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Revisiting Schrödinger’s Cat

Put me in that box at your own risk!

If there is one area of quantum mechanics that has generated more writing — both sense and nonsense — than any other, it’s the thought experiment known as “Schrödinger’s cat”. I dithered over whether I should even write on the subject, since so much has been said already and I don’t know that I have anything particularly new to contribute. On the other hand, with a cover story in Scientific American about the intersection between quantum physics and what we traditionally think of as the macroscopic world, that mythical cat seems as relevant as ever, so here goes….

Erwin Schrödinger was one of the founders of quantum mechanics; as I write this, I have a roomful of students taking an exam in which they have to solve Schrödinger’s equation, which is the primary equation governing quantum systems. However, like Einstein, Schrödinger never was comfortable with the standard interpretation of quantum mechanics (as a lot of physicists aren’t), and his “cat” thought experiment was designed to illustrate exactly why.

In brief, the standard interpretation of quantum physics — known as the Copenhagen interpretation because it was developed by Danish physicist Niels Bohr — says that because microscopic objects such as electrons, atoms, etc. are inaccessible to us except through experiments, it makes no sense to talk about those objects having an independent reality. We only see electrons through measurement, and their characteristics depend strongly on what kind of experiment we do — in some experiments, electrons are very wave-like, experiencing diffraction and interference, while in other experiments, electrons are particle-like, colliding and scattering and following straight-line trajectories. In addition, it’s impossible to perform an experiment without affecting the system being studied: shining light on atoms to measure their properties inevitably changes the state the atoms are in. (Macroscopic experiments also affect what is being measured, but in many cases the effect is small enough we neglect it.) Both of these aspects are unambiguous, although their interpretation is not.

The standard interpretation says this is as far as we can go: it doesn’t make sense to talk about electrons having a definite position before they are measured, or atoms existing in a particular state before the experiment. Instead, the best quantum mechanics can do is provide probabilities that the microscopic system has particular characteristics — the uncertainty isn’t a lack of knowledge on our part, but a fundamental property of nature. We cannot know the electron’s position because it doesn’t have one.

So how can we understand the process of determining an electron’s position? Our equipment is in some sense macroscopic: we manipulate it on our own scale, but it is made up of microscopic pieces. The challenge is how to understand that interaction between the equipment and the microscopic system being studied, and how the microscopic and macroscopic merge. As the Scientific American article I mentioned earlier points out, it’s not strictly true that quantum theory is only microscopic: a lot of macroscopic systems are quantum in nature, bringing the weirdness into the human scale.

But what about a mixed system, in which there is an undoubtedly quantum system interacting with something that doesn’t really need quantum mechanics to describe it. And so we return to Schrödinger’s cat: imagine a closed box containing a radioactive substance and a live cat. (Warning to animal lovers: this isn’t a very nice experiment, but admittedly it’s only a thought experiment — it’s intended to be an illustrative example rather than a practical design.) There is also a vial of cyanide gas connected to a radiation detector, so that when the detector triggers, the vial opens, releasing the gas and killing the cat.

It’s simple enough: the rules governing the radioactive substance are quantum in nature, described using probabilities. The cat’s state of being doesn’t need quantum mechanics to understand: either it’s alive or it’s dead. (Other aspects of the cat aren’t important for this problem.) However, to describe the entire system using quantum mechanics leads to a paradox: the cat’s state of being is undetermined in the same way that an electron’s position is. Until you take a measurement (by opening the box in this case), the cat is in what is known as a superposition of states: it must be considered to be neither dead nor alive.

Schrödinger argued that this was an inherent problem in interpreting quantum mechanics. We know the cat has to be alive or dead — there’s no meaningful sense in which it is both dead and alive at the same time. I won’t get into the question of whether the cat itself constitutes an observer, able to perform the “measurement” itself. If you want to remove consciousness (and cruelty) from the question, you could imagine a tiny bomb triggered by the radioactive device, so that the bomb is in a superposition of exploded and non-exploded states. Similarly, you could argue that the Geiger counter that triggers the release of the cyanide gas constitutes a measurement, but you could imagine a simpler experiment in which the radiation directly explodes the bomb, removing the middle piece. The paradox remains. The main thing is that there should be a non-quantum system forced into a superposition by virtue of its interaction with a quantum system.

Lest you think this is purely academic, the physics version of asking how many angels can dance the Watusi on the head of a pin, we have a lot of devices that require quantum mechanics to work. Inside an ordinary digital camera, a small semiconductor device triggers when a photon — a particle of light — collides with it. Electrons are steered magnetically to trace patterns on the front of television screens to produce the images you see. (I had an argument with a Creationist colleague at Lambuth University about whether electrons could be said to exist or if they were speculative objects, since they cannot be seen directly. My response in part was to ask whether he had ever watched television.) The invisible and tiny manifest themselves macroscopically, whether or not a conscious observer is inserted into the system. Although these ordinary things don’t produce the kind of paradoxes inherent in the Schrödinger cat thought experiment, we still haven’t fully resolved the problem of interaction between quantum systems and things that don’t really require quantum physics to describe.

I alluded in my previous post to the fact that I studied quantum measurement theory in my undergraduate days. That’s a fairly esoteric area of physics that actually grapples with these questions, and no real consensus exists on how to resolve the paradox. I won’t lie: I don’t have a nice simple pat answer to wrap things up, but I do think that old cat still has some life in her.

