diff --git a/README.md b/README.md
index 370ce69b..72fc0b9f 100644
--- a/README.md
+++ b/README.md
@@ -38,7 +38,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1042 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1045 checks (GM_STEPS=20 for one step)
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
 node tools/seed-actor.mjs && node tools/smoke-actor.mjs
diff --git a/package-lock.json b/package-lock.json
index 33cae88a..8d9bf398 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -8,6 +8,7 @@
       "name": "game-maker",
       "version": "0.1.0",
       "dependencies": {
+        "@dimforge/rapier2d-compat": "^0.20.0",
         "pixi.js": "^8.6.0"
       },
       "devDependencies": {
@@ -47,6 +48,12 @@
         "keyv": "^5.6.0"
       }
     },
+    "node_modules/@dimforge/rapier2d-compat": {
+      "version": "0.20.0",
+      "resolved": "https://registry.npmjs.org/@dimforge/rapier2d-compat/-/rapier2d-compat-0.20.0.tgz",
+      "integrity": "sha512-FFYwGrfJov7d5kn4fxblG/ip5C+sYQL+Jl/hqLvDvu4BcMD/Mhrz7cTvnZTtuIRDXY8CLuraObqB/prip4wdWQ==",
+      "license": "Apache-2.0"
+    },
     "node_modules/@esbuild/aix-ppc64": {
       "version": "0.25.12",
       "resolved": "https://registry.npmjs.org/@esbuild/aix-ppc64/-/aix-ppc64-0.25.12.tgz",
diff --git a/package.json b/package.json
index 233da6e5..dc6dc1af 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,7 @@
     "vitest": "^3.0.0"
   },
   "dependencies": {
+    "@dimforge/rapier2d-compat": "^0.20.0",
     "pixi.js": "^8.6.0"
   }
 }
diff --git a/src/editor/App.ts b/src/editor/App.ts
index 71495273..1ffc41b7 100644
--- a/src/editor/App.ts
+++ b/src/editor/App.ts
@@ -2,6 +2,7 @@ import type { Application } from 'pixi.js';
 import { HttpFileStore } from '../core/store/HttpFileStore';
 import { HttpProjectStore } from '../core/store/HttpProjectStore';
 import type { StoredTexture, UploadedTexture } from '../core/store/TextureStore';
+import { loadPhysics } from '../runtime/physics/engine';
 import { requestRender, type VStack } from '../ui';
 import { fontLibrary } from '../viewport/FontLibrary';
 import { soundLibrary } from '../viewport/SoundLibrary';
@@ -42,5 +43,6 @@ export async function startEditor(app: Application, root: VStack): Promise<void>
   fontLibrary.onChange(() => requestRender());
   installDebugHook(app, state, editor);
   await editor.boot();
+  loadPhysics().catch((error: unknown) => console.error('[physics] Rapier could not be loaded:', error));
   await startPlugins(API_ENDPOINT, state, editor);
 }
diff --git a/src/editor/debugHook.ts b/src/editor/debugHook.ts
index 1ea04fc4..a2f15f84 100644
--- a/src/editor/debugHook.ts
+++ b/src/editor/debugHook.ts
@@ -6,6 +6,7 @@ import { findAsset, findAssetOf, findObjectHolder } from '../core/model/Project'
 import type { FrameRect } from '../core/model/texture';
 import { findLayer } from '../core/model/tilemap';
 import { effectiveScale, gameClock } from '../core/time';
+import { physicsStatus } from '../runtime/physics/engine';
 import {
   activeDrag,
   afterFrames,
@@ -641,6 +642,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     sceneSelect: () => ({ ...centreOf(editor.sceneButton), enabled: editor.sceneButton.enabled }),
     /** Behaviors running without Play (`Run in editor`), and how long they have been at it. */
     editorRun: () => editor.editorRunInfo(),
+    physics: () => physicsStatus(),
     /** Rows the hierarchy shows – collapsed subtrees are not in here. */
     hierarchyOrder: () => hierarchy.rowOrder(),
     hierarchyChevron: (id: string) => hierarchy.chevronPosition(id),
diff --git a/src/runtime/physics/engine.ts b/src/runtime/physics/engine.ts
new file mode 100644
index 00000000..49c1fd58
--- /dev/null
+++ b/src/runtime/physics/engine.ts
@@ -0,0 +1,40 @@
+import type RAPIER from '@dimforge/rapier2d-compat';
+
+export type Rapier = typeof RAPIER;
+
+/** Everything outside the engine is in pixels; Rapier is tuned for metres. */
+export const PIXELS_PER_METRE = 100;
+
+export const toMetres = (pixels: number): number => pixels / PIXELS_PER_METRE;
+export const toPixels = (metres: number): number => metres * PIXELS_PER_METRE;
+
+export type PhysicsStatus =
+  | { state: 'idle' | 'loading' }
+  | { state: 'ready'; ms: number; version: string }
+  | { state: 'failed'; error: string };
+
+let status: PhysicsStatus = { state: 'idle' };
+let loading: Promise<Rapier> | null = null;
+
+/**
+ * Rapier is its own chunk with the WebAssembly inside. Every caller shares the one load, and a
+ * failed load stays failed rather than being tried again on each call.
