# Schroedinger's cat is always dead

post by PhilGoetz · 2011-08-26T17:58:39.387Z · score: -12 (14 votes) · LW · GW · Legacy · 56 comments

Suppose you believe in the Copenhagen interpretation of quantum mechanics.  Schroedinger puts his cat in a box, with a device that has a 50% chance of releasing a deathly poisonous gas.  He will then open the box, and observe a live or dead cat, collapsing that waveform.

But Schroedinger's cat is lazy, and spends most of its time sleeping.  Schroedinger is a pessimist, or else an optimist who hates cats; and so he mistakes a sleeping cat for a dead cat with probability P(M) > 0, but never mistakes a dead cat for a living cat.

So if the cat is dead with probability P(D) >= .5, Schroedinger observes a dead cat with probability P(D) + P(M)(1-P(D)).

If observing a dead cat causes the waveform to collapse such that the cat is dead, then P(D) = P(D) + P(M)(1-P(D)).  This is possible only if P(D) = 1.

If you don't say that only conscious agents can collapse waveforms, then you have to agree that something in the box collapses the waveform as seen from inside the box, while it's still uncollapsed to Schroedinger. And Schroedinger's opening the box collapses that waveform for him; but it is still uncollapsed for someone outside the room.  This seems like it might be equivalent to many worlds - all possibilities already exist; you just haven't chosen which one you're going to access until you open the box.

But if you do say that only conscious agents can collapse waveforms, then it's something about their mental processes that does the collapsing. This could mean their beliefs matter. And then, the cat is always dead.

ADDED:  People.  Read the entire post before responding.  I am not claiming that the cat is always dead.  I am not claiming that consciousness collapses waveforms.  I am claiming that there are only 2 known alternatives:

1. Interactions collapse waveforms, regardless of whether conscious entities are involved.  It is not possible, under this view, for any waveform to be either collapsed or not collapsed, because there will be some viewpoints from which it has collapsed, and some from which it hasn't.  So this appears to be equivalent to many-worlds.
2. Consciousness collapses waveforms.  Leading to weirdness such as, potentially, the cat always being dead.

If you can't produce another alternative, and you don't believe in many-worlds, you owe me an upvote.

Finally, this post is supposed to be fun!  You are crushing all whimsy and playfulness on LessWrong when you pile downvotes like bricks on anything playful because it does not provide a complete and satisfactory resolution.

comment by Mitchell_Porter · 2011-08-27T05:31:58.775Z · score: 5 (9 votes) · LW(p) · GW(p)

Pardon me for saying this in capitals, but this thread is full of people getting it wrong.

THE COPENHAGEN INTERPRETATION DOES NOT SAY THAT CONSCIOUSNESS COLLAPSES THE WAVEFUNCTION.

THE COPENHAGEN INTERPRETATION DOES NOT SAY THAT CONSCIOUSNESS COLLAPSES THE WAVEFUNCTION.

THE COPENHAGEN INTERPRETATION DOES NOT SAY THAT CONSCIOUSNESS COLLAPSES THE WAVEFUNCTION.

At least, not in the sense that everyone is assuming, according to which the wavefunction is a real thing.

In the Copenhagen interpretation, the wavefunction is not a physical entity. It has the same sort of ontological status as a probability distribution. It is not itself a probability distribution, but it is the mathematical object from which probability distributions are derived. And it also has this in common with probability distributions, that when new information about the world is obtained, you have to update it. This is the only sense in which "consciousness collapses the wavefunction" in the Copenhagen interpretation.

comment by PhilGoetz · 2011-09-21T20:23:17.455Z · score: 1 (1 votes) · LW(p) · GW(p)

In the Copenhagen interpretation, the wavefunction is not a physical entity. It has the same sort of ontological status as a probability distribution. It is not itself a probability distribution, but it is the mathematical object from which probability distributions are derived. And it also has this in common with probability distributions, that when new information about the world is obtained, you have to update it.

It sounds like you're saying the wavefunction represents our uncertainty. My impression is that you know more about grand theories of physics than I do; but I'm confident that the wavefunction does not represent mere uncertainty. Entanglement, the double-slit experiment, the uncertainty principle, etc., can't be explained by uncertainty.

comment by wedrifid · 2011-09-21T20:52:25.662Z · score: 4 (4 votes) · LW(p) · GW(p)

You can tell there is really something wrong with the naming conventions when the Uncertainty Principle can't be explained by uncertainty.

comment by wedrifid · 2011-08-27T05:55:03.947Z · score: 0 (4 votes) · LW(p) · GW(p)

At least, not in the sense that everyone is assuming, according to which the wavefunction is a real thing.

