Physics/Astronomy 152i

Freshman Seminar in Astrophysics

Half credit course intended primarily for prospective physical science majors.
Prerequisite: Physics 105a (or 101a) or the Bryn Mawr equivalent.
Co-prerequisite: Physics 106b (or 102b) or the Bryn Mawr equivalent.

Instructor: Professor Stephen Boughn, 610-896-1146,

Textbook: "Just Six Numbers" by Martin Rees.

Course Description:

The advertised purpose of this course is to introduce beginning physical science students to some of the exciting new developments in astrophysics. The overall theoretical understanding of many of these topics can be expressed in terms of fundamental physics at the level of an introductory course (e.g., Phys 105-6), and much of the class will involve the application of fundamental physical principles to various astrophysical phenomena. Since the details of these phenomena are quite complicated, it will be necessary to construct simple models and make judicious approximations. However, this shouldn't be viewed as a limitation of the course; in fact, such activities are precisely what working physicists often do. The picture of physics you may have acquired so far, i.e., a series of "laws" (and corresponding equations) and "problems" to be solved with those laws, is usually very far from the reality of "doing physics". The study of physics is a never-ending series of constructing models to describe physical phenomena and then attempting to apply these models to new situations. Ultimately all these models are "wrong" in that they invariably fail to properly describe or predict some new phenomenon (this was true for Newton's law of gravitation which was surpassed by Einstein's theory of general relativity, which in turn will have to be generalized to include quantum mechanics). Therefore, the approach in this course will, hopefully, give you a better idea of how physics really works.

The unity of physics, i.e., the interrelation of different areas of physics, is also something that is readily apparent in dealing with complex astrophysical phenomena. This is also a crucial aspect of the study of physics that is not often emphasized in standard courses (for understandable reasons). For example, to understand why stars shine, one needs to invoke thermal and statistical physics, gravitational physics, hydrodynamics, electrodynamics, nuclear physics, scattering theory, quantum mechanics, etc. While we will necessarily make gross simplifications (I assume that you have not yet mastered the above topics!), you should begin to get a feel for how physicists pursue such a problem.

Finally, it is hoped that this course will provide you with insight on how to go about "solving" quite general physical problems. This is also not the standard fare in most courses in which you are required to apply restricted techniques to solve a set of restricted problems in, for example, electrodynamics. Physicists, astrophysicists in particular, often choose problems for study not because they involve applications in a particular branch of physics but rather because they are inherently interesting (or important). They then use those areas of physics that are required to solve "the problem". Of course, this isn't easy and the present class won't, by any means, make you an expert in doing it. However, any insight you gain here will undoubtedly help you in your future study of physics. Among useful skills are: the ability to look at a complicated problem and "see" the most important feature of it; the ability to build simple models that capture the salient features of a problem; the ability to make approximations; the ability to do simple, "back-of-the-envelope" calculations; and the ability to judge what's a reasonable result, i.e., to know when you've made a mistake.

The supplementary text, Martin Rees's, "Just Six Numbers," is not a resource for achieving the above goals; in fact, it is intended primarily for the non-technical reader. However, it will provide a backdrop for putting into context the topics listed below. It also will provide examples of the inter-relatedness of physics. Finally, it tells a fascinating story of how a few (six) seemingly unrelated quantities have conspired to allow the existence of just the kind of Universe that can have such creatures as us in it. Some of the discussion seems more philosophical than physical and illustrates Rees's particular take on the subject. His multiverse thesis is fascinating, and we will have the opportunity to discuss some of its implications.

Course Format:

In addition to the first introductory and organizational lecture, there will be seven classes that meet ~ every other Monday evening. (During this course, you'll learn to know and love the symbol "~".) A reading assignment will be given in advance (from Rees, your current physics text, and from a few other sources). You will be assigned questions and problems based on these readings that will be due at the beginning of each class. The workshop nature of the course necessitates that assignments be completed before class. Solutions for each assignment will be handed out at the beginning of each class and no late assignments will be accepted. These assignments constitute an integral component of the course and it is very important that you put considerable effort into completing them. An ample number of office hours will be scheduled in case you find that you need help with the assignment. However, it is important that you make a considerable effort before seeking help.

Each class will begin with an introductory lecture that, in addition to the readings, will help set up the topics. The remainder of the class will be a workshop in which we will discuss the topic and break up into teams to work on several defining problems. During the workshop there will be a coffee/cookie break during which you may consult with other teams. The workshop problems will not be typical textbook problems, but rather questions like, "What can we deduce about the mass content of the local universe from the dipole moment in the Cosmic Microwave Background radiation?" and "Why is there a maximum mass of a Neutron Star, what is this mass, and what happens when this mass is exceeded?" These will entail marshaling information from the written assignments, the readings, your overall knowledge of physics, and your ability to make simple models of the systems–in short, "doing physics". One student from each team will be called on to report to the class on their deliberations and another team member will be appointed to write up the solution to the team's workshop problem. These solutions will be distributed to the entire class. The only exam will be the final: a combination of multiple choice questions and a few, short, back-of-the-envelope problems. While all of this may sound like a lot of work for a half-credit course, the readings will not be excessive, the assignments are relatively short, and the class only meets ~ every other week. However, because of the workshop nature of the course, class attendance is mandatory. Final note on written assignments: Problems on the written assignments occasionally conclude with the statement, "Comment". Remember to do so. I want you to think about the implications of what you've just computed. Pondering these implications is often as important as working the problems. To help you remember this, I'll take off a point if you do not "comment".

Tentative Course Schedule

Jan 23
Introductory Lecture / Workshop
Rees, Chapter 1
Feb 4
Relativity Theory
selected from physics text
Feb 18
Let's Build a Star
Rees, Chapters 2-4; selected from physics text
March 4
Collaspsed Stars and Black Holes
selected from physics text and the Web
March 25
The Big Bang
Rees, Chapter 5; selected from physics text; "The Evolution of the Universe"
Apr 1
Dark Matter
Rees, Chapter 6; selected from physics text; "Dark Matter in the Universe"
Apr 15
Structure Formation and the Accelerating Universe
Rees, Chapters 7-8; "The Quintessential Universe"
Apr 29
Cosmic Rays and Wrapping It All Up
Rees, Chapter 9-11; "Cosmic Rays and the Energy Frontier"


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