Science › Classical physics
Isaac Newton
1687· Person
ProvenTested many times; part of working physics.
English mathematician and physicist (1643–1727) who wrote the Principia, the founding book of modern physics.
See it on the Science map →How it works
Newton showed that the same few rules govern a falling apple and the orbiting Moon. In the Principia (1687) he set out three laws of motion and a universal law of gravity, then used them to derive the planets' orbits, the tides and the paths of comets. He also co-invented calculus, split white light into colours with a prism, and built the first practical reflecting telescope. He worked by pairing careful measurement with relentless mathematics.
Why it matters
He made the universe predictable: one set of equations for heaven and earth. Engineering, spaceflight and most of physics until 1900 rest on his framework.
How the work held up
Halley's comet returned in 1758 as his laws predicted. Neptune was found in 1846 where Newtonian calculations said an unseen planet must be. NASA still navigates most spacecraft with his equations.
Still a mystery
He admitted he could not explain how gravity acts across empty space. Einstein answered that in 1915; Newton's laws also fail at near-light speeds and atomic scales.
Connections (3)
▸Isaac Newton wrote Newton's laws of motionProven
The relationship
Newton took Galileo's insight about inertia and Kepler's planetary data and condensed them into three general laws. He framed motion in terms of forces: anything that changes an object's speed or direction. One method now described cannonballs, pendulums, tides and planets, replacing a patchwork of separate rules.
How the link was found
Newton published them in the 'Philosophiæ Naturalis Principia Mathematica' in July 1687, prompted and paid for by Edmond Halley.
Why it's proven
Centuries of precise predictions, including spacecraft navigation. Limits found in the 20th century (relativity, quantum mechanics) refined rather than overturned them at everyday scales.
Open questions and mysteries
Why objects have inertia at all, and what ultimately sets their mass, are only partly answered, through the Higgs field and debates over Mach's principle.
▸Isaac Newton discovered Newton's law of gravityProven
The relationship
Newton realised the force pulling an apple down could reach to the Moon, weakening with distance. He calculated that an inverse-square force keeps the Moon in orbit and, with his laws of motion, reproduces Kepler's elliptical planetary orbits. One law replaced separate explanations for falling objects and heavenly motion.
How the link was found
Newton worked on the idea from about 1666. Halley's 1684 question about orbit shapes prompted him to publish full proofs in the Principia (1687). Robert Hooke disputed credit for the inverse-square idea.
Why it's proven
Accurate planetary predictions, Halley's comet's return (1758), Neptune's discovery (1846), and Cavendish's 1798 lab measurement of the pull between lead spheres.
Open questions and mysteries
Newton could not say how gravity acts instantly across empty space. Einstein's curved spacetime answered that; how gravity works at the quantum level is still unknown.
▸Isaac Newton co-invented Calculus: the math of changeProven
The relationship
Newton needed math for quantities that change continuously, like the speed of a falling body or a planet's changing direction. He developed 'fluxions' (rates of change) and their reverse to handle this. Gottfried Leibniz developed the same ideas independently, with clearer notation. Calculus became the toolkit that turned Newton's physical insights into precise calculations.
How the link was found
Newton worked it out around 1665–1666 while Cambridge was closed by plague, but published little. Leibniz published in 1684. A 1712 Royal Society report, secretly drafted by Newton, accused Leibniz of plagiarism.
Why it's proven
Historians now agree both invented it independently. Newton's notebooks document his early work, and Leibniz's papers show a separate path.
Open questions and mysteries
The Principia is written mostly in geometry, not calculus, and historians debate how much Newton relied on calculus to find his results.