Time travel is one of the most worn of SF tropes but its appeal is enduring, witness the revived Doctor Who. The fascination of being able to go into the past, perhaps to alter it, or to see the future (you do of course reach a small part of that in the normal, slower way) seems to strike deep. Many stories focus on the paradoxes that could arise; say by inadvertently causing one of your ancestors not to be born will that mean you yourself will never exist? (In which case you could not possibly go back to the past to change it.)

A classic of the time travel sub genre is Robert Heinlein’s “All You Zombies” where the narrator, Jane, by time travelling and sex change, turns out to be her own mother and father – which as an idea for a story would be hard to top.

Time travel fiction’€™s offshoot, alternate history (counterfactual history as it is sometimes called) is a safer bet as it allows the writer to meddle in this way without affecting real history. Each such story is also a thought experiment which explores the ways in which our own world could have been different – for better or worse.

The advent of many worlds theory offered a get out clause for possible time travel paradoxes in that the paradox is avoided if you can only travel back to a different (albeit only slightly) universe, not your own. It also legitimises any alternate history story as it could (must?) be set in a different universe.

We can of course not yet travel in time in the real world; only in fiction. It is possible that in the future we may develop the ability. I seem to remember reading a speculation that only when time travel has been invented will people be able to travel back in time but only to that point and no further back and hence this is why no one has yet travelled back to our time. We haven’€™t invented it yet.

This seems to me a bit like the Fermi Paradox. (In my opinion the reason we don’€™t have evidence of aliens is that we don’€™t have evidence of aliens, end of. When we do, we will. If we don’t, we won’€™t.

The fact that no one has yet travelled back in time to us would tend to suggest, though, that time travel is absolutely impossible, even between universes. This does not, however, prevent us thinking about it and writing stories utilising it.

Yet there is one aspect and complication to possible travelling in time that I have never seen explicitly addressed anywhere and which may be why all those attempts to reach us from the future fail.

They end up in the wrong place.

Consider. The Earth spins on its axis once a day (give or take.) On this count alone, if you travelled back in time 12 hours you would end up on the other side of the planet.

But: The Earth orbits round the Sun once a year (give or take.) This equates to 67,000 miles per hour in a circular (actually elliptical) fashion.

So go back in time one day and stay at the same coordinates relative to the Sun, and the Earth will be 1,608,000 miles away!

We’€™re not finished. The whole solar system is spinning round the centre of the galaxy at about 1 million miles a day (give or take) in a rotational time scale of the order of millions of years.
Travel back one year and you will be 365 or so million miles away from where Earth is (was.)

The galaxy itself along with its neighbours is moving at about 1,300,000 miles per hour. One year’€™s travel here is 11,388,000,000 miles!

We do not notice these relative motions as they are all at constant speeds. Think how you only really notice a car or train’€™s motion when it accelerates, brakes or changes direction.

Eric Idle put this all quite succinctly in The Meaning Of Life.

In sum, to travel in time you must therefore also travel in space, and at what are effectively unattainable speeds. The fastest human-made object (one of the Voyager probes) has a human-imparted speed that is but a fraction of all this.

To time travel and still end up on Earth you must factor in all those vector equations to your time machine and somehow give it the speed to get it where you want to go.

(The fictional Time Lords must have got this point; the complication I’€™ve outlined is implicit in the acronym TARDIS.)

Of course, now that I’€™ve written this post, by morphic resonance the relevant calculations will have been enabled.

So, if someone from the future turns up on your doorstep tomorrow (or yesterday,) blame me.