In the 19^{th} century Michael Faraday undertook experiments to explore the relationship between electricity and magnetism. These experiments demonstrated that a magnet moving through a wire coil causes an electric current to flow through the wire and conversely an electric current flowing through a wire coil causes a magnetic compass to deviate from pointing north. These phenomena are utilised extensively today in the generation of electricity and the conversion of electricity into circular motion (an electric motor). Faraday was a great experimentalist but it was James Clerk Maxwell who extended these principles into a complete theory of electricity and magnetism. When Maxwell applied his theory to the properties of empty space (with no positive/negative charges or north/south poles present) not everything in the equations disappeared. What remained was a description of a wave propagating at a very fast speed. This is light.

In 1905 Albert Einstein published four scientific papers. One of these papers titled ‘On the Electrodynamics of Moving Bodies’ outlines his theory of special relativity. Einstein was interested in Faraday’s experiments and Maxwell’s theory. Particularly the fact that it doesn’t matter if the magnet moves inside the coil or the coil moves around the magnet, as long as there is motion between the two an electric current will flow through the coil. In special relativity, Einstein uses two assumptions to establish a new foundation for physics. The original foundation leads to inconsistencies to explain this phenomenon between electricity and magnetism. In classical mechanics, as developed by Galileo Galilei and Isaac Newton, there is no speed limit. Objects can travel at any speed. This original foundation also contains a concept called universal time, which involves time flowing at the same rate for all objects. If one object is travelling very fast and another is not travelling at all then both will still agree how long it took for the minute hand of a clock to make one complete revolution, one hour.

One assumption Einstein imposed is the speed of light is the fastest possible speed any object can travel. As a result of this restriction the concept of universal time had to be abolished. No longer would everyone agree on the time taken for the minute hand to make one complete revolution. Imagine two objects equip with clocks, one travels close to the speed of light and another stays at rest. Upon comparing the clocks we would find time has passed more slowly for the object travelling fast compared to the object that stayed at rest. If the clock for the object at rest shows that one hour has passed, then the object travelling fast would show that less than one hour has passed. Time slows down the faster you travel with the amount it slows down proportional to how fast you travel. Unstable particles, produced in space, don’t not have enough time, before they decay into other particles, to travel the distance from space to sea level where they are detected. When the time taken for these particles to decay at rest is measured in the laboratory they are found to decay more quickly. Time must slow down for the unstable particles as they travel close to the speed of light, from their perspective not enough time has passed for them to decay. These effects only become noticeable when objects travel close to the speed of light.

Einstein wanted to apply these same principles to gravity with an aim of removing the inconsistencies plaguing the motion of the planet Mercury around the Sun, as predicted by Newton’s theory of gravity. In 1915, after ten years working on this problem, Einstein presented his general theory of relativity to the scientific community. Since this time most of the predictions made by Einstein’s theory have been tested and verified. Some predictions, such as gravitational waves, have yet to be detected. General relativity is one of the greatest achievements in human history and continues to enlighten our understanding of the Universe.

**Stephen will be teaching a 5-week course titled ***100 Years of General Relativity* from Tuesday 28 April. To join him for this one hundred year anniversary course to celebrate and explore Einstein’s theories of special and general relativity you can book your place by clicking here http://goo.gl/H3LM3a .