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I know that a wave function "collapses" when it is observed. One of the interpretations invokes consciousness as the cause. However, how do we know that observing is really only observing? How do we know that we aren't forcing the wave function to collapse because the observation adds something to the system (energy, constraint,etc)?

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  • $\begingroup$ An observation is, by definition, something that returns a number. $\endgroup$ – WillO Oct 19 '16 at 2:35
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    $\begingroup$ Wavefunction collapses when it is 'measured' according to the copenhagen interpretation! Not observed, measured!. And by that we mean we dont know what the hell is going on. What constitutes a measurement is an unsolved problem in physics. All we know is that the act of measurement messes with the delicate quantum system causing it to collapse to one of its eigenstate. $\endgroup$ – Prasad Mani Oct 19 '16 at 2:36
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    $\begingroup$ A wavefunction is not, as far as know, an observable. A particle's position is an observable as is a particle's momentum. If an observable, such as position or momentum, is measured, the measurement process 'collapses' the state to an eigenstate of the observable, i.e., just after a position measurement, the wavefunction is a position 'eigenfunction'. But the wavefunction has not been observed whatever that might mean. $\endgroup$ – Alfred Centauri Oct 19 '16 at 2:44
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    $\begingroup$ Exactly. If it helps, it CANNOT be observed anyhow since it is a complex quantity and is just a mathematical construct. $\endgroup$ – Prasad Mani Oct 19 '16 at 2:52
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Collapse of a wave into an observed particle -

Suppose the entity moving through double slit is an electron (in wave form). Let us observe it by shining a laser on it. If we were to observe the electron with the laser, then the laser has to be reflected off the electron. Right? But, in order to reflect the laser, the electron has to turn into a particle. Why? Because a wave can not reflect off a wave. A wave (laser) can only reflect off a particle. Therefore, in order to reflect the laser, the electron must turn into a particle; otherwise, the observation is just not possible using a laser. Therefore, if there is a measurement via laser, the electron must behave like a particle, not a wave. This way, observation is not just observation, it has to force the wave to collapse first.

Why the Collapse of a wave becomes mandatory on detection?

Why the laser is reflected off the electron in the first place? The actual reason for this can be the fact that the specific laser frequency that detects the electron can not coexist with electron wave at the same point of space time. So, when they meet, one has to collapse. Laser being more fundamental wave, it is the electron wave (less fundamental wave) that gives way and collapses. When collapsed, it becomes a particle and may reflect the laser depending upon where the particle is materialized per density distribution. This is why when we try to observe the electrons in either of the slits, it causes interference pattern to disappear irrespective of whether the electron was seen or not.

Laser (EM) has the fundamental property of a wave - fixed speed. Electron can travel at various speeds. Therefore laser is more fundamental wave as compared to electron wave. Electron wave has some wave properties, but not all of them. Fixed speed is one such example property.

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