Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Schrodinger's Cat - An Alteration

  1. Jan 12, 2013 #1
    I am a complete amateur, but I have been reading about quantum physics and I have a question (so I apologize in advance if the question has an obvious flaw). In Schrodinger's famous thought experiment, a cat is put into a box and quantum effects can cause poison to be emitted in such a way that the cat is seemingly both dead and alive until the box is opened (under the Copenhagen interpretation).

    Let's alter the experiment. Suppose that it is not a cat in the box but a live human. Further, it is not poison that is emitted, but a device is implanted that causes a sharp pain in his/her arm. Why, in all probability, would the human being experience only one state before the box is opened and a measurement has taken place? In other words, it would be highly doubtful that the person would experience both having a sharp pain and not having a sharp pain in the same way that the cat would be both dead and alive. Further, why should a human being in the box or the fact that they are not killed make a difference to the principles of the experiment? Both the cat and the human would be conscious, living, breathing individuals, each just as capable of making observations inside the box. The only difference is that the human would be able to speak of his/her experiences, while a cat could not, and the human, unlike the cat, would always survive. In other words, is the Cophenhagen Interpreatation of the thought experiment flawed, or am I missing something important? (Thank you in advance for any responses.)
    Last edited: Jan 13, 2013
  2. jcsd
  3. Jan 13, 2013 #2
    A very good question—and, yes, one that physicists and philosophers of physics have discussed at length. There is a thought experiment very similar to yours called "Wigner's friend" you might wish to check out, in which a friend of the experimenter in the Schrödinger's cat scenario checks whether it's alive or dead, and we're then asked to consider what the state of the friend is before the experimenter (Wigner) asks him about the outcome. The motivation of the question is largely the same as yours, and the answer is equally far from cut-and-dried:

    The "orthodox" Copenhagen answer, if such a thing exists, is simply that there is no issue—physical or philosophical—with a person being in a state of superposition, provided you treat them as two properly independent "selfs" who aren't aware of each other. In your thought experiment, your subject doesn't both feel pain and not feel pain. Rather, there's a superposition of two versions of him: one that feels pain and one that does not. Fundamentally, this is the same as the Copenhagen answer to the original Schrödinger's cat: one single cat was never actually both alive and dead; there was just a superposition of a live cat and a dead cat. It's a subtle but important difference. However—and this is the real legacy of the Wigner's friend thought experiment—this raises the question of how, if ever, the wavefunction actually collapses to a definite state. If Wigner's friend didn't make it happen and just created another superposition state, then why is Wigner so special? If you follow this to its seemingly logical conclusion, it's enough to make a solipsist out of anyone! This is called the measurement problem and various possible resolutions have been debated. There is, at least, partial salvation through something called decoherence, which demonstrates that large classical-scale systems (like cats and physicists) interact with their environment in such a way that they can't really sustain the same kind of stable superposition states as electrons and photons can. Their wavefunctions naturally evolve into states like a bit like wavefunction collapse—but it's not quite the same thing. Many people think it's progress though. In any case, different candidate interpretations of QM will generally discuss how they purport to handle this and other questions.

    It's a deep and interesting field to jump into, but do proceed with caution: any discussion of quantum mechanics and consciousness invariably attracts a large proportion of cranks and quacks. A place for reliable (but dense and sometimes hard to read for non-specialists—hell, even for specialists sometimes) information is the Stanford Encyclopedia of Philosophy (which has that and many other articles about interpretations of quantum physics, measurements, wavefunctions, and consciousness).
    Last edited: Jan 13, 2013
  4. Jan 13, 2013 #3
    As a side note, you might wonder why we bother with something as confusing as quantum mechanics when it doesn't seem like we can handle even very simple challenges to its foundations. The short answer is that, as a predictive theory, it works. Whatever superposition actually means, taking it as true allows some staggeringly accurate quantitative predictions of reality to be made. This assures us it does at least make sense to accept the outrageous claims of quantum mechanics, even if we don't (yet, one hopes) understand their full implications. This leads to a very minimalist "interpretation" (scare quotes since it's essentially an insisted absence of interpretation) of quantum mechanics called the "shut up and calculate" approach. Physicists taking this line are of the opinion that we should just accept that whatever QM means, all that matters is that it implies a mathematical framework that makes testable predictions, it has so far passed those tests with flying colours, and so the rest can be for the philosophers to quibble over. While maybe not a very satisfying answer, it is at least a good reminder to not get too invested into questions like these: they're certainly worthwhile to consider, but at the end of the day, quantum mechanics as a predictive theory will continue chugging happily along as we slog along in ignorance of some of the metaphysics.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Schrodinger's Cat - An Alteration
  1. Schrodinger's Cat (Replies: 3)

  2. Schrodinger's cat (Replies: 1)

  3. Schrodinger cat (Replies: 2)