Nature has written a wonderful mystery. The plot continually changes and the most important clues come from seemingly unrelated investigations. These sudden and drastic changes of scientific scene appear to be Nature's way of revealing the unity of all fundamental science.
The mystery begins in the middle of the nineteenth century with the puzzle: How does the sun shine? Almost immediately, the plot switches to questions about how fast natural selection occurs and at what rate geological formations are created. The best theoretical physics of the nineteenth century gave the wrong answer to all these questions. The first hint of the correct answer came, at the very end of the nineteenth century, from the discovery of radioactivity with accidentally darkened photographic plates.
The right direction in which to search for the detailed solution was revealed by the 1905 discovery of the special theory of relativity, by the 1920 measurement of the nuclear masses of hydrogen and helium, and by the 1928 quantum mechanical explanation of how charged particles get close to each other. These crucial investigations were not directly related to the study of stars.
By the middle of the twentieth century, nuclear physicists and astrophysicists could calculate theoretically the rate of nuclear burning in the interiors of stars like the sun. But, just when we thought we had Nature figured out, experiments showed that fewer solar neutrinos were observed at earth than were predicted by the standard theory of how stars shine and how sub-atomic particles behave.
At the beginning of the twenty-first century, we have learned that solar neutrinos tell us not only about the interior of the sun, but also something about the nature of neutrinos. No one knows what surprises will be revealed by the new solar neutrino experiments that are currently underway or are planned. The richness and the humor with which Nature has written her mystery, in an international language that can be read by curious people of all nations, is beautiful, awesome, and humbling.