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Schrodinger's cat? Really?

  1. Aug 2, 2013 #1
    I can't seem to wrap my head around the basic concept of the Schrodinger's cat experiment, the one in which a radioactive particle is either emitted or not emitted, a device is either triggered or not triggered, and the cat is both alive and dead (in a superposition of states) until one of us opens the box and looks inside. It seems anthropocentric to think that nature needs us to observe something in order to have a definite outcome. It's as though the universe should not have existed until conscious life evolved in order to witness it.

    For one thing, I've always assumed that the term "observer" really meant "interactor" or, any photon or particle that interacts with another photon or particle. I assumed that the thing that would collapse the superposition is the atom that absorbs the radioactive particle (or doesn't) and triggers the cat's death (or doesn't), if not the atom emitting said particle, let alone the cat itself, or the atoms making up the box itself, which would interact with the heat (or lack thereof) or even the minute amount of gravity produced by either the dead or live cat.

    By the logic used in this thought experiment, if we put the box with the cat inside an even bigger box with a human in it, then the human could look in the box and collapse the superposition for him or her, but to everyone else outside the second box, the cat is both alive and dead and the human who looked inside the box has seen both a live and a dead cat...at least until another human looks inside the box containing the first human along with the box containing a cat. And if the human in the second box is to broadcast the results to everyone on the planet, then to those aboard the International Space Station (who have not heard the news), does the whole planet Earth exist in a superposition of two possible states where the entire human population has heard of both the dead and the live cat? Where does it end? If this is not the case, then neither can it be the case that we need to look inside the box to have a definite live or dead cat. Could someone explain what I'm missing?
    Last edited: Aug 2, 2013
  2. jcsd
  3. Aug 2, 2013 #2


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    That's OK, you aren't supposed to be able wrap your head around it :smile:. Schrodinger proposed the thought experiment not because he or anyone else was seriously suggesting that the cat was ever both dead and alive, but to point out weaknesses in the then-current understanding of quantum mechanics. The things that you're finding hard to swallow are precisely the sorts of problems that Schrodinger wanted to draw your attention to.

    You might try Googling around for "decoherence" - what you're describing is not far off from more modern thinking about how to connect the quantum world to the classical world in which we and our measuring instruments live. (One small point though: The notion of "wave function collapse" is itself part of the problem - it works mathematically but doesn't provide a lot of insight into exactly what's going on with the cat at the moment of decay. It is possible to formulate and reason about QM without ever invoking the notion of collapse so.... if it doesn't work for you, you don't need to use it).
  4. Aug 2, 2013 #3


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    This is one of the key issues that Everett attempted to address with his Many Worlds interpretation (MWI).
  5. Aug 2, 2013 #4


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    That's exactly it.

    Schrodinger's Cat is utterly trivial under the Copenhagen Interpretation where we know about the QM world when it makes its appearance here in the classical world. In Schrodinger's Cat that occurs at the particle detector - everything else is common sense classical from that point on. The cat is alive or dead when you open the box - not in some weird superposition of states.

    What it drew attention to was the need for a quantum theory of measurement that treated the whole thing Quantum Mechanically without this division between Quantum and Classical. Since then a lot of work has been done doing just that and much progress has been made particularly in the area of decoherence. A few issues still remain but things are now much clearer eg:

    'Fortunately, there is a way out of this illogical outcome. Every real system, whether quantum or 'classical' (such as a life-sized cat), is in contact with an external environment -- a messy, noisy collection of atoms whose state can never be perfectly known. This coupling between a quantum system in a superposition and the environment in which it is embedded leads the system to 'collapse' or decay over time into one state or another. This process is known as decoherence.'

    And that is the answer to Schrodinger's Cat from the modern viewpoint where we now have a quantum theory of measurement and a better understanding of decoherence. The cat is being observed all the time by the environment and that collapses the wave function so it is never in this weird state of being alive and dead. It is known that even the cosmic background radiation is enough to do this - and very quickly to boot.

    Interestingly, while it is very difficult to isolate objects from their environment, requiring things like temperatures near absolute zero, it is not impossible and some very strange phenomena occur when you do that eg:

    Einstein once said to Bohr - Do you believe the moon isn't there when you are not looking to which Bohr replied (in answer to that and other questions such as God does not play dice) stop telling God what to do. The joke is though neither were correct - the moon is there when you are not looking because it is never not being looked at - the environment is doing it all the time - and it is this that gives the world its classical properties.

    Last edited: Aug 2, 2013
  6. Aug 2, 2013 #5
    I found these more sceptical comments by Leifer on this topic of decoherence kinda of interesting and arguably can be seen by some as arguing that there's still a long way to go:
    What can decoherence do for us?
  7. Aug 2, 2013 #6


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    In the area of QM foundations and interpretation expecting agreement on anything is well nigh impossible. What I said is a consensus view of people active in the area as, for example, detailed in Schlosshauer's textbook on the subject where I learnt much of this stuff from:

    There are a number of papers online that give the full gory detail so you can make up your own mind eg:

    I personally hold to the decoherence ensemble interpretation believing it solves the essential mystery:

    'Postulating that although the system-apparatus is in an improper mixed state, we can interpret it as a proper mixed state superficially solves the problem of outcomes, but does not explain why this happens, how or when. This kind of interpretation is sometimes called the ensemble, or ignorance interpretation. Although the state is supposed to describe an individual quantum system, one claims that since we can only infer probabilities from multiple measurements, the reduced density operator is supposed to describe an ensemble of quantum systems, of which each member is in a definite state.'

    This is the key issue with decoherence and the very essence of the debate about if it solves the measurement problem. Most physicists believe, strictly speaking, it doesn't, as do I. What it does however is gives the appearance of wave-function collapse so observationaly its equivalent to solving the problem. And that is the key issue - are you willing to accept that as good enough? I am - others aren't. But either way it has greatly clarified and elucidated the essential mystery.

    Other issues like the so called factoring problem remain but I believe they will be resolved in time leaving what I said above as the central issue. Still one never knows.

  8. Aug 3, 2013 #7
    http://aspelmeyer.quantum.at/docs/82/downloads/exp.pdf [Broken]
    "This is at the heart of the so-called “quantum measurement problem”, also known as Schrödinger’s cat paradox. Another question is whether quantum superposition states of massive macroscopic objects are consistent with our notion of space-time or whether quantum theory will break down in such situations"

    "Physical theories are developed to describe phenomena in particular regimes, and generally are valid only within a limited range of scales. For example, general relativity provides an eective description of the Universe at large length scales, and has been tested from the cosmic scale down to distances as small as 10 meters [1, 2]. In contrast, quantum theory provides an eective description of physics at small length scales"

    "Our knowledge is ultimately restricted by the boundaries of what we have explored by direct observation or experiment"

    "On one hand, quantum theory excellently describes the behaviour of physical systems at small length scales. On the other hand, general relativity theory excellently describes systems involving very large scales: long distances, high accelerations, and massive bodies."

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