Physics for the 21st Century
Introduction To Online Text By Christopher Stubbs
1. The intellectual heritage of Modern Physics
Physicists have been trying to figure out the world around them for centuries. Our society has inherited a rich intellectual heritage from this ongoing effort. The accumulated knowledge, the mathematical framework, and the concepts that we have inherited from giants like Galileo, Newton, and Einstein are traditionally taught in a roughly historical sequence. It takes most people many years to make the progression from mechanics (forces, masses, and accelerations) to electromagnetism (fields, charges, and potentials) to quantum mechanics (propagating waves of probability) to the current research frontier of physics. Most people claw their way to the boundary of modern knowledge only after a few years of graduate study.
The approach pursued here is different. We intend to jump directly to “the new stuff.” The goal of this course is to present some of the fascinating topics that are currently being actively studied by today’s physics community, at a level that is accessible to an interested high school student, or the high-school-student-at-heart.
The course has three components: i) written material, arranged as units, ii) video segments that present case studies related to the units, and iii) interactive Web modules. The different units can be approached in any desired sequence, but taking the time to explore the video and interactive segments associated with the units you find the most interesting is recommended.
The choice of research subjects presented here is representative, not exhaustive, and is meant to convey the excitement, the mystery, and the human aspects of modern physics. Numerous other threads of modern research could have been included. Hopefully, the topics selected will provide incentive for the reader to pursue other topics of interest, and perhaps it will prove possible to cover a number of these other topics in subsequent versions of the course.
2. 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.
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.
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.
While physicists have had remarkable successes in understanding the world in terms of its basic constituents and their fundamental interactions, physicists also now recognize that the reductionist approach has very real limitations. For example, even if we knew the inventory of all the objects that made up some physical system, and their initial configuration and interactions, there are both practical and intrinsic limitations to our ability to predict the system’s behavior at all future times. Even the most powerful computers have a limitation to the resolution with which numbers can be represented, and eventually, computational roundoff errors come into play, degrading our ability to replicate nature in a computer once we are dealing with more than a few dozen objects in a system for which the fundamental interactions are well-known.
As one of our authors, David Pines, writes:
An interesting and profound change in perspective is the issue of the emergent properties of matter. When we bring together the component parts of any system, be it people in a society or matter in bulk, the behavior of the whole is very different from that of its parts, and we call the resulting behavior emergent. Emergence is a bulk property. Thus matter in bulk acquires properties that are different from those of its fundamental constituents (electrons and nuclei) and we now recognize that a knowledge of their interactions does not make it possible to predict its properties, whether one is trying to determine whether a material becomes, say, an antiferromagnet or a novel superconductor, to say nothing of the behavior of a cell in living matter or the behavior of the neurons in the human brain. Feynman famously said: “life is nothing but the wiggling and jiggling of atoms,” but this does not tell us how these gave rise to LUCA, the last universal ancestor that is the progenitor of living matter, to say nothing of its subsequent evolution. It follows that we need to rethink the role of reductionism in understanding emergent behavior in physics or biology.
Understanding emergent behavior requires a change of focus. Instead of adopting the traditional reductionist approach that begins by identifying the individual constituents (quarks, electrons, atoms, individuals) and uses these as the basic building blocks for constructing a model that describes emergent behavior, we focus instead on identifying the collective organizing concepts and principles that lead to or characterize emergent behavior, and treat these as the basic building blocks of models of emergence.
Both the reductionist and the scientist with an emergent perspective focus on fundamentals. For the reductionist these are the individual constituents of the system, and the forces that couple them. For the scientist with an emergent perspective on matter in bulk, the fundamentals are the collective organizing principles that bring about the emergent behavior of the system as a whole, from the second law of thermodynamics to the mechanisms producing the novel coherent states of matter that emerge as a material seeks to reduce its entropy as its temperature is lowered.
4. The plan of this course
The course begins with the classic reductionist perspective, the search for the basic building blocks of nature. Their identification is described by Natalie Roe, in Unit 1, and the fundamental interactions on the subatomic scale are reviewed by David Kaplan in Unit 2. On human length scales and larger, one of the main forces at work is gravity, discussed in Chapter 3 by Blayne Heckel.
