The Big Questions: Physics (15 page)

The same effect, he has pointed out, might be achieved by firing super-energetic particles towards each other so that they missed by only a tiny distance. Their energy would warp the space–time around each particle, and when those warped space–times met, they could form a loop in time. It’s not a loop that you could enter and walk round, however. Much more interesting – and feasible – is the wormhole time machine drawn up by American astrophysicist Kip Thorne.

You will have almost certainly heard of wormholes because they are a staple of science fiction. But that is entirely justifiable: although they have occupied countless hours of research time, this method of time travel was actually inspired by a sciencefiction story. When the cosmologist Carl Sagan was writing his novel
Contact
, he wanted to find a plausible way to send his heroine to Vega, a star 26 light years away, in an instant. Sagan asked Thorne’s opinion, and Thorne set to finding a solution. He found it in a paper that Einstein had written with Nathan Rosen in 1935.

They had solved a problem associated with black holes, the remnants of stars that have collapsed under their own gravity. At the core of a black hole is a ‘singularity’, a breakdown in space and time. Einstein and Rosen had imagined this central core connecting with another region of space–time. This is the Einstein–Rosen bridge. Thorne soon realized this could be the answer Sagan needed.

Think of a railway engineer trying to lay track to the other side of a hill. You could lay it up one side and down the other. You could divert the track around the hill. But if there was a tunnel through the hill, that would be a shorter, more direct route. Because time and space are so closely related in relativity (physicists put them together and talk about the fabric of the universe as ‘space–time’), you can do for time what railway engineers already do for space: you can engineer a short cut. Subsequent analysis of the geometry of these shortcuts through space–time has shown that they would work for time travel.

To specify a point in space–time, you give a position and a time: St Paul’s Cathedral at noon today, for instance. If this is one mouth of the wormhole, the other might be St Paul’s Cathedral at noon yesterday. Walk into the mouth today, and you would walk out at the same point in space, but 24 hours earlier. It might take you some finite time to move from the wormhole’s entrance to its exit, but that needn’t be a problem. In theory you could jump in at one mouth, emerge in the past and hang around to watch yourself jumping into the wormhole.

It’s not quite as easy as that, of course: there are several obstacles to overcome. One is, where do you find a wormhole? Though they exist as solutions to Einstein’s equations, there is no evidence they exist naturally. There is a remote possibility that we could create one by colliding fast-moving subatomic particles. Various theoretical ideas indicate that their highly concentrated energy might warp the fabric of space–time enough to tear a hole in it. But even then we would not be in control.

Space–time is like elastic: it doesn’t like to be stretched. The way space–time tears to create a wormhole creates an energy imbalance that tends to pull the mouth of the wormhole shut. The only way to keep a wormhole mouth open, physicists reckon, is to pack it with ‘negative energy’ that pushes against the natural closure. Though it is possible that a material carrying negative energy exists, we have no idea what it might be or where we might look to get some. Assuming we can keep the wormhole open, who is to say that the hole would bridge to another area of space–time? And if it did, would it be where we wanted to go?

The best solution to this problem (given the existence of a wormhole and fantastical technological abilities with negative energy) seems to involve anchoring one end of a wormhole to a neutron star. A neutron star is an amazingly dense object. Though only around 20 kilometres (12.5 miles) across, a neutron star weighs more than the sun. In Earth’s gravitational field, one teaspoonful of neutron star material would weigh a billion tons.

This concentration of mass has a profound effect upon the space–time around a neutron star: it warps it severely. One of the results is that time slows down in the vicinity of a neutron star. Near a neutron star, time runs at about 30 per cent of the speed it runs on Earth. Tow one end of a wormhole to a neutron star, then, and let the other end sit in empty space, and a time-shift would develop between the two mouths of the wormhole. In theory, that means you could enter the wormhole at a time after you emerged at the other end.

OK, so none of this is easy. Why not? It’s not because creating a time machine breaks some fundamental law of physics. A better suggestion is that we operate by rules that ‘conspire’ against time travel. Perhaps, as Gödel and Einstein suggested, the disturbing implications of someone travelling into their past are a way of alerting us to the fact that something in the universe makes it impossible.

As every Hollywood screenwriter knows, time travel to the past certainly throws out some weird and wonderful dilemmas. The classic one is known as the ‘grandfather paradox’. What if you went back in time, and killed your grandfather when he was a young boy? That would mean one of your parents was never born – would it also negate your own existence? Would you be rubbed out of reality?

There are three possible solutions to this. The first, and most plausible to physicists who think about time travel a lot, is known as the ‘chronology protection conjecture’. It was conceived by Stephen Hawking in 1992, and suggests that some as yet unknown aspect of the natural world will kick in if the flow of cause and effect is ever threatened. Basically, the laws of physics conspire to protect the past. It’s a neat idea.

Everywhere physicists look, there certainly seem to be unanticipated factors weighing down any attempt to create a time machine. There is the need for negative energy for wormholes. Gott’s cosmic string time machine seems to suffer from the setback that the universe conspires against you ever assembling enough mass in a small enough location. There are even hints that quantum versions of time machines, proposed to incorporate physics that is not yet properly understood but will one day have to be taken into account in considerations of time travel, have their own brick walls.

