Observer effect in quantum world Why do the observer effect happens? Why do when seen a photon becomes a particle and when not seen shows wave nature? And why does not it happen on macroscopic level?
 A: Here is a simple example of an experiment, the double slit of single photons at  a time:


Figure 1. Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

The human observers are you and I, the experiment though is recorded and not affected by you and I. A single photon interacts with the slits (photon scattering off double slit) and then interacts with the screen, which is the first and permanent observer recorded for us.
Note that the wave nature of the photons is seen in the accumulation of single spots, i.e. it is in the probability of a photon hitting the $(x,y)$ spot on the screen. The spots seem random until enough photon footprints are accumulated.
Thus, observer at the quantum level is equivalent to interaction, not human eyes or ears.
Macroscopically one can observe quantum phenomena only in very particular cases. Due to the great number of particles in a mole. of order of Avogadro's number $N_A = 6.02214076×10^{23}\ \mathrm{mol^{−1}}$quantum mechanics is lost in the incoherence of the independent atomic and molecular motions. Superconductivity is a special case where quantum mechanics relations can be observed, but that is another story.
