Year 2015 is the centenary year of relativity – especially general relativity. The, was put forth in two parts. The first one, Special Relativity, came out in 1905 and the second, General Relativity, in 1915.

However, relativity was not a new concept even before 1905. Even in Galileo’s time, the relativity principle saw the daylight, but Einstein’s relativity was quite distinct from that of Galileo. Einstein’s Relativity included relativity of time besides velocity. He could do this very minutely because he took into consideration transit time of light from any object to the observer, which Galileo did not.

A very simple example illustrates the relativity of time as per Einstein’s Relativity. Imagine two distinct stars – A and B – at distances which light takes two and three hours to travel from them, respectively, to the observer on the earth (light travels at a speed of 3,00,000 km per second).

If the observer looks at them through a telescope simultaneously at 10 am, do you think that he is observing their states at the same time? No. He is observing the state of A at 8 am and of B at 7 am. Though for the observer, he watches the stars simultaneously, the states of the two stars are not simultaneous really.

Einstein also stressed the fact that the speed of light in vacuum is absolute. It remains constant whichever fast moving platforms it is sent from. For example, light sent from a rocket moving at a speed of 5000 km per second, still remains the same (It was known in Galileo’s time that light travels with a definite velocity. But, it was thought to be so high that the time of transit was considered negligible).

Einstein also put forth one very important fact that no physical phenomenon for effect can travel faster than light. This means that it takes a definite time for gravitational force to be experienced between two bodies kept at a large distance from each other. This time is the time for light to travel between the two bodies. For example, if a star is born now at a distance such that it takes 12 hours for the light to reach it from here, we will experience its attraction only 12 hours after its time of creation. Similarly, electrical and magnetic forces also are time delayed.

The essence of Special Relativity is the space-time equivalence as per which any length can be expressed in terms of time to travel it. Just like the equivalence of two different currencies. So, the location of any event or any body can be specified accurately not only by its position but also by the time light takes to travel up to the body. The most important inference of Einstein is the conversion of mass into energy (E = mc2). The release of nuclear energy whether by fission or fusion is the immediate fallout of the above equation.

Phenomenon of gravity

The General Relativity phenomenon of gravitation was not taken as a distinct property of matter but of the geometry of space-time. For example, when you throw an object from the ground at an angle, the object follows a parabolic curve and falls to the ground. Similarly, while travelling in a bus, when the bus negotiates a sharp turn, you are likely to be thrown out from the seat along a curvilinear path. These two phenomena are, by and large, due to gravitation and we see a geometrical curve in them.

Einstein applied geometry of space-time to Newton’s gravity and boldly proclaimed that the force of gravity is like any other force caused by acceleration. This along with the time dimension and one more space dimension is the starting point of Einstein’s equation in general relativity.

Astrophysics and cosmology took birth and new chapters were added. Black holes and white holes were the by-product of this intellectual exploration. During the World War I itself, the supremacy of General Relativity was established by calculating drift in the path of a planet, which agreed very accurately with this observation.

Besides being a scientist, Einstein was a peace lover. During the Nazi regime, he openly canvassed for the non-cooperation of scientists with the Nazis who were about to destroy the peace of the world. He was forced to leave Europe for the United States in the 1930s due to the warmongering Nazis and join the establishment of Princeton University.

Efforts by the US in nuclear technology was started with his blessings but the classified secr-ets of the Manhattan project was kept far from him and the bom-bing of Japan was done without his knowledge. He declined when he was requested to take over as the president of the newly formed state of Israel in 1949.

When Mahatma Gandhi was assassinated, Einstein responded spontaneously (not verbatim), “Future generations would wonder whether such a man existed on earth!” Our and future generations, similarly, knowing well that spirituality of Einstein behind this thinking will marvel whether such a scientist existed on earth!

(The writer is retired Professor of Physics, National Defence Academy, Khadakwasla, Maharashtra)