According to the theory of relativity, the speed of light in a vacuum is considered an absolute constant, denoted by 'c,' which is approximately 299,792,458 meters per second. This means that the speed of light is the same for all observers, regardless of their relative motion.
When two photons pass each other in opposite directions, their relative speed is still equal to the speed of light 'c.' This is because the speed of light is independent of the motion of its source or the observer. No matter how fast the source of light or the observer is moving, the speed of light remains constant.
It is important to note that the theory of relativity states that nothing with mass can travel at or exceed the speed of light. Photons are massless particles, and they always travel at the speed of light in a vacuum. From the perspective of an observer, if they were to measure the relative speed between two photons passing each other, they would find that the relative speed is '2c' because the photons are moving in opposite directions. However, this does not violate the principles of relativity because the speed of light is constant in all reference frames.
In essence, the theory of relativity does not allow for objects to travel faster than the speed of light, even in relative terms. The speed of light sets an upper limit on the velocity that any object or information can attain.