Tuesday, June 06, 2006

Superman explains double slit experiment

If you're a reader who does not know how quantum interference works, an old supersymmetric man, also known as superman with a supercharge "Q" on his shirt, explains it in this

If you have two minutes, you can also learn what is string theory from a

Sorry, Steve, I just copied the description at Google's website and of course disagree with it! :-) For those who found the previous videos too complicated or separated from reality, here's a

but it is more physical than this blog, especially in the context of mechanics. Finally, you should certainly avoid searching for

because otherwise you may find a calculus tutorial by an MIT alumnus which could be a problem. ;-)


  1. During a single run of a double-slit experiment (leading to one more detection event), do you think that

    (1) the particle actually goes through both slits

    (2) the particle actually goes through only one slit

    (3) the question is not allowed

    (4) something else?

    I think (2) is the sensible answer but people say many things about this, and I'm not sure what you will say.

    By the way, I do know the rules for making a quantum calculation, e.g. an irreversible interaction at the slit would destroy the quantum interference. This is a conceptual question about how you think of the physical reality between observations.

  2. Dear mitchell,

    sorry, these are somewhat verbal games. But the point of 1) and 2) is clearly designed for the reader to imagine a classical "reality" which either has a real, objective object that goes through one slit, or both slits.

    But that's not how this world - a quantum world - operates. So answers 1,2) are gone.

    Because I support the freedom of speec, I must also reject the answer 3).

    Thankfully, you have given us another choice, option 4).

    The correct answer is that a single particle must be treated as going through one slit only, but all histories contribute to the probability amplitudes via Feynman's path-integral prescription, and the resulting amplitude that has contributions from all the histories must be squared (in absolute value) and only be interpreted as the probability, and this probability is the only thing that QM (the most complete possible theory) can predict.

    So physical laws can only predict statistical properties of many similar experiments, not the outcome of one particular experiment.

    I can also describe the situation without Feynman's approach. The most complete knowledge about the particle is described by a wave function that is nonzero in both slits. But the wave function "is not" the particle itself and it is not a real wave. It is a tool to calculate probabilities, and the rest goes just like in the paragraphs above.

    Best wishes

  3. Lubos:

    I didn't quite understand that explanation, it sounds a awful lot like Many Worlds, but at the same time it didn't.

    How would that interpretation differ from the Many histories interpretation/decoherent histories interpretation that Hartle and Gell Mann are supporters of where the others worlds are actual?

  4. Dear I Do Not Give a F***,

    what I write is true and completely independent of someone's preferred interpretation as long as the interpretation is consistent with the known observations.

    The wave functions interfere (i.e. add from both slits); they only determine probabilities that can only be checked when the same experiment is repeated many times; the particle is always seen at one place.

    In many worlds, one imagines that all the histories with the final outcome "exist" somewhere in "parallel universes". I personally prefer consistent histories as the most comprehensive interpretation.

    But once again, phenomena such as decoherence are real phenomena that exist regardless any interpretation as long as the interpretation takes experimentally verified 25-year-old realizations into account. They can be observed, they can be calculated and predicted, and they describe many things such as the boundary between the classical and quantum intuition.

    Physics is not about vacuous philosophical flapdoodle. Physics is about understanding and predicting phenomena. I told you how this should be done properly, what can be done, and what can't be done. Everything you try to add is pure rubbish and you're clearly dissatisfied only because I don't want to add any rubbish of this kind - which is too bad.

    Best wishes