>>7624618This is precisely the confusion. What does it mean to measure something? You have to interact with it somehow. Your eyes don't passively observe the world without interacting, you need photon interactions to see. Measurement and interaction/disturbance of the system are synonymous in this case (though you can certainly look at different kinds of interactions, measurements will have to be some kind of interaction).
It's counter-intuitive because at these beyond-microscopic levels there is no way to observe a system without perturbing it. You can't just 'watch' QM happen, you have to probe it somehow and, in doing so, you'll collapse perfect states or introduce terms into the Hamiltonian of the system.
Your last statement is spot on in the sense that outside particles can come in and bother the other particles out of their special states. Of course, you can consider systems of several particles with special mixed states--but if an outside particle not of this system comes in it can mess everything up. The whole problem is you have a special hamiltonian due to a specific setup. If you try and measure this system or an outside particle comes into the picture, it messes up this hamiltonian and subsequently messes up the quantum states within.
Maybe this will help: just think of a house of cards. Certainly there are forces within a house of cards and it can get very complicated how they all interact, but this state is still quite special. Now let's say you want to measure some properties of these cards with tools (measurement via interaction), or bring this house of cards to your windy lab for study (change the environment)--then the house of cards will be blown over by the wind, or you'll probably knock it over with your hammers and saws.
