If you want to see fear in eyes of an quantum physicist, just mention the measurement problem. Basically the problem is how does a wave function collapse. When microscopic particles are not observed their wave function happily obey Schrödinger wave equation. But when we observe the microscopic particle, its wave function collapses. But this contradicts Schrödinger wave equation as it does not allow wave function to collapse*.
Here is what Schrödinger’s cat paradox is:
We take a box and we place in it an unopened container of cyanide.
Connect it to a radiation detector of some radioactive material.
Now when the radioactive substance decay, its decayed material is detected by radiation detector. This caused cyanide to evaporate in environment and anything living (the cat in this case) inside the box will instantly die if it inhales the vapours.
Next we place a cat in the box, and quantum mechanics takes over instantly we close the lid of the box.
When we close the lid, the inside of the box becomes dark. Since we are not observing the atoms of the radioactive material inside, it has a choice; whether to decay or not decay, whether to release cyanide and if cat is dead or not. So until and unless we open the lid and make a conscious measurement, cat is both dead and alive at the same time, which is really a paradox.
Albert Einstein, who was impressed by the ability of the thought experiment to highlight these issues**. In a letter to Schrödinger dated 1950, he wrote:
You are the only contemporary physicist, besides Laue, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established. Their interpretation is, however, refuted most elegantly by your system of radioactive atom + amplifier + charge of gunpowder + cat in a box, in which the psi-function of the system contains both the cat alive and blown to bits. Nobody really doubts that the presence or absence of the cat is something independent of the act of observation.
It is mysterious fog which prevails in the quantum world. A hundred years and we still need many questions to answer.
Here are some links to know more on Quantum nature.
** EPR Paradox