Knocking on Heaven's Door - Lisa Randall [13]
I remember feeling a tad betrayed when my high school teacher, after devoting months to Newton’s Laws, told the class those laws were wrong. But my teacher was not quite right in his statement. Newton’s laws of motion work at the distances and speeds that were observable in his time. Newton thought about physical laws that applied, given the accuracy with which he (or anyone else in his era) could make measurements. He didn’t need the details of general relativity to make successful predictions about what could be measured then. And neither do we when we make the sorts of predictions relevant to large bodies at relatively low speeds and densities that Newton’s Laws apply to. When physicists or engineers today study planetary orbits, they also don’t need to know the detailed composition of the Sun. The laws that govern the behavior of quarks don’t noticeably affect the predictions relevant to celestial bodies either.
Understanding the most basic components is rarely the most efficient way to understand the interactions at larger scales, where tiny substructure generally plays very little role. We would be hard pressed to make progress in atomic physics by studying the even tinier quarks. It is only when we need to know more detailed properties of nuclei that the quark substructure becomes relevant. In the absence of unfathomable precision, we can safely do chemistry and molecular biology while ignoring any internal substructure in a nucleus. Elizabeth Streb’s dance movements won’t change no matter what happens at the quantum gravity scale. Choreography relies only on classical physical laws.
Everyone, including physicists, is happy to use a simpler description when the details are beyond our resolution. Physicists formalize this intuition and organize categories in terms of the distance or energy that is relevant. For any given problem, we use what we call an effective theory. The effective theory concentrates on the particles and forces that have “effects” at the distances in question. Rather than delineating particles and interactions in terms of unmeasurable parameters that describe more fundamental behavior, we formulate our theories, equations, and observations in terms of the things that are actually relevant to the scales we might detect.
The effective theory we apply at larger distances doesn’t go into the details of an underlying physical theory that applies to shorter distance scales. It asks only about things you could hope to measure or see. If something is beyond the resolution of the scales at which you are working, you don’t need its detailed structure. This practice is not scientific fraud. It is a way of disregarding the clutter of superfluous information. It is an “effective” way to obtain accurate answers efficiently and keep track of what is in your system.
The reason effective theories work is that it is safe to ignore the unknown, as long as it won’t make any measurable differences. If the only unknown phenomena occur at scales, distances, or resolutions where the influence is still indiscernible, we don’t need to know about them to make successful predictions. Phenomena beyond our current technical reach, by definition, won’t have any measurable consequences aside from those that are already taken into account.
This is why, even without knowing about phenomena as substantial as the existence of relativistic laws of motion or a quantum mechanical description of atomic