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# Spacetime interval

nosplit "Four-momentum" words: 73
In the Galilean transformation, there are two separate invariants that two inertial frame of reference always agree on between two separate events:
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: $$cΔt2−(Δx2+Δy2+Δz2) (11)$$
Note that this distance can be zero for two events separated.