Schrödinger’s Cat and the quantum mechanical "observer"

[tim314]

I’m sure many of you are familiar with the Schrödinger’s Cat thought experiment. (Note: thought experiment . . . no cats were harmed, yada yada.) First, you put a cat in a box. Then you take a single atom of some radioactive isotope, and set up a mechanism so that if the atom decays while the mechanism is turned on, it will trigger and kill the cat. You set the mechanism to run for one half-life of the isotope before turning off (so there will be a 50% chance of a decay and a dead cat), and you then leave the room. You wait for the mechanism to do its thing. Here’s the weird part: according to quantum mechanics, until you observe the atom, it is in a superposition of decayed and undecayed states. This means that until you observe the cat, he’s in a superposition of alive and dead states. This is often used as an example of the bizarre consequences of quantum mechanics.

Of course, the moment you look into the box you see either a living or a dead cat. And the process of making this observation collapses the wave function of the cat to either the "living" or "dead" state, and likewise collapses the wave function of the atom to either the "decayed" or "undecayed" state.

At least, this is how the thought experiment has usually been presented to me.

My question is, why doesn't the cat's observation count? I mean, either the cat observes himself to be still alive, or he doesn't, right? Isn't this just as much a measurement as me looking in the box? I mean, surely quantum mechanics doesn't distinguish between human observations and cat observations, does it? In fact, it seems to me that even the mechanism that has to either kill or spare the cat is making a measurement, and that this is just as much an observation as anything a person could do. So is the whole Schrödinger’s Cat thought experiment a bunch of nonsense, and really you never get a superposition of living and dead cats? Or am I misunderstanding what the thought experiment is supposed to convey?

[Small Clanger]

I think Schrödinger intended the Cat example to demonstrate that the whole concept of superposition was absurd. No-one thinks that any macroscopic object (let alone a cat) can exist in a state of superposition, but quantum entities can.

The collapse-of-the-waveform problem (what exactly causes the cat death or not) continues to be a. . . problem.

You want real quantum weirdness? Look for texts on the 'EPR paradox' and 'Bells inequality'.



Once more: S.C. steps back and waits for the physicists turn up and make things all complicated

[alimarx]

Wasn't there a thread (http://boards.straightdope.com/sdmb/showthread.php?t=279698) on this recently? Oh.... nevermind...

[erislover]

It is just an illustration. An "observation" is any interaction that would reveal information about a quantum state, AFAIK.

[The Chao Goes Mu]

[QUOTE=Small Clanger]I think Schrödinger intended the Cat example to demonstrate that the whole concept of superposition was absurd. No-one thinks that any macroscopic object (let alone a cat) can exist in a state of superposition, but quantum entities can.

The collapse-of-the-waveform problem (what exactly causes the cat death or not) continues to be a. . . problem.

You want real quantum weirdness? Look for texts on the 'EPR paradox' and 'Bells inequality'.



This is where It gets wonky for me, exactly how or why can quantum entities exist in a state of superposition? What is the mechanism that allows for this to occur?

[chrisk]

Wasn't there a thread (http://boards.straightdope.com/sdmb/showthread.php?t=279698) on this recently? Oh.... nevermind...

Actually, there was... I started it - got very complicated. :D

Basically, the answer I was able to understand was 'yes, the cat's observations count... but not to you.' :)

[MaryEFoo]

There was a description in (Nature? ??) magazine some time ago that seemed to be clearly stating that, as long as there is no way of observing the cat or the particle, and no way, direct or indirect, of finding out the state of affairs, the cat is definitely not alive and definitely not dead. That we know for a fact the cat has entered a third state, the indeterminate. And the atom has definitely not decayed and also is definitely not non-decayed, it is indeterminate instead.

And indeterminate, of course, has never been observed by anyone except maybe an all-knowing entity such as God.

I have problems with this. I could understand the wording of this if they changed it to: the particle is in an unknown state (call it indeterminate if you want, but the definition would have changed), it's in a don't-know state, a can't-know state, a don't-care-because-it-makes-no-difference state, until some observation finds what it is. But it's in one of two real states, not a mysterious third one.

[The Chao Goes Mu]

There was a description in (Nature? ??) magazine some time ago that seemed to be clearly stating that, as long as there is no way of observing the cat or the particle, and no way, direct or indirect, of finding out the state of affairs, the cat is definitely not alive and definitely not dead. That we know for a fact the cat has entered a third state, the indeterminate. And the atom has definitely not decayed and also is definitely not non-decayed, it is indeterminate instead.

