Quantum Question

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Ixtellor
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Quantum Question

Postby Ixtellor » Fri Dec 01, 2017 8:21 pm UTC

So as a casual fan...

It is my understanding that the concept of reality not existing until it is observed has been proven. And based on what I read, seems like this is 'true' and defendable.

My question is : who does the observer have to be? Microbes, plants, sentience? Does a tree falling in the woods actually occur if no sentient being is there to observe it or does basic life with the ability to sense qualify as observation?
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Re: Quantum Question

Postby Zohar » Fri Dec 01, 2017 8:38 pm UTC

You're misunderstanding the phenomenon.

The reality is the universe exists as a set of probable situations. All these situations exist, and are real, they just all exist at the same time, with certain probabilities. Once observed, reality collapses into a randomly chosen situation (based on the previously described probability). But it doesn't mean what happened before the observation wasn't real or that reality has always been this way.
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Re: Quantum Question

Postby doogly » Fri Dec 01, 2017 8:40 pm UTC

An observer is a Hermitian operator.

But yeah so this illustrates a problem with developing an ontology based off of your approximation. The statement "nothing becomes real until it is observed" could be more precisely rephrased as "nothing behaves classically until it interacts with something classical." The fundamental description never switches over from wacky quantum --> nice and real, it is always wacky quantum. *That* is the real reality, we now know.

But once your electron finishes its slit business and has its position measured (by a metal plate, which is not by any measure sentient, so no worries about that mysticism), it can then be described by nice, familiar, old style reality. Which is comforting, if boring.
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Re: Quantum Question

Postby Xanthir » Fri Dec 01, 2017 9:09 pm UTC

A perhaps more accurate analogy (and I'll let doogly yell at me about this) is that "classical" physics (everything has a single well-known value, there's no randomness, etc) is, roughly, the "averaging" of quantum physics, in the same way that, no matter what distribution of random variables you start from, if you sample it a ton and then take the average, the average will always follow a normal distribution. That is, you can have variables with wacky distributions, like flat, or two humps, or much more likely on the extreme ends rather than in the middle; but if you take a couple of samples and average them, the averages always come out with the normal distribution, a single nice smooth hump centered on the mean (if you're averaging enough of them, but "enough" is a fairly small number). Similarly, "classical" physics is just the "average" of whole lots of quantum interactions; because each individual interaction is too small for us to see normally, we never notice the weirdness underneath, only the "average" behavior that arises from the collective interactions. (Experiments showing off weird quantum stuff are due to us *very carefully* isolating some quantum variables, so they *don't* interact with lots of other things, and it doesn't just dissolve into averages.)

...diving into stats for analogies is maybe not the most effective way to simplify things.
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Re: Quantum Question

Postby doogly » Fri Dec 01, 2017 9:40 pm UTC

I think for the most part I don't have to yell at Xanthir. I think it just shows that it's important that when you want to use words like "reality," you shouldn't use them for the parts of your story that are the averaged, processed, coarse grained, etc, bits. It's the wacky bits underneath.

The word "real" is just mostly fraught. I can't recommend it.
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Re: Quantum Question

Postby Ixtellor » Fri Dec 01, 2017 10:02 pm UTC

Zohar wrote:You're misunderstanding the phenomenon.

The reality is the universe exists as a set of probable situations. All these situations exist, and are real, they just all exist at the same time, with certain probabilities. Once observed, reality collapses into a randomly chosen situation (based on the previously described probability). But it doesn't mean what happened before the observation wasn't real or that reality has always been this way.


This is how I understood it, I just didn't want to type out all this stuff. Read many many articles on the double slit theory which led to many more articles.

Just not sure what constitutes observation. So does 'observation' include when particles interact with other particles meaning to atoms of H are within the proximity of each other that would result in 'contact' based on the infinite number of positions/spins etc?
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Re: Quantum Question

Postby ucim » Fri Dec 01, 2017 10:34 pm UTC

Ixtellor wrote:Just not sure what constitutes observation.
Consider the Schrodinger's cat experiment; it is "observed" when the box is opened (and the inside interacts with the outside). Now, put that experiment (and, if you like, some cat-loving scientist) in its own box.

If you open the outer box without opening the inner box, the cat is still superposed.

If you open the inner box and open the outer box, the cat is observed by "our" outside world.

What happens when you open the inner box but leave the outer box closed? The cat-lover in the box observes the {living|dead} cat, and knows how it turned out. But we don't. So, is the cat alive or dead? Or what?

