How Long is a Piece of Spacetime

 On November 25th 1915 Albert Einstein submitted his famous paper on the General Theory of Relativity. David Hilbert also derived the General Theory in November 1915 using quite different methods. In the same year Emmy Noether derived her remarkable ‘Noether’s Theorem’ which lies at the heart of much modern Physics. 1915 was a very good vintage indeed. We will take a brief walking tour of General Relativity using some of the ideas of Noether, Hilbert and Einstein to examine gravitational redshift, gravitational lensing, the impact of General Relativity on GPS systems and high precision atomic clocks, and Black holes all of which can be summarised by asking ‘how long is a piece of spacetime?’