Special Time Travel Section
Time Traveling with Oedipus Rex
In the article, "The Chronology Projector Conjecture," Michael Shermer neglects a possible consistent version of time travel. This is the case where the equations governing the evolution of the universe allow closed time loops, but only consistent solutions can be "real" universes. Universes in which you travel back in time and kill your father are not allowed. Some movies actually handle this situation pretty well, e.g., in the Bruce Willis 12 Monkeys film, in which he travels back and causes the disaster that made his trip back in time necessary.
There are interesting paradoxes with this solution to time travel, but I think no inconsistencies. For example, imagine you are walking through the forest and you meet an old man who hands you a package. On opening the package you discover the plans for a time machine. You build the time machine and travel back in time. After various adventures (enough to fill about 90 minutes), you encounter a young man in the forest and give him the plans for the time machine. Where did those plans come from?
One can imagine an interesting variation on Oedipus Rex where he not only kills his father and marries his mother, but he is his father and kills himself This plays havoc with our grandest illusion--free will--which Sophocles would likely have appreciated.
--Arthur Snyder, Stanford Linear Accelerator Center, firstname.lastname@example.org
Skeptical of the Time Travel Skeptics by James Richmond
As a physicist, I was excited to see that Vol.10, No.1 (2003) included a special section on time travel. In light of the current state of knowledge of the physics of time travel, however, it seems to me that the authors have been a little too hasty in dismissing the possibility. In this response, I hope to show skeptic readers that theories of time travel are not quite ready for the wastepaper basket.
Michael Shermer's "The Chronology Projector Conjecture"
Michael Shermer is prudent in being skeptical of the possibility that we might one day be able to build time machines. However, he is incorrect when he writes that "not only does [time travel] violate numerous physical laws, there are fundamental problems of consistency and causality." The kind of time travel described by physicist Kip Thorne violates no known physical laws.
Thorne's description of a wormhole time machine is consistent with all known laws of physics. It is true that many physicists have reservations about whether this method of backwards time travel could work in practice; potential problems include keeping the wormhole stable enough to allow a traveler to pass through unharmed. Apparent paradoxes of causality (e.g. the "matricide" paradox), which might occur if the past was altered by a time traveler, led Stephen Hawking to postulate the "chronology protection conjecture." But it is important to remember that, at present, this is conjecture, not a conclusion backed by physical theory.
Shermer asks, "if time travel were possible, where are all the tourists from the future?" According to Thorne's theory, a time traveler using a wormhole time machine could only travel back to the moment of the creation of the wormhole. Therefore, one possibility we must take seriously is that we haven't seen any tourists from the future simply because no wormhole time machines have been built yet!
Ted Dace's "Days of the Future Past"
Ted Dace's review of Paul Davies' book, How to Build a Time Machine, ends with an implied accusation that Davies and other physicists do not fully appreciate the nature of time. Unfortunately, in his review, Dace himself displays a less-than-perfect understanding of time as it is pictured in Einstein's theory of General Relativity, in which time is an additional coordinate added to the three familiar spatial coordinates. An event in spacetime is specified by giving its position in space and its location in time. …