The answer is "what". Its state is superposed with that of the cat-lover, whom we cannot observe until we open the outer box. But the cat lover knows what happened, because it did happen. Or did it?

It's turtles all the way down. Every (i.e. subatomic) interaction is an "observation", but only by and of the particles that actually were involved. To particles not involved, no observation took place and the entanglement merely extends outward. We are a cat lover in a box in a box in a box....holding a box in a box... with a cat. Do we exist? The answer is... "to whom?"

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Re: Quantum Question

Postby speising » Fri Dec 01, 2017 10:43 pm UTC

the word "observed" was an unfortunate choice in QM. better to use "interact", which doesn't seem to imply a consciousness.

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Re: Quantum Question

Postby Pfhorrest » Fri Dec 01, 2017 11:25 pm UTC

And the apparently collapse of the wavefunction only happens from the perspective of whoever is interacting with the thing in question. But there is a whole lot of constant interaction happening all the time in most mundane circumstances. The reason why weird Schrodinger's Cat type things don't seem to happen in normal life is that it's really really hard to be not observing the cat at all. Yeah, it's in a box your eyes can't see through, but nevertheless information about what's inside that box is reaching you, even if you're somewhere else entirely unaware of any cat experiment happening, so in the relevant sense -- the sense in which you are indirectly interacting with the cat -- you are still observing it.

To someone who is somehow not observing it, and consequently not observing you (because to do so would be to indirectly observe the cat), the cat is still neither alive nor dead, and you are simultaneously in the different states you would be in depending on whether you observed the cat to be alive or dead (e.g. if you're actually looking in the box, you're simultaneously in the states of thinking "the cat is alive" and "the cat is dead").
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Re: Quantum Question

Postby doogly » Sat Dec 02, 2017 3:03 am UTC

Yeah, the cat would need to be in a sufficiently low pressure and temperature box that odds are really good it is dead. Much better than even.

I think Ucim is off, above. If you have a cat in a box and your friend Alice is watching the box, but you put Alice in a box of her own, and aren't observing her (but you know that she has opened the box), you have Classical Ignorance as to her result. You know she got one or the other. Before she opened the box, she (and you, if you know what her plans are) would say the state is 1/sqrt(2)( |dead> + |alive>). She has no uncertainty, she knows what the state it is. It's not an eigenstate. She can calculate that if she opens her box, the results will be dead or alive, since the states are orthogonal, each with probability 1/2. She gets nice classical probabilities in her prediction, but in her statement of "what is going on," she has a quantum state.

Then she opens her box. You know she has because she promised to do it at noon, and she keeps her promise. (Apply an uncertainty factor to that, if she's not a dependable person. But that would be sad. She has One Job, and it's so simple that Schrodinger could do it.*) You would describe the state as being maybe dead, maybe alive, fifty fifty. But definitely, right now it is one of those two! (Unless Alice sucks at this, it's possible.)

This looks like splitting hairs, and if you are just doing one measurement, it is. That's why things get really interesting and fun only when you send things through a few lenses, and let the uncertain states interfere. EPR is the real sauce, Schro's cat and is the opening act. Double Slit, maybe halfway between.

*Schrodinger was pretty mediocre, but, I mean, that's fine. A tier or two below Dirac isn't shabby. But he had trouble with grokking Hilbert space. He tended to think about wave functions as a fuzzy function defined over physical space, when that is not the real deal.
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Re: Quantum Question

Postby ucim » Sat Dec 02, 2017 4:44 am UTC

doogly wrote:...and aren't observing her (but you know that she has opened the box)
How do you know she opened the box? Her promise to do so is insufficient; after all, you could rig the cat box the same way. From the POV of the cat, it's either alive or dead, and not superposed. Inside the cat box, the cat knows, but Alice doesn't. Inside the Alice box, Alice knows, but you don't.

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Re: Quantum Question

Postby Pfhorrest » Sat Dec 02, 2017 5:12 am UTC

Yeah I'm confused about the state of this conversation, doogly sounded like he agreed with me, but what you're saying Jose sounds like the same thing I was saying, and it sounds like he's disagreeing with you.
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Re: Quantum Question

Postby doogly » Sat Dec 02, 2017 6:37 am UTC

Let's say Alice is p unreliable. I give her a 50/50 shot. That is classical. It is not then 1/sqrt(2)( |alice does> + |alice don't>). She's not in a quantum state, she's not entangled with anything, I just don't know what she's going to do. Quantum mechanics is about more than just ignorance.
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Re: Quantum Question

Postby Pfhorrest » Sat Dec 02, 2017 6:49 am UTC

Okay yes that's true, but then if Alice is in the magical impossibly sealed Schrodinger's Cat box instead of the cat, she is in a superposition just like the cat would be (ignoring for the moment that, like you said before, the conditions of any real such box would just kill her, like they would the cat; say it's a magic interaction-shielded box with normal livable conditions inside it). So if, inside the Alice box, is another Schrodinger's cat box, and Alice in her box open's the cat box, she is now in a superposition of having seen a live cat and having seen a dead cat, from our perspective outside the Alice box. That's the main point Jose seems to be making, and the one that I was making.

