Maxwell’s equations describe light as a wave so in the nineteenth century, a superfine ether was assumed to propagate it in space. If the earth orbits the sun to give the seasons, and spins to give night and day, the ether wind can’t always be stationary (Figure 5.1), so the speed of light should vary: light going against the wind should go slower, and light going with the wind should go faster. However in 1887, Michelson and Morley found, to everyone’s surprise, that the speed of light was the same in every direction. There was no ether wind because the earth’s movement didn’t change the speed of light!
Later, in 1904, the Lorentz transformation allowed the speed of light to stay the same in all inertial reference frames, and in 1905 Poincare deduced the relativity principle, that the laws of physics were the same in every reference frame, so a ball thrown up in a moving car behaves the same as in a stationary car. In general, throwing a ball, swinging a pendulum, or shining a flashlight is the same on a satellite orbiting the earth at thousands of miles per hour as it is on earth.
This is fortunate, because our earth actually is like a planetary satellite platform carrying us through the cosmos. Its spin whirls us around at about 1,000mph, it goes around the sun at about 66,000mph, and around the galaxy at an amazing 483,000mph. Some estimate our speed relative to the cosmic background radiation at about 1,300,000mph, yet science still works on earth as it does in the rest of the universe. We live on a moving planet but our reality bubble is still maintained, and Einstein wondered why?