A bit more on philosophy and time travel: It seems consistent to suppose that a time traveler can affect the past but not change it. Perhaps I will invent a time machine tomorrow and race heroically back to 1865 to save Lincoln from John Wilkes Booth. I might arrive at Ford’s Theater and race up to Lincoln’s box; I might even wrestle dramatically with Booth in the hallway. But we know in advance that I won’t be successful, because history tells us that Booth did shoot Lincoln that night.

This way of looking at it entails no paradoxes, but it does create a problem. If time travel is possible then presumably hundreds of well-intentioned time travelers converged on Lincoln’s box that night, all determined to save the president and all somehow slipping on banana peels at the wrong moment. This is not impossible, but it seems terrifically unlikely — so much so that the very fact of Lincoln’s death seems to imply that time travel is not possible.

But University of Sydney philosopher Nicholas J.J. Smith points out that we don’t quite know this: A time machine may be invented a century from now with a backward range of only 50 years. In that case we have no experience from which to judge these matters. “One cannot conclude from the supposition that local backward time travel would bring with it what we ordinarily regard as improbable coincidences, that such time travel will occur only rarely: for the only reason we regard the events in question as improbable coincidences is that within our experience, they have not occurred very often — and our experience does not (apparently) encompass backward time travel.”

(Nicholas J.J. Smith, “Bananas Enough for Time Travel?”, The British Journal for the Philosophy of Science, September 1997.)