In Unit 4, Shamut Kachru takes on the issue of developing a theoretical framework that might be capable of helping us understand the very early universe, when gravity and quantum mechanics play equally important roles. He describes the current status of the string theories that seek to develop a unified quantum theory of gravitation and the other forces acting between elementary particles. Note, however, that while these exciting developments are regarded as advances in physics, they are presently in tension with the view that physics is an experiment-based science, in that we have yet to identify accessible measurements that can support or refute the string theory viewpoint.
Conveying the complexities of quantum mechanics in an accessible and meaningful way is the next major challenge for the course. The beginning of the 20th century was the advent of quantum mechanics, with the recognition that the world can’t always be approximated as collection of billiard balls. Instead, we must accept the fact that experiments demand a counter-intuitive and inherently probabilistic description. In Unit 5, Daniel Kleppner introduces the basic ideas of quantum mechanics. This is followed in Unit 6, by Bill Reinhart with a description of instances where quantum properties are exhibited on an accessible (macroscopic) scale, while in Unit 7, Lene Hau shows how the subtle interactions between light and matter can be exploited to produce remarkable effects, such as slowing light to a speed that a child could likely outrun on a bicycle.
Emergence is introduced in Unit 8, where David Pines presents an emergent perspective on basic topics that are included in many existing courses on condensed matter, and then describes some of the exciting new results on quantum matter that require new organizing principles and the new experimental probes that have helped generate these. The methodology of physics, the instrumentation that is derived from physics labs, and the search for the organizing principles responsible for emergent behavior in living matter, can provide valuable insights in biological systems, and the extent to which these are doing so is discussed by Robert Austin in Unit 9, “Biophysics.”
About 90% of the mass in galaxies like the Milky Way comprises “dark matter” whose composition and distribution are unknown. The evidence for dark matter, and the searches underway to find it are described by Peter Fisher in Unit 10.
Another indication of the work that lies ahead in constructing a complete and consistent intellectual framework by which to understand the universe is found in Unit 11, by Robert Kirshner. In the 1920s astronomers found that the universe was expanding. Professor Kirshner describes the astonishing discovery in the late 1990s that the expansion is happening at an ever-increasing rate. This seems to come about due to a repulsive gravitational interaction between regions of empty space. Understanding the nature of the “dark energy” that is driving the accelerating expansion is a complete mystery, and will likely occupy the astrophysical community for years to come.
5. An anthropic, accidental universe?
The long term goal of the reductionist agenda in physics is to arrive at a single, elegant, unified “theory of everything” (TOE), some mathematical principle that can be shown to be the single unique description of the physical universe. An aspiration of this approach is that the parameters of this model, such as the charge of the electron, the mass ratio of quarks to neutrinos, and the strengths of the fundamental interactions, would be determined by some grand principle, which has, so far, remained elusive.
In the past decade, an alternative point of view has arisen: that the basic parameters of the universe come about not from some grand principle, but are instead constrained by the simple fact that we are here to ask the questions. Universes with ten times as much dark energy would have been blown apart before stars and galaxies had a chance to assemble, and hence would not be subjected to scientific scrutiny, since no scientists would be there to inquire. Conversely, if gravity were a thousand times stronger, stellar evolution would go awry and again no scientists would have appeared on the scene.
The “anthropic” viewpoint, that the basic parameters of the universe we see are constrained by the presence of humans rather than some grand physical unification principle, is seen by many physicists as a retreat from the scientific tradition that has served as the intellectual beacon for generations of physicists. Other scientists accept the notion that the properties of the universe we see are an almost accidental consequence of the conditions needed for our feeble life forms to evolve. Indeed, whether this issue is a scientific question, which can be resolved on the basis of informed dialogue based upon observational evidence, is itself a topic of debate.
Exploring the anthropic debate is arguably a sensible next step, following the dark energy discussion in Unit 11.
Welcome to the research frontier.