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What makes a particle distinguishable from another one? It seems, two

  1. Mar 1, 2012 #1
    What makes a particle distinguishable from another one? It seems, two different kinds of particles ought to be distinguishable due to different mass or charge or other observable property. But, if particles can change into each other, like neutrinos, it's not clear that we can ever be 100% sure that we can distinguish them. Even heavier particles like neutrons and protons--can we distinguish a neutron from a proton and an electron? Does the concept of distinguishability have a fuzzy boundary, or can it be exactly decided?

    Is this the same problem as the question of quantum collapse? In a neutron decay scenario, a neutron can be distinguished from a proton and an electron when a measurement is made which collapses the wavefunction. So it seems like objects are distinguishable if we have previously collapsed them into states that ought to look sufficiently different over the course of the experiment. But if we wait too long without making a measurement, we couldn't really distinguish a hydrogen atom from a neutron, could we?

    Quantum collapse seems to make indistinguishable things into distinguishable. Or maybe, there are no distinguishable things-they only seem so over a certain span of time.
  2. jcsd
  3. Mar 1, 2012 #2
    Re: distinguishability?

    Well in the proton/neutron example (which aren't fundamental particles by the way, but that doesn't matter), protons have an electric charge and neutrons don't, and its very easy to distinguish between a particle with a charge and one without.
  4. Mar 1, 2012 #3


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    Re: distinguishability?

    In the well-known OPERA experiment, a beam of muon neutrinos travels from CERN to Gran Sasso. Some of them turn into tau neutrinos along the way, and are detected. How do you tell whether a neutrino is a muon neutrino or a tau neutrino? By their interaction. If a muon neutrino strikes an atom in the detector it produces (guess what!) a muon. A tau neutrino on the other hand produces a tau particle, which leaves a totally different track.
  5. Mar 2, 2012 #4
    Re: distinguishability?

    Vorde, I meant to compare neutron to the set (proton and an electron) which has 0 charge.
  6. Mar 3, 2012 #5
    Re: distinguishability?

    A neutron and the set you are indicating (I think you're meaning proton and electron bound state, say an hydrogen atom in his fundamental state) have very different properties that makes them distinguishable.
    Neutron and hydrogen fundamental state have, for example:

    - different masses;
    - different charge distribution;
    - different electro-weak and strong interaction.

    There are many other differences, but I think you got the point :smile:

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