The Sagnac Interferometer

Georges Sagnac built it in 1913 hoping to prove the aether existed, which by then was a minority position, and instead produced the one interferometer that measures something no other instrument can get at without looking outside. Split a beam and send the two halves round the SAME closed loop in opposite directions. If the loop is stationary the two paths are identical and the fringes sit still. If the loop is rotating, the beamsplitter has moved slightly by the time each beam returns — further away for the beam travelling with the rotation, nearer for the one going against — and the fringes shift in proportion to the rate. What makes it extraordinary is what is absent from the formula. The shift depends on the enclosed AREA and the rotation rate, and on nothing else: not the shape of the loop, not what it is made of, not the refractive index, and not anything outside it. There is no reference and nothing to see. A sealed box can know it is turning. A bench-sized loop sees a turntable and cannot possibly see the Earth. The fix is to enclose the area many times over — a kilometre of optical fibre wound on an eight-centimetre former gives four thousand turns, and that instrument detects the Earth's rotation. Ring laser and fibre optic gyroscopes have flown aircraft since the 1980s, and there is nothing inside them that moves.