Physics Engines

Overview

Physics engines handle collision detection, rigid body dynamics, joints, ragdolls, cloth, and other physical simulations that modern games require. Early games implemented physics case-by-case — Pong had ball-and-paddle collision; Pinball Construction Set (1983) had remarkably good ball physics for its era; Marble Madness (1984) handled isometric ball-on-slope dynamics. By the late 1990s the physics-as-middleware market emerged, with Havok (1998), MathEngine (1998), and PhysX (originally Ageia, 2005) competing for licences.

This entry surveys the landscape. For specific engines see Havok; for ragdolls see Ragdoll Physics; for collision see Collision Detection.

Fast facts

Purpose: Simulate physical behaviour — Newtonian rigid bodies, soft bodies, fluids, cloth, articulated characters.
Core components: Collision detection, dynamics, constraint solving.
Major middleware: Havok, PhysX, Bullet, Jolt, Box2D, Chipmunk.
Built-in: Unity (PhysX-based + DOTS), Unreal (Chaos), Godot (Jolt), all modern engines.
Performance: Single-thread → multi-thread → GPU-accelerated → modern multi-threaded SIMD.

Physics types

Type	Simulation domain	Examples
Rigid body	Solid, non-deforming objects	Falling boxes, stacked crates, tumbling barrels
Soft body	Deformable objects	Jelly, sponge, deformable terrain
Ragdoll	Articulated characters	Death animations, body reactions
Cloth	Fabric simulation	Capes, flags, banners, character clothing
Rope / chain	Chain-of-segments	Rope swings, vehicle tow lines
Fluid	Water, smoke, particles	Pool water, smoke effects, blood spatter
Particle	Small mass-points	Sparks, debris, weather
Vehicle	Car / bike / aircraft dynamics	Specialised modules in most engines

Major physics engines

Engine	Type	Origin	Notable for
Havok Physics	Commercial	1998, Dublin	Half-Life 2, Skyrim, Halo 3, dozens of AAA
PhysX	Commercial → free	Ageia 2005, NVIDIA 2008	Unreal Engine 4 default, Borderlands, Witcher 3
Bullet	Open source	Erwin Coumans, 2003	GTA V, Red Dead Redemption 2, Blender, Maya
Box2D	Open source 2D	Erin Catto, 2007	Angry Birds, countless 2D indies
Chipmunk2D	Open source 2D	Scott Lembcke, 2007	Crayon Physics, Limbo, indie 2D
Jolt Physics	Open source	Jorrit Rouwé, 2022	Horizon Forbidden West, Godot 4
MathEngine	Commercial (defunct)	1998	Early competitor to Havok
ODE (Open Dynamics Engine)	Open source	Russell Smith, 2001	Education, robotics, early indie
Newton Game Dynamics	Open source	Julio Jerez, 2001	Mid-2000s indie

Integration approaches

How games use physics:

Approach	Detail
Middleware licence	Pay per platform / per game (Havok, original PhysX)
Engine-integrated	Unity, Unreal, Godot bundle physics
Open source	Bullet, Box2D, Jolt — free for any use
Custom	Bespoke physics for specific gameplay (Trials, Worms, racing games)
Hybrid	Middleware for general physics + custom for specific systems

What an engine does each frame

Simplified physics-engine update loop:

each frame:
    1. Update transforms from animation / scripts
    2. Apply external forces (gravity, wind, scripted impulses)
    3. Broad-phase collision detection (quick rejects via spatial hashing)
    4. Narrow-phase collision detection (precise contact generation)
    5. Constraint solver (resolve collisions + joint constraints)
    6. Integrate velocities → positions
    7. Sleep idle bodies (don't simulate stationary objects)
    8. Output transforms for renderer

The constraint solver is the maths-heavy core — typically iterative (run multiple passes until converged) and sensitive to timestep. Stable physics requires careful timestep management; fixed timestep with multiple sub-steps per frame is common.

Performance considerations

Optimisation	Effect
Spatial partitioning	Don't check every pair against every other pair
Sleep / island detection	Stop simulating objects at rest
Substepping	Smaller timesteps for stable simulation, especially with high velocities
CCD (Continuous Collision Detection)	Prevent fast objects from tunnelling through walls
Multi-threading	Solver across multiple cores
GPU acceleration	PhysX and others can offload to GPU
Determinism	Reproducibility for replays / multiplayer

Notable physics-driven games

Games where physics is the gameplay rather than just visual flavour:

Game	Year	Physics use
Pinball Construction Set	1983	Surprisingly good 1980s ball physics
Marble Madness	1984	Isometric marble dynamics
Lemmings	1991	Per-pixel terrain physics
Worms	1995	Artillery + destructible terrain
Half-Life 2	2004	Gravity gun + interactive props
Trials series	2008+	Pure motorbike physics gameplay
Angry Birds	2009	Box2D + projectile physics
Crayon Physics Deluxe	2009	Player-drawn physics shapes
Bridge Constructor	2011+	Engineering simulation
Kerbal Space Program	2011+	Newtonian orbital mechanics
Besiege	2015	Build-and-test physics machines
Human: Fall Flat	2016	Active ragdoll as movement
Boneworks / Bonelab	2019, 2022	VR full-body physics
Teardown	2022	Voxel-based destruction physics

Simulating the real world