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It started with a simple experiment that was all the rage in the early 20th<\/sup> century. And as is usually the case, simple experiments often go on to change the world, leading Einstein himself to open the revolutionary door to the quantum world.<\/p>\n <\/p>\n Here\u2019s the setup. You take a piece of metal. You shine a light on it. You wait for the electrons in the metal to get enough energy from the light that they pop off the surface and go flying out. You point some electron-detector at the metal to measure the number and energy of the electrons.<\/p>\n Done.<\/p>\n The bizarre thing about this experiment, measuring what\u2019s known as the photoelectric effect, is that the electrons didn\u2019t behave as they should. If light is made of waves of electricity and magnetism, as Maxwell taught us, then the electrons should be able to slowly accumulate energy until they get hot enough to go flying off, regardless of the frequency of light. And brighter\/more intense light should lead to more energetic electrons, because there\u2019s more energy available. But neither of those expected outcomes happen (as an aside, I\u2019ve personally performed this experiment myself, not out of any great love for landmark physics experiments, but because it\u2019s a mandatory piece of undergraduate physics training). Instead, only light above a certain frequency could get electrons to dance, and those electrons always left the metal with the same energy, no matter how intense the light was.<\/p>\n \n