  We put forward an idea that physical phenomena have to be treated in
5-dimensional space where the fifth coordinate is the interval S. Thus, we
considered the (1+4) extended space G(T;X,Y,Z,S). In addition to Lorentz
transformations (T;X), (T;Y), (T;Z) which are in (1+3)-dimensional Minkowski
space, in the proposed (1+4)d extended space two other types of transformations
exist in planes (T,S); (X,S), (Y,S), (Z,S) that converts massive particles into
massless and vice versa. We also consider energy-momentum-mass 5-d space which
is conjugated to the time-coordinates-interval (T;X,Y,Z,S) space. The mass in
above mentioned 5-d energy-momentum-mass space corresponds to the interval S in
5-d space (T,X,Y,Z,S). Well known energy-momentum 4-vector P(1+3)=(E/c;
px,py,pz) in the Minkowski (T;X,Y,Z) space is transformed to the the 5-vector -
P(1+4)=(E/c; px,py,pz,mc) in the extended space G(T;X,Y,Z,S) and becomes
isotropic for 5-vectors of the massive as well as for the massless particles.
