Comment by nine_k | Hacker News Reader

Comment by nine_k | Hacker News Reader

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According to modern QFT, there are no particles except as an approximation. There are no fields except as mathematical formalisms. There's no locality. There is instead some kind of interaction of graph nodes, representing quantum interactions, via "entanglement" and "decoherence".

In this model, there are no "split particle" paradoxes, because there are no entities that resemble the behavior of macroscopic bodies, with our intuitions about them.

Imagine a Fortran program, with some neat index-based FOR loops, and some per-element computations on a bunch of big arrays. When you look at its compiled form, you notice that the neat loops are now something weird, produced by automatic vectorization. If you try to find out how it runs, you notice that the CPU not only has several cores that run parts of the loop in parallel, but the very instructions in one core run out of order, while still preserving the data dependency invariants.

"But did the computation of X(I) run before or after the computation of X(I+1)?!", you ask in desperation. You cannot tell. It depends. The result is correct though, your program has no bugs and computes what it should. It's counter-intuitive, but the underlying hardware reality is counter-intuitive. It's not illogical or paradoxical though.

This is incorrect. There are particles. They are excitations in the field.

There still is the 'split particle paradox' because QFT does not solve the measurement problem.

The 'some kind of interaction of graph nodes' by which I am guessing you are referring to Feynman diagrams are not of a fundamental nature. They are an approximation known as 'perturbation theory'.

I think what they must be referring to is the fact that particles are only rigorously defined in the free theory. When coupling is introduced, how the free theory relates to the coupled theory depends on heuristic/formal assumptions.

We're leaving my area of understanding, but I believe Haag's theorem shows that the naïve approach, where the interacting and free theories share a Hilbert space, completely fails -- even stronger than that, _no_ Hilbert space could even support an interacting QFT (in the ways required by scattering theory). This is a pretty strong argument against the existence of particles except as asymptotic approximations.

Since we don't have consensus on a well-defined, non-perturbative gauge theory, mathematically speaking it's difficult to make any firm statements about what states "exist" in absolute. (I'm certain that people working on the various flavours of non-perturbative (but still heuristic) QFT -- like lattice QFT -- would have more insights about the internal structure of non-asymptotic interactions.)

Though it doesn't resolve whether a "quanta" is a particle or a measurable convergence of waves, Electrons and Photons are observed with high speed imaging.

"Quantum microscopy study makes electrons visible in slow motion" https://news.ycombinator.com/item?id=40981054

There exist single photon emitters and single photon detectors.

Qualify that there are single photons if there are single photon emitters:

Single-photon source: https://en.wikipedia.org/wiki/Single-photon_source

QFT is not yet reconciled with (n-body) [quantum] gravity, which it has 100% error in oredicting. random chance. TOD

IIRC, QFT cannot explain why superfluid helium walks up the sides of a container against gravity, given the mass of each particle/wave of the superfluid and of the beaker and the earth, sun, and moon; though we say that gravity at any given point is the net sum of directional vectors acting upon said given point, or actually gravitational waves with phase and amplitude.

You said "gauge theory",

"Topological gauge theory of vortices in type-III superconductors" https://news.ycombinator.com/item?id=41803662

From https://news.ycombinator.com/context?id=43081303 .. https://news.ycombinator.com/item?id=43310933 :

> Probably not gauge symmetry there, then.

> This is a pretty strong argument against the existence of particles except as asymptotic approximations.

I think it's also a pretty strong argument against the mathematical well-definedness of typical (interacting) QFTs in the first place.

Perhaps a better way to say it is that particles are not longer small balls of dirt [1], but a mathematical construction that is useful to generate an infinite serie [2] to calculate the results.

Since in some conditions these mathematical tricks behave very similar to small balls of dirt, we reused the word "particle" and even the names we used when we thought they were small balls of dirt.

[11] We probably never thought they were made of dirt, and in any case the magnetic moment is the double of the value of the small ball of dirt model.

[2] That has so many infinites that would make a mathematician cry.