And indeterminate, of course, has never been observed by anyone except maybe an all-knowing entity such as God.

I have problems with this. I could understand the wording of this if they changed it to: the particle is in an unknown state (call it indeterminate if you want, but the definition would have changed), it's in a don't-know state, a can't-know state, a don't-care-because-it-makes-no-difference state, until some observation finds what it is. But it's in one of two real states, not a mysterious third one.

See, I understand what you are saying. I don't understand the "how" of it. What proofs are the Ubiquitous "THEY" using to explain this phenomenon? How do they know this? It makes me nuts. It's like asking 'if a tree falls in a forest.....etc.' ::::::::::::::banging head against wall:::::::::::::::::

[Bear_Nenno]

You don't need a decaying atom to create an "unknown" state. Flip a coin and catch it in your hand. Before you open your hand up and observe whether it is heads or tails, it's in an unknown state. It is both heads and tails. It is also Not heads OR tails. It's not until you observe it and collapse one of the wave forms that it is truly one or the other...

[Sean Factotum]

There was a description in (Nature? ??) magazine some time ago that seemed to be clearly stating that, as long as there is no way of observing the cat or the particle, and no way, direct or indirect, of finding out the state of affairs, the cat is definitely not alive and definitely not dead. That we know for a fact the cat has entered a third state, the indeterminate. And the atom has definitely not decayed and also is definitely not non-decayed, it is indeterminate instead.
As I've always understood it, the cat is in neither state. That doesn't mean a third state has been created - it just means that without an outside observer (hence the cat's POV being irrelevant), there is no conclusion to the experiment. The how of it is that at the quantum level there are no results until it is measured (aka observed.)

As for using a cat, Schrödinger was merely using an object with which people could identify. He could have easily substituted a goat, a flaming pig, or William Shatner.

[tim314]

I have problems with this. I could understand the wording of this if they changed it to: the particle is in an unknown state (call it indeterminate if you want, but the definition would have changed), it's in a don't-know state, a can't-know state, a don't-care-because-it-makes-no-difference state, until some observation finds what it is. But it's in one of two real states, not a mysterious third one.

Look, the particle isn't in one state or the other prior to observation. It's in a superposition of two states. The act of making a measurement actually changes the state of the particle, so that rather than being a superposition of eigenstates it's in a particular eigenstate of whatever measurement you're making.

In layman's terms, an eigenstate means "a state that the measurement doesn't change." So, say I want to measure whether a particle is spinning clockwise or counterclockwise around a certain axis. In reality, when we talk about the "spin" of a particle, we don't mean that it's spinning in the classical sense -- it's not -- but I'm trying to give a simple example that is comprehensible to those who haven't studied much physics. The true state of the particle might be 70% clockwise / 30% counterclockwise. That means that if you put a particle in that state and measured which way it's spinning, you'd measure clockwise 70% of the time, and counterclockwise 30% of the time. However, let's say you make the measurement, and find out it's spinning clockwise. The act of making this measurement changes the state of the particle so that it is now in an eigenstate of our "which way is it spinning" measurement. So, each subsequent time that we make the measurement, the particle will still be spinning in the clockwise direction, and the state won't change (since it's an eigenstate). So it's not that we didn't know which way the particle was spinning -- it wasn't in a state of definite spin before we made the measurement! In other words we aren't just measuring something, we're changing something!

To those who are having trouble wrapping their heads around this, my suggestion would be to stop thinking of particles as actual particles in the classical sense. In quantum mechanics, a particle can be represented by a wave packet -- basically, think of a wave that has its amplitude maximized at a certain point, with the amplitude falling off to zero on all sides of that. So the envelope of the wave is like a three dimensional gaussian distribution (a "bell curve"). Now, suppose I asked you "Where is this wave?" Your answer would have to be "Well, it doesn't have a single defined position -- it just has different amplitudes at different points in space." So, there is some uncertainty in it's position -- nothing so strange about that. Now, suppose you tried to measure the exact position of the wave (even though it doesn't really have one). You'd be most likely to measure it right at the peak of the distribution -- but there's a chance you'd measure it a little to one side or another from the peak. We can only talk about the probability of you measuring a certain position.

