Gravity’s secrets: From Newton’s apple to black holes, it is still a mystery

Yet, gravity never entered human consciousness till the 17th century. Until the great Isaac Newton introduced one of the most profound ideas in science — that the same force pulling apples to the ground keeps the Moon in orbit around the Earth.

Newton himself came up with this anecdote about it,  and it has grown with retellings. The story has grown with retelling.  Around 1666, as he sat in the garden of his family home in Lincolnshire, he saw an apple fall from a tree. That prompted him to wonder: if an apple falls to the ground, might the Moon be falling too, perpetually pulled toward the Earth but kept aloft by its sideways motion?

From this thought came the law of universal gravitation: every object in the universe attracts every other with a force proportional to their masses and inversely proportional to the square of their distance. With one sweeping insight, Newton explained both earthly and celestial motions.

If I have seen further it is by standing on the shoulders of giants:

Isaac Newton once wrote to Robert Hooke. It was this humility which led him to figure out one of the fundamental forces of nature

Einstein and the fabric of spacetime

For two centuries, Newton’s law reigned supreme. But it could not explain certain anomalies, like Mercury’s wobbling orbit. In 1915, Albert Einstein transformed our understanding with his general theory of relativity. Gravity, he argued, was not a force pulling objects together but the bending of spacetime itself by mass and energy. Planets move as they do because they are following the curves of this cosmic fabric.

A useful way to picture this is to imagine spacetime as a stretched fabric — like a sari pulled taut. Place a heavy tennis ball on it, and the cloth dips, pulling smaller objects nearby toward the depression. In much the same way, massive bodies like stars and planets warp spacetime, guiding the motion of everything around them.

When Einstein presented his theory in Berlin, even seasoned physicists were stunned. The mathematician David Hilbert, who nearly beat him to the equations, reportedly remarked that such a union of physics and geometry was a work of “the highest artistry.”

As physicist John Wheeler later summed it up:

Spacetime tells matter how to move; matter tells spacetime how to curve.

Gravity is a weak force

Unlike the electromagnetic force or the strong nuclear force, gravity is universal: it acts on all matter, across all distances, and it never switches off. But, despite ruling the cosmos, gravity is astonishingly weak compared to the other forces of nature. To see this clearly, physicists compare how different forces act on the same particles.

Take a proton and an electron: the electromagnetic force of attraction between them is about  times stronger than their gravitational attraction. That’s why atoms, molecules, and chemistry are governed by electromagnetism, while gravity is irrelevant on such small scales.

This extreme weakness has profound implications. Gravity only becomes dominant when enormous masses are involved — stars, planets, galaxies. At the particle level, its effects are drowned out by other forces. And this is why physicists have found it so difficult to quantize gravity. The graviton – the particle hypothesised to mediate the force of gravity — if it exists, would interact so faintly that no experiment could possibly detect it with today’s technology.

Gravity’s weakness may be the very reason it is both the grand architect of galaxies and the last fundamental force resisting our attempts at unification.

From falling apples to black holes

Gravity governs everything from the everyday to the extreme. It shapes the planets, guides comets, and binds galaxies together. But in the densest corners of the cosmos, it creates exotic objects like neutron stars and black holes — places where spacetime bends so strongly that not even light can escape.

Einstein’s theory predicted these monsters, though he himself doubted nature would allow them. Today, telescopes like the Event Horizon Telescope have imaged a black hole’s shadow, confirming that Einstein’s equations still hold in the most extreme conditions we can probe.

What began as an apple’s fall now informs climate science and planetary defense. But despite its central role, gravity remains the least understood of the fundamental forces. Physicists have unified electromagnetism with the weak force, and hope to one day unify all forces into a “theory of everything.” Yet gravity resists quantization. Efforts like string theory and loop quantum gravity are attempts to bridge this gap, but no experiment has yet revealed how gravity works at the tiniest scales.

The Last Word: Gravity is still a mystery

From Newton’s orchard to Einstein’s curved spacetime, gravity has reshaped our understanding of the universe. And yet, it remains elusive at its deepest level. Newton himself admitted his discomfort with its strangeness:

That one body may act upon another at a distance through a vacuum… is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it.

Gravity is both the most familiar and the most profound of nature’s forces — the invisible thread stitching together the story of the cosmos.

Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.