What is the Cooper pair paradox in superconductivity?
I've heard this called a paradox, but it's really a misunderstanding. The idea is that two electrons, both negatively charged, should repel each other—so how can they form a bound state? The answer is that the binding is mediated by the lattice. An electron deforms the lattice, creating a region of positive charge that attracts a second electron. This effective attraction overcomes the Coulomb repulsion, but only at very low temperatures where lattice vibrations are weak. The pair is very loosely bound, with a size of about 100 nanometers—much larger than the atomic spacing. So it's not a paradox; it's a subtle interplay of forces. In physics, we must be careful not to confuse the map with the territory. The Cooper pair is a well-defined quantum state, and its existence is confirmed by experiments like flux quantization. It's a beautiful example of emergent behavior.
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