Computational solution to quantum foundational problems | Hacker News Reader

Computational solution to quantum foundational problems

arxiv.org

30 pointsfrisco19 comments

Scott Aaronson has commented on the paper; he considers it as garbage:

> http://www.scottaaronson.com/blog/?p=1767#comment-103591

Also look at the update at the beginning of http://www.scottaaronson.com/blog/?p=1767

I was actually asked to referee this article. (I think I'm allowed to say that since I declined, I won't say which journal it was, and it's on the arXiv.) Although I agree with everything Scott says, I also recall another fundamental confusion. The author makes a grossly unjustified jump from (a) assuming the Universe as a "computing device" is "performing a calculation"--when in fact this mean our method for making predictions employs this calculations--to (b) concluding that you can construct a physical device capable of harnessing that power so as to perform similarly difficult computations. In fact, all sorts of problems come up in trying to make this leap that have to do with the finite precision of realistic preparations and measurements. The author did not acknowledge them.

For the sections where the paper was strictly discussing my area of expertise (decoherence), the author avoided egregious errors but basically just summarized some well-known ideas in a haphazard way that didn't advance his argument.

OP here, just saw that. Knew I should have checked Scott's blog before submitting to HN. Or just have asked myself what I was doing before submitting an ArXiv link. :(

Yea. Keep in my that almost anyone can put stuff on the arXiv. If you found the paper on a random person's personal website and you wouldn't post it, you shouldn't decide to post it just because its on the arXiv.

Does somebody know of good papers about potential links between computation complexity and why we do not observe large scale quantum behaviours ? As a basic computer guy, I have always suspected that the large computer that is running our universe does not have the resources to handle quantum entanglement on a large number of particules, and has to fallback to some classical approximation to be able to run the full simulation in real time ;-)

Superconduction and superfluity are examples of macroscopic quantum phenomena. I’m not sure they can be classified as entanglement, but the fundamental reason is at least as weird as entanglement and I think they would be also very difficult to simulate.

There is also a correction of the law for Ideal gases. It’s more important when the temperature is low and the quantum effects are more visible. Essentially, let’s suppose that you have a lot (~1E23) of identical atoms/molecules in a gas. In some gases the atoms/molecules can’t be in the some quantum state at the same place (fermions) and in other gases they can be (bosons). Moreover, the particles are all identical, so you can’t label them and can’t track them. This causes an “apparent repulsion” in the fermions gasses and an “apparent traction”. It’s not a real force, it’s only a statistical effect that looks like a attraction/repulsion. (It’s a little more complicated because the positions and velocities are not discrete, so “the same place” is more difficult to define.) (I can’t find a link to a direct simple measurement.) More info: http://en.wikipedia.org/wiki/Fermi_gas and http://en.wikipedia.org/wiki/Bose_gas

Of course there are some large scale (~150km http://iopscience.iop.org/1367-2630/8/9/193/fulltext/ ) or large number of particles quantum effects that are usually very carefully crafted , but the majority of the matter that surround us does not show such behavior, so I still wonder about this ;-) That said, thanks for the reminder !

I don't know about links to computational complexity, but in the many worlds interpretation the "classical approximation" is explained by "quantum decoherence".

1: http://en.wikipedia.org/wiki/Quantum_decoherence 2: http://lesswrong.com/lw/pp/decoherence/

why would it have to run it in 'real time'? what's the difference between this universe being simulated by a very slow computer in the super-verse, vs being simulated by a very fast computer in the super-verse? for us, I'd argue there's no difference at all.

Not always a fan of linking xkcd, but:

http://xkcd.com/505/

That's right, thanks for this point.

If someone's interested in a hard problem: design a density functional that's exact with regards to strong correlation and you have my respect.

It's instructive to remember at this point that Feynman used to call the exchange-correlation energy "the stupidity energy" out of frustration. Many Bothans died to give us PBE and B3LYP (at least it seems like it)

I actually never read that quote before, thanks. I'm a 1st year M. Sc. student in a theoretical chemistry group. Although most members in the group do DFT, I don't. My knowledge of the literature extends to some Perdew, Yang, HK and KS.

What do you do then? H-F? In any case, do not just stick to DFT. I can't emphasize enough how essential a book like Negele and Orland is to make yourself useful in dealing with strongly correlated systems -- running codes on these things is going to fail badly in terms of results deliverables useful to experimentalists or for further theory unless you can do some model building yourself. Way too many of these groups turn out people whose only skill is running ABINIT or VASP or QUANTUM-ESPRESSO or maybe writing some multi-threaded FORTRAN if they're lucky (shudder). Endeavor to make contact with developments in modern condensed matter theory (it doesn't have to be holography or AdS or anything that hardcore, just get used to the idea of a field as something other than what a plow goes into)

The project I was assigned requires only Hückel level theory, for the moment. I'm concerned with molecular electronics, specifically trying to use CAPs (complex absorbing potentials) to model conduction through aromatic molecules.

Thanks for the recommendation. The group I'm in is definitely focused on method development. Calculations are mostly done on atoms and diatomic molecules to benchmark the functionals.

You may want to have a look at some of Jorge Hirsch's papers on complex aromatics and ring currents. The guy is a bit of crackpot on superconductivity, but what he says on aromatics might be of interest to you.

I´ll repeat one of my comments in a previous submittion of the same paper:

There are some experiments where it’s easy to see quantum effects in “big” things. There are experiments that show explicitly the wave behavior of helium atoms (they have like 2 electrons and 12 quarks, and a lot of gluons, they are not a single particle) http://en.wikipedia.org/wiki/Helium_atom_scattering . I think there are bigger examples, but I should google a little to find them.

Also, the superconductivity and superfluity effects are due to quantum mechanics effects (a lot of properties of solids only can be explained correctly using quantum mechanics, but they are not so evident).

There is more hand waving that a proof here. Obligatory Xkcd: http://xkcd.com/1240/

Don't forget the obligatory Wikipedia article: https://en.wikipedia.org/wiki/Macroscopic_quantum_phenomena

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