What is the temperature in space?
D.02 Have physical constants changed with time? The fundamental laws of physics, as we presently understand them, depend on about 25 parameters, such as Planck’s constant h, the gravitational constant G, and the mass and charge of the electron. It is natural to ask whether these parameters are really constants, or whether they vary in space or time. Interest in this question was spurred by Dirac’s large number hypothesis. The “large number” in question is the ratio of the electric and the gravitational force between two electrons, which is about 10^40; there is no obvious explanation of why such a huge number should appear in physics. Dirac pointed out that this number is nearly the same as the age of the Universe in atomic units, and suggested in 1937 that this coincidence could be understood if fundamental constants—in particular, G—varied as the Universe aged. The ratio of electromagnetic and gravitational interactions would then be large simply because the Universe is old. Such
