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

Quantum Mechanics: Paradoxical?

  1. Apr 17, 2003 #1
    First let me define my use of the word paradox, in this thread. A paradox is a self-contradictory statement, or one that cannot be true, but also cannot be false.

    Alright, now, some have said that Quantum Mechanics meets this criteria, because it states that something can be both alive and dead at the same time. If this is what it said, then I would agree that it was paradoxical, because "dead" is the opposite of "alive" and so you cannot be both.

    However, it is my current opinion that that is not what QM implies. I have read that Schrodinger's (sp?) Cat is not alive or dead, it is 1/2 of each. It is in a half-way state, until an observation is made, and then it becomes one or the other.

    Is my understanding of "Schrodinger's Cat" correct. If not, is Quantum Mechanics paradoxical?
  2. jcsd
  3. Apr 17, 2003 #2
    The example you use (Schodingers cat) is an interpretation of Quantum physics that is not supported by all physicists. Why would the cat depend on it being observed before it can be determined if the cat would be alive or not. Why hasn't the cat itself observer states? etc.

    I think it is an uneasonable interpretation of quantum physics. Quantum physics takes place in the micro universe, and not in the macro universe. If this interpreation would be true, then it leads to several other paradoxes. For instance the observer that first sees the cat, would determine by his/her observation (the collapse of the wave function) if the cat is alive or not. But why wouldn't the observer itself be in a same kind of state as the cat and the experiment itself. Being halfway observing a dead cat, and halfway being observing a life cat. And so on.

    For this obvious reasons, this kind of interpretation is assumed wrong.
  4. Apr 17, 2003 #3

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    One important point is being missed here.

    "In his original thought experiment, Schrodinger imagined that a cat is locked in a box, along with a radioactive atom that is connected to a vial containing a deadly poison. If the atom decays, it causes the vial to smash and the cat to be killed. When the box is closed we do not know if the atom has decayed or not, which means that it can be in both the decayed state and the non-decayed state at the same time. Therefore, the cat is both dead and alive at the same time - which clearly does not happen in classical physics."

    Because the observer is not coupled to a quantum mechanical system that can kill him (namely, a radioactive atom).


    The above makes me wonder: Why introduce the cat at all?

    I mean, the state of the cat is supposed to be analogous to quantum mechanical wave functions, but the fate of the cat is controlled by none other than a quantum mechanical wave function of a radioactive nucleus! Why not just look at that directly?
    Last edited: Apr 17, 2003
  5. Apr 17, 2003 #4
    Yes, but the cat is an observer. Would it be different if the cat were a human? The cat observes, prior to possible being killed, the espace of gas or not. So, why wouldn't that collapse the wave function?

    And the explample of the (second) observer, opening the box, that would suppose in this example to cause the collapse of the wave function. Why only then? It can be argues as well, this observer then becomes part of and connected to the system in a dual state, and becomes part of that dual state: observing a dead cat or observing a life cat.
  6. Apr 17, 2003 #5
    This statement is not quite correct either. Bose-Einstein condensates, for example, have been created which incorporate hundreds of molecules. What seems to matter is the extremes of the situation, not simply size. At extremely cold temperatures, high energy levels, small sizes etc. quantum effects can be observed in any system. According to calculations and observations, certain neutron stars displays Quantum effects on a truly large scale.

    As for Shrondenger's Cat, that is merely one effect of the central paradox of Quantum Mechanics, the Uncertainity Principle. This couples all the observable characteristics of particles in ways which make no common sense whatsoever. Mass and momentum, spin and momentum, etc. are all intimately connected so that it is impossible to effect one without effecting the other.

    What's more, this effect is precisely balanced and not simply arbitrary. If a flying ball hits a wall it may fall to the floor resting in one spot while still spinning, but not so with Quanta. The energy they contain, including their momentum, is quantized.

    This a bit along the lines of Zeno's paradox. If I take a step and then half of that and so on I will never reach my destination. However, Quantum Mechanics demonstrates it is impossible past a certain point to further subdivide my steps. What's more, as something influences my steps it also influences everything else about me.

    Some have suggested that what Quantum Mechanics really describes are really waves, that everything is made out of waves of energy and this explains how influencing one characteristic of a "particle" effects all the others. However, ordinary waves can theoretically be infinitely subdivided and do not display both the properties of a particle and a wave.

    Is it mass, is it energy, is it a particle, a wave, alive, dead, a thing, or a non-thing? No one can say for sure. Thus they are described as Indeterminate and paradoxical. All that can be said for sure is they obey stritch rules of indeterminacy which defy everyday experience.