Note that particles are not just for perturbation theory. There is a particle whenever there exists a particle annihilation/creation field configuration. A proton is a particle so writing down its creation/annihilation field configuration is in theory possible, though maybe not in practice.

Another point is that infinities do not necessarily make mathematicians cry. Abraham Robinson is quite pleased with them. It seems a possible hypothesis that at least some QFT are mathematically well-defined using non-standard analysis. Where 'some QFT' at least renormalizable and perhaps also asymptotically free. I don't know enough about it to know how the Haag theorem, mentioned in another comment impacts this.

Another analogy (flawed as any of them). Sports teams "exist" in a sense. They meet one another in well-defined interactions, called matches, and such an interaction can be described as if teams were well-defined atomic entities, producing a score.

But a sports team is not atomic, not a "final reality" entity. A sports team can pass through one gate, or through several gates, when entering a stadium. From a doctor's perspective, the team "does not exist", a doctor only operates in terms of individual players' organisms.

Particles are an approximation to the actual behavior of the field, and are used in perturbation theory to calculate the more complicated field behavior.

This works well when interactions are weak. Electrons do not couple strongly to the electromagnetic field, so it makes sense to view electrons as particles. However, quarks couple very strongly to the strong force (hence the name), so the perturbative approach breaks down, and it makes less sense to view quarks as particles.

So in a non-perturbative QFT calculation which has a well defined particle-number operator, that's just "an approximation" within the theory? What is it approximating?

Energy capacity?

I'll bite: Explain yourself.

Also, for context, my question was posed because the idea of "particle number" as well as "quantum states of particles (which are countable) represented in a Fock space" and in general the idea of particles are, like, page 2 of any QFT textbook. It doesn't approximate anything in the theory. Creation and annihilation of particles (and hence the well-defined concept of a particle) is fundamental to the construction of the theory itself, perturbative or not.

Particles are page 2 of any QFT textbook because the free particle is the only system we can exactly solve. In practice, that solution is usually used as the basis for a perturbative expansion.

That doesn't validate your assertion that particles are just "an approximation". Just because it's used in perturbation theory doesn't mean it's exclusive to it.

You're also manifestly wrong on "the free particle is the only system we can exactly solve".

Okay, it's not "the only" system we can exactly solve, but it's 99% of what we solve in practice, and it's the exact solution you'll see over and over again in QFT 1.

The free particle solution is an approximation to reality, because reality includes interactions. There's a mathematical formalism to this that we'd agree on, but you might disagree about how to describe it in words.

False in multiple ways.

QFT doesn’t discard local fields and replace them with only nonlocal graph nodes.

Maybe this is coming from some speculative quantum gravity ideas.

>There's no locality.

How so? QFT is Lorentz invariant. Even has such a thing as the norm flux.

My bad; QFT actually postulates locality. I was thinking about the casual set theory which strives to solve some of the QFT's difficulties, and where locality is an emergent / statistical phenomenon rather than a postulated condition.

> Lorentz invariance is also violated in QFT assuming non-zero temperature.

Source: https://en.wikipedia.org/wiki/Lorentz_covariance

If you couple your system to a heat path that is at rest wrt a specific Lorentz frame, you of course lose Lorentz incariance. On the other hand the lagrangian of the standard model itself is to my knowledge fully Lorentz invariant.

I don't know what they talk about there, but it sounds like some kind of thermodynamic approximation is involved there. Does thermodynamics survive Lorentz transformation?

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.

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.

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.

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.

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.

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.

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.

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

Technology is made iteratively by repeated trial and then observed error in the physical structures we've created (i.e. we build machines and then watch them fail to work properly in a particular way).

Technology that works in a different universe without atoms, would require us to be able to experiment within that universe if we wanted to produce technology that works there with our current innovation techniques.

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.

> have no understanding of quantum physics

But you know about the Anthropic Principle :)

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.

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).

>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.

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.

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.

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.

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.

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.

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.

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".

>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.

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

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

>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?

s/FOR/DO/

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