I thought Einstein was more important

None of this produces the slightest change in the world of 1980. Bewildered, now, he revisits 1775 and shoots Washington again, while the earlier version of himself, there on the same errand, looks on in annoyance. Then he heads for the early 1940s, where Enrico Fermi is working on the atomic bomb that Hassel knows had been invented forty years before by Madame Curie, and shoots him too. Nothing he does has any impact on the shape of the future. And then he encounters a fellow time-traveler, this one a professor of mechanics from Yale, who also has been roving through time unsuccessfully trying to change the course of events.

"I got Columbus," says Hassel.

"I got Marco Polo."

"I got Napoleon."

"I thought Einstein was more important."

"Mohammed didn't change things much -- I expected more from him."

"I know. I got him too."

Indeed, they had both assassinated Mohammed, nineteen months apart.

Hassel says, "But how could you have killed him after I killed him?"

"We both killed him."

"That's impossible. And the Yale man replies, "

My boy, time is entirely subjective. It's a private matter -- a personal experience. There is no such thing as objective time, just as there is no such thing as objective love, or an objective soul." They both had roved up and down the time-stream, making radical changes in history -- but only in their own subjective sense of history. Everybody else's history went right on, unaltered.

The Bester story is good sardonic fun, and I have read it many times with much enjoyment. But it doesn't, I think, stand up to serious logical analysis. I prefer to believe that if you change the past you irrevocably change all of time to come -- unless, of course, some other time-traveler
revokes the irrevocable by looping back in time to catch you in the act and cancel out your history-changing deed....