- #1
- 6,724
- 429
The standard definition of coordinates on Penrose diagrams seems to be something like [itex]\tan(u\pm v)=x\pm t[/itex]. This is what Wikipedia gives, and Hawking and Ellis also give a transformation involving a tangent function, although I haven't checked whether the factors of 2, etc. agree. Neither source comments on why a tangent function is used. It's clear that if the transformation is going to be of the form [itex]f(u\pm v)=x\pm t[/itex], then f has to be a homeomorphism from a finite, open interval of the reals onto the whole real line. But it seems to me that we could just as well have used [itex]f=\tan^3[/itex], or [itex]f(x)=-x/[(x-1)(x+1)][/itex]. Is there anything about the tangent function that makes it especially desirable? Any transformation of the form [itex]f(u\pm v)=x\pm t[/itex] will preserve the shape of light-cones, since it sends curves x-t=const to curves u-v=const, and similarly for x+t and u+v. I believe that f=tan makes particles at rest have world-lines that look like hyperbolas, but is there some special reason that a hyperbola is a desirable result? World-lines of moving particles are funky S-shapes.