Magnitude of Vectors in Special Relativity

MrBillyShears
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So for finding the magnitude of a vector, velocity for example, we use v=√(vx2+vy2+vz2), but in special relativity, velocities can not exceed c. Is their a different formula for magnitude in SR, or could a velocity like(in natural units) v=(.9,.9,.9) not exist, since the magnitude comes out to be about 1.5588c, which obviously exceeds c?
 
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That vector could not be a velocity. Velocity is limited by v_x^2+v_y^2+v_z^2<1.
 
Ok, thanks I get it now.
 
In special relativity, all vectors should really be regarded as 4 dimensional. UltrafastPED's reference give the method for how to get the magnitude of a 4D vector.

Chet
 
How much about 4-vectors do you already know?
 

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