I think Jose only mentioned the (un)reliability of Alice opening the cat-box as promised as a concession to the point that we, outside the Alice box, don't really know if she opened the box or not. But if she did as promised, then she's in a superposition of having seen a live cat and having seen a dead cat, to us.
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Re: Quantum Question

Postby Tchebu » Sat Dec 02, 2017 3:30 pm UTC

You can definitely just trust Alice to open the box at a given time. This simply corresponds to knowing the Hamiltonian (i.e. the rules of time-evolution) of the Alice+cat system which is part of the assumptions in the thought experiment. "Opening the box at noon" just corresponds to turning on a strong enough interaction between Alice and the cat at noon. This is part of the definition of your system, which you know ahead of time.

If you have a cat in a box and your friend Alice is watching the box, but you put Alice in a box of her own, and aren't observing her (but you know that she has opened the box), you have Classical Ignorance as to her result.


Just thought I'd clarify this point, which may also help the rest of the discussion. This statement can be right or wrong depending on exactly what is meant...

The initial state of the Alice+cat system is
|Alice doesn't know> (|alive cat> + |dead cat>)
up to factors of sqrt 2 and whatnot.

Opening the inner box (the one with the cat) turns on an interaction, which by construction ensures that Alice's state now correlates with the cat's, so it quickly turns the above state into

|Alice saw alive cat> |alive cat> + |Alice saw dead cat> | dead cat>

this is a superposition state (because it's a sum of eigenstates of our chosen observable) in which Alice and the cat are entangled (because it can't be factorized into the form |Alice's state>|Cat's state> ), and in which Alice knows the cat's state. At this point we have to be careful about what questions we, as outside observers, ask. There are several questions we might be tempted to ask before opening the box:

1) Is the cat dead or alive?
2) What did Alice see when she opened the box?

3) What's the quantum state of the cat?
4) What's the quantum state of Alice?
5) What's the quantum state of the Alice+cat system?

The first two questions are typical questions about observables and are answered by opening the outer box. For each question there are two possible answers with 50% probability and that probability has quantum mechanical origins. In this sense, the quoted statement above is incorrect.

Question 5 is trivial. The state of the total system is the thing I wrote above.

Questions 3 and 4 are more subtle. The entangled state doesn't factorize into the form |Alice's state>|Cat's state>, so there's no way to actually assign a definite quantum state to either Alice or the cat. However what we CAN do is assume total ignorance of what's going on with Alice and sum over all of Alice's possible states. This will result in a classical distribution for the cat's state, which will unsurprisingly be that the cat has a 50% classical probability to be alive/dead. Similarly we can instead sum over all the cat's possible states and get a classical probability for Alice's quantum state. In this sense the quoted statement above is correct, but the classical probabilities only appear once we sum over the subsystem that we claim total ignorance of.
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Re: Quantum Question

Postby gmalivuk » Mon Dec 04, 2017 2:45 pm UTC

Ixtellor wrote:Just not sure what constitutes observation.
Interacting with something classical.

Anything with lots of particles moving and interacting a lot tends to be classical, and our brains definitely have a lot of particles interacting a lot, but so do cats and, for that matter, boxes.
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Re: Quantum Question

Postby Xanthir » Tue Dec 05, 2017 5:36 pm UTC

doogly wrote:
Then she opens her box. You know she has because she promised to do it at noon, and she keeps her promise. (Apply an uncertainty factor to that, if she's not a dependable person. But that would be sad. She has One Job, and it's so simple that Schrodinger could do it.*) You would describe the state as being maybe dead, maybe alive, fifty fifty. But definitely, right now it is one of those two! (Unless Alice sucks at this, it's possible.)


Just to make sure I'm not misunderstanding you (because if I am, then you'd be saying something that really confuses me):

Alice interacts with a definitely live or dead cat, when she opens the box. Her states mesh with the cat's states, superposition dissolves into the averages of classical mechanics.

