Next, Alex encountered a photomultiplier tube, which was emitting electrons when illuminated with light. A physicist named Einstein (the same one she met earlier) appeared and explained that this was an example of the photoelectric effect. He showed her that the energy of the electrons emitted depended on the frequency, not the intensity, of the light.
As they chatted, a high-speed train whizzed by, and Alex noticed that time seemed to slow down for the passengers on board relative to her own frame of reference. Einstein smiled and said, "Ah, yes! That's time dilation in action." He handed Alex a PDF manual titled "Solutions to Relativity Problems" and told her to study it carefully.
As Alex continued her journey, she met a quirky physicist named Schrödinger, who was busy solving a puzzle. He introduced her to the concept of wave-particle duality, where particles like electrons can exhibit both wave-like and particle-like behavior.