As part of my series on the Higgs boson, I mentioned that it used to be that we thought of objects in the macroscopic (‘classical’) world as consisting of either particles or waves while in the microscopic (‘quantum’) world, we had wave-particle duality, where entities had both properties. Both worlds were governed by different laws.
The more modern idea that everything at every size consists of quantum fields eliminates that problem. There is no ‘wave-particle duality’ (as formerly understood) and this implies that there is no quantum to classical transition. But it also implies that even macroscopic objects must obey the laws governing quantum fields, such as having a non-zero ground state energy because of having to obey the Heisenberg uncertainty principle. So the search has been on to test this idea for larger and larger objects.
Here is an ingenious experiment that shows that a small square mirror of size about 10 micrometers (which is very large when compared to atoms or elementary particles and would normally be considered to be of a size where classical laws applied) behaves according to the laws of quantum mechanics.