My favorite experiments are not necessarily the groundbreaking ones. I love those too, don't get me wrong; but I like the ones that make me take a good hard look at the way in which I picture the physics.
One critical skill in physics is knowing what to leave out. For instance, if I can predict and describe a physical system with classical physics, why add quantum mechanics? But a recent paper highlights that it is always important to bear in mind that every classical picture has a quantum background. You may be able to neglect that background, but should never forget that it is there.
Cavities and quantum states
The experiment involves thinking about optical cavities, which are sort of on the border between quantum and classical worlds. Normally, we think about optical cavities in terms of the color, or wavelength of light that an optical cavity will accept. The distance between the mirrors must be commensurate with the wavelength. This can be described by both classical and quantum physics. The amount of light in the cavity, however, is almost always thought of in terms of classical physics.
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