Hello from the past, ladies and gentlemen. My name is Alasdair Richmond, I teach philosophy at the University of Edinburgh. And I specialize in that branch of metaphysics that looks at philosophical questions to do with time, and in particular, time travel. And the aim of this talk is to give you a short overview of the philosophy of time travel and some of the philosophical problems that this topic throws up. The key text for this talk is a 1976 paper by David Lewis, 1941 to 2001, called The Paradoxes of Time Travel. And in that paper Lewis tries to defend the logical possibility of backwards time travel. So a really good way into the debate is to begin by looking at Lewis's definition of what time travel might actually involve. And Lewis says time travel involves a distinction between two ways in which time could be registered. What you might call external time on the one hand and personal time on the other. External time is time as it's registered by the world at large. Which might be time as it's registered by the movement of the tides. By the rotation of the sun, by the movement of the air through space. By the sun rising and setting. So external time is simply time as it's registered by the majority of the non time traveling universe. Time for everybody. Personal time, on the other hand, is time as it's surveyed just by a particular person or a particular traveling object. And so personal time, for example, might be time registered by the traveler's watch. Time registered by the accumulation of the traveler's memories. Time registered by the accumulation of the traveler's digestive products. Time registered by the traveler's hair graying or cells dying. Now for most of us, and I'm going to assume for the purposes of this talk that most of us aren't time travelers, external time and personal time march instead. If five minutes elapses for you as recorded by your watch, by your digestion, by your blood circulating, you should typically find that five minutes has passed in the external world. But in cases of time travel, personal time and external time diverge. And there are two ways in which this could be imagined as happening. In cases of forward time travel, personal time and external time share the same direction but they have different measures of duration. Suppose, for example, I depart from January, 2013 in my time machine and I arrive in January of 2063. So 50 years has elapsed in external time. But in my frame of reference aboard the machine, measured by my watch and my digestion and the accumulation of my memories, all the processes that travel with me, only five minutes has elapsed. So I get out of the machine five minutes older in personal time only to find that 50 years has elapsed in the external world. So this would clearly be a case of time travel, five minutes of personal time corresponds the same measures 50 years in external time. So in cases of forward time travel, personal time and external time share the same direction but different duration. Cases of backward time travel, personal time in external will time again diverge. Remember that Lewis thinks that divergence is between personal time and external time, not actually constitutive of time travel. So in backward time travel personal time and external time diverge but they diverge in direction. Suppose now that my five minute personal time jumping from January, 2013 doesn't take me into the future, it takes me into the past. So I activate the machine, five minutes passes for me, my watch shows an increase of five minutes. Five minutes seems to elapse in my frame of reference, but when I arrive I've arrived in jaggedy 1863, so a journey that has five minutes positive personal duration has 150 years negative external duration. So in backward time travel, personal time and external time diverge in direction. So in this case, five minutes positive. Personal type measures the same as 150 year negative unit of external time which only begins after it ends. The journey ends in 1863 but it begins in 2013. So, says Lewis, given a distinction between personal time and external time, it is at least possible to imagine both forward time travel and backward time travel. Now the details of this are slightly technical, but forward time travel seems to be a very deeply embedded phenomenon, and one of our best supported physical theories. Einstein's special theory of relativity predicts that the rate at which time passes is not an absolute, not an invariant, but varies according to relative speed. In other words, the greater the relative velocity between two systems, the closer that that relative velocity comes to the speed of light, the more that the rate of temporal passage diverges in those two frames of reference. Now, if I'm lucky, I maybe have 40 or even 50 years of personal time ahead of me. And the Special Theory of Relativity says that if I travel fast enough relative to the solar system. I can make that 40 or 50 year interval of personal time comprise tens, or hundreds or millions or even billions of years of external time. Provided that I travel fast enough, I can make my 40 years of personal time extend through the entire future history of the sun. So forward time travel is very deeply embedded in Einstein's Special Theory of Relativity and we have decades and decades of very well supported physical results that suggest these divergences between frames of efforts, really, really occur. Backward time travel is a bit more speculative. And whether physics permits backward time travel is still something that's naturally, hotly contested. But the general theory of relativity seems to predict that under certain circumstances, given an enormous amount of mass or an enormous density of mass or enormously rapid movement of mass, it's possible to create circumstances where personal time and external time diverge. Not only in duration but in direction. There's a famous model universe by the Austrian mathematical physicist Kurt Godel which describes a relativistically possible universe where it's possible to journeys into the local future that can visit any point in the external past. So the General Theory of Relativity seems to underwrite the kind of personal time, external time discrepancies there's a constitutive of backward time travel this way.
