What Is Gravity?
Gravity is the force that keeps your feet on the ground, the Moon in orbit, and galaxies from flying apart. It is the most familiar of the four fundamental forces — and the least understood at the quantum level.
Gravity is the fundamental force of attraction between all objects with mass or energy. On Earth's surface, it accelerates objects downward at approximately 9.81 m/s². Newton described it as a force proportional to mass and inversely proportional to distance squared. Einstein's general relativity reinterpreted gravity as the curvature of spacetime caused by mass and energy — objects "fall" because they follow curved paths through warped spacetime.
Two Descriptions of Gravity
🍏 Newton's Law of Universal Gravitation (1687)
Every object attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of the distance between them: F = Gm₁m₂/r². The gravitational constant G = 6.674 × 10⁻¹¹ N·m²/kg². This model works brilliantly for everyday calculations, orbital mechanics, and engineering.
🌌 Einstein's General Relativity (1915)
Mass and energy tell spacetime how to curve; curved spacetime tells matter how to move. Objects in free fall follow geodesics — the straightest paths through curved geometry. This explains phenomena Newton could not: the precession of Mercury's orbit, the bending of light around the Sun, gravitational time dilation, and the existence of black holes.
Key Facts About Gravity
- Weakest fundamental force: About 10³⁶ times weaker than electromagnetism. A small refrigerator magnet can overcome the gravitational pull of the entire Earth on a paperclip.
- Infinite range: Gravity extends across the entire observable universe, decreasing with distance but never reaching zero.
- Always attractive: Unlike electromagnetism (which can attract or repel), gravity only pulls objects together. There is no "anti-gravity" in known physics.
- Shapes the universe: Gravity forms stars, planets, galaxies, and galaxy clusters. It drives stellar fusion, creates tides, and determines the ultimate fate of the cosmos.
Gravity on Different Worlds
- Earth: g ≈ 9.81 m/s² (your normal weight)
- Moon: g ≈ 1.62 m/s² (1/6 of Earth — you could jump 6× higher)
- Mars: g ≈ 3.72 m/s² (38% of Earth)
- Jupiter: g ≈ 24.8 m/s² (2.5× Earth — you'd feel crushed)
- Sun's surface: g ≈ 274 m/s² (28× Earth)
- Neutron star: g ≈ 10¹² m/s² (incomprehensible — a marshmallow would hit with the energy of a nuclear bomb)
💡 Key concept
Gravity is not a force in general relativity — it is geometry. A falling apple is not being "pulled" — it is following the straightest possible path through spacetime that has been curved by the Earth's mass.
Unsolved Questions
- Quantum gravity: General relativity and quantum mechanics are incompatible. A complete theory of quantum gravity remains one of the biggest open problems in physics.
- Gravitons: The hypothetical quantum particle of gravity has never been detected.
- Dark energy: The universe's expansion is accelerating — an effect that opposes gravity on cosmic scales. Its nature is unknown.
Gravitational waves — ripples in spacetime predicted by Einstein in 1916 — were first directly detected by LIGO on September 14, 2015, from two black holes merging 1.3 billion light-years away. The signal stretched and compressed LIGO's 4 km arms by less than 1/10,000th the width of a proton.
People Also Ask
What causes gravity?
In general relativity, mass and energy warp the geometry of spacetime. Other objects then move along the curves in that geometry. At the deepest level, the mechanism connecting mass to spacetime curvature is described by Einstein's field equations, but why mass curves spacetime remains an open philosophical question.
Is gravity the weakest force?
Yes — by far. The gravitational attraction between two protons is about 10³⁶ times weaker than their electromagnetic repulsion. Gravity dominates at astronomical scales only because large objects are electrically neutral, so electromagnetic forces cancel while gravitational forces add up.
Can gravity be shielded or blocked?
No known material or method can shield against gravity. Unlike electromagnetic radiation, which can be blocked by conductors (Faraday cages), gravity penetrates everything. This is a consequence of it always being attractive and having spin-2 (in the hypothetical graviton model).
What is zero gravity?
"Zero gravity" (or microgravity) occurs in free fall — when an object and its surroundings accelerate together under gravity. Astronauts on the ISS experience microgravity not because there is no gravity (it is about 90% of surface value) but because they are in continuous free fall around Earth.
References and further reading
- Young, H. D. & Freedman, R. A. University Physics with Modern Physics, 15th ed. Pearson, 2019.