"Man ... can go up against gravitation in a balloon, and why should he not hope that ultimately he may be able to stop or accelerate his drift along the Time-Dimension, or even turn about and travel the other way."

H.G. WELLS, The Time Machine


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Time travel is a much loved concept by science fiction writers, and Star Trek fans, but is it possible within the known laws of physics?

Some of the greatest minds in history have struggled with the formidable question: is time travel possible? Isaac Newton said no; he believed that time is absolute. It is like an arrow that, once fired, soars in a straight line.

Albert Einstein had a more dynamic theory. He understood that time is more like a river; meandering and flowing around galaxies, sometimes faster and sometimes slower depending on gravity. A fellow scientist at Princeton, Kurt Godel, expanded this to allow for "whirlpools" to develop, where moving along the direction of rotation would send the observer back in time.

Fueled by modern physics, the great Stephen Hawking was torn. He once vetoed the idea, stating that future time tourists would have made themselves known by now. But it is argued that they may simply not be interested in doing so.

Further study convinced Hawking that time travel is possible – albeit impractical. Fellow scientists now agree that the potential exists. Like space, it is theoretically possible to shift through different points in time. The debate now lies with the ability for a person to move through and interact with this fluid construct.

One of the stock answers to the question of time travel is to suggest that if time travel were possible it would have already happened. By way of explanation let's say that at some time in the future a clever scientist invents a time machine that can travel through time in any direction, just like the fictional time machine created by H. G. Wells. So where is our intrepid time traveller? History shows that no time traveller has ever visited us from the future, therefore it's never going to happen. This appears at first glance to be a good solid argument against the possibility of time travel, but the argument is flawed. It may be that time travellers from the future have visited us, but have not revealed themselves in order to avoid changing the future. Alternatively, time travel may be possible, but only into the future, the past already having been determined. Finally, time travel into the past may be possible, but only into a different (alternative) universe, thus avoiding paradoxes. The fact that we have no record of having been visited by time travellers does not exclude its possibility. Having dealt with that little problem we can now move on the fun part, paradoxes.



This is where we start to run into some of the problems posed by time travel. The most commonly posed paradox is known as the 'grand parent' paradox. This states that if you could travel back in time you could murder your grand parents and thus prevent your existence, thus rendering it impossible for you to have gone back in time and killed them... no need to draw you a picture. Even more to the point perhaps, you could travel back in time and kill the person responsible for time travel before they discover it! However, popular though the grand parent paradox is, it only reveals the tip of the iceberg. Let's examine a theoretical time travel situation in more detail in order to highlight some of the problems involved.

Imagine that today you travel back in time to August 2001 and warn the authorities that the World Trade Centre in New York is going to be attacked on September 11th. They take the necessary action and as time unfolds the disaster is eventually averted. You are still in August 2001 at this point and the disaster has not yet been prevented, but it will be eventually because of the chain of events that you have put into motion, and you now wish to return to the time you came from. Here comes the Big Question - can you return to where you came from? Where you came from the attack had taken place, therefore where you came from no longer exists! If you do return to your original time, the World Trade Centre will still be standing (because you prevented the attack), so it cannot be where you came from. You will have changed the course of history, not just for yourself, but for the entire world. The question is, will the World Trade Centre still be standing when you return to your starting time? This is a major point of contention. According to one theory it both will and it won't! This is because your actions will have created an alternative possibility and in the process an 'alternative' universe, where we now have one universe with the World Trade Centre intact, and one with it destroyed, and we will all exist in both. The theory is firmly rooted in quantum theory that states that ALL alternative outcomes are possible. However, there is no evidence at this stage that the strange phenomenon found in quantum theory can be applied to the larger world.

Yet another theory suggests that alternative universes will not be created because events will conspire against you that prevent you (as in this scenario) from warning the authorities and changing history. This 'theory' it isn't really a theory as such, more an expression of a desire, because it is believed to be impossible (and very bad manners from the point of view of historians) to change history. The theory has no explanation as to how this 'prevention' could actually come about and has no solid theoretical basis.

