The Big Questions: Physics (14 page)

And there ‘you’ are. The chance that something with exactly your genetic make-up will appear on a blue-green planet somewhere in another universe seems infinitesimally small. But that tiny probability is converted into certainty when we allow the existence of an infinite number of universes. Not that you two could ever meet. When a new universe bubbles out and pinches off, contact is lost for ever. You are trapped in your own space and time; your twin is in a separate and unreachable sphere.

The weirdness of a bubbling universe pales beside your third and final chance for multiplicity, however. Quantum theory provides not just a chance of there being another you, but an argument that there are a near-infinite number of you. The twist is, each one has made a different choice in life. This is the ‘many worlds’ interpretation (MWI) of quantum theory, and it is truly mind-bending.

There are a number of interpretations of quantum theory, and each one has to explain the inexplicable. The theory allows for quantum particles – atoms, electrons, the bullets of light energy known as photons – to exist in more than one state at any one time. This phenomenon is known as superposition, and it is a profound mystery. An electron can spin clockwise and anticlockwise at the same time, for example. A photon can be simultaneously here and there. An atom can hold two different energies.

In the classic demonstration of superposition, physicists fire electrons at a screen that is scored by two narrow vertical slits. The stream of electrons is so slow that there is only one particle in the apparatus at any one time. To our thinking, the electron will go through one or other of the slits. Place a phosphorescent screen, rather like the screen of a cathode ray tube television, behind the slits, and we should see two sets of glowing dots where the electrons land: one behind the left slit, and one behind the right slit. We don’t. We see a series of glowing bands known as an interference pattern.

Interference is something we associate with waves. Ocean waves interfere with each other: when the crests meet, they reinforce and the water piles up higher. When two troughs – effectively, negative quantities of water – meet, an even deeper trough is the result. When crest meets trough, they cancel out to give flat water.

The same is true for light, as Thomas Young demonstrated two centuries ago. Young was demonstrating that light is a wave, overthrowing Newton’s particle theory of light. In an arrangement like the double slit experiment described above, but with light passing through the slits, Young’s screen showed a series of light and dark bands, something that could only be achieved if the slits both acted as secondary sources of light, with the two emerging light waves interfering.

Going back to the single electron in the double slit experiment, then, how to explain an interference pattern? How can there be interference when there is only one particle? The answer is that, although we think the electron has to go through one or other of the slits, it actually goes through both. An electron might be a particle, but it is also a wave.

There is no easy resolution to this paradox, and the world’s greatest minds have debated it endlessly since quantum theory was invented. In the 1950s, however, Hugh Everett came up with a radical new take on the problem. At the time it was much
derided, but today it is gaining support. The idea is simple. Every time a quantum particle faces a choice, new worlds are created – worlds in which every option is realized.

It’s easy to see why scorn was poured on Everett’s idea: who can stomach the notion that a world is created every time a photon is spat out by a star, or absorbed by an atom in a human retina? These are both quantum events, where one quantum particle is absorbed by another. Can we really believe that just looking at the heavens forces a new universe into existence? Everett left physics shortly after publishing this idea, but it has nevertheless found a series of champions. That is largely because, strange as it seems, it actually offers a reasonable solution to the strangeness of the quantum world.

In Everett’s many worlds interpretation, the electron doesn’t form a superposition state when faced with a choice of two slits, but splits the world into two. In one world, it goes through the left slit. In the other world, it goes through the right hand one. Though we have no consciousness of the different worlds, quantum particles such as electrons feel their influence from across the divide. The pattern we see results from interference between electrons in different worlds. In this view, what we think of as reality is just one of an infinite number of realities, each one slightly different from the next. And each one will contain a version of you.

The MWI seems to have a slow-growing following amongst physicists; a 1995 poll of physicists attending a conference on quantum theory found that 60 per cent believed it to be the correct interpretation of the theory. Such polls are unscientific, though, and not an indication of the ‘rightness’ of everything. Which is why, if you are really intent on finding out the truth about that other you, you have to consider a radical proposal: quantum suicide.

Don’t try this at home, but the protocol of this experiment is fairly simple; it could even be done using currently available
technology. You hold a loaded gun to your head, but rig it up so that pulling the trigger prompts a measurement on a quantum particle – determining the spin of an electron, for example. If the result is ‘clockwise’, those standing around watching hear a click. If it’s ‘anticlockwise’, they see the gun fire. Not a pretty sight.

But here’s where perspective becomes everything. If Everett was right about the existence of many worlds, there will always be a world in which the gun doesn’t fire. Your conscious existence will, therefore, never know of the gun firing. After a dozen clicks you’ll be convinced that quantum suicide is actually a route to appreciating not only the multiplicity of your existence, but also your immortality. Not that you’ll be able to share that viewpoint with anyone. What’s more, you can have your cake and eat it. You have found that other you, but you can also leave it behind, hopping from world to world like Alice in a quantum wonderland.

