Time as a Ruler: Gravity Measures Itself
A beam of light travels through the void, and its own 'ticking'—an invisible clock embedded in the very fabric of spacetime. Physicists have taken this beam and allowed it to measure itself: the gravitational field on the beam is quantized, and the role of the clock is played by the so-called dressing time — the time that the field 'puts on' like a coat.