These days, people are likely to be familiar with Albert Einstein's theories of special relativity and general relativity, even if only in name. However, many of the same people don't know that there was a theory of relativity that came long before Einstein.
Named for its originator, the 17th-century Italian astronomer Galileo Galilei, Galilean relativity dictates that any two observers moving at constant speed and direction will obtain the same results for all mechanical experiments. Another way of stating the theory is simply that the laws of physics remain constant for all inertial frames. This means that a game of ping-pong played inside the cab of a moving train will be exactly like a game played in a parlor or laboratory that remains perfectly still -- provided, that is, that the train is moving in a straight line and at a constant speed. Acceleration, deceleration or a bend in the tracks will suddenly remind the ping-pong players that the train is in motion.
While this theoretical example holds up perfectly well in the abstract, there are a few caveats to its actual application. For one thing, most moving vehicles cannot move at a perfectly constant speed or in a perfectly straight trajectory. Also, the "perfectly still" laboratory or parlor is not actually perfectly still. At the equator, the surface of the planet Earth rotates at approximately 1,000 miles per hour (about 1,600 kilometers per hour), while the entire Earth itself is orbiting the sun at something like 67,000 miles per hour (about 108,000 kilometers per hour) -- not to mention that our entire solar system is flying through an expanding universe of empty space at an astonishing rate [source: Butterworth and Palmer]. So, if we're moving at such an incredible velocity at all times, why do we ever feel the sensation of stillness? It's because of Galilean relativity -- we are part of a vast inertial framework that anchors us all to a common experience of stillness.
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