Time Travel and Time Machines

Abstract

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.

Spooky Action at a Temporal Distance

“…Indeed, the argument is superficially similar to a well-known argument due to Gödel, in which he argued that time cannot be absolute in general relativity because “the compatibility with the laws of nature of worlds in which there is no distinguished absolute time ... throws some light on the meaning of time also in the worlds in which an absolute time can be defined” [ 34 ]. This argument is regarded as problematic: in particular, the modal step has been challenged by Earman, who pointed out that it is not clear that “absoluteness”, must be an essential property of time, and therefore perhaps we should simply say that the status of time varies along with the distribution of matter, so time is absolute in worlds where there is a distinguished absolute time and not absolute in worlds where there can be no distinguished absolute time [ 35 , 36 ]. In the same way, one might object to our appealing to worlds with different constraint equations but the same dynamical equations on the grounds that perhaps the dynamical equation should be varied along with the allowed kinematical set in such a way to ensure a suitably simple GR-like theory.…”

Lewisian Time Travel in a Relativistic Setting

“…While there has been some crossover in these discussions in the philosophy of science literature (e.g. Maudlin 1990Maudlin , 2007Arntzenius 2006;Smeenk and Wüthrich 2011), metaphysicians have generally continued their discussions independently. We need to ensure we're all employing compatible conceptions of time travel; were we using different conceptions, a single spectrum for all possible cases would be inappropriate.…”

Global Spacetime Structure

“…See alsoEarman and Wüthrich (2010) andSmeenk and Wüthrich (2011).14 Here we abuse the notation somewhat. Properly, we require that every physically reasonable maximal extension of (int(D(S)), g ab|int(D(S)) ) must contain a chronology violating region V .…”