• Question: Why do photons behave differently when observed?

    Asked by atomicalex to Ben, Jony, Katharine, Mark, Peter on 19 Nov 2011.
    • Photo: Ben Still

      Ben Still answered on 17 Nov 2011:

      Photons, as all particles, are only physically real when we observe them. In between times they are just balls of possibility, only probabilities of being in a certain place at a certain time. Observing is the collapse of the wave like behaviour into particle like behaviour.

    • Photo: Mark Basham

      Mark Basham answered on 17 Nov 2011:

      A good example of this happening is the double slit experiment, there is some info about it here http://en.wikipedia.org/wiki/Double-slit_experiment

      As to why they behave differently, I don’t think anyone knows, its just the way they are.

    • Photo: Peter Williams

      Peter Williams answered on 19 Nov 2011:

      There is a whole field of physics dedicated to just this question – look up “Quantum decoherence”.

      You can think of it in terms of thermodynamics, the measurement is a thermodynamically irreversible process. In this process information leaks out from the system into the environment. In the case of the photon the information is each individual path it can follow. The measurement “decoheres” the system – it forgets about the other paths.

      People are trying to manipulate this process to produce quantum computers

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