The weird thing in quantum mechanics is that your measurement changes the wave. After you measure it to be at a certain point, the wave function collapses to an infinitesimally narrow spike at that point (called a Dirac delta function.) So, at that moment of your measurement, the wave function really does represent a particle located at a single point in space. After the measurement, the delta function gradually expands back out into a wave packet.

My point is, there's nothing weird about talking about something in a superposition of states and whose properties don't have definite values. I can construct an ordinary macroscopic wave packet (out of say, sound waves) that has neither definite position nor definite momentum. The weird thing in quantum mechanics is that if we measure its position or momentum (or anything else) the function changes so that that particular measured quantity does have a definite value -- namely whatever one we measured.

[tim314]

Getting back to my original post, I don't buy the excuse that the Schrodinger's Cat thought experiment is just an analogy to what happens on the quantum level. I have always had this presented to me as an actual hypothetical example of how one can create a superposition of states on a macroscopic level. If it were just an analogy, you could just say "Particles in quantum mechanics are like cats that are both alive and dead." But this isn't what is said. Instead, a specific way of producing a cat that is "both alive and dead" is described. I'm saying, this seems like bull****, that wouldn't give you a cat that's both alive and dead, because an observation has been made by both the cat and the apparatus itself in the process. I'm still waiting for someone with a physics degree to come in here and tell me definitively whether I'm right or I'm wrong.

I'm not trying to denigrate the people who have already replied -- I appreciate your responses -- but I still feel like my original question hasn't been answered in any sort of definitive way. I'm hoping for something like "Yep, that's an observation" or "No, that doesn't count as an observation, because an observation must satisfy this specific criteria that that fails to meet."

[tim314]

Actually, erislover gave me a pretty good definition of an observation.An "observation" is any interaction that would reveal information about a quantum state, AFAIK
This is basically what I thought as well. But if so, the oft repeated claim that the thought experiment describes a "cat which is both alive and dead" is total BS, isn't it? It just describes a means of killing a cat that will have a 50% success rate, and this is not profound. If that's all we're doing, I might as well let kitty play Russian roulette with a half-emptied revolver, and leave quantum mechanics out of it.

[betenoir]

You don't need a decaying atom to create an "unknown" state. Flip a coin and catch it in your hand. Before you open your hand up and observe whether it is heads or tails, it's in an unknown state. It is both heads and tails. It is also Not heads OR tails. It's not until you observe it and collapse one of the wave forms that it is truly one or the other...

No it isn't.

It is in fact either heads or tails. And just one of those. There is no wave form to collapse. As Small Clange put it so accurately to begin with...Shrodinger's actual point to this thought experiment was to convey the absurdity of relating quantum particle level paradoxes to macro level reality.

[betenoir]

This is basically what I thought as well. But if so, the oft repeated claim that the thought experiment describes a "cat which is both alive and dead" is total BS, isn't it? It just describes a means of killing a cat that will have a 50% success rate, and this is not profound.

Sure it is. Because if you don't leave quantum mechanics out of it, it forces you to think about the quantum level of reality where things work in ways that make no sense one our macro level. But apparently that's how they do work. Shrodinger is just providing a metaphor for people to understand just how alien the micro level really is.

[tim314]

You don't need a decaying atom to create an "unknown" state. Flip a coin and catch it in your hand. Before you open your hand up and observe whether it is heads or tails, it's in an unknown state. It is both heads and tails. It is also Not heads OR tails. It's not until you observe it and collapse one of the wave forms that it is truly one or the other...

I'm not so sure of this. If (as I suspect) an observation truly means any interaction whose result is dependent on the state of an object, then the light bouncing off the coin as it flips is observing it's state, the air being displaced by the coin is observing its state, etc.

The difference between a coin (made up, obviously, of very many particles) and a single subatomic particle is that a single particle can fly through space for a while and not interact with anything. In other words, they can truly go unobserved. Heck, there are millions (thousands? billions? I don't know the order of magnitude) of neutrinos flying through you right now, and not interacting with anything.

It seems absurd to me to claim that the state of a coin (or anything else) is changed not by the light bouncing off of it, but by the fact that at some later time that light happens to strike the eye of a sentient being.

In fact, even if I think observation means "observation by a sentient being" (say, myself), the state of the coin is changed (to whatever the observed state is) at the time the light I'm seeing struck the coin (since I only have knowledge of its state at that time), not at the current time. But if this claim is true, my seeing the coin caused the state to change at some earlier time. This isn't just absurd, it's causality violating.

