In the Galilean transformation, there are two separate invariants that two inertial frame of reference always agree on between two separate events:

- time
- length, given by the Pythagorean theorem

However, in special relativity, neither of those are invariant separately, since space and time are mixed up together.

Instead, there is a new unified invariant: the spacetime-interval, given by:

$cΔt_{2}−(Δx_{2}+Δy_{2}+Δz_{2})$

Note that this distance can be zero for two events separated.

- Four-momentum | 0, 116, 4
- Relativistic mechanics | 0, 116, 5
- Special relativity | 260, 3k, 34
- Theory of relativity | 0, 3k, 40
- Particle physics | 137, 28k, 449
- Physics | 276, 39k, 680
- Natural science | 0, 48k, 1k
- Science | 0, 52k, 1k
- Ciro Santilli's Homepage | 262, 181k, 3k