    As for the "observer" and Shrondenger's Cat observing itself, recent experiments suggest it is not so much an observer collapsing the wave function but environmental noise. The state of somehow alive and dead last for perhaps a few femto seconds until environmental noise effects it. In other words, the weird indeterminate state of quanta increases with isolation, yet another extreme.
  7. Apr 17, 2003 #6
    That is what I think indeed happens. The kind of interpretation this experiment initially was given, serves a philosophical viewpoint of Idealism (which states that an objetive outside reality does not exist, and is dependend on the mind/observer). However, any observation is an act of "the flesh" not of "the mind", and thus does not raise the kind of paradox suggested here.
  8. Apr 17, 2003 #7
    No, it doesn't raise the kind of paradox referred to here, but it doesn't discount it either or that of the Hegelians' fascination with infinity either. If anything, it focuses the discussion more on seriously self-referential and self-contradictory paradox itself.

    Ninty five percent of all the scientists that have ever lived, are alive today. Despite this and the inordinate amount of money spent on science, the paradox of existence has only deepened. Thus it presents a ground for simultaneously unifying all these disparate views and placing them at odds to each other. In other words, progress as we've come to know the meaning of the word. :0)
  9. Apr 17, 2003 #8
    i like the noise theory, it makes sense.. but the many-worlds theory should be mentioned.

    let's say we are all together when we set up the box.. everybody leaves and i'm the first one back after an hour. i'm so curious, i open the box and find the cat dead - theoretically collapsing the wave-function and "making" the cat dead. then i seal the box again.

    i leave and someone else comes back and opens the box - the cat will be dead. BUT the 2nd person didn't collapse the wave-function... i did.

    the many-worlds theory allows for this, and also allows for the cat to act as an observer by stating that all potentialities actually do exist and we are constantly splitting paths.. like, the "you" that almost got hit by a car actually did get hit by a car in MANY other "worlds", but the "you" that is sitting here now is your current reality.

    it's crazy, but not impossible. as my username suggests, i'm into buddhism and this theory fits nicely with the philosophy..

    it's almost like your consciousness is a bubble surfing through this one set of many-worlds and when you die, you could be born again in a this "world", but that world would diverge depending on how you choose to live.

    it's like a video game where you play over and over again and can sometimes follow similar paths and sometimes play a completely different character..

    i hope i made sense, i haven't slept in awhile.. but this theory is one that has come up to eliminate the paradox..

    but, like i said, the environmental noise theory sounds much more realistic....
  10. Apr 17, 2003 #9

    I think you are not understanding the intent of the experiment. I agree with Tom. It didn't need to be a cat. It could have been anything. The point of this thought experiment was to show how absurd the copenhagan interpretation was. It was an attempt to link the weird events in the quantum world with the classical world that we know, to illustrate the absurdity. The fact that the macro world object was a cat, which can be argued to be conscious, has nothing to do with the experiment at all.
  11. Apr 17, 2003 #10
    Personally, I think Tom really cleared the whole matter, with the analysis he presented with the excerpt from the book.

    Further, I should like to make clear the fact that for one, cats, including any other living thing under the curse of death, is dying. Yet, a "living thing" is considered to be alive, although it is dying ("dying" should not be misconstrued to be portending a future event, but it should contrued to specify a present state). So, I guess with such minute reasoning, one can conclude it is vaguely possible to be "alive and dead" at the same time?(not rhetorical)
  12. Apr 18, 2003 #11
    Is this experiment demonstrating
    that in quantum mechanics things
    are too small and moving too fast to
    "open the box" and see atoms dead and
    alive all the time? Is it that they say the
    cat is half dead and half alive to mean
    that we can't see all the atoms all the time so we give them probabilities and possibilities? If so I think it's a very round about way of saying this.
  13. Apr 18, 2003 #12
    Welcom to PFs:wink:

    Didst thou read this, Jammieg? (forgive me if I have mispelled your screen name). I think Tom's post explains it very well.

  14. Apr 18, 2003 #13


    User Avatar
    Staff Emeritus
    Gold Member

    Are you talking about decoherence? AFAIK, no experiment can be interpreted as supporting such idea. It is an interesting interpretation, I agree, but its predictions do not differ from those of any other interpretation of QM (since the formal machinery is the exact same one).
  15. Apr 18, 2003 #14


    User Avatar
    Staff Emeritus
    Gold Member

    Why is it not coupled?

    Both the cat and the observer are aggregates of a very big number of quantum systems, each of which follows the rules of QM (unitary evolution).

    Think of the wavefunction associated to such aggregate (the "wavefunction of the observer"). After opening the box, such wavefunction will have very different spacetime probability distributions: in one case it will have a big probability of smiling, and of feeding and playing with a cat after the experiment; on the other, such probability is zero.