But I, sitting outside the quantum-isolated room that Alice is in, will see Alice in a superposition of "Alice sees a live cat" and "Alice sees a dead cat". (Or rather, the careful weak measurements I make of the inside of the room that would let me distinguish the two situations without strongly interacting with them return superposed results.) Effectively identical to the results of "Alice and a cat are in a quantum-isolated room, with a cat-killing poison canister connected to a radioisotope", removing the intermediary isolated box that the cat+poison were wrapped in.

(But I'll never see "Alice interacts with a superposition'd cat".)

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Re: Quantum Question

Postby doogly » Wed Dec 06, 2017 1:12 am UTC

Yeah, that seems right.

It's weird. There are good reasons why you can't experimentally get cats into such a state. I'd be really pleased to see like, a couple grains of sand in such a state.
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Re: Quantum Question

Postby jewish_scientist » Wed Dec 06, 2017 1:15 am UTC

Imagine that you have a bag that contains 3 green balls and 2 red balls and you take a ball out of the bag without looking at it. Using only the available information, you can conclude that the ball has a 3/5 chance of being green and a 2/5 chance of being red. You look at the ball and it's red. Using only the available information, you can conclude the ball has a 0 chance of being green and a 1 chance of being red.

Before you look at it, the ball has a probability of being be green or red although it cannot be either at the same time. Since that is a real mouthful, lets just say that the ball is in a "superposition" between green and red. After you look at it, these probabilities disappear and are replaced by certainties. The "superposition", which only exists because there were probabilities, has been destroyed (a.k.a. collapsed) by you viewing (a.k.a. observing) the ball.

This is kinda sorta how it works.

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Re: Quantum Question

Postby doogly » Wed Dec 06, 2017 1:32 am UTC

No that is a completely classical situation you described.

The difference between classical ignorance and quantum indeterminacy is not readily apparent in things like the above situations, but when you include repeated measurements it matters, this is the Bell inequality business.
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Re: Quantum Question

Postby ucim » Wed Dec 06, 2017 3:20 am UTC

jewish_scientist wrote:you take a ball out of the bag without looking at it
"looking at it" isn't what you think it means, at least on a quantum level. You have to not only "not look at it", it has to be impossible for you to be able to have interacted with it. This means that the ball (and the other ball) has to not have any interactions with any particles that have any interactions with..... any particles that interact with you. That's really hard to do if there are more than a handful of particles.

When you open the box, it is interacting with the air and sending photons all over the place, which are hitting your (closed) eyelids but still affecting you. The very delicate quantum superposition is destroyed. It's only if you can remain actually isolated from all this that you could claim superposition... but when you open the box, the superposition isn't between red and green balls, it's between that and all the particles in the air and the room and the furniture and all.... and that yuge superposition looks very classical.

But that's what "classical" is... it's a yuge superposition with everything around it.

In the case of the cat in a box, in a box with Alice, the isolation can be maintained in the same way that it's maintained in the original cat box (cats being macroscopic too). As far as the cat is concerned, it's dead or alive, but not both. Alice doesn't see this; as far as Alice is concerned, the cat in the box is "superposed".(*) Inner box opened, Alice sees an actual dead (or live) cat. We see nothing of the kind. As far as we (outside the outer box) are concerned, nothing has happened. We "see" (and "saw") a superposition of {{dead|live} cat and Alice not looking}, {dead cat and Alice looking}, and {live cat and Alice looking}. When the outer box is opened, we will see, actually, either Alice looking at a dead cat, Alice looking at a live cat, or Alice mad at us for opening the box before she got a chance to see the cats, which are still superposed.

(*)Note: This is not quantum mechanics, this is an interpretation of what quantum mechanics means. There are others, equally weird. "Superposition" isn't a thing... it's a mathematical expression. Many worlds isn't a thing either. We don't know what the thing is, and it's an act of faith that there even is a thing (aside from the cogito ergo sum thing). But that's how the math works out.

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Re: Quantum Question

Postby Tchebu » Wed Dec 06, 2017 3:37 pm UTC

d. We "see" (and "saw") a superposition of {{dead|live} cat and Alice not looking}, {dead cat and Alice looking}, and {live cat and Alice looking}.

