
Quantum effects in relatively large systems - JumpCrisscross
http://nautil.us/issue/29/scaling/how-big-can-schr246dingers-kittens-get
======
biswaroop
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](https://www.youtube.com/watch?v=2Z6UJbwxBZI)

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

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

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

~~~
jessriedel
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.

~~~
biswaroop
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.

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

------
amelius
Perhaps also interesting: [1]

[1]
[http://www.ted.com/talks/aaron_o_connell_making_sense_of_a_v...](http://www.ted.com/talks/aaron_o_connell_making_sense_of_a_visible_quantum_object)

------
RobertoG
Why is Everett (many worlds) interpretation so unpopular?

Frequently is not even mentioned, as it was not so plausible, or so
implausible, as Copenhagen.

~~~
twoodfin
The most common objection I've heard is that physicists are strongly biased
towards 'conservative' laws. And for good reason: Assuming that physical
quantities are conserved and then looking for apparent violations of that
assumption has been an extremely fruitful theoretical and empirical
enterprise.

"Many worlds" suggests the opposite: That physical quantities such as mass are
forever increasing, albeit in a way we can't experimentally detect.

~~~
repsilat
> That physical quantities such as mass are forever increasing

I think that's a little uncharitable. In classical terms you could say that
each "universe" is associated with some weight or probability, and those
probabilities always sum to one.

I guess you could say that the "number" of things increases (provided that
number is countable and finite, I guess, which seems a bit silly), but the
probability-weighted mass of things going on stays the same.

Of course, it isn't terribly useful to talk about all of the inaccessible
universes taking up the majority of the probability, so we condition on our
observations (i.e., we do a Bayesian update, or "collapse the wavefunction".)

Then if we talk about "our universe" having probability 1 given our
observations, and surmise that people in another parallel, inacessible
universe would also say that theirs has probability 1, we might say "Aha!
There is now a total of 2, where once there was a total of 1!" It should be
clear how this is an error, though.

------
hyperion2010
> The interference patterns can be washed out by decoherence: They vanish as
> the researchers admit gas into the apparatus, increasing the interactions of
> the molecules with their environment.

I know that this has been know for a long time, but it is nice to see a clear
experiment that shows that decoherence has nothing to do with 'measurement' or
'looking' like most stories about quantum phenomenon try to imply. Nice to see
this particular abuse of the anthropic principle laid to rest.

