By Bruce R. Linnell, PhD

The following files represent
a *summary* of general relativity, broken into bite-size pieces of * roughly* 10 pages each
(except GR2f, which is more like 20). In all these files, items in blue text
are the most important things, items in green text are "gotchas" for the
beginning student of general relativity (except for in GR1f), and items in
gold are things I am 99% (but not 100%) sure of.

GR1a – a brief review of vectors and coordinate systems; an introduction to tensors, index notation, and Einstein

summation; definitions of manifolds, tangent vectors and tangent planes

GR1b – a brief refresher of special relativity; basic terms and concepts of general relativity; the metric tensor; an

introduction to covariant and contravariant tensors; introduction to parallel transport

GR1c – the Riemann, Ricci, and Weyl tensors; the Ricci scalar; an introduction to the stress-energy tensor or

energy-momentum tensor (it goes by both names); Einstein’s general relativity equation and some

explanations/interpretations of it

GR1d – tensor notation is incredibly “dense”, so this shows Einstein’s equation (in terms of the metric) expanded

into its full glory

GR1e – some visualizations of common spacetimes including Schwarzschild (non-rotating mass), Kerr (rotating

mass) and a moving mass (*the
visualizations of the Kerr spacetime are my own, and try to show in *

*several different ways how spacetime is stretched around
a spinning mass*); other spacetimes such as

wormholes and warp-drives; a discussion of the many tests general relativity correctly predicts

GR1f – general relativity as applied to cosmology, and why we think the universe is filled with “dark matter” and “dark

energy” (and all the assumptions that go into that)

These next files go into *much* more mathematical depth and detail,
explaining things well enough so that you could (hopefully) understand scholarly papers and articles __about__
general relativity.

GR2a – miscellaneous background info : more on vectors and tensors (wedge product, dyadic product, Levi-Civita symbol,

etc.); more general relativity terms and definitions; intrinsic/internal vs. extrinsic/external curvature; various kinds of “flat”

spacetimes

GR2b – much more on special relativity : spacetime diagrams; simultaneity and cause/effect; light cones; Minkowski

diagrams; Lorentz transformations; doing physics in special relativity (velocity, acceleration, momentum, force, etc.)

GR2c – all about derivatives : path tangent vectors; scalar field derivatives; directional scalar derivatives; vector field

derivatives; affine connections; Christoffel symbols; covariant derivatives; directional covariant derivatives; Lie

derivatives; *the explanations of the Christoffel symbols
and covariant derivatives contain *

*as-clear-as-I-can-make-them explanations with good
visuals*

GR2d – all about covariant and contravariant tensors : how they are defined and how they transform; the Jacobian;

one-forms; exterior derivatives; and a recap