Dancing in the Dark: The End of Physics? Page #2

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they're in this room.

Actually, there would be billions

going through you every second.

You don't notice, but they're there.

These theoretical dark matter

candidates are called WIMPs -

weakly interacting

massive particles.

But because they interact weakly

with ordinary matter,

the stuff from which we and

scientific instruments are made,

catching them is about

as straightforward

as trapping water in a sieve.

In fact, in the early days of dark

matter, these particles were

so theoretical that no-one had any

idea at all about how

they might get hold of one,

even in theory.

Then, in 1983, freshly minted

theoretical physicist

Katie Freese had an epiphany.

I was at a winter

school in Jerusalem

and that's where I got into

the dark matter business.

I met a man named Andre Drukier.

He's a brilliant, eccentric person.

He's Polish,

he speaks English, French,

German, Polish,

all at the same time.

And he knew where to

go for the New Year's party.

And he started, believe it or not,

in that evening,

over the cocktails -

cocktails have always been

good for science -

started telling me about

work that he'd been doing.

Drukier had hit upon a way of

detecting neutrinos, real particles

that share some characteristics

with the proposed WIMPs.

So what we realised is you could

use exactly that same

technique for WIMPs.

WIMPs have the same

kind of interactions,

they have the weak interactions,

the same ones that the neutrinos do.

I, at the time, was a post-doc

at Harvard and I convinced

Andre to come to Harvard for

a few months. And there, we also

worked with David Spergel, and the

three of us wrote down some of the

basic ideas for what you might do

if you wanted to detect the WIMPs.

WIMPs, the particles that could

be dark matter, are like ghosts.

They travel through ordinary matter.

But they are particles,

so every once in a while,

one of them

should collide with

the nucleus of an atom, in theory.

What's more, the theoretical

collision should release

a photon, a tiny flash of light -

dark matter detected.

Simple, in theory.

If you were to try to build one of

these experiments on a table top

or in a laboratory on the

surface of the Earth,

then your signal would be completely

swamped by cosmic rays.

These would just ruin your attempt

to do the experiment,

because the count rate from the

cosmic rays would be so high

that you'd never be

able to see the WIMPs.

So what you have to do

is go underground.

It is because of the ideas that

Katie had in the 1980s that

thousands of scientists have

been scurrying underground

in search of the dark ever since.

Juan Collar is one of them.

His search for dark matter has

taken him to Sudbury, a small

town in Canada, perched just above

the North American Great Lakes.

To look at it now,

you wouldn't think that this place

owes its existence to

one of the most catastrophic

events the world has ever witnessed.

Millions of years ago, a gigantic

comet crashed into what is

now Sudbury, creating, to date,

the second largest crater on Earth.

The comet brought with it

lots of useful metals that ended up

under what became

known as the Sudbury Basin.

When humans became clever enough,

they sunk holes into the crater

so they could get the metals out.

The area's nickel mines

are responsible for, amongst other

things, the town of Sudbury's main

tourist attraction, the Big Nickel.

What they're less well

known for is the part

they play in the search

for dark matter.

Juan and his colleagues

regularly make the two-kilometre

descent into the darkness in pursuit

of the universe's missing mass.

He's been making

the journey for some time.

How long have you been doing

experiments underground?

In my case, since 1986.

It's been a while.

So you haven't found anything yet?

No.

Do you ever feel like giving up?

Well, after walking a mile

underground like this...

This is not the right time to ask me

that question, don't you think?

There's ups and downs,

of course, but, yeah.

Every so often you have to

wonder about the fact that we may be

looking in the wrong place, right?

But someone has to do that job.

I mean, in physics a negative

result is also important.

You close a door,

and then we can get to work looking

for other possibilities.

The scientists are heading

for an underground

laboratory in which it is hoped

that the super-shy dark matter

particle may one day show its face.

Because anything brought

in from the outside world could

give off radiation that might look

a bit like dark matter,

every trace must be

removed before entering the lab.

No-one is allowed

near the ultra-sensitive detectors

without being thoroughly cleaned

and given a special

non-radiating outfit to wear.

Here in this near-clinically clean

environment is a bewildering

collection of experiments,

some of them several storeys tall,

all designed to catch dark

matter in the act of existence.

Most of the experiments intend to

record the hoped-for

flash of light, produced

when WIMPs collide with atoms.

But Juan's experiment

works in a totally different way.

Juan has decided to listen, rather

than look, for dark matter.

So, Peter, this is the

inner vessel of Pico-2-L,

what we call this project.

And it goes inside that big

recompression chamber.

We have cameras that look inside

and the principle of operation

of this detector is the following -

we put a liquid in there that is

a rather special liquid. It's what

we call a super-heated liquid.

It makes it sensitive to radiation,

so when particles like the liquid

that goes in there normally - it's

now empty - they produce bubbles.

The number of bubbles tells us

about the nature of the particle

that interacted.

You can see these copper things

here. These are electric sensors.

They are very sophisticated

microphones and through sound

we are actually able

to distinguish...

differentiate between different

types of particles as well.

What sound would dark matter make?

It's actually very soft.

It's not the loudest.

So if you find a WIMP

it'll have a wimpy noise?

Very wimpy indeed, yes.

Juan has scaled up this

idea in his latest detector.

Because a bigger detector

means a greater hit rate.

Assuming, of course, that there's

anything doing the hitting.

So this is 260.

It's a much larger bubble chamber,

about 30 times larger

in active volume than

the one we were looking at before.

We explore the same principle.

We listen to the

sound of particles, etc.

It's just a much bigger version.

In some of the models they have

developed for these dark matter

particles, the rate of interaction

is as small as one interaction,

one bubble in our case, per

tonne of material per year, or less.

Confident?

Confident? Not really.

You do your job the best you can

and then you hope

for the best, but...

..nobody knows if there's WIMPs

out there or not. We're trying.

But confidence is not something that

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