Energy from the Sun to Earth

It is a feature of the way the world is made that two protons together have less mass than two protons separately. This is a startling but indisputable fact. Weigh two protons separately, then weigh them together: the numbers don't match. The numbers differ by about 1 percent. This curious difference is not to be explained by some law of nature; it is a law of nature, as basic to the way the world works as any fact in our possession. The mass discrepancy is equivalent to an amount of energy g...