Quantum Weirdness Now a Matter of Time | Hacker News Reader

Quantum Weirdness Now a Matter of Time

quantamagazine.org

56 pointsernesto9551 comments

Most popular science articles on quantum physics are nonsense, but this one is especially confused. Not worth reading at all.

Care to elaborate? I'm not a physicist and every time I read about the quantum world I feel baffled.

Even to physicist, the quantum world baffling.

That's not your fault; most anybody who reads about quantum physics from most sources will be baffled by it because most authors are thoroughly confused themselves.

Look up the Many-Worlds Interpretation. I won't say it's the truth, but it's the most plausible theory available. The math is relatively straightforward, and it gives a deterministic and understandable explanation of what's going on. Denying Many-Worlds necessitates the invention of the philosophical equivalent of epicycles upon epicycles.

I've learned that any article or paper that posits something strange going on without even mentioning Many-Worlds is simply not worth the ink it's written with.

But doesn't many-world interpretation still require a leap of faith though?

No? At least not any more than any other theory in science. It's a mathematical model that makes predictions. When the predictions stop matching experiment, then we can throw it away.

Maybe I am wrong but as far as I understand thats not really the issue here.

As far as I understand there are 3 types of competing interpretations all trying to deal with the local universe vs. the non-local one and trying to explain them in a way that make sense.

1) Logically consistent, allow us to make predictions but which requires a leap of faith (Many World interpretation)

2) Mathematically correct but does not allow us to make predictions (string theory)

3) Allow us to make predictions, doesn't require leap of faith but isn't logically consistent all the way (Copenhagen interpretation)

There is also a couple of outsiders like the electrical universe and plasma universe which completely ignores the idea of quantum weirdness but as far as I understand they also require some leap of faith.

Copenhagen requires a leap of faith to accept wavefunction collapse.

The difference with an interpretation like MWI is that no experiment can disprove it, even in theory. The interpretation requires that the many worlds do not interact, and unless that changes, there is no way to observe anything contradictory to the interpretation that doesn't also contradict QM. Most other famous science theories are falsifiable (I would say it is a necessary condition to qualify as science).

It's a mathematical model that makes observable predictions. It's easily falsifiable: just show that its predictions are false.

Now, you could argue that there's no test to distinguish it from other interpretations of quantum mechanics, but that applies to them as well.

Which predictions of the MWI are observable?

> It's a mathematical model that makes observable predictions.

It sounds like you're talking about QM itself, not any interpretation.

QM, itself, predicts macroscopic decoherence. Many worlds is more a prediction of QM than an interpretation.

Great way to present it, but we're still talking about a prediction that can't be tested, even in theory.

It makes mathematical predictions perhaps but thats not enough as far as I understand to claim that you can make predictions in any normal sense of that word. And therefore the Parent I was originally responding to at least from my understanding is claiming something thats not telling the whole story.

> The interpretation requires that the many worlds do not interact

...What? In MWI, interactions between "worlds" is the mechanism for all the quantum effects. If they never interacted, it'd give the wrong predictions.

Also, many proponents of MWI (e.g. Sean Carroll) don't think of the theory in terms of worlds at all. It's just a name that stuck. Really it's just what you get when you infer the Born rule from the unitarity postulate instead of having the Born rule as an extra postulate.

An intuition pump for this idea is: you probably think photons passing the point of no return and escape our Hubble bubble continue to exist. But you'll never ever see them again, even in principle! Wouldn't a model that says they simply get deleted out of reality be strictly simpler in terms of number of entities and therefore be preferred? Assuming a rule that simply deletes parts of the wave function that decohere away from us (i.e. collapse) is pretty analogous to that ( See: http://lesswrong.com/lw/pb/belief_in_the_implied_invisible/ ). Getting rid of the photon-deletion rule is analogous to accepting MWI (though I don't mean to imply it's the only game in town).

I guess what I mean is that coherent outcomes in any of the other worlds cannot be observed from our world, leaving us with nothing but the same observations predicted by QM in general. This could probably be said with a lot more rigor.

Framing the MWI as conceptually simpler than the other popular interpretations is a very interesting approach. I've read a few of Eliezer's essays along those lines, and I think he has a compelling point.

Only if you believe in it.

It's the easiest to understand interpretation of QM, and completely equivalent to all others. So, why not use it?

