## MAT5600 Differential Geometry Spring 2013 [last offered: Spring 2008] subtopic: Einstein for Dummies? [oops that title is already taken][idiot's guide too] Instructor: Robert Jantzen

Course Catalog Description:

 MAT 5600 - Differential Geometry Description: Geometry of curves and surfaces, curvature, first and second fundamental forms, minimal surfaces, use of MAPLE [forget minimal surfaces!] Prerequisites: MAT 2500 [MAT2705 too!] Corequisites: Credit Hours: 3.0 Last Offered: Spring 2008 by Robert Jantzen, Spring 2001, Fall 1996 by Klaus Volpert

Spring 2013 MWF 9:30 - 10:20.

Updated for 2013 with extra material incorporated involving symmetry groups, the hyperbolic geometry of special relativity and simple black hole examples [see below]. The course will start with a simple exercise from multivariable calculus that enables us to compute the so called geodetic precession effect measured by the GP-B satellite experiment. We will review some of multivariable calculus that we need so we can all start on the same page. Physics majors who want to understand the language of modern theoretical physics and math majors who like multivariable calculus and differential equations and linear algebra applied to some whimsical topics in geometry are the target audience for this course. Maple will be a welcome tool in working problems in this course, and in generating pretty graphics. For a sample see pasta design (Pasta by Design, CBS Sunday morning spot)  or what, relativistic pasta? Are you kidding, bob? No. Let's have fun for a change. Isn't that why we were attracted to mathematics in the first place?

## What is this stuff?

Differential geometry is the mathematics used to describe curved spaces, building on the foundations of multivariable calculus and linear algebra.

For the very first time, this course will be an elementary introduction to differential geometry that will not only to deal with the geometry of curves and surfaces but will have an eye towards simple 2-dimensional applications to the flat geometry of special relativity and to the curved geometry associated with a spherical black hole. Problem sets will be assigned throughout the semester to make lectures, notes and readings more concrete through calculation, with some guided assistance from Maple when helpful.

I am thinking that this course will start off by revisiting how one treats the geometry of curves and surfaces in Multivariable/Vector Calculus, and by slightly adjusting the familiar details (we'll remember them!), one can extrapolate easily to handle both curved spaces and see how the flat hyperbola geometry associated with the hyperbolic functions usually ignored in calculus describes very nicely the heart of special relativity (time dilation, length contraction, Lorentz transformations). Similarly building on the use of polar, cylindrical and spherical coordinates in flat space, one can easily handle curved spaces with the same tools, enabling one to understand the usual suggestive surface representations of the deformation of the Euclidean plane by the presence of a black hole [see the illustration above above]. You don't have to be an Einstein to comprehend these elementary ideas! We will try to have some fun.

Watch a woman astrophysicist talk about how and why she uses this mathematics.

## Some history [more depth]

Newton and Liebnitz were the leading thinkers behind calculus back in the 17th century (the 1600s!). Although the curvature of a curve was contemplated long before calculus entered the picture, and afterwards as well, it took some time before calculus was used to explore the complete geometry of curves by a couple of French mathematicians Frenet and Serret around 1850 and it was even later that this was presented in the vector calculus style of modern textbooks. This geometry is important in elementary physics to describe centripetal accelerations and centrifugal forces that we all (well most of us) have direct experience with from driving cars around curves in the road or riding roller coasters in an amusement park.

However, once curves were investigated, it was natural to move on next to surfaces. In the mid 1800s Gauss, one of the most prolific mathematicians of all time, pioneered this field, but it was his student Riemann who got serious about curved spaces of any dimension. Unfortunately, he did not live long enough to fully develop the tools of Riemannian geometry, which was left to others, most notably some Italian mathematicians Ricci and Levi-Civita who were the principle thinkers behind tensor calculus at the turn of the 19th (to 20th) century, ready just in time for Einstein to apply their machinery to his curved spacetime theory of gravitation called general relativity. Einstein, however, as a physicist who did not like to write lots of equations, was not big on abstract mathematics and had to be introduced to this stuff by a friend who was a mathematician (Grossmann) in the early 1900s. After some rumination time, general relativity was born in 1916. This pushed curved geometry into the public imagination and physicists continued using this kind of geometry to unify their force fields for the rest of the 20th century. String theory is the latest part of the program to geometrize the forces of nature.

Interestingly enough GPS (gravitational positioning system) is an acronym which has quickly become a part of our daily life, most commonly in car navigational systems, but more and more available in our cell phones and other technology devices. GPS would not work without special and general relativistic effects that differential geometry provides the calculational tools for evaluating.

However, you don't have to be interested in describing the fundamental forces of nature to use differential geometry—it has lots of more mundane applications. For example a flower that is shaped like a trumpet horn initially growing outward from the base is symmetric about an axis of symmetry, but as it bends outward while growing, eventually its curvature will cause it to no longer be rotationally symmetric but develop ruffles [see the article Crumpling, buckling, and cracking: elasticity of thin sheets, Physics Today, February 2009 (comment), followup: Custom shapes from swell gels ].

Differential geometry and physics?
Better yet: Differential geometry, fiber bundles and physical theories by Isadore M. Singer, Phys. Today 35, 3, 41 (1982)
or Reflections on Relativity (on-line book) by Kevin Brown.

## cute stuff

• dr bob show and tell 2011-11-18: gravity orbits and donut geodesics: the same problem nicely handled by Maple (including GR)

• 2012-04-19talk General Relativity, Cosmology and Pasta?
contains link to  [torus animation page]

• 2012 Fall fun: Cavatappo 2.0 [follows from: ../../notes/torus/2012-04-19talk/helixtubegeos.pdf, in turn from ../../notes/torus/index.htm]
But...we can be serious with this too.
The crazy cavatappo surface stuff can be adjusted to represent the Born rigid transport of a circular body in its rest frame orbiting the central axis in a 3d spacetime in which the vertical screw symmetry axis  is the time axis and the spatial projection is a circular orbit around the spatial origin. A pedagogical example, of course. The horizontal cross-sections of the world tube of the body would then exhibit Lorentz contraction. [Pasta by Design in the NYTimes]

electromagnetic stuff (another talk)
Not only that, such a tubular "world sheet" is the history in spacetime of a "closed string" in the same sense that a world line is the history of a point. A world line must have a timelike tangent vector to be interpreted as the world line of a particle with nonzero rest mass, and the world history of a string must similarly contain a timelike direction in its tangent space. One can also easily evaluate the surface area of the surface over one revolution to find that the theorem of Pappus also holds in this Lorentzian case: the total surface area equals the circumference of the orthogonal circular cross-sections times the total arclength (proper time) along the world line of the central helix. The surface area integral serves as the Lagrangian for the string field equations. An accessible discussion of such a toy model may be found in a recent book Differential Geometry and Manifold Theory by Stephen T. Lovett.

Don't be put off by the physics jargon. We'll learn about what we need to play around in this area.