2+1 is not always 3: In the microworld unity is not always strength
In the microscopic world, as long as there are only two particles, things are relatively simple. When other particles are introduced, however, the situation soon becomes more complicated. Imagine there are two people pushing a broken-down car: The total force is the sum of their forces. Similarly, if there are three people, the total force would be the sum of the force of these three people, and so on. Now imagine a colloid, a solid particle of a few thousandths of a millimeter, immersed in fluid. Imagine also that just ahead, there is a similar particle. If there are "critical" thermal fluctuations in the fluid that separates them, the two particles will either repel or attract each other without even touching, a phenomenon caused by the fluctuations alone. In other words, an interaction force called the "critical Casimir" force emerges, as if the particles were connected by an invisible spring....