Small nitpick. You'll only get this particular superposition if you somehow rig the lid of the box to only open when a radioactive nucleus decays, independent of Alice's actions, wishes, desires, hopes, philosophical outlook on life etc. Otherwise, if opening the box is just a part of your experimental protocol, then this is simply part of your definition of the system. The system is rigged to turn on a strong Alice-Cat interaction at some time and this will necessarily evolve the system towards a subspace of states where Alice has looked at the cat.
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Re: Quantum Question

Postby ucim » Wed Dec 06, 2017 7:56 pm UTC

"Alice looking" == "Inner box opened".

What is rigged up has nothing to do with Alice; it is merely that if the nucleus decays, poison is released and the cat dies. Nucleus decays => cat dies. The poison turns on on a strong nucleus-cat interaction. The box isolates the (inner) system from Alice, but opening it turns on a strong Alice-cat interaction.

Alice waits a while and opens the box. As far as Alice is concerned, before opening the (inner) box the nucleus is in a superposed "did it? didn't it?" state, (the exact value of which depends on the time and the half-life) and thus so is the cat ("is it? isn't it?"). As far as the cat is concerned, the nucleus is always in an actual intact (or decayed) state... and in the case of the decayed state, the cat isn't concerned for very long.

So long as the outer box remains closed, there is no strong Alice-us interaction (nor is there a cat-us interaction or a nucleus-us interaction). So, what's inside the (outer) box remains superposed as far as we are concerned.

Again, it's not "what happens", it's how we interpret what happens. With QM, it's not even clear that "happening" is a thing.
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An analogy that comes to mind, which is (at least for me) easier to grok, is the idea of simultaneity. Two events can appear simultaneous at one velocity, but in a frame moving at a different velocity, they would not appear to be simultaneous. Simultaneity "isn't a thing".
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Re: Quantum Question

Postby jewish_scientist » Thu Dec 07, 2017 3:47 pm UTC

ucim wrote:
jewish_scientist wrote:you take a ball out of the bag without looking at it
"looking at it" isn't what you think it means, at least on a quantum level.

Lesson 1: What is a Superposition?
Lesson 2: What Qualifies as an Observation?
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Re: Quantum Question

Postby ucim » Thu Dec 07, 2017 4:10 pm UTC

jewish_scientist wrote:Why would I try to squish these two lesson together if I do not have to?
So that you could understand them. They provide context for each other.

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Re: Quantum Question

Postby Pfhorrest » Thu Dec 07, 2017 6:22 pm UTC

"Observation" is basically equivalent to one of the following (which may be equivalent with each other):

the observer taking the observed thing out of superposition
the observer joining the observed thing in its superposition

To "observe" something is to interact with it, and two interacting systems are either both in superpositions or neither, as to "interact" just is to have your quantum state entangled with its, which means either its wavefunction collapses out of superposition to one observed state, or your wavefunction becomes a superposition of having observed different possible collapses.
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Tchebu
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Re: Quantum Question

Postby Tchebu » Fri Dec 08, 2017 6:47 pm UTC

So, what's inside the (outer) box remains superposed as far as we are concerned.


Yes, but in the usual setup, where opening the box is not governed by a quantum event, that superposition doesn't include states in which Alice hasn't looked at the cat.
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Re: Quantum Question

Postby ucim » Fri Dec 08, 2017 7:58 pm UTC

Tchebu wrote:Yes, but in the usual setup, where opening the box is not governed by a quantum event, that superposition doesn't include states in which Alice hasn't looked at the cat.
Is it really necessary that opening the box be governed by a (single) quantum event, or is it sufficient that the outer box be completely isolated from the outside? If the outer box is completely isolated, then isn't its contents in superposition (though a huge one)?

As I understand it, the point of the nucleus decay trigger is to give a comprehensible quantum state to macroscopize. But in the end, anything isolated is in superposition of all its possible states, until the isolation ends through interaction with the "outside" world, however we define it.

Nest paw?

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Re: Quantum Question

Postby Tchebu » Fri Dec 08, 2017 10:39 pm UTC

Not necessarily "all it's possible states", at least certainly not with equal coefficients. Which superposition you'll get depends on the Hamiltonian (i.e. the time-evolution rules) of the system (and the initial state, of course).

Compare Schroedinger's cat to its more fortunate sibling put in a similar quantum-isolation box, but without the poison bottle. The latter's quantum state will not pick up any "deadness" component and Alice will have zero probability to find a dead cat when she opens the box.

Or for that matter consider Shroedinger's cat at different times. At early times the cat's state is almost purely "alive" and gradually rotates towards the "dead" state, only passing through the equal probability state at a specific time. Contrast this with a setup where the poison bottle is just triggered to break via stopwatch. Then the evolution from |alive> to |dead> basically happens over the short time that it takes for a cat to die from poison via classical mechanics.

