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I How does measurement work in a two particle universe?

  1. Jul 28, 2017 #1
    Say that they are not entangled. Assume classical quantum laws still hold but that there are only two particles in this universe with nothing else.

    How would one of the particles measure the other?
    Both particle's location and momentum are nonexistent until a measurement occurs by one on the other. Presumably, a particle may be considered an observer.
  2. jcsd
  3. Jul 28, 2017 #2


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    I would say that "measurement" is necessarily a macroscopic interaction, and that it's not defined for a small number of particles. Measurement requires irreversibility.

    I should add that not everybody uses this criterion. There are articles that talk about undoing measurements, but that's a matter of semantics. Usually, they're talking about doing something that would be an initial step of a measurement, such as using a photon to detect the presence of an electron. But I would say that until the photon is actually observed, the measurement isn't actually completed.
  4. Jul 28, 2017 #3
    Do you mean observed as in determined by a macroscopic system or a conscious observer?

    From my understanding, wave function collapse happens because the particles become entangled with the environment, hence cementing their ontological status. And if two particles interact, they become entangled as well. So it seems to be the same thing. But for two particles entangled, while they are correlated, they are still in some sort of existential limbo. Does that imply the same thing happens macroscopically?

    If so, physical reality of far away objects becomes less concrete from our vantage point unless there is some form of entanglement between say earth and that something far away. I'm probably over reaching with this since I'm assuming no boundaries between the quantum world and the macroscopic world.
  5. Jul 28, 2017 #4
    So essentially what I'm saying is that because experimenter and the particle are entangled upon measurement, the experimenter can make definite statements about particle locality. But for an hypothetical alien outside our cosmological horizon, which cannot become entangled with us, cannot make definite statements about the experimenter/particle quantum system. From his perspective, the experimenter and the particle are entangled in a quantum blur, but the system might as well not even exist since the alien cannot possibly make become entangled with said system.
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