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I've read it said that decoherence is the reason we don't observe macroscopic superpositions. I find this very confusing... what exactly does it mean to observe a macroscopic superposition? Doesn't the superposition go away upon observation?

What types of behavior would macroscopic objects exhibit if decoherence wasn't happening... what types of things would we actually see?

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When we say we "observe" that an object has a particular quantum state, we do not typically imply that we do so with a single measurement. Instead, we prepare the desired state many times, and make many measurements, so that we have a probability distribution of observed eigenstates. We can take the probabilities we get from our series of measurements and convert them, up to an overall phase, into coefficients in the original state wavefunction.

A question such as "What would we see if decoherence wasn't happening?" is moot, because seeing an object requires that photons from the environment constantly interact with that object, which cause decoherence.

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    $\begingroup$ Ok, I'll put it differently. Suppose we were back in time before decoherence was understood. What was the expected behavior of macroscopic objects according to quantum mechanics without decoherence? I assume there was a difference between this prediction, and actual behavior of macroscopic objects, which is what led to the discovery of decoherence right? $\endgroup$ – Ameet Sharma Jul 19 '17 at 5:57
  • $\begingroup$ @AmeetSharma This might be better asked at History of Science and Mathematics SE. I don't know what the reasoning was back then. $\endgroup$ – probably_someone Jul 19 '17 at 5:58

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