Similarly, compare Alice, who is fully briefed to open the inner box at noon, to Bob, who is told to stare at the box, which should open at some point via Geiger counter trigger. These two systems have different Hamiltonians, which generically will lead to different states after a given time. Alice's quantum state will sharply transition at noon to a state where Alice looked at the cat. Bob will only gradually move from |didn't see cat> to |saw cat> through various superpositions of these two states.
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Re: Quantum Question

Postby ucim » Fri Dec 08, 2017 11:48 pm UTC

Tchebu wrote:Not necessarily "all it's possible states", at least certainly not with equal coefficients.
I didn't say with equal or constant coefficients. Nonetheless, "classical" is just "quantum with too many states to bother keeping track of". And a cat without the poison can die of other causes too. The poison and trigger just makes the case obvious.

Point is, it's the isolation that makes superposition a reasonable (if sometimes unnecessary) POV. So long as its isolated, it's in (perhaps horrendously complex) superposition. Once isolation ends, so does superposition (replaced by classical ignorance).

Given this, I posit that superposition (or not) isn't a thing. It's a POV. But that's more philosophy (like any interpretation of QM) and not QM itself.

Jose
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Re: Quantum Question

Postby Tchebu » Sat Dec 09, 2017 3:08 am UTC

Classicality isn't about having "too many states to bother keeping track of" it's about the uncertainty in our observables. For a cat dying via classical poisoning, its "health" observable has very small uncertainty throughout the entire process. I can do the experiment 100 times, opening the box at a given time and will predictably find the cat exhibiting the same symptoms corresponding to a very particular stage of poisoning as outlined in a veterinary textbook. Same goes for any other "natural" cause.

For Shroedinger's cat, the uncertainty in the "health" observable becomes very large, such that half of my cats won't show any symptoms at all and the rest will be at various stages of the poisoning.

This is the crucial difference that makes Shroedinger's cat a "quantum" system and the cat in the box with a timer a "classical" one. Granted, there's no sharp boundary between the two regimes, but hopefully you're willing to treat 10^-34 error bars on a cat's blood pressure as being "classical enough".

I really don't understand what you mean by "superposition isn't a thing". Your comparison to simultaneity is not quite correct, because there are no physical phenomena that depend on two things being simultaneous, whereas there are plenty of phenomena that rely explicitly on systems being capable of quantum superposition.
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Re: Quantum Question

Postby ucim » Sat Dec 09, 2017 4:16 am UTC

Tchebu wrote:I can do the experiment 100 times, opening the box at a given time and will predictably find the cat exhibiting the same symptoms corresponding to a very particular stage of poisoning...
Only if the cat was poisoned. But you could also open the box and find the cat was not poisoned at all, but was just mad at being kept cooped up in a box. I'm not sure which is worse, but that's neither here nor therea superposition of here and there
Spoiler:
Yeah, I know it isn't. But like the reversed biased diode....
What makes one system (mostly) quantum and the other (mostly) classical is that the trigger is clearly quantum, and we don't interact with the system, preserving its purity. Remember, the cat doesn't see it as quantum at all once the trigger is triggered.

Tchebu wrote:I really don't understand what you mean by "superposition isn't a thing"s
Substitute "many worlds". Which one is "right"? Nothing explicitly depends on quantum superposition; they can all be equally explained by many worlds. And of course my analogy isn't perfect; if it were, it wouldn't provide any insight at all.

Jose
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Re: Quantum Question

Postby Pfhorrest » Sat Dec 09, 2017 4:40 pm UTC

Superposition is a feature of many worlds too, there’s just no collapse of it; the whole universe is in one giant superposition.
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Re: Quantum Question

Postby Tchebu » Sat Dec 09, 2017 6:03 pm UTC

^ that.

Also,
Only if the cat was poisoned. But you could also open the box and find the cat was not poisoned at all, but was just mad at being kept cooped up in a box.

