Published by Australian Academy of Science
Simply astronomical – the Square Kilometre Array Box 3 | Is Einstein still right?

In 1905 Albert Einstein published a revolutionary theory of space and time known as special relativity. Special relativity states that the speed of light is constant, regardless of how fast an observer is moving relative to another. An important consequence of Einstein's theory is that distance and space aren't fixed, they are relative or dependant on the speed of the observer. For example, you may think that the length of a ruler is always the same, but according to Einstein the size of a ruler will appear shorter and time will go slower at faster speeds.

Einstein spent the next ten years developing special relativity into a new theory of gravitation, known as general relativity. This states that, like motion, gravity can also affect time and space. Gravity pulling in one direction is completely equivalent to acceleration in the opposite direction. So an elevator accelerating upwards feels the same as the gravity pulling you to the floor.

A key prediction of general relativity is that light travelling close to a massive object will bend slightly due to gravity. In 1919 this prediction was tested when astronomers observed light from stars passing near the sun during a solar eclipse. Amazingly, they found the light bent as predicted. Overnight Einstein became an international celebrity.

General relativity's ultimate test will be in the extreme gravitational fields around black holes.

Related sites

• Einstein, Friedman and relativity (CSIRO, Australia)

• Relativity and the Cosmos (Einstein's big idea, NOVA, USA)

• Testing Einstein (National Radio Astronomy Observatory, USA)

• Strong field tests of gravity using pulsars and black holes (Square Kilometre Array)

• Putting relativity to the test (National Centre for Supercomputing Applications, USA)

Other boxes

Box 1. What is a radio telescope?

Box 2. Spectroscopy – a vital tool