Hammar Experiment - The Experiment

The Experiment

Using a half-silvered mirror A, he divided a ray of white light into two half-rays. One half-ray was sent in the transverse direction into a heavy walled steel pipe terminated with lead plugs. In this pipe, the ray was reflected by mirror D and sent into the longitudinal direction to another mirror C at the other end of the pipe. There it was reflected and sent in the transverse direction to a mirror B outside of the pipe. From B it traveled back to A in the longitudinal direction. The other half-ray traversed the same path in the opposite direction.

The topology of the light path was that of a Sagnac interferometer with an odd number of reflections. Sagnac interfometers offer excellent contrast and fringe stability, and the configuration with an odd number of reflections is only slightly less stable than the configuration with an even number of reflections. (With an odd number of reflections, the oppositely traveling beams are laterally inverted with respect to each other over most of the light path, so that the topology deviates slightly from strict common path.) The relative immunity of his apparatus to vibration, mechanical stress and temperature effects, allowed Hammar to detect fringe displacements as little as 1/10 of a fringe, despite using the interferometer outdoors in an open environment with no temperature control.

Similar to Lodge's experiment, Hammar's apparatus should have caused an asymmetry in any proposed aether wind. Hammar's expectation of the results was that: With the apparatus aligned perpendicular to the aether wind, both long arms would be equally affected by aether entrainment. With the apparatus aligned parallel to the aether wind, one arm would be more affected by aether entrainment than the other. The following expected propagation times for the counter-propagating rays were given by Robertson/Noonan:

where is the velocity of the entrained aether. This gives an expected time difference:

On September 1, 1934, Hammar set up the apparatus on top of a high hill two miles south of Moscow, Idaho, and made many observations with the apparatus turned in all directions of the azimuth during the daylight hours of September 1, 2, and 3. He saw no shift of the interference fringes, corresponding to an upper limit of km/s. These results are considered a proof against the aether drag hypothesis as it was proposed by Miller.