Please explain if you understand Einstein's theory of relativity.

The relative speed of the two opposite beams is 2c, no problem. This is not called superluminal, because it is the relative speed of two other objects standing in a reference frame. Of course, you haven't observed anything faster than light, because you think their speed is C, but if you look at another beam from the perspective of one beam, the speed is C, and there is no faster than light. Of course, Lorentz velocity transformation Vx' = (Vx-U)/( 1-U/C 2) is needed in the proof process, where Vx' is the velocity observed by the observer in the K' system, Vx is the velocity observed in the K' system, and U is the velocity of the K' system relative to the K system. For example, if your stationary reference frame is K system, the beam 1 propagates to the right, and if it is K' system, then u=c, and the beam 2 propagates to the left. Vx=-c observed in the steady-state K system, and Vx' observed in the K' system where the beam 1 is located, so VX' = (VX-U)/ is substituted.

For another example, from the perspective of earthlings, the distance from the earth to another planet is 8 light years. At present, there are two spaceships starting from the earth and the planet at the same time, with the speed of 0.8c From the perspective of the earthlings, they met eight light years later/1.6c=5 years later, and there is no problem of superluminal speed here. The relative speed is 1.6c, which is the relative speed of two spaceships standing in a reference frame, but if you stand in one spaceship and look at the speed of the other, it is not 1 .6c. In this problem, let the earth be in the K system and the right spaceship1be in the K' system, then u = 0.8c. At this time, spacecraft 1 thinks that the speed of spacecraft 2 is VX' = (VX-u)/(1-uvx/C2) =-1.6c/1.64, which is still less than the speed of light. So the same event is considered to have gone through five years in the earth reference system, but the people in the spaceship think that it has not gone through five years, because he thinks that the earth and the planet are moving at a speed of 0.8c, so the distance from the earth to the midpoint of the connecting line between the earth and the planet is 4 light years from the earth's point of view, but it is shrinking from the spacecraft's point of view and becomes L' = L * (1-u 2/. This is studied from the perspective of length contraction. This problem can also be seen from the clock delay effect (also called the moving clock slow effect), because the clock in the reference system of the spacecraft 1 is always motionless in the spacecraft from departure to encounter, so the time seen in the spacecraft is the inherent time, also called the static time, and the inherent time is the shortest (that is, the slowest running). From the point of view of earthlings, it is believed that the clock in the spaceship appears in two different places from the departure to the meeting, that is, the moving clock, so the moving clock runs slower than the clock on the earth, and the contraction factor is also (1-u 2/c 2) 1/2 = 0.6, so earthlings think it can be explained that the spaceship has only gone through three years. This problem can be explained from any angle, so the theory of relativity can still be established. Generally speaking, people who think that relativity is wrong are basically people who don't understand relativity well (not excluding some experts), but understanding relativity can't be fully understood by reading two pages in three days or two. Sometimes you think you understand, but you may not understand when you encounter problems. This requires constant thinking and a long process to understand, so it is not surprising that novices raise various doubts.