I read that measurement changes things because we have to bounce photons off an object to 'see' it and that changes its position, momentum etc... But on the other hand, Griffiths' QM book seems to suggest we don't know what it is about measurement that changes the state of something. We don't know what's special about measurement, or what exactly constitutes measurement. The photon idea sort of makes sense to me, so if it isn't actually the accepted answer, then why not? It's just that the photon thing and Griffiths seem to say different things. From quickly googling this, most people seem to say that the act of measurement will always disturb the thing being measured. That doesn't seem particularly strange, does it? But until now, having only read books for laymen, I'd got the impression that the fact measurement changed things was bizarre and no-one knows why it changes things or even what exactly constitutes measurement... I'm quite confused.