The basic principle of relativity

In Galileo Galilei’s 1632 Dialogo, the progressive Salviati explains relativity to the open-minded Sagredo and the orthodox Simplicio:

“Motion is motion, and as motion it acts, inasmuch as it has relation to things that lack it; but among things that all participate equally in it, nothing acts, and it is as if it did not exist. And so the merchandise, with which a ship is loaded, moves insofar as leaving Venice it passes through Corfu, through Candia, through Cyprus, and goes to Aleppo, where Venice, Corfu, Candia, etc. remain and do not move with the ship; but for the bales, chests, and other packages, with which the ship is loaded and stowed, and with respect to the ship itself, the motion from Venice to Syria is as if it were nothing; and nothing alters the relation that exists between them; and this is because it is common to all and equally shared by all; and when, amongst the goods in the ship, a bale has shifted from a chest by a single finger, this alone will have been a greater movement for it in relation to the chest than the voyage of two thousand miles made by them together.”

To which Simplicio replies: “This is good, solid, and entirely peripatetic doctrine.”

Galileo helpfully summarises Salviati’s exposition with an aside: “For the things that are equally moved by it, motion is as if it did not exist, and in the meanwhile it operates insofar as it has relation to things that are lacking in it.” Galileo was imprisoned in 1633 for this and many such empirical observations which supported Copernicus’ 1543 heliocentric theory, in contradiction to Catholic orthodoxy, and his Dialogo was placed on the Index of Forbidden Books, where it remained for 200 years.

Things you can do while in constant linear motion:

• You can play your usual game of tennis on the deck of Galileo’s ship as it plies its trade across a calm sea;
• You can swim back and forth with equal ease in the ship’s swimming pool, and dive off its high platform without fear of missing the pool below;
• You can toss a ball straight up in the air while walking, and it will fall back into your hand just as it would when standing still, whereas for a stationary passerby the ball will follow a parabolic path;
• You can juggle equally as capably – or ineptly – while walking as while standing, although slowing to a standstill adds difficulty;
• You can talk normally to a passenger in your car, yet a pedestrian overhearing you will experience a change of pitch - a Doppler shift - as your voice moves past them.

There is nothing you can do from within a frame of reference in constant linear motion that will prove its motion. Hence the enduring struggle to reject conclusively Ptolemy’s geocentric theory, embraced by an anthropocentric Catholic Church, of the Sun and whole Universe revolving around a fixed Earth, in favour of the simpler heliocentric theory of Earth orbiting the Sun. Not until 1838 did astronomers have sufficiently sensitive instruments to measure the small parallax shifts in relative positions of stars over the course of a year which could only result from angular motion of Earth around the Sun.

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