h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Time does not exist. We create time because we think. We think in sentences so we have to be able to remember the beginning and anticipate the end of our sentences. We observe processes (because we can remember) and we use time to explain these processes, and we predict the future course of these processes based on our explanation.

So time is a useful concept. But there is no back there that we could travel to, or a future either. Space time is actually a very very thin film, with lots of ripples and distortions, but it's actually only space. We add the time so we can watch the movie and press rewind (in our heads) when we want. What we are reviewing is simply a now-existing pattern of neurons or microtubules or whatever in the brain, just as when we watch a film we are looking at a now-existing pattern of light projected through a now existing strip of celluloid.

So we cannot travel in time, either backward or forward. That said the past is actually here with us. The background cosmic radiation left over from the Big Bang appears on our TVs. Every atom that made up the physical body of say Jesus Christ is still (probably) floating round the planet. There might at some future date be a way of reconstituting previous entities from the information that remains about them in the present.

eg I liked the urban myth that suggested it was possible to "play back" a pot made in Babylonian times to hear what was going on when the potter was throwing the pot on his wheel. The suggestion was that the sound vibrations would in some way be stored in the pot as it was being turned on the wheel (say through minute vibrations in the potter's fingers) and then "hard coded" in the firing process. We just need to figure out what sort of "needle" we'd need to read back the pot.

But we'd still only be getting the information out of the present ie in the old pot we'd found in Ur or wherever. The point being that the old pot is just much as in the present with us, as a new pot made this morning, and it's nowhere else.