[tim314]

Sure it is. Because if you don't leave quantum mechanics out of it, it forces you to think about the quantum level of reality where things work in ways that make no sense one our macro level. But apparently that's how they do work. Shrodinger is just providing a metaphor for people to understand just how alien the micro level really is.

I assume you mean "sure it is profound" not "sure it is B.S." (a little ambiguous given the post you were replying to.) But I don't see how. Either you have a living cat or a dead cat, never a superposition of cats, right? (Or are you saying we do have a superposition of cats, in which case my question "why doesn't the cat's own observation collapse it into a state of definite deadness" still needs an answer.) But if we don't have a superposition of living and dead cats, then how is the macroscopic cat state at all analogous to anything happening on the microscopic level? It's just an ordinary cat that might or might not be dead, with nothing quantum mechanical about it.

[Bear_Nenno]

No it isn't. ...Shrodinger's actual point to this thought experiment was to convey the absurdity of relating quantum particle level paradoxes to macro level reality.Sorry, I thought that's what I was doing too.

[MaryEFoo]

Let's try, how this indeterminate state is used by physical scientists.

We can see that a lot of very smart people who have dedicated their lives to this field will tell us that a particle can be decayed and un-decayed at the same time, or spin can be 70% clockwise and 30% counter- at the same time, until it's observed or interacted with.

Why don't they just use statistics? Ie, the spin has a 70% likelihood of being clockwise when it's measured, or of these thousand particles, approx. 700 spin clockwise? Or, this particle has a curve of probability of location, and the curve is shaped like XXX until we measure it? (sorta like playing Battleship).

What makes the indeterminacy approach be more desirable than the statistical approach?

[Omphaloskeptic]

Let's try, how this indeterminate state is used by physical scientists.

We can see that a lot of very smart people who have dedicated their lives to this field will tell us that a particle can be decayed and un-decayed at the same time, or spin can be 70% clockwise and 30% counter- at the same time, until it's observed or interacted with.

Why don't they just use statistics? Ie, the spin has a 70% likelihood of being clockwise when it's measured, or of these thousand particles, approx. 700 spin clockwise? Or, this particle has a curve of probability of location, and the curve is shaped like XXX until we measure it? (sorta like playing Battleship).

What makes the indeterminacy approach be more desirable than the statistical approach?The statistical approach fails to capture all of the information about the quantum system. This is kind of hard to describe, but it relies on the fact that there are several different measurements you could make on a system, each of which will disturb the system so as to preclude making any of the other measurements. In particular, we can sometimes choose a measurement of a quantum system that will always give the same answer, even if other measurements only give statistical information. This means that a quantum superposition is distinguishable from a statistical mixture.

A more detailed answer: For a particle with "spin," think of the "spin" as counterclockwise rotation around some axis. (The particle isn't really spinning, of course, but for the purposes of this post you can think of it that way.) The "counterclockwise" rotation is then spin about an axis pointing vertically upward; the "clockwise" rotation is spin about an axis pointing down. But the particle could also be spinning about an axis pointing to the left, or up and to the right, or in any other direction.

It turns out that if you take a particle that's really spinning along one axis "u", and measure its spin along a different axis "v", you will always find that the particle is spinning either "up" along v or down along v (that is, spinning about v or spinning about -v); you will measure each of these with a probability dependent on the angle between u and v. If u and v are perpendicular, then the probability is 50%-50%; but if u and v are the same, then you'll always measure spin along v, and if u and v are oppositely directed, you'll always measure spin along -v.

So if you measure the spin along the z axis for a particle whose spin is "really" along the x axis, you'll get a 50/50 mix of "up" and "down." But you can't think of this particle as a statistical mix of 50% z-up and 50% z-down, because if you measure the same particle along the x axis, you'll always measure "up".

[JasonFin]

Let's try, how this indeterminate state is used by physical scientists.

We can see that a lot of very smart people who have dedicated their lives to this field will tell us that a particle can be decayed and un-decayed at the same time, or spin can be 70% clockwise and 30% counter- at the same time, until it's observed or interacted with.

Why don't they just use statistics? Ie, the spin has a 70% likelihood of being clockwise when it's measured, or of these thousand particles, approx. 700 spin clockwise? Or, this particle has a curve of probability of location, and the curve is shaped like XXX until we measure it? (sorta like playing Battleship).

