Max Tegmark: It's goodbye to the universe - hello to the multiverse

What is the universe made of? The ancient Greeks conceived of the "atom", the indivisible unit of matter. Today's physicists talk of smaller particles - quarks and electrons, neutrinos, Higgs Bosons and photons. Understand them - and the forces that hold everything together - and we may finally get a handle on what makes it all tick.

The trouble is, the more we drill down into the subatomic world, the more complexity we find. The bedrock of reality seems as elusive as ever. Perhaps, says a leading theoretical physicist, we do not live in a world of particles and forces at all - but of pure mathematics.

Max Tegmark, professor of physics at the Massachusetts Institute of Technology, claims that at the heart of everything are numbers. On one level this is uncontroversial; the whole point of physics is that we can use mathematics to describe the world around us.

But Tegmark goes further. His Mathematical Universe Hypothesis (MUH) states that not only does maths describe the world we live in, it is the world we live in. "If you grant that both space and everything in space is mathematical," he says, "then it begins to sound less insane that everything is mathematical."

Tegmark, a tall, affable Swede and as far removed from the popular image of the geek as you can imagine, is one of the rock gods of cosmology, a select group of thinkers who are using their mathematical prowess to tear up all our cherished notions about the universe, and replace it with a cosmos that is so bewilderingly weird that it makes the plot of most science-fiction novels look like an Ikea instruction leaflet.

Outlandish ideas that would once have been dismissed as insane are being substantiated by new discoveries all the time, the most recent - that of gravity waves - announced last month. Tegmark and others believe that the universe is far grander - and far stranger - than science has been willing to admit. Spend any time with one of these new-wave cosmologists and you will be left reeling as all your conceptions of reality are swept away.

Meeting Max Tegmark

I meet him in a swanky London bar. He is here to publicise his book about mathematics and reality and as we talk over a nice Riesling, the clatter around us dissolves into a Narnia-like vision of parallel worlds, infinite space and places where an identical me and an identical him are having the same conversation in an identical bar - but on a planet ten-to-the power-of-ten-to-the-power-of-123 light years away.

The philosopher-kings of the 21st century are unafraid to speculate on the wildest shores of physics - so much so that many have been accused of peddling outlandish ideas that are more metaphysics - magic, even - than science.

Tegmark bridles. "We are not talking about vague philosophical notions but mathematical theories," he says. "It is not my job to tell the universe how it should be."

The MUH is the latest refinement of a long-running revolution that has overturned the classical cosmology which has held sway since the time of Copernicus and Galileo. This states that the universe - all there is - is basically what you see when you stare up at the night sky. We live in a huge sphere of expanding space about 90 billion light years across, which contains maybe half a trillion galaxies.

Impressive, but a mere gnat on an elephant's back compared to the multiverse, the name given to the concept that states that there is - must be - far, far more "stuff" out there than even our best telescopes can see, or ever will be able to see.

Multiverse hypothesis

The multiverse hypothesis has been knocking around for decades, but only recently has it become respectable enough to come out of the closet. Leading physicists such as Tegmark, Brian Greene of Columbia University and Astronomer Royal, Lord Rees, are fully signed-up multiversers.

The multiverse comes in several "flavours", or levels, including Tegmark's hypothesis - and all may be correct at the same time.

In fact, you now struggle to find a physicist who is not a multiverse-believer. "We cannot opt out of ideas because we don't like them," says Tegmark. "We make assumptions, then test them. This is science."

Some evidence for the multiverse comes from observation. Since the Big Bang 13.8 billion years ago, there has been plenty of time for space to look very different in various directions. Huge clusters of galaxies in one direction, vast empty voids in another. But it does not.

The best explanation is that the early universe underwent a period of unimaginably fast expansion, called inflation. By postulating a period of rapid expansion - a faster-than-light stretching of the fabric of space-time itself - in the first few quadrillionths of a second after the the Big Bang, inflation meant that regions of space now tens of billions of light years apart were once connected.

Last month, a team of scientists at the South Pole announced that their telescope, Bicep 2, had discovered gravity waves, colossal ripples in space time. The finding, which has been disputed and has yet to be confirmed, would not only back up our models of the Big Bang but also provide strong evidence for inflation and the reality of the multiverse.

Recent discoveries like this suggest space may have inflated to an infinite extent after the Big Bang. Physicists believe that the inflationary process was capable of creating matter as well as space, in similarly infinite quantities.

In which case, go far enough and you will see an exact "repeat" of the stars and galaxies we see around us, including another planet Earth and another you - and every variation thereof, including worlds where Elvis lives, where Hitler won the Second World War, and where strange creatures like unicorns graze on alien pastures. This is a respectable idea. "To get rid of that conclusion," Tegmark says, "either inflation is wrong ... or space is in fact not infinitely stretchy."

The multiverse "zoo" gets more bewildering. Some physicists believe that inflation happens all the time, and that an infinite number of "bubble universes" are being spawned, of which ours is only one. One interpretation of quantum physics states that every "quantum event", such as the decay of a radioactive atom, causes a new universe to split off and grow in a new direction.

For Tegmark, if something is mathematically consistent, it exists - all possibilities that do not break the laws of mathematics are as real as the ground under your feet. The universe - these universes - are numbers, not merely describable by them. "But surely you need stuff, for the maths to work on?" I ask. "No. What you call 'stuff' is a mathematical construction," he says.

Tegmark's universe of gyrating numbers, of equations literally coming to life, is one of bizarre possibilities and incomprehensible scale. Perhaps his most shocking argument concerns the role of humans, and consciousness in all this. Far from being insignificant monkeys in an ordinary little planet, we may be, he says, the only things that give the universe its meaning.

Again using mathematics, he argues that ET probably does not exist, at least in the observable part of "our" universe. If it is just us, then this has profound implications Tegmark says: "All those galaxies only became beautiful 400 years ago when someone saw them for the first time. If we humans wipe ourselves out, then the entire universe becomes a huge waste of space."

One day, he says, if we avoid destroying ourselves, humans will probably colonise the cosmos. And whether we do this will depend, in large part, on the choices we make - decisions about nuclear weapons, climate change and all the existential threats on the horizon. If he is right, we are the meaning of life, the universe and everything, and ours is the most significant century in nearly 14 billion years.

Our Mathematical Universe, by Max Tegmark, is published by Allen Lane