From a lot of time discussions about SRT ant Relativity in this forum are circling, as I see it, around one and the same mis-interpetation - the relative relocity. I will try to explain this with the only purpose to decrease amout of posts about SRT n this forum (take care, the database can crash again Please Register or Log in to view the hidden image! ) First, how can be defined 'Frame of Reference /FR/', when two objects are in one and the same FR. For example if we have three objects A,B and C and : A B---------->v C--------> v A v<----------B v<--------C it is known that B and C are going away or are approaching tio A with one and the same velocity B and C are in one FR. What will happens however in the following case: v<----------B A C----------> C and B are moving in opposite directions? Are they in one and the same FR? The answer is yes, B and C are again in one and the same FR. Evaen if they move with the speed of light they are in one and the same FR no matter in what direction they are moving wrt A. This is very important because the velocity v which is included in gamma is not the same mechanical velocity which is used in classical mechanics, in another words it is possible two frames to move one wrt other with velocity bigger then c (in the above case) but their relative velocity used by SRT will be 0! the relative velocity between two frames can be found only if we has third FR in rest, then: VBC(relative) = |VAB| - |VAC| the modul velocities - |VAB| and |VAC| are the magnitudes of velocities of frames B and C wrt A not caring of direction of movement. In the same way as we calculate kinetic energy T we are not caring about the direction of movement only by the magnitude! Please try to understand what 'relative velocity' means in SRT.