Absolute zero is unattainable. Temperature of 0K impossible to obtain any way. This is the third law of thermodynamics. Photons can exist only in the moving at speed of light. At this speed the rest mass (invariant mass) of the photon approaches zero. It is derived from the Lorentz transformations (special theory of relativity). Also there is a particle having a negative and imaginary rest mass. But that does not mean that a photon can not have mass. For example the light beam is deflected by massive celestial objects. The fact is that there are several varieties of types of mass. There are inertial mass, passive gravitational mass, active gravitational mass, invariant mass. In classical physics, they are equal, but in modern physics are different concepts. And in different frames of reference mass of the same object will be different.
