XX-I.NOVIKOV-NOTP&CWCI

Notion Of The Past & Can We Change It?
by I. Novikov
Theoretical Astrophysics Centre, Copenhagen


At the very end of the Millennium there is a lot of discussions and speculations about the most daring dreams of physicists, about their most challenging ideas. Among them probably the most outstanding is the possibility of travelling to the past.

What can be said from the scientific point of view about the possibility of flights into the past?

First of all it is necessary to note that we ourselves cannot get younger in any ‘flight’ voyage. In any one of us, in any human being and any system, time can only flow forward, only from youth to old age. As Alice says to Humpty Dumpty in Lewis Caroll’s Through the Looking Glass, ‘one can’t help growing older’. We know the law of increasing disorder, increasing entropy, which dictates the aging of an organism.

Nevertheless, it is possible to imagine that using specially designed machinery, a human being could get into a special ‘tunnel’ in which he moves backwards with respect to time in the external space, and emerges in the past when passing through the other mouth of this tunnel. Obviously, the traveler through time does not get younger at all. However, having sneaked into the past, he can find himself, for example, in the time of his youth or even in an epoch before he was born!

This journey looks, to a certain degree, like diverting a small fraction of the discharge of a powerful river, pumping this rivulet through a pipe along the bank in the direction opposite the river flow, and then returning this water to the main flow far upstream.

Pure theoreticians, mathematicians rather than physicists, have already dealt for a considerable time with bizarre fantastic worlds in which travel back through time is allowed. These worlds are generated by solving systems of equations of general relativity. It appears that the general opinion has been that these solutions have no connection whatsoever with reality, despite being of great interest for studying the structure of the theory itself.

All this looked very funny. For theorists, this solution was a veritable mathematical toy. No more than a toy, though.

Only rather recently this idea started to be treated seriously.

In spite of many works on the subject the possibility (in principle!) to create a time machine remains unclear.

If time travel becomes reality (in remote future) its impact on our society would be so great that we cannot even imagine it. Thus it is very interesting to discuss whether and how the laws of physics can deal with a time machine.

The argument that is especially popular in debates of this sort is the so-called ‘grandfather paradox’. It goes roughly like this: "If I could go back into the past in which my grandfather was very young, I could kill him and thereby make my own birth impossible". Or another version of the same paradox: "I return into my own past, meet myself in my youth and kill my younger version".

In both cases this unnatural homicide generates complete nonsense. Should we infer that such an event is impossible? But why? I have my ‘free will’, don’t I? Hence I can realize this ‘free will’, at least in principle.

Science fiction writers have scrutinized all possible versions of this scenario. But here we return to physics.

Does the ‘grandfather paradox’, or other similar paradoxes, demonstrate that travel through time is not allowed? Indeed, it seems logical that having gone back in time and eliminated the cause of a phenomenon that has already taken place in the present, we thereby violate the fundamental principle of science: causality!

But is this true? I doubt it, and suspect that the argument as given above is flawed. What has physics to say about the likely consequences of meeting oneself (or one’s grandfather) in the past?

Obviously, a physicist (at least our contemporary physicist) is unable to perform an exact calculation of the actions of a human being. This is the field for psychology and sociology, not for physics. However, a physicist can give a strict calculation of what happens to simple physical objects after they pass through a time machine. Such calculations have been performed, and it was demonstrated how physics could deal with such unusual situation without any paradoxes. We will not go into details here, but restrict ourselves by a few remarks.

First of all if somebody use a time machine it means he/she makes a ‘loop of time’. I wish to attract the readers attention to one totally new factor that arises here. If a ‘time loop’ exists, the events on this loop cannot be separated into future and past. To clarify this statement, let us consider the following example.

Imagine that I walk in a long string of people moving along a straight line. I can definitely say which of them is in front of me and who is behind. If, however, we all follow a circle, I can say ‘ahead of me’ or ‘behind me’ only about my nearest neighbors but not about the entire line of people. Regarding people further and further ahead of me, I ultimately go around the entire circle and reach myself from behind. This is why people moving on a circle cannot be divided into those ‘moving in front’ and those ‘walking behind’.

The same is true for the ‘time loop’. We can say which of the nearest events belong to the future and which to the past. But this division cannot be applied to the time loop as a whole. The loop has no clearly defined future and no past, and all events affect one another on a circle. Briefly and metaphorically speaking, we are under ‘double’ strong influence: without the time machine events are influenced by the flow of data from the past (but not from the future! this is the gist of the causality principle), while events on the loop respond to information coming from both the past and the future.

Therefore, with the time machine, today’s events must be consistent with (i.e. be determined by) not only the past but also the future! I formulated this self-consistency principle many years ago and now it appears to be accepted by everyone who works in the time machine field. Recently I and my colleagues were able to provide that this principle can be deduced from the fundamental laws of physics.

Let us recapitulate.

With the time machine becoming a reality, the future starts to affect the past. All events occur in such a way that this influence is taken into account. However, once an event has taken place (it was influenced by the events both in the past and in the future), that’s the end, it cannot be altered. ‘What has really happened cannot be undone’.

Still, how about the assassination of the grandparents? Could this extravagant crime be committed using the time-machine? The answer is a categorically NO. The American physicist Kip Thorne puts it this way:

" ...something has to stay your hand as you try to kill your grandmother. What? How? The answer (if there is one) is far from obvious, since it entails the free will of human beings. The compatibility between free will and physical law is a terribly muddy issue even in the absence of time machines.

"As for the constraints of ‘free will’, the reader should notice that even without a time machine, ANY LAW OF PHYSICS places limits on ‘free will’. Say, I might wish to walk on the ceiling (without special equipment): my ‘free will’ prompts me to. This, however, is forbidden. The law of universal gravitation limits my ‘free will’ and there is nothing I can do about it.

In the presence of the time machine the constraints on ‘free will’ are, of course, somewhat different, but they are not, in principle, anything extraordinary in the physics of our time.

I will conclude this brief discussion of the limitations imposed on ‘free will’ with a remark made by Einstein and which may be of interest to those readers who find time mull over problems of this type. Schopenhauer once remarked: "A man can do what he wishes but he is not free to wish what he wants" (Epilogue. A Socratic dialogue in M. Planck: Where is science going? London, 1933 p.210)