Online research in philosophy

In 1949 the great logician Kurt Gödel constructed the first mathematical models of the universe in which travel into the past is, in theory at least, possible. Within the framework of Einstein’s general theory of relativity Gödel produced cosmological solutions to Einstein’s field equations which contain closed time-like curves, that is, curves in spacetime which, despite being closed, still represent possible paths of bodies. An object moving along such a path would travel back into its own past, to the very moment at which it “began” the journey. More generally, Gödel showed that, in his “universe”, for any two points P and Q on a body’s track through spacetime (its world line), such that P temporally precedes Q, there is a timelike curve linking P and Q on which Q temporally precedes P. This means that, in principle at least, one could board a “time machine” and travel to any point of the past. Gödel inferred, in consonance (as he observes) with the views of Parmenides, Kant and the modern idealists, that under these circumstances there could be no such thing as an objective lapse of time, that time or, more generally, change, is an illusion arising from our special mode of perception. For consider an observer initially at point P (with time coordinate t seconds as indicated by his own clock). At point Q (with time coordinate t′) he boards a time machine and travels back to point P, taking time t′′ to do so. In that case, according to his own clock, t′ – t + t′′ > 0 seconds have elapsed, and yet an identical clock left at P would show that 0 seconds have elapsed. In short, there has been no “objective” lapse of time at all. Gödel remarks that in his universe this situation is typical: for every possible definition of an “objective” time one could travel into regions which are past according to that definition. He continues.

This paper presents a critical examination of claims advanced by several philosophers to the effect that 'time travel' represents a physical possibility and that the interpretation of certain actually observed phenomena in terms of 'time travel' is both legitimate and advantageous. It is argued that (a) no convincing motivation for the introduction of the time travel hypothesis has been presented; (b) no coherent and interesting sense of 'going backward in time' has been supplied which makes 'time travel' compatible with Special Relativity; (c) even the conceptual possibility of 'time travel' is an unsettled and somewhat nebulous question.

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn't this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

Thinking about time travel is an entertaining way to explore how to understand time and its location in the broad conceptual landscape that includes causation, fate, action, possibility, experience, and reality. It is uncontroversial that time travel towards the future exists, and time travel to the past is generally recognized as permitted by Einstein’s general theory of relativity, though no one knows yet whether nature truly allows it. Coherent time travel stories have added flair to traditional debates over the metaphysical status of the past, the reality of temporal passage, and the existence of free will. Moreover, plausible models of time travel and time machines can be used to investigate the subtle relation between space-time structure and causality.

It surveys some philosophical issues concerning time travel and should serves as a quick introduction. It includes a new and improved way to define a time machine.

Suppose that you travel back in time to talk to your younger self in order to tell her that she (you) should have done some things in her (your) life differently. Of course, you will not be able to make this plan work, we know that from the many versions of 'the grandfather paradox' that populate the philosophical literature about time travel. What will be my centre of interest in this paper is the conversation between you and ... you – i.e. the older you that travelled back in time and the younger you, when you first meet. As we shall see, given this situation, endurantists will have to endorse a strange consequence of their view : you will turn out to be a universal while your properties will turn out to be particulars.

This column is a followup to a previous Alternate View column [Analog, June-'89] about "wormholes", faster-than-light travel, and time machines, which was based on a spectacular theoretical breakthrough in general relativity. It described how a sufficiently advanced civilization might construct a stable wormhole (a curved-space shortcut between one region of space and another) and use it both for faster-than-light travel and for time travel, with no laws of physics violated in the process except causality (the principle that a cause must precede its effects).

Wormholes are shortcuts through space time, constructs of general relativity (GR) that appear to offer a physics foundation for faster than light travel and even for travel back in time. They first appeared in the physics literature in 1935, when Albert Einstein and his colleague Nathan Rosen discovered that implicit in general relativity is a tunnel like structure in the topology of space time connecting two separated regions. Einstein and Rosen were actually trying to explain fundamental particles like electrons and proton. They suggested that if lines of electric flux were threaded through such a structure, the flux would be trapped and one end would appear to be an isolated positive charge and the other end would appear to be a negative charge. Later, however, general relativity was used to calculate the masses of such “particles” and it was realized that they would be have a mass of at least a few micrograms, far heavier than the mass of an electron or proton.

In this note I raise a new problem for backwards time travel, and make some first suggestions as to how it might be solved. I call it the motivation problem. It is not a logical or a metaphysical problem, but a psychological one. It does not impact upon the possibility, or even the likelihood, of backwards time travel. Yet it is deeply puzzling, and we will have no idea what time travel would actually be like until we explore it. Thus, where other problems for backward time travel assume that we know what time travel would be like, and argue that we cannot have it, this new problem gives us no reason to think that we cannot have time travel, but argues that we have much less idea than we usually suppose about what it would really be like to travel back in time.

In this course we will examine several philosophical puzzles concerning time. We all seem to experience time in a very fundamental and direct way. Yet once we begin to reflect on what time really is, it is easy to feel as puzzled as St Augustine was, who wrote: “If no one asks me, I know what [time] is. If I wish to explain it to him who asks me, I do not know.” The first set of issues we will discuss concern the question whether time is ‘real.’ Time appears to consist of past, present and future. But do the past and the future exist in the same way as the present or is only the present real? Does time ‘flow’? In what ways is time different from space? What would it be to ‘spatialize’ time? Next we will ask whether certain views of time imply that there can be no freedom of the will. One might worry that if facts about the future (including facts about what I will do tomorrow) already existed in the same way as facts about the present exist, then I could not be free to choose what I will do. After all, how can I be free to decide to skip class tomorrow, if it is ‘already’ a fact today that I will attend class? What, if anything, is the connection between various views of time and ‘fatalism’? The third topic we will discuss is time travel. First we will ask whether time travel is a conceptual possibility. As we will see, there are certain conceptual puzzles associated with the possibility of time travel. For example, one might think that if time travel is possible, then I should be able to travel back in time and kill my father before the date of my conception. But this scenario seems to lead to a contradiction. Some have taken considerations such as these to argue that the very idea of time travel is incoherent. Is this right? If not, why not? Then we will look at what the theory of relativity says about the nature of time in general and about the physical possibility of time travel more specifically. Finally we will examine several issues concerning the asymmetry of time..