The beam of light from a laser on a rotating turntable casts into space. At some distance, the beam moves across space faster than ?c?. Why does this not contradict relativity?
Solution 11E Step 1: In this problem, they have provided that, the beam of light from a laser light on rotating turn table is projected into space. So, at some instant, the beam will be appeared to move faster than the speed of light. We should not get confused about the speed of laser light with this. They have clearly asked about the condition of light beam travelling across the space which gets projected from the laser on the turntable. So, the circular motion of the table in fact creates the condition of violating principle of relativity. So, in the region with dotted lines, the speed will exceed c. But it does not contradicts relativity because, the beam of laser can produce a “dot” in all the areas as illustrated in the diagram. The “dot” it self is not a physical entity. The laser light is considered to be a physical entity. Since, the dot is not a physical entity, it may violate the laws of physics. So, we are interested in the behaviour of the objects and particles which are physical entities. The dot is a spreaded image of photons. Conclusion: The laws of physics are applicable only to physical entities. Since the dot itself cannot be considered as a physical entity, we cannot tell that it contradicts the principle of relativity and the photons from laser are moving with the speed “c” only.