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Reading a somewhat long and argumentative thread here inspired the following unrelated question in my mind:
Where does a 2 dimensional flat Lorentzian geometry depart from Euclidean geometry as axiomatized by Euclid? I.e. Euclid's axioms (in modern language) can be taken to be:
We can interpret straight lines in Euclidean geometry as "geodesics" in the Lorentzian geometry. I believe the difficulties start to arise in axioms 3 and 4. A meaningful notion of a "circle" becomes difficult, perhaps we'd replace it with a hyperbola? The concept of rapidity between time-like geodesics is promissing, but doesn't seem to be a sufficiently general replacement for "angle".
Where does a 2 dimensional flat Lorentzian geometry depart from Euclidean geometry as axiomatized by Euclid? I.e. Euclid's axioms (in modern language) can be taken to be:
1. A straight line segment can be drawn joining any two points.
2. Any straight line segment can be extended indefinitely in a straight line.
3. Given any straight line segment, a circle can be drawn having the segment as radius and one endpoint as center.
4. All right angles are congruent.
5. If two lines are drawn which intersect a third in such a way that the sum of the inner angles on one side is less than two right angles, then the two lines inevitably must intersect each other on that side if extended far enough.
We can interpret straight lines in Euclidean geometry as "geodesics" in the Lorentzian geometry. I believe the difficulties start to arise in axioms 3 and 4. A meaningful notion of a "circle" becomes difficult, perhaps we'd replace it with a hyperbola? The concept of rapidity between time-like geodesics is promissing, but doesn't seem to be a sufficiently general replacement for "angle".