Particle physics [and relativity]
Relativity theory was originally formulated just for mechanics and electromagnetism. But its predictions like E = m c2 were immediately applied for example to radioactivity, and soon it came to be assumed that the theory would work for any system at all—unless it involved gravity. So this has meant that in particle physics c2 t2 - x2 - y2 - z2 is at some level the only quantity that ever appears. And to make mathematical work easier, what is very often done is to carry out the so-called Wick rotation t t—so relativistic invariance is just independence on 4D orientation. (See page 1061.) But except in rather simple cases there is practically no evidence that results obtained after Wick rotation have anything to do with physical reality—and certainly the transformation removes some very basic phenomena such as particle propagation. One feature of it, however, is that it maps the equation for quantum mechanical time evolution into the equation for probabilities in statistical mechanics, with imaginary time corresponding to inverse temperature. And while it is conceivable that this mapping may have some deep significance, none has so far ever been identified.