No, no I couldn't. If I set up the experiment as intended and made no mistakes, with the poison bottle being triggered by a timer, there is zero probability (in the ideal limit, and abysmally small in a realistic setting) that I will find an unpoisoned cat in the box. And if I do make a mistake and mess up the triggering mechanism, for example, then I'll get a zero probability to find a poisoned cat. In either case, the uncertainty on the cat's health is small, indicative of the fact that the whole system is well described by a purely classical story : "I put the cat in the box, the timer triggered the mechanism, poison was released and the cat died" (or in case of a mistake "I put the cat in the box, the timer went off, but the poison wasn't released, cat is healthy but annoyed"). I can test this description, by preparing 100 such cats and opening the boxes at different times to see if they're following the above script, and I'll find that they do.
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Re: Quantum Question

Postby ucim » Sat Dec 09, 2017 8:04 pm UTC

Maybe we're thinking of different experiments? In the one in my mind, there is a cat, a nucleus that may (or may not) decay during the experiment, a trigger that breaks the poison vial when the nucleus decays, and there is a lid which can be opened from outside. The outside does not know when (or whether) the nucleus decays until the box is opened, but the cat finds out right away. I was not talking about a trigger that is set off by a timer. Such a thing could be easily described classically, and the point of it all is to link quantum effects to macroscopic results.

Pfhorrest wrote:Superposition is a feature of many worlds too, there’s just no collapse of it; the whole universe is in one giant superposition.
Well, if it never "collapses", then it's an unnecessary feature, and can be discarded without effect. Occam and all...

Jose
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Re: Quantum Question

Postby Pfhorrest » Sat Dec 09, 2017 9:52 pm UTC

It’s the classical world that’s discarded in manyworlds, not the superposed stuff.
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Re: Quantum Question

Postby Tchebu » Sun Dec 10, 2017 12:26 am UTC

ucim wrote:Maybe we're thinking of different experiments?


Well we've gone through many different variations of it at this point. My original objection was to you including "Alice hasn't looked" as part of the superposition in the "nested boxes" variation, where I pointed out that it will only be part of the superposition if the opening of the inner box is linked to a quantum event, similar to how the cat's fate is linked to the nuclear decay in the inner box.

ucim wrote:The outside does not know when (or whether) the nucleus decays until the box is opened, but the cat finds out right away.


I'd just like to point out that "the cat finds out right away" is not synonymous with "the contents of the inner box is no longer in a superposition relative to the 'dead/alive' basis". Would you agree with this?

ucim wrote:
Pfhorrest wrote:Superposition is a feature of many worlds too, there’s just no collapse of it; the whole universe is in one giant superposition.
Well, if it never "collapses", then it's an unnecessary feature, and can be discarded without effect. Occam and all...


It may not collapse, but it still responsible for yielding the correct statistical distributions of experimental outcomes, so it's not an unnecessary feature.
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Re: Quantum Question

Postby ucim » Sun Dec 10, 2017 3:01 am UTC

Tchebu wrote:it will only be part of the superposition if the opening of the inner box is linked to a quantum event, similar to how the cat's fate is linked to the nuclear decay in the inner box.
I guess I don't see how the opening of the inner box needs to be quantum-linked, so long as the cat-poisoning is linked to one. I supposed the box-opening needs to be quantum-isolated; I'll go along with that. The idea is that upon opening the box (inside another closed box), Alice is quantum-entangled (from the POV of the Outside) with the cat's fate. But once the box is opened, the cat's fate is sealed (from the POV of Alice).

Tchebu wrote:I'd just like to point out that "the cat finds out right away" is not synonymous with "the contents of the inner box is no longer in a superposition relative to the 'dead/alive' basis". Would you agree with this?
"In superposition" can refer to two things; one is the Hamiltonian and all the math behind it. I don't disagree with the math. But the other thing is the interpretation of what this math means. Assuming you're not one of the "shut up and calculate" crowd, the way I see it is that from the outside of the box, the cat is in superposition so long as the box is shut. But from the inside of the box, the cat is dead (and the body showing evidence of decay for some amount of time). Or alive (and probably pissed off). But not both. This makes superposition a POV thing.

Tchebu wrote:It may not collapse, but [superposition] is still responsible for yielding the correct statistical distributions of experimental outcomes, so it's not an unnecessary feature.
The Hamiltonian is still responsible.... but (I maintain) "superposition" is an interpretation of the Hamiltonian. It's what we tell ourselves in order to make sense of the math.

So consider....
Inner box contains a cat, a clock, and a camera, along with a quantum-triggered poison.
Outer box contains Alice and the inner box.
At 2:00 the inner box is closed, as is the outer one.
At 2:30, what is the state of the cat? (the superposition indicated by the Hamlitonian? Dead? Alive? Does it matter who's asking?)
At 3:00 the inner box is opened, to reveal a dead cat and an image of the poison flask in the process of breaking, with a clock in the background indicating 2:15.
At 3:00, what was the state of the cat at 2:30? Does it matter who's asking?
at 4:00, the outer box is opened. Now what was the state of the cat at 2:30?

