3.2. The origin of inertial mass and equation E = mc²

In order to explain inertial mass effect, let’s consider a wave having momentum pf, and “trapped” in a selected direction in a segment which length is equal L. We will consider behavior of this wave when our segment L travels in the same direction at a velocity = v.
The time required for this wave to travel through whole segment L at the speed of light in the same direction of velocity v is equal to:
,
For the direction opposite to velocity v the time needed is:
.
Total travel time through segment L in both directions is equal to:
.
The wave traveling in a direction opposite to v has a momentum opposite to a momentum of the wave traveling in a direction of v. We can then calculate the average momentum based on the respective times of travel:
.

Taking the definition of a momentum  p=mass‧ velocity=m‧ v , we can see that inertial mass  m_i  is expressed as momentum   p_f   of “trapped” wave divided by speed of light c.
Consequently, an energy contained in a wave is equal to: E=p_f ‧ c  thus
we obtain  E=m_i‧ c² .