The same arguments can be applied to travel into the future. Suppose I travel 24 hours into the future, armed with a loaded gun, (just in case, you never know) and happen to meet myself sitting at my computer, (can you meet and interact with yourself?) also armed with a gun, which would be most unusual for me as I do not own one. I am alarmed to see that the gun is pointed at me and warn my future self to put it down or I will shoot him. He prepares to fire the gun so I shoot him dead at the computer, then return. On my return, when tomorrow comes, I am sitting at my computer and receive a visit from 'me' from my past. He warns me that he will shoot me. I of course knew this would happen and took the precaution of keeping the gun beside me and aimed at the spot at the time I knew 'he' would arrive, and I shoot him first. Can I do that? That isn't what happened in my future, it was the 'me' at the computer that died. Even if I don't manage to shoot 'him' in time and still die at my computer, which one of us exactly is it that is still alive? We have created a nonsense scenario. Now this may sound like nonsense, and perhaps it is, but if the future really does exist then it does raise the prospect of meeting and interacting with yourself.

If we accept the idea that the future does exist, because time is just another dimension as discussed in the previous section, then these paradoxes would be unavoidable. If however, we take the view that only the dimension of time itself exists, and not the events within it, then this would suggest that time travel is impossible. Why? Because if future events do not exist, then it would be impossible to travel into the future to witness them, there would be no 'future' to visit.

We either have to accept that if time travel is possible we will, by our actions , create all sorts of paradoxes, or accept that we will create alternative universes. The paradox problem will not go away.


 Alternate Universes

The idea of having alternate Universes very conveniently solves the paradoxes raised by time travel, but is it a serious possibility? It sounds a bit far fetched, like something out of a science fiction paperback, but try not to dismiss the idea out of hand, it has some strong supporters.

The alternative universe theory (also known as the multiverse) suggests that for every possible outcome of an event, an alternative universe is created, with the result that somewhere out there I won the lottery last week, and so did you. If only we knew how to get there! This theory has the problem that there must be an infinite number of alternative universes existing to cover every possible outcome of every event. Which one, if any, is the 'real' one, or are they all equally 'real'? Furthermore, where does all the energy and mass come from that creates all these alternative universes? To try and find answers to these questions we have to return briefly to quantum theory.

We learned from the cat-in-the-box thought experiment that prior to observation the cat is in, what is termed in quantum speak, a 'superposition of states'. In plain talk, it is either dead or alive until the moment of observation, its fate is not determined beforehand. The explanation given for this strange state of affairs was the behaviour of the electron's probability wave spreading through first the box, then the room, and finally collapsing into an electron at the moment of observation. A different approach however, is to suggest that at the moment of observation both (or all, as the case may be) possibilities become realities. This is achieved by the creation of an alternate universe, in which the cat is in the opposite state to the cat in our universe. A refinement of this theory suggests that there are always two universes involved, but prior to the experiment they are identical in all respects. If three different experimental outcomes are possible, then we would have three initially identical universes, one of which would change. In general, we would need an infinite number of universes to cover all possibilities. When we were looking at the section Can anything 'real' be infinite? we discovered that if the universe were infinite in size then every possibility of every possible outcome would exist within the universe. Perhaps the universe IS infinite?

We have looked at some of the problems associated with time travel, it is now time to look at the laws of physics.


 Quantum theory

We shall now study another aspect of quantum theory, one that is directly related to time travel, and learn that particles can travel backwards in time!

We can look at an event that begins with a photon and an electron, and ends with a photon and an electron. We can say that what has taken place is a photon is absorbed by an electron, the electron continues on a bit, and a new photon comes out. This process is called the scattering of light. When calculations are made for scattering, we must include some peculiar possibilities. For example, the electron could emit a photon before absorbing one. Even more strange is the possibility that the electron emits a photon, then travels backwards in time to absorb a photon, and then proceeds forwards in time again. The path of such a 'backwards-moving' electron can be so long as to appear real in an actual physical experiment in the laboratory. The backwards-moving electron when viewed with time moving forwards appears the same as an ordinary electron, except it's attracted to normal electrons - we say it has a 'positive' charge. For this reason it's called a 'positron'. The positron is a sister particle to the electron, and is an example of an 'anti-particle'. This phenomenon is general. Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle. When a particle and its anti-particle collide, they annihilate each other and form other particles.

We have now learned that time travel is not only possible, it is a perfectly normal phenomenon, at least for particles. But is it possible in the macroscopic world?


