Meaning of ct in Lorentz transformation - In Lorentz transformation matrix, the first column is defined as - ct, not t itself. Is it because ct satisfies the units of x, y, z? Or, simpler Lorentz transformation matrix will be derived? The idea of 'ct', instead of t, is quite abstract for me. Not sure whether it is conceptually correct to consider ct as - a meter of x*(0) corresponds to the time it takes light to travel 1 meter in vacuum. Derivation and use of 4-vector scalar product - Why is the scalar product defined in such way, where square of 'ct' has a minus sign? How to prove its invariant property for any inertial frame? Already in a big mystery.... How about the momentum 4-vector? What does p0 in the first row mean? Thanks a lot.