A world exactly, to the minutest detail, a duplicate of our own, but as
we shall be five hundred, or from that to twenty thousand, years hence.
Let there be also another world, a duplicate of what we were five hundred
to twenty thousand years ago. There should be many worlds of each kind
at different dates behind us and ahead of us.
I send a visitor from a world ahead of us to a world behind us, after
which he comes to us, and so we learn what happened in the Homeric age.
My visitor will not tell me what has happened in his own world since the
time corresponding to the present moment in our world, because the
knowledge of the future would be not only fatal to ourselves but would
upset the similarity between the two worlds, so they would be no longer
able to refer to us for information on any point of history from the
moment of the introduction of the disturbing element.
When they are in doubt about a point in their past history that we have
not yet reached they make preparation and forecast its occurrence in our
world as we foretell eclipses and transits of Venus, and all their most
accomplished historians investigate it; but if the conditions for
observation have been unfavourable, or if they postpone consideration of
the point till the time of its happening here has gone by, then they must
wait for many years till the same combination occurs in some other world.
Thus they say, “The next beheading of King Charles I will be in Ald. b.
x. 231c/d”—or whatever the name of the star may be—“on such and such a
day of such and such a year, and there will not be another in the
lifetime of any man now living,” or there will, in such and such a star,
as the case may be.
Communication with a world twenty thousand years ahead of us might ruin
the human race as effectually as if we had fallen into the sun. It would
be too wide a cross. The people in my supposed world know this and if,
for any reason, they want to kill a civilisation, stuff it and put it
into a museum, they tell it something that is too much ahead of its other
ideas, something that travels faster than thought, thus setting an
avalanche of new ideas tumbling in upon it and utterly destroying
everything. Sometimes they merely introduce a little poisonous microbe
of thought which the cells in the world where it is introduced do not
know how to deal with—some such trifle as that two and two make seven, or
that you can weigh time in scales by the pound; a single such microbe of
knowledge placed in the brain of a fitting subject would breed like wild
fire and kill all that came in contact with it.
And so on.