 Einstein's theory of relativity

Black holes, according to relativity theory, warp spacetime with their enormously powerful gravitation field. The effect of this gravitational field is that if an astronaut were to cross the event horizon of a black hole, time would slow down on board his spacecraft as he approached the singularity and eventually come to a stop. Similarly time slows down in proportion to speed, the faster our astronaut travels the slower time runs. The closer the astronaut travels to the speed of light the more time slows, until at the speed of light, time would stop. Both these effects of time being affected by speed and gravity have been discussed in the previous section, all of which illustrates that time is not a fixed constant, but is affected by gravitational fields and relative speed in the same manner as the other three dimensions of space.

The solutions to particular equations of the Special Theory of Relativity can be expressed mathematically in any direction of time without running into any problems. Does this mean that time travel is possible? There is nothing in relativity that rules out time travel, it would appear to be theoretically possible.


 Constructing a time machine

Research carried out in the late 1980's showed that genuine time travel is not forbidden by the known laws of physics. This means that it may be possible to build a time machine, but not that it may be easy. Help may be at hand though, it is possible there are naturally occurring objects in the universe that act as time machines.

There are at least two ways to build a time machine. Frank Tipler published a possibility in the highly respected journal Physical Review in 1974. This involves making a naked singularity, a singularity that is not concealed from view behind the event horizon of a black hole. To make a naked singularity involves rotating a singularity extremely rapidly, and if rotated sufficiently fast it would fling away the event horizon and exposes the singularity. We know that spacetime is extremely distorted by the singularity's strong gravitational field and the effect of this rotation would be to twist spacetime, and tip it over so that one of the dimensions of the space dimensions is replaced by the time dimension. A carefully piloted spaceship taken close to the singularity would enter the time dimension and journey through time instead of space, although to the astronauts all would appear as normal. When the spaceship moved away from the distorted area around the singularity, it would be in a different time from when they had entered the area.

According to Tipler's calculations, the same effect could be achieved with a cylinder about 100 km long and about 10 km across, made of material compressed to just over the density of a neutron star, and rotating twice every millisecond. It would be like ten neutron stars joined pole to pole and given a strong twist. Curiously, there are objects in the universe which nearly fulfill the other requirements - so-called millisecond pulsars are known which contain almost the right density of matter and spin once every 1.5 milliseconds, at one-third the speed needed to make a time machine. Such objects are so close to being time machines that they hold out the tantalising possibility that an advanced civilisation might be able to tweak them up in the right way to allow time travel.

That such things as naturally occurring time machines exist in the universe, with only a little tweaking needed, raises the prospect that an advanced civilisation may have already done the trick! This raises the interesting possibility that such a civilisation would have the capacity to travel between the galaxies; a journey of a few million light years would be as nothing. Something for the UFO brigade to mull over!

The other possibility for building a naturally occurring time machine involves worm holes - tunnels through spacetime which may, according to relativity, connect a black hole in one part of the universe to a black hole in another part of the universe. Before the mid-1980's physicists believed that such objects as wormholes could not 'really' exist, and that a better understanding of Einstein's equations would prove this. They were forced to change their minds as a result of careful investigation of wormholes carried out by Kip Thorne and his colleagues at Caltech in 1985. It is interesting to note that this research was triggered by Carl Sagan, a well known scientist, who was writing the science fiction novel 'Contact', a best seller that went on to become a highly successful film. Sagan wanted his wormhole to be as scientifically accurate as possible and approached Thorne to check out the idea as presented in the book. What neither Sagan nor Thorne first realised from the results of Thorne's study was that this short-cut through space would also work as a short-cut through time. In 1988 Morris, Thorne and Yurtsever (Morris and Yurtsever were students of Thorne) published their conclusions in the journal Physical Review Letters, that Einstein's equations really did allow for the existence of wormholes that link different times, and could be used as time machines.

We have seen that the laws of physics do not preclude the possibility of time travel, and further, that it may be possible to construct a time machine by tweaking naturally occurring objects in the universe. It would appear that we only need the technology to make time travel a reality.

On waking, every conscious being is aware of passing through time effortlessly, day after day, year after year, at exactly the same rate. The difficulty lies with moving through time faster than everyone else.

There is no doubt that more time travel theories will unravel. The Multiverse Theory itself is a far fetched notion, and some scientists object to the absurdity of infinite parallel universes. Nevertheless, nature is defined by quantum mechanics, so future time travel theories must evolve out at the quantum level. And that will inspire all kinds of exotic theories.




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