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Introduction To Online Text by Christopher Stubbs

The "physics approach" to understanding nature: simplicity, reductionism, and shrewd approximations

So what is physics, anyway? It's an experiment-based way of thinking about the world that attempts to make shrewd simplifying assumptions in order to extract the essential ingredients of a physical system or situation. Physicists try to develop the ability to distinguish the important aspects of a problem from the unimportant and distracting ones. Physicists learn to factor complex problems into tractable subsets that can be addressed individually, and are then brought back together to understand the broader system. An important ingredient in this approach is the attempt to find unifying principles that apply to a wide range of circumstances. The conservation laws of quantities like energy and electric charge are good examples of these broad principles, that help us understand and predict the properties of systems and circumstances.

laser clock in Jim Bergquist's lab

Figure 2: Laser clock in Jim Bergquist's lab.

A core ingredient in the way physicists look at the world is the central role of experiment and observation to determine which concepts or theories best describe the world we are privileged to inhabit. While there are many possible theories one might conjecture about the nature of reality, only those that survive confrontation with experiment endure. This ongoing interplay between theory and experiment distinguishes physics from other intellectual disciplines, even near neighbors like philosophy or mathematics.

Physics has had a strong tradition of reductionism, where complex systems are seen as aggregates of simpler subunits. All the substances you see around you are made of compound substances that are combinations of the elements (carbon, oxygen, hydrogen... ) that comprise the periodic table. But the atoms in these elements, which are defined by the number of protons in the respective atomic nuclei, are themselves made of protons, neutrons, and electrons. We now know that protons and neutrons are in turn composite objects, made up of yet more elemental objects called quarks. If the basic ingredients and their mutual interactions are well understood, the properties of remarkably complex situations can be understood and even predicted. For example, the structure of the periodic table can be understood by combining the laws of quantum mechanics with the properties of protons, neutrons, and electrons.

lab

Figure 3: Jim Bergquist with laser clock.

Physical systems that contain billions of atoms or particles acquire bulk properties that appear at first sight to be amenable to the traditional reductionist approach, although concepts like temperature and entropy are really only meaningful (or perhaps more accurately, useful) for these aggregate systems with large numbers of particles. The behavior of these many-body systems can often be described in terms of different levels of abstraction. For example, some aspects of the complicated interactions between light and glass can be summarized in terms of an index of refraction, that is independent of the details of the complex underlying phenomena.