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Isaak Moiseevich I-Aglom, Simple Non-Euclidean Geometry and Its Physical Basis: An Elementary Account of Galilean Geometry and the Galilean Principle of Relativity

Galilean geometry is the study of the x-t plane under the motions that correspond to changes in time origin and change of uniform motion coordinate system. Although we introduce this geometry on physical grounds, we quickly move on to study this geometry in its own left without any physical motivation. Distance in Galilean geometry should mean distance in space and not in time so the distance between two points is the distance between their projections on the x-axis. Therefore circles in Galilean geometry are pairs of lines t=constant. The angle between two lines can be defined in terms of the section they cut out of a circle centred at their intersection, so in this way we have angles in Galilean geometry. But circles can also be defined in terms of angles: a moving point on a circle makes a constant angle with any two fixed points of the circle (half the central angle). Applying this definition in Galilean geometry gives a new type of curve which we call cycles. Circles and cycles together are in many ways the natural analog of Euclidean circles. We see this in particular when we study circle inversion. Euclidean circle inversion can be understood in terms of stereographic projection: put the plane tangent to the south pole and the eye at the north pole and project up on the sphere, then interchange the plane and the eye and project back. In Galilean geometry the sphere must be replaced by a cylinder (since, e.g., the left of the circle t=-c, t=+c is the line t=0 and thus maps to a line at infinity). In the chapter called "Conclusion" we look at how our ideas carry over to the geometry of space-time in special relativity, again without much concern for physics. As before, we use physics only to derive the motions of our geometry, in this case the Lorentz transformations, and then quickly go on to consider things of purely geometric interest. For instance, we see that the difference in signs between space and time components of a Lorentz transformation implies that a circle will be a hyperbola x^2-t^2=0 and the analog of stereographic projection requires a hyperboloid.

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