The light clock

To reason cleanly about time it helps to build a clock stripped down to its essentials, using only light. Imagine an ideal device called a light clock: two parallel mirrors placed a distance $d$ apart, between which a beam of light bounces back and forth. Each “tick” is a round trip of the beam; since light travels at speed $c$ and covers a total distance $2d$, each tick lasts

$\Delta\tau = \frac{2d}{c}$

The light clock: a beam bounces between two mirrors a distance $d$ apart. A round-trip tick lasts $\Delta\tau = 2d/c$.

The merit of this clock is that its “hand” is light itself: since $c$ is fixed by Einstein’s postulates, the clock’s rhythm depends only on the distance $d$ between the mirrors. This is exactly what is needed to put the idea of absolute time to the test: we need only ask how long the same tick lasts as seen by someone watching the clock in motion.

▶ Length contraction and time dilation — MinutePhysics

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Topics: Special relativity Concepts: Time dilation Methods: Minkowski diagram Objects: Light clock

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