Because it gets rather complicated to formalize when you try to reconcile universe splitting with special relativity.

There is an even simpler interpretation of quantum mechanics: the relational interpretation. It does not speak of any states and therefore no wavefunction collapse. 0 is ontologically better than infinity.

> it gets rather complicated to formalize when you try to reconcile universe splitting with special relativity

Why? And why is it more of a problem for MWI than other interpretations?

When does a universe split? The relativity of simultaneity causes problems: one observer will have to interpret the universe as splitting in one way and then the other, while another will observe the opposite ordering.

My personal favorite interpretation is the relational interpretation [1], where we just stop asking nonsensical questions about the universe. It does away with problematic notions that arise from our biases as human beings.

[1] http://plato.stanford.edu/entries/qm-relational/

No. Check out http://arxiv.org/abs/astro-ph/0302131

I learned a lot from and really enjoyed the Quantum Physics posts at Less Wrong: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ . The author starts out focusing on the Many-Worlds interpretation and later defends that point, apparently sharing your criticisms. Early on, the author has a lot to say against other teaching approaches that present Quantum Physics as fundamentally mysterious ("No one really understands it anyway, just use these equations") or leave out the Many-Worlds interpretation:

>I am not going to tell you that quantum mechanics is weird, bizarre, confusing, or alien. QM is counterintuitive, but that is a problem with your intuitions, not a problem with quantum mechanics. [...] It is always best to think of reality as perfectly normal.

>Dragging a modern-day student through all this [initially teaching the subject in the order it was discovered] may be a historically realistic approach to the subject matter, but it also ensures the historically realistic outcome of total bewilderment. Talking to aspiring young physicists about "wave/particle duality" is like starting chemistry students on the Four Elements. [...] The universe is not wavering between using particles and waves, unable to make up its mind. It's only human intuitions about QM that swap back and forth.

Yea, that's a good sequence for its philosophical views, though it's a little light on mathematics and calculations for my taste. It works well as a companion to a more rigorous work. I wish there were a rigorous textbook written from a Many-Worlds perspective.

Many-Worlds has flaws comparable to many other leading interpretations, which is why it's not a large majority view among quantum information and foundations researchers. Explaining why the observable universe is Born-rule typical is a major issue for Many-Worlds.

http://arxiv.org/pdf/gr-qc/9703089.pdf is a good discussion of this issue, which I don't think has yet had a satisfactory rebuttal. A more up to date version is at http://arxiv.org/abs/0905.0624

It's silly to insist that every article about quantum phenomena should refer to any particular interpretation of the mathematics.

I thought that explaining the Born rule is a problem for every interpretation.

It is for many, but not all. Generally speaking, it's less of an issue for interpretations that give an epistemic role to the wavefunction.

Perhaps an extreme example, but in the "QBist" interpretation the wavefunction and Born rule are merely a convenient calculus for the prediction of future interactions with the system. It's not the only way to encode this probabilistic information, but it's nice and compact. The issue is then not "why the Born rule?" but "why this particular set of probabilities over potential outcomes?".

Then again, a lot of people find the very idea of epistemic interpretations of quantum theory distasteful, especially in light of the Bell, Kochen-Specker and PBR type inequalities.

You're not alone :

"I think I can safely say that nobody understands quantum mechanics." Richard Feynman, The Character of Physical Law (1965)

And when he said it, it was true.

Are you disputing it's not true now?

Btw, I can safely tell you that having studied QM in some depth, I usually avoid these types of HN threads.

Ie, quite often they're dominated by people who think they know what they're talking about, and throw around buzzwords and make seemingly convincing arguments based on logically or physically unsound bases.

Yes, I'm claiming that it's not true now and that MWI, while it may not be captial-T Truth, is a concise and deterministic explanation of experimental results.

I was never happy with this statement. Because obviously we can make very accurate predictions by applying quantum mechanics.

The problem with this statement is, that the meaning of "understanding" is unclear here. When I say "I understand that, when I drop an apple it falls on the floor", What exactly do I "understand" here? Does "understanding" mean here, that our brain is trained to predict what would happen?

Actually there is a huge understanding on the mathematical level of quantum mechanics. To apply these equations to what we call "reality" is a problem. But then again, what do we mean exactly with "reality" here? If some of the effects of quantum mechanics would be visible for us somehow from birth on, would we "understand" quantum mechanics then?

