Quantum Physics and What Our Animals See When We're Not There.

Factual Questions
1

I would be the first person to admit, I am not a quantum physicist. I think that is probably true for most of the people on these boards. But I try to understand it, as people try to explain it in simple terms.

There is just one thing I don’t understand. According to quantum physics, different quantum realities only “collapse” into one when they are being directly observed. And the thought experiment, Shroedinger’s Cat, seems to suggest the observer has to be human.

Does that mean when I am not at home, my cats are seeing all kinds of bizarre phenomenon? Matter phasing in and out, different realities coming into being, then disappearing? Because I find that hard to believe. When I come home, my cats don’t seem to be traumatized in any way, as I would expect if this were the case.

What about when I leave a room? What happens then? If I leave the tv or radio on, I still hear it in the next room. And everything is exactly where I left it. What if anything is actually happening when I leave a room (or in my top example, my pets alone in the house)?

I’m confused. Someone please allay my confusion:)

2

I have only the vaguest concept about quantum physics. But I can help a bit:
A better way to describe the quantum collapse is not to say that it only happens when a person observes it. A better way to imagine it is to say that the collapse occurs any time that one of the particles leaves any evidence of itself. (And at that moment, the particle stops being a weird quantum thing ,and becomes a real part of our world.)
The evidence could be what you see, as a human observer. But the evidence could also be any other observer, such as a piece of lab equipment that detects the particle. Or maybe just the fact that a couple particles bumped into each other, and interacted in a way that made them somehow act as normal objects in our world.
You might think of it as the two particles, by bumping into each other, “observed” each other, so they stopped being quantum, and became real objects in our normal world.

I’m sure what I just wrote would make a real scientist laugh*…but it’s the only way I can make any kind of sense aboout quantum phyiscs.

*(fellow dopers, please go easy on me…and try to correct me using short words. "Cause I really cant grok quantum stuff.)

3

You’re not alone, people have been arguing about this since 1924. What you’re talking about is the “Copenhagen Interpretation”:

But it’s not the only explanation of Quantum mechanics and it’s not universally agreed on by physicists.

I’m not aware the observer has to be human, any act of “measurement” constitutes an observation which collapses the wave function. A cat can be an observer just as much as a machine recording to a HD can be. Defining what constitutes a measurement and who counts as an observer is one of the problems with the copenhagen interpretation.

So if a cat is an observer, is a Frog one? What about a Plankton? A bacteria? A virus? Some of the other interpretations of QM attempt to resolve this. See here for a list:

4

It is only one amongst many interpretations of QM that holds that a conscious observer is necessary to collapse the wave function. Although it used to be a widely held interpretation, it is my impression that it has been going out of favor amongst physicists (or those of tehm who care about the interpretation QM, which may be only a small minority, since it really makes little or no difference to how one uses the theory) for a long time now.

Anyway, the vast majority of wave function collapses would take place on a micro-level, inaccessible to ordinary, casual human observation. In most circumstances, differences different collapses might give rise to would largely cancel each other out, and could only be detected (if at all) with sophisticated instruments. Neither you nor your cat would notice any difference, just looking normally. (The Schroedinger cat ‘experiment’ is, however, set up to be an exception to this general rule.)

I do not think anybody has ever held that only a human observer could collapse a wave function. If it did require observation to cause collapses, cats could do it as well as we could. I suppose that you are thinking that the cat is observing itself inside the box the whole time, and thus should be collapsing the relevant wave function, regardless of whether any human looks inside. I think you are probably right about this, but what you have uncovered is simply a flaw in the rhetoric of the thought experiment, not in the underlying reasoning. Schroedinger only talked about a cat in his box, being killed or not killed, for dramatic effect. You do not need to have a cat there at all. All you need is a setup where a quantum level event (a radioactive decay of a single atom, in this case) can cause an effect large enough to be easily observable by a human. In the original story, the radioactive decay, if it occurs, is detected by a Geiger counter which causes a vial of cyanide to be spilled into the box, killing the cat. You could just as well have the decay cause a vial of ink to spilled into the box, making the box all inky inside when somebody looks. This raises just the same issues about the proper interpretation of QM as the cat version of the experiment, and avoids the problem you have pointed to (but that was ignored or unnoticed by Schroedinger) about the cat itself being an observer. Unfortunately it does not make for such a compelling, memorable story.