"Everyone" is assuming no such thing.

comment by Mitchell_Porter · 2011-08-27T06:39:00.448Z · score: 0 (2 votes) · LW(p) · GW(p)

Here is what I see people saying:

"observation provokes collapse to a random eigenstate, and the result of the collapse determines the result of the observation"

"If you don't say that only conscious agents can collapse waveforms, then you have to agree that something in the box collapses the waveform as seen from inside the box"

"I'm betting that interacting with another subatomic particle is what collapses a waveform"

"The Copenhagen interpretation does say that a certain class of interactions - measurement interactions - produce collapse"

But in the Copenhagen interpretation, "collapse of the wavefunction" has exactly the same sort of reality as "collapse of the probability distribution" does. It is not supposed to be a physical process like "apple falls to the ground".

comment by PhilGoetz · 2011-08-27T15:44:18.634Z · score: 3 (3 votes) · LW(p) · GW(p)

But in the Copenhagen interpretation, "collapse of the wavefunction" has exactly the same sort of reality as "collapse of the probability distribution" does. It is not supposed to be a physical process like "apple falls to the ground".

I don't understand the distinction. If the probability distribution collapses, why is that not a physical process? Is it an immaterial, spiritual probability distribution?

Anyway, Schroedinger is the person who came up with the cat thought-experiment; which shows that he thought at the time that the collapse of the wavefunction had tangible, physical reality.

comment by Mitchell_Porter · 2011-08-28T03:49:46.894Z · score: 5 (5 votes) · LW(p) · GW(p)

You may wish to read Schrödinger's original article. It is about whether quantum mechanics could possibly be a complete description of reality. He considers two options: either that the wavefunction is a statistical description of an ensemble of possible states, or that the properties of microphysical systems are objectively undefined prior to observation. The cat shows up (in part 5) as a demonstration that the second option is absurd. This is the part where it may sound (to a modern reader, versed in MWI-think) that he is talking about a physical process of wavefunction collapse, but he's not. He's discussing a situation where the cat goes from "neither alive nor dead" to one or the other, at the moment of observation. At this stage, the wavefunction is not being proposed as a description of the cat's physical state.

Parts 7 through 9 are where he addresses the Copenhagen interpretation, and contrasts it with wavefunction realism: "psi-function as expectation catalog", versus "psi-function as description of state".

If the probability distribution collapses, why is that not a physical process? Is it an immaterial, spiritual probability distribution?

Flip a coin but don't look at it. Then look at it. The probability distribution of the coin just "collapsed". The collapse of the wavefunction in the Copenhagen interpretation is the same thing. A wavefunction is like a prior; you adjust it on the basis of information acquired.

comment by wedrifid · 2011-08-27T07:33:49.403Z · score: 0 (4 votes) · LW(p) · GW(p)

I challenged the universality of a claim ("everyone") and you have attempted to support it by selecting anecdotes. This is obviously a mistake. Not only that, the examples don't even claim what you say they do. Although the second example at least alludes to it none of them assumes that:

THE COPENHAGEN INTERPRETATION DOES [...] SAY THAT CONSCIOUSNESS COLLAPSES THE WAVEFUNCTION.

Say that some people are confused about what the Copenhagen Interpretation says, by all means. But if you are going to throw about grossly exaggerated accusations for the sake of emphasis then at least refrain from trying to defend them!

comment by Mitchell_Porter · 2011-08-27T08:26:08.820Z · score: 5 (9 votes) · LW(p) · GW(p)

"Everyone" is an anaphor for the "people getting it wrong" mentioned in my first sentence.

the examples don't even claim what you say they do

They all affirm that (quoting myself) "the wavefunction is a real thing". So perhaps I should have shouted that part?

THE COPENHAGEN INTERPRETATION DOES NOT SAY THAT THE WAVEFUNCTION IS A REAL THING.

But then - although the Copenhagen interpretation features in Phil's first sentence - it's not ubiquitous in the discussions. So maybe I should just say

THE WAVEFUNCTION MIGHT NOT BE A PHYSICAL THING! MOST PHYSICISTS THINK OF IT AS JUST LIKE A PROBABILITY DISTRIBUTION!

The problem is not just a misapprehension about what the Copenhagen interpretation says. The problem is the ubiquitous background assumption of "wavefunction realism". Either wavefunctions are real, and they collapse under observation, or spontaneously, or because of interaction; or wavefunctions are real, and they don't collapse, and there are many worlds.

You can't obtain clarity about quantum mechanics until you at least understand the perspective, historically identified with the Copenhagen interpretation, according to which wavefunctions are not real, and just provide a formal calculus for obtaining probabilities pertaining to the things that are real, the "observables". Observables are where reality lies in quantum mechanics, not wavefunctions.

Once you understand that, you are then free to note that quantum mechanics is incomplete, you can run off and try to make a better theory in terms of actually existing wavefunctions or in terms of something else, etc. But most discussions on this site are starting off confused, not just about what Copenhagen says, but about what quantum mechanics says, and it's because they start from the premise of wavefunction realism.

comment by PhilGoetz · 2011-08-27T15:45:51.317Z · score: 2 (4 votes) · LW(p) · GW(p)

THE COPENHAGEN INTERPRETATION DOES NOT SAY THAT THE WAVEFUNCTION IS A REAL THING.

Then why did Schroedinger talk about the cat in the box?

I think you must be talking about some new interpretation unknown to Schroedinger.

comment by prase · 2011-08-27T21:42:30.349Z · score: 2 (2 votes) · LW(p) · GW(p)

I think you must be talking about some new interpretation unknown to Schroedinger.

