I was wondering if quantum superposition occurs only on an atomic level, or does it apply to all matter, regardless of the size?
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Generally, quantum superposition is noticeable only among a few coherent particles at the atomic level. However, quantum superposition exists at the macroscopic level also. It just isn't noticeable since you have bazillions of wave functions that are all added together where you lose all noticeable coherence. Think of a couple of rocks thrown in a pool of water. You can easily see (and measure) the interference pattern of the water waves from the two rocks. But if you dumped trillions of rocks into the same pool, you would have an incoherent mess and would be hard pressed to identify any coherent interference pattern.