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Airline passengers need to know where the exits are, just in case. Time travelers, as well, should consider what might go wrong -- or weird.

Buckle in -- here's the stewardess to explain some of the scientific mind-benders that could result from a voyage through time:

• The grandfather paradox. This puzzle imagines that a man goes back in time and kills his then-childless grandfather. Consequently, the time-traveler prevents his own birth. How could he have gone back in time?

"Somewhat disquieting," as physicist Carl Sagan said of the prospect in a public television interview.

Various theories attempt to untangle the knot. One is that history cannot be changed, and the traveler would find that his plan to alter the past simply won't work. He would be like the Coyote in a Road Runner cartoon, doomed to one mortifying flub after another.

• The butterfly effect. This idea holds that even a tiny disturbance in the past could bring about big changes in the future. In Ray Bradbury's short story, "A Sound of Thunder," for example, a man goes back in time to hunt dinosaurs. He accidentally steps on a butterfly. Returned to his own time, he finds the world is nothing like he left it.

• Causal loop. A future event causes a past event, which causes the future event, which causes a headache.

• The Fermi paradox, so-called for physicist Enrico Fermi. If time travel will be possible thanks to advanced technology in the future, then why don't any time-traveling tourists come back for a visit?

But suppose the tourists of tomorrow still wear cargo shorts and fanny packs. How would anyone know these strange visitors from another time aren't just regular tourists?

-- Ron Wolfe

Style on 11/20/2016

Print Headline: We're all tourists through time

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