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# simulationRootEntity

The root simulation entity associated with the simulation that raised the event.

```
let simulationRootEntity: Entity?
```

## Discussion

This entity is `nil` if the physics simulation is the default global one that covers all entities in the hierarchy.
Use [`PhysicsSimulationComponent`](/documentation/RealityKit/PhysicsSimulationComponent) to construct a custom physics simulation for a subtree of entities
that you have access to.  The event from this custom simulation reports a non-`nil` root simulation entity.

---

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