Light traveller

H.G. Wells’ story on time travel makes very interesting reading: but does it touch reality at some stage? Is it possible for someone to travel into the past or future given a suitably designed vehicle? In an article in this series (Who is faster… you or light?, November 23, in this newspaper), I had referred to the limerick about Miss Bright, who travelled faster than light and managed to get into the past.

The reason for recalling the adventures of Miss Bright was the claim by a group of scientists at CERN, the European laboratory for fast-moving subatomic particles, that they had found a bunch of neutrinos travelling faster than light. Would such particles suffer a
time-warp like that experienced by Miss Bright?
That claim was, of course, examined carefully since its veracity would have brought Albert Einstein’s special relativity under a dark cloud of suspicion. For, relativity theory rules out particles that cross the light speed barrier. To energise an ordinary particle to a speed beyond that of light would demand infinite energy. So it would have been an awkward piece of evidence to explain, had the result stayed as correct. It now seems that there were experimental errors that made the original conclusion inoperative.
In general, we may add that the special theory of relativity, originally proposed by Einstein in 1905, does allow travel into the future. If you leave the earth in a spaceship moving at, say, 80 per cent of the speed of light, you will age more slowly than the inhabitants of our planet, so much so that when you come back, say, after 30 years by your watch, the people here would have aged by 50 years. Such results have been observed for fast-moving subatomic particles.
Before we enter into the science-fiction territory, it should be noted that from the beginning, the neutrinos have been particles that are hard to catch. Theoretically, the simplest model of a neutrino has the particle always travelling with the speed of light. However, it interacts with other matter very weakly. Since any detector of neutrinos must employ particles which interact with the neutrino, such detectors are hard to come by. Which is why claims about neutrinos are difficult to verify.
To return to our time machine, the theory of gravity proposed by Albert Einstein in 1915, the general theory of relativity, made it possible for scientists to think of space and time structures that had sub-regions where time warps could exist. For, the Einstein theory tells us that the geometry of space and time ceases to be the geometry of Euclid that we studied at school provided the region under study has strong gravitational effects arising from large conglomerations of matter.
Imagine an experiment in which we station three observers A, B and C around the Sun and ask them to send light signals to one another. Light, proverbially travels in a straight line, and so the rays AB, BC and CA will form a triangle in space. What will its three angles add up to? Not to two right angles as our school texts assure us, but to slightly more. The discrepancy may be slight, but it is important. It tells us that geometries in which Euclid’s basic assumptions fail might be relevant wherever gravity is present.
Mind you, these changes from the Euclidean to non-Euclidean are very small but they open out the possibility of space and time very different from Euclid’s if there exist rather unusual matter distributions. And out of these speculations have emerged the notions of black holes and wormholes.
Black holes are comparatively less esoteric and easier to understand.
If a massive object begins to shrink under its own gravitational attraction it ultimately becomes a black hole. In such a state its surface gravity is so powerful that it attracts the light escaping from the surface of the black hole. This means that the object is invisible to an outside observer. Astronomers are indeed finding highly compact objects close to the black hole state in the nuclei of galaxies.
Wormholes are not so easy to define or locate in the universe. In a crude sense, they provide connection between two points in space and time via a special tunnel. To have this kind of geometry is not, however, easy. In fact, to date no wormhole has been found in the universe.
If a wormhole were actually found, then it may provide us with a time warp. It may be possible for an observer going through the wormhole passage to visit the past. However, such a possibility may raise many practical issues.
For example, if our observer goes far enough back in time, he might meet his grandfather as a boy. If the observer kills that boy, there is an obvious end to the lineage. So that boy could not have produced descendents like the observer. But the observer does exist. In short, we would run into a paradox in which we cannot decide if the observer exists today or not!
This paradox is typical of time travel into the past. The usual assumption that cause precedes effect gets violated, leading to nonsensical results. To get round this difficulty elaborate safeguards are needed. For example, Igor Novikov, a Russian astrophysicist, has shown that one can have wormholes in which a past-going observer may get close to his grandfather, but not close enough to produce a causal effect. In short, his grandfather is safe from any evil designs of the grandson.
Such scenarios are, however, very much contrived and one may wonder how they came about if indeed they did come about. On a broader canvas, one may look at the issue from the point of view of how the laws of physics operate in nature. There is a certain predictive power in those laws, which is their strength. That will disappear in chaos if free interaction between past and future were allowed. So it is just as well that we do not have time machines at hand.