What Gives Gold that Mellow Glow?
In discussions of special relativity, you occasionally encounter a claim like, “The effects of special relativity only matter to particle physicists and others working with extreme energies and velocities. Relativity has no consequences in everyday life.” Well, these days, anybody who relies upon the Global Positioning System(GPS) to navigate their car or the airliner in which they're travelling uses both special and general relativity, because without correction for their effects, GPS would be so inaccurate as to be useless. But GPS is a recent innovation, and the relativistic corrections are both complicated and hidden from the user in the software in the receiver and on board the satellites. But there's an effect of special relativity which was observed, if not understood, by the ancients: the yellow gleam of gold.
With an atomic number of 79, gold is in the last row of theperiodic table containing stable elements, and only four stable elements (mercury, thallium, lead, and bismuth) have greater atomic number. With 79 protons in its nucleus, the electrons of the gold atom are subjected to an intense electrostatic attraction. Using the naïve Bohr “solar system” model of the atom for the moment, electrons in the 1s orbital, closest to the nucleus, would have to orbit with a velocity v of 1.6×108metres per second to have sufficient kinetic energy to avoid “falling into” the nucleus. This is more than half the speed of light: c≈3×108 m/s