Hacker News Reader

Comment by Vampiero | Hacker News Reader

Hearing at all implies sound waves. If there are soundwaves there is air inside the box. If there is air inside the box the system can't be in a superposition because it has long since decohered.

In other words it's not the act of "listening" that collapses the wave function but the interaction of air molecules that creates the sound in the first place, even if no one is there to listen.

A "measurement" or an "observation" doesn't actually require a human, or a living thing. It's just a physical interaction.

View on HN

This does not help at all, quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition. The box and the air and the cat can interact all they want, you will still have a superposition of cat dead - and box and air in some state - and cat alive - and box and air in some other state. And no point are we addressing how we get rid of one half of the entangled state. And even if I see the box wiggle a bit, we only declare that me watching the box somehow was an measurement and that collapsed the wave function without answering the question why I did not simply get entangled with the box, too.

> quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition

This is wrong, they do collapse, you can test this using the double slit experiment. No human has to be there for the wavefunction to collapse and thus break entanglement.

If what you said was true then this wouldn't be a part of physics, physics is only about stuff we can measure not nonsense that has no meaning.

It's not at all clear how, when, and why wave function collapse happens. The only known quantity there, decoherence, is related to interaction between a coherent quantum system and a noisy decohered environment, which causes the original system to decohere itself.

Theoretically, as far as it is known today, it is possible for a quantum system of any size, and at any temperature, to remain in a coherent state, if it could be perfectly isolated from the rest of the world.

Any limits to this process, where a perfectly isolated system would undergo wave function collapse without any outside interference, would be a Nobel-prize worthy discovery and would constitute a new theory of physics: quantum mechanics and QFT allow no such process.

Edit: and the double slit experiment is unrelated - the screen and the slit are noisy classical systems that are not in any way isolated from the rest of the classical world. In principle, if you performed the experiment using a screen and slits made entirely of atoms entangled with the emitter, and isolated perfectly from the rest of the world and the CMB and everything, you'd get a different result: this is what QM predicts.

> quantum mechanics and QFT allow no such process

Double slit experiments shows collapse happens without any outside interference, nothing outside of the room has to do anything for the resulting measurements to show collapse happened. You can then afterwards enter the room, look at the recorded data and see that collapses did happen.

So you must have misunderstood what those theories says about collapses. They don't say exactly when collapses happens or what causes them, that is the unsolved problem, but we can know that they do happen.

In the normal experiments we perform, the screen is already a classical system, collapsed / entangled with the rest of the "classical world" (depending on the interpretation). When the particles interact with the screen, of course their wave function collapses as well.

But what if instead you had a screen that was itself a quantum system? You'd then use the Schrodinger equation without any collapse (without the Born rule) to describe the results. You'd have to perform other measurements on the quantum screen to obtain a classical result that can be interpreted, of course. But nothing in QM says that there is some size limit, so in principle it should be possible to prepare a quantum system of any size, including the size of a typical double slit experiment screen, that doesn't collapse.

Quantum mechanics, i.e. the Schrödinger equation, and wave function collapse are incompatible. While not measured a quantum system evolves unitarily, when measured the quantum system ends up in the measured Eigenstate which is a non-unitary change. But any measurement device is also a quantum system and therefore the combined system should also evolve unitarily. And now we have a serious problem to which nobody has a generally accepted answer.

What is deficient about the original explanation that the wave function is not real but rather a scientific theory? The issue has been unresolved for about a century now, it's relatively unsatisfying but hasn't held the science back in any way.

The problem with that idea is that we don't have any other model, and little room for it. We already know from Bell's inequality and the experiments that verified it that there can't be underlying variables that we haven't detected yet (or if they exist, we have to rethink special relativity and by extension QFT). It's also quite hard to explain why the wave function is so good at predicting the results of experiments if it's not a part of the underlying reality.

There is a thing we call the universe and it does what universes do. What physics does is searching for models - mostly described in the language of mathematics - that describe the universe or parts of it. Some parts of the universe are well modelled by the Navier-Stokes equations, we also call those parts of the universe water [1]. But upon closer inspection one notices that description is not perfect, one can obtain a more accurate model if one treats the water bits of the universe as a collection of an enormous number of particles, water molecules. And we can keep going, treat molecules as collections of atom, break atoms into electrons and nucleons, split the nucleons into quarks and gluons, who knows how deep the rabbit hole goes.

Maybe we will even eventually find the ultimate model, a model that describes every aspect of the world perfectly, even though it is not obviously clear how we would determine that we did not miss some aspect of the universe in our models. But even then the map would not be the territory, the model would not be the universe or a piece of it. Imagine a wave function would perfectly describe an electron, would we be justified to say that an electron is a wave function? No, because for them to be the same thing, it is not sufficient that the wave function has all the properties of the electron, it would also have to have only the properties of the electron. But the wave function is an equation, I can write it down or square it, things I can most certainly not do with the electron.

