[soup10]

just because QFT follows an internal logic, doesn't mean the jump from macro physics to quantum physics itself is logical. In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics.

[perlgeek]

The Universe is fundamentally quantum in nature; if anything, we'd need a model that explains why classical physics works so well most of the time.

[pinkmuffinere]

As a naïve fool with no understanding of quantum physics, I want to take a stab at this! Here’s my hypothesis:

Consider a world in which everything is “very quantum”, and there are no easy approximations which can generally be relied on. In such a world, our human pattern-matching behavior would be really useless, and “human intelligence” in the form we’re familiar with will have no evolutionary advantage. So the only setting in which we evolve to be confused by this phenomena is one where simple approximations do work for the scales we occupy.

Sincerely, I don’t think this argument is super good. But it’s fun to propose, and maybe slightly valid.

[perlgeek]

The main objection is: if there wasn't a classical limit, our brains would have evolved differently.

So yes, we can use the antrophic argument as evidence for the existence of the classical limit, but it doesn't have explanatory power for why there is a classical limit.

[movpasd]

This is called the anthropic principle. I personally have objections to it, specifically that due to emergence it is hard to make definitive statements about what complex phenomena may emerge in alternate universes. However, it's taken seriously by many philosophers of physics and certainly has merit.

[GoblinSlayer]

Isn't that argument from ignorance? You can consider a class of physics similar enough to our physics, it should give enough space for research.

[movpasd]

My point is that it isn't possible to determine the emergent behaviour of a complex system from first principles. So arguments of the type "these physics don't result in atoms being produced, so life can't emerge" doesn't imply that other complex structures _like_ life don't emerge.

[GoblinSlayer]

Then how do we make technology if we don't know the result?

[Gibbon1]

I'm a fool too but two things I remember. One was a paper discussing the thermodynamics of groups of particles. When they have strong interactions with nearby particles classic behavior emerges very quickly as the number of particles increases. And not n equals 1 million, or 1000, but more like two dozen.

And then there was Feynman asked to explain in layman's terms how magnets work. And he said I can't. Because if I taught you enough to understand you wouldn't be a layman. But he said it's just stuff you're familiar with but at a larger than usual scale. And he hinted even then one level down and you run out of why's again.

[groestl]

> have no understanding of quantum physics

But you know about the Anthropic Principle :)

[xtacy]

We do have a model. That’s statistical physics.

Any standard course goes over various derivations of classical physics laws (Newtonian dynamics) from quantum mechanics.

[perlgeek]

I did study physics, and our statistical physics lecture only derived thermodynamic laws.

We also had a somewhat shoddy derivation of Newton's Laws from the Schrödinger equation, but wasn't really satisfactory either, because it doesn't really answer the question when I can treat things classically.

What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant).

(Another thing that drove me nuts in our QM classes where that "observations" where introduced as: a classical system couples to a quantum system. Which presupposes the existence of classical systems, without properly defining or delineating them. And here QM was supposed to be the more fundamental theory).

[vonneumannstan]

>What I'd really like (and haven't seen so far, but also haven't searched too hard) is the derivation of an error function that tells me how wrong I am to treat things classically, depending on some parameters (like number of particles, total mass, interaction strength, temperature, whatever is relevant).

There are plenty of ways to do this and things like Wigner functions literally calculate quantum corrections to classical systems.

But generally if you can't even measure a system before it's quantum state decoheres then it's quantum status is pretty irrelevant.

I.e. the time it takes for a 1 micrometer wide piece of dust to decohere is ~10^-31 s and it takes a photon ~10^12s to cross it's diameter. So it decoheres 10 billion billion times faster that a photon could even cross it.

[GoblinSlayer]

The error is usually taken as ratio of wavelength to your desired precision, but in general depends on your use case, sometimes you have full precision all the way down, sometimes you have insufficient precision on astronomic scale. Quantum physics doesn't have an absolute scale cutoff.

[soup10]

i started writing a response about how the human brain is designed to operate in an environment where classical physics is the norm, so we need to bridge the deviations from that if we are to really understand the world. But I don't know how much that's really true if you consider neural biology and I won't claim to know where quantum stops and classical begins as it relates to brain function.

[perlgeek]

You need quantum physics to understand how chemistry works.

So, given that chemistry plays a huge role in how the human (or any) brain works, it would be quite a stretch to argue that the brain works with classical physics.

We are often sloppy and sort all the chemistry in with classical physics, but that's a very human-centric approach. In reality, the Universe doesn't have different "domains" with separate rules for chemistry and physics; it evolves according to the Schrödinger equation, and we use Chemistry as an abstraction to not have to deal with nasty mathematics to predict how certain reactions will work.

[anon84873628]

I think the parent was really referring to "mind" instead of "brain". It's not the hardware of the brain that's classical, but our sense perception and model of the world.

[kybernetikos]

I do think there's something to this approach though - our sensory organs and processing ability are not abstract powers of understanding the universe - they developed exactly to give us enhanced survival chances. We should not expect to even be able to detect (let alone intuitively understand) aspects of reality that can't be used for survival.

[hnlmorg]

I do understand the point you’re making but my counter argument to that would be that physics hasn’t relied on our sensory input for a hundred years or more.

It’s been almost entirely based on maths and careful measurements from machined instruments purpose built for observing phenomena.

So at this point you’d hope the limitations of our biological senses would have been long surpassed.

[anon84873628]

It's impossible to surpass the physical limits of our biological senses. Can a person blind from birth form visual mental images?

We can try to retrain and reuse the sense for other purposes though. I'm reminded of that film "The Zero Theorem".

[Xmd5a]

>our [...] processing ability are not abstract powers of understanding the universe

Neural nets are called universal approximators for a reason. If what you guys are discussing is true, then a neural net would not be able to learn from a dataset about quantum experiments. I doubt this is the case. Also there is quantum cognition, and by that I mean the fact some researchers figured out a lot of puzzling results from experimental cognitive science seem to make more sense once analyzed from a quantum perspective.

[GoblinSlayer]

Humans are affected by prejudice, so learning straight from a dataset is easier said than done, but possible, yes.

[Gravityloss]

Yes, though our cells have machinery that does use quantum phenomena.

[vonneumannstan]

>In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics.

I think you have this backwards. QM IS the law of the universe and Classical Physics is just a high mass low energy approximation of it. In any case there doesn't need to be a logical explanation at all, the laws of physics are as they are. Why is the value of the fine structure constant what it is?