**Dr Irengbam Mohendra Singh**

This piece could be a way of evoking a parenthesis of balance in the incredibly disorientating theories of gravity. Physicists say gravity is the weakest force among the four physical forces in nature. That’s why we are able to lift our feet to walk. We believe them. But some scientific theories change over time, not necessarily because they are wrong.

For two centuries, an Englishman Isaac Newton was the undefeated master of classical physics. He and his contemporary French philosopher and mathematician Gottfried Leibniz were considered the greatest intellectuals of the 17th century. They often argued as to who discovered Calculus first. In fact ,they discovered it independently.

Newton had an edge over Leibniz more as a physicist with his theory of gravity, space and time, and the three laws of motion that remained unchallenged for two centuries. Leibniz was better known as a mathematician and philosopher.

I was quite happy with Newton’s theory of gravity in my school, though he didn’t explain what is the force that pulls objects to each other. I’m still happy with it, though Einstein’s new theory General Relativity has made a mess of his theory of gravity. I can understand how big objects like the planets and stars bend Einstein’s spacetime, but , I can’t quite fathom how Newton’s apple or a sewing needle falls to the ground by bending spacetime or that, you and I are standing on Earth by bending spacetime continuum, like a marble automatically rolling down a funnel. It sticks in my craw.

I think Newton’s theory of gravity is still applicable, say, in the intense singularity inside a Black Hole where there is no Einstein’s spacetime continuum. I am sure, we need a new law of gravity that combines Newton’s and Einstein’s theories of gravitation together. Newton constructed his Law of universal gravitation and published it in 1687. The law states that a particle attracts every other particle in the universe using a force (he didn’t know what, and left it to a metaphysical agent) that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Newtonian physics explains the theory of space, time and motion. Newton considered space distinct from body and that, time passes uniformly unhindered by anything in the world. So he regarded space and time as absolute, though he didn’t regard space and time as genuine substances.

His theory remained the basis of physical science until the beginning of 20th century, when Einstein changed the gravitational part of his theory.

Newton discovered his law of gravity by studying the apparent complexity of the motions of the planets around the Sun. He constructed a simple equation that is capable of explaining a wide range of phenomena in the universe, such as that, the Earth is spherical because it is formed by the action of gravity, and because of gravity, planets orbit around the Sun. His Second Law of Motion is expressed by the equation: F = ma. Force is equal to mass multiplied by acceleration that relates to gravity. His gravitational force can easily explain how our solar system was formed from collapsing cloud of gas and dust after the Big Bang, because of their gravitation pull. The Sun formed first, followed by the Earth and other planets.

Newton’s Gravitational Constant (GC) is one of the fundamental physical constants. It describes a property of our Universe that can be measured. It’s an empirical physical constant, used in calculations of gravitational effects, by Newton in his Law of Universal gravitation, as well as by Einstein in his General theory of relativity.

Newton’s gravity, in lay person’s day to day experience, keeps us on the ground, preventing us from flying off to somewhere in space. Australians stand upside down and the Indian Ocean hugs the Earth, and the Pacific ocean does not fall off. It’s the gravity of the Earth why rain falls and we have atmosphere containing oxygen to breathe. If we kick a football into the air, the length of time it will travel depends on a competition between the force with which it is kicked and the gravitational pull of the Earth.

We know a medium sized star like our Sun, collapses and dies due to gravitation pull inside it as the nuclear fuel in its core runs out. I can use Newton’s gravity here, but I can’t bring Einstein’s spacetime to work inside of the star. It’s something quite weird that an astronaut walking on the moon bend spacetime much less than when he walks on the surface of the Earth. Einstein’s General Relativity can explain why everything falls at the same rate, and why gravity can be removed by the act of falling. That may be true, but it does not explain everything.

Newton’s gravity plays a very vital role in our Universe, from the smallest dust to the massive stars, and for you and me, on this Earth. Because of the Earth’s gravitational force the Earth moves around the Sun in an orbit, receiving its energy. It is the Earth’s gravity that gives us weight, which is much lighter on the surface of the moon as it has weaker gravity, not because of less pliable spacetime.

It’s the Earth’s gravity that made Newton’s apple fall to the ground. It’s the gravitational pull of the moon towards it, which causes ocean tides. Gravitation creates stars and planets like ours, by pulling together from matter that was created after the Big bang. It’s because masses attract each other. Anything that has mass has also gravity. The gravity of Earth is caused by its mass and all its mass gives a combined gravitational pull.

Albert Einstein changed all these by postulating that, gravity is a curvature in spacetime that wraps around an object, such as a star or a planet. And, if an object is nearby, it is pulled into the curvature spontaneously by rolling down like a marble in a funnel. Einstein also discovered that gravity pulls on light and time.

A Black Hole packs so much mass and so tightly because of their combined gravity that even light cannot escape, and so it’s very black. A few weeks before Einstein presented his general relativity papers to the Prussian Academy in 1915, astronomers detected the presence of Black Holes throughout the cosmos. Einstein didn’t believe they would exist. He was wrong.

Karl Schwarzschild, a World War I veteran and a German astronomer, who worked out solutions to Einstein’s complicated equations for the spacetime geometry around a massive sphere, described the first mathematical step towards describing black holes in space. Unfortunately he didn’t pursue the topic as he died shortly after a skin disease. In late 1960s, black holes emerged again, as part fiction, part science, and as an offshoot of general relativity.

Potholes on Newton’s theory of gravity began to emerge in 1859 when astronomers discovered, well before general relativity, that the orbit of Mercury doesn’t quite conform to the path predicted by Newtonian gravity, unless it was caused by another nearby planet. Howsoever they tried, they couldn’t find another planet, whose gravity could cause deviation in Mercury’s orbit. The orbit of Mercury is an ellipse with a long axis that points in a particular direction.

In Newtonian gravity, the direction of the axis does not change but general relativity predicts that it changes by 43 arc-seconds per century. Einstein used his new theory of gravity to calculate Mercury’s orbit and found that it explained the discrepancy precisely. That helped to convince other physicists that his theory was correct and the Newtonian theory was wrong. However, Einstein’s special relativity does not apply everywhere. It fails at singularities like the centre of black holes. Singularity is an event in which fundamental physical quantity such as the density of matter or the strength of the gravitational field becomes infinite. This may occur in a region of space in which the gravitational attraction of a star has caused it to collapse to infinite density as in the black hole.

Einstein differed from Newton in that Newton didn’t believe space was affected by objects in it, but Einstein did. Einstein theorised that a mass like a planet can warp it, bend it, push it or pull it. Newton described gravity with a constant gravitational force. Einstein thought gravity was just a natural outcome of a mass’s existence in space. Further, his General Relativity did not explain everything but explained something Newton didn’t.

Einstein’s gravity in the black holes is made more confused by another weird theory, the “Cosmological Natural Selection (CNS) theory, proposed by an American theoretical physicist Lee Smolin in his book, The Life of the Cosmos (1997). It’s all about Darwin’s biological evolutionary ideas of “reproduction” and “mutation” that are applicable to our Universe. He says black holes have a role in natural selection. It causes the emergence of a new universe on the “other side”, whose elementary particles may differ slightly [mutation] from those of the universe where the black hole collapsed. Each universe thus gives rise to as many universes as it has black holes.

I think Newtonian gravity still works in small things, while Einstein’s general relativity extended the range to include high gravitational field.

(The writer is based in the UK. Email:[email protected] Website: www.drimsingh.co.uk)