So the wave function is of course only a mathematical description of a quantum system and not a real thing out there in the universe. But that is not the problem, the problem is that we have two seemingly contradictory descriptions of quantum systems. We have the Schrödinger equation describing the unitary evolution of isolated quantum systems and it works really well. But we also have the Born rule and the collapse postulate which tell us the probability distribution of measurement outcomes and how measurement changes the state into the measured Eigenstate, a non-unitary change, and this also works really well. But there is the problem, on the one hand a measurement - whatever that exactly is - seems to change the state non-unitarily but on the other hand a quantum system together with a measurement device is just a bigger quantum system and should therefore evolve unitarily. Something has to give.

As you say, that is very unsatisfying, but it was also not a complete roadblock, shut up and calculate worked pretty well. But I would not confidently declare that this does not hold us back at all, who knows what progress can be unlocked by solving the issue? It might of course also be mostly inconsequential but I guess that is something we will only find out after the fact.

[1] Or fluids if you want to be pedantic.

Hey thanks for your thoughtful response.

In terms of what we may/not be missing out on, I say that because it seems few of the possibilities remaining after a century of investigation (many worlds, relational/information based approaches, etc) contain any new physics or scientific insight.

Eg if we could somehow tell that MW or pilot wave theory is "correct", we would still have nothing to do but shut up and calculate the same things, no?

If unitary evolution continues in some sense, we would still need to throw factors away in order for the theory to remain predictive to us.

Particles are only observed to be in a single place at a single time. My understanding is that it may be that we lose the perspective necessary to see them elsewhere, since we are embedded in the same eigenstate (or something close to it). But the task of science is confined to observables. I question, what are the scientific questions that collapse postulate holds back from being solved?

> evolve unitarily and do not destroy a superposition

Unitary evolution can destroy superposition of the small subsystem by introducing negligible amount of superposition into large system.

Not sure if that is true, but assuming it is, does this really count as destruction instead of moving it around and spreading it? After all unitary evolution is reversible and I could therefore reconcentrate the superposition into the small system. It would however seemingly be a good explanation how to get from the decayed non-decayed superposition of a few atoms to an almost alive or almost dead cat. This is however also what makes me doubt your statement, if true, why are we still puzzled about Schrödinger's cat?

Well the naming is a different question, but it is consistent with observation of what we call wavefunction collapse.

In Schroedinger cat experiment, the evolution is not thermalization but rather entangling the whole cat with atom state. I don't think we're really puzzled about it now, since we know that macroscopic objects can be in superposition. So cat, theoretically, can be in superposition of alive and dead until measured. I don't think there is any contradiction or confusion here physics-wise.

This isn't quite right, wave function collapse is relative so while within the box an observer's wave function of the box will be in a definite eigenstate, the wavefunction of the box for an observer isolated from the box will still be in a superposition of its states.

In other words, the fact that an observation takes place from within the box and collapses the wave function for an observer in the box does not mean that the wave function for every observer also collapses. A wave function collapse for one observer does not imply a wave function collapse for any other observer.

> wave function collapse is relative

No it isn't, why do you think this? If it was we wouldn't be able to see it in our experiments, as where the particle ends up would depend on which observer is looking, which would get absurd consequences.

I provided my explanation for why this is the case. A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device.

Of course, a second measurement device that's completely isolated from the first measurement device can eventually interact with the first measurement device and the two will converge and share the same wave function. However, until the second measurement device interacts with the first one or with the original quantum system being observed, then from the point of view of the second measurement device, the first measurement device is in a superposition of all the possible states that it could have observed.

This can continue on and on, with a third measurement device which is entirely isolated from the second measurement device having a wavefunction that's a superposition of all the possible observations that the second measurement device will observer...

> A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device.

Wave function collapse is a real measurable phenomena, what you describe is not measurable so just a belief, don't mix in your beliefs with actual results.

Wave function collapse is one of many equivalent (so far as we know) interpretations of quantum mechanics and not all interpretations have wave function collapse whatsoever. With that said even in the Copenhagen interpretation which is the most popular interpretation of quantum mechanics, the moment in time when the wave function collapses is relative to each measurement device. It is not the case that just because some measurement device that has been entirely isolated from me happens to perform a measurement on a quantum system, that my wave function of that quantum system will also collapse. On the contrary my wave function remains exactly as is until the moment I perform a measurement on the quantum system which collapses my wave function of said system.

Double slit experiment disproves you here, the observed pattern is the same for every observer and shows a collapse happened.

One common view is that there is THE wave function of the universe. If it collapses, it collapses, there is no way it can collapse for some observers and not others. You might not know that it has collapsed but that does not make it not collapsed.

There is a different view that the wave function is just an representation of the knowledge someone has about the state of the universe or some part of it, in which case it can collapse for some and not others. On the other hand there is not really any collapse at all in this case, it is just a fancy term for updating once believe about the state of the universe or some system.

