Relativistic speeds going perpendicular

[Swankie]

Couldnt find anything from 15min of forum searching...

If 2 objects are traveling along say, an x and y axis(or just 90 degrees apart really) at close to c, how do they interpret each others speeds? 0<theta<90 are easy enough. Couldnt find anything about perpendicular though.

A Newtonian version of this problem is simple enough but I am not sure when you are going at the speeds I am tlaking about.

 

[ghwellsjr]

They're just going to interpret the other one as moving directly away at slightly closer to c.

 

[jtbell]

You need to use a vector version of the relativistic "velocity addition" formula:

http://en.wikipedia.org/wiki/Velocity-addition_formula#Vector_notation