Jose
Order of the Sillies, Honoris Causam - bestowed by charlie_grumbles on NP 859 * OTTscar winner: Wordsmith - bestowed by yappobiscuts and the OTT on NP 1832 * Ecclesiastical Calendar of the Order of the Holy Contradiction * Please help addams if you can. She needs all of us.

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Re: Quantum Question

Postby Tchebu » Sun Dec 10, 2017 10:00 pm UTC

I guess I don't see how the opening of the inner box needs to be quantum-linked, so long as the cat-poisoning is linked to one.


If you want to end up with a superposition of "Alice looked" and "Alice hasn't looked", you need to link it to a quantum event, for exactly the same reason that you need to link the cat if you want to put it into a superposition of "dead" and "alive". It's literally the same exact argument just one box higher. The isolation is necessary to maintain that superposition, but is not responsible for creating it.

[...] the way I see it is that from the outside of the box, the cat is in superposition so long as the box is shut. But from the inside of the box, the cat is dead (and the body showing evidence of decay for some amount of time). Or alive (and probably pissed off). But not both. This makes superposition a POV thing.


You seem to be subscribing to the Relational QM interpretation. But even there, different observers never disagree on whether the same system is in a superposition, provided they can both actually measure that system in principle. The apparent POV dependence you seem to be getting comes from thinking of Alice or the Cat as "self-observing" but that's explicitly excluded in RQM. The subtlety is that you have to really distinguish Alice, the Cat, and the combined Alice+Cat system. Out of these 3, Alice can only measure the cat, so the only thing we need to check is whether Alice and Bob agree on when the cat's in a superposition and when it isn't, and they do.

To elaborate:
Spoiler:
If Bob is sitting outside the outer box. His impression of the story is that the full state of the system starts of as
2:00 |Oblivious Alice>| Alive Cat>
then evolves to
2:59 |Oblivious Alice> ( |Alive> + |Dead>)
then upon opening the box entangles Alice with the cat to yield
3:00 |Happy Alice>|Alive> + |Sad Alice> |Dead>
And finally upon opening the outer box at 4:00, we see Alice and the cat to be in one of those two situations with suitable probability, that we can confirm by repeating the experiment many times.

The first two states factorize into an "Alice" part and a "cat" part. In the first case, neither are in a superposition, in the second case, Alice is not in a superposition, but the cat is. In the third case the state doesn't even factorize so we can't even talk about the states of Alice and the Cat as separate things. We can calculate their density matrices, and find that they have a classical probability to be in a purely "happy/sad" or "dead/alive" states. Since those options are not superpositions, we can say with certainty that the neither Alice, nor the cat, are in a superposition, even though we don't know which state they're in exactly.

So from Bob's perspective, as far as superposition goes, the story is this
2:00 Alice not in superposition, Cat not in superposition
2:59 Alice not in superposition, Cat in superposition
3:00 Alice not in superposition, Cat not in superposition, Alice+Cat in a superposition
4:00 Alice+Cat successfully observed. Alice+Cat no longer in superposition (due to wavefunction collapse, if you wish)

Compare this to Alice's perspective. She can't measure herself, or the combined Alice+Cat system. So the only things we get to compare is what she says about the cat's superposition. But that story is very simple:

2:00 Cat not in superposition.
2:59 Cat in superposition.
3:00 Cat successfully observed. Cat no longer in superposition.

So on the one thing that Alice and Bob can actually compare notes, which is the superposition status of the cat, there's no disagreement!! Superposition is not a POV thing.
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Re: Quantum Question

Postby ucim » Sun Dec 10, 2017 11:32 pm UTC

While I ponder this, what about the Historical Documents? At the end of the experiment, everyone can examine the image taken by the camera, of the flask breaking and the clock indicating 2:15. It's a fast acting poison, so it's fair to infer that the cat is dead by 2:30. (Alternatively, the image could show a dead cat.)

At 2:30, was the cat dead at 2:30, or in superposition?
At 4:00, was the cat dead at 2:30, or in superposition?

Jose
Order of the Sillies, Honoris Causam - bestowed by charlie_grumbles on NP 859 * OTTscar winner: Wordsmith - bestowed by yappobiscuts and the OTT on NP 1832 * Ecclesiastical Calendar of the Order of the Holy Contradiction * Please help addams if you can. She needs all of us.


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