Wednesday, August 21, 2013

Celebrating the quantum cat

screen-shot-2013-08-12-at-10-22-45-am goog schr catA Google doodle celebrates Schrödinger's cat. Time magazine explains:
Put a cat in a box, he proposed, and rig up a Rube Goldberg contraption involving a hammer, a vial of poison and a quantum triggering device. If an electron is in one position, the hammer will remain safely cocked. But if the electron moves into the opposite location, the hammer will drop, smashing the vial and killing the cat. The laws of quantum mechanics hold that as long as the electron remains undisturbed, it hangs in limbo, occupying both its possible states. The cat, by extension, is both dead and alive.
This is always presented as some profound quantum mechanical truth, but the pre-quantum explanation is not any different from the quantum explanation. There is not a shred of empirical evidence that a cat can be half dead and half alive. For all we know, the above ket (wave function) is just a mathematical shorthand for our lack of knowledge about whether the cat is dead or alive. Some people say that the cat is dead in one world, and alive in another. But that explains nothing. In quantum mechanics, a state is often written as a superposition of possibilities, even tho a measurement would constrain the state to one of those possibilities. The Schroedinger cat is just an analogy for thinking about such superpositions. But it is not to be taken too seriously.

Cosmologist Sean M. Carroll has put his Quantum Mechanics Made Easy online. It is not bad. He is interested in the arrow of time, and he notes that collapse of the wave function is irreversible. There is no agreement on what the collapse means, or whether the wave function really collapses. He also encourages describing quantum mechanics in 5 words:
Interfering probability theory.
Everything is waves, until measured.
representation theory of Poincaré group
unintuitive doesn't mean not comprehendable
You understand it? Me, neither.
multiple outcomes to single measurement
Probabilities on orthomodular proposition lattices
Imperfectly predictable and unpredictably perfect.
Schizophrenic states waiting for measurement.
nonrelativistic particles with discrete energies.
They're everywhere. Until you look.
Everything not forbidden is mandatory.
All knowledge is precisely uncertain.
Stop dictating to God, Einstein!
I like the "waves until measured".


  1. well - "This is always presented as some profound quantum mechanical truth, but the pre-quantum explanation is not any different from the quantum explanation." - I am finding Your lack of faith disturbing... This is simply not true. In classical probability, You have 2 states - Cat is dead - D and Cat is alive - L. If You have no knowledge about interior of the box, You may describe a Cat by some probability equations, and You will have separate results for D and L states. But in quantum mechanics You have additional term - interference L and D. And it may be empirically observed - of course not for cats, but for electrons for example. If You like to see it - just take a look on one of Yours pictures below Your photo. This is it.
    So quantum picture is quite different...

  2. Yes, we know electrons have wave properties because we can observe the interference. But interference between a live and dead cat? That is the sort of science fiction you might find on Star Trek, but never in the lab.

  3. You miss the point.
    1. Schroedringer cat is gedankenexperiment and it was provided by Schroediger - in order to show that quantum mechanics is wrong. And it became the Scholar-Book-Example of how it works.
    2. it has nothing to do with "wave" properties of electrons, because it is not electron what interfere here, but his wave functions. There is no single experiment where "electrons interfere". You have serious lack of knowledge. If You measure electrons hitting the wall, You will always see that it hit certain, small place on it.
    Sorry but You should rework Your QM course...

  4. See the Wikipedia article on the Double-slit experiment, where it explains that the popular interpretation is that because of wave properties of electrons, each electron is interfering with itself.

    The Schroedinger cat is indeed a thought experiment about probability, but it does not prove quantum mechanics right or wrong. Any other theory would have the same dilemma.

  5. I don't need five words to describe Quantum Mechanics, I can describe Quantum Mechanics in two words: Half baked. If you like, I can quote Feynman himself (his own two words) and call it a "Dippy Process".
    Any theory or "interpretation" (a la Copenhagen) of a theory which relies upon a paradox or contradiction to get off the ground is already useless as science since it has about as much mechanical and logical rigor as "..and then a miracle occurred! Praise be Paradox!"
    How about this interpretation of your interpretation "We have observed phenomena which we pitifully can not explain logically, and have been told by our elders there is no better explanation possible, so we will abide by their judgement and not try to explain it rationally or seek to discover a better answer, but accept irrational paradox as an answer and claim understanding while telling everyone else how profound it all is."
    Or, by some chance you actually realize you are supposed to be doing physics, not dualistic mysticism, and start looking at an actual three dimensional particle with some non-zero extension moving through three dimensional space, not an imaginary point particle of non-physical abstraction in two dimensions, and realize, hey, if an actual particle is spinning quite quickly while it travels, does it not describe a spiral to some degree? What would the spiral trajectory of a football look like if you viewed it from the side as a two dimensional perspective??? hmmm.... Hey, it kinda looks like a wave...hmm, I wonder..." Try looking at your own posted Bubble Chamber tracks for goodness sakes...notice the spirals? They aren't produced by virtual non dimensional points parading around part time as waves, but actual spinning things with actual physical extension and mass.
    Just because Niels Bohr had no imagination or any ability to visualize (which he himself practically admitted) doesn't mean there was no other explanation than his self professed "mystical duality" Copenhagen interpretation.
    As for the joke Schroedinger made about QM involving a cat, Einstein once said "I like to think that the moon is there even if I am not looking at it", which speaks volumes about what he thought of QM, however politely, and how infantile the CI really is.

  6. The theory does not rely on any paradox or contradiction to get off the ground. You do not need any mysticism to compute predictions and verify them.