## Geometric Structure

The way to determine distance, volumes and transformations in spacetime.
- Presumed
- Newton: time and space are absolute, no t/R rotations (boosts).
- Special relativity: Only inertial observers.

- Background chosen

Maxwell: Could be any metric, you choose.

- Background-Free

GR: Solve field equations for the metric.

Any theory for gravity must be background-free.

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