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Table 7.5
Non-historical, retrospective view of the role of the interferometer experiments
Result
Hypothesis:
Lorentz hypothesis
+
Special
Newtonian relativity
relativity
No displacement
0.01 (error)
0.90
Small displacement
0.09
0.09 (error)
Predicted displacement
0.90
0.01 (error)
Table 7.6
Early stages of the development of the interferometer experiments
Result
Hypothesis
Apparatus effect
(temperature, vibration)
Ether drag
(no apparatus effects)
No ether drag
No displacement
(Helmholtz)
0.50
0.1
0.9
Small displacement
(error)
0.50
0.1
0.1
Predicted displacement
(Michelson, Lorenz)
0.00
0.8
0.0
differences in the relative velocity of the earth and the luminiferous ether (Swenson
1970
). Michelson and Morley's experiments showed a much smaller effect than that
predicted. In 1905 Einstein's special theory of relativity made the ether unnecessary.
It included the radical proposal that the velocity of light is a constant so that if the
ether existed the velocity-dependent effects could not detect it. Table
7.5
assigns a
small probability to the occurrence of all three types of result (including the result
predicted by the rival hypothesis).
The Michelson-Morley experiment is often presented as a crucial experiment
that confirmed Einstein's theory (Holton
1973a
). Yet for many, the non-existence
of 'ether' (and therefore, an interaction with light) remained beyond consideration.
There is a long history of hypotheses about the behaviour of the apparatus and ad-hoc
hypotheses dealing with the negative result before and after 1905. In the early stages
of his search for evidence of ether-drift, Helmholtz advised Michelson that unless
temperature gradients within the interferometer apparatus could be eliminated, dif-
ferential expansion of the material would mask any effects indicating ether effects.
Fitzgerald later proposed a similar effect of the ether on the interferometer arms in
order to explain the null-results. Table
7.6
summarises this simplified version of the
early situation. It illustrates the range of possibilities that could be expected from
the Michelson-Morely experiment. Here we treat the Helmholtz' criticism as a hy-
pothesis about the experimental apparatus (no displacement). Fitzgerald introduced
an ad-hoc explanation of the negligible result; according to this the ether affects the
length of the interferometer arms.
