Steven Weinberg: What Is Quantum Field Theory, and What Did We Think It Is? (1997) | Hacker News Reader

Steven Weinberg: What Is Quantum Field Theory, and What Did We Think It Is? (1997)

arxiv.org

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This article came out about the same time as the first volume of Weinberg's textbook The Quantum Theory of Fields. It's a nice semi-technical distillation of the core argument in his textbook: We see a world made of particles, but Poincare invariance and cluster decomposition (the fact that widely separated systems behave approximately independently) force us to organize these particles into fields.

As I'm not deep into this matter, can someone share how this theory solves the "action from distance" problem of entanglement?

The collapse is described as either non-local, or non-existent (many worlds intepretation). I'm not sure adding fields about every particle solves this, but again, I'm ignorant on the details.

There is no action at a distance problem, because no influence travels between the particles.

The whole point of entanglement is that measuring one of the pair affects the outcome for the other one. Whether the influence "travels" between the particles is subject to debate, but there is influence.

Sorry, no. There is no debate on this point. The confusion here may be merely linguistic, but there can be no instantaneous influence, in the sense of a causal effect or transmission of information, from one particle to a distant entangled particle. If there were, that would violate everything we know is true about causation, and therefore reality. Nonlocal entanglement does not, and can not, involve an instantaneous influence.

You're arguing about something I'm not. The fact the waveform collapse appears instant doesn't violate causality at all. To break causality the collapse should be usable for communication, but it isn't.

Yet it does appear instant, from all data we have. And that is not explained at all by our current understanding of quantum physics. It's "explained" as in we observe it, and can calculate how it works. But it's not explained as in how come that collapse happens instantly regardless of the distance.

Therefore spacetime is either non-local, or we need another interpretation like many worlds that avoids the collapse and replaced it with neverending waveform, or something else.

To say there's nothing here left to explain to me says you don't understand the problem in the first place. Either that, or you know something that leading physicists don't know right now.

> Therefore spacetime is either non-local

You might be interested in Travis Norsen's book "Foundations of Quantum Mechanics - An Exploration of the Physical Meaning of Quantum Theory" (2017) where he argues that quantum mechanics (in the orthodox Copenhagen interpretation) is not even non-local because the word doesn't even make sense in quantum mechanics.

> there can be no instantaneous influence, in the sense of a causal effect or transmission of information, from one particle to a distant entangled particle

Didn't the Bell's theorem prove that wrong though? I.e. that there needs to be some sort of "hidden variables" for it to work?

More like the opposite of that. Bell’s theorem tells us that there are two choices: either QM is inherently non-local, or there are influences (hidden variables) that can operate faster than light. Although both choices are in a sense disturbing, most people realize that we have to accept the non-local option. If we do, everything still works, even if we might feel uncomfortable. If, however, we accept faster-than-light influences, nothing works.

Makes sense. And unless I'm mistaken, to be entangled, they have to be very close to each others at some point, right? And then we can send one of the pair far away and they'd still be entangled. I can imagine some sort of more fundamental link connecting the two, but it's still not breaking ftl

Yes, the particles are prepared together, then separated. Experiments on entangled particles have now managed to keep them entangled while separating them by huge distances—many kilometers. Of course there is a link connecting them, which we express by saying they are “entangled”. But we may never be able to connect this with concepts that are intuitive from our experience with the classical world.

Non-locality is not just a problem of the classic world. There's no notion of it in the quantum mechanics world either. There's nothing in the math that suggests how can particles kilometers away can be connected "outside" space.

And technically if they were, this would be apparent in many more ways than entanglement experiments, and it WOULD violate causality.

General relativity also has no such concept as "simultaneous events" at distance. The very point of the theory is that event simultaneity is a meaningless concept at distance (any distance, but especially at large distance).

So from scale cosmic to scale quantum, "non-local" is a meaningless concept, yet we have to just assume none of this matters and call the wave function collapse non-local, because that's the only way we decided the experiments make sense.

So you're trying to handwave an issue away that's bigger than you admit. It implies space beyond space and time beyond time, where everything in the universe can happen simultaneously for two entangled particles regardless of where and when in that universe they happen to be.

