The
UNEXPLAINED

Mysteries of Mind Space and Time

A nearby artificial satellite draws a streak of light across the background of thousands of distant stars revealed by a long-exposure photograph. Many astronomers believe that, in the immensity of our Galaxy, there are numerous black holes. Will they one day be harnessed for time travel?If time travel is possible in principle, how can it be achieved? JOHN GRIBBIN explains how super-dense neutron stars could be the means by which, one day, space travellers will be able to journey into the past

THE PHILOSOPHICAL DEBATE about the paradoxes of time travel makes one thing clear: it is possible to imagine ways in which the Universe may be constructed so that time travel, of some kind or another, may be allowed by the laws of physics. That leaves the big question: is time travel really possible? And) if so, how could a time machine be built?

During the 1970s, there was a great deal of excitement about the implications of the existence of black holes for time travel. A black hole is a region of space in which enough matter is concentrated for gravity to make a 'hole' in space-time, a region from which no energy - light energy included - can escape (see page 30). The idea of black boles as holes in space is an entirely respectable part of modern astrophysics, and they are invoked by scientists to explain a number of strange phenomena. Energy released from black holes could, for instance, explain the genesis of high-energy x-rays that have been observed in our own Galaxy and, on a much grander scale, the energy sources that appear to lie at the centres of certain galaxies. A black hole is a hole in space - but, as Einstein taught us, space and time are inseparable, two facets of space- time. It seems reasonable to suppose that a hole in space must also be a hole in time,whatever that means - and if it were possible to travel 'through' a black hole, we might emerge in some other place, perhaps even another universe, and at some other time. Unfortunately, however, such speculation seems to be nothing more than science fiction, because the journey through the black bole would crush you to death and rip you apart simultaneously. But nevertheless, the discovery of black holes has set a whole new generation of theorists thinking about the possibility of time travel.

Two possible sites for black holes. In the computer processed image of M87, one of the largest galaxies known (left), a hot bright jet of matter, emitted from the galaxy's core, can be seen. The jet sends out enormous quantities of energy in the form of light and radio waves. The source of the energy may be a black hole at the galaxy's centre. The same explanation has been invoked for the enormous explosion that is occurring at the centre of the galaxy M82 in Ursa Major (below). Like all proposed black hole identifications, these are highly controversial

According to modern theories of space time - refinements of Einstein's theory of general relativity - the key to time travel is not black holes, but what are called naked singularities. A singularity is a place where the laws of physics as we know them break down - as in the heart of a black hole, for example, or in the Big Bang in which the Universe was created - and where space and time cease to exist. In a black hole, the singularity is 'veiled' by an event horizon from which no information can escape. Some scientists, supporters of the 'cosmic censorship' theory, believe that this is always the case - nature always hides its singularities. Others, however, believe that singularities can sometimes become visible: theorist Stephen Hawking has shown that black holes gradually 'evaporate' to expose the singularities - naked singularities - at their hearts, and some scientists believe that when matter rotates too fast to become a normal black hole, it can become visible from certain directions - that is, it does not form an event horizon (see page 41).

Now if a naked singularity exists in the Universe, then it is possible, according to the laws of physics, to travel in an orbit 'around' the singularity - an orbit that takes you first into the future or past, then back home to the time and place you started from. Such an orbit is called a closed timelike line - and, of course. it implies the possibility of time travel and all the philosophical problems it entails. It is small wonder that it has caused dissension among the scientists. One school of thought believes that. because naked singularities imply the existence of closed timelike lines, naked singularities cannot exist. Another group of theorists points out that the Big Bang in which the Universe was born may have been a bursting out from a singularity, like a black hole in reverse; thus at least one naked singularity may not only have existed, but may have been the cause of our Universe coming into existence. And if a naked singularity once existed, inside the Universe, then, because the closed timelike lines associated with it remain fixed for all time, it influences the whole future of space time. If we could only find it, we would have a ready-made time machine with which we could visit anytime since the beginning of the Universe.

But most startling of all are the ideas of a maverick American theorist, that not only may closed timelike lines exist, but it might one day be possible for mankind to create them. With 20th-century physics and engineering as his guide, Frank Tipler has sketched out plans that a future civilisation might use as a blueprint in building a time machine.

Tipler's work, published in respectable journals such as the Physical Review and Annals of Physics, is necessarily highly mathematical and not always easy for the uninitiated to follow. But when asked straight out 'Is time travel possible?', he replied: 'My current view is that there is indeed a real theoretical possibility for causality violation in the context of relativity theory.' Remember that, in scientific terms, 'causality violation' implies 'time travel'! The tricky mathematical part of Tipler's work is the calculation that time travel is allowed within the laws of relativity theory. The Universe does 'allow' time travel after all, and there is no known reason why closed timelike lines should not exist in the real world. The next question Tipler looked at was whether the conditions appropriate for time travel to take place would arise naturally in the Universe. Finally, he has looked at the possibility of building an artificial time machine.

