Experiments spell doom for physical-collapse explanation of quantum weirdness • Hacker News Board

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#111

Post by abecedarius » Fri, Oct 21, 2022, 4:20 PM UTC

causi wrote: post #57

ThalesX wrote: post #6

Earlier quoted context omitted.

Having been passionate about this for awhile, I have a layman’s surface overview of the state of physics, or I like to believe so. It amused me to no end when my elderly aunt came to ask me for a tl;dr; on Quantum Physics. She stopped me shortly asking how she can control the universe with her mind. I tried to explain a bit about the famous interpretation (Copenhagen, not Von Neumann–Wigner) but she would have none o…

What I don't understand is why we call it "observation" when it should really be "interaction". The quantum weirdness resulting from the collapse of probabilities has nothing to do with a conscious observer, just whether and at what point the phenomena in question interact with something else in an observable way.

Well, an interaction that leaves a persistent change in state in the "observer".

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#112

Post by GoblinSlayer » Fri, Oct 21, 2022, 4:21 PM UTC

tsimionescu wrote: post #103

fallingfrog wrote: post #90

Earlier quoted context omitted.

Well, the thing is that the MWI is actually simpler than all the other interpretations, because it simply removes the idea of wave function collapse entirely. Any theory involving wave function collapse is adding something extra to quantum physics that can't be demonstrated experimentally. Because we live in the macro world, it feels like a single unitary reality is simpler, but actually, the MWI makes fewer assumpti…

It is not simpler, since MWI still needs the Born postulate to actually predict the results of quantum experiments. It replaces this idea of the wave function collapsing with a redefinition of what a measurement apparatus / observer is (in MWI, a measurement apparatus exists in a single "branch" of the wave function). This is especially problematic because QM doesn't predict any particular decomposition of a quantum…

>This is especially problematic because QM doesn't predict any particular decomposition of a quantum state into particular classical states.

It does predict it, this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#113

Post by pdonis » Fri, Oct 21, 2022, 4:26 PM UTC

jfengel wrote: post #92

dzdt wrote: post #4

There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas. My feeling is that Evere…

That's right: the "collapse" interpretation is an approximation. You can derive if from the MWI. We already know that the "worlds" separate and almost completely cease interacting. The interactions decrease to the order of "two to the power of Avogadro's number" almost immediately. The collapse interpretation rounds that to zero (at some arbitrary point). To make that more concrete you have to add a term to the equat…

> You can derive if from the MWI.

Not as an actual physical process, no. "Collapse" in the MWI just means that, once the "worlds" have decohered, within each "world" the wave function can be collapsed to the one that describes the measurement result observed in that "world", for purposes of predicting future results. But in the MWI this is just a mathematical convenience and doesn't correspond to anything physically happening.

In the theories being tested by the experiments described in this article, though, collapse is an actual physical process: there is only one "world". This contradicts the MWI.

> They've detected the other branches, albeit in a very indirect way.

What experiments are you referring to here?

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#114

Post by tsimionescu » Fri, Oct 21, 2022, 4:27 PM UTC

wongarsu wrote: post #12

ThalesX wrote: post #6

Earlier quoted context omitted.

Having been passionate about this for awhile, I have a layman’s surface overview of the state of physics, or I like to believe so. It amused me to no end when my elderly aunt came to ask me for a tl;dr; on Quantum Physics. She stopped me shortly asking how she can control the universe with her mind. I tried to explain a bit about the famous interpretation (Copenhagen, not Von Neumann–Wigner) but she would have none o…

To be fair, the Copenhagen interpretation doesn't really fit today's world view. We have largely moved away from viewing humans or "consciousness" as something special, so the notion that an observer collapses the wave function just seems weird now. "Many worlds" or "the wave function never collapses, you are in a superposition" both make much more sense with how we currently view the world

The CI has absolutely nothing whatsoever to do with consciousness. The CI view is simply "when a quantum system interacts with a measurement device, the wave function of the quantum system changes to one of some subset of real values, with a probability given by the modulus of the amplitude of that value in the wave function". Exactly what constitutes a measurement apparatus is undefined, but it certainly doesn't involve a human.

In fact, it is precisely the MWI that requires human observers for its explanations, at least to some extent, as it makes the entire notion of a classical world a fiction that only exists inside your own head because you are the one that's getting entangled with a quantum system. Any probability you compute in MWI is relative to you personally, since in MWI any possible event happens with probability 1 when checking at the universe level.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#115

Post by pdonis » Fri, Oct 21, 2022, 4:28 PM UTC

iainmerrick wrote: post #58

herdcall wrote: post #54

Earlier quoted context omitted.

