I have often seen the following claim (for instance on Wikipedia: https://en.wikipedia.org/wiki/Von_Neumann%E2%80%93Wigner_interpretation):
In his 1932 book The Mathematical Foundations of Quantum Mechanics, John von Neumann argued that the mathematics of quantum mechanics allows for the collapse of the wave function to be placed at any position in the causal chain from the measurement device to the "subjective perception" of the human observer.
But I simply don't understand how this can be true. To me this reads as if it would have no experimental consequence where the point of collapse (between measurement device and observer). But how can this be? For instance, for a given quantum experiment we could imagine two hypothesis:
- It is really the measurement device that causes the collapse (due to some proprety of the way it measures), completely independent on any human observer. The human observer just plays the role of observing whatever resulting value was recorded due to the collapse.
- The human observer plays a crucial role in causing the collapse (through consciousness or in some other way) so before the human gets involved, the measurement device has now entered into a superposition itself based on the possible states of what it measured. Only when the human observes the measurement device does the collapse take place.
To me it seems it must be possible, for any given experiment, to change the experiment to find out if one or the other case is true.
For instance, if case 2 is true, the whole experimental setup (including measurement device) retains the superposition of states until the human observes it. This should make it possible to elucidate the well-known quantum effects ("quantum weirdness") even after the measurement by the measuring device. For instance if the experiment being measured involved a binary state of potentially negative amplitudes then the measurement device would now also be in a binary state with potentially negative amplitudes. If the measurement device is made to emit, say, a photon with different polarization based on the binary value the device observed, then the device will (still assuming case 2) be emitting photons also in superposition with potentially negative amplitudes. These photons can then be used in a second experimental setup that would show the effects of destructive interference among particles (exploting the negative amplitudes). The human then observes the effect of the second experiment: If the destructive interference is seen, this confirms case 2 because it can be concluded that the measurement device did not collapse the wave-function. On the other hand, if the interference effects are not observed, it means that the measurement device itself actually did collapse the wave function even before the human looked at the measurement, thus causing the photons emitted of being in a state with only positive amplitudes (0,1) or (1,0) meaning that destructive interference was not impossible, human observer or not.
Since there are obvious experimental differences it follows that this cannot just be an interpretational isuse. What am I missing?