|Hibernation in 2001, a Space Odyssey|
The obvious answer to this question, such as it is formulated, is yes, of course it’s possible! We all travel in time at the rate of 24 hours a day.
Naturally, this is not what comes to mind when this question is asked. What is usually meant is this: will we be able someday to make sudden jumps in time, either forward (to the future) or backward (to the past)?
There are several schemes for traveling to the future. If they aren’t possible just now, one day they could become so. For example, perhaps human beings will be frozen and remain in suspended animation, to wake up and resume their ordinary lives a hundred years later. Or they could take passage on a spacecraft, make a trip at relativistic speed, and return to their starting point a century later, whilst for the travelers the elapsed time would have been just one year. In both cases, from the point of view of the persons in question, this would have been a trip forward in time, but in reality no sudden leap would have happened, for time would have kept going on for the hibernated body, though the mind would not be aware, and also for the relativistic traveler, although in this case time would have been accelerated.
But when we speak of time travel, we do not refer to these cases, which are possible in principle. We mean disappearing from the present and appearing in the past or in the future. Will we be able to do this someday?
In the previous article I mentioned that there are two philosophical theories about time: the A-theory and the B-theory. If the A-theory is true, time travel would automatically be impossible, because according to this theory the past no longer exists and the future does not yet exist; only the present exists. Obviously you cannot travel to what does not exist. So, for time travel to be possible, the B-theory must be true.
The problem is, this is not enough. Assuming that it were possible to travel back in time, we find so many possible paradoxes, that the universe would be an incoherent place. Here are some of those paradoxes:
· The suicide paradox: the time traveler goes back in time and kills himself as a child. Since he has been killed, he won’t reach adult age, and therefore cannot travel in time to murder himself. An equivalent version is the grandfather paradox, where the traveler kills his own grandfather as a child; therefore neither his father nor he himself would have been born.
· The predestination paradox: the traveler goes back in time to prevent a catastrophe, for example, a train accident. Let us suppose that he succeeds. But if the disaster did not happen, he would have no reason to travel back in time to avoid it, so the catastrophe won’t be prevented.
· The existence of objects without a cause: the inventor of the first time machine travels forward in time, 100 years into the future, and finds there a statue that has been erected in his honor. He takes the statue, goes back 100 years into the past, and places it in the same place he found it, where it will remain for 100 years. But who made the statue? Nobody. It is an object without a cause that exists only during a temporary loop. (This is the plot of the story Find the sculptor by Sam Mimes, a writer of science fiction).
· Time travel to the past, and human freedom, appear to be inconsistent. I considered this question in two previous articles in this blog: the time machine problem and its solution.
It is less known that travel into the future controlled from the future can also cause paradoxes. For example, the inventors of a time machine could use it to transfer a person from their past to their own present (the future of the transferee). Let's look at one of them, similar to the suicide paradox:
· The inventor could use the machine to move himself, as a child, to his current present. In that case, he would not have invented the machine, because he would not have lived in the meantime, so he would not have been able to move himself in time.
One of my own science fiction novels (A face in time) uses precisely this type of time travel, towards the future, controlled from the future. A young student in the year 2089 falls in love with a girl who lived 300 years earlier, and moves her to his own present. Although there is a passing mention of the fact that this can lead to paradoxes, while writing the novel I did not get into detail, for I did not want to spoil my own story-line.
But the worst argument against the possibility of time travel is a version of the Fermi paradox. If time travel might become possible in the future, where are the time travelers? Why aren’t they here? Why is there no trace of their presence during the whole history of mankind?