Right: a pulsar lying among stars in the constellation of Vela flashes on and off. These pictures were made with sensitive electronic detectors connected to a telescope. 'Noise', similar to television interference, obscures the original eight pictures. The computer- enhanced pictures flanking the central sequence show the pulsar at maximum and minimum brightness. In the left-hand picture it is a faint dot just below the centre that is absent in the right- hand picture. Theoretically pulsars could be harnessed in the future for purposes of travel forwards and backwards through time

The crucial factor, Tipler discovered, is the existence - or creation - of a massive, rotating object. A large amount of matter concentrated in one place distorts the fabric of space-time by gravity (see page 42). If the object also rotates, then space-time is twisted still more by the rotation, as the firm grip of gravity tries to carry the fabric of space-time round with the rotating mass. For some combinations of mass, density and angular velocity a singularity can be formed without creating an event horizon. The region of distorted space-time around this naked singularity is called, logically enough, the 'strong field' region, referring to the gravitational field. Tipler has proved that a traveller from a weak field region - the Earth, say - could go to a strong field region near such a rotating object, move in the direction of negative time, and then return home to the weak field region without ever violating any known physical law. He would return to find himself in the past, returning to Earth, if he wished, before he left. Building a time machine is impracticable now. But that may not always be the case.

A working time machine can be constructed, in principle, by taking a sufficiently massive but compact lump of material - any material - and making it spin fast enough. Strictly speaking, the rotating material ought to be in the form of an infinitely long cylinder, but Tipler believes that a cylinder 10 times longer than its diameter probably has the right proportions to do the trick. How much mass do you need? Astronomers know that in the last stage of compression before the black hole state, objects called neutron stars exist in the Universe. Such a star might contain as much matter as our Sun, packed into a volume no more than 15 miles (24 kilometres) across; the name 'neutron star tells us that the density of such a star is roughly the same as the density of neutrons in the heart of an atom. A neutron star is, in effect, a whole star compressed to the density of an atomic nucleus. Keeping the mass of the Sun as an approximate guide, our working time machine could be made from a neutron star squeezed into the shape of a cylinder 7 miles (10 kilometreS across and 70 miles (100 kilometres) long, rotating twice every millisecond, with the rim of the cylinder moving at half the speed of light.

In space-time diagrams two space dimensions are shown; the third represents time. The 'light cones' (left) represent a light wave converging and spreading out again. Only journeys within cones (that is, slower than light) are possible. But near a massive rotating object (right) light cones are tilted. Journeys can lie within a cone, yet go backwards in time

It sounds outrageous. But in the late 1960s, astronomers discovered sources of rapidly pulsing radiation in our Galaxy. These sources, dubbed pulsars, are now explained as rapidly rotating neutron stars. Some of them spin at a rate of once very few milliseconds; most have a mass about the same as our Sun. Of course, the spin isn't quite fast enough to make them time machines, and these spinning neutron stars are not stretched into elongated cylinders. But the description of a pulsar is tantalisingly close to Frank Tipler's description of a working time machine.

Right: Dr Who, one of the most famous of fictional time travellers, with his somewhat impractical design of time machine Above: How 'hey met themselves, by Dante Gabriel Rossetti. The startled couple encounter apparitions of themselves - but time travellers could meet their physical selves according to Frank Tipler's theory

Here, perhaps, is the way some future spacefaring civilisation might set about making a time machine. First, choose a suitable pulsar. Stretch it into an elongated cylinder, increase its spin appropriately, and you can open for business. There are just two remaining snags. First, such a time machine works only from the moment of its creation. Once it is made, you can use it to go forward in time and return to your starting time, but you can never go back to a time before the naked singularity came into being. Secondly, says Tipler,time travel might need rather a lot of energy. If you imagine sending a physical object on a closed timelike loop so that it ends up alongside itself. then in effect you have created a duplicate of the original object. Einstein's equations tells us that mass is equivalent to energy, according to the familiar equation E=mc2. Creating amount m of mass requires energy mc2 and c is the speed of light,186,000 miles per second (300,000 km/h). So Tipler believes that a working time machine could most probably be used in practice only to send messages, not material bodies, through time. On the other hand, a civilisation capable of sculpting neutron stars into time machines might have energy enough to squander on physical time travel as well.


The problems of causality violation remain. One of the shortest stories ever published by science-fiction writer Larry Niven has the title 'Rotating cylinders and the possibility of global causality violation'. Niven stole the title, he acknowledges, from one of Tipler's erudite articles in the Physical Review. The story describes a future civilisation that tries to take Tipler's work to its logical conclusion and build a working time machine. Every time the work nears completion, some catastrophe strikes the builders. The punch line comes when another race discovers a half-built time machine and decides to finish the job; at the moment the leader of the civilisation decides to proceed with the project, the sun in the sky of his planet blows up as a nova and his civilisation is destroyed.

Further reading
John Gribbin, Timewarps, J.M. Dent 1979 ,
John Taylor, Black holes, Souvenir 1973
S. Weinberg, The first three minutes, Deutsch 1977

Violating causality

Niven's frightening story highlights the implication of the argument that causality violation is abhorrent to nature, taken to its logical conclusion. The illogicality of the story may be a kind of double bluff, representing Niven's own attempt to make us, the readers, say 'but that's ridiculous', leaving us with the only possible alternative - that causality violation really is possible after all. Tipler, cautiously, says only that 'we are a very long way from completely resolving the causality violation question.' But that, coming from a respected mathematical physicist with a thorough knowledge of Einstein's theories, speaks volumes. At the very least, if anyone ever tells you flatly that 'time travel is impossible', you can be sure that he doesn't know what he is talking about. The fairest answer modern science can give to the question, 'Is time travel possible?' is 'We don't know.' And that, of course means that perhaps it is possible to travel in time after all.

Timeless What is Time? The Flow of Time Entropy and the Arrow of Time Timeslips Time paradoxes

Reproduced from THE UNEXPLAINED p1534