The first, and often only description we get of friction is
that of a force which opposes the motion of an object. However, this description lacks on several
fronts. The main objection is that no
one really knows where that force comes from.
More subtly, and at smaller scales, a particle should never be
absolutely halted by friction. These
particles should instead move randomly, in a process called Brownian motion, in
thermal equilibrium with their surroundings.
Brownian motion can be added to the traditional description of friction
by adding a force that changes in strength and direction randomly. However, this still does not get to the
heart of the issue: what is friction?
At its heart friction has to be a quantum mechanical phenomena since the forces that oppose motion and generate Brownian motion are the direct result of interactions between the surface atoms or molecules. Unfortunately, directly mapping the phenomenological description of friction onto quantum mechanics results in nonsense. Other more complicated approaches result in sensible answers but have uncomfortable implications. For instance, the present macroscopic frictional force may depend on not only the present, but also past quantum states. Recent research has presented a novel approach to the problem. Their description of friction comes down to the quantum properties of measurement. Each collision between two surface atoms is a measurement of the others momentum and position, which can be measured simultaneously as long as the measurement is not too accurate. These measurements have the effect of constraining position and momentum, leading to a force that opposes the motion (friction) and also naturally results in Brownian motion.
It is nice to see that some of the commonly used but little understood notions of physics are getting some attention. However, it boggles the mind that friction could be due to measurement.