In the 1930s, Hans Bethe and Carl Friedrich von Weizsäcker had hypothesized that the source of the sun's energy was fusion reactions in its core. While this hypothesis was widely accepted for decades there was no way of observing the sun's core and directly testing the hypothesis. Ray Davis's Homestake Experiment was the first to detect solar neutrinos, strong evidence that the nuclear theory of the sun was correct. Over a period of decades the Davis experiment consistently observed only about 1/3 the number of neutrinos predicted by the Standard Solar Models of his colleague and close friend John Bahcall. Because of the great technical difficulty of the experiment and its reliance on radiochemical techniques rather than real time direct detection many physicists were suspicious of his result.