Incidentally, it is worth noting that, contrary to most tellings of it, Schroedinger intended his story, and particularly the consequence of a cat being both dead and alive at the same time (or a box all inky and all clean inside at the same time) to be seen as absurd. He told the story to ridicule the idea that the wave functions is collapsed only by an act of observation by a conscious observer, and to suggest that some alternative interpretation of QM theory was needed. his opponents however, mostly embraced and gloried in the absurdity, and, at the time at least, they mostly prevailed.

5

IANAQP but my guess would be that a cat would qualify as an observer. Cats are aware enough to notice their environment. So are dogs, monkeys, mice, horses, birds, etc. At some point you might get down the line far enough that you have living things that are unaware of their surroundings - stuff like plants maybe - and they might not function as an observer. They’re basically oblivious to events happening around them.

6

BTW, the entire point of of Schroedinger’s oft-misunderstood thought experiment was to demonstrate how ridiculous quantum physics would be if you tried to apply it at a macro scale. Your cat does not see random alternate realities because the effects of quantum mechanics are happening at a particle and sub-partical scale.

7

Being an observer in quantum theory is not a question of awareness, or complexity, or any other attribute of a physical system (i.e. there is no class of things such that those things are ‘observers’), but rather, a role played in a certain kind of physical interaction called a ‘measurement’.

This is illustrated even more starkly by the elaboration on Schrödinger’s thought experiment known as Wigner’s friend: consider one experimentator (A) outside of the Schrödinger-box who, after the alotted time, takes a look into the box and observes either a dead or life cat. In this setup, he is the observer, and thus, his act of observation leads to the ‘collapse’ of the wave function.

But then stipulate that his lab is itself perfectly shielded from the environment until such a time as another experimentator (B) enters it, and learns from A about the outcome of his measurement. For B, the whole situation must be describable using the ‘ordinary’ quantum evolution until the time he checks the measurement result himself, and thus, after A has looked into the box, the whole lab must be in a superposed state consisting of equal parts of the poison having been released, the cat having died, and experimenter A being sad to find out his pet is dead, and the poison not having been released, the cat being alive, and experimenter A being happy and hopefully resolving not to engage in any further animal cruelty. Only once B learns the measurement outcome does the wave function collapse to either of those possibilities, because in the setup so conceived, he is the observer.

The problem is, of course, that those two accounts are flatly inconsistent with one another: A will see a definite cat-state upon looking into the box, while B will describe the system of poison+cat+A as being in a superposition and hence, A as not having any definite state (such as the state of seeing the cat alive). This is what prompted Wigner to entertain the possibility of conciousness causing the collapse (CCC), as it seemed too much of a stretch to him to suppose that A could be in such an ‘uncertain’ state.

This is not identical with the Copenhagen interpretation, by the way, and in fact, should not properly be considered an interpretation at all: since if the wave function objectively collapses upon A opening the box, and thus, A is in a definite state afterwards, and B’s description is simply mistaken, then B could—in principle—carry out a measurement on the whole system, letting it ‘interfere with itself’, and observe by the lack of interference that it is in fact in a definite state; so CCC actually makes different predictions from ordinary QM (even though these predictions will never be checked, since it is impossible to shield any system of such a complexity from the outside well enough to not have the interference essentially be destroyed by decoherence effects).

So the answer to the OP, to cut short a story that basically is still being told, is that your cat, considered as an observer, will not see any weird quantum stuff happening, but if you could isolate your house well enough, then you could by means of an impossibly accurate experiment verify (if ordinary quantum mechanics holds) that the inside must be in a huge superposition of all its possible states. How to reconcile these two perspectives is essentially the problem of finding the right interpretation of quantum mechanics.