Even if Schrödinger hadn't known some interpretation when he devised his feline thought experiment in 1935, I don't think it justifies calling it new.

comment by PhilGoetz · 2011-08-27T15:01:55.452Z · score: -2 (4 votes) · LW(p) · GW(p)

Look: I did NOT CLAIM THAT CONSCIOUSNESS COLLAPSES THE WAVEFUNCTION. Just READ the post. I am not going to explain it again. It is VERY CLEAR that it is presenting TWO alternatives, ONE of which is that consciousness collapses the waveform.

Almost everyone simply chooses one of the two alternatives, and says, "That's impossible!".

Please stop doing that. In general. This is the nature of the post: You have only 2 obvious alternatives, and BOTH of them are impossible.

I am NOT proposing a solution. I am NOT saying that the cat is always dead. I am pointing out that there is no currently viable solution.

If you can't resolve the situation BY PICKING ONE OF THE TWO ALTERNATIVES - NOT by dismissing one and ignoring the other - you should change your downvote to an upvote.

Note:Mitchell Porter is very smart, and did not actually say that I said that consciousness collapses the waveform.

comment by Mitchell_Porter · 2011-08-28T03:55:09.059Z · score: 0 (0 votes) · LW(p) · GW(p)

Phil, this comment was not directed specifically at you. It was my way of saying that all the discussion in the comments was based on the false premise that the choice is between "wavefunctions are real and they collapse" and "wavefunctions are real and they don't collapse". Your post offered that as the choice too, but you didn't originate that misconception, you were just expressing the local paradigm.

comment by satt · 2011-08-26T19:45:18.963Z · score: 3 (3 votes) · LW(p) · GW(p)

If observing a dead cat causes the waveform to collapse such that the cat is dead,

Unless I've misunderstood the Copenhagen interpretation, it doesn't. (Then again, it's not a brilliantly well-defined interpretation to start with.) It's not that one observes X and therefore compels the wavefunction to collapse to state X rather than state Y; it's that observation provokes collapse to a random eigenstate, and the result of the collapse determines the result of the observation (not the other way round). As such, you can't derive the statement that P(D) = P(D) + P(M)(1-P(D)), and so there's no constraint forcing P(D) = 1.

comment by Davorak · 2011-08-26T18:10:55.989Z · score: 3 (5 votes) · LW(p) · GW(p)

There seem to be several problems with the reasoning displayed in your post.

Could you communicate what you want people to take a way from this so I can put the post in a proper context and decide how to communicate the problems I see?

comment by PhilGoetz · 2011-08-26T18:15:36.531Z · score: 0 (0 votes) · LW(p) · GW(p)

My guess is that the Copenhagen interpretation isn't supposed to talk about what your beliefs are; it's just supposed to talk about entanglement of waveforms. So Schroedinger's beliefs about whether the cat is alive or dead don't matter.

But I've heard people talk about such situation as if Schroedinger's belief that the cat was alive or dead was important. Especially in connection with the idea that a waveform only truly collapses when an observation is made by a conscious agent.

If you don't say that only conscious agents can collapse waveforms, then you have to agree that something in the box collapses the waveform as seen from inside the box, while it's still uncollapsed to Schroedinger. And Schroedinger's opening the box collapses that waveform for him; but it is still uncollapsed for someone outside the room.

But if you do say that only conscious agents can collapse waveforms, then it's something about their mental processes that does the collapsing. This could mean their beliefs matter. And then, the cat is always dead.

The whole business seems murky and mysterious to me, and I hope for some enlightenment. And if it is not enlightening, it can at least be entertaining.

comment by Davorak · 2011-08-26T18:48:07.524Z · score: 1 (1 votes) · LW(p) · GW(p)

But I've heard people talk about such situation as if Schroedinger's belief that the cat was alive or dead was important. Especially in connection with the idea that a waveform only truly collapses when an observation is made by a conscious agent.

No. Strong evidence for consciousness being a fundmental part of reality would be a huge deal.

The whole business seems murky and mysterious to me, and I hope for some enlightenment. And if it is not enlightening, it can at least be entertaining.

It is often not so entertaining for the person trying to explain because it takes most people serious effort to understand, something most are unwilling to do for amusement sake. In person it can be more productive in my opinion, but I have not had much success online.

QED by Feynman is a decent place to start if you want to learn more about quantum mechanics.

comment by Eugine_Nier · 2011-08-27T07:11:51.407Z · score: 0 (0 votes) · LW(p) · GW(p)

But I've heard people talk about such situation as if Schroedinger's belief that the cat was alive or dead was important.

In the consciousness-collapsing-waveform interpretation of quantum mechanics (which is not the same as the Copenhagen interpretation BTW) it is observation that collapses quantum states not belief. In particular the fact that Schroedinger misinterpreted his observation is irrelevant to how the wave function collapses.

comment by ata · 2011-08-26T18:27:21.408Z · score: 2 (4 votes) · LW(p) · GW(p)

P(M) > 1

Typo?

If observing a dead cat causes the waveform to collapse such that the cat is dead, then P(D) = P(D) + P(M)(1-P(D)). This is possible only if P(D) = 1.

