Let us start by saying what a great pleasure it is to write for StarTrek.com. We have both been Star Trek fans for a long time, and have been inspired by the show in some of our scientific endeavors. Indeed, we both watched Star Trek: The Original Series, when it first aired. That shows how far backward in time we go! However, we are also science educators and the authors of the new book Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space, and as such, take this opportunity to describe for the fans some of the things that Star Trek does not get quite right regarding time travel.
First, Star Trek seems to be unaware of, or at least does not explore, the connection between faster-than-light travel (e.g. warp drive) and backward time travel. It tends to treat them as different phenomena. Usually the Star Trek characters need some special method for travelling backward in time, such as colliding with a “black star” (black hole?), or relying on something like the Guardian of Forever. It turns out that there is a very deep link between travel faster-than-light travel and backward time travel. If you can do one, you can do the other. By travelling faster than light in one direction, reversing course and then travelling faster than light in the opposite direction (i.e., by making a closed loop in space), it would be possible to return to one’s starting point in both space and time.
Second, with a time machine it would only be possible to travel back to the time when the time machine was first created. So if a time machine was created, say, in the year 2060, then after that you could use the time machine to travel from any time in the future to as far back as 2060, but no farther. This is because the machine was activated (i.e., became a “time machine”) at a certain time in the past. Since the machine must always travel along its own worldline, or path in spacetime, it cannot go back to a time prior to 2060, since the device, as a functioning time machine, did not exist before then. So you can’t use a time machine to go back and kill Hitler or visit the dinosaurs. One caveat to this would be if an advanced alien civilization had built a time machine long ago (say several million years ago) and conveniently left it around for us humans to use.
Third, and probably most significant from a plot point of view, Star Trek repeatedly assumes that, if you go back in time, you have the possibility of changing history. Many Star Trek plotlines involve going back and “undoing” spurious changes in the timeline, to restore the “original” timeline. That’s not how it works. It is logically impossible for an event to both happen and not happen in the same universe. There are two possibilities. One is that you go back in time and try and change the past, but you can’t because what has already happened has happened. In this scenario, a time traveler could be a part of past events but cannot change them (a conclusion which Spock briefly mentions in the episode “Assignment: Earth.”) This possibility was also explored in a Deep Space Nine episode in which Sisko and Bashir travel backwards to the time of a great social activist named Gabriel Bell. Unfortunately, shortly after their arrival, Bell is killed. As the plot unfolds, it is Sisko who assumes the identity of Bell and so becomes the cause of the past events he travelled back to witness. (But…wouldn’t they have had pictures of Bell before and after his murder, and see that it was not the same person?)
Another possibility is that one could travel back in time and, for example, kill Hitler and prevent World War II. This cannot happen in one and the same universe because of the logical impossibilities we discussed earlier. At the moment of the murder, the universe “splits” in some sense, and the killing of Hitler takes place, not in your original universe (the one you started out from) but in a different universe. In that universe, you would be seen to appear out of the time machine, kill Hitler, and live the rest of your life in the new universe. In that universe you have no past history, only a future history. In the original universe you started from, you would have a past history, enter the time machine, and never be seen again. So in the original universe, you would have a past history, but no future history. (This is the basis of the plotline in the most recent Star Trek motion picture.) Note that there is no possibility of “restoring the timeline” in this scenario. If now, in the new universe, you wanted to go back to your original universe and restore the timeline, you can’t. Instead, you would create yet another new universe, in which you first appear out of the time machine, and then disappear back into it.
In the usual interpretation of quantum theory, called the Copenhagen Interpretation in honor of the Danish physicist Niels Bohr, quantum mechanics is taken to predict the relative probabilities of the possible outcomes of some physics experiment. However, there is an alternative interpretation, called the “many worlds interpretation”, introduced in a Reviews of Modern Physics article in l957 by Hugh Everett (no relation to Allen as far as we know). In this interpretation, when an experiment is done, all possible outcomes occur, but they occur in different “worlds” which cannot interact with one another because of a quantum mechanical phenomenon called “decoherence.” Effectively, this results from the fact that, while the different worlds seem to match one another on large (macroscopic) scales, the individual coordinates of the huge numbers of individual atoms and molecules do not. Thus at the atomic, or microscopic level, the Everett worlds are quite different from one another. Following the branching which occurs when the experiment is performed, there will thus be many copies of “you,” all of which would be indistinguishable to the naked eye. The probability with which a given copy winds up in a particular Everett world is equal to the probability of the corresponding experimental result predicted by the Copenhagen interpretation. Thus there is no way of distinguishing between the two interpretations experimentally. The choice between them is a matter of personal preference, and is the subject of many lunch table discussions between physicists in university faculty dining rooms.
