirty-
It's actually sort of orangey
but then it takes
the blue part of the spectrum of the light
and converts blue into green!
There are about 2000 liters of water in there.
If you follow me...
here we have
two valves, which we can open
and control how much is going through
each inlet. We have two inlets in our tank
We feed in the water from this side
and also from the other side,
and so we have this rectangular tank
and we fill in the water, this way.
So by feeding it in this particular way,
You give it angular momentum.
And then we have a hole in the center
(these are exchangeable center holes)
The water is coming in
and because of the way we feed it in
it starts spinning
and it's draining in the center.
So it's a 'stationary draining fluid flow',
or just a 'bath tub vortex flow'.
So this is not a black hole for me or you,
that's a black hole for small excitations in this system
So if you look at some waves or little ripples 
on the surface
(this is what we are interested in)
they have a certain propagation speed,
and if you set up fluid flows where the
background velocity of the flow
exceeds the propagation speed 
of the little ripples,
you can set up what's called
'analogue horizons'.
So suppose you're in a 1-dimensional
fluid flow,
and you look at small excitations,
and they can be moving up or downstream of the flow.
But if you have a region
where the fluid flow exceeds
the speed of the little ripples on the free surface
then it means that
if you are in this region,
you cannot propagate against the flow.
So let's say if you would be
in this region where the flow is 'supercritical'
(so, faster than the perturbations)
I am in the region
where it's 'subcritical',
and we want to exchange information
only using the ripples on the free surface,
then I can send information to you downstream,
but you cannot send any information upstream,
because the flow is faster going downstream
so that nothing can counter-propagate.
So in the region where exactly
the two speeds
the perturbation speed of the ripple
on the surface
and the background flow are equal
that's your analogue black hole horizon.
So this is more of a difficult system:
it's not 1-dimensional:
it has a 2-dimensional velocity component,
meaning: it has a radial component 
(the water is draining),
and it has an angular moment
(the water is spinning)
And what you can see is there
are two horizons
And I don't know if I can visualize it
I can try
You can see, I am looking for
two horizons in there:
there's one when the total velocity
of the fluid flow
(so radial *and* angular momentum)
exceed the little ripples on the surface,
and this is what we call an
'ergo horizon'.
So any fluctuation that comes inside
this ergo horizon
cannot counter-propagate against the fluid flow
it gets dragged along
(it's called 'frame-dragging')
but it can still escape.
There's another point
where the radial velocity
only the radial component
is faster than the little ripples on the surface
and that's called the 'analogue event horizon'.
So if you are in a flow here,
you see nothing special is happening, right?
Perturbations can go in each direction.
If I am going to a point
where the background flow
exceeds the perturbations,
you see a little cone opening in the back
and this cone opens
if and only if the background flow
exceeds the speed of the perturbation.
I don't know if you can see it, can you see it?
BRADY: Oh yeah I see it now.
DR WEINFURTNER: So now, if I manage
And I need to now go up all the way here
then you see the cone nicely here
you see how the cone tilts
more, and more, and more
and at some point
this cone tilts completely inside
and once this cone is not like this,
it tilts like this
-that's where the 'effective black hole horizon' is.
And between these two horizons here
there are some very interesting
scattering processes happening.
A big part of that is to set up this
background flow in the best way for the
purposes of of mimicking rotating black
holes, when I say that I have something
very particular in mind: a very bizarre
process that happens around rotating
black holes.
So, a rotating black hole has to have a mass and an angular
momentum
what can happen, because you have these two horizons, you have this 'ergosphere', and you
have this black hole horizon, and you
have this frame-dragging region, so if
something comes into this region -a small
perturbation- it scatters with this black
hole and it extracts a little bit of
angular momentum from the rotating black
hole, and so this is what's called a
'superradiant' scattering process:
you can come out with more energy
(loosely speaking) and this energy comes
from the angular momentum of the
(in our case analogue)
rotating black hole.
What is really interesting is because
when you have a real-life setup -and
even a very controlled real-life setup-
you have to come up with ways to
actually detect this effect because they
are very small. And also this is perhaps a really
old-style fluid flow experiment
people have been experimenting with for a hundred
years probably, what they didn't have
back then and what we have now is the
technology.
So what we're using is top-of-the-art
cameras with a very high frame rate
per second and a very high spatial
resolution. We are hunting for the very
small -not so small like the gravitational wave but it's still very small- and I can
tell you how small:
if you look at that flow and i'm in a
machine where this analogy should hold,
I'm looking at small excitations on the
top like that... I can stimulate them for
example these are not very small but
it's better to see it like that and usually
what you want is that this little
amplitude of this guy that it is small
compared to the height of the fluid flow
so what we want to send in is a wave with
an amplitude of a millimeter. The change
is tiny so the increase in the amplitude is
at best a tenth, so 0.1 millimeter.
what we really want to see are tiny
changes in the amplitude of those waves.
What I want to see is when the wave
scatters with this vortex -with this analogue
rotating black hole- that it increases
its energy; by interacting with this
effective rotating black hole
it gains energy, and that's exactly what
should happen around astrophysical
rotating black holes: an astrophysical rotating black hole loses its
angular momentum and one way of losing
that is through this superradiant
scattering process. So, whatever comes it
scatters with it and it can actually
transfer some of the angular momentum
from the rotating black hole onto for
example a gravitational wave or other
objects. But this is more a test of
principle, to study this effect which has
not been seen before, in a very
controlled way and it's true we are
we are going to improve
this much further, that's just the first
step.
People usually like it because it is
very hands-on. One of the huge challenges
to overcome was to measure the free
surface over a relatively large area and
you can see the area now indicated by
this light
where the
bright green is. Then what you see above
here, what we developed here is an
air-water interface sensor, and what it
does:
in the middle there is a
blue LED light and it projects a random
pattern changing with 500 frames per
second onto this free surface. And then
you have two cameras on the edges of the
sensor and then you
also take 500 images per second
the first step is you take a
series of images to find out which
point corresponds to the same physical
point in these two images, and you look at a
correlation and it works exactly the same way as the eyes
you see everything as two images but
your brain converts it into one, so this is
what we do and with this information we
can reconstruct the free surface. Here on
the screen you see the reconstructed
image through this process
it's a little bit better than our eyes
because I get exact amplitudes here.
You can see all the waves coming in and
you can see how these waves scatter
here and how they get deformed. I have to say
that we started a year ago
so before that [this] was a storage room. What
we have put together in one year
I think it's really incredible. We
have great support here. So what we're
doing right now: we are testing. We are
testing how good the sensor is.
So this is a nice stage, it's also a
little bit of a nerve-racking stage,
because we could be very close to seeing
the effect or we could maybe be a year
maybe two
I don't think- I hope not but we could be-
one doesn't know when we will see,
so it's an exciting time!
PROF. COPELAND: ... intricate link between the matter and the space time. Just trying to
imagine these two huge objects - what
they're doing to the matter, to the
space-time around it as they- and the
space-time must be going,
"Oh my god!"