+ */
+export function loadPhysics(): Promise<Rapier> {
+  if (loading) return loading;
+  const start = performance.now();
+  status = { state: 'loading' };
+  loading = import('@dimforge/rapier2d-compat').then(async ({ default: rapier }) => {
+    await rapier.init();
+    status = { state: 'ready', ms: Math.round(performance.now() - start), version: rapier.version() };
+    return rapier;
+  });
+  loading.catch((error: unknown) => {
+    status = { state: 'failed', error: error instanceof Error ? error.message : String(error) };
+  });
+  return loading;
+}
+
+export function physicsStatus(): PhysicsStatus {
+  return status;
+}
diff --git a/tests/core/physics-engine.test.ts b/tests/core/physics-engine.test.ts
new file mode 100644
index 00000000..b8fa4d71
--- /dev/null
+++ b/tests/core/physics-engine.test.ts
@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest';
+import {
+  loadPhysics,
+  PIXELS_PER_METRE,
+  physicsStatus,
+  type Rapier,
+  toMetres,
+  toPixels,
+} from '../../src/runtime/physics/engine';
+
+const STEP = 1 / 60;
+const GRAVITY = 980;
+
+/** A 400 × 20 px floor whose top is at y = 290, and a 32 px crate dropped from y = 0. */
+function crateOverFloor(rapier: Rapier) {
+  const world = new rapier.World({ x: 0, y: toMetres(GRAVITY) });
+  world.timestep = STEP;
+  const floor = world.createRigidBody(rapier.RigidBodyDesc.fixed().setTranslation(0, toMetres(300)));
+  world.createCollider(rapier.ColliderDesc.cuboid(toMetres(200), toMetres(10)), floor);
+  const crate = world.createRigidBody(rapier.RigidBodyDesc.dynamic().setTranslation(0, 0));
+  world.createCollider(rapier.ColliderDesc.cuboid(toMetres(16), toMetres(16)), crate);
+  const run = (steps: number) => {
+    for (let i = 0; i < steps; i++) world.step();
+  };
+  const y = () => toPixels(crate.translation().y);
+  return { world, crate, run, y };
+}
+
+describe('physics engine', () => {
+  it('loads once and reports the Rapier version', async () => {
+    const first = loadPhysics();
+    expect(loadPhysics()).toBe(first);
+    const rapier = await first;
+    expect(rapier.version()).toBe('0.20.0');
+    expect(physicsStatus()).toMatchObject({ state: 'ready', version: '0.20.0' });
+  });
+
+  it('converts at 100 px to the metre', () => {
+    expect(PIXELS_PER_METRE).toBe(100);
+    expect(toMetres(980)).toBe(9.8);
+    expect(toPixels(toMetres(123.5))).toBeCloseTo(123.5, 10);
+  });
+
+  it('lets a crate fall freely in pixels per second squared', async () => {
+    const { run, y, crate } = crateOverFloor(await loadPhysics());
+    run(30);
+    // ½ g t² = 122.5 px after half a second; the integrator lands about a pixel past it.
+    expect(Math.abs(y() - 0.5 * GRAVITY * 0.25)).toBeLessThan(1.5);
+    expect(toPixels(crate.linvel().y)).toBeCloseTo(GRAVITY * 0.5, 3);
+  });
+
+  it('keeps a crate resting on the floor', async () => {
+    const { run, y, crate } = crateOverFloor(await loadPhysics());
+    run(180);
+    expect(Math.abs(y() - 274)).toBeLessThan(0.5);
+    expect(Math.abs(toPixels(crate.linvel().y))).toBeLessThan(0.5);
+    run(120);
+    expect(Math.abs(y() - 274)).toBeLessThan(0.5);
+  });
+});
diff --git a/tools/e2e/41-physics.mjs b/tools/e2e/41-physics.mjs
new file mode 100644
index 00000000..ec993de2
--- /dev/null
+++ b/tools/e2e/41-physics.mjs
@@ -0,0 +1,28 @@
+/**
+ * The physics engine (Rapier) loads in the editor, in the background after the first project is
+ * open, from a chunk of its own – the editor bundle does not carry it.
+ */
+export async function physics(t) {
+  const { check, json, waitUntil, consoleLog } = t;
+
+  await waitUntil(`['ready', 'failed'].includes(__gm.physics().state)`, { timeout: 15000 });
+  const status = await json('__gm.physics()');
+  check(
+    'Rapier loads in the editor without Play',
+    status.state === 'ready' && status.version === '0.20.0',
+    JSON.stringify(status),
+  );
+  const scripts = await json(
+    `performance.getEntriesByType('resource').map((e) => e.name).filter((n) => n.endsWith('.js')).map((n) => n.split('/').pop())`,
+  );
+  check(
+    'it comes from a chunk of its own',
+    scripts.some((name) => /^rapier-.+\.js$/.test(name)),
+    JSON.stringify(scripts),
+  );
+  check(
+    'loading it writes nothing to the console',
+    !consoleLog.some((line) => /\[physics\]|rapier/i.test(line)),
+    consoleLog.filter((line) => /\[physics\]|rapier/i.test(line)).join(' | '),
+  );
+}
diff --git a/tools/e2e/index.mjs b/tools/e2e/index.mjs
index 3c93c44d..b68285ec 100644
--- a/tools/e2e/index.mjs
+++ b/tools/e2e/index.mjs
@@ -43,6 +43,7 @@ import { nineSlice } from './37-nine-slice.mjs';
 import { plugins } from './38-plugins.mjs';
 import { tile } from './39-tile.mjs';
 import { tilemap } from './40-tilemap.mjs';
+import { physics } from './41-physics.mjs';
 
 export const STEPS = [
   hierarchy,
@@ -85,4 +86,5 @@ export const STEPS = [
   plugins,
   tile,
   tilemap,
+  physics,
 ];