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Introduction To Online Text by Christopher Stubbs

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.

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Figure 6: Penzias and Wilson horn antenna at Holmdel, NJ.

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.

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Figure 7: Meissner experiment.

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 is unknown. The evidence for dark matter, and the searches under way 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.