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24 Responses to “Revisiting Schrödinger’s Cat”


  1. 1 Kristy King May 19, 2011 at 14:07

    Alright, so I’m a political theorist and not a physicist but I always struggle with this stupid cat thing (when I have to teach the play “Copenhagen”). This whole epistemological approach seems deeply arrogant to me because the claim is that all existence is a function of human knowing. Maybe I’m being overly literal about the cat metaphor but the thing is that the cat IS either alive or dead regardless of whether the scientists taking bets know it or not. It seems dangerously hubristic to me to claim that knowledge literally creates the world and to make all existence dependent on human apprehension. It builds upon a dualistic view of the world in which humans exist outside the nature that they describe, bring into being and, of course, dominate. What am I missing?

    • 2 Matthew R. Francis May 19, 2011 at 15:48

      I don’t think it’s quite so bad as that, but it is still highly subjective. Some of the language Bohr used seems downright vitalist to me (though I would hesitate to say he was actually a vitalist). The main thing is that he wasn’t a Realist in any sense: he didn’t believe in a physical reality independent of experience and experiment. To Bohr, the cat is neither dead nor alive until the box is opened.

    • 3 Matthew R. Francis May 19, 2011 at 16:20

      Another thought: it’s probably a mistake to say that the experimenter “causes” the cat to be either dead or alive within the Copenhagen interpretation. It’s more that the action of measurement dictates the system go into one possibility over the other.

      As I said in my earlier post, I’m not a fan of the Copenhagen interpretation. I honestly haven’t spent enough time thinking about this stuff since my undergrad days to say what I would prefer, given the alternatives, but I do consider myself a Realist. (I’ve even been known to kick rocks while shouting, “I refute you thus!” It’s embarrassing.)

    • 4 furyguitar August 12, 2012 at 11:22

      Please, correct me of I’m wrong, as I am merely a music teacher with an interest in physics, astronomy etc – but my understanding is that observation does in fact play a key role. Perhaps not in a macro system such as a car being alive or dead, but I once learned that the double-slit experiment showed drastically different results when putting detectors in place to record with path was taken (diffraction pattern vs seemingly random). In that case, it was machine observing, not a living person, but the outcomes were affected nonetheless.

      I believe in Sean M Carroll’s “From Here to Eternity,” he stipulates that “an observer” need not be a human, nor necessarily something living. So it may not be as hubristic as it might appear.

      • 5 Matthew R. Francis August 12, 2012 at 11:43

        As I said in the post, measurement is a key aspect of quantum mechanics, however you want to interpret it. (I prefer “measurement” to “observation” for exactly the reason you bring up: “observer” seems to indicate a mind, which isn’t needed to make everything work.) Measurement may in fact be *the* key to understanding quantum theory (since it lies at the heart of quantum entanglement, the wave-particle duality, and so forth), so I’m certainly not denying its importance in the microscoipc world!

        The problem as I see it in the Schrodinger’s cat thought experiment is that we have a mixture of microscopic – where measurement is described by quantum mechanics – and macroscopic – where we rightly resist the idea that the cat’s state (living vs. dead) depends on measurement.

  2. 6 Richard May 19, 2011 at 21:48

    I am now a lot closer to understanding this than I was. Thanks.

  3. 7 Kristy King May 21, 2011 at 15:03

    Matthew, just curious: does ‘realist’ in your world mean that you take for granted the existence of a material world outside of human consciousness? (I have no guesses about what a ‘vitalist’ might be to a physicist! Is ‘vitalist’ in diametrical opposition to ‘realist’ or do you also have some kind of ‘idealist’ category?)

    • 8 Matthew R. Francis May 21, 2011 at 19:51

      OK, I might be getting in over my head (not being a philosopher), but to me “realist” means someone who thinks material objects have an existence independent of our observation and mathematical description. Electrons are real in some sense; our measurement may affect them, but they are there whether we perform experiments on them or not. Our theoretical models may or may not be particularly good, but there is something there.

      I’m using “vitalist” to mean that there is something important about being alive to the observation process. Go too far that route and you end up in New Age woo. Again, I worry I’m getting too much beyond my area to define things; maybe the opposite of realist would be a radical empiricist, for which everything is subjective and relative to the observer with no ability to extrapolate. Any philosophers out there care to school me?

    • 9 hhh June 8, 2011 at 13:44

      “Stupid Cat” ” Your World”…. You do not take into consideration other people eh?

      • 10 hhh June 8, 2011 at 13:46

        And Mathew, I am not a viable Source, But it sounds to me like You’re doing a fine Job.. Whether Completely Right or not!

  4. 11 Andrew DeFaria June 9, 2011 at 11:30

    I figured it out one day. Didn’t need to open the box either. I was sitting there and pondering how I could determine that the cat was alive or dead when wham! The cat meowed! It’s alive! IT’S ALIVE!

    ;-)

  5. 13 peyami August 12, 2012 at 10:43

    Reblogged this on peyami.

  6. 14 Achille Talon August 12, 2012 at 14:18

    The Copenhague interpretation doesn’t say the world exists because a consciencious mind observe/measure it. It says, the state of the world as we observe it depends on the observation. It is quite different. The consciencious mind doesn’t make the world a reality thru observation, it makes its interpretation of the world thru observation. An unobserved world is a probable world until we observe it. But, the world is in some state at every given moment.

  7. 15 Bolivar "The Stainless Steel Rat" August 15, 2012 at 15:52

    The world I observe inside my computer has too many cats and babies!


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