Gravity isn’t a “pull” — it’s curved spacetime

In General Relativity, mass and energy change the geometry of spacetime. Objects move along the “straightest possible paths” in that curved geometry (called geodesics).

Goal: Build an intuitive picture using a simple “ball on a sheet” visual — then explain what it gets right, and what it gets wrong.
Spacetime Curvature Geodesics Orbits Light bends too

One-sentence summary

Matter tells spacetime how to curve, and curved spacetime tells matter how to move.

Accessible reading tip

The core idea (in plain language)

Newton’s picture

Gravity is a force pulling two masses together. It acts at a distance.

This works extremely well in everyday life — and even for many space missions.

Einstein’s picture

Gravity is what “straight-line motion” looks like when spacetime itself is curved.

Planets orbit because they’re following straightest paths through curved geometry, not because a mysterious pulling force reaches out through space.

Diagrams: flat space vs curved space

These are simplified diagrams similar to what many short videos show: a flat surface with a grid, then a heavy ball “denting” it to represent curvature.

1) “Flat spacetime” (no mass nearby)

Flat space diagram: evenly spaced grid A rectangular grid with straight, evenly spaced horizontal and vertical lines, representing a flat geometry. Straight lines stay straight
In a flat geometry, “straight” paths don’t curve toward anything.

2) “Curved spacetime” (mass/energy present)

Curved space diagram: grid distorted toward a central ball A grid whose lines bend inward toward a central shaded circle representing a heavy mass, illustrating how mass is associated with curvature in a common analogy. Mass / energy Example “straightest path”
The grid is distorted around mass. Objects follow the straightest paths available in that geometry.

Limits of the “ball on a sheet” analogy

In the demo, balls roll inward partly because Earth pulls them “down” the dent. In real general relativity, there isn’t a hidden downward direction — the curvature is the story.

The sheet is a helpful picture, but it’s an “embedding diagram.” The real geometry involves time as well, and time curvature is essential for explaining many effects.

A key idea: even light bends (its path curves) near massive objects — not because light has mass, but because the geometry it travels through is curved.

Quick recap

1) Mass changes geometry

More mass/energy ⟶ more curvature.

2) Motion follows geometry

Free-fall is “straightest possible motion” in curved spacetime.

3) Orbits are not “pulling”

An orbit is a geodesic-like path around a curved region.

Back to diagrams Back to the idea

General Relativity

A responsive, accessible gallery of images. Select a thumbnail to view it larger.

Credited to Chris Ferrie.

Tip: Tab through images, Enter/Space to open.

This is a ball.

The ball has mass.

More mass. Less mass.

Mass warps space.

Less mass, less warp.

More mass, more warp.

It wants to go here.

But it can't.

It must follow the shortest path through curved space.

The ball is spinning.

Mass drags space.

Space drags mass.

This is flat space from a different angle.

See mass warp space from this angle.

Shrink a large mass enough to make it a black hole.

A black hole is a large amount of mass in a relatively small area.

A black hole has so much mass that not even light can escape its warp.

The center of a black hole is called a singularity.

Two black holes can spin around each other.

They send ripples though space called gravitational waves.

These waves stretch and squish space throughout the universe.

Know you know General Relativity!