Consider an experiment in which a closed box is set up with a system inside that is initialised at time t=0 with state psi(0). A measurement of quantity w of the box’s contents is made at time T, that is observed by scientist S1. There is another scientist S2 that can, by pressing a button, cause a measurement to be recorded of quantity q of the box’s contents at time T/2. q may or may not be the same as w, eg they may be the same variable, they may be non-commuting variables, or they may be commuting variables. S1 does not know whether S2 has pressed the button, and S2 does not look at the recorded measurement of q prior to when S1 looks at the measurement of w. Say this experiment is conducted 200 times, with S2 pressing the button exactly half of those times, but S1 doesn’t know on which occasions the button was pressed. This gives us 100 trials in which w was measured at time T in a system at which q had been measured at time T/2 - call these ‘pre-measured trials’, and 100 trials that are ‘non-pre-measured’. I have the following questions: 1. Does S2 pressing the button change the probability distribution of the measurement of w? ie does the difference between the distributions of w results from the 100 pre-measured trials and those from the 100 non-pre-measured trials have a nonzero expected value for any w? 2. Would the answer to 1 depend on whether S2 ever looks at the results of her q measurements (ie was the value recorded at T/2 a measurement that caused the wavefunction to collapse, if nobody looked at it?), or when she looks at them? 3. Does it depend on what the variables w and q are? eg what if they are (i) the same variable (ii) non-commuting variables such as position and momentum (iii) commuting variables such as spin and position What I’m trying to understand here is whether wavefunction collapse affects subsequent measurements. I would have thought that it would do so. Also I’m wondering what makes something count as a measurement, in particular, whether recording a value with a recording device but never looking at it constitutes a measurement that will make the wavefunction collapse.