The situation we have just discussed is that which prevails if we assume that backward time travel is not possible. This assumption is quite possibly correct. However, it might not be. If it is not, and if the many worlds interpretation is correct, then Professor David Deutsch of the University of Oxford, showed in a 1992 paper, later elaborated on by Allen, that there was an interesting possibility for avoiding the paradoxes associated with backward time travel. Suppose in the case of backward time travel that the time traveler arrives in a different Everett world. This would lead to just the situation discussed above in which no paradox occurs. Hitler is both killed and not killed, but these contradictory events happen in two parallel but different Everett “worlds”, and any given observer will see only one of them.
In summary, there are only two possibilities for accommodating backward time travel within a viable set of physical laws. One is that the laws of physics ensure that any actions of the time traveler, rather than changing past history, are in fact part of that history. The second is that the time traveler arrives in a second universe, unobservable from the first except by the time traveler. While her actions cause the timeline in the second universe to be different from that in the first, any given observer, including the time traveler, will have a unique history.
However, there are additional problems with both of these scenarios. In the first, it turns out that one runs into a very peculiar situation. Namely that, if one wants to preserve the known (and experimentally verified) laws of quantum mechanics, then an experiment done in the present can be affected by whether or not a time machine will be invented in the, possibly distant, future! In the second scenario, as shown in a paper by Allen, the time traveler will get “sliced and diced”, with different parts of the time traveler appearing in different universes. (These ideas are discussed more fully in our book.)
We hope you have enjoyed this brief excursion into the realm of time travel physics.
Allen Everett has been chasing space pirates since he discovered Buck Rogers in the 25th Century on the radio when he was about 9 years old in 1942. About the same time, he discovered he liked doing math problems when his father let him “help” with his income tax return. This led him to an undergraduate degree in Physics at Princeton in 1955, followed by a Ph. D. in theoretical nuclear physics from Harvard in 1960. He then accepted a position in the physics department at nearby Tufts University where he remained until his retirement in 2004. As particle accelerator energies increased, so did the energies of his professional interests. Eventually, like many in the field, he became interested in theoretical questions considering energies beyond the range of terrestrial accelerators. These included problems related to what are called cosmic strings, involving the overlap between ultrahigh energy particles and the cosmology of the very early universe.
Along the way he did not lose his zeal for doing in the space pirates. Indeed, in 1966, as he and his brilliant, beautiful, and in all ways quite wonderful new wife, Marylee, were unpacking in their new apartment, they decided to take a break and watch a new TV series which was starting that night, called Star Trek. Both Allen and Marylee fell in love again, this time with the program, and Star Trek night became a regular occurrence in the Everett household through all the series yet to come.
Moreover, prodded by a former graduate student, Adel Antippa, Allen developed a sort of professional side interest in a hypothetical particle, called a “tachyon,” so that Allen developed a professional interest in the highly speculative subject of superluminal speed. This is of course reflected in the title of this book, since “warp drive” was the name given to the otherwise unspecified system which allowed the starship Enterprise to exceed the speed of light. This later led Allen to further work on problems involving “Time Travel and Warp Drives,” and the subject became his principal scientific interest.
Tom Roman started watching Star Trek when it first aired, while he was in high school. Like many others, his original interest in time travel started with the H.G. Wells novel, The Time Machine. Around the same time, Tom read a piece in an Isaac Asimov popular science book which discussed Einstein’s special theory of relativity, and in particular, the subject of time dilation. (This is an effect predicted by Einstein’s special theory of relativity, which allows time travel into the future). What impressed Tom at the time was that these were not idle speculations, but in fact were backed up by rigorous scientific experiments. He came to the epiphany that everything he (and all his friends and relatives) knew about space and time was completely wrong. Tom realized that if he didn’t understand space and time, he didn’t understand anything. So he made up his mind then and there to pursue a study of Einstein’s theories of relativity.
Tom received a bachelor’s degree in physics from the University of Maryland. He later got his Ph.D. in physics at Syracuse University under the guidance of Peter G. Bergmann. Bergmann was a research assistant to Albert Einstein in Princeton during the years 1936-1941, where they worked on the subject of unified field theories. After graduate school, Tom taught for several years at the University of Hartford, before later moving to Central Connecticut State University. He is currently a professor in the Mathematical Sciences department at CCSU. Like Allen, Tom has taught an undergraduate course in time travel physics.
Tom still remembers with fondness, and still watches, all those old Star Trek episodes of the original series, as well as many of the newer series. And, after all these years, he still doesn’t understand space and time!
Time Travel and Warp Drives is available now from The University of Chicago Press. For additional details about the book, and to purchase it, click HERE.