We have now encountered a number of ways in which contemporary physics threatens common-sense views of space and time. With str comes the inter mingling of space and time and the abandonment of absolute simultaneity; with gtr comes curved spacetime; there are interpretations of quantum theory that posit backward causation, multiple universes and instantaneous action-at-a-distance. Mention has also briefly been made of an attempt to integrate these two theories. We saw in §3.8 that canonical quantum gravity theory has significant and potentially disturbing implications for time, even if one interpretation of what these are is empirically suspect (see §6.9 and §7.8). There is another theoretical programme aimed at integrating (or replacing) gtr and quantum theory that more than merits a mention: string theory. This too has radical implications, but they concern space rather than time. If current versions of string theory are to be believed, we live in a universe of at least nine but possibly ten (or more) spatial dimensions, not just three.

String theory is still in the early stages of development. New versions are continually being developed, and there is as yet no empirical evidence that strings themselves exist. Given their size (see below), direct detection may never be possible. But although the theory is far from universally accepted, it is also taken seriously, and in the opinion of many it is our best prospect for a theory of everything.