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

The problem I have with fictional time travel is that the “where” is not explained.

In “The Time Machine” Wells tells how the machine dropped a little, because the land it had been standing on had eroded away when it moved into the future. But how did the time travel “know” where the land had been? The Earth is rotating rather quickly. If you are in a helicopter hovering above the ground over London and it travels back an hour in time, do you find yourself hovering over Germany? Not only is the Earth rotating around its axis, it is in orbit around the sun. The orbit round the sun is not perfect, it will be perturbed by the positions of other celestial objects in all the positions they occupy for the duration of your time jump. Then the Sun is moving in a rotating galaxy, which is itself moving relative to other galaxies. So if you travelled back a day in time and found yourself in space, where the Earth had been, where exactly would “had been” be relative to? The only reference point I could think of that makes fictional sense would be the origin point of the Big Bang, from which the Universe is expanding.

Being able to somehow move anywhere in space at the same time as travelling in time overcomes this fictional problem, but introduces another one. I’d like to go to Hawaii, so why don’t I just move myself back in time half a second, and move to Hawaii at the same time. Surly this would be the preferred mode of transport of anyone possessing a time machine, but it is never used in time travel fiction.

The only method I have seen this adequately explained by is with the use of machines such as The Tardis which in the original Doctor Who programmes relocated somewhere in time and space, but the destination was not under the control of the voyagers.

I then hit my second “where” problem. What would have happened in The Time Machine if the level of the land had risen? The bottom of the machine would be occupying the same space as the land below it. There would be no time for the land to move out of the way, as it arrived at that time instantly. Being able to control where your machine materialises does not help, as assuming you have chosen somewhere on the surface of a planet, there will be an atmosphere there. When an aircraft travels faster than the speed of sound, there is a sonic boom because the air molecules do not have a wave of air to push them out of the way, they are hit by the aircraft. However they are at least hit by the aircraft moving into the space they occupy and are then smashed out of the way. With time travel, your craft will not move into the area where the molecules of the atmosphere are, you just arrive all in one go. I don’t think the results would be very pretty. Moving into space for your time shift does not really help, as space is not a perfect vacuum. I long to see a fictional explanation of how this is managed.

Welcome to h2g2 radiofour. Terrific post.

Yes the 'spatial' issue of time travel is a thorny one. One top of the Earth's rotation there are also man-made changes to the landscape. Buildings being put up and pulled down, forests being cleared, etc. What stops you materialising in the middle of a block of flats? Back to the Future would have ended very quickly and suddenly if the DeLorean had driven straight into a tree or house that wasn't there in 1985...