clock paradox definition of clock paradox in the Free Online Encyclopedia.

clock paradox [′kläk ‚par·ə‚däks]

(relativity)

The apparent contradiction between the principle of relativity, which asserts the equivalence of different observers, and the prediction, also part of the theory of relativity, that the clock of an observer who passes back and forth will be slower than the clock of an observer at rest. Also known as twin paradox.

Clock paradox

The phenomenon occurring in the special theory of relativity wherein two observers who start together with identical clocks and then undergo different motions can have different total elapsed time on their clocks when they rejoin later. This effect is a well-defined, mathematically consistent prediction of special relativity which has been verified by experiment but, historically, it has been referred to as a paradox because of erroneous reasoning in the manner in which the effect is commonly analyzed. The clock-paradox phenomenon arises because there is no notion of absolute simultaneity in the theory of special relativity.

The clock-paradox effect is illustrated by the following hypothetical example. (In the context of the example the effect usually is referred to as the twin paradox.) Two identical twins are separated as young adults. One of the twins remains on Earth and, for the purposes of the discussion, is assumed to undergo inertial motion. (In fact, in the context of general relativity, the twin on Earth would be viewed as accelerating—only freely falling observers would be viewed as inertial—but in this example the corrections made to the effect by performing a proper, general-relativistic analysis would be negligible.) The other twin is placed in a rocket ship which accelerates very rapidly away from Earth until it is receding from Earth at nearly the speed of light. After coasting for a while at this speed, the rocket ship turns around and accelerates very rapidly back toward Earth, so that it soon is traveling at nearly the speed of light. The rocket ship then lands on Earth and the twins rejoin. The twin on Earth is old (and everything else on Earth has aged considerably), but the twin in the rocket ship (and everything else therein) has barely aged at all.

The clock-paradox phenomenon has been observed directly in an experiment performed in 1971 by J. C. Hafele and R. E. Keating, who observed differences in elapsed times of atomically stabilized clocks flown in airplanes as compared with ones on the ground. In this experiment the special relativistic effect is so small, since the velocities achieved by airplanes are much smaller than c, that the tiny corrections due to general relativity cannot be neglected, so the experiment actually must be viewed as verifying the analogous clock effect in general relativity rather than purely the clock effect of special relativity. See Relativity, Space-time

Mentioned in	?		References in periodicals archive	?		Encyclopedia browser	?		Full browser	?
No references found	Albrecht calls this time-choice conundrum the clock paradox, described with his UC Davis collaborator Alberto Iglesias in a paper published this year in Physical Review D. It's likely that times are changing: a century ago, mathematician ... by Siegfried, Tom / Science News	clo Cloaca cloacal bladder cloacal gland cloak rail cloaking cloakroom CLOB clobber clobbering memory clochan cloche clocher clock Clock and Watch Industry clock control system Clock Correction Clock Diagram clock doubling Clock Drive clock frequency clock gene clock method clock motor clock oscillator clock paradox clock pulse clock pulses Clock Rate clock signals clock speed clock star clock tick clock time clock track clock tripling clock watch Clock, Astronomical clock-doubled clock-tripled clock/calendar clocked flip-flop clocked logic clockless computing Clocks Clocks and Watches clockwork Clockwork Orange, A clod Clodia Clodion	clock face Clock frequency Clock Gate Factor Clock gene Clock gene Clock gene Clock Generation Unit clock golf clock in clock in clock in or on Clock Input Clock Master Unit clock method Clock Mode Clock Module clock motor Clock multiplier Clock multiplier Clock Multiplier Unit Clock Native Clock network clock off clock off or out clock on clock oscillator clock out clock out clock out clock out clock paradox clock pendulum Clock Per Instruction Clock Phase Clock pulse Clock pulse Clock Pulse Generator clock pulses clock radio clock radios Clock rate clock ratio Clock Reference Board Clock Reference Oscillator Clock Restriction Diagram Clock Run In Clock Security System Clock Select Register B Clock signal Clock signal clock signals Clock speed clock speeds clock star Clock Status Receiver Clock Stopping Clock Switch Card

Mentioned in

References in periodicals archive

Encyclopedia browser

Full browser

No references found

Albrecht calls this time-choice conundrum the clock paradox, described with his UC Davis collaborator Alberto Iglesias in a paper published this year in Physical Review D.

It's likely that times are changing: a century ago, mathematician ... by Siegfried, Tom / Science News