Is there a force called gravity?

What is gravity? Is it a force, a pseudo force, or something else? This question is being debated vigorously by physicists since 2015, the 100th anniversary of Einstein's Theory of General Relativity.

"Gravity really does exist," said Newton after the publication of his famous Universal Law of Gravitation in 1687. While the oft-told story of an apple falling on his head is apocryphal, what Newton did was to work out the mathematical law that governs the gravitational attraction between two bodies.

To make his concept of gravity universal, Newton needed a universe where everything needs to know where everything else is so that gravity can act with the appropriate amount of force. Accordingly, his concept of gravity is a beautiful synthesis between terrestrial and celestial phenomena that "abundantly serves to account for all the motions of the celestial bodies," reaching across the vast expanse of the universe. The law works with clockwork precession as long as the masses have low density and move slowly. For example, the Earth and other planets orbit the Sun because of the gravitational force exerted by the Sun.  

Despite the amazing success of the law, Newton was never completely satisfied with his theory. There are many anomalies it cannot explain satisfactorily. One of them is the mystery of "action-at-a-distance," a concept involving two bodies interacting with each other without being in physical contact. He even wrote to a colleague, telling him that any "competent thinker shouldn't believe his theory."

The others are: how is the force transmitted through space? How do planets know that they have to move in elliptic orbits around the Sun? Besides, unlike magnets that can attract as well as repel each other, why does gravity always pull?

Moreover, Newtonian gravity does not work well for objects with enormous mass density or objects moving with speeds comparable to the speed of light. It also cannot explain the observed bending of light. Nevertheless, how philosophically unsatisfying the theory is, gravity seems to be a force of some sort.

Enter Einstein into the equation. He trashed Newton and claimed that there is no such thing as "gravitational force." According to his Theory of General Relativity, published in 1915, gravity is a manifestation of the curving of the fabric of space-time by matter. Simply put, space―length, width, and height―becomes curved and time slows down in the presence of matter. The more the mass, the greater is the curving and slower the clocks will tick. This is why astronauts, who are moving very fast in space, age a little bit more slowly than people on Earth.

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As proof, former NASA astronaut Scott Kelly's historic 520 days in orbit aboard the International Space Station zooming around the Earth at 17,500 mph in 2015 to 2016 made his six minutes older twin brother, Mark Kelly, even older by five milliseconds.

To see the relationship between gravity and space-time curvature, consider a stretched rubber sheet. If we roll a marble across the sheet, it would move in a straight line, just like an object would move in the absence of gravity. Now place a bowling ball in the middle of the sheet. The sheet would become distorted, looking like a big bowl, or a valley ("gravity well") with the ball at the centre. The heavier the ball, the steeper the sides of the well. If we now release a marble on this sheet, we would see it curving toward the ball. The marble is responding to the curvature of the sheet produced by the ball. It is not attracted by the ball in any way. Replace the ball by the Sun and the marble by the planets. Voila! We have the planetary orbits.

More specifically, the marble's path would depend on the speed and direction with which it is released. If released relatively slowly and closer to the centre of the ball, it would follow circular or elliptic orbits, while marbles released further away or at higher speeds could loop around the centre in unbound parabolic or hyperbolic paths, such as those of comets.

The most conclusive evidence of space-time curving is the phenomenon of gravitational lensing―deflection of light by the Sun's gravitational curving of space, confirmed in 1919 by the British Astrophysicist Arthur Eddington during a solar eclipse. Newton's law has no provision for massless particles to feel the effect of gravity.

Next, why does gravity always pull us down and not push us up? Now imagine someone went under the rubber sheet and pushed it up, resulting in a "gravity hill." The ball would roll away!  As far as scientists know, matter always makes gravity wells, not gravity hills. If gravity hills do exist, then Earth would be pushing us out into space.

Gravitational energy across the universe is transported by gravity waves which are tiny ripples in the fabric of space-time travelling outwards from the gravitating sources at the speed of light, stretching the space in one direction and compressing in another direction. They were detected in 2015 at the Laser Interferometer Gravitational Wave Observatories at Louisiana and Washington in the US.

Although the problem of action-at-a-distance is not yet resolved, there are conjectures that the force is carried by yet-to-be-discovered massless particles called gravitons. In other words, when we throw something up and it comes back down, it is the gravitons that pulls it down.

So, can we now say that we know what gravity is? Frankly, at the most fundamental level, we still do not know what it is. Einstein probably comes closest to answering the question about gravity. It is not an ordinary force à la Newton, but rather a geometric property of space and time. In Einstein's own words, "Time and space and gravitation have no separate existence from matter. Space-time tells matter how to move, and matter tells space-time how to curve."

When the curvature is small, Einstein and Newton agree. What goes up must come down can be explained by both. As the gravitating masses increase, Einstein dethrones Newton and begins to rule the Universe. However, both theories fail miserably in the microscopic world.

Finally, Theory of General Relativity may be Orphic, but it is real; not a gimmick, or an abstract mathematical theory. It is at once simple and elegant, but also maddeningly non-intuitive. Thanks to Einstein, we live in a universe of curved spaces and altered time.

Dr Quamrul Haider is professor emeritus at Fordham University in New York, US.

Views expressed in this article are the author's own. 

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