Given Einstein’s universally-acknowledged genius and the reputation for intractability still enjoyed by general relativity a century after its birth, this work is a paragon of clarity and simplicity of explication. It is very light on mathematics, only requiring basic algebra, and works almost entirely through illustrative models such as that of an observer on a moving train. Einstein was one of those who had been forced to rethink our universe based on the contradictions raised by the apparent anisotropy of the fixed speed of light in vacuo on a moving Earth. His solution was simple but profound, representing another Copernican revolution, in that he abandoned the twin presuppositions of fixed time and position. As he writes here, everything else in Special Relativity follows as a matter of simple logical consistency, and his new universe is perfectly coherent, if weird.
The General form extends this to the abandonment of a fixed coordinate system altogether, hence the name of “General Relativity”, but it entails some important further considerations. GR space is Gaussian rather than Euclidean. Its geometry is Riemann, and as a profound consequence we can infer that the Universe is finite but unbounded: Look far enough into distant space and you would see the back of your own head. Hereby is an important inconsistency arising out of Newtonian physics dissolved, as a Newtonian Universe would require a centre and a finite size; a sphere uniformly filled will at sufficient size exert infinite gravitational forces at its bounds. Our Universe can be unbounded and uniformly filled at the largest scales without contradiction. Gravity – and acceleration – also bend space. Measure pi by sitting on a spinning disk and measuring along the circumference and perpendicular across the axis and the foreshortening along the direction of motion will yield a lower than Euclidean value – space has curved for the fly on the wheel.
Einstein’s work is unquestionably one of the two or three most successful in scientific history. He explains and predicts phenomena that could be observed at the time, such as the precession of Mercury’s orbit and the gravitational shift in the positions of stars close to the line-of-sight of the Sun. Yet he also makes predictions which could not have been observed at the time, and I think it is correct to say that all have since been confirmed. QED and QCD might compete with Relativity for predictive and explanatory power, but they were the work of many great minds. Einstein, along with perhaps Newton and Darwin, earns unique credit for being the single mind associated by history with the entire edifice, even if this picture is not entirely fair in all cases. It is a joy to find he also writes so gracefully.