Introduction For S4-6

What does space do? Until just over 100 years ago, the answer to this question would have been something like “nothing really”. Space was considered as little more than a set of axes e.g. x,y,z with which an object’s position could be described using a set of coordinates. Similarly time was considered to be something which did nothing more than tick forward regularly at the same rate, no matter where you happened to measure it. This was entirely sufficient for Newton’s laws of gravity, discovered in the 17th century, and based on the idea that two masses exert an attractive force on each other. Newton’s laws had been used very successfully to describe gravitational effects on Earth and beyond e.g. the motion of the planets around the Sun.

The person who changed our understanding of space, time and along the way, gravity , was Albert Einstein. With his theories of Relativity (Special Relativity in 1905 and General Relativity in 1915) he transformed our understanding of fundamental ideas such as measuring time, simultaneous events, and how to explain what we observe as gravitational effects e.g. planets orbiting the Sun. General Relativity explains the orbit of the Earth around the Sun as being due to the enormous mass of the Sun curving space (and time, more accurately spacetime) in its vicinity to produce a “dent” or dip , which the Earth becomes trapped in, causing it to move in orbit around the Sun. It makes us think about space and time in a completely different way than was done previously. In Einstein’s theories, space and time can now do more – their behaviour can be influenced by mass. Spacetime is curved by mass, and the shape or curvature of this spacetime in turn guides the motion of other masses and light. These ideas sound strange and unfamiliar but experimental tests have confirmed the predictions of General Relativity and shown it to give a more correct description of gravitational forces and effects than Newton’s laws.

One of the new phenomena predicted by General Relativity was gravitational radiation, the ability of a disturbance in a region of spacetime to generate waves, ripples in spacetime, which can then travel through completely empty space. Numerous attempts were made to measure these waves over the years, but the incredibly weak nature of these spacetime vibrations meant that it was only very recently, in September 2015, that their existence was proved using the extremely high sensitivity of detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO). Just as our understanding of the universe was transformed in the 20th century by the development of new telescopes using radiation from different parts of the electromagnetic spectrum e.g. radio waves, gravitational waves give us an additional tool with which we can listen to the universe, and promises new insights into its behaviour and history.

With thanks to Dr Gordon Robb, Strathclyde University.