Thane Plambeck

Gravitational wave takeout

Gravitation

I've been learning some physics, spending more and more time with Misner, Thorne, and Wheeler's 1973 book on general relativity. Its no-nonsense title is Gravitation.


It's hard for me to look my copy without having words like "gigantic," "imposing," or "heavyweight" come to mind. It's got to be the Muhammad Ali of General Relativity books. Gravitation weighs in at 1,272 pages and has the same height, weight, and reach as a large city's Yellow Pages telephone directory. One could expect a loud thud were it ever to be dropped to the canvas by some future, new and improved theory of gravitation. It also seemed well beyond my possible comprehension when I first peeked into it, relying as it does on the "modern" language of differential forms and a geometric treatment of tensors.

But I wouldn't recommend Gravitation as a book to learn general relativity from. Instead, there is this excellent book by Bernard F. Schutz, which is much more to my liking.

Time machines

No one should accuse today's physicists of ignoring the problems we're all interested in. What could be more interesting than TIME TRAVEL?

Hint in reading the following: Roughly speaking, if there are "closed timelike curves," (CTCs), it means that in principle, time travel to the past is possible, and you could go back and shoot your grandpa. (Not that you would want to, of course).

From the conclusion to the paper
"Energy-Momentum Restrictions on the Creation of Gott Time Machines,''
by Sean M. Carroll, Edward Farhi, Alan H. Guth, and Ken D. Olum.

Complete paper:

http://arxiv.org/abs/gr-qc/9404065

The role played by closed timelike curves is an important issue in classical general relativity, and may be important in an ultimate quantum version of the theory. The general theorems of Tipler and Hawking are strong statements about the difficulty of creating CTC's, but incomplete in that they do not specify what will go wrong with any particular attempt at time machine construction. In this paper we have studied some specific obstacles to the creation of time machines of the type discovered by Gott [8], using the considerable simplification accorded by working in the toy model of (2+1)-dimensional gravity. These obstacles are most easily understood by considering the anti-de Sitter geometry of the 3-dimensional Lorentz group, in which we find that Gott time machines cannot lie to the past of collections of particles with timelike momentum and deficit angle less than 2 pi. We then use this fact to show that a Gott time machine cannot be created in an open universe with a timelike total momentum, essentially because there can never be enough energy in an open universe to achieve the Gott condition.

OK, I know—it sounds like a negative result. It's at least encouraging that people are working on the right problem.

further reading

A research paper

Electron Band Structure in Germanium, My Ass