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POYNTING's THEOREM 121

but in 2005 scientists at the University of Darmstadt in Germany have stopped light for one
minute (!) trapping it inside the crystal and thereby creating the optical memory prototype. Note
also another curious quantum phenomenon - electrostatic and magnetostatic fields consist
(according to the quantum picture) of boiling sea of virtual photons constantly pop in and out
of existence, while the free flow of real photons forms EM wave. If for some reason such virtual
photons leave their virtual state and become real, we immediately may detect the birth of EM
wave and vice versa. That makes the “state line” between static and dynamic fields rather
blurring.

It is worthwhile to point out that all concepts of photon movement are quantum in nature.
Nevertheless, we will try to use quasi-classical analogy balancing on the brink of failure. The
unsatisfied reader can study the specialized literature [8]. Since the rest mass of photon in a
vacuum is zero the mass m in Einstein’s equity is called relativistic mass. As such, the photon
according to quantum physics should manifest this non-zero mass in the same manner as any
other particle with non-zero rest mass. It means that the total energy carried by photon should
be sub-divided into two portions, kinetic and potential. Additionally, it must carry two portions

Figure 3.1.4 Photon’s momentum illustration: a) Common case, b) Spin Angular
Momentum (SAM), c) Orbital Angular Momentum (OAM)

of momentum, linear and angular. Mechanics teaches us that the linear momentum is a vector
quantity and product of photon’s mass and its velocity = [kg·m/s] where is the
∥
∥
velocity vector in straight line motion, as shown in Figure 3.1.4a . The second momentum
5
called angular = is the photon tendency to have imaginary spin under influence the
⊥
torque force T depicted in this figure by green vector. Figure 3.1.4b, c demonstrates the two
distinct form of angular rotation. As long as the photon travels said along z-axis and
instantaneously “rotates”, i.e. spin, around its own axis (r = 0), as Figure 3.1.4b depicts, it
carries so-called Spin Angular Momentum (SAM). Meanwhile, photon with shifted center of
rotation ( ≠ 0), as Figure 3.1.4c shows, possesses so-called Orbital Angular Momentum
(SAM).

Let us start with the kinetic energy and linear momentum. Suppose the sunlight or powerful
laser illuminates huge (around several square kilometers for large spacecraft) and perfectly

5 Public Domain Image, source: https://en.wikipedia.org/wiki/Angular_momentum