What makes the indeterminacy approach be more desirable than the statistical approach?Put simply, the statistical approach is inconsistent with experiment. In reply to a similar question in early 2003, our (former?) moderator Jesse gave a better reply than I ever could.

I'd post a link to the thread, but it doesn't seem to be around anymore. I saved Jesse's post because he quoted a particularly good explanation, but it appears hyperlinks weren't preserved. :(
This is something I wrote a while ago for another thread about whether particles could "really" have well-defined simultaneous values of things like position and momentum or whether the "fuzziness" is truly fundamental:

Although people usually focus on indeterminacy when discussing the mysteries of quantum mechanics, the real mystery is that QM poses a lot of problems for a realist view of reality, i.e. one where the properties of the world exist independently of our measurement. This is most obvious in the EPR experiments where properties of entangled particles are measured at different locations. What we find is that there are regular correlations between measurements made on particle A and measurements made on particle B which are inexplicable if we picture the particles as classical objects with definite properties that cannot communicate faster than light--this is what Einstein called "spooky action at a distance."

It is sometimes imagined that the uncertainty principle, which prevents us from knowing simultaneously the value of two noncommuting variables (like position and momentum), is just a limitation on measurement ; maybe the particle has a definite position and momentum at any given time, but each time we try to measure the position it changes the particle's momentum in a random way, and each time we measure the particle's position it offsets the momentum. However, the EPR experiment shows it is much worse than that. The correlations between entangled particles are such that they cannot be explained by any picture of the world in which the particles have definite values for each noncommuting variable at every time, unless the particles can somehow communicate instantaneously so as soon as you measure one the other "knows" which property you measured and adjusts its own properties. This is the result known as "Bell's Theorem," which says that no local theory of hidden variables can explain the results of the EPR experiment.

In Huw Price's Time's Arrow and Archimedes' Point he offers a little story to help us see what's so strange about the EPR results:

quote:
By modern standards the criminal code of Ypiaria [pronounced, of course, "E-P-aria"] allowed its police force excessive powers of arrest and interrogation. Random detention and questioning were accepted weapons in the fight against serious crime. This is not to say the police had an entirely free hand, however. On the contrary, there were strict constraints on the questions the police could address to anyone detained in this way. One question only could be asked, to be chosen at random from a list of three: (1) Are you a murderer? (2) Are you a thief? (3) Have you committed adultery? Detainees who answered "yes" to the chosen question were punished accordingly, while those who answered "no" were immediately released. (Lying seems to have been frowned on, but no doubt was not unknown.)

To ensure that these guidelines were strictly adhered to, records were required to be kept of every such interrogation. Some of these records have survived, and therein lies our present concern. The records came to be analyzed by the psychologist Alexander Graham Doppelganger, known for his work on long distance communication. Doppelganger realized that among the many millions of cases in the surviving records there were likely to be some in which the Ypiarian police had interrogated both members of a pir of twins. He was interested in whether in such cases any correlation could be observed between the answers given by each twin.

As we now know, Doppelganger’s interest was richly rewarded. He uncovered the two striking and seemingly incompatible correlations now known collectively as Doppelganger’s Twin Paradox . He found that

(8.1) When each member of a pair of twins was asked the same question, both always gave the same answer;

and that

(8.2) When each member of a pair of twins was asked a different question, they gave the same answer on close to 25 percent of such occasions.

It may not be immediately apparent that these results are in any way incompatible. But Doppelganger reasoned as follows: 8.1 means that whatever it is that disposes Ypiarians to answer Y or N to each of the three possible questions 1, 2, and 3, it is a disposition that twins always have in common. For example, if YYN signifies the property of being disposed to answer Y to questions 1 and 2 and N to question 3, then correlation 8.1 implies that if one twin is YYN then so is his or her sibling. Similarly for the seven other possible such states: in all, for the eight possible permutations of two possible answers to three possible questions. (The possibilities are the two homogeneous states YYY and NNN, and the six inhomogeneous states YYN, YNY, NYY, YNN, NYN, and NNY.)

Turning now to 8.2, Doppelganger saw that there were six ways to pose a different question to each pair of twins: the possibilities we may represent by 1:2, 2:1, 1:3, 3:1, 2:3, and 3:2. (1:3 signifies that the first twin is asked question 1 and the second twin question 3, for example.) How many of these possibilities would produce the same answer from both twins? Clearly it depends on the twins’ shared dispositions. If both twins are YYN, for example, then 1:2 and 2:1 will produce the same response (in this case, Y) and the other four possibilities will produce different responses. So if YYN twins were questioned at random, we should expect the same response from each in about 33 percent of all cases. And for homogeneous states, of course, all six posible question pairs produce the same result: YYY twins will always answer Y and NNN twins will always answer N.

