According to the theory of special relativity, the speed of light in a vacuum, denoted as "c," is considered to be a fundamental constant and the maximum speed at which information or matter can travel. In this framework, it is not possible for any object with mass to reach or exceed the speed of light.
When you consider the relative motion of two photons passing each other in opposite directions, their relative velocity is not additive in the way you might intuitively expect. According to the principles of special relativity, the relative velocity between two objects cannot exceed the speed of light. Therefore, the relative speed between the two photons would not be 2c or twice the speed of light. Instead, it would still be c, the speed of light.
The reason for this is that special relativity incorporates time dilation and length contraction effects that occur as objects approach the speed of light. These effects prevent objects with mass from reaching or exceeding the speed of light, as it would require infinite energy to accelerate them to that point.
It is important to note that the statement "all things are relative" refers to the notion that motion and speed are only meaningful in relation to other objects. However, the fundamental speed limit of the universe is the speed of light, and no matter how you perceive the motion of two photons passing each other, their relative speed will not exceed c.