The two slit experiment is shattering for several reasons, one of which is oftern talked about and another very rarely. The one that is talked about is that wave-particle duality. The other one that is less discussed is that up to then (and also Einstein) science worked hard to reduce the significance of the observer, whether we are talking laws of motion, biological laws operating before people existed, earth being just another planet in the solar system .... always moving away from the person being of as little criticality as possible. Relativity made a serious dent in that approach, but it is the two slit experiment that really puts that specific act of observation central. And we haven't fully got to the bottom of this aspect, though sticking to the maths does allow us not to reflect on it too much.