Sky rockets

摘要

Speed is relative. There is no absolute standard for "stationary" in the universe. Perhaps the nearest thing is the all-pervasive cosmic microwave background radiation. Its Doppler shift across the sky - blue in one direction, red in the other - reveals that relative to the CMB, the solar system is rattling along at 600 kilometres per second. Microwaves are rather insubstantial, though, so we don't feel the wind in our hair.