If we could travel into the past, it's mind-boggling what would be possible. For one thing, history would become an experimental science, which it certainly isn't today. The possible insights into our own past and nature and origins would be dazzling. For another, we would be facing the deep paradoxes of interfering with the scheme of causality that has led to our own time and ourselves. I have no idea whether it's possible, but it's certainly worth exploring.
—Carl Sagan, NOVA interview, Oct. 12, 1999
The Time Machine: A comic book story about time travel, circa 1950s, based on H.G. Wells 1895 novel.
My 10-year-old son said to me recently that it would be great if someone invented a time machine; he also added that his friend at school had plans for one. Why is it important to build one? I asked with interest. "Then we could have prevented Hitler's birth, the Second World War, and the killing of all the Jews" he said matter-of-factly. That's an understandable sentiment, trying to rewrite history to avoid a genocide. But it raises all kinds of intractable problems, not least of which is the morality of summarily executing someone before he commits any crime. I also explained to my son, "That technology used for good can also be used for evil purposes." He walked away shaking his head in agreement; even so, I am not sure if he was convinced.
Nevertheless, I, too, have shared my son's fascination with time travel and the limitations imposed by the laws of physics, notably the laws of causality, and whether the application of modern engineering principles can lead to the building of such a machine. So, for now at least, time machines only exist in science fiction, an idea advanced in the human imagination by The Time Machine, a science fiction novella that H. G. Wells published in 1895. Those were heady times, full of optimism; anything was possible and science would be the ticket to travel.
Time travel has appeal other than the larger sentiments to rewrite history, notably the evil aspects of it, which are many. There are smaller, more personal matters of the heart that we would like to change. These centre on our personal relationships. All humans make mistakes in their relations to other humans; the closer the relationship the greater the possibility of the mistake and hurt. Like my son who wished for a time machine for the right reason—to erase a mistake of history—some would wish for a time machine to erase our mistakes in human relationships.
But that is not possible; there is no delete key in life. Not really. Memories exist and so do the recollections of painful moments. That is what makes us human. As well, knowing that you can make mistakes allows us to consider more carefully, after making many mistakes, that what you say and what you do can and will hurt another person.
While there are no time machines, and despite my son's testimony of his friend's plans to build one, there are no large-scale projects to build a machine to break the time barrier. It's interesting to note that there are no scientific limitations to travelling backwards in time—Einstein's theory of relativity does not forbid it. It might actually encourage it, since physical reality in Einstein's thinking is bounded by four dimensions—the three physical ones plus time and that it is malleable. Einstein writes in The Principles of Relativity (1954):
Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.The debate among scientists and philosophers continues if this is indeed the case; physicists face a number of intractable problems, most noteworthy being on how to marry Einstein's relativity with quantum mechanics and how to explain the existence of dark matter. Even if these can eventually be unravelled, time travel raises practical engineering-type considerations and working around the laws of causality. That is one event happens after another, says University of California at Santa Barbara physicist Andrew Cleland in a Discovery News article (April 22, 2010):
Cleland also points out that the fundamental principle of causality stands in the way of travel into the past. The entire universe, as we understand it, is beholden to this rule."Something occurs first and the outcome of that occurrence happens afterward," Cleland says, "and there has never to my knowledge been an experiment that came out different from that. I am not aware of any experimental tests of quantum mechanics that have shown any violation of causality, in spite of the fact that many experiments could reveal such a violation."One science-fiction writer avoided this issue in a particular way. In a Library of America (LOA) post, Fritz Leiber explains his way of dealing with the issue of causality in his Hugo Award–winning The Big Time (1958):
To dramatize the effects of time travel, science fiction usually assumes that if you could go back and change one crucial event, the entire future would be drastically altered. . . . But that wouldn’t have suited my purposes, so I assumed a Law of the Conservation of Reality, meaning that the past would resist change (temporal reluctance) and tend to work back quickly into its old course, and you’d have to go back and make many little changes, sometimes over and over again, before you could get a really big change going—perhaps the equivalent of an atomic chain reaction.For now, apart from science fiction films and books, we have to be content with time moving in only one direction: forward.