Miller was an advocate of aether theory and absolute space and an opponent of Albert Einstein’s theory of relativity. In 1900, he began work with Edward Morley on the detection of aether drift, at the time one of the “hot” areas of fundamental physics. Following on with the basic apparatus as the earlier Michelson-Morley experiment, Miller and Morley published another null result (the proposed result was absent) in 1904.
These experimental results were later cited in support of Albert Einstein’s theory of relativity. Miller continued to work on refining his experimental techniques after 1904, conducting millions of measurements on aether drift, and eventually developing the most sensitive interferometer in the world at that time.
Dayton Miller performed over 326,000 turns of interferometer with 16 readings each one, (more than 5,200,000 measurements). They showed what appeared to be a small amount of drift (about 9 km/s, 1/3 of the velocity of the earth around the sun): With white light and 32 m arms he could see nearly always the same result:
- A shift amplitude of 0.12 ± 0.01 fringe, incompatible with zero.
- A shift phase which points to an apex in the constellation “El Dorado”.
The amplitude analysis suggests a drag of aether. But the analysis of phase suggests that the Solar System goes towards the constellation Dorado (Mahimahi) at a speed of 227 km/s.
These results were presented by Miller as a positive indication of the existence of an aether drift. However, the effect Miller saw was tiny - much smaller than would be expected for a stationary aether. In order for these results to be consistent with an aether, it had to be assumed that the aether was dragged along with the earth to a much greater extent than aether theories typically predicted. Furthermore, the measurement was statistically far from any other measurements being carried on at the time. Fringe shifts of about 0.01 were being observed in many experiments, while Miller’s 0.08 was not duplicated anywhere else—including Miller’s own 1904 experiments with Morley, which showed a drift of only 0.015.
Based on an error analysis, Miller’s critics argued that he overestimated the precision of his results, and that his measurements were actually perfectly consistent with a fringe difference of zero - the null result that every other experiment was recording. However, Miller continued to defend his results, claiming that the probable reason for the so-called null results were that they were not being done at high locations (such as mountain tops) where the aether wind (drift) was supposedly much higher due to less aether drag. Miller did experiments that gave positive results high up on the top of Mount Wilson (California) where the aether wind was presumably about 10 or 15 times higher than results obtained by Miller (and Joos) in urban cities where the aether wind is supposedly more entrained (more drag).
Einstein was interested in this aether drift theory and acknowledged that a positive result for the existence of aether would invalidate the theory of special relativity, but commented that altitudal influences and temperatures may have provided sources of error in the findings. Miller commented:
The trouble with Professor Einstein is that he knows nothing about my results. […] He ought to give me credit for knowing that temperature differences would affect the results. He wrote to me in November suggesting this. I am not so simple as to make no allowance for temperature.
During the 1920s a number of experiments, both interferometry based, as in Miller’s experiment, and others using entirely different techniques, were conducted and these returned a null result as well. For example, Georg Joos reprised Miller’s experiment using a very similar setup (the arms of his interferometer were 21m vs. the 32m in the Miller experiment) and obtained results that were one fiftieth the magnitude of those from Miller’s. However Miller claimed that the explanation for results of the experiments of Georg Joos were because they were done at low altitude in the interior of a building where the aether wind was very low.