Sorry if I'm missing something, but are you implying that the Copenhagen interpretation implies that the waveform collapse happens so as to retroactively make the cat dead if Schrödinger would have mistaken the cat for dead? Why would the sort of model that forms in Schrödinger's brain after the fact control what did in fact happen, even given the Copenhagen interpretation? (I didn't think it was quite that silly.)

comment by PhilGoetz · 2011-08-26T18:29:03.887Z · score: 0 (0 votes) · LW(p) · GW(p)

Typo.

comment by Jack · 2011-08-27T01:27:06.615Z · score: 1 (5 votes) · LW(p) · GW(p)

One can twist the Copenhagen Interpretation to say just about anything you want it to say. That's because it isn't a proper interpretation but basically a commitment to not think about the inherent contradiction involved in considering measuring devices to be classical systems. It was basically just a way to let physicists do the math without worrying about broader ontological implications- in no way is it a serious candidate for a true interpretation of quantum mechanics. It's best thought of as the problem statement, not a possible solution.

comment by Douglas_Knight · 2011-08-27T06:09:56.062Z · score: 2 (2 votes) · LW(p) · GW(p)

Once, the Copenhagen really was an interpretation. As physicists shied away from the mysticism of the founders, they refused to admit their deviation and redefined the Copenhagen interpretation to be what you say.

comment by NancyLebovitz · 2011-08-26T19:14:42.259Z · score: 1 (1 votes) · LW(p) · GW(p)

Pure verbal reasoning here-- I don't know the math.

It seems extremely unlikely that conscious agents are required to collapse waveforms. If nothing else, consciousness is too vaguely defined to be that important to subatomic processes. (IIRC, Egan's Quarantine has a good bit of fun with consciousness and collapsing waveforms.)

I'm betting that interacting with another subatomic particle is what collapses a waveform, though even that seems rather weird as I type it. Are some (all?) particles sufficiently particle-like to collapse waveforms, or can two waveforms collapse each other, or is there no point in trying to do this in words?

I don't think a moderately clueless Schrodinger is enough to turn a sleeping cat into a dead one with the awesome power of Not Knowing What's Going On. People make perceptual mistakes all the time, and it probably isn't bending reality, though I'll grant that if it were fairly rare, it would be very hard to find out.

comment by pragmatist · 2011-08-26T19:21:30.113Z · score: 1 (1 votes) · LW(p) · GW(p)

If interacting with another subatomic particle were sufficient to collapse a waveform, then you couldn't prepare two particles in an entangled state. But entangled particles are regularly prepared in the lab, so your conjecture seems to have been refuted.

I think the best bet for a collapse interpretation of quantum mechanics is some form of spontaneous collapse theory, like GRW.

comment by NancyLebovitz · 2011-08-26T21:59:25.470Z · score: 1 (1 votes) · LW(p) · GW(p)

I had no idea the measurement problem existed. I knew there was a question about what waveform collapse meant, but not that people knew so little (nothing?) about the collapse itself.

In the pre-google state of gazing at 'GRW' I came up with General Relativity Wuxia (wrong for all letters). Perhaps waveforms collapse when there's a duel with a definitive outcome. Excuse me, I'm feeling slightly giddy. There's got to be a cool science fictional theory, a ridiculous fantasy theory, and an embarrassing new age theory to be fitted into a really remarkable blank spot.

comment by PhilGoetz · 2011-08-27T15:17:42.926Z · score: 0 (0 votes) · LW(p) · GW(p)

Right - but that's part of the point of the post. You read the post, and then reasoned your way from its beginning, to its halfway point.

The post is not claiming that the cat is always dead.

comment by Nisan · 2011-08-26T19:05:00.395Z · score: 1 (1 votes) · LW(p) · GW(p)

I think you could set up an experiment where only one bit escapes from the box, a 1 signifying "alive" and a 0 signifying "maybe dead". But then on observing a 0 the cat would still be in a superposition of states.

Caveat: I am not a physicist. I got 3 A's in quantum physics, but it wasn't at Caltech.

comment by orthonormal · 2011-08-28T01:24:59.451Z · score: 1 (1 votes) · LW(p) · GW(p)

Actually, this would be like the case where you shine a light in the two-slit experiment to see which slit the electron passes through, but the light is dim enough that it won't interact with the electron every time. You get the sum of an interference pattern and a classical double Gaussian in that case.

In the experiment you suggest, the experimenter's wavefunction would be the sum of one where the cat is dead and the experimenter observes a 1, and one in which the experimenter observes a 0 and the cat remains in dead-alive superposition from the observer's perspective. (That is, the weighted sum of a factorizable wavefunction with a non-factorizable one.)

comment by prase · 2011-08-26T23:44:42.983Z · score: 0 (0 votes) · LW(p) · GW(p)

But then on observing a 0 the cat would still be in a superposition of states.

Exactly.

comment by Raemon · 2011-08-26T19:02:53.595Z · score: 1 (1 votes) · LW(p) · GW(p)

This would be probably be a good place for me to inquire what the hell Schrodinger's Cat actually means. I've never fully understood it.

