What is the stuff in the universe made of?

Our high-school physics tells us atoms, with 90 natural elements building the world around us. But what are these atoms made of?

Over the last century, we've ripped atoms apart, eventually finding electrons and quarks, the fundamental building blocks of stuff. But are they really fundamental? Have we truly reached the bottom?

Not according to string theory.

String theory is the idea that everything in the universe, every particle of light and matter, is comprised of miniscule vibrating strings.

These strings are truly tiny, many billions of times smaller than an individual proton within an atomic nucleus.

And they vibrate at countless billions of times per second in ten dimensions of space, or maybe eleven. Or it might be twenty-six dimensions.

Why would physicists, a seemingly sensible bunch, think this is the way the universe operates?

To understand why, we need to step back more than 100 years.

Two beautiful, incompatible theories

At the end of the 1800s, physics was riding high. Scientists thought they understood gravity, electricity, magnetism, heat and gases.

In 1900, Lord Kelvin apparently retorted that "there is nothing new to be discovered in physics".

But soon after the dawn of the 20th century, this cosy situation began to fall apart.

Einstein rewrote the very notions of space and time with his special and general theories of relativity, whilst Planck, Bohr and Heisenberg revealed that the world of the very small obeyed the apparently nonsensical rules of quantum mechanics.

By the mid-1900s, our view of the universe had been utterly revolutionised. And these new strange physical theories were yielding incredibly accurate predictions for experimental tests.

But despite this success, physicists were unhappy.

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The problem was with forces. It was realised that there were four fundamental forces that underpin the universe: gravity, electromagnetism, strong nuclear force and weak nuclear force.

Of these, the strong and weak forces only operate at subatomic scales. At a larger scale, the rest of the universe is a never-ending battle between gravity and electromagnetism.

Gravity is described by Einstein's beautiful mathematics of curved space and time. But the other three forces are written in the language of quantum mechanics. And these two methods are completely incompatible.

This situation frustrates physicists as they have to remember two independent sets of mathematics to describe the physics of the universe.



It also worries them, as they are unable to describe physical processes in situations when the fundamental forces are battling for dominance, such as at the birth of the universe or in the centre of black holes.

So, they have searched for one set of mathematics to describe all of the forces, a Grand Unified Theory that will mean that there is less to remember.

The search for this Grand Unified Theory is not new, and many have tried and failed. Einstein was searching for a way to unify gravity and electromagnetism until his dying days.

This search for grand unification brings us to string theory.

Vibrating strings each playing a cosmic note

Like a lot of science, the birth of string theory was messy.

It was born in the post-war explosion in particle physics which led to the discovery that the universe appeared to be built of a small family of fundamental particles — quarks, leptons and force-carrying bosons.

This helped make sense of the ever-growing zoo of particles flung out of high-powered accelerators, but physicists asked whether the apparently fundamental quarks, leptons and bosons were themselves made of similar stuff?

Sorry, this video has expired The Standard Model of physics explained

Scrabbling within the mathematics, physicists started to find similarities in the particles, representing them as one-dimensional loops of stringy-stuff.

Different vibrations of this stringy stuff correspond to each of the fundamental particles; one note played on a fundamental string is an electron, another note is a quark, another is a photon, the particle of light.

The strings themselves are not made of anything smaller —they are the true fundamental pieces of the universe.

But the mathematics of string theory is a little strange, and in putting the pieces together, physicists needed to add more and more dimensions of space to make their theories work, many more than the three we experience in our everyday lives.

If string theory is correct, more convoluted mathematical trickery is required to hide these extra dimensions from us.

String theorists are built of stern stuff and working with complex vibrations in multiple dimensions didn't daunt them.

With its simplicity as the underlying idea that can explain everything in the universe, string theory has proven very seductive.

Since its crystallisation in the 1980s, it has continued to grow and evolve into a group of ideas — known as "M-Theory" — although no-one seems to know what M stands for.

M-theorists are confident that they are on the correct road towards grand unification, and soon will be able to pull together all of the individual threads and declare victory over the fundamental forces.

Mathematically elegant, but impossible to prove

Not everyone is convinced.

Whilst scientific journals contain many thousands and thousands of pages of mathematical investigations of string theory, they all lack one important thing — predictions that can be tested through experiments.

To probe the tiny scale of strings we would need to build a huge version of the Large Hadron Collider — at least as large as our Milky Way galaxy, if not the observable universe.

As you can imagine, unless something radically changes in the mathematics of string theory, experimental verification will remain forever out of reach.

Again, string theorists remain undaunted. Even if we cannot test the theory, they say, we should continue our efforts on M-theory as the idea is so beautiful, it just cannot be wrong.

With a bit more effort, they say, we will hold in our hands the theory to describe everything.

But to others, this is going too far. Science without predictions is not science, it's simply mathematics.

And if string theory is never tested against nature, then it will never be science.

String theory has made other physicists grumpy, with its fame overshadowing their own search for a Grand Unified Theory

But their approaches can seem as strange and weird as string theory, with ideas such as "Loop Quantum Gravity" which suggests even space and time are built from fundamental bits.

These physicists feel that the theorists on the string theory bandwagon are heading down a dead-end path in the search for ultimate physics.

Is string theory, or its many descendants in "M-theory", on the right track towards grand unification?

In truth, we simply don't know. We don't know if any of our ideas are truly inching a way towards the ultimate theory, or if a completely different approach is required.

In fact, we simply don't know if a grand unified theory even exists.

But this will not stop physicists from continuing their search.

Professor Geraint Lewis is an astrophysicist at the University of Sydney with expertise in dark energy, gravitational lensing and galactic cannibalism.