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

World lines cannot be logical if they do not embody not only the rotation of the earth and the orbit of the earth around the sun, but also the motion of the sun around the galactic centre and the 'motion' of the galaxy in regard to space-time expansion.

If one postulates, as many in SF do, the possibility of time travel, then surely the effect of world lines *must* be taken into account and dealt with, not merely ignored because inconvenient to handle. Unfortunately, I have yet to encounter any SF writing, anywhere, which formulates a process whereby the time traveller is moved through time and simultaneously is moved through space along a world line. If the traveller is merely displaced in time, then the destination in space-time is at a location which is no longer coincident with the surface of the world.

If instead, one postulates that the traveller automatically follows their own world line, then where and what is the mechanism which ensures this? Worse, there are huge problems associated with both forward and backward travel on this basis. Forwards, that world line simply does not exist. Backwards, it is occupied by the traveller already, and in effect peters out at the point of conception.

What SF authors seem to be implying, however, is neither of the above postulates (neither purely time travel nor travel along one's own world line). What they all imply, is that the traveller stays with the world line of the piece of ground upon which he is standing at the moment of departure. It seems to me that we need to develop a rationale for this, as it too has problems going backwards, as man-made and geological events will be encountered at various points. And again, going forward there is not yet an extant world line. Mathematics will provide a locus of movement for some point on the earth's surface, in terms of the various astronomical motions I noted earlier. What though of geology, let alone the effects of man?

Perhaps in the coming years we will benefit from the rise of some great thinkers, who will solve these difficulties in SF for us. I sure hope so!

Does anyone else feel the way I do on this?

This complaint is quite often leveraged against HG Wells' "The Time Machine", but since that book was more of a "social critique" than pure sci-fi, the flaw can probably be excused.

Personally, I use wormholes for my time travel. Wormholes can lead through both time and space. There is a fixed displacement in time. As for space, the two 'mouths' of the wormhole are just like other objects in space (think of them as the mouths of black holes). If you put a wormhole in Paris that goes to Moscow 50 years back in time, then the Paris end will stay in Paris and the Moscow end will stay in Moscow, in the same way that the Eiffel Tower will stay in Paris and will not somehow stay fixed in space-time as the Earth rotates off it (that would be funny!). I think that makes sense.