    While the box is closed, the wavefunction for the radiactive atom is such that

    |state at probing time> = |decayed> + |notDecayed>

    The cat+atom system is set so that its state rigth after that time is

    |decayed,dead> + |notDecayed,alive>

    And the cat+atom+observer system is such that, once opening the box, it should be

    |decayed,dead,grieving> + |notDecayed,alive,playingWithCat>

    However, our perceptions apparently show that, at some point, the system went from the above state to one described by only one of the two terms.

    It is not just a matter of knowing vs not_knowing, since quantum superposition does have physical effects (as in the double slit experiment).

    Just for the dramatic effect it has. People may have no problem with an atom behaving in an odd way, but having a half-alive, half-death animal was better for Schrodinger to make people think twice about the Copenhagen interpretation.

    We can also think about arranging the experiment so that, if the cat dies, a city is built in his name; if not, then a cruise ship is built for the cat to take a vacation.

    The QM description of the full system after 10 years would then be

    |decayed,dead,grieving,CatCity> + |notDecayed,alive,playingWithCat,USSfelineVacation>

    Or isn't it? did the first observer collapse it? the cat? the construction workers? why?

    An observer who stayed in Australia for those ten years would describe the full system as a superposition (why would he stop the unitary evolution description?), and yet, once he returns to LabCity, the state has to be described by only one of those terms.

    If those terms could interfere, then a "double slit experiment" would be able to show a physically real interference pattern on a TV screen until the moment he looks at the result; from then on, the interference pattern would dissappear, and he would see only a big spike in the slit corresponding to, let say,

  16. Apr 18, 2003 #15
    I thank you all for your replies.

    However, no one has yet answered the real question of this thread: Is "Schordinger's Cat" paradoxical?

    with.budda, I like that you brought the many-worlds idea, because that seems to resolve this problem. In the many-worlds reality, there is not paradox.
  17. Apr 18, 2003 #16
    So... a cat being both alive and dead, a particle being also a wave, mass being energy, etc. all based on the most accurate physical theory in history which no one can say exactly what the theory is about.......is not paradoxical. This is rhetorical nonsense that flies in the face of science and philosophy.
  18. Apr 18, 2003 #17


    User Avatar
    Staff Emeritus
    Gold Member

    If we use words in a loosely defined manner, then it is easy to form phrases like

    "According to QM, the atom has decayed and has not decayed"

    Which is a blatantly self-contradictory statement. However, such description is a childishly oversimplifying rendering of what QM actually says about the state!

    It is analogous to going from: "You have a wet tongue and a dry skin" to a cheating description like "you are dry and wet", and then happily saying "you are paradoxical!".

    QM is definitely not paradoxical in the predictions it makes. If it was able to produce mutually contradicting predictions, it would have no use as a physical theory.

    Much to the contrary, the theory is extremely well defined, and it clearly states what we will find out of an experiment.

    The problem comes when people try to describe the outcome in terms of everyday-life words, which are of course derived from everyday-life objects and interactions.

    Instead of saying "it is a paradox, as everything else... beware the power of paradox!", I find it much more useful to try and understand the subtleties in the process of translating measurements into interpretations. Such approach sheds light on the understanding of the subatomic world, the process of assigning descriptions, and the construction of interpretations, instead of stopping at the awe produced by a "paradox" based on ambiguous concepts, imprecise meanings and incomplete descriptions.
  19. Apr 18, 2003 #18
    If you are going to be picky about semantics and use of the word paradox, you should be careful yourself. What you are talking about the application of the theory, not the theory itself or what it describes.
  20. Apr 19, 2003 #19


    User Avatar
    Staff Emeritus
    Gold Member

    Despite popularization books, QM is a physical theory. It does not ever talk about dead cats and paradoxical decayed-and-not atoms. It has to do with physical states, Hilbert spaces, probabilities and experimental predictions. There are no inconsistencies or "paradoxes" in it.

    What you may be refering to are the informal descriptions portrayed on popularization books and tv programs, in which the puzzling aspects (what you may call "paradoxical features") of the theory are strongly emphasized in order to keep the attention of the general public.

    As I said, these "paradoxes" are just the result of a faulty description.

    The theory does reflect the fact that microscopic ojbects behave in a way that conflicts daily life notions, but why would you expect otherwise?
  21. Apr 19, 2003 #20
    This "popular" misconception you speak of extends to the greatest founding members of the theory including Heisenburg, Einstein, and Feignman and extends to such renouned institutions as Standford univeristy. Deny it all you want, it is widely recognized within scientific and academic circles.

    The mathematics of the Hilbert spaces, probabilities, and predictions you speak of are based on paraconsistent logic
    ( http://plato.stanford.edu/entries/logic-paraconsistent/ ) repleate with contradictions which defy classical Aristotelian logic. Hence, QM is considered a break from classical physics and is referred to as modern physics.

    To contest these facts is as absurd as the theory itself is.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Quantum Mechanics: Paradoxical?
  1. Quantum Mechanics (Replies: 2)