Many people hear fundamental physics and think of theoretical physics like string theory. In fact, physics research might be fundamental, or it might be theoretical, or even both, or neither. Pop culture often glosses over the fact that the two terms are not synonymous, they are in fact distinct descriptors and refer to different aspects of the research. I find it useful to illustrate this distinction by classifying research on a 2D grid where the vertical axis goes from experimental to theoretical, and the horizontal one goes from fundamental to applied, as in diagram below.
I don’t mean to classify research topics, such as atoms or galaxies or superconductors, which often grow tentacles in more than one quadrant, rather research types, which are more related to the specific techniques employed. Experimental research typically involves building instruments and collecting and analyzing data to check theories, while theoretical research involves proposing new theories or making predictions from pre-existing ones using analytic or numerical calculations. On the other axis, fundamental research involves the basic laws of physics and the fundamental building blocks of the universe, while applied research involves systems or models more immediately applicable society.
Let’s consider the two quadrants of fundamental research. The fundamental — theoretical quadrant contains work like string theory and supersymmetry, and well as simulations to understand how galaxies arise from fundamental physics. This research often involves long calculations and heady math. Theorists often work in office complexes with endless supplies of white boards, and progress is made by thinking about a problem for a long time. Shoes aren’t required, though many wear socks. To be sure, modern society would not exist without this quadrant. From Maxwell’s electromagnetism to Einstein’s relativity, what was originally fundamental and theoretical eventually became experimental and applied with the advent of radio, optics, and GPS technology. In other fields, such as quantum mechanics, fundamental theories and experiments progressed in concert, but in all cases, the theoretical — fundamental understanding was a crucial piece of the puzzle.
Let’s move on to the experimental — fundamental quadrant, which contains experimental work conducted to shed light on fundamental theories of nature. Sometimes the lines become blurry. Does chemistry belong here? Does biology? I would argue that because the understanding in these fields is largely empirical, and because they have yet to reach the point where a first principles approach is useful, these fields do not belong on our diagram of physics research. Astronomy is another difficult case. Even as recently as a century ago, the classification of galaxies into essentially arbitrary shape categories constituted a breakthrough. Over the past century though, astronomy has undergone a transformation into a field where practitioners observe astrophysical systems and test quantitative predictions from first principles-based theories, or perhaps try to learn first principles theories from observations. Indeed many now refer to it as astrophysics.
Experimentalists build and use complex apparatuses to establish controlled experimental conditions which isolate some fundamental property of nature. Many work in labs and shoes are nearly always required. We are lucky to have the generous support of institutions such as the Department of Energy, NASA, the National Science Foundation, as well as some private foundations, to pursue this often expensive research which is almost always years, decades, or even centuries away from application.