It's such an outrageous concept to introduce in this special case, with zero explanation, and without considering the larger implications it has, it honestly borders on The Avengers techno-babble about time travel in terms of how much sense it makes.

Accepting the non-local option is not an ultimatum, given there are multiple competing interpretations of these mechanics, and non-locality is just ONE of them.

You continue to insist the world has agreed in unison with the interpretation you personally like, but that's not the case.

Do you realize many worlds doesn't require collapse?

I’ve heard of it: https://arstechnica.com/science/2017/07/a-brief-history-of-q...

Frankly, I don’t understand anything you‘re trying to say in your multiple replies to my comments, and I suggest you spare yourself any further effort in this direction.

Some work much more recent than 1997 with ADS/CFT correspondences shows that entanglement may manifest physically as wormholes. That is, there becomes a way for entangled particles to be local in spacetime despite them appearing to be distant in space. (ER = EPR conjecture).

It doesn't solve it. The action at a distance is not a problem with quantum theory. This is because it both is predicted by quantum theory and also observed in experiment. It is a fact of nature and must be accepted. Entanglement does not violate special relativity, and QFT happily merges both special relativity and quantum mechanics into one theory without issue.

"It's a fact of nature and must be expected" is the Copenhagen interpretation. Which is well-known to be incomplete and basically referred to as the "shut up and calculate" interpretation.

The issue still remains.

My high school science teacher explained it well with something a long the lines of "You can't grog it since you're big and stupid". Technically even normal measurement is action at a distance since we're collapsing a wave function that covers an area into a small point instantaneously. In entanglement the wave function is just spread out even further, but if we already accept wave functions over areas for particles it's not that big a stretch. Problem is that it obviously doesn't jive with what we have experience with everyday as big stupid people.

The issue of waveform collapse is fundamentally different than "it collapses". The fact it collapses is the easy part. The hard part is why it collapses simultaneously for two particles which can't communicate at the same(ish) moment due to light speed limitations.

No, there is no issue. And the Copenhagen interpretation isn’t just “shut up and calculate” even though people may humorously refer to it as such.

Basically what we have is that if you had a model of reality where small invisible angels tug at the feet of people and thereby cause gravity, if I come along with a Newtonian model of gravity, then you can’t dismiss my theory because “It doesn’t explain the angels!”

Classically you want to assume that there are small local particles traveling along always in one specific position with one specific speed. QM instead postulates that this is not the case, but that reality can accurately be described based on probability wave distributions that when we do the math accurately predict your particle observations. But does so without explaining “the angels” and without bridging what you perceive as a gap between localized classical particles being the ground truth and what QM postulates. But the incorrect notion of localized particles behaving classically was never a universal truth that we have to include. It was just an incorrect model that seems to give us correct predictions in some but not all cases. QM now gives us both the same predictions when they were correct, but also correct predictions in all the cases where it fails.

You spent a lot of time mocking the idea of strawman gravity angels and zero time explaining when the waveform function collapses and why this happens seemingly FTL.

Wave function collapse is a convenience for calculation. There is, as far as I know, no reason to think it is real.

The wave function collapse is quite real in that before measurement a particle interferes with itself and exists in a superposition, and after measurement it doesn't interfere with itself and is not in a superposition (from our perspective, at least).

Does the wave function collapse, or we get entangled with a specific outcome, or something else happens is subject to debate and is the substance of my question.

But "it acts as a wave, then acts as a specific point in space" part is quite real as it's what we observe in experiments.

Physicist here. I don't understand to what exactly "the angels" are supposed to refer here. Could you elaborate?

It seems it's some arbitrary strawman, so the overall point is my question about how we interpret the apparent waveform collapse is stupid by implication.

The 'action at a distance' referred to here it not the one that crops up in entanglement. In entanglement, the nature of our knowledge changes instantaneously when we make a measurement. The action at a distance problem Weinberg refers to relates to the speed at which one object influences a distant one. For example, in Newtonian gravity, with its V = GM1M2/r potential, the speed of propagation of gravity is effectively infinite.

Heres a better intro - https://m.youtube.com/watch?v=zNVQfWC_evg

https://arxiv.org/abs/hep-th/9702027

@dang the article link is broken, the above works for me.

Fixed now. Thanks!

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