Time Travel and the Downing Street Dilemma

This entry – the first in my “Quarks to Quasars” blog – is adapted from an essay I originally wrote for the website of Pages Books in Toronto earlier this month.

For more than a hundred years – ever since H.G. Wells penned his groundbreaking novel The Time Machine – people have speculated on the possibilities and paradoxes of time travel. There are, presumably, two ways in which such travel could be realized: Human beings could eventually build a time machine, or we could stumble across a pre-existing machine built by a more advanced civilization. Both possibilities have received much attention – indeed, I focus on such questions in Chapter 7 of my latest book, In Search of Time: Journeys along a Curious Dimension (McClelland & Stewart, October 2008). But the question of whether time travel may eventually be possible is almost beside the point. Merely considering such travel raises a host of mind-bending philosophical problems, enough to challenge the most careful thinkers. So this essay will not be about wormholes or rotating cosmic strings or any of the other exotic physical structures that may one day be exploited to bring about time travel. Instead, I want to focus on one particular philosophical objection time travel – the famous “grandfather paradox.” I will examine one proposed resolution of the paradox, and some difficulties with that purported solution that have perhaps been overlooked. If you’re even an occasional reader of science fiction, you’re probably familiar with the grandfather paradox. The basic idea is this: suppose you travel back to the time when your grandfather was a young man, and kill him (either by accident or by a willful act; the logical conundrum is the same). He never meets his future wife, never has children, and so you are never born – so how could you have traveled through time? One solution – if that’s what it is – is the idea that history must be “consistent.” The idea is that you can do as you please when you travel back in time, so long as those events correspond to what actually happened. This rules out killing your grandfather in a rather straightforward way: You can’t kill him, because you didn’t. But if grandpa has always told stories about a crazed young man who took pot shots at him with a rifle back in the1930s, then you could indeed be that crazed man; history would be consistent. You can try to kill him; you just can’t succeed. In a nutshell: You may have some degree of freedom on your jaunt through time, but you can’t change history. As physicist Brian Greene puts it: “If you time-travel to the past, you can’t change it any more than you can change the value of pi.” But there are problems here. Does it matter how well we know the relevant history? That is, is our knowledge of past events somehow relevant? Would someone who failed History 101 be free to run amok at the Battle of Hastings, while an A-student would have to tread carefully so as to not interfere with the death of Harold and the victory of William? The answer, presumably, is: “No, because the rest of us know what happened there.” But what about a visitor to the Jurassic era? Could our time traveler do as she pleases, simply because so few records from that era survive? And then there’s the flip-side, in which we have too much knowledge of a particular time and place. This brings us to what I call the “Downing Street dilemma.” Suppose you travel back in time just a few years, to 2005, and tried to visit Downing Street in London. Home to the British Prime Minister and many top government offices, the street must surely be one of the most intensely-monitored thoroughfares in the world: No doubt there are dozens of closed-circuit TV cameras on every block, and this must already have been the case by 2005. Suppose you have a time machine at your disposal, but, before you begin your journey, you scan the videotapes that show Downing Street circa 2005. And suppose, as you can the tapes for some particular day, you see no sign of yourself: You did not, it seems, walk along Downing Street on the selected day in 2005. Already thing are getting complicated: We have to consider not only “2005 you,” but “time-traveling you” as well. Let’s say you’re 33 now (in 2008), which means you were 30 in 2005. If 30-year-old you walked along Downing Street in 2005, you would surely remember it in 2008 – but that’s not the point. Time-traveling you has no option of “becoming” that person. Rather, what you can “become” – if that is the right word – is the 33-year-old traveler who may appear on the videotape at some point. Let’s consider the first case: the scenario in which the tape for some particular day doesn’t show you at all. How does that impact your proposed journey? Does it mean you simply cannot visit Downing Street on that day in 2005 with your time machine? Is such a trip ruled out before it can even begin? Now imagine the alternative scenario: Suppose the tapes reveal that time-traveling you did in fact take a stroll along Downing Street on some particular day back in 2005. This is hardly more appealing: Now, not only can you take the trip, but, presumably, you must; and when you get there, every step, every muscle twitch, must accord precisely with what was recorded by the cameras! Free will would seem to be greatly compromised, if not utterly extinguished. (Actually, Downing Street has been closed off by a gate, and guarded by police, for some time. But we could simply replace Downing Street with some other un-gated but heavily-monitored area – Trafalgar Square, for example.) The “consistent history” argument says, roughly, that if you travel back in time, you can only do things that are consistent with the overall history of the universe. Or, put more simply: If you travel into the past, you will only be able to what you actually did. This rules out many kinds of trips, or at least curtails the activities a time traveler can engage in while visiting the past. So what, exactly, are we left with? Must our time traveler experience an endless series of puzzling coincidences, peculiar constraints, and a bizarre absence of free will? (Must every rifle aimed at grandpa misfire? Must every attempt to stroll along Downing Street go awry?) Perhaps, as a number of scholars have suggested, such questions put the cart before the horse. As philosopher Nicholas Smith has argued: “If a time traveler is going to travel to some past time, then she has already been there. If she is going to save a life or prevent a birth when she gets there, then she has already done so.” From this perspective, the abundance or accuracy of the “records” of the period we travel back to (the amount of video surveillance, etc.) is simply irrelevant. The question isn’t what the historical record can show us; it’s what actually happened, recorded or not. Presumaby Smith would counter my Downing Street dilemma as easily as any of the other standard scenarios: If the time traveler took a stroll along Downing Street in 2005, then he surely had free will at that time. And – crucially – it is a mistake to think that he walked along the street again a few years later with the help of the time machine: That trip simply was the original 2005 trip. Smith refers to this mix-up as the second-time-around fallacy. It is a mistake, he argues, to think that some particular event happens, and then “happens again” when experienced by a time traveler. It simply happens, period. If a time traveler is on hand, so be it; but that means he was there all along. Smith’s argument is simple and powerful, but it still leaves a great deal to ponder. What does it mean to say that a time traveler had free will when he visited X, if he has yet to jump into the time machine to carry out his visit to X? As you climb into the time machine for a visit to Downing Street circa 2005, your intuition says that you ought to be able to do as you please throughout your journey – yet history demands that you are restricted to doing what you actually did. The problem arises because, in this bizarre scenario, your future lies in my past (or simply, “the past”). To you, these are future events which can unfold in one way or another depending on your actions; to the rest of us, they lie in the past and are fixed, unchangeable facts about the world. And then there is the awkward problem of having two “yous” in the world at the same time. What if the time traveler encounters himself? Surely in that case the moment is indeed “experienced” twice – once by the young version of the traveler and again (or should we say “simultaneously”?) by the older version. (Based on an e-mail I received from Professor Smith, I think he would concede this point.) Mind you, if you did walk along Downing Street in 2005, at age 30, and met another you who was three years older – that is something you would surely remember! At the very least, having two entities both claiming to be “you” does seem to raise some perplexing philosophical problems. What happens to our notion of “self”? Rather than having solved the grandfather paradox, we may have compounded it. Of course, I have not exhausted the proposed “solutions” to the grandfather paradox here; I haven’t even mentioned, for example, the much-discussed “parallel universes” scenario. (I do try to go through all of them fairly thoroughly in Chapter 7 of my book.) But I think this series of simple thought experiments shows that even when we think we have a straightforward “solution” to the paradoxes of time travel – “don’t worry, the time traveler can do as he pleases so long as history is consistent” – the solution may not be quite as satisfying as we had hoped.