we have wondered how they got there -

and what lies beyond them.

Scientists have long searched for a simple theory

that explains how the universe works,

a theory of everything.

Many theories have been tested

and so far, every one of them has failed to fully explain

what we see in our universe.

But there is one particularly enticing idea

that some theoretical physicists think might just be 'it',

String Theory.

String theory is finite.

It doesn't blow up.

It doesn't collapse on itself.

That's why we believe in it.

Other theories do collapse. Other theories do blow up.

But string theory does not.

Dr Michio Kaku has spent decades

grappling with - and attempting to answer -

some of life's biggest questions.

What happened before the Big Bang?

Are there other universes?

What lies on the other side of creation?

The other side of a black hole?

Are gateways possible?

Wormholes? Higher dimensions?

Do we live in a multiverse?

All these questions cannot be answered

using our present-day understanding.

When Dr Kaku talks about our present-day understanding,

he means our current best theories about the way the universe works.

In truth, they contradict each other.

And sometimes even give conflicting results -

Albert Einstein's general theory of relativity works perfectly well

for predictions about the movements of stars and galaxies,

but doesn't work when applied to the behaviour of subatomic particles.

Conversely, quantum theory is great with atoms

but predicts the whole universe should collapse into a black hole,

which it clearly doesn't because we're all still here!

So, will we ever find a single theory that explains everything?

Two thousand years ago, Pythagoras asked himself that question.

The great Greek mathematician

said that there must be a unifying principle,

a paradigm by which to summarise the vast creation

that we see all around us of the universe that we know.

And he looked around and saw a lyra string.

You pluck a lyra string and you get a note,

you pluck another one, you get another note.

And then he said, aha, the mathematics of music is rich enough

to explain the diversity of everything we see around us.

And only recently have we come up with a new idea

based on the Pythagorean idea of music.

In other words, string theory.

So how do we go from Pythagoras' musical musings to proper physics?

The first port of call would be a particle accelerator

like the Large Hadron Collider at CERN in Switzerland,

where tiny particles are smashed to pieces in high energy collisions

and then closely studied.

These experiments are our best way of testing theories

about how the universe works.

So what does string theory predict we would see?

We now believe that all these subatomic particles,

hundreds of subatomic particles

that we get by smashing protons at the Large Hadron Collider,

they're nothing but musical notes as Pythagoras believed.

If I had a super microscope

and I could peer into an electron,

what would I see?

I would see a rubber band,

a vibrating rubber band.

Of course, Dr Kaku doesn't mean an actual rubber band.

But rather something rubber-band-like.

Or to put it another way - the strings in string theory.

And just like the strings on a musical instrument,

if you put some energy into them, they vibrate.

It vibrates one way, we call it an electron.

We vibrate it another way, it's called a neutrino.

We vibrate it another way, it's called a quark.

But it's the same rubber band.

So, string theory offers a tantalising possibility -

an explanation for the vast variety we see in the universe,

from the collisions of stars to the collisions of atoms.

Of course, string theory is just that - a theory.

Its critics point out that many of its predictions are untestable -

something Dr Kaku himself acknowledges.

Its proponents though, consider it to be

the best hope of unifying physics.

Dr Kaku believes string theory

could even explain the mystery that is dark matter.

Dark matter makes up most of the matter of the universe.

It is invisible and it holds the galaxies together,

but how do we prove it?

We think that dark matter could be the next octave of the string.

If you could magnify all the particles you see around us,

we would see a lot of rubber bands,

a lot of rubber bands vibrating at different frequencies.

But the rubber band has higher octaves.

That, we think, is dark matter.

If Dr Kaku is right, the huge complexity of the entire universe

could be reduced to the simple and elegant vibration of strings.

I think the one thing that people should realise

is that physics, at the fundamental level,

gets simpler and simpler the deeper we go,

but becomes more powerful, the deeper we go.

The universe is simpler than we thought.