Relatively straightforward

Science News, Sept 10, 2005 by Arra Avkain, P. Weiss

In "Realistic Time Machine? New design could forgo exotic ingredient" (SN: 7/16/05, p. 38), you refer to a standard concept of a person speeding "in a rocket traveling slightly less than the speed of light" and say that "motion at such enormous speeds drastically slows the clock for the traveler." That reasoning, which is common, troubles me. If the traveler is traveling speedily with respect to an observer, then, clearly, the observer is traveling speedily with respect to the traveler (who is observing the observer). Since all motion is relative, why shouldn't the observer's clock slow down as observed by the traveler?

ARRA AVKAIN, FRESNO, CALIF.

The difference is subtle, but it's because the traveler changes direction--turning around is a form of acceleration--that less time passes for that voyager than for the observer.--P. WEISS