Gravity is the invisible force that keeps our feet on the ground, planets in orbit, and galaxies bound together. As one of the four fundamental forces of nature, gravity governs the motion of objects on Earth and across the cosmos. From a falling apple to the dance of distant stars, gravity is a universal force that shapes our reality.
Gravity is described by the general theory of relativity, proposed by Albert Einstein in 1915, which describes gravity in terms of the curvature of spacetime, caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing, not even light, can escape once past the black hole's event horizon. However, for most applications, gravity is sufficiently well approximated by Newton's law of universal gravitation, which describes gravity as an attractive force between any two bodies that is proportional to the product of their masses and inversely proportional to the square of the distance between them.
Gravity is the term used to describe a fundamental physical law, a physical interaction that primarily derives from mass, and the observed consequences of that interaction on objects. Gravity is the law that every object with mass attracts every other object in the universe in proportion to each mass and inversely proportional to the square of the distance between them. The force of gravity, F, is written using the gravitational constant, G, as
F= G. ( mm′)/ r^2
for two masses, m, and m′ separated by a distance r.
Gravity is considered one of the four fundamental forces.
The nature and mechanism of gravity were explored by a wide range of ancient scholars. In Greece, Aristotle believed that objects fell towards the Earth because the Earth was the center of the Universe and attracted all of the mass in the Universe towards it. He also thought that the speed of a falling object should increase with its weight, a conclusion that was later shown to be false. While Aristotle's view was widely accepted throughout Ancient Greece, there were other thinkers, such as Plutarch, who correctly predicted that the attraction of gravity was not unique to the Earth.
Although he did not understand gravity as a force, the ancient Greek philosopher Archimedes discovered the center of gravity of a triangle. He postulated that if two equal weights did not have the same center of gravity, the center of gravity of the two weights together would be in the middle of the line that joins their centers of gravity. Two centuries later, the Roman engineer and architect Vitruvius contended in his De architectura that gravity is not dependent on a substance's weight but rather on its "nature". In the 6th century CE, the Byzantine Alexandrian scholar John Philoponus proposed the theory of impetus, which modifies Aristotle's theory that "continuation of motion depends on continued action of a force" by incorporating a causative force that diminishes over time.
In 628 CE, the Indian mathematician and astronomer Brahmagupta proposed the idea that gravity is an attractive force that draws objects to the Earth and used the term gurutvākarṣaṇ to describe it.
In 1684, Isaac Newton sent a manuscript to Edmond Halley titled De motu corporum in gyrum ('On the motion of bodies in an orbit'), which provided a physical justification for Kepler's laws of planetary motion. Halley was impressed by the manuscript and urged Newton to expand on it, and a few years later, Newton published a groundbreaking book called Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). The revolutionary aspect of Newton's theory of gravity was the unification of Earth-bound observations of acceleration with celestial mechanics. In his book, Newton described gravitation as a universal force and claimed that it operated on objects "according to the quantity of solid matter which they contain and propagates on all sides to immense distances always at the inverse square of the distances". This formulation had two important parts. First was equating inertial mass and gravitational mass. Newton's 2nd law defines force via F=ma for inertial mass; his law of gravitational force uses the same mass. Newton did experiments with pendulums to verify this concept as best he could.
Einstein's general relativity redefines gravity not as a force, as Newton described, but as the curvature of spacetime caused by mass and energy. Massive objects, like planets or stars, warp the fabric of spacetime around them. This curvature tells objects how to move, creating the effect we perceive as gravity. For example, the Sun’s mass curves spacetime, causing Earth to orbit in a path along that curvature. The effects of gravity are equivalent to acceleration. For instance, being in a rocket accelerating at 9.8 m/s² feels the same as standing on Earth’s surface under gravity’s pull. Objects in free fall follow the shortest paths (called geodesics) in curved spacetime. This explains why planets move in elliptical orbits or why light bends near massive objects. Time runs slower in stronger gravitational fields. For example, a clock near a black hole ticks more slowly than one farther away. In 1971, scientists discovered the first-ever black hole in the galaxy Cygnus. The black hole was detected because it was emitting bursts of X-rays as it consumed a smaller star, and it came to be known as Cygnus X-1. This discovery confirmed yet another prediction of general relativity, because Einstein's equations implied that light could not escape from a sufficiently large and compact object.
On Earth, gravity is always present. It causes objects to fall, keeps the atmosphere in place, and drives the tides through the Moon’s gravitational pull. Engineers rely on gravity when designing bridges, buildings, and vehicles, ensuring stability against its constant pull. Earth’s force of gravitation is 9.8 m/s^2 as measured. Our universal mass is constant, but our weight is not. This is due to the force of gravitation. A person’s weight on Earth and the Moon may not be the same because both of these bodies have different gravitational forces. Moon’s force of gravitation is 1/6th of the Earth’s. Hence, a person may feel lighter on the Moon’s surface than on Earth.
Gravity acceleration is a vector quantity, with direction in addition to magnitude. In a spherically symmetric Earth, gravity would point directly towards the sphere's centre. As the Earth's figure is slightly flatter, there are consequently significant deviations in the direction of gravity: essentially the difference between geodetic latitude and geocentric latitude. Smaller deviations, called vertical deflection, are caused by local mass anomalies, such as mountains.
Scientists are still unraveling gravity’s secrets. At the quantum level, gravity remains incompatible with quantum mechanics, prompting research into theories like string theory and loop quantum gravity. Additionally, dark matter and dark energy, mysterious entities that influence gravity and the universe’s expansion, pose significant questions. Dark matter strengthens gravitational effects in galaxies, while dark energy accelerates cosmic expansion, counteracting gravity. In conclusion, Gravity is more than a force; it’s a fundamental principle that connects the smallest objects to the vastness of the universe. From Newton’s apple to Einstein’s spacetime, our understanding of gravity has reshaped science and technology. As researchers probe deeper into its mysteries, gravity continues to inspire awe and curiosity, reminding us of the intricate forces that bind our universe together.
Sources:
10.5: The Source of Gravity - Physics LibreTexts
(2) If light has no mass, why is it affected by gravity? General Relativity Theory - YouTube
(2) General Relativity Explained simply & visually - YouTube
-By Apoorva Wayse