When we talk about ‘the theory of relativity’, we are actually referring to the ideas contained within two distinct papers, written eleven years apart, by the German physicist Albert Einstein. Taken together, the papers turned upside-down our previous assumptions about physics and cosmology, and brought about a genuine scientific revolution, the full implications of which are still being explored today.
The first paper appeared in 1905 and outlined Einstein’s ‘Special Theory of Relativity’, which emerged from a thought experiment he conducted when he was just sixteen. A thought experiment is an experiment carried out solely in the mind – usually because the physical evidence to prove or disprove the hypothesis is unavailable or too difficult to obtain. In Einstein’s case, he imagined riding alongside a light beam and came to the conclusion that if he could keep up with it, the beam would appear to be stationary – in just the same way as if you were sat in a railway carriage, staring at another train going in exactly the same direction and at the same speed as you. Yet he knew that such a phenomenon ran contrary to established scientific theories, so he carried out another thought experiment.
This time he imagined a moving train struck by lightning at both its ends at precisely the same moment. Einstein wondered how the event might seem to a stationary observer on an embankment and to someone aboard the train. The person on the embankment, in line with the middle of the train as the lightning strikes, sees the two bolts simultaneously. However, the person on the train is travelling in the direction of the bolt that hits the front of the train, so sees that one momentarily before the other bolt. In other words, the exact same event appears to happen at different times to different observers.
The Special Theory
Drawing on his knowledge of the work of, for example, James Clerk Maxwell, Henri Poincaré and Heinrich Lorentz, Einstein reached the somewhat esoteric conclusions that the laws of physics are the same for all observers moving at constant velocity relative to each other, and that the speed of light in a vacuum is constant. So, what is it that makes the Special Theory so important? It’s because, for centuries, the world had accepted Isaac Newton’s proofs that space and time were absolutes, and Einstein was showing that they were not. In Einstein’s own words, the Special Theory of Relativity ‘employs a modification of the theory of space and time’.
Einstein wanted to establish rules that were general and universal, and the Special Theory frustrated him because it applied to only very specific conditions (where motion is at a constant velocity and in a straight line). So he conducted yet another thought experiment – this time about a man floating freely in an enclosed box as it freefalls through space. If the man in the box takes off his watch, that too floats freely beside him. It feels to the man as if he is inside a box sitting still in a non-gravitational field, even though gravity is pulling the box towards the Earth. Equally, if the box was speeding up through space far from the pull of gravity, the subject inside would be pushed to the floor just as if he were being pulled by gravity. Traditionally, gravity and acceleration were regarded as different phenomena, though both related to mass. Einstein now realized that gravitational mass and inertial mass are equivalent – an idea that he called the Equivalence Principle.
Thinking about the freefalling box again, he also concluded that if you pierced a hole in one of its sides, a beam of light would hit the opposite wall at a higher point than it entered, its trajectory having been bent by gravity. In other words, light under the influence of gravity does not always – as had been assumed until then – travel in straight lines. Now he could extend the Special Theory to relate it to any sort of motion, whatever the acceleration and direction.
The General Theory
The General Theory showed how gravity warps both time and space, and also provided the field equations to explain how gravity acts on matter and how matter generates gravity by curving space-time. To picture this last concept, think of a basketball bouncing on a trampoline. The fabric of the trampoline bends as the ball travels across it and comes to rest. Add a second ball and it rolls until coming to rest next to the first ball. This is not because the first ball exerts some mysterious force on the second, but simply because the trampoline fabric has been warped by Ball One.
Among its many contributions to science, the General Theory opened the way to our greater understanding of phenomena including black holes, wormholes and the Big Bang. In the words of physicist Max Born, the theory counts as ‘the greatest feat of humans thinking about nature – the most amazing combination of philosophical penetration, physical intuition and mathematical skill’.