Quantum effects in relatively large systems | Hacker News Reader

Quantum effects in relatively large systems

nautil.us

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

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.

Perhaps also interesting: [1]

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

Why is Everett (many worlds) interpretation so unpopular?

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

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.

> 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.

Everything is being conserved perfectly well in MW/UWF, since you're just evolving a pure wave function with the Hamiltonian operator, guaranteeing conservation of everything that's meant to be conserved.

Part of it is, we can't even imagine a plausible way for all those worlds to be created. That they just pop into existence for every collapse of the wavefunction is an unsatisfactory explanation. The Copenhagen interpretation is more palatable because then we're dealing with just one Universe.

At least is a materialist theory.

The other one is about the observer affecting the universe in an unknown way.

>the observer affecting the universe in an unknown way.

there is no pure "observation" in QM like the one we can imagine in classical case. Any observation in QM is actually an interaction. While i'm not a fan of Copenhagen, there is nothing un-materialistic that an interaction with the system affects its state, like a coin balancing on its edge finally would finally fall on one side upon you touching it.

There is no wavefunction collapse in many-worlds.

I don't think Many Worlds/Universal Wave Function is actually unpopular amongst physicists. There was a survey (I'll have to find it) that showed it was one of the most popular interpretations.

It's appealing for its mathematical simplicity, giving unitary wave function evolution with the reasonable assumption that the universe has a wave function. It eliminates wave function collapse from the axioms, and possibly eliminates the Born rule.

Talking about its 'complexity' in the 'creation' of parallel worlds isn't much of an issue, you can look at simple systems like the von Neumann measurement model to see how they naturally arise through superposition.

The main issue is how the Born rule arises. That is, why the probability of an 'observer' ending up in a particular branch of the wave function is |psi|^2 isn't clear, but then it isn't clear what probability means in this context...

Here is a paper about a poll of physicists. http://arxiv.org/abs/1301.1069

Question 12: What is your favorite interpretation of quantum mechanics?

Top Three:

Copenhagen: 42%

Information-based/information-theoretical: 24%

Everett (many worlds and/or many minds): 18%

> 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.

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