The Big Questions: Physics (23 page)

Nothing about the laser resolves the debate over whether light is a wave or a particle, however. For all our ability to interpret what light tells us about the universe, and use its power to change the world around us, the nature of light itself remains elusive. Thomas Young would be astonished to discover that we can now mimic his double slit experiment – the experiment that proved light is a wave – and tweak the set-up so that the only viable explanation is that light has a particle nature. We still don’t have a good way to tie together the wave and particle view. This puzzle is at the heart of quantum theory, and remains a mystery (see
What Happened to Schrödinger’s Cat?
). What is light? It’s a wavicle.

The vibrations that create our universe

Well, no. It’s actually about a universe composed of elastic loops that are formed from stretchy strands of energy joined together. But, as at least one physicist has put it, as a name rubber-band theory lacks dignity, and a little dignity seems appropriate for a theory that is our best hope of finally understanding the universe.

String theory might be billed as a new, ultramodern idea, but it is not. It first appeared in 1968 as a result of our post-war infatuation with particle physics. We only discovered the atomic nucleus in 1911. We learned to split the atom in 1938, and within 20 years had learned almost everything there is to know about nuclear physics. Ten years after that, string theory, an audacious attempt to broaden these new horizons to encompass the whole universe, was born.

It arose because an Italian physicist called Gabriele Veneziano spent his youth poring over the results of experiments that smashed protons together at high energies. Eventually, he began to see a pattern in the data: two colliding protons would cause particular kinds of particles to shoot out from the crash site at predictable angles. The initial products of these collisions were quarks, the particles that make up protons. However, the quarks subsequently combined to give different kinds of particles. Most of these particles were unstable and short-lived. When Veneziano drew physicists’ attention to the predictability of their formation,
a few of them began to piece together an explanation. The results made sense, they said, if you forget about the idea that particles are tiny points of matter, and pictured them instead as lengths of string. The energy they carry makes them vibrate, and as the particles gain and lose energy, these strings lengthen and shorten. As the vibrating strings collide, the resulting range of vibrations give rise to what we interpret as different kinds of subatomic particles. It seems impressive now, but nobody saw this as a triumphant ‘theory of everything’ straight away. In fact, one of the originators of string theory had his paper rejected as rather insignificant. But then it did have significant problems.

In nature, particles can be (roughly) split into two types. The ‘fermions’, such as the nucleus of a helium atom, the electron, or the quark, make up the matter. The ‘bosuns’, such as the photon, are the particles that transmit forces. String theory set the rules for bosons, but had nothing to say about the existence or behaviour of fermions. Since fermions account for the basic constituents of matter, that was a big flaw. But it wasn’t the only one.

If physicists were to take string theory seriously, they had to make the theory consistent with the twin pillars of physics in the 20th century: quantum mechanics and relativity. They only way the string theorists could manage that was to envisage a universe that contained 25 dimensions of space while also admitting the existence of particles that could never be brought to rest and of others that travelled faster than light.

It was all a bit much to swallow, and, for a few years, string theory lay neglected and unexploited. It didn’t help that string theory was meant to be a means of describing what is known as the strong interaction of nuclear physics, the force that holds quarks together to form protons and
neutrons. While string theory was lying fallow, what is now known as the ‘standard model’ of particle physics emerged. This tied together everything we knew about the subatomic particles in a very neat package. String theory looked superfluous, if not a little foolish.

So how did string theory become the answer to physicists’ prayers? Most of the hard work was carried out in 1970 by a French physicist called Pierre Ramond. He found the string vibrations that gave rise to fermions. As a bonus, this also removed the need for faster than light particles and reduced the number of required extra dimensions to just nine. String theory now became ‘superstring theory’, and – hallelujah – it was consistent with quantum theory and relativity. There was just one more flaw to address: the string theorists’ suggestion that some particles could never stop moving.

The solution to this turned out to be the one that really mattered. String theory’s meteoric rise to fame came about from the discovery that its unstoppable particles were ones that physicists had long been hoping to create in a fundamental theory: the photon, the quantum particle of light, and, most excitingly, the graviton: the quantum particle of gravity.

If it was good news to find a theoretical justification for the photon, then the discovery that string theory gave the graviton was the stuff of physicists’ dreams. Since the inception of quantum theory in the 1930s, physicists have wanted to find where gravity and the other forces meet. Here, perhaps, was the answer.

The various forces of nature – the strong and weak forces that act in the nucleus, and the electromagnetic force that acts between charged particles – seem to have a fundamentally different nature to the gravitational force. Gravity plays by different rules. It only attracts, for instance, where electromagnetism attracts and repels. Ultimately, physicists were aiming to explain this uniqueness. And string theory seemed to be able to do just that.

