Longer periods away from home - just over two weeks in Japan and Korea in this case - always make me think about time and our experience of it. On the larger scale, one experiences the elasticity of time complicated by homesickness, jet-lag, work schedule and the personal, emotional shape of absence (the last couple of days always fly by as far as I am concerned). On the smaller scale, trips away are almost the only periods in which I experience regular, abstract exercise, usually swimming. If I swim 20 minutes a day for a week or more, the weirdness of time very quickly becomes apparent: the subjectivity of the time I experience, weaving in and out of my thoughts, is totally at odds with the stop-clock ahead of me, ticking away the seconds, and the ordinary clock to my left or right, converting time elapsed into portions of a circle, slices of a pie.

This is, it might seem, pretty banal stuff. The subjectivity of our experience of time is widely acknowledged. As we get older, time seems to go faster - or is it that we seem to move faster in time? The spatial metaphors we use are confused and confusing. The theories to explain this change range from the physiological (the body cools as we age) to the arithmetical (each moment is a smaller proportion of a lengthening lifespan).

If time is so mutable, so much a matter of the ebb and flow of consciousness, is it in fact illusory? The commonsense view has long been that of classical science. Isaac Newton contrasted "absolute, true, and mathematical time" which "in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration" with what he called "relative, apparent and common time". This is the view he bequeathed to the industrial age, the world of clocks, measurement and effective time management, but one which was exploded in its metaphysical aspects by Einstein's musings on relative motion and the speed of light, by the space-time continuum, and the uncertainties of quantum mechanics.