Doppler effect, though relative velocities need to be handled with more care since distances can be defined in different ways in expanding metrics. At the same time, other explanations were proposed that did not concord with general relativity. Edward Milne proposed an explanation compatible with special relativity but not general relativity that there was a giant explosion that could explain redshifts (see Milne universe). Others proposed that systematic effects could explain the redshift-distance correlation. Along this line, Fritz Zwicky proposed a "tired light" mechanism in 1929. Zwicky suggested that photons might slowly lose energy as they travel vast distances through a static universe by interaction with matter or other photons, or by some novel physical mechanism. Since a decrease in energy corresponds to an increase in light's wavelength, this effect would produce a redshift in spectral lines that increase proportionally with the distance of the source. The term "tired light" was coined by Richard Tolman in the early 1930s as a way to refer to this idea. Helge Kragh has noted "Zwicky’s hypothesis was the best known and most elaborate alternative to the expanding universe, but it was far from the only one. More than a dozen physicists, astronomers and amateur scientists proposed in the 1930s tired-light ideas having in common the assumption of nebular photons interacting with intergalactic matter to which they transferred part of their energy." Kragh noted in particular John Quincy Stewart, William Duncan MacMillan, and Walther Nernst. Tired light mechanisms were among the proposed alternatives to the Big Bang and the Steady State cosmologies, both of which relied on the general relativistic expansion of the universe of the FRW metric. Through the middle of the twentieth century, most cosmologists supported one of these two paradigms, but there were a few scientists, especially those who were working on alternatives to general relativity, who worked with the tired light alternative. As the discipline of observational cosmology developed in the late twentieth century and the associated data became more numerous and accurate, the Big Bang emerged as the cosmological theory most supported by the observational evidence, and it remains the accepted consensus model with a current parametrization that precisely specifies the state and evolution of the universe. Although the proposals of "tired light cosmologies" are now more-or-less relegated to the dustbin of history, as a completely alternative proposal tired-light cosmologies were considered a remote possibility worthy of some consideration