Gravity

Gravity (also known as gravitation) is the force of attraction that acts between any objects with mass. It is one of the four fundamental forces in physics. It is the weakest of the fundamental forces when compared with the others, but it acts over vast distances and accumulates, thereby shaping the structure of the universe: the Earth, the Moon, the stars, and galaxies.

Gravity is why falling objects come to rest on Earth, why the Moon orbits the Earth, and why the Earth orbits the Sun. It also plays a major role in ocean tides and the weight of bodies.

Newton's Explanation

In the 17th century, Isaac Newton formulated the universal law of gravity. According to this law, two bodies attract each other with a force proportional to the product of their masses and decreasing as the square of the distance between them — specifically, the force decreases with the inverse square of the distance. The standard equation is F = G·m₁·m₂/r², where G is the gravitational constant.

General Theory of Relativity

In 1915, Albert Einstein put forward the general theory of relativity, which explains gravity not as a direct force acting across distance, but as the curvature of spacetime caused by mass and energy. Objects follow the straightest possible paths through this curved spacetime. This theory accounted for phenomena that Newton's framework could not, such as the bending of light as it passes near a massive object and the precession of Mercury's orbit.

Gravity on Earth

At Earth's surface, the acceleration due to gravity (g) is approximately 9.8 meters per second squared (m/s²). This value varies slightly from place to place — it is slightly greater at the poles than at the equator, because of Earth's shape and rotation. It also decreases with altitude.

Modern Research

In 2015, gravitational waves were detected for the first time — ripples in spacetime produced by massive events such as the collision of black holes. An outstanding challenge remains the reconciliation of gravity with quantum mechanics; a unified theory has not yet been achieved.