This paper is an enquiry into the logical, metaphysical, and physical possibility of time travel understood in the sense of the existence of closed worldlines that can be traced out by physical objects. We argue that none of the purported paradoxes rule out time travel either on grounds of logic or metaphysics. More relevantly, modern spacetime theories such as general relativity seem to permit models that feature closed worldlines. We discuss, in the context of Gödel's infamous argument for the ideality (...) of time based on his eponymous spacetime, what this apparent physical possibility of time travel means. Furthermore, we review the recent literature on so-called time machines, i.e., of devices that produce closed worldlines where none would have existed otherwise. Finally, we investigate what the implications of the quantum behaviour of matter for the possibility of time travel might be and explicate in what sense time travel might be possible according to leading contenders for full quantum theories of gravity such as string theory and loop quantum gravity. (shrink)

It has been suggested that there is something worrisome, puzzling, or incomprehensible about the sorts of causal loops sometimes involved in backwards time travel. This paper disentangles two distinct puzzles and evaluates whether they provide us reason to find backwards time travel incomprehensible, inexplicable, or otherwise worrisome. The paper argues that they provide no such reason.

What is our conception of the temporal priority of cause over effect? It is that a causal chain runs always in the earlier‐to‐later direction. Each link in the chain is a process, whose initiation is the immediate, and thus simultaneous, effect of the arrival at a particular stage of the process that constitutes the preceding link. It is the fact that it is the subsequent continuation of the process, once initiated, that calls for no explanation, which gives a temporal direction (...) to the causal chain. If there are causal chains running in the reverse as well as in the usual direction, there is a possibility of causal loops. (shrink)