Lawrence M. Krauss in Nautilus:
Surely the same restriction doesn’t hold true in science, you might say. After all, for centuries after the introduction of the scientific method, conventional wisdom held that there were theories that were absolutely true for all scales, even if we could never be empirically certain of this in advance. Newton’s universal law of gravity, for example, was, after all, universal! It applied to falling apples and falling planets alike, and accounted for every significant observation made under the sun, and over it as well.
With the advent of relativity, and general relativity in particular, it became clear that Newton’s law of gravity was merely an approximation of a more fundamental theory. But the more fundamental theory, general relativity, was so mathematically beautiful that it seemed reasonable to assume that it codified perfectly and completely the behavior of space and time in the presence of mass and energy.
The advent of quantum mechanics changed everything. When quantum mechanics is combined with relativity, it turns out, rather unexpectedly in fact, that the detailed nature of the physical laws that govern matter and energy actually depend on the physical scale at which you measure them. This led to perhaps the biggest unsung scientific revolution in the 20th century: We know of no theory that both makes contact with the empirical world, and is absolutely and always true. (I don’t envisage this changing anytime soon, string theorists’ hopes notwithstanding.) Despite this, theoretical physicists have devoted considerable energy to chasing exactly this kind of theory. So, what is going on? Is a universal theory a legitimate goal, or will scientific truth always be scale-dependent?
More here.