Comment by TTPrograms | Hacker News Reader

Comment by TTPrograms | Hacker News Reader

Every now and then people (on HN and elsewhere) like to rediscover pilot-wave theory and other similar ideas that posit that billiard-ball-like-particles still describe the universe. In these interpretations the wave-function, which is the probabilistic field that modern physics uses to model particles, is just an effective representation about our knowledge about the system. I tend to see these theories as paying a price of drastic increase in complexity to retain classical intuition about the system.

The mainstream view in modern physics (for the past 30 or so years, I believe) is rather that the wavefunction is the fundamental description of reality, at least in the sense that classical effective knowledge theories are not plausible models.

My interpretation of this paper on quick glance is that it is an attempt to experimentally demonstrate that only effective knowledge theories with very strange properties would suffice to explain observations, thus strengthening the "wave-function is fundamental" viewpoint.

I'm an enthusiast at best, but I don't think that's quite right. The dominant interpretation that treats the wavefunction as made of "probability stuff" is surely the statistical/ensemble interpretations, or even "shut up and calculate." Billiard ball theories like de-Broglie Bohm are definitely a minority viewpoint, and not representative of those that deny ontological status to the wavefunction.

We might separate interpretations according to their ontology:

1. de-Broglie Bohm: The particle has a real (though hidden) configuration.

2. Many Worlds: The wavefunction is real and fundamental.

3. Ensemble/Consistent histories/Copenhagen/"Shut up and calculate": Nature is fundamentally probabilistic. Measurements have ontological status, and talking about what happens between measurements is suspect (in various ways, depending on the theory).

From what I can tell most physicists fall into category #3.

To be clear, my statement about the wave-function being probabilistic is intended to be a Copenhagen reference. My impression is that this paper intends to refute 1). Personally I think 2 and 3 are pretty much equivalent, as we don't have any reason to think we could run an experiment that would distinguish between universe splitting and wavefunction collapse.

I'm unclear on which point you disagree.

My point of disagreement is giving the mainstream view as "the wavefunction is the fundamental description of reality". I claim that the mainstream view, as seen in type 3 theories, treats the wavefunction as a mathematical object, not a fundamental (i.e. physical) one.

For example, Griffiths (who introduced consistent histories) describes the wave function as "a mere mathematical tool or pre-probability for calculating probabilities which can, if one wants, be obtained using different solutions to Schrödinger’s equation." He goes on to contrast this to MWI and Bohmian mechanics, which treat the wave function as fundamental, are both deterministic, and are minority viewpoints. [1]

1: http://arxiv.org/pdf/1105.3932v2.pdf

I would tend to agree -- the way I always learned quantum mechanics was that the wavefunction was merely a representation that happens to give the right answers when operated on by the Hamiltonian.

In much the same way, I don't think that particle physicists really think that matter is made of tiny strings.

I might actually buy a copy of this paper, I'd be curious to see what new experiments they have done to more closely demonstrate the physicality of the wavefunction.

> From what I can tell most physicists fall into category #3.

Most 'media physicists' fall into category #3. It is from my understanding that most 'practicing physicists', by association, fall into category #1 (QFT).

What do you think is the best way to learn of these things, i.e. physics fundamentals and all in the modern state of knowledge?

I ask from the perspective of a B.S. Physics cum computer software person... which, I think, might not be too far removed from the perspectives of most HN peeps...

Understanding current thought in the field for me is based on skimming many academic papers. That's kind of different, though.

Getting a solid understanding of the fundamentals of the various fields is more of a coursework sort of thing. There's a bunch of quantum lectures on MIT's open courseware - anything Allen Adams will be great. If you're a book kind of person I believe I enjoyed Shankar but it's been a while.

>> Every now and then people (on HN and elsewhere) like to rediscover pilot-wave theory and other similar ideas that posit that billiard-ball-like-particles still describe the universe.

Have you seen the experiments of Yves Couder?

Hypothetically, if a model like that produced an exact mathematical equivalent of quantum theory wouldn't that be more satisfying? It would also destroy some of the notions that have been mathematically proven - like EPR and Bells' inequality. A lot of people are unsatisfied with QM and looking for something more tangible - including physicists.

Apparently pilot wave theory was never soundly rejected, the crowd just went in a different direction.

Yves Couder has demonstrated that you can construct a physical system that obeys the mathematics of pilot wave theory. This has zero bearing on if pilot wave theory is an accurate description of subatomic physics.

A lot of people are unsatisfied with QM and looking for something more tangible - including physicists.

While I would also prefer that quantum mechanics turns out to be quite classical at its heart - with objective reality, locality and all that - it seems a pretty bad idea to expect or even assume that nature works the way we prefer it to and that it will be easily understandable for human brains. Actually, the longer we struggle to make sense of all of that, the more likely it seems to me that everything is actually radically different from the way we have looked at it for the past century.

>Hypothetically, if a model like that produced an exact mathematical equivalent of quantum theory wouldn't that be more satisfying? It would also destroy some of the notions that have been mathematically proven - like EPR and Bells' inequality.

destroying Bell's inequality would be the key. It would be very satisfying. I'd say Nobel prize and forever in history books satisfying. Unfortunately (as i myself is on the ERP side and came to a something like pivot wave on my own while trying to explain the double-slit) there is no reasonable explanation for observed Bell's violations except for the explanation by QM (which for a "realist" like me sounds just like "will of God spreading FTL" :)

Note: actually i haven't been able to find good (for my criteria) Malus law verification for single photons - the law is the basis of Bell's violation claims in photon polarization experiments. Slight modification of the law on top of Einstein's proposed modification - with resulting law being pretty reasonable for single photon situation while still preserving classic law in the case of classic light ray - actually produces the Bell violation in the classic "local realism" explanation. So i still hope a bit :)

Bell's inequality is, as you say, mathematically proven - and experimentally verified. Destroying it is about as likely as finding out that 2+2=5.

Pilot wave theory produces the same predictions (as do all interpretations of QM), so in some sense it's valid, but it's an ugly attempt to brush the fundamentals under the carpet; it makes some simple cases simpler, but it makes the really interesting QM harder. It's not wrong, in the same way that constructing Newton's laws in a rotating frame of reference is not wrong - but it's less pleasant to work with for most interesting problems, and obfuscates the physical reality.

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