Is the cat, according to the theorized (proven?) model supposed to be literally alive and dead, or is that just a metaphor for a deeper level physics thing that doesn't have an obvious analog in the typical human's model of the world? If it's a metaphor, or a partial metaphor, what exactly is it a metaphor for?

comment by Zed · 2011-08-26T20:48:43.227Z · score: 1 (3 votes) · LW(p) · GW(p)

Schrödinger's cat is a thought experiment. The cat is supposed to be real in the experiment. The experiment is supposed to be seen as silly.

People can reason through the math at the level of particles and logically there should be no reason why the same quantum logic wouldn't apply to larger systems. So if a bunch of particles can be entangled and if on observation (unrelated to consciousness) the wavefunction collapses (and thereby fully determines reality) then the same should be able to happen with a particle and a more complex system, such as a real live cat. After all, what is a cat except for a bunch of particles? This means the cat is literally both alive and dead until the superposition resolves.

The problem is that philosophers have sometimes abused this apparent paradox (both alive and dead!?) as some sort of Deep Mystery of quantum physics. It's not a deep mystery at all. It's just something that illustrates that if you take the Copenhagen interpretation literally then you have to bite the bullet and admit that a cat (or a human, etc) can be both alive and dead at the same time. Not just seemingly so, but actually so in reality. As that's the only thing that's consistent with the small scale quantum experiments. Schrödinger came up with this thought experiment because he realized the implications of the Copenhagen interpretation and concluded the implications were absurd.

If you're not willing to bite that bullet (and most quantum physicists nowadays aren't) then you have to look at other possibilities. For instance that the world splits and that in one world the cat is alive and in the other the cat is dead. In one world you'll observe the cat being alive and in the other world you observe the cat as dead. Both worlds are equally real and in both worlds you have the sensation of being in the only real world.

(I only have an elementary understanding of QM)

comment by Eugine_Nier · 2011-08-27T07:28:02.731Z · score: 1 (1 votes) · LW(p) · GW(p)

For instance that the world splits and that in one world the cat is alive and in the other the cat is dead. In one world you'll observe the cat being alive and in the other world you observe the cat as dead. Both worlds are equally real and in both worlds you have the sensation of being in the only real world.

Don't take the "splitting" too literally either. Otherwise you've merely replaced the problem of when a wave function collapses, with the problem of when the worlds splits.

comment by Zed · 2011-08-27T10:04:49.476Z · score: 1 (1 votes) · LW(p) · GW(p)

The collapse of the wave function is, as far as I understand it, conjured up because the idea of a single world appeals to human intuition (even though there is no reason to believe the universe is supposed to make intuitive sense). My understanding is that regardless of the interpretation you put behind the quantum measurements you have to calculate as if there are multiple words (i.e. a subatomic particle can interfere with itself) and the collapse of the wave function is something you have to assume on top of that.

8 minute clip of EY talking with Scott Aaronson about Schrödinger's Cat

comment by Mitchell_Porter · 2011-08-27T10:29:38.777Z · score: 1 (5 votes) · LW(p) · GW(p)

My understanding is that regardless of the interpretation you put behind the quantum measurements you have to calculate as if there are multiple worlds

You have to do this in any probabilistic calculation, especially when you have chains of dependent probabilities. The mere fact that, e.g., the behavior of a ball bouncing around on a roulette wheel can be understood in terms of branching possible worlds, is not usually interpreted as implying that those possible worlds actually exist, or that they interact with this one.

The peculiarity of quantum probability is that you can get cancellation of probability amplitudes (the complex numbers at the step just before probabilities are computed). Thus in the double slit experiment, if you try to analyze what happens in a way analogous to Galton's Quincunx, you end up saying that particles don't arrive in the dark areas, because the possible paths 'cancel' at the amplitude level. This certainly makes no sense for probabilities, which are always nonnegative and so their sum is monotonically increasing - adding a possible path to an outcome can never decrease the overall probability of that outcome occurring. Except in quantum mechanics; but that just means that we are using the wrong concepts to understand it, not that there is such a thing as a negative probability.

However, it is not as if we know that the only way to get quantum probabilities is by supposing the existence and interaction of parallel worlds in the multiverse, and in fact all the attempts to make that idea work in detail end up in a conceptual shambles (see: measure problem, relativity problem, preferred basis problem). We don't need a multiverse explanation; we just need a single-world explanation that gives rise to the same probability distributions that are presently obtained from wavefunctions. The Nobel laureate Gerard 't Hooft has some ideas in this direction which deserve to be much better known; they are at least as important as anything in the "famous" interpretations associated with Bohm, Everett, and Cramer.

comment by Zed · 2011-08-27T11:29:21.592Z · score: 1 (1 votes) · LW(p) · GW(p)

Thanks for the additional info and explanation. I have some books about QM on my desk that I really ought to study in depth...

I should mention though that what you state about needing only a single-world is in direct contradiction to what EY asserts: "Whatever the correct theory is, it has to be a many-worlds theory as opposed to a single-world theory or else it has a special relativity violating, non-local, time-asymmetric, non-linear and non-measurepreserving collapse process which magically causes blobs of configuration space to instantly vanish [...] I don't see how one is permitted to hold out any hope whatsoever of getting the naive single world back."

