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The fractional quantum Hall effect (FQHE) is a physical phenomenon in which the Hall conductance of 2D electrons shows precisely quantised plateaus at fractional values. Should this be considered evidence that electrons are composite?

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No, it is not a proof of that. As Robert Laughlin demonstrated,

http://en.wikipedia.org/wiki/Laughlin_wavefunction

one may explicitly write down a wave function involving electrons, particles with charge $-|e|$, that reproduces all the fractional observations. He got a Nobel prize for this wave function in 1998.

It may be useful to preemptively notice that the fractional Hall effect is a long-distance effect. In physics, at least up to the Planck scale, long-distance dynamics is derived from shorter-distance dynamics. In plain English, larger objects are composed of smaller building blocks (and interactions between them), not vice versa.

For electrons, including those in the fractional quantum Hall effect, one may experimentally show that the short-distance particles (revealed by high-energy collisions) are electrons of charge $-|e|$. For hadrons, one may repeat the same experiments and see that the short-distance, fundamental particles are quarks with charges such as $+2|e|/3$ or $-|e|/3$. So the situations are fundamentally different even though each particle, regardless of the charge, may create various bound states and manifest itself in ways that "look like" a different charge at longer distances.

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