What Is Gravity?
Gravity is why apples fall, why the Moon orbits Earth, and why galaxies hold together. But what is it, really? Newton described it with a neat formula — Einstein revealed it as a curvature in the fabric of space and time itself.
Gravity Is Everywhere
Gravity is a fundamental force of nature — one of just four (alongside electromagnetism, the strong nuclear force and the weak nuclear force). Unlike the other three, gravity acts over any distance, no matter how vast.
Every object with mass attracts every other object with mass. Right now, you and this screen are gravitationally attracting each other. The pull is vanishingly small because you're both tiny compared to, say, the Earth — but it exists.
Newton's Law of Gravitation
In 1687, Isaac Newton published his law of universal gravitation. The story of the apple may be embellished, but the insight was real: the same force that makes a ball fall also keeps the Moon in its orbit.
Where:
- F — gravitational force (in Newtons)
- G — the gravitational constant ≈ 6.674 × 10⁻¹¹ N m²/kg²
- m₁, m₂ — the masses of the two objects
- r — the distance between their centres
The inverse-square law (the r² in the denominator) means: double the distance, the force drops to one quarter; triple the distance, it drops to one ninth. Gravity weakens rapidly with distance but never reaches exactly zero.
How Orbits Work
An orbit is a continuous free-fall — but sideways. Imagine throwing a ball horizontally. Earth curves away beneath it as it falls, and if you throw it fast enough (about 7,900 m/s at Earth's surface), the ground curves away at exactly the rate the ball falls. The ball never lands. It orbits.
The Moon is falling toward Earth right now — it just keeps missing because it's moving sideways at about 1,022 m/s. Newton realised that the Moon's "fall" and an apple's fall were the same phenomenon.
Kepler's laws describe orbital shapes and periods, but Newton's law provides the deeper "why" behind all three.
Einstein's Revolution
Newton's law worked brilliantly for 230 years. But in 1915, Albert Einstein published the General Theory of Relativity, revealing a more profound picture: gravity is not a force at all. It is the curvature of spacetime.
Mass and energy warp the fabric of space and time around them — like a heavy ball resting on a stretched rubber sheet. Smaller objects (and even light) follow the shortest paths (called "geodesics") through this curved spacetime. What we perceive as gravitational attraction is actually objects following straight paths through curved space.
Consequences of Curved Spacetime
General Relativity makes predictions Newton's theory couldn't:
- Gravitational time dilation: Clocks tick slower deeper in a gravitational field. GPS satellites must correct for this constantly or navigation errors would accumulate at ~11 km/day.
- Light bending: Even massless photons follow the curved spacetime. During a solar eclipse in 1919, Eddington observed stars appearing shifted around the Sun — confirming Einstein's prediction.
- Gravitational waves: Accelerating massive objects send ripples through spacetime itself. In 2015 LIGO detected the merger of two black holes, 1.3 billion light-years away.
- Black holes: Enough mass in a small enough volume curves spacetime so severely that even light cannot escape — the event horizon.
Try It Yourself
You can play with gravitational simulations right in your browser:
- N-Body Gravity Simulation — Watch stars and planets interact under Newtonian gravity. Create stable orbits, binary stars, or chaotic three-body systems.
- Solar System — Realistic planet orbits with accurate mass ratios and orbital periods.
- Orbital Mechanics — Gravitational slingshots, Lagrange points and transfer orbits.