9D Chapter 1-3 Notes

9D Chapter 1-3 Notes - Class Notes for Modern Physics, Part...

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Unformatted text preview: Class Notes for Modern Physics, Part 1 J. Gunion U.C. Davis 9D, Spring Quarter J. Gunion What is Modern Physics? The study of Modern Physics is the study of the enormous revolution in our view of the physical universe that began just prior to 1900. At that time, most physicists believed that everything in physics was completely understood. Normal intuition and all experiments fit into the context of two basic theories: 1. Newtonian Mechanics for massive bodies; 2. Maxwells Theory for light (electromagnetic radiation). Consistency of the two required that there be a propagating medium (and, therefore, a preferred reference frame) for light. However, even a little thought made it clear that there was trouble on the horizon. And then came many new experimental results that made it clear that the then-existing theoretical framework was woefully inadequate to describe nature. In a relatively short period of time, physicists were compelled to adopt: J. Gunion 9D, Spring Quarter 1 1. the theory of special relativity based on the idea that there was no propagating medium for light (so that light traveled with the same speed regardless of the frame from which the light was viewed); 2. the theory of quantum mechanics, according to which the precise position and precise momentum of a particle cannot both be determined simultaneously. In fact, one must think of particles not as particles, but as waves, much like light. 3. At the same time, experiments made it clear that light comes in little quantum particle-like packets called photons. 4. In short, both particles and light have both a particle-like and wave-like nature. It is useful to focus first on the inconsistencies of the ether picture and of the above-outlined naive picture of space and time. This will lead us to the theory of special relativity. The latter inconsistencies are revealed by thinking carefully about Galilean transformations between coordinate systems that underpinned the pre- relativity view of space and time. J. Gunion 9D, Spring Quarter 2 Before proceeding, let me just emphasize that in this course we will be embarking on an exploration that has been repeated in a certain sense several times now. Indeed, the business of looking for inconsistencies in existing theories now has a long history of success, beginning with the development of special relativity, general relativity, and quantum mechanics. We have learned not to be arrogant, but rather to expect that the best theories of a given moment are imperfect and to look for difficulties (perhaps subtle ones) or extensions that are suggested by thought experiments that push the theories into a new domain. As an example, the development of the Standard Model of fundamental interactions (that you may have heard of) began with the realization that the theory that was developed to explain the weak interactions would violate the laws of probability conservation when extended to high energies....
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9D Chapter 1-3 Notes - Class Notes for Modern Physics, Part...

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