Hence, Doppelganger realized, we should expect a certain minimum correlation in these different question cases. We cannot tell how many pairs of Ypiarian twins were in each of the eight possible states, but we can say that whatever their distribution, confessions should correlate with confessions and denials with denials in at least 33 percent of the different question interrogations. For the figure should be 33 percent if all the twins are in inhomogeneous states, and higher if some are in homogeneous states. And yet, as 8.2 describes, the records show a much lower figure.

Doppelganger initially suspected that this difference might be a mere statistical fluctuation. As newly examined cases continued to confirm the same pattern, however, he realized that the chances of such a variation were infinitesimal. His next thought was therefore that the Ypiarian twins must generally have known what question the other was being asked, and determined their answer partly on this basis. He saw that it would be easy to explain 8.2 if the nature of one’s twin’s question could influence one’s own answer. Indeed, it would be easy to make a total anticorrelation in the different question cases be compatible with 8.1—with total correlation in the same question cases.

Doppelganger investigated this possibility with some care. He found, however, that twins were always interrogated separately and in isolation. As required, their chosen questions were selected at random, and only after they had been separated from one another. There therefore seemed no way in which twins could conspire to produce the results described in 8.1 and 8.2. Moreover, there seemed a compelling physical reason to discount the view that the question asked of one twin might influence the answers given by another. This was that the separation of such interrogations was usually spacelike in the sense of special relativity; in other words, neither interrogation occurred in either the past or the future light cone of the other. (It is not that the Ypiarian police force was given to space travel, but that light traveled more slowly in those days. The speed of a modern carrier pigeon is the best current estimate.) Hence according to the principle of the relativity of simultaneity, there was no determinate sense in which one interrogation took place before the other.


This is the problem posed by the EPR experiment in a nutshell, but instead of twins we are talking about entangled particles and instead of answers to questions we are talking about measurements of the particles' "spin" along their 3 axes (there is an uncertainty relation between these spins). As the great physicist Richard Feynman said, "Nobody understands quantum mechanics…do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will go 'down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that." The weirdness of QM, and the difficulty with imagining "how can it be like than" in a way consistent with the view that reality exists before we observe it, extends to other famous experiments and thought-experiments, like the Double-slit experiment (from this page which introduces a lot of important QM concepts, including Bell's theorem) and the Schroedinger’s Cat thought-experiment. (is the cat ‘really’ alive or dead before it is measured?) But Bell's theorem and the EPR experiment show most clearly what the basic problem here is for a realist.

A number of different "interpretations" of quantum-mechanical weirdness have emerged over the years, with none yielding any new physical predictions (and thus being experimentally indistinguishable) but each offering a different way to conceptualize what’s "really" going on in these sorts of experiments. Here is a page which gives some good links on these various interpretations, and I’ll attempt my own summary here:

1. The Copenhagen Interpretation

Basically, the Copenhagen interpretation says that we shouldn’t worry about how what’s really going on in the first place—science can only deal with correlating and predicting the results of various measurements, but it can’t tell us anything about what goes on when we’re not looking. This is basically a logical positivist perspective, and it was preferred by Bohr.

2. "Objective Collapse" interpretation

Here the wave-particle duality is taken literally—the world exists as a wavelike potential when it’s not being observed, but somehow measurements periodically "collapse" the wavefunction into a definite state. Some versions of this suppose that it’s consciousness that does the collapsing, others suppose that an entangled system collapses once it reaches a certain limit in mass. Unlike the other interpretations, these might actually be expected to yield different predictions than orthodox QM—so far, there’s no evidence for anything like this though.

3. The Bohm-de Broglie interpretation

Bell’s theorem shows that no local hidden variable theory can explain the results of the EPR experiment, but that leaves open the possibility of a nonlocal hidden variables theory where particles can communicate faster than light. This is the route taken by Bohm and de Broglie’s interpretation. In the Ypiarian story, this would be like the twins having a psychic link which allows one to know what question the other was asked, and adjust his own answer accordingly.

