Schrödinger's Cat: Could someone explain what this is supposed to mean?

Factual Questions
21

Maybe someone who understands the issue better than I could characterize what happens if the box contains a sensor that registered the time that the poison was released… as observers, we perceive that the system is in multiple possible states for the entire hour we wait for the experiment to run its course. Upon opening the box, however, we have evidence that this was not actually true, that the event occurred 9 minutes after the start.

What do we say happened there? Was the timer itself in an indeterminate state until we observed it? Or was the indeterminacy of the state nullified by the fact that the automated timer was “observing” it the whole time? It makes me think there must be some way of describing layers of indeterminacy… for example, take the whole human-box-cat system, and put it inside another human-box-cat system, etc…

22

In this case, as I understand it, the timer observes the decay, so there isn’t any indeterminate state - at one point the atom hadn’t decayed, and then, 9 minutes in, it did. And was observed. This is like what was said about the observer not having to be human.

23

Right you are.

24

Oh yes, Penrose is still emphasising the idea; it gets an entire chapter in The Road to Reality (Cape, 2004). Though, in fairness, the interesting recentish development is that he’s now proposing a space-based cat-like experiment called FELIX that might be able to detect such “objective reduction”. A bit of a long shot in my opinion, but the practical challenges involved are such that I can see some experimentalists being willing to see if they can be overcome.

25

Nope. Quantum mechanics is very accurately describing reality at the subatomic level. The point of this thought experiment is to show just how different reality is at a subatomic level from the level we’re used to.

26

So if my cat dies, I can put him in a box, and as long as I don’t open the box, he might still be alive?

27

Conveniently, superstring theory (at least as of the 1999 date of the link, which I believe is also when The Elegant Universe was published) suggests that there are 9 spatial dimensions - the three we can sense, plus six that are curled up. Considering that a cat spends at least two thirds of its existence curled up, that seems to fit quite nicely.

28

Well, that’s the whole issue that the cat thing was trying to get at: What is an ‘observation’?

Conventional quantum theory not only says that the atom is in a superposition state, but when it interacts with another particle, now both particles are in a superposition state. And when either of those particles interacts with a third, now all three are in the superposition state, so on, and so on, so that, theoretically, not just the radioactive atom, but also the detector, the gas, and the timer will be in a superposition state until it’s ‘observed’. When it’s ‘observed’ then the waveform collapses to one or the other, rather than a superposition. This is very strange of course, but when thinking about how strange it is, most people get distracted enough by the weirdness that they ignore the whole issue of when exactly something is ‘observed’.

But Heisenberg pointed out that logically, the same superposition/collapse should apply to a living object (same kinds of atoms as a dead object, after all), so now the cat is in a superimposed state. Not only is this just completely wacky, it raises questions about the ‘observation’ : Can the cat make an ‘observation’ and collapse the waveform by getting poisoned? Or does the cat stay superposed until a human opens the box and observes it? What if a human was originally in the box instead of the cat? What if the human was asleep? Or completely brain-dead and on a respirator?

Now, it’s not so clear what an ‘observation’ is. In fact, it seems impossible to define, so there’s obviously a little problem there with quantum mechanics. That’s the whole reason all those cats have been poisoned/not poisoned.

29

Actually, it’s quite clear what an observation is: any kind of interaction that communicates information between two systems. The only confusion arises because, as Cecil so eloquently put it in the poem linked to above, “The act of observing disturbs the observed.” So yes, the cat can observe whether or not the gas canister has broken, but this in turn creates a new system: the cat-and-canister system, with which we presume nothing else has interacted. If something other than my eyes interacted with it, it would indeed collapse the wavefunction, but a new one would arise: the cat-and-canister-and-something-else system. If, however, I open the box, then I collapse the wavefunction, but from someone else’s point of view I have also created the cat-and-canister-and-me system, whose wavefunction is intact.

30

That’s exactly the main outstanding problem in quantum philosophy: the so-called “measurement problem”. The Copenhagen interpretation says that there are two types of change going on in the system – a unitary one and the nonunitary “quantum leap” when a system is observed. There’s really no good way to say what causes the leaps as opposed to the smooth evolution of the state over time.

31

I know the experiment you mean. It’s basically an interferometer bouncing photons off a crystal in such a way as to push it exactly half the distance between the crystal atoms, creating as large as possible a difference in the gravitational energies in the two superposed states (hit-by-photon/not-hit-by-photon), and measuring certain characteristics of the photons that come out. It’s been in five or six lectures I’ve seen him give since 1997.

