Highly strung

It came out of nowhere like an express train in the night. Only it wasn't an express train - it was an entire universe, hurtling towards our own from a higher dimension. Before the collision, our universe was an empty, aching void. Afterwards it ignited, exploding outwards in an unstoppable firestorm of light and matter.

It came out of nowhere like an express train in the night. Only it wasn't an express train - it was an entire universe, hurtling towards our own from a higher dimension. Before the collision, our universe was an empty, aching void. Afterwards it ignited, exploding outwards in an unstoppable firestorm of light and matter.

Is this a description of the Big Bang? Is everything we see, out to the very farthest reaches probed by our telescopes, merely the wreckage of a titanic collision between universes? A group of physicists from Britain and America think it is. They call the colliding-universe scenario the "ekpyrotic universe", from the Greek for "born out of fire". They say the cosmic smash that triggered the Big Bang may not have been the first. "Before the Big Bang, there was another Big Bang and, before that, another, stretching way back into the mist of time," says Neil Turok from the University of Cambridge.

It's an awe-inspiring vision. And it comes from "string theory", one of the hottest topics in cosmology. String theory views the fundamental building blocks from which everything is made not as tiny, point-like "particles" but as impossibly small "strings" of super-dense matter. The strings - about 10 trillion trillion times smaller than atoms - vibrate exactly like strings on a violin. And each note they create corresponds to a different microscopic particle such as an "electron" or a "quark". The higher the pitch of the note, the more energy in the vibration and the heavier the particle.

Experimental physicists have so far discovered a host of fundamental particles, which are "glued" together by four fundamental forces, which in turn are transmitted between the particles by a host of "force-carrying" particles. In order that strings can faithfully mimic all the myriad properties of such a zoo of fundamental particles, they must be free to vibrate in a large number of different ways. This can only happen, theorists have discovered, if strings inhabit a world with an incredible 10 space-time dimensions.

A 10-dimensional world is a slight embarrassment for physicists. After all, the familiar world appears to have only four space-time dimensions - north-south, east-west, up-down, past and future. Proponents of string theory, however, have refused to be fazed by this contradiction between theory and reality. They maintain that there are additional space dimensions, currently hidden from our view. Whereas the familiar space dimensions extend across billions of light years of space, these dimensions are "rolled-up" so incredibly small that they have so far gone unnoticed.

Why go to all the trouble of inventing impossible-to-see entities quivering in impossible-to-detect dimensions? The answer, of course, is that there is a pay-off. String theory holds out the tantalising hope of solving one of the greatest outstanding problems in science. That problem is how to unite Einstein's general theory of relativity - which describes gravity - with quantum theory - which describes the other three fundamental forces of nature.

Einstein's theory explains the behaviour of bodies with a lot of mass such as planets circling the Sun and even the evolution of the whole universe. Quantum theory's domain of expertise, on the other hand, is the world of small things such as atoms and their constituents. Because the domains of the two theories are so different, there is generally no overlap, so each can be used independently of the other. A serious difficulty arises, however, when people turn their attention to the first moments after the Big Bang. At that time the universe was both very massive - the domain of Einstein's theory - and smaller than an atom - the domain of quantum theory. This means that neither Einstein's theory, nor quantum theory, is individually up to the job of illuminating this period. What is badly needed is a hybrid of the two - an over-arching theory that meshes both together into a quantum theory of gravity.

Devising such a theory is hard, but string theory offers hope. One of the possible string vibrations turns out to have all the properties of a "graviton", the hypothetical carrier of the gravitational force. Consequently, string theory - which is inherently a "quantum" theory - contains within it a theory of gravity. This is why many theorists see it as the best chance for a unified theory.

It turns out, however, that one-dimensional strings are not the only entities which can pop up in string theory. Since there are 10 space-time dimensions to play with, the theory can support the existence of more complicated objects, dubbed "branes", with many more dimensions. A string, to use the terminology, is one-brane, whereas a more general brane, with "p dimensions", is a p-brane (and a physicist's very weak pun).

The existence of branes raises an interesting possibility. Perhaps our universe is a four-brane, or four-dimensional "island universe" adrift in a 10-dimensional space-time. This possibility raises another scenario. Perhaps, out there in the unimaginable 10-dimensional void of string theory, other island universes may be floating. As the science fiction writer Arthur C Clarke wrote: "Many and strange are the universes that drift like bubbles in the foam upon the River of Time."

The possibility of other island universes has captured the imagination of a team led by Turok and Paul Steinhardt of Princeton University. For, if there are other universes out there, then it stands to reason that they might occasionally bump into each other. And this, at long last, might explain what the big bang was.

Of course, there could be countless island universes besides our own lurking out there in higher-dimensional abyss. However, the simplest case is always the easiest to deal with mathematically. Also, nature, for reasons nobody really understands, invariably chooses the simplest option. Turok, Steinhardt and their colleagues assume that, in the whole multi-dimensional universe, there are just two four-branes - ours and one other.

Four-dimensional objects are of course impossible for human beings to visualise. Turok and Steinhardt therefore visualise the four-branes as two-dimensional objects - like the two slices of bread in a sandwich. Again, for simplicity, they assume that the two slices of bread are infinite in extent, so that they form the ultimate boundaries of the universe. They also assume they are without matter or light - and you can't get much simpler than that.

Between the two slices of bread, where the filling would go, is the 5th dimension. And it's along this dimension - which we can no more perceive than a blind person can experience colour - that the two brane universes hurtle towards each other.

In physics there is a cast-iron rule called the "conservation of energy". It asserts that energy can never be created or destroyed, only transformed from one form into another. In a light bulb, for instance, electrical energy is converted into an equivalent amount of light energy and heat energy.

Moving bodies possess energy merely by virtue of their motion. Wander into the path of a speeding cyclist and you will be left in no doubt about this. Consequently, the fast-approaching branes have tremendous "energy of motion" along the 5th dimension. When they collide, the conservation of energy ensures that this energy is dumped into their four-dimensional interiors as surely as the energy of motion of two colliding express trains is dumped into the twisted wreckage. And this energy sets the branes expanding violently. "It creates the headlong explosion of space we have come to call the Big Bang," says Turok.

Of course, the Big Bang was a bit more than a violent expansion of completely empty space. We, and the matter we are made of, are testimony to that. Here, Einstein has something important to say. In 1905, he discovered that mass is a form of energy. Consequently, not only can mass be turned into other forms of energy - for instance, the scorching heat of a nuclear fireball - but other forms of energy can be converted into mass.

So, the energy of the colliding branes ends up not only in the furious expansion of empty space but also in the creation of matter - a blistering hot fireball of fundamental particles. It creates the "hot" Big Bang.

If this scenario is correct, then the subsequent evolution of our universe is pretty much the same as is widely accepted. As the space of our brane exploded in size, the expansion rapidly cooled the Big Bang fireball. Eventually, when it was cold enough, galaxies and stars congealed out of the shimmering debris. And, one day, after 13.7 billion years, a group of physicists on the third planet of an unremarkable star in a nondescript galaxy called the Milky Way hit on the idea that a collision between peculiar entities called branes might at last provide an explanation for everything we see around us.

Marcus Chown is the author of 'The Universe Next Door'