4. Transactional interpretation

The EPR experiment can also be explained if you assume the future can affect the past, so that the particle’s original properties are affected by the measurements that will be made on them later, once they are separated. In the Ypiarian story, this would mean that the twin’s choices to commit or not commit various crimes would be affected by which questions they would be asked much later when they’re interrogated. This isn’t as strange as it sounds, since all the laws of physics we currently know of are time-symmetric (they look the same forwards as they do backwards) and apparently the apparent "arrow of time" emerges solely from statistical mechanics, perhaps because the universe started off in a very low-entropy state. Huw Price’s book Time's Arrow and Archimedes' Point , which I quoted from above, deals with this problem, and he favors a version of this interpretation.

5. The Many-Worlds interpretation.

This interpretation takes the mathematical formalism of QM literally and proposes that the wavefunction is all there is. This means that when I measure the state of a particle that’s in superposition, instead of "collapsing" it into a definite state, I just become entangled with it and enter into a superposition myself; basically, I "split" into two versions of myself, one of whom observes one state and another of whom observes another. In popular accounts this is sometimes explained in terms of the entire universe splitting into parallel histories all the time, but it’s a bit more subtle than that, since different "worlds" can interfere with each other and cannot be viewed as totally "parallel,’ although thermodynamics may explain the appearance of splitting through a phenomenon called decoherence. For technical reasons this interpretation preserves locality (see question 12 of the Everett FAQ ), and it’s also 100% deterministic to boot (although it suggests an odd kind of subjective indeterminacy in which my first-person experience randomly chooses which split copy to become—hence a variation of this interpretation is the many-minds interpretation which deals with this issue a little more explicitly). This interpretation is seen as being the most theoretically elegant one by a number of physicists, and it seems that it is sometimes implicitly assumed in quantum cosmology, although physicists are often agnostic about whether other worlds/histories are actually "real." Many-worlds could also make sense of quantum computation, which some physicists believe can be understood in terms of the quantum computer performing different computations in different worlds and then combining the results through interference.

So, those are the various interpretations…as I said, the main problem is that none of them really gives any new testable predictions, which is a bit unsatisfying. There’s some good reason to think that a theory of quantum gravity would transform our understanding of QM somewhat, so perhaps such a theory will depend on a modified version of one of these interpretations that is testable in some way. In any case, Bell’s inequality shows definitively than no classical, realist picture of the world can explain the EPR results, so whatever the truth turns out be, it’s guaranteed to violate our cherished assumptions in one way or another (faster-than light signalling, the future affecting the past, parallel universes…take your pick!)

[toadspittle]

It just describes a means of killing a cat that will have a 50% success rate, and this is not profound. If that's all we're doing, I might as well let kitty play Russian roulette with a half-emptied revolver, and leave quantum mechanics out of it.

Actually, I thought the original thought experiment specifically required quantum mechanics--the cat is killed/not killed as a direct result of subatomic actions:

A cat is placed in a sealed box. Attached to the box is an apparatus containing a radioactive nucleus and a canister of poison gas. The experiment is set up so that there is a 50% chance of the nucleus decaying in one hour. If the nucleus decays, it will emit a particle that triggers the apparatus, which opens the canister and kills the cat. (According to quantum mechanics, the unobserved nucleus is described as a superposition (mixture) of "decayed nucleus" and "undecayed nucleus".) However, when the box is opened the experimenter sees only a "decayed nucleus/dead cat" or a "undecayed nucleus/living cat."

This isn't coin toss stuff--the cat dies because of a decaying atom. You can't get much more quantum than that.

[tim314]

Actually, I thought the original thought experiment specifically required quantum mechanics--the cat is killed/not killed as a direct result of subatomic actions:


This isn't coin toss stuff--the cat dies because of a decaying atom. You can't get much more quantum than that.

There's no doubt in my mind that you need quantum mechanics to describe the state of the atom. I'm saying the state of the cat is no different than if you'd just shot him, or not shot him. It's not a superposition, because your aparatus already made a measurement on the atom and collapsed the whole thing down to a single outcome, and likewise the cat made a measurement of his own aliveness or deadness by continuing to be alive or dead.

If I'm right, and both of those things constitute a measurement, then there's nothing particularly quantum mechanical about the state of the cat. In which case, what was the point of the cat in the first place? What was the point of the whole thought experiment?