And yet Hawking’s chronology protection conjecture is still just an idea – a way of sidestepping awkward questions about the grandfather paradox without forcing physicists to give up looking
at time travel. The second possibility for protecting your grandfather comes from the quantum world, where weird problems can always find similarly weird solutions. In this case, the idea is quite simple: everything that happens creates a new universe that has no connection to any other universe.

This idea, dreamed up by Hugh Everett in the 1950s is known as the ‘many worlds hypothesis’ (see
Am I Unique?
) and is used to resolve a long-standing problem in quantum theory. Its application to the paradoxes of time travel is equally simple – and equally exasperating. If you go back in time and kill the young boy you think to be your grandfather, you enter a different, parallel world, one where your only existence is that of the time traveller, a totally separate existence from the grandson. There is no ‘other you’ whose existence can be called into question. Paradox resolved.

But, again, not in a particularly satisfactory way. The third idea is simply that we do not have the control over the external world that we think we do. This approach to the paradox says you do not have free will and would not be able to kill your grandfather even if you wanted to. This is a complicated area, and raises philosophical questions that physicists are not equipped to answer. If they really want to know how the grandfather paradox plays out, they need to get on and build a time machine.

All of this seems like it is leading to the conclusion that we cannot travel through time. But nothing could be further from the truth. We know time travel is possible because we have already done it.

The
Apollo
astronauts who flew on rockets to the moon and back became, effectively, the world’s first time travellers. The world’s greatest time traveller is Russian cosmonaut Sergei Krikalev, who circled the Earth for about 800 days at 17,000 miles an hour. Krikalev is now one forty-eighth of a second into the future.

You don’t even have to be a cosmonaut to travel through time. Experiments with highly sensitive atomic clocks flown around the Earth have shown that they moved into the future. A round-the-world trip on an aeroplane might gain you something on the order of a few billionths of a second. Why? The answer lies with Einstein’s first relativity theory: special relativity.

Special relativity (see
What is Time?
), which was published in 1905, says that the passage of time for any person or object is relative, and depends on motion. If you blast off on a rocket bound for Alpha Centauri, say, your watch will run slow compared to the clocks back on Earth. If your rocket travels at close to the speed of light, that difference in measured time could be profound. In a long but fast return journey, it is possible that you could arrive back on Earth just a few years older, but find that everyone who stayed home has aged much more.

In this scenario, if you have a twin, they would no longer be the same age as you. This bizarre result, known as the twin paradox, is fully allowed by the laws of physics. The truly remarkable thing is that this difference in the passage of time means the travelling twin has travelled into Earth’s future. When you return from your travels in space, you find that you have also travelled in time: more time has passed on Earth than has passed for you. We can conclude therefore that we can indeed travel through time – and some humans have already done so. However, this travel into the future is relatively easy. It is travel to the past that is proving so difficult. Will we conquer these difficulties? Only time will tell.

Drifting poles, the planet’s churning core and the threat to life on Earth

Can we avoid the fate that befell Mars? The Red Planet’s magnetic shield failed, and its atmosphere was blown away by the sun, leaving it a barren, sterile wasteland. Is Earth heading the same way?

Earth’s magnetic field, known to scientists as the magnetosphere, has been an integral part of the biosphere since life on the planet began. Bacteria, plants and animals are known to be affected by its orientation. Many species of birds would quite literally be lost without it – it is the cornerstone of migration strategies that allow them to escape harsh northern winters, for example.

Humans cannot consciously sense magnetism in the same way as many animals, but we still gain enormous benefit from the Earth’s field. Not only does it appear to keep our atmosphere in place, it also protects us from intense solar radiation and electrical storms that would otherwise play havoc with our electrical grids, satellites and aircraft communications. If Earth’s magnetic shield is failing, we need to know sooner rather than later.

We may never know which human civilization was the first to make explicit use of Earth’s magnetic field. Until relatively recently, it was thought to be the Chinese, who used magnetic minerals known as ‘south-pointing fish’ to align their buildings in accordance with the principles of feng shui. However, reliable
evidence for this practice dates back no further than 400
BC
which means the oldest magnetic artefact is most probably a piece of the mineral magnetite found in Veracruz, Mexico, home of the Olmec.

The Olmec were, it is thought, the first civilization of the New World, existing between 1000 and 1400
BC
. The piece of magnetite, unearthed in the early 1970s, had been fashioned into a bar that would offer little friction when set on the ground, and scored with a groove in the middle of one end. It looks, to all intents and purposes, like a compass needle.

When physicist John Carlson reported the discovery of the Olmec magnetite, he pointed out that the Olmec people constructed their buildings on alignments 8 degrees west of north. This, he said, was ‘a curiosity’. But taken with other evidence gathered over the subsequent centuries, it is more than a curiosity – it is evidence that the Earth’s magnetic field is far from constant. And that is why we think it may currently be failing.