After a measurement, the wave function of a particle, and of any particles entangled with it, collapses. But this isn't itself a measurable difference. For exa

After a measurement, the wave function of a particle, and of any particles entangled with it, collapses. But this isn't itself a measurable difference. For example, when performing measurements on two entangled particles at very far away places, the wave function collapses instantly, across any distance, even light years away. After that experiment, the result on the other side is 100% determined (if performed in the same basis). But there is no experiment whatsoever that could be done to tell if the collapse has happened or not from the other side. This is the resolution of the famous EPR paradox: wave function collapse is non-local (instantaneous), but it is probably impossible to send classical information with it faster than the speed of light limit.