My level of understanding is insufficient to debate QM on a serious level, but I'd be very interested in a high level exchange about QM here on LW. If you disagree with Eliezer's views on QM I think it is a good thing to say that explicitly, because when you study the different interpretations it's important to keep them apart (the subject is confusing[1] enough as is).

[1] a property of yours truly

comment by wedrifid · 2011-08-27T14:07:20.750Z · score: 3 (3 votes) · LW(p) · GW(p)

My level of understanding is insufficient to debate QM on a serious level, but I'd be very interested in a high level exchange about QM here on LW. If you disagree with Eliezer's views on QM I think it is a good thing to say that explicitly, because when you study the different interpretations it's important to keep them apart (the subject is confusing[1] enough as is).

I agree that such an exchange would be useful. Unfortunately it would be hard to have with Mitchell_Porter because of the reputation he has gained for his evangelism of qualia and Quantum Monadology. People who have sufficient knowledge and interest in physics to be useful in such an exchange are less likely to become significantly involved if they think they are just arguing with a crackpot (again).

comment by Zed · 2011-08-27T15:34:32.458Z · score: 0 (2 votes) · LW(p) · GW(p)

Yikes! Thanks for the warning.

comment by Mitchell_Porter · 2011-08-28T03:16:58.264Z · score: 0 (0 votes) · LW(p) · GW(p)

I almost added this warning myself, though it would have been with a different emphasis:

Such debates about MWI as I have had here, in the past, have often not been a clean discussion of the merits of MWI versus some other interpretation, because I won't shut up about these other issues, which are far more interesting and important. There are severe problems awaiting anyone who wants to explain consciousness in terms of interactions between distributed, coarse-grained physical states; there is an interesting possibility that it could instead be explained in terms of a single, microphysically exact entangled state; that is my preoccupation. The debate over MWI is just a sideshow.

MWI looks bad from my ontological perspective, because I say we should take the apparent ontology of the self more seriously, as its actual ontology, whereas MWI extends the dismissal of conscious appearances further. But MWI also looks bad from a pure physics perspective, which just wants an exact mathematical description of the world that works, and cares nothing about its relationship to the "subjective world" of "lived experience". The most shocking feature of MWI, once I really understood it, is that it cannot by itself make any correct predictions at all, because the entire predictive content of QM comes from the Born rule (or projection postulate), and no derivation of the Born rule within MWI exists. You often hear people saying "all the interpretations of QM make the same predictions", but this is not true for MWI. You could say it makes no predictions (since it has no substitute for the Born rule), or that it makes wrong predictions (if you just count the worlds naively), but the only version of MWI which makes the same predictions as QM is the, so far imaginary, version which contains a derivation of the Born probabilities.

It's almost comical, how new problems for MWI keep appearing, the more I discuss it with people. For example, the standard lay understanding of MWI is that there are well-defined worlds, and they split into more worlds when there are quantum events. But among the informed defenders of MWI, it's usually considered desirable to reject the idea of a "preferred basis", such as would be implied by a canonical division of the wavefunction into a unique set of worlds. Instead, it's considered a feature, not a bug, that you can express a wavefunction as a superposition of basis functions in many complementary ways. But what I've realized is that you can extend this perspective into the interior of a person. You can consider the density matrix of my left brain hemisphere, and the density matrix of my right brain hemisphere, and you have the same freedom to choose basis functions for each of them. So, in an MWI without a preferred basis, you can't even say that there is a specific set of copies of you in definite states, spread across the multiverse. We can describe your left hemisphere in the position basis, and the right hemisphere in the position basis, and that will produce one set of copies; or we could describe your left hemisphere in the position basis, and your right hemisphere in the momentum basis, and that will produce a differently defined set of copies.

This isn't even a physics debate, in the sense of making calculations and comparing their results with each other and with reality. MWI exists mostly as a verbal construction by means of which people try to make sense of QM. But if you go and chase down the implications of what is being said, you end up with nonsense. Of course, the more advanced MWI advocates like Robin Hanson, David Deutsch, etc, do have something quantitative to say; though often the key to debunking them still revolves around seeing past the equations, to the plain meaning of what they are arguing or asserting. But the debate on LW isn't at that level.

ETA: It might seem that the Gell-Mann–Hartle formalism of decoherent histories offers a derivation of the Born rule. I would argue that the procedure whereby an absolute prior for a set of decoherent histories is obtained, is an adaptation of the Born rule to the GM–H formalism, and that it faces the same problem of motivation or interpretation, as does any attempt to just add the Born rule to MWI: why do some worlds count for more than others? GM–H provides a slightly novel way to get the right probabilities, but it still hinges on attaching unequal weights to the worlds, and what this could mean, in a multiverse context where all the worlds exist equally, is left unexplained.

comment by Mitchell_Porter · 2011-08-27T13:03:52.018Z · score: 3 (5 votes) · LW(p) · GW(p)

