In which situations will the ‘normal’ laws of physics no longer apply, but those of quantum mechanics?

Answer

Dear Roel,

Contrary to how you phrase it in your question, there is no hard “boundary” from where you have to jettison the classical laws of physics in favor of quantum mechanics. Indeed, quantum mechanics is most visible at the nanometer scale, but the laws are equally valid for macroscopic things. On a macroscopic scale, the de quantum mechanics into classical physics: this is what it is called correspondence principle.

For example, according to quantum mechanics, the light coming from your desk lamp is made up of photons: many small, separate “packets” of energy. The light is therefore quantized: a whole number of photons comes out of your desk lamp/half a photon does not exist. According to classical physics, however, light is a continuous quantity: there is no question of quantization. An amount of light just comes out and we can use the lamp, for example. dim continuously.

At first glance, these two approaches are fundamentally different and cannot coexist. However, if you know that from your desk lamp (40W or something?) about 10¹⁹ photons per second, you see that classical physics can still be used. No one will notice if a photon more or less comes out of your lamp. The quantization only becomes important (= measurable, visible) when we talk about very weak light sources. That’s why we can (fortunately) continue to use the laws of classical physics for many macroscopic things: The quantum mechanical effects here are often so subtle that they don’t matter anyway.

Quantum mechanics is of course very important for nanotechnology and we cannot ignore these subtle effects. An example of this is the tunneling effect in which an electron, for example, passes through an insulator, while it does not have enough energy to do so. Engineers use this for example in all kinds of memory chips (eg USB stick). On a macroscopic level, such a thing is unthinkable. If you walk to the wall you will not suddenly end up on the other side of it: a sore head is the only thing you will get from this experiment. Still, according to quantum mechanics, there is a chance that it will work one day, but it is so small that I wouldn’t try it…