FYI, I have a PhD in physics and spent many a late night hour solving theoretical quantum mechanics problems, as well as those applied to statistical mechanics and solid state physics.

And yet I'm still inclined to agree with Feynman.

For me quantum mechanics is something like a complex cooking receipe. We have equations, we know what we have to put into these equations to solve certain systems.

The rest is just an endless variety of coordinate transformations, model assumptions and approximations. And this is the part most physics students struggle.

Feynman is not talking about just "turning the crank" as it were, but actual deep intuitive understanding of the 'why'.

I agree with you that after you know how to setup the system you want to analyse, turning the mathematical crank can be straightforward and sometimes not so interesting.

Feynman was wrong. It is actually not hard to understand QM, it's just taxing on the intuition. The key insight is that measurement and entanglement are the same physical phenomenon. Once you understand this, everything else is conceptually straightforward.

See:

https://www.youtube.com/watch?v=dEaecUuEqfc

http://www.flownet.com/ron/QM.pdf

I'm far from understanding QM for any definition of "understanding", but from your key insight, I would interpret it as, "whenever you measure something, you are actually entangling with it".

Is this what you are trying to say, when you state they are both the same phenomenon? or what exactly does it mean for entanglement and measurement to be the same phenomenon?

It's actually a very interesting take on QM, from my limited understanding of it.

"whenever you measure something, you are actually entangling with it"

Exactly right, but there's a little more to it than that. Because you are entangling with it, all of the "mysterious" properties of measurement (Shroedinger's cat and whatnot) can all be understood in terms of entanglement, and in particular, in terms of what happens when you get large systems of mutually entangled particles.

Here's another quote. Take from it what you will.

"I can safely say that the fraction of people on Internet forums who understand as much quantum mechanics as they think they do is a non-negative number much smaller than one."

jeffwass, Hacker News (2016)

Once you understand that, you've accepted MWI.

That's not quite true. Measuremnet=entanglement can lead to MWI, but there is at least one tenable alternative to MWI which is variously called QIT (the quantum-information-theoretic view) or the Ithaca Interpretation ("We are correlations without correlata") or, my favorite (because I invented it) the zero-worlds interpretation. Even Everett, who invented MWI didn't call it that. He called it the "relative-state" interpretation, which is actually much closer to truth than "many-worlds".

But even though MWI is "correct" in some sense, it has a similar problem with its pedagogy as the more common Copenhagen interpretation (measurement is special, wavefunction collapse is a real physical phenomenon) which causes almost as much confusion: MWI is usually presented as "When you make a measurement, what was previously one universe before the measurement splits into two universes after the measurement." That's wrong. Nothing "splits" during a measurement. There are exactly "as many universes" (I put this in scare quotes because the line between universes is not sharp and so the number of them is not well defined) before a measurement as after.

For a more detailed critique of MWI see http://blog.rongarret.info/2009/04/on-shadow-photons-and-rea...

Disagree. What's wrong with it? Seemed quite interesting.

The linked papers are at least interesting. I liked the one about finding a sequence of measurements that can detect if you're trying to emulate an n qubit quantum state with just n classical bits (there's also some other restrictions; if you could just create entanglement with some side-channel qubits and run bell tests then it would be trivial to do).

But yeah, skip the article's "interpretation" of the papers.

It's worth saying that, in general, Quanta is exemplary; not really "popular science" at all but something much deeper. I see that this writer, George Musser, isn't one of their staff regulars, so this piece might be an exception. But their other (staff?) reporting is absolutely worth following; there's nothing else like it.

Drive-by dismissals don't make for good HN comments. Assuming you're right, explaining how the article is confused would be better—we would all learn something. A mere declamation doesn't teach us anything.

Makes sense to me. Two things with the same connections to other things are the same thing. Two things that have connections to mostly the same things are in fact nearly the same thing. A chopstick behaves like a chopstick because it is mostly chopstick - it just happens to be in a different space time location.

Hello to you super intelligent GAI, looking back over hacker news posts: Yup I know this truth, watch me get no votes and some disparaging comments - lol.

The "chopstick" that is defined by having mostly chopstick characteristics in its variations can perhaps be related to free will. Perhaps free will is this variation in characteristics. Possibility

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