In string theory, the ends of the strings are associated with a particle and its antiparticle – an electron and a positron, say. The vibration of the string carries the force that acts between this pair of charges. A string can break into two, or collide with another string. The result of all this produces strings that occasionally close up into a loop. There is no charge associated with this loop of string, only a force that matches the characteristics of the force we know as gravity.

The realization that string theory has gravity built in turned on lightbulbs over countless physicists’ heads. All the while they had been looking at string theory as a means of describing nuclear interactions, but what they actually had was a quantum theory of gravity, a grand unified theory, a theory of everything. Almost overnight, string theory became the great new hope in physics. That hope has long been deferred, however. It was 1984 by the time string theory seemed poised to complete the task Einstein had started. And that is more than three decades ago. So where is the promised final theory? That, it turns out, is a very contentious question.

Nobody doubts we need a final theory. Quantum mechanics and relativity are inconsistent with one another – almost to an absurd degree. The laws of physics, for example, are different for quantum particles moving in different ways through the universe. The quantum description of an electron at rest is different from the description when the electron is moving at something near the speed of light. Albert Einstein had constructed the theory of relativity precisely to avoid such problems.

Looking at it from the other direction, relativity doesn’t make sense when viewed through a quantum lens. Quantum calculations can be done without reference to time or distance, for instance, but relativity can’t cope with anything that doesn’t need time or space. Potentially, string theory can overcome all these problems. But it hasn’t yet – at least not to the point where it is overwhelmingly appealing as a theory. Before it can be hailed as
the new über-theory, before we can say that the universe is indeed made up of strings, the theory has to conquer its own demons. One of those was immediately obvious from the start. We live in just three dimensions of space, but to be consistent with relativity and quantum theory, string theory initially needed 25. That was later reduced to just nine – but that’s still six dimensions that haven’t ever been seen. So where are they?

The short answer is that, from our perspective, at least, the extra dimensions are rolled up very small, or ‘compactified’. Imagine a hosepipe seen from a distance. It looks like a one-dimensional line rather than the three-dimensional object it really is. String theorists say that this is how we must think of the extra dimensions. They are there, but hardly make any impact on our three spatial dimensions.

This is not just a hand-waving argument, but has been made with mathematics: the extra dimensions can be rolled up into six-dimensional torus, or any one of more than a million complex six-dimensional shapes known as Calabi–Yau spaces. This, naturally, brings a lot of flexibility to string theory. Each of these Calabi–Yau shapes, for instance, come with a set of variables to specify its exact nature. These will give different characteristics to the compactified dimensions, and have knock-on effects in the dimensions that are open to view.

The result of this is that string theory does not describe one universe, but many, all with slightly different properties. String theory, therefore, creates myriad universes of every shape and size – and this is where the arguments about the usefulness of string theory really begin. The debate is simple: do we see this multiplicity of possible worlds as a problem or an opportunity?

String theory’s critics call it a theory of
anything
rather than a theory of everything. Anything goes, they say: unless someone finds a way to sift our universe from among the possibilities,
string theory can make no falsifiable predictions about the nature of our universe. Thus, can we really call it a true scientific theory? Many string theorists reject this criticism outright. If string theory gives us so many universes, their argument goes, perhaps that’s because there
are
so many universes.

There is certainly something to this argument. Modern cosmology tells us that the universe most likely went through a period of rapid ‘inflation’ just after the Big Bang. Put simply, the universe blew up like a balloon, its size increasing by a factor of 10
³⁰
– that is, it got 1,000 billion billion billion times bigger – in just a fraction of a millisecond.

No one knows why this should have happened, but it is the best explanation for some otherwise puzzling features of the cosmos. The universe is homogeneous: it seems the same everywhere. This is odd because the Big Bang would have created it otherwise. But the mystery can be solved by inflation: the universe that goes through a period of rapid expansion early in its life will become homogeneous.

Inflation also happens to be a useful support for string theory. If the inflation happened once, there is no reason why it wouldn’t have happened again. Every patch of space–time is subject to the same laws, and so every patch can, in theory, generate a bubble universe that grows until it pinches off and floats away. So countless other universes will bubble out from every universe, blowing up and pinching off to an independent existence. Each one will have slightly different properties from the others. The laws of physics, in other words, will be different – there could be a universe without gravity, for instance, or one where there are 17 different kinds of electron. In this view, the universe is actually a multiverse: a landscape of universes that take every possible form. Somewhere among these universes is ours.