# 1911-1921

## The book of science

Tom Sharp

 Max von Laue, Georges Sagnac physics

## Sagnac effect

• In 1913, Georges Sagnac split a beam of light
• and sent each half in opposite directions
• around a ring. When he rotated the ring he observed
• an interference pattern in the recombined beams
• and concluded the aether is stationary relative to earth.
• Max von Laue had predicted the interference in 1911,
• but explained it is because light travels at a fixed speed
• no matter how fast the source of the light travels
• (according to Einstein’s theory of special relativity),
• and because when the ring is rotated,
• although the two beams travel along the same path,
• this path is lengthened in the direction of rotation
• and shortened in the opposite direction.

## Experiments

• Sagnac’s experiments in 1913
• continued the work by Michelson and Morley in 1887
• that compared the speed of light in perpendicular paths
• in order to detect stationary aether.
• The idea was that since light propagates as a wave,
• something that we can’t see or feel must be waving,
• and they called this medium “luminiferous aether.”
• Stationary aether
• is aether that moves
• with earth in its orbit.
• Michelson and Morley didn’t detect stationary aether,
• so Sagnac tried to find evidence of aether
• moving relative to the earth, and didn’t find it,
• so he thought he had proved the aether was stationary.
• Meanwhile Franz Harress tried to measure
• the Fresnel drag of light through rotating glass in 1914
• and found the same effect as Sagnac, but couldn’t explain it,
• describing it as an systematic bias in his measurements.
• The effect of the glass was supposed to be the same
• as the effect of any medium such as water or aether.
• The speed of light was supposed to be
• increased by the glass moving it its direction,
• and impeded by the glass moving against it.
• In the Sagnac and Harress experiments,
• the opposite occurred; it took longer to traverse the path
• for the beam moving in the direction of rotation
• and shorter for the beam moving in the opposite direction.
• Von Laue came back in 1920
• to explain the Sagnac effect in Harress’s experiment.
• The fact that the speed of light is reduced
• when propagating through glass had nothing
• to do with the fact the glass was moving,
• and nothing to do with the interference pattern.

## Interferometry

• Hologram
• Laser gyroscope
• Laser Interferometer Gravitational-Wave Observatory
• We have
• widely applied this trick
• for detecting small differences, extremely small differences.
• We could
• hear the breathing and sighing
• of a mite that crawls upon the chin hair of a flea.
• We can
• hear you murmur in your sleep
• from the vibrations of the window in your room.
• If that’s
• not a fine enough interference,
• wait until we can tell what you are dreaming.

Albert Einstein refuted Michelson, Morley, Sagnac, and Harress; the theory of special relativity made the search for luminiferous aether unnecessary.