The Earth attracts the apple, but the question is: how does the Earth know that there is an apple?
The idea that two bodies, widely separated with nothing between them except a vacuum, still attract each other is not easily digestible. Newton called it “action at a distance”, but how and why this force acts between them without any delay is so troubling that Newton himself found his idea to be philosophically indefensible. However, his equation could explain planetary motion nicely, so it was accepted without much fuss.
There were, however, some cracks in Newton’s explanation of gravity. The first appeared in 1895, when the French astronomer Urbain Le Verrier, while observing the motion of the planet Mercury, noticed that Mercury’s elliptical path is not fixed but that the point closest to the Sun precesses very slowly. This precession of Mercury’s orbit cannot be explained by Newton’s law of gravity.
According to Albert Einstein (1905), nothing can travel faster than light, neither matter, nor energy, nor any kind of information. The speed of light is not just a speed; it is a limit woven into the very structure of reality itself. The distance between the Earth and the Sun is so vast that if the Sun were to cease to exist now, its effect on the Earth would be observed only after eight minutes, the time taken by light to travel. However, according to Newton, the force of gravity acts instantaneously, so the extinction of the Sun should be felt on Earth immediately.
Suppose you are in a closed room with no windows, in deep space, where there are no stars, planets, or any other objects around. With no source of gravity, you would feel weightless; everything in your room would be floating. You would have no idea which side is up or down; every direction would be equal. Now, suppose a rocket engine fitted to your room starts up and pushes the floor with the same acceleration you are used to feeling on Earth. You would now feel your weight, and everything in the room would appear to be falling downwards. You would not be able to distinguish between the Earth’s gravity and the acceleration of your room. This is called the equivalence principle by Einstein.
According to Einstein, gravity is not a force; it is geometry. A body produces a curvature in the space-time fabric, and objects moving within this curved space-time follow its shape. All bodies, whether a steel ball or a feather, must travel along the same curved path. This explains Newton’s observation that all objects fall with the same acceleration when air resistance is ignored.
A laser shot from one wall would hit the opposite wall at the same height. However, in the accelerated room, during the time taken by the light to travel from one wall to the other,
the room would have moved upwards. As a result, the laser beam would hit at a lower point. From your perspective, the light appears to bend due to gravity. Since mass curves space-time, the greater the mass, the greater the curvature, and light travelling through space bends towards the mass.
This idea, that light bends due to gravity, was experimentally confirmed in 1919 by observing the bending of light during a total solar eclipse, led by Arthur Eddington and Frank Watson Dyson. This gave strong support to Einstein’s theory. It also explains the anomalous orbital behaviour of Mercury.
Of all the forces in nature, gravity is the weakest, so weak that the gravitational force of the entire Earth cannot even snatch a pin from the grip of a magnet. Yet it governs the entire universe and remains the least understood by physicists. Anyone who can fully explain gravity will surely win a Nobel Prize.