32

So let me get this straight. You’re sitting in the lab and you open the box. From your perspective the cat has either died or not. But from my perspective, standing in the hallway, the entire particle-canister-cat-lab-you system is indeterminate until I open the door and ask you “How’s the cat?” Could the waveform collapse differently for me than it did for you? Would this place us in different universes?

33

I think I’m getting this…

When cats are not curled up, they do love to play with string…

Nope, it’s gone now…

34

You win! :wink:

35

Do the experiment yourself:
http://www.phobe.com/s_cat/s_cat.html

Hey! the creator of that site links to the Straight Dope Classic on this subject!

36

Alan Smithee’s question is essentially the same thing I’m wondering about (and Spatial Rift 47 provides one answer, but I don’t know if it’s the generally accepted one).

Does a system have different wavefunctions relative to different observers? That is to say, is the wave function of a system a matter of perspective? Specifically, is the wave function of the system collapsed for observers who are themselves entangled with the system, while uncollapsed for observers who are not entagled with the system? Is there any sort of general concensus among physicists with regard to these questions?

37

It is not an absolute truth that “the wave function collapses”. That is just one possible interpretation. Albeit a popular one.

It is true that the “problem of measurement” has occupied the great minds of quantum mechanics. Here is a somewhat more rigorous explanation.

Alas, the great minds have also made mistakes on occasion. Also, it seems they can be prone to “stuck paradigms”.

QM says that you need to perform a measurement (open the box) in order to observe a value (whether the cat is alive). The traditional interpretation says that until you perform a measurement, the value “isn’t decided yet”. It’s not that you merely don’t know whether or not the cat is dead - tradition says that the cat is both dead and alive, awaiting the final decision which happens the instant you look.

It goes further. Tradition says that if you think the cat is either definitely dead or definitely alive before you look, you are factually wrong. You can actually perform an experiment that proves that idea wrong. You can do an experiment that proves the cat cannot be either definitely alive or definitely dead. (This is extremely paraphrased; I am deliberately confounding this with a different phenomenon, for the sake of explanation.)

However, the traditional interpretation of QM is not the only correct one.

A different interpretation of quantum mechanics exists that is entirely consistent with the fundamental formulas of QM. This interpretation does not entail “collapse” of the wave function. It does say that observations have definite values all the time. The cat starts out alive, and if the radioactive decay occurs at some point, it kills the the cat.

This alternative interpretation makes QM entirely deterministic and objective. It also neatly removes the problem of measurement. It is called Bohmian Mechanics, or Bohm’s interpretation.

Bohm’s formulas make the exact same predictions as fundamental QM. Predictions that have been thoroughly confirmed in actual experiments.

It is still true that some of the phenomena are “strange”. Bohm’s interpretation reflects that. (I.e. Contextuality and Nonlocality.) This is not a problem with Bohm’s interpretation. It is a “problem” with the Real World. Personally I have never had any trouble with this. We can do the experiments and observe the outcome right in front of our eyes. In fact, we observe the “strangeness” of QM all the time. Consider all the materials we have in the world - all the chemical reactions - all of that is deeply connected with Quantum Mechanics and its strange principles.

Here is a nice dissertation on the “paradoxes” of QM and the case for Bohm’s interpretation. It is heavy on the math but the abstract and summary might give good impressions.

38

I always thought that the point was that when, after a sufficiently long time, we open the box and find a dead feline, we can then honestly say that “curiosity killed the cat” :smiley:

ducks & runs

39

To continue with the point made by Lumpy: “if atomic and subatomic particles exist in superposition, then why doesn’t the macroscopic world”, doesn’t that go along with the question of why do subatomic particles behave fundamentally differently than those on the macroscopic world.

I think that it has something to do with quantum physics having a different set of laws that only apply at the subatomic level. Another explanation is that the cat has nine lives.

40

But that’s exactly the problem that the Cat addresses: it’s perfectly possible to couple a microscopic system with a macroscopic one. Nature has no way of drawing a “dividing line” that can tell when to use one set of rules and when to use another.

Really, the answer is that macroscopic systems do exist in superposition, but that almost all the states cancel each other out and what’s left over is classical physics as we know it.