# School of PhysicsLecture Demonstration Manual

## Aa-3 Gravity Drum

Published: Monday 22 June 2009 - Updated: Thursday 31 March 2011

### Aim

To demonstrate a model of gravity being the result of the curvature of space/time.

### Apparatus

•  Gravity Drum
•  Large mass shot-put
•  Small ball-bearings

### Diagrams

Click on pictures to enlarge.

 Gravity Drum

### Description

Isaac Newton worked out that the same force that makes apples fall to the ground also makes the Moon fall towards the Earth along a curved path, called an orbit. But his formula for the gravitational force between two masses is silent on the precise mechanism responsible for this force. Albert Einstein recognised that the gravitational force can be cancelled by allowing a body to fall. When falling, either as an apple falls to the ground or an astronaut in orbit, the body becomes weightless. This gave Einstein the idea that the mechanism for transmitting the gravitational force arises from a distortion of the structure of space and time around the Earth, apple and astronaut.
In the gravity drum we have a two dimensional model of a three dimensional projection of a four dimensional slab of spacetime. Initially the drum is flat representing space without a mass present. Circles drawn on the flat drum will have a circumference equal to two pi times the radius. Putting a heavy shot-putt onto the centre of the drum stretches and distorts the flat surface in the same way mass distorts real spacetime. Now small marbles rolling across the curved drum will "fall" or even "orbit" the heavy mass on the curved surface.
Close inspection of the curved drum next to the heavy mass will reveal that the surface has a saddle shaped curvature. Convex in a plane perpendicular to the surface oriented radially, concave in a plan tangential to a circumference. Circles drawn on such a surface will have a circumference greater that two pi times the radius. The means the heavy mass has created more space! The planet Mercury, orbiting deep in the gravitational well created by the gravity of the Sun, has an orbit that can only be explained by including the curved distortion of spacetime.