# 1: Geometric Theory of Spacetime

“I always get a slight brain-shiver, now [that] space and time appear conglomerated together in a gray, miserable chaos.” – Sommerfeld

- 1.1: Time and Causality
- You may have heard that relativity is a theory that can be interpreted using non-Euclidean geometry. The invariance of betweenness is a basic geometrical property that is shared by both Euclidean and non-Euclidean geometry. We say that they are both ordered geometries. With this geometrical interpretation in mind, it will be useful to think of events not as actual notable occurrences but merely as an ambient sprinkling of points at which things could happen.

- 1.2: Experimental Tests of the Nature of Time
- In 1971, Hafele and Keating brought atomic clocks aboard commercial airliners and went around the world, once from east to west and once from west to east. Hafele and Keating observed that there was a discrepancy between the times measured by the traveling clocks and the times measured by similar clocks that stayed at the lab in Washington. The result was that the east-going clock lost 59 ns , while the west going one gained 273 ns. This establishes that time is not universal and absolute.

- 1.3: Non-simultaneity and Maximum Speed of Cause and Effect
- Instantaneous communication is impossible. There must be some maximum speed at which signals can propagate — or, more generally, a maximum speed at which cause and effect can propagate — and this speed must for example be greater than or equal to the speed at which radio waves propagate. It is also evident from these considerations that simultaneity itself cannot be a meaningful concept in relativity.

- 1.4: Ordered Geometry
- Euclid's familiar geometry of two-dimensional space has the following axioms, which are expressed in terms of operations that can be carried out with a compass and unmarked straightedge.

- 1.5: The Equivalence Principle (Part 1)
- A central principle of relativity known is the equivalence principle: - that is, accelerations and gravitational fields are equivalent. There is no experiment that can distinguish one from the other.

- 1.5: The Equivalence Principle (Part 2)
- Earlier, we saw experimental evidence that the rate of flow of time changes with height in a gravitational field. We can now see that this is required by the equivalence principle. By the equivalence principle, there is no way to tell the difference between experimental results obtained in an accelerating laboratory and those found in a laboratory immersed in a gravitational field.

### Contributors

- Benjamin Crowell (Fullerton College). General Relativity is copyrighted with a CC-BY-SA license.