Diffraction of Electrons.
According to the poll of the greatest physicists conducted by The New York Times, the experiment with electron diffraction is one of the most astonishing studies in the history of science.
There is a source that emits a stream of electrons onto photosensitive screen. And there is an obstruction in the way of these electrons, a copper plate with two slits. What kind of picture can be expected on the screen if the electrons are imagined as small charged balls? Two strips illuminated opposite to the slits. In fact, the screen displays a much more complex pattern of alternating black and white stripes. This is due to the fact that, when passing through the slit, electrons begin to behave not as particles, but as waves (just like the photons, or light particles, which can be waves at the same time). These waves interact in space, either quenching or amplifying each other, and as a result, a complex pattern of alternating light and dark stripes appears on the screen. At the same time, the result of this experiment does not change, and if electrons pass through the slit not as one single stream, but one by one, even one particle can be a wave. Even a single electron can pass simultaneously through both slits (and this is also one of the main postulates of the Copenhagen interpretation of quantum mechanics, when particles can simultaneously display both their “usual” physical properties and exotic properties as a wave).
But what about the observer? The observer makes this complicated story even more confusing. When physicists, during similar experiments, tried to determine with the help of instruments which slit the electron actually passes through, the image on the screen had changed dramatically and became a “classic” pattern with two illuminated sections opposite to the slits and no alternating bands displayed. Electrons did not seem to show their wave nature under the watchful eye of observers. Is this some kind of a mystery? There is a more simple explanation: no observation of a system can be carried out without physically impacting it.