Try and Change the Past

Try and Change the Past by Fritz Leiber - There is an infinite number of paths through a lens between an object point and its image point–and no matter how many changes of path you make, have you changed anything?

No, I wouldn't advise anyone to try to change the past, at least not his personal past, although changing the general past is my business, my fighting business. You see, I'm a Snake in the Change War. Don't back off-human beings, even Resurrected ones engaged in time-fighting, aren't built for outward wriggling and their poison is mostly psychological. "Snake" is slang for the soldiers on our side, like Hun or Reb or Ghibbelin. In the Change War we're trying to alter the past–and it's tricky, brutal work, believe me–at points all over the cosmos, anywhere and anywhen, so that history will be warped to make our side defeat the Spiders. But that's a much bigger story, the biggest in fact, and I'll leave it occupying several planets of microfilm and two asteroids of coded molecules in the files of the High Command.

Change one event in the past and you get a brand new future? Erase the conquests of Alexander by nudging a Neolithic pebble? Extirpate America by pulling up a shoot of Sumerian grain? Brother, that isn't the way it works at all! The space-time continuum's built of stubborn stuff and change is anything but a chain-reaction. Change the past and you start a wave of changes moving futurewards, but it damps out mighty fast. Haven't you ever heard of temporal reluctance, or of the Law of the Conservation of Reality?