8

It was a thought experiment intended to argue against the Copenhagen interpretation of QM. I dare say our OP did a fine job of it as well. “Do our pet cats see a blurry quantum world when we aren’t around to observe things and collapse the wave functions for them?” It sort of shows how silly it is to think of conscious observation being the important factor.

9

Your cats are not traumatized because this sort of thing is normal for cats. Why do you think they stare wildly at nothing, sit facing the wall, and/or run around like crazy late at night? Duh.

10

Naw…think of it as a computer simulation. The collapse only happens when it needs to otherwise it just runs more general and less computationally intensive shortcut algorithms…

:stuck_out_tongue:

11

Ha ha, I came in to say the same thing; thought I had my chance until I saw the last post. My cat is frequently defending us tooth & nail against various malevolent no-see-'ems.

12

Nice idea, the problem is just that the way a quantum system normally evolves is actually more computation intensive (exponentially so, in fact) than explicitly calculating a single ‘exact’ (i.e. ‘always collapsed’/classical) history of the world. It’s part of the reason why quantum computers outperform classical ones: they can simulate quantum systems (by simply being quantum systems) without the exponential slowdown any classical simulation incurs.

13

Well. IF you have a quantum computer…yea… :slight_smile:

If you don’t…

14

One interesting thing is measurement doesn’t even need interaction! E.g. if you know a particle is certain to be in either region A or region B, failing to detect it in region A would be the same as detecting it in region B.

15

What we now know is that, for all practical purposes, at a macroscopic level, quantum mechanics does not apply to cats, so stop worrying about the little buggers. They certainly don’t care.

Pure quantum states, even of tiny systems, are extremely fragile. When they interact with other systems, particularly ones with 10^23 degrees of freedom (i.e. macroscopic objects), the simple, coherent properties of pure quantum states are buried in a complex web of correlations that we call entanglement or decorrelation.

Collapse of the wave function does not occur at a precise time or location, it is simply a statement that it is hopeless for us to do an experiment which untangles all the correlations. Instead, when we want to, we can choose to collapse the wave function and ignore all those correlations. Collapsing the wave function is a promise to ourselves that we won’t try to disentangle the correlations.

This gives us a much clearer view of how the quantum world yields to the classical world for macroscopic systems. There are still conceptual difficulties, which gives ample room for weird philosophical interpretations like the popular many worlds interpretation.

16

The observer doesn’t have to be aware at all - a simple particle detector is an observer that’s sufficient to turn the wave function into a particle.

17

The problem with decoherence, however, is that it doesn’t help you explain why the cat has any definite perceptions at all—even if its state becomes massively entangled with the environment, that doesn’t make it a definite state, but rather, a superposition of the state of both the cat and its environment. If the superposition of the cat together with the vial of poison (or whatever else it may have interacted with) was cause for concern, then so is the superposition of cat+environment, even though, if you trace out (forget about) the latter, the cat is formally described by a state lacking quantum coherences. What decoherence explains is the lack of interference between macroscopic objects due to their being highly entangled with environmental degrees of freedom, but not the existence of any definite perceptions at all.

18

I’m convinced my cats are up all night, debating about what I see when they’re not in the room. Assuming I qualify as an observer.

19

The problem with this idea is that it says ignoring correlations which asymptotically vanish is collapse, whereas collapse is essentially ignoring all the possibilities you could’ve measured but didn’t (which isn’t really a choice) and ignoring the former is quite different from ignoring the latter.

20

Here, let me fix that for you:

“The observer doesn’t have to be aware at all - a simple particle detector is an observer that’s sufficient to turn the wave function into a particle.”

This whole thread is based on a popular misunderstanding of quantum physics as ascribing a special role to conscious observers in its laws, such that it is necessary to understand whether a cat has the same kind of consciousness as a human in order to answer the OP. But that is not the case, at all.

(Which is not to deny that there are perplexing questions as to how to interpret collapse/measurement/observation/…; they just don’t have to do with answering the question “What counts as conscious?”)