*The following is based on part seven of Jacob Bronowski’s BBC series on the history of science and invention, “The Ascent of Man” (1973). This part is about the physics of Newton and Einstein:*

**Newton **was born on Christmas Day in the year that Galileo died, 1642. He got his university degree at Trinity College, Cambridge but then came **the plague years: 1665 and 1666**. He went to live with his mother in the country. There he **made his great discoveries in physics and mathematics**.

From his notebooks we know that he was badly taught: he had to work out mathematics for himself. But along the way he discovered a new form of mathematics: **calculus**. It became his secret weapon.

Copernicus and Kepler told us how the planets move but could not say why. Newton could: gravity. With his **law of gravity** he could work out how fast an apple fell from a tree and how many days it took the moon to go round the earth. Utterly amazing.

But none of it was made public till 20 years later. In the meantime Newton made his name in **optics**: he showed how white light is made out of coloured light. He became a professor at Cambridge and a leading light of science in Britain.

**Then one day ****Edmund Halley came to Cambridge to ask Newton a question** about physics. Halley loved his answer but then asked, “How do you know?” Newton said he would send him the proof. That proof took three years and was so long it became a book: the **“Principia”** (1687). It laid out his physics. Our idea that there are **laws of nature** comes from that book.

**Newton’s physics** was a wonder of the age, yet it **assumed that time and space are absolute**, that they are the same for all observers. Still it stood for 200 years. Then in **1881 Michaelson** found the first hole in it: light always went at the same speed no matter what. No one knew what to make of it until **Albert Einstein** came up with his **theory of special relativity** in **1905**.

Einstein would think about stuff like this: **Suppose **you get on the tram at the town clock to go to work and **your tram went the speed of light**. What would you see? If you looked back at the clock you would see that time had stopped – and yet for the people on the street the hands of the clock are still moving! Strange. That means the closer you get to the speed of light, the more time slows down. Time is not absolute. Nor is space: if you push the example further you find that the tops of the buildings will look like they are bending over the street and passers-by will look tall and thin.

Einstein worked out his physics along those lines and, **while his conclusions were strange, he was proved right** in the course of his life. Even the bit about the edge of a phonograph record ageing more slowly than the centre.

**See also:**