Loopholes and the 'Anti-Realism' of the Quantum World | Hacker News Reader

Loopholes and the 'Anti-Realism' of the Quantum World

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49 pointsjonbaer16 comments

> The photon, in other words, has definite reality at the beginning and end.

That's not true. In a low-flux time-delay interferometer you cannot assign a definite emission time to a detected photon.

http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzl...

Also to what extent does “beginning and end” apply to a photon, which exists on a null geodesic? I realize that a photon can have no perspective, so I won’t try to talk about what a photon “sees,” and from our perspective a photon is emitted, travels, and is absorbed. Still, if you look at a spacetime diagram emission and absorption exist along the null axis.

Talking about photons hurts my brain. ><

I don't know why you're getting down-voted. It's a perfectly reasonable question.

> emission and absorption exist along the null axis

Only if the photon travels through free space. If it's reflected or refracted this is no longer true.

Serious question: Doesn't it seem to be a deep and cutting flaw, that an entire branch of physics attempts to draw conclusions about subatomic behavior based on the macroscopic interference noted when photons pass through polarized sunglasses, or perhaps electrons travel through the diffraction grating of a cathode ray tube?

It seems fundamentally incorrect to show hobbyists and laymen these partial examples, and then cross ones arms and say "that's the way it is" and venture no further.

Polarization experiments and the double slit experiment are continually hauled out onto the stage, again and again, and never enlighten anyone. Taking a relativistic particle, and shooting it through or bouncing it off of a standing wall of atoms (e.g. polarization filters and diffraction gratings), doesn't really introduce an air of mystery into anything, because the wall of atoms operates its own subatomic state, which is selectively ignored. It's basically like setting a murder mystery inside a super max prison.

I think a lot of these scientists would rather focus on solving these theoretical problems than trying to explain to random laypeople the nuances of quantum physics.

I've spent hundreds of hours studying particle and quantum physics and I still have a multitude of unanswered questions because there is a certain mathematical wall you hit where real-world experiments that could be explained to regular folk are simply not useful as models, and until I master those mathematics, it doesn't matter how long a particle physicist tries to explain things to me.. I won't get it.

EDIT: If you're interested in other kinds of experiments I would check out the work being done with Quark–gluon plasma, specifically with lattice structures that cause quantum perturbations to scale to a macro level where we can perform experiments and get some pretty cool results.

The general idea is creating a state of extreme entanglement by homogenizing as many properties between the particles in the lattice as possible. This, combined with an near absolute-zero temperature and a lattice so flat it could be considered two-dimensional, means that the entropy in the system is low enough that you can watch small-scale perturbations bubble up unimpeded by surrounding entropy.

Further reading: https://arxiv.org/pdf/1611.01533.pdf

I'm not sure I understand the question. Is it a flaw that we are trying to draw conclusions about things we can't observe, indirectly through things we can observe? No. I can think of no other way to do it.

To put it into layman's terms (as much as I understand it, which is not really very much at all): You know how you can run up to a fence where the gaps in the fence are smaller than you are? And you know how when you run through the fence, there is a different chance that you ran through one gap or the other (which are smaller than you, I should reiterate)? And then if you look at it a different way, even though you went through one gap or the other (somehow), you know how it turns out that you went through both at once? It's totally, exactly like that.

While I may have got that completely wrong, I think you see the point. We have no frame of reference for talking about quantum mechanics at the macro level. Quantum mechanics doesn't make sense at the macro level. This isn't as much of a problem as you might imagine. There are lots of things that don't make sense when you take them out of the context where they make sense.

For example take a piece of paper and a pencil. Draw a small box. Beside it, draw a big box. Erase the small box. Beside your remaining box, draw a bigger box. Which box is small? Now erase the bigger box. Is the existing box big or small? Well, the question doesn't make any sense without the context. It was big originally. But then it was small. And now with nothing to compare it to, it is neither big nor small.

I think people take quantum physics too literally. It's a wave. It's a particle. It hits the mirror. It doesn't hit the mirror. From a macro perspective, these concepts have meaning. I don't see why they have to have any meaning at all from another perspective. You only need to have a system that, when viewed from the macro perspective, allows the macro phenomena to exist. Apart from that, it could be anything at all. Do particles exist in an actual space, or do they just have some chance to react with each other in such a way that it is consistent with there being space when viewed in that way? Does a particle need to exist when it doesn't interact with anything? I don't really see why it does. Why can't the universe have lazy evaluation? And if all particles interact with other particles from their own context, I don't see why a particle can't exist for some contexts, but not exist for others -- just as long as there is eventual consistency. Hey, I'm a programmer. This stuff happens to me all the time :-)

At least that's the way I think about it. I'm not sure if that's the kind of thing you were getting at.

Your claim in the first link seems consistent with this recent experiment: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...

Indeed, it's wrong to think of "a" photon emitted at a particular time, and it takes either path. The emission is a quantum event with a probability distribution over time, capable of interfering with earlier or later versions of itself.

I think if you use a fast optical switch to allow photons into the experiment for only a short window of time, and that window of time is less than the path difference, you won't see interference. That's different from a low-flux laser, because each photon's probability envelope is truncated in time by the shutter.

TL;DR: There was a loophole found in an older thought experiment that indicated that at the quantum level, nothing is real until it is observed.

A discovery was recently made that found a way to add an unknown variable into the mix such that an expiriment where the future seems to affect the past was possibly explained away by a classical model.

However, a new experiment was proposed to show whether the new classical model actually was the explanation — and it turns out this classical “loophole” can not explain it.

So we’re now back to the beginning where it once again appears that nothing is real until observed at a quantum level which is also referred to as “anti-realism”.

Pretty sure local hidden variable theories were ruled out as an explanation of quantum mechanics a long time ago. What's new here?

They weren't. There was an error or oversight in the proof.

Wikipedia says that this is the conclusion of Wheeler's experiment:

> Any explanation of what goes on in a specific individual observation of one photon has to take into account the whole experimental apparatus of the complete quantum state consisting of both photons, and it can only make sense after all information concerning complementary variables has been recorded. Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely.

Local hidden variable theories were disproved by violations of Bell's inequality, not by a thought experiment.

https://en.wikipedia.org/wiki/Bell_test_experiments

Are you saying that all of these experiments were flawed in some way?

No, I'm saying that the Wheeler experiment seems to be more concerned with the wave/particle duality, not with the hidden variables.

And as this article shows, the Wheeler experiments are not thought experiments, they are very real ones.

Wheeler’s delayed choice excitement is referred to as a thought expirement both by the physicists in the article as well as the first line in its Wikipedia entry. Like Einstein, he worked through many thought excitements that suggested an outcome, but couldn’t be tested at the time. However, many of Einstein’s proposed experiments that have since been observed are still referred to as his thought expiriments.

We may be saying the same thing, but worth making the distinction.

For the other poster (diggsey), it will be difficult to answer your question about the novelty of these newly run expiriments without a full read of the article.

Local hidden variables are only ruled out if you reject superdeterminism. At least one Nobel prize winning physicist is developing a superdeterministic theory of QM.

And of course, non-local classical theories like de Broglie-Bohm are just fine, once again.

> nothing is real until it is observed.

What do the terms "real" and "observed" mean in this sentence?

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