eli5: Why is it significant that light is both a wave and a particle?
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For a long, long time, it was believed that we had more or less figured out the basic principles of science and further details would just support what we already believed: as one scientist said in the late 1800s: “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”
In physics before this point, everything’s behavior could be described either by the rules for particles, or the rules for waves. It seemed like the world fit in these nice boxes.
But light–to paraphrase Einstein–sometimes acts as one thing, sometimes the other, sometimes neither, sometimes both.
The duality of light was one of the earliest indications that physics was not, at all, close to completion.
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