Check out "superdeterminism," first proposed by Bell (of Bell's inequality fame) himself and currently being vigorously pursued by Sabina Hossenfelder. There is no "collapse" per se according to this model and everything is really classical with hidden variables, and the entanglement comes because everything shared the same origin (the big bang). A very fascinating and in my opinion the best explanation, though it's…

I have never fully understood that idea, even after reading Hossenfelder's posts about it. The idea that there's a "block universe" and everything is predetermined is comprehensible, sure; but what's the exact mechanism by which Bell's inequality is violated in experiments? Doesn't that require the universe to have been carefully set up to look as if it's acting in a weird non-local way, on an experiment-by-experimen…

> Doesn't that require the universe to have been carefully set up to look as if it's acting in a weird non-local way, on an experiment-by-experiment basis?

Yes. That is a big reason why most physicists do not favor superdeterminism.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#116

Post by dchichkov » Fri, Oct 21, 2022, 4:30 PM UTC

jfengel wrote: post #92

dzdt wrote: post #4

There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas. My feeling is that Evere…

That's right: the "collapse" interpretation is an approximation. You can derive if from the MWI. We already know that the "worlds" separate and almost completely cease interacting. The interactions decrease to the order of "two to the power of Avogadro's number" almost immediately. The collapse interpretation rounds that to zero (at some arbitrary point). To make that more concrete you have to add a term to the equat…

If quantum computers would work, it would be very difficult to explain "where" the computation happens without having some sort of MWI into picture. But if, somehow, quantum computers keep their noise levels up without producing any interesting acceleration I'll expect that a theory would emerge at some point explaining that. And it may disambiguate which interpretation is the one.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#117

Post by macrolocal » Fri, Oct 21, 2022, 4:30 PM UTC

rubidium wrote: post #35

dzdt wrote: post #4

There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? Some smart people have tried to make theories where wave function collapse is a real thing. These new experiments are working to rule out those ideas. My feeling is that Evere…

Many worlds is just some peoples favorite because they get too weirded out but indeterminism. They prefer a billiard ball universe and so invented the hairbrained many worlds theory.

Eh, these are models, not reality. The Many-Worlds interpretation makes the fewest assumptions, so it's probably the least prone to over-fitting.

Until we find philosophically or experimentally distinguished ways to pare down its ontological cost, working with most general theory is just keeping an open mind.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#118

Post by pdonis » Fri, Oct 21, 2022, 4:30 PM UTC

GoblinSlayer wrote: post #112

tsimionescu wrote: post #103

Earlier quoted context omitted.

It is not simpler, since MWI still needs the Born postulate to actually predict the results of quantum experiments. It replaces this idea of the wave function collapsing with a redefinition of what a measurement apparatus / observer is (in MWI, a measurement apparatus exists in a single "branch" of the wave function). This is especially problematic because QM doesn't predict any particular decomposition of a quantum…

>This is especially problematic because QM doesn't predict any particular decomposition of a quantum state into particular classical states. It does predict it, this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.

> this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.

No, that's not what decoherence is. Decoherence is the spread of entanglement over a very large number of untrackable degrees of freedom (usually referred to as the "environment"). This makes interference effects unobservable. But decoherence itself does not involve any "splitting"; the entanglement that decoherence spreads over a very large number of untrackable degrees of freedom has to already be there before decoherence can act on it.

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#119

Post by pdonis » Fri, Oct 21, 2022, 4:32 PM UTC

TheOtherHobbes wrote: post #96

fallingfrog wrote: post #90

Earlier quoted context omitted.

Well, the thing is that the MWI is actually simpler than all the other interpretations, because it simply removes the idea of wave function collapse entirely. Any theory involving wave function collapse is adding something extra to quantum physics that can't be demonstrated experimentally. Because we live in the macro world, it feels like a single unitary reality is simpler, but actually, the MWI makes fewer assumpti…

You don't think postulating an unknown number of extra universes with a very poorly defined relationship with each other qualifies as an extra assumption?

The MWI does not postulate extra universes, although that pop science description is unfortunately common even though it is wrong. The MWI just says that the one universe that exists is actually very, very different from what we perceive. (To be clear, I'm not saying this correct description makes the MWI any more palatable. I am just clarifying exactly what kind of unpalatableness the MWI requires.)

Re: Experiments spell doom for physical-collapse explanation of quantum weirdness

#120

Post by kgwgk » Fri, Oct 21, 2022, 4:36 PM UTC

iainmerrick wrote: post #58

herdcall wrote: post #54

Earlier quoted context omitted.

Check out "superdeterminism," first proposed by Bell (of Bell's inequality fame) himself and currently being vigorously pursued by Sabina Hossenfelder. There is no "collapse" per se according to this model and everything is really classical with hidden variables, and the entanglement comes because everything shared the same origin (the big bang). A very fascinating and in my opinion the best explanation, though it's…

I have never fully understood that idea, even after reading Hossenfelder's posts about it. The idea that there's a "block universe" and everything is predetermined is comprehensible, sure; but what's the exact mechanism by which Bell's inequality is violated in experiments? Doesn't that require the universe to have been carefully set up to look as if it's acting in a weird non-local way, on an experiment-by-experimen…

> what's the exact mechanism by which Bell's inequality is violated in experiments?

It’s not violated - it simply doesn’t apply because it’s predetermined that the measurements that are done work well together. The experimenters follow the script.