It is also important to note that a collapse is not a total collapse into a point. Measuring different things will collapse the wave function along different dimensions, only measuring everything in the universe would collapse the wave function along all dimensions into a point, or as far as the uncertainty principle permits.

Imagine you have a lamp that sends a single photon, a double slit, and a screen with silver or something to catch the photon. All of that is inside a black box. You press the button to send the photon, open the box and see the screen.

There are 5 possible scenarios:

1) The wave function collapsed when the photon colides with the screen. You can calculate the time using distance/c. All the other steps are classic. You just open the box and see where the spot was.

2) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hits the screen there is a collapse and ow you have a classic spot in a classic screen. Later you just open the box.

3) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hit the screen, the gas gets entangled and later the walls of the box get entangled. There is no collapse until you open the box. Anyway, when you open the box everything get's classic and you see the usual result.

4) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hit the screen, the gas gets entangled and later the walls of the box get entangled. There is no collapse. When you open the box your eyes and brain get entangled. There is no collapse! Anyway, each of the versions of you see the expected result because each one is entangled with a screen that has a spot as expected.

5) Shut up and calculate!

I'm a strong proponent of 5 because people pay me to calculate the distribution of the probability of the spot and not to tell weird stories.

The easy way to get the result of 5 is to use the scenario in 1, and that is what is teach, and that is essentially the Copenhagen interpretation and anyone that is not insane will use that scenario to make the calculations because it's "obvious" that all the other add negligidle corrections and are infinitely more difficult.

Here the problem is to define what is a measurement. It's "obvious" that the screen is good enough, but there is no good technical definition of what "obvious" means.

The problem is that the only non magical scenario is 4, where there is no collapse and you get entangled too. There are still many technical details to solve, but if it's real you would see the same experimental results than in 1, 2, 3 and 4, but the advantage is that there is no magic rule of collapse. In this scenario, measurement does not collapse wave functions, it just an illusion when some of the parts is big enough to get decoherence and make all the versions so different that you get (almost) no interference effect.

Anyway we are still not sure, so you can believe any of them until someone solves the problem, probably in 50 or 100 years.

This is actually an open problem in QM. We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed). And given special relativity, it is definitely possible that two different observers could disagree on the order that two independent measurements of the same system happened.

Do note that the result of the experiments is not going to differ, it is guaranteed to be consistent by the fact that both experiments are measuring the same quantum system. It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly.

And in the many worlds interpretation, it is indeed believed that apparent wave function collapse is a relative phenomenon, caused by entanglement with the classical environment, which spreads out at the speed of light.

> We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed)

We do know that they can't disagree as collapsed particles behave differently than uncollapsed particles.

> It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly.

Quantum systems aren't about knowledge or statistics, the wave function is an actual physical thing that changes how the particle behaves. If the wave function has collapsed it no longer behaves the same as before, so what you said here is wrong.

You're now arguing against claims no one has made, which is apparent from the fact that you seem to only quite very tiny snippets of what's said while ignoring the main argument.

You seem to just want to argue for the sake of arguing.

Wasn't the argument that two observers can disagree whether a collapse has happened? I offered evidence that they can't, as otherwise they would see different results in the double slit experiment.

There is nothing in the double slit experiment that makes collapse necessarily an objective reality. There is in fact no way to check if collapse of the wave function has happened or not.

I imagine you're thinking of the version of the double slit experiment with a detector installed next to one of the slits, which prevents the interference pattern from forming. However, this only applies for a "measurement device" installed at the slits. That is, a classical system interacting with the photon/electron/etc before it hits the classical screen. But this is not mysterious at all: the classical device causes decoherence of the wave function and thus prevents self-interference of the beams traveling through the two slots. That is true even in interpretations that consider that wave function collapse is not a physical phenomenon, such as Many Worlds.

But this doesn't prove much. We already know that a photon hitting an atom doesn't cause wave function collapse, while a photon hitting a detector does. We can say that an atom is not a measurement device and an observer is, but this immediately raises the question of why. Or, we can choose the MWI route and say that the wave function never collapses, and the only difference between an atom and a detector is that we are not entangled with that atom, but we are entangled with the detector. And if so, it immediately follows that if we could get a detector that we are not entangled with, we would not see the same results as we do in the traditional double slit + detector on a slit experiment.

After a measurement, the wave function of a particle, and of any particles entangled with it, collapses. But this isn't itself a measurable difference.

For example, when performing measurements on two entangled particles at very far away places, the wave function collapses instantly, across any distance, even light years away. After that experiment, the result on the other side is 100% determined (if performed in the same basis). But there is no experiment whatsoever that could be done to tell if the collapse has happened or not from the other side.

This is the resolution of the famous EPR paradox: wave function collapse is non-local (instantaneous), but it is probably impossible to send classical information with it faster than the speed of light limit.

Catch 22: and if there is no air inside the box, then you are certain the cat is dead.

Reply on news.ycombinator.com