Einstein had already revolutionised physics back in1905 when he had published four hugely important scientific papers, all in the renowned Annalen der Physik, Germany then being the world’s leading centre for physics. The first was ‘On a Heuristic Point of View Concerning the Production and Transformation of Light’. This proposed that electromagnetic radiation was composed of photons or quantums. This formed the basis for quantum theory. The second was ‘On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat’. This explained the phenomenon of Brownian Motion (the motion of minute particles suspended in a stationary liquid, but visible through a microscope). The fourth was ‘Does the Inertia of a Body Depend upon Its Energy Content?’. This contained what was to become the most famous equation in the world: E=mc2.

The third of these seminal papers was ‘On the Electrodynamics of Moving Bodies’, which launched the theory of Special Relativity. Before going further, it should be noted that Einstein based Special Relativity on two fundamental assumptions: first, that any experiment performed by two separate researchers will give the same result, even if one is moving at a constant velocity relative to the other; second, that the two would come up with the same measurement of the speed of light, even if one was moving.

Special Relativity introduced the concept of relativity, but only in a very specific context, that of reference-bodies, to use Einstein’s own words, “in a state of uniform rectilinear and nonrotary motion with respect to [reference-body] K . . . The validity of the principle of relativity was assumed only for these reference-bodies, but not for others (for example, those possessing motion of a different kind). In this sense, we speak of the special principle of relativity, or special theory of relativity.”

Even so, Special Relativity introduced the concepts of time dilation, length contraction, the increase of mass of a body as it accelerates, and the speed of light in a vacuum as the universe’s speed limit. Time dilation, to put it simply, means that the faster you go, the slower time flows for you (relative to someone who is stationary), although you would not notice it while you were travelling. Length contraction – as a body (say, a spaceship), goes faster, it seems to get smaller, relative to a stationary observer. As already mentioned, as a body accelerates, its mass increases, by the same degree as time dilates and its length contracts. At the speed of light, anybody would have an infinite mass, requiring infinite energy to continue to propel it, as, by definition, there is no infinite supply of anything, no body can reach the speed of light in a vacuum. And it was in Special Relativity that the old concept that space and time were separate phenomena was replaced by the realisation that they were a single phenomenon: spacetime.

Now, the lives we lead do not see us attain speeds at which such effects become discernible to our senses (although they are discernible to sensitive instruments). This might change in the not-too-distant future with more efficient space propulsion systems, such as ion drive, which, in principle, can develop tremendous velocities over time.

But, back in 1905, all this was absolutely mind-boggling. But what about bodies in motion that was not uniform, rectilinear and nonrotary? That is what Einstein was addressing when he developed his General Theory of Relativity. (Of course, during this process, he discussed matters with, and bounced his ideas off, friends and colleagues.) An inspiration for General Relativity was the realisation that, in a small volume of space, it is impossible to tell the difference between acceleration and the effects of gravity. And he wanted to determine how spacetime would be affected by mass. His objective was to develop more general equations that would, in addition to other things, incorporate the effects of gravity.

He succeeded. And again the results were mind-boggling. Mass alters the geometric properties of spacetime. Or, to put it differently, mass bends spacetime. And because of that, mass bends light, even though light has no mass. (Prior to Einstein, physics had been dominated by the work of Sir Isaac Newton, whose equations are still valid for most nonrelativistic calculations and who conceived of gravity as a force; if gravity was a force, it should not be able to bend light.) The bending of light (from distant stars, by the sun) was established by observation in 1919.

Although it has since experienced some modifications, General Relativity still remains a central pillar of astrophysics and cosmology. It is the irreplaceable foundation for the standard model of cosmology. It has become what scientists classify as a “highly successful theory” because it has passed a large number of tests, both observational and experimental. It has been and is successfully applied to many different cosmological phenomena. It remains a live sphere of research and continues to inspire new experiments. It forms the framework for the contemporary popular conception of the universe, including black holes. Its development was of enormous intellectual importance. It is a living legacy of Einstein.