Thoughts on physics: a broad overview

Posted by Matt

As I've probably mentioned here, I'm in the process of trying to decide where to go to graduate school. For me this involves trying to absorb a sizable fraction of the papers written by the particle theorists at the six schools I'm considering (Cornell, Harvard, Stanford, Berkeley, U. Washington-Seattle, and Chicago), as well as talking to people at the schools, to try to get a feel for what both life and research would be like at each. It also involves a lot of thinking about what sort of physics I would like to do. With all of this on my mind, and schoolwork over for the quarter (except for a commutative algebra final on Wednesday), this seems like a good time to write down some thoughts about high-energy physics.

If you're not too familiar with physics, the things that come to mind might be the subjects of an introductory course -- blocks on planes, rotating objects, simple circuits -- and while these things are of practical interest, and introduce some important concepts, they don't give much of a feel for what a typical physicist is interested in. Broadly speaking, physicists who are not astrophysicists usually fall under the rubric of either "condensed matter" physicists or "high energy" physicists. [This is oversimplifying: atomic/molecular physics, for instance, is somewhere in between.] Condensed matter physics concerns itself with systems in which there is a large amount of matter in a relatively small volume; solid-state physics, for instance, is a subset. This area borders on chemistry and on many engineering applications, but it also includes some fascinating abstract and theoretical topics.

High energy physics is the physics of high energies, or of small distance scales. In some vague sense, these are equivalent. To see things at small distances, you need to probe them with something of small wavelength, and wavelength and energy are inversely related. High energy physics also is intimately related to cosmology, the study of the early universe and the evolution of the universe over time. This all sounds very vague, but hopefully over the course of several posts I can explain some of these ideas and why they are so fascinating to me.

First, let's take a historical perspective on the development of particle physics. "Atomic theory" as a philosophical idea is very old, but in scientific form (i.e., as a precise and empirically validated statement about nature) arose only recently in history, with the early chemists. Over the course of the 19th century this became much better understood: the matter we see around us is made up of discrete building blocks, with properties that recur in interesting patterns. The periodic table displays these strange recurring properties of the elements. A modern understanding reveals that these patterns arise from the solutions of Schroedinger's equation; rather than simply categorizing elements, we now understand underlying principles that explain the properties. Gell-Mann in the 1960s similarly systematized the classification of certain particles called hadrons that were seen in particle accelerators. In the case of the periodic table, the explanation for the pattern comes from the fact that atoms are a positively charged nucleus (as Rutherford found) surrounded by a cloud of electrons (first discovered by J. J. Thomson). In the case of hadrons, the properties come from the fact that they are made out of quarks. What seems to be just the boring work of classifying a list of constituents of matter always turns out to be not so boring: the categorization reveals mathematical patterns, symmetries, that point to a deeper understanding of nature.

High-energy physics is our current continuation of this process of investigating the underlying constituents of matter. Our current understanding has changed sharply since around 1970. At one time it was possible to debate whether quarks, for instance, are "real" or merely a mathematical bookkeeping device. This question seems rather silly now. The modern perspective, due to people like Wilson and Kadanoff, is that the theories we deal with in high-energy physics are "effective field theories." This means that they are theories that are valid over some particular regime of energy scales, not "the" underlying theory. For instance, in a superconductor, electrons pair up into "Cooper pairs." Imagine that there is some life form that lives in superconductors, and at some point they start doing physics. They will conclude that Cooper pairs are one of the fundamental constituents of matter. Cooper pairs are elementary quanta in the effective field theory describing superconductors. Of course, we know that these are "really" just bound states of electrons. But it is entirely possible that in a more fundamental theory -- one that holds at higher energies and reduces to our current theory at a certain scale -- electrons are also not fundamental.

I would say that if any physicist tells you he or she is trying to find "the" theory of everything, you should be rather skeptical. It is an attitude that one hears sometimes -- but I think that we should always say that we are looking for a better, more generally valid, effective theory. The goal is always to better understand the universe we live in, but we should not expect to attain a final answer. In this sense the distinction between high-energy physics and condensed matter is somewhat blurred, as both are taking some system and trying to construct effective pictures describing it.

As for the relation to cosmology: we have a great deal of evidence, dating back to Hubble, that the universe used to be much smaller and hotter. Penzias and Wilson discovered the "cosmic microwave background" that gives further evidence of this "Big Bang" hypothesis. Perhaps some day I'll get around to trying to explain why this background arises. The reason particle physics has something to tell us about cosmology is that quantum mechanics predicts that the number of particles in the universe is not constant. Particle/antiparticle pairs, for instance, can be produced from the vacuum. In the early, extremely hot, universe, a large number of particles of all types would be produced by the huge amounts of energy available. The initial number of them would be controlled by the laws of thermodynamics. Over time they would interact according to the laws of particle physics, changing the ratios of different types of particles available. For instance, a huge mystery is why the universe seems full of matter but not of antimatter. We do know of one source of an asymmetry between them -- called CP violation -- but the known types of CP violation are not nearly large enough to explain the discrepancy. A better understanding of this could come either from particle physics, or from cosmology. Particle physics tells us something about the large-scale matter content of the universe. One of Einstein's greatest insights was that gravity is just the geometry of spacetime, and that this is influenced by all sources of energy and momentum. Thus the particle content created by the Big Bang feeds back on the geometry of spacetime, a process that continues to this day. The fairly recent discovery of dark energy shows that there is some constituent of the energy of the universe that we don't understand. This has to fit in somehow with our picture of particle physics, but we don't know how. This makes it a very exciting research topic.

Hopefully, this rather rushed summary gives you some idea of why particle physics and cosmology are closely bound together. You'll note that I tend to use "particle physics" and "high-energy physics" roughly interchangably. This is pretty common -- although recently high-energy theory has developed a picture in which, beyond just particles, strings and higher-dimensional objects ("branes") are also important. On the other hand, there are signs that stringy theories tend to be equivalent to ordinary quantum field theories of point particles, and this relates to the fascinating notion of "holography." But, I am getting ahead of myself. I'll talk more about these things later.