No wonder physicists can't explain past, present and future. The passage of time is just an illusion that we can't live without, says Marcus Chown

DEEP in the Amazon rainforest, a tree frog sits on a log watching a fly. A genetic fluke has furnished the frog with a brain that perceives its surroundings as they were a second ago. When the fly comes within range, the frog lunges. But, with its out-of-date observation, it misses. Weakened by a rarely sated hunger, the frog falls off the log and dies.

It's a sad story. But if you think it is completely fanciful, think again. There is nothing in the laws of physics that says all creatures have to process data about their environment in the same way as we do. A "behind the times" perception like that of our deceased frog is only ruled out by the handicap it imposes. "Natural selection has equipped people and frogs to experience the world in the most effective way for their survival," says James Hartle of the University of California, Santa Barbara. "A frog that calculates the trajectory of a fly from the most recent data, eats; one that doesn't, starves."

In a phenomenon that has scientists puzzled, the Earth is right on schedule for a fifth straight year.

Experts agree that the rate at which the Earth travels through space has slowed ever so slightly for millennia. To make the world's official time agree with where the Earth actually is in space, scientists in 1972 started adding an extra ``leap second'' on the last day of the year.

Theoretical consequences of the gravitational origin of inertial reaction forces, that is, Mach's principle, are explored. It is argued that Mach's principle leads to the conclusion that time, as we normally treat it in our common experience and physical theory, is not a part of fundamental reality; the past and future have a real, objective existence, as is already suggested by both special and general relativity theory. A laboratory scale experiment whereby Mach's principle, and thus radical timeless ness, can be established is mentioned.

Key words: times, relativity, Mach's principal.