I have posted here, on this topic (MWI), perhaps a hundred times. There are many comments from me in the Quantum Physics sequence. Two years ago I made a top-level post in favor of the rather anodyne position that MWI is not the favored interpretation, it's just one among many. Now I would take a much stronger line, that MWI has very little going for it. It cannot even reproduce the predictions of QM, which derive from the "Born rule" that MWI discards, in favor of having only the Schrodinger equation. Instead, the ideological stance is adopted that Only The Wavefunction Exists, and the recovery of the Born probabilities, which contain the whole of QM's empirical content, is left for future research. Or, even worse, it's just assumed. But this is a problem because, if you count the branches of the wavefunction, they should all count for the same, which would mean that the probabilities of all outcomes are equal, which would mean that MWI is falsified. Robin Hanson dreamed up an idea for how to get the right multiplicities of worlds, but it means that the individual worlds are somewhat messy superpositions. There are various other claims in the physics and philosophy literature of having recovered the Born rule, none of them satisfactory. One should be aware, especially in the era of arxiv.org - which is not peer-reviewed - that bad papers are available in abundance; though in this area, even good physicists produce bad papers advancing bogus arguments.

In the quotation above, Eliezer is once again assuming that wavefunctions exist and that the only alternative to MWI is wavefunction collapse. "Blobs of configuration space" don't "vanish" if they were only ever domains in a probability distribution; see my remarks elsewhere on this page on the necessity of understanding that wavefunctions need not exist. I have made these points in the past ( 1 2 3 ).

comment by Manfred · 2011-08-27T18:32:22.214Z · score: 0 (0 votes) · LW(p) · GW(p)

If you're not willing to bite that bullet (and most quantum physicists nowadays aren't)

Incorrect. Most physicists today would tell you that schodinger's cat is |alive>+|dead>.

If the world simply "splits," then you've got a hidden-variable theory, which has been ruled out by Bell's inequality measurements. Instead what happens is more complicated, and is mathematically equivalent to one-world quantum mechanics.

comment by Raemon · 2011-08-26T20:57:21.591Z · score: 0 (0 votes) · LW(p) · GW(p)

That makes reasonable sense, but I assume that the "box" can't just be a box, it has to be a completely sealed environment, where the cat particles can't even react with each other? Or at least with any adjaecent gas particles or passing neutrinos or whatever?

comment by Zed · 2011-08-26T21:32:37.189Z · score: 1 (1 votes) · LW(p) · GW(p)

Yep, the box is supposed to be a completely sealed off environment so that the contents of the box (cat, cyanide, Geiger counter, vial, hammer, radioactive atoms, air for the cat breathe) cannot be affected by the outside world in any way. The box isn't a magical box, simply one that seals really well.

The stuff inside the box isn't special. So the particles can react with each other. The cat can breathe. The cat will die when exposed to the cyanide. The radioactive material can trigger the Geiger counter which triggers the hammer, which breaks the vial which releases the cyanide which causes the cat to die. Normal physics, but in a box.

comment by orthonormal · 2011-08-28T01:18:15.737Z · score: 2 (2 votes) · LW(p) · GW(p)

Clarification: the outside world does interact with the inside, but not in any way that depends on whether the cat is alive or dead. (If the contents of the box are positively charged electrically, they can continue to exert a force on objects outside. But if the cat is positively charged†, then the box needs to shield its influence on the electromagnetic field so that you can't tell from outside if it's moving or not.)

† That is, if it's a cation.

comment by Zed · 2011-08-28T02:15:56.125Z · score: 0 (0 votes) · LW(p) · GW(p)

I think that what you're saying is technically correct. However, simplifying the thought experiment by stating that the inside of the box can't interact with the outside world just makes the thought experiment easier to reason about and it has no bearing on the conclusions we can draw either way.

comment by orthonormal · 2011-08-28T02:35:23.046Z · score: 0 (0 votes) · LW(p) · GW(p)

It's a distinction with a difference: the point is that a closed system means a factorizable wavefunction, not lack of interaction. (The latter is strictly impossible!)

comment by PhilGoetz · 2011-08-26T19:51:19.710Z · score: 1 (1 votes) · LW(p) · GW(p)

I'm pretty sure it's meant to be literal. Also, that it's not proven.

comment by thakil · 2011-08-26T20:02:10.172Z · score: 1 (1 votes) · LW(p) · GW(p)

It was invented by shroedinger to describe his discomfort with the copenhagen interpretation

comment by pragmatist · 2011-08-26T19:08:15.005Z · score: 0 (0 votes) · LW(p) · GW(p)

If you don't say that only conscious agents can collapse waveforms, then you have to agree that something in the box collapses the waveform as seen from inside the box, while it's still uncollapsed to Schroedinger.

I don't understand this claim. If I think collapse can occur without conscious observation (say by the interaction of microscopic and macroscopic systems, however that distinction may be drawn, or by some spontaneous dynamical process) why would I have to agree that the waveform is still uncollapsed to Schroedinger? You seem to be assuming here that Schroedinger's epistemic state is relevant to whether or not the waveform is collapsed to him, but I thought the whole point of this option was to render Schroedinger's epistemic state (and, indeed, his status as a conscious agent) irrelevant to the situation.

comment by PhilGoetz · 2011-08-26T19:49:10.782Z · score: 1 (1 votes) · LW(p) · GW(p)

The idea of the non-collapsed waveform is that there are two systems that haven't interacted; and each is indeterminate to the other until they interact.

