Science › Classical physics
Galileo Galilei
1632· Person
ProvenTested many times; part of working physics.
Italian astronomer and physicist (1564–1642), a founder of the experimental method and early champion of a Sun-centred solar system.
See it on the Science map →How it works
Galileo tested ideas against measurements instead of ancient authority. He rolled balls down ramps to show that falling objects speed up at a steady rate regardless of weight, air resistance aside. From 1609 his telescope revealed Jupiter's moons, the phases of Venus and mountains on the Moon. His 1632 Dialogue defended Copernicus and argued that motion is relative, which led to his trial and house arrest in 1633.
Why it matters
He made experiment the judge of physical truth and supplied the motion rules Newton built on. Inertia and relative motion both start with him.
How the work held up
Others repeated his ramp and pendulum timings, and Jesuit astronomers in Rome confirmed his telescope observations by 1611. The phases of Venus ruled out the old Earth-centred Ptolemaic model.
Still a mystery
He wrongly thought the tides proved Earth's motion and dismissed Kepler's elliptical orbits. Whether he really dropped balls from the Leaning Tower of Pisa is doubtful.
Connections (1)
▸Galileo Galilei proposed Galileo's relativity: motion is relativeProven
The relationship
Galileo needed to answer the strongest argument against a moving Earth: if we were spinning and orbiting, wouldn't we feel it? His ship thought-experiment showed that steady motion leaves no trace inside. That made a moving Earth physically reasonable, and established that only relative motion and changes in motion can be measured.
How the link was found
Galileo, in his 1632 'Dialogue Concerning the Two Chief World Systems', through the character Salviati's description of a cabin below deck.
Why it's proven
Experience on any smoothly moving vehicle confirms it, and Newton's laws build it in. No experiment has found physics that differs between steadily moving labs.
Open questions and mysteries
Einstein extended the principle to light in 1905, which forced time and space to become relative. Why nature has this symmetry at all is unexplained.