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Comment by biswaroop | Hacker News Reader

Superfluidity is briefly mentioned in the article, but it's a quantum effect that's visible in very large systems.

A system exhibits zero viscosity when its particles condense into a single quantum mechanical state. Superconductivity is an instance of this, where electrons pair up and flow without resistance.

Buckets of liquid helium show superfluidity. [1]

Labs cool blobs of alkali gas to superfluidity. [2]

Entire cores of neutron stars are superfluid. [3]

There's even a theory of quantum gravity where physical vacuum is a superfluid. [4]

[1] https://www.youtube.com/watch?v=2Z6UJbwxBZI

[2] https://www.youtube.com/watch?v=1RpLOKqTcSk

[3] http://arxiv.org/abs/1012.0045

[4] https://en.wikipedia.org/wiki/Superfluid_vacuum_theory

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This isn't really more "macroscopically quantum" than spectroscopy or the stability of matter. (Neither of those would be possible in a normal classical universe either.) You're just noticing indirect quantum effects on large scales, but there is no macroscopic coherence.

On the other hand, the interferometery experiments like MAQRO discussed in the article do actually exhibit long-range coherence of large amounts of matter.

Although liquid helium is kind of a partial BEC, isn't a BEC one of the best examples of a coherent state? Except for certain quantum phase liquids, superfluid systems often exhibit long-range phase coherence.

I'd actually be curious to know the quantum discord of a superfluid Fermi gas.

No, the spatial coherence of a BEC isn't larger than the ground state wavefunction.

At that rate isn't superconductivity a "quantum effect"? If electron orbital transitions are governed by quantum effects, what common phenomenon isn't a quantum effect? I know of a biochemistry lab cohort which would be deeply offended if photosynthesis wasn't classified as a quantum effect.

> a quantum effect that's visible in very large systems

Well, in a certain sense, chemistry and solid state, and the very existence of atoms are also macroscopically-visible quantum phenomena.

You do a science class with kids in school and show them static electricity, little pieces of paper dancing in an electric field - that's quantum. That's electric fields interacting, which is an exchange of virtual photons, which is a quantum phenomenon.

We really are jut waves on the surface of an ocean of quantum phenomena.

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