But you can have any number of nested systems, each of which may or may not have interacted with the system it is contained in. If the cat in the box can be in an indeterminate state to Schroedinger, then Schroedinger and the cat can be in an indeterminate state to someone out in the hallway. Whether Schroedinger has opened the box or not.

The Copenhagen interpretation doesn't admit of the possibility that every waveform is either collapsed or not collapsed. If that were the case, every atom's waveform would always collapse as soon as it interacted with another atom!

It can only be collapsed with respect to some observer. UNLESS you make the stipulation that only very special observers, like conscious agents, can collapse a waveform.

comment by pragmatist · 2011-08-26T20:18:40.675Z · score: 1 (1 votes) · LW(p) · GW(p)

Here is how I interpret your claim here: The only way the Copenhagen interpretation could be an absolute state theory - i.e. a theory where the quantum state of a system is absolute, not relative to some other system - is for collapse to be caused by conscious agents. Am I misinterpreting you?

If I'm not, I don't see why you believe this. The Copenhagen interpretation does say that a certain class of interactions - measurement interactions - produce collapse. And I acknowledge that it cannot maintain that all physical interactions are measurement interactions. That view has been conclusively refuted empirically. However, why think that the only alternative is that measurement interactions must involve conscious observation? Bohr, as far as I can tell from his mysterious proclamations on the topic, seemed to think that any interaction with a macroscopic system is a measurement interaction. He didn't think that consciousness played any essential role in his interpretation. I think Wigner was the one who emphasized consciousness.

Now you could say that Bohr's interpretation is untenable, since microscopic/macroscopic is a continuum, not a binary distinction. Also, macroscopic systems are just built out of microscopic systems, so why think the measurement problem doesn't apply to them? I agree! But the exact same criticisms can be raised about consciousness, so Wigner's interpretation is not on sounder footing here. So I guess I'm not seeing why you think a Wigner-type delineation of measurement interactions is the only way to avoid Copenhagen collapsing into Everett.

comment by PhilGoetz · 2011-08-27T15:21:41.457Z · score: 0 (0 votes) · LW(p) · GW(p)

Here is how I interpret your claim here: The only way the Copenhagen interpretation could be an absolute state theory - i.e. a theory where the quantum state of a system is absolute, not relative to some other system - is for collapse to be caused by conscious agents. Am I misinterpreting you?

You got it! Thank you!

If I'm not, I don't see why you believe this. The Copenhagen interpretation does say that a certain class of interactions - measurement interactions - produce collapse. And I acknowledge that it cannot maintain that all physical interactions are measurement interactions. That view has been conclusively refuted empirically. However, why think that the only alternative is that measurement interactions must involve conscious observation? Bohr, as far as I can tell from his mysterious proclamations on the topic, seemed to think that any interaction with a macroscopic system is a measurement interaction.

There might be some other type of measurement, not involving consciousness, that would occur rarely enough to work. I'm just not aware of any.

comment by Manfred · 2011-08-27T04:06:58.307Z · score: -1 (1 votes) · LW(p) · GW(p)

How many legs does a dog have if you call the tail a leg? Four. Calling a tail a leg doesn't make it a leg.

Abraham Lincoln

comment by PhilGoetz · 2011-08-27T15:16:01.818Z · score: 0 (2 votes) · LW(p) · GW(p)

See comment above to Mitchell. Read the post. It is NOT claiming what you think it is claiming.

I don't understand how all you people can read the post and think that it's claiming that the cat is always dead. The title is a lead-in. You don't read the title and then stop thinking.

comment by Manfred · 2011-08-27T18:16:49.231Z · score: 1 (1 votes) · LW(p) · GW(p)

I'll be more direct. You misused the word "observes." It has both a colloquial meaning and a meaning as a piece of physics jargon. Only one of the definitions works when you apply it to physics. Although you succeeded in being wrong in the post, you in fact over-succeeded and were too wrong to be useful.

comment by PhilGoetz · 2011-08-27T20:18:39.398Z · score: 0 (0 votes) · LW(p) · GW(p)

I'm sorry I was rude. I don't understand this last comment of yours.

I think that what you call a misuse of the word observes, is the interpretation under which consciousness mysteriously interacts with the world. I expect, if I had to guess, that most people educated in the matter don't believe that happens. But it's still in the running, because the other viewpoint also leads to bizarre conclusions.

(Another way of saying this is: I am not convinced that the view that waveforms collapse, in a way that does not involve consciousness, doesn't have a secret implied invocation of consciousness buried in it. I don't know how a single-world interpretation can get away from requiring something like consciousness to do very selective collapsing of waveforms.)

So, using the word "observes" as it would be used under an interpretation that you disagree with, isn't a "misuse".

comment by Manfred · 2011-08-27T22:02:10.307Z · score: 0 (0 votes) · LW(p) · GW(p)

Hmm, it looks like wikipedia does give the "consciousness causes collapse" position more credence than I had expected. But that may just be the equal time fallacy, as its cited proponents have titles like "founding director of the Center for Ecoliteracy." I've never run into it in person among physicists.

EDIT: Of course, what you used in the article wasn't the "consciousness causes collapse" position at all - not even those people, by and large, would agree that if you mistake a sleeping cat for a dead one, that kills it.