Introduction For S1-3

Our understanding of how things move and why they move has changed greatly over time. For centuries, people thought that the motion of the stars and planets were governed by different laws from those which described the motion of objects on Earth. This way of thinking was changed completely in the 17th century by Isaac Newton. He understood that objects fall to Earth due to the force of gravity, pulling them towards Earth. He also realised that the motion of the planets could be explained using these same laws, and that the reason why the planets orbit the Sun is also because of gravity, with the Sun’s enormous mass exerting a gravitational pull on the planets and causing them to orbit around it.

Newton’s picture of gravity works very well for describing gravitational effects involving quite small masses, and weak, everyday gravitational forces. However it does not work so well if we need very accurate predictions of gravitational effects, such as those required by satellite navigation systems (sat-navs) in cars. They require extremely accurate measurements of the position of GPS satellites, and these measurements are affected by Earth’s gravity. Newton’s picture of gravity is not capable of describing how GPS systems work, or describing effects produced by very strong gravitational forces and enormous masses, such as those of stars and galaxies.

To describe these we need a different, better picture of gravity and its effects. Just over 100 years ago just such an improved picture of gravity was produced by Albert Einstein and named General Relativity. It completely changed the way people think about gravity, space and time and has been shown to describe many things which Newton’s picture of gravity cannot, including how GPS works. Einstein’s new picture of gravity can appear strange, but works wonderfully – it keeps us on the right road.


With thanks to Dr Gordon Robb, Strathclyde University.