[Omphaloskeptic]

Getting back to my original post, I don't buy the excuse that the Schrodinger's Cat thought experiment is just an analogy to what happens on the quantum level. I have always had this presented to me as an actual hypothetical example of how one can create a superposition of states on a macroscopic level. If it were just an analogy, you could just say "Particles in quantum mechanics are like cats that are both alive and dead." But this isn't what is said. Instead, a specific way of producing a cat that is "both alive and dead" is described. I'm saying, this seems like bull****, that wouldn't give you a cat that's both alive and dead, because an observation has been made by both the cat and the apparatus itself in the process. I'm still waiting for someone with a physics degree to come in here and tell me definitively whether I'm right or I'm wrong.

I'm not trying to denigrate the people who have already replied -- I appreciate your responses -- but I still feel like my original question hasn't been answered in any sort of definitive way. I'm hoping for something like "Yep, that's an observation" or "No, that doesn't count as an observation, because an observation must satisfy this specific criteria that that fails to meet."The Schrödinger's cat "paradox" arises because quantum mechanics places no obvious maximum limit on the size of a quantum system. Quantum mechanics seems to predict that the cat should be in a superposition of "alive" and "dead" states (just as the state of a single photon might become correlated with that of the decaying atom); yet we never see such superpositions.

There are several possible resolutions of this paradox.
Quantum mechanics may be incomplete; new physics takes effect somewhere between the microscopic scales at which quantum effects are easily observed and the macroscopic scales at which we don't see quantum effects and explains why we don't see macroscopic superpositions.
A nonobvious emergent property of quantum mechanics may inhibit the formation of macroscopic superpositions.
Something may prevent us from observing macroscopic superpositions, even when they exist.(This list is not intended to be exhaustive, and the alternatives are not mutually exclusive.)

Penrose, for example, argues for #1 (in The Emperor's New Mind and Shadows of the Mind), proposing quantum gravity as the required "new physics."

An explanation which IME is more popular with quantum physicists is a combination of #2 and #3 called "environment-induced decoherence." The essential idea is that the "environment" is very good at interacting with systems, and especially with macroscopic systems: for example, through absorption and emission of photons, interaction with molecules of gas surrounding the system, etc. The environment thus tends to constantly measure systems. Because these interactions tend to be localized in space and time, the systems tend to become correlated with the environment in a position basis. This means that macroscopic superpositions separated in space will be very hard to observe; the universe is constantly trying to correlate the position state of the experimental system with your state. Like the cat, you become correlated with the state of the nucleus. This is essentially a many-worlds interpretation: the wavefunction never actually "collapses" into a single eigenstate; it just becomes entangled with larger and larger portions of the universe.

In this view, from the cat's perspective it has "measured" the atom and knows its state (well, in the sense that it's either alive or dead). From your perspective outside the box, the entire atom+cat system inside the box is still in a superposition of states (at least, if the box is very well insulated from the outside world). Now, when you look into the box, your state becomes correlated with the state of the cat (and the atom). To you, it appears that you have collapsed the wavefunction of the cat. To someone outside your lab, the atom+cat+you system inside your lab is still in a superposition; and so on. You never "see" any superpositions, because one of your states sees a living cat and the other sees a dead cat.

Essentially, this argument boils down to what chrisk said:
Basically, the answer I was able to understand was 'yes, the cat's observations count... but not to you.'

[Mathochist]

What is the mechanism that allows for [superposition] to occur?

I don't think anyone really knows, but nobody really knows a "mechanism" for QM at all. It's just an assumption that things behave more or less linearly, so linear combinations of states are also states.

[Chronos]

One can't really use quantum mechanics, or any physical theory, to discuss what's happening in an unobserved system. The role of physics is to predict (at least statistically) the outcomes of experiments. So it's perfectly legitimate to say "When I open the box and make the measurement, there's a 50% chance that I'll observe a dead cat and a 50% chance I'll observe a live cat". It is not legitimate, however, to say anything about the condition of the cat before it's observed, or even to ask questions about such. Such questions are not questions of physics.

[Mathochist]

One can't really use quantum mechanics, or any physical theory, to discuss what's happening in an unobserved system. The role of physics is to predict (at least statistically) the outcomes of experiments. So it's perfectly legitimate to say "When I open the box and make the measurement, there's a 50% chance that I'll observe a dead cat and a 50% chance I'll observe a live cat". It is not legitimate, however, to say anything about the condition of the cat before it's observed, or even to ask questions about such. Such questions are not questions of physics.

I generally accept this premise, but it's important to keep in mind that it is a philosophical premise nonetheless, which may in the end be proven wrong.