Where relativity meets science fiction

‘Scientific people know very well that time is only a kind of space. We can move forward and backward in time just as we can move forward and backward in space.’ This might sound like a claim from the future, or at least the present, but it comes from the past.

It is spoken by the Time Traveller in H.G. Wells’s
The Time Machine,
which was published in 1898. The truly remarkable thing is Wells’s prescience: nearly twenty years passed before Albert Einstein published the theory that made such time travel theoretically possible – and even then it took years before anyone noticed.

Oddly, Wells’s Time Traveller only ever travels to the future. Now, however, we know that the laws of physics allow for travel forwards and backwards in time. If you can handle ideas such as infinitely long rotating cylinders the size of galaxies, wormholes held open with exotic forms of negative energy, and having to choose between never having been born or losing your free will, then you might just be able to handle the science of time travel. As thrill-rides go, it’s a little bumpy. But, given the prize, it’s definitely worth it.

Time travel is so fascinating because we are trapped by time. We cannot choose how we move through it, as we do with the other
dimensions. But Wells’s idea that if we only knew how, we might be able to treat time just like space, was right on the money.

‘Scientific people know very well that time is only a kind of space. We can move forward and backward in time just as we can move forward and backward in space.’

THE TIME MACHINE
H.G.WELLS

In 1915, Einstein published his general theory of relativity. This described the universe as a four-dimensional fabric made up of three dimensions of space, and one of time. Every piece of matter and energy in the universe warps the fabric, changing the shape of the universe in a way that causes matter and energy to experience the pull we call gravity. The sun, for instance, creates a kind of well in the fabric, into which nearby planets would fall, if it were not for their momentum. The result is that the planets orbit the sun in the same way that, in a casino, a speeding ball orbits the centre of a spinning roulette wheel.

It is easy to imagine how the undulating landscape of gravity affects motion through space. But the same is true of motion through time: this undulates too. Pack enough mass and energy into a small enough region of space and you can even bend time into a loop – it is rather like rolling up a sheet of rubber so that the ends meet and you can walk around the surface without ever reaching an endpoint. In this configuration of the universe, a moment repeats itself endlessly.

The first person to notice that general relativity allows the creation of loops in time was the Austrian mathematician Kurt Gödel. In 1949, in a review article describing how the invention of relativity had changed our perception of the universe, he wrote that it ‘is possible in these worlds to travel into any region of the past, present and future, and back again, exactly as it is possible in other worlds to travel to distant parts of space.’

Gödel had solved Einstein’s equations and found that, if the universe is rotating, time can flow in loops. He was alarmed
by this, and being a close friend and colleague of Einstein, Gödel showed him the result. Einstein said he too was ‘disturbed’ by the possibility. ‘It will be interesting to weigh whether these are not to be excluded on physical grounds,’ he wrote in a reply to Gödel’s paper. Gödel seemed to have a similar view: something, he suggested, must stop such things happening. The universe surely cannot be allowed to have people travelling through time.

In some ways, Einstein need not have worried. Gödel’s work was solid, but useless. The motions of the galaxies tell us that our universe is not rotating, so loops in time will not naturally exist. If we are to build a useful time machine, we will have to create those loops for ourselves.

‘It is possible in these worlds to travel into any region of the past, present and future, and back again, exactly as it is possible in other worlds to travel to distant parts of space.’

KURT GöDEL

But we do have ideas for how to accomplish that. The first came in 1976, when Frank Tipler of Tulane University in New Orleans, Louisiana, drew up the blueprints for a time machine. Tipler showed that an extremely massive and infinitely long, fast-rotating cylinder would warp the fabric of the universe enough to create a loop in time.

Again, though, this has little future as a time machine. It is certainly not the kind of thing that Wells envisaged: his Time Traveller builds a time machine that fits into his house. Infinitely long cylinders are hardly likely to fit in any factory, however large. There is another option: use time machines that nature has already built. In 1991, Princeton astrophysicist J. Richard Gott showed that the universe might contain material that could act as the raw material for a time machine. The material is a super-dense strand of ‘cosmic string’.

According to some theories of how the universe formed, cosmic strings would have formed in the earliest moments of
creation, and might still be hanging around the universe today. They are, essentially, defects in space, something like scar tissue that formed when the universe was going through a period of rapid change. A cosmic string is a fearsome beast: though less than the width of an atomic nucleus in diameter, it stretches across the universe. Unsurprisingly, turning one into a time machine is not for the faint-hearted. For a start, you need a pair of them.

The extreme density of each of the strings warps space–time in a way that means you can create a loop in time by placing them side by side, then moving them rapidly apart. Travel in a loop around these moving cosmic strings and every time you return to where you started, you will find yourself at an event in your past. Gott compares it to an Escher drawing. Just as Escher warped perspective to create geometrically impossible effects, the strings warp the geometry of the space–time around them so much that it no longer follows the rules that are familiar to us.