Summer 1666, a young Cambridge University physics student by the name Isaac Newton was sitting under an apple tree in his mother’s garden at Woolsthorpe Manor in Lincolnshire, England. An apple fell from the tree on the ground and that triggered him to think: why did the apple come down straight to earth, not go sideways or upwards? That question led him to delve deeper into the mystery of attraction between two bodies and to come up with the law of gravity. He published his research work in “The Principia Mathematica” in 1687, where he described, among other things, this seminal work on the law of universal gravitation.

This law of gravitation tells us that two bodies attract each other with a force which is proportional to the product of the masses of the bodies and inversely proportional to the square of the distance between them. This simple empirical formula was astonishingly successful in calculating the force of attraction between two bodies on earth. This law was also applied to calculate the attractive force between the earth and the moon and to the orbital motion of the moon round the earth. The law was quite accurate in defining the orbits of many other celestial bodies, although in few cases the law was somewhat inaccurate.

This law was and still is the centre piece of what is now known as the ‘classical physics’ or ‘Newtonian Physics’. We all studied this law, Newton’s laws of motion, properties of matter, electricity and magnetism, heat and thermodynamics, optics etc in our schools and they gave the grounding for advanced physics.

For nearly 300 years this law was supreme and explained how the force of gravity controls the motions of all celestial bodies. However, the law did not say anything about the nature of this force or how the force is propagated through space; it just stated that the force diffuses through space without leaving any trace. As the predictions of the law were right in most of the cases, nobody bothered too much about these minor details or even ignored some minor discrepancies.

At the turn of the 20^{th} century, a patent clerk by the name Albert Einstein was working on patent submissions of electrical devices in Bern, Switzerland as a ‘Technical Expert, Third Class’ and in his spare time he was working on gravitational problems. The Patent Office work took up 48 hours of his time per week over six days. Einstein described his work load subsequently as, it left him with ‘eight hours for fooling around each day and then there is also Sunday!’.

In 1905 he published a technical paper outlining the Special Theory of Relativity giving revolutionary scientific ideas and concepts. In this paper, he introduced two fundamental concepts: the principle of relativity and the constancy of speed of light. The speed of light was stated to be independent of the speed of the observer. In other words, whether the observer moves in the direction of light or opposite to it, he would see the speed of light always remaining constant (c= 3*10^{8} m/s). (It may be mentioned that in 1905, Albert Einstein produced three more monumental papers of enormous significance: (i) the mass-energy equivalence (E=mc^{2}), (ii) Brownian motion of small particles, and (iii) photoelectric effects. For his work on photoelectric effects showing the particulate nature of light, which laid the foundation for quantum mechanics, he was awarded Nobel prize in 1921. As mentioned above, the year 1905 was extremely productive for Albert Einstein.

Einstein developed his relativity concept even further and produced the General Theory of Relativity in 1915. In this General Theory of Relativity, he advanced the principle of spacetime, not space and time. He stipulated that the three dimensions of space (such as X, Y and Z dimensions of cartesian coordinates) and one dimension of time are not independent of each other, but intricately linked to form a single four-dimensional spacetime continuum.

This concept of spacetime continuum was revolutionary at that time and even now they make human beings baffled. The relativistic consideration has produced what is now called the time dilation. The passage of time is relative and so it depends on the motion of an observer relative to a stationary observer. Also, the passage of time depends on the location in a gravitational field. For example, a clock attached to an observer in a spaceship will tick slower than that of a clock attached to an observer in a stationary position. Also, a clock in a higher gravitational field, such as at the surface of earth, will tick slower than that of a clock in lower gravitational field such as the top of a mountain.

Let’s take an example. There were three men in the UK, all of them exactly of the same age, say, 25 years. They decided to offer themselves as guinea pigs for a research on gravity. One was asked to stay in Lincolnshire, England (not too far from Newton’s famous apple tree), the other was told to go and live high up in the Himalayan mountains and the third, most adventurer of the lot, got the opportunity to have a space travel in a superfast spaceship. The spaceship travelled fast and so his clock was ticking slowly. Let’s say his spaceship was so fast that one year in the spaceship clock was equal to five earth years and the mountain man clocked 10 minutes more than the Lincolnshire man in five years. When after five years they met on earth, they found that the mountain man was 10 minutes older than 30 years, Lincolnshire man was 30 years of earth age and the spaceship man was whopping four years younger than 30 years! To the spaceship man, it would look like he had come back four years in the future!

Einstein stipulated that the gravitational field creates space and the bodies with masses bend and warp space; more like massive bodies create curvature in a trampoline. All less massive bodies fall into the curvature in the trampoline created by the massive bodies. When there are a large number of bodies warping the space, the space becomes jagged and celestial bodies move around in tortuous paths. There is no force of gravity pulling objects towards each other; just the bodies move around the jagged curved space along the path of least resistance.

Space, like any other force field, is discreet, quantised and granular. The quantum of space is dubbed as graviton, similar to the term photon in electromagnetic field, and it is so small that we cannot feel its discreteness, as we cannot feel the discreteness of photons of light or discreteness of atoms in a solid body.

Exactly hundred years after Einstein’s General Theory of Relativity, experimental evidence of gravitational field and gravitational wave have been produced by the LIGO (Laser Interferometer Gravitational-wave Observatory) experiment and shown that space is modulated by the gravitational field. A monochromatic laser beam of light was split and sent at right angles to each other along two arms, each of 4 km long. These beams were reflected back along the same path and allowed to interfere back at source. If there is no distortion or modulation of the path lengths, the two beams would interfere in anti-phases and there would be no interference patterns.

When two super massive black holes some 1.3 billion light years away merged and produced a gigantic massive black hole, an enormous amount of energy, equal to three solar masses, was produced and sent out as gravitational energy. It rippled through the whole universe in the form of gravitational wave at the speed of light and deformed the spacetime fabric. That deformation in spacetime was detected by the LIGO experiment in the form of interference pattern and that proves that gravitational waves modulates the space.

The implication of Einstein’s spacetime is that at the very beginning when even the ‘Big Bang’ did not take place, there was no spacetime. Spacetime came into existence following that ‘Big Bang’, when gravity came into play along with other forces such as electrical force, strong nuclear force and weak nuclear force. If at the end, as Physics predicts, the whole universe starts to collapse, there would be what is called the ‘Big Crunch’ and the spacetime would collapse too and disappear. There will be nothing, no material, no space and no time. These are the predictions of scientific theories as exist today.

In 1930 when Einstein came to London as the guest of honour at a fundraising dinner to help the East European Jews, George Bernard Shaw, the chief guest, said humourously, “Ptolemy made a universe which lasted for 1400 years. Newton made a universe which lasted for 300 years. Einstein has made a universe, and I can’t tell you how long that will last.” The audience laughed loudly, but none louder than Einstein.

**Dr A Rahman is an author and a columnist**