Hello from the past ladies and gentlemen. My name is Alasdair Richmond and 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, 1941-2001, 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' 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, times 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 registered by a particular person or a particular traveling object. 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 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 in-step. 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 case there's a 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 elapse in external time. But in my frame of reference, aboard the machine measure by my watch, my digestion, the accumulation of my memories, all the process that travel with me, only five minutes has elapsed. So I get out the machine five minutes older, in personal time, only to find that fifty years has elapsed in the extent of the world. So this would clearly be a case of time travel. Five minutes of personal time corresponds the same journey measures fifty years of external time. So in case there's a forward time travel, personal time and external time share the same direction, but different duration. Cases of backward time travel, personal time and external time again diverge. Remember that Lewis thinks that divergences between personal time and external time aren't 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 5 minute personal time journey 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 January, 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 time, measures the same journey as a 150, year negative interval of external time. Now a backward time journey has the peculiarity that in external time the journey 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 forty or even fifty 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 forty or fifty 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 decade and decade of very well supported physical results that suggests that these divergences between frames of reference 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 relativistic possible universe where it's possible to t-, jump these 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 as well.
