Physics 264: Optics and Modern Physics

Physics 264 is the physics course most students at Duke take after their introductory survey course. The course is intended to demonstrate why physics is a beautiful subject (and so encourage students to major in physics) by discussing in some depth two great achievements of 20th century science, Einstein's theory of special relativity and quantum mechanics.

Physics is much more than a collection of many formulas in which you plug in numbers to get some answer. In 264, you will see a theme that is repeated in other core and upper-level physics courses: that there are elegant satisfying insights that suggest why the laws of physics have the forms that they do, and that there are concise mathematical ways to capture these insights, from which one can understand conceptually and quantitatively a great variety of phenomena.

Physics 264 mainly addresses two basic questions about nature. One question concerns the implications of a maximum possible speed on the properties of space, time, and motion. Here you will learn about Einstein's theory of special relativity, with its many strange predictions, for example that mass is a form of energy (the famous equation E=mc2) and that energy can be transformed into matter (say particles and antiparticles); that the rate at which time passes is physically different in certain circumstances and that time travel into the future is possible and has been done; that the speed of light paradoxically always has the same value, even if you are moving in a rocket toward a flashlight at nine-tenths the speed of light; and that objects physically become shorter in the direction of motion, as measured by someone watching the objects move by. Crazy stuff but all confirmed in detail by experiments and now the foundation of many areas of modern science and engineering.

The second question addressed in 264 concerns what are the properties of matter on atomic and subatomic scales, the weird but fascinating regime of quantum mechanics. Quantum mechanics is perhaps most student's first exposure to how strangely Nature behaves for objects the size of atoms or smaller. For example, a molecule can simultaneously have multiple values of a particular quantity such as energy, a particle can have particle or wavelike properties depending on what experiment is carried out on the particle, there is a hard limit to what the human race can know about nature (the uncertainty principles), and, perhaps most disturbingly, that nature is intrinsically random and unpredictable at its most fundamental level which implies that there are details of the universe that no one will ever be able to predict with certainty.

An example of an interesting calculation that students in 264 learn how to carry out is what determines the decay time of a radioactive nucleus: how is it possible that some radioactive nuclei decay in microseconds while others decay over 10 billion years? In 264, you will learn that the answer is quantum tunneling of an alpha particle through a barrier, and you will learn further that quantum tunneling is a general phenomenon of substantial importance to many areas of science and engineering, for example biology (tunneling of hydrogen atoms in hydrogen bonds), electrical engineering (tunnel diodes), metrology (Josephson junctions), condensed matter physics (atomic force microscopy), and astrophysics (stars could not fuse hydrogen efficiently without tunneling).

For those of you who like to work with your hands or who are curious about how one obtains insights about nature through physical measurements, Physics 264 is also rewarding because you learn how to carry out and analyze several experiments that are substantially more interesting and satisfying than the labs of most introductory physics courses. Some of the 264 experiments in fact reproduce (using modern electronics) key experiments that motivated scientists to develop special relativity and quantum mechanics, and in two cases led to Nobel prizes. One example is Millikan's photoelectric effect experiment which confirmed Einstein's crazy hypothesis that light was not a wave but consisted of particle-like photons. The physics of the photoelectric effect later blossomed into many applied areas of physics, engineering, and consumer goods via the invention of the charge coupled device (CCD) that lies at the heart of modern digital cameras and video devices.

Another 264 lab concerns the Michelson-Morley interferometer, which provides a highly accurate way to measure the tiny wavelength of visible light and that was used in another context to prove that space was not filled with some fluid-like ether, a key result related to Einstein's theory of special relativity. The technology of the interferometer experiment has also spread to many branches of engineering and science and is the foundation for today's gravity wave detectors like LIGO, arguably the most sensitive experimental device yet built by the human race. LIGO and its cousins might revolutionize astronomy and physics within your lifetime by examining the gravitational waves emitted when a star supernovas and collapses into a black hole, or when neutron stars in a binary star system collide to form a black hole, or the gravitational waves emitted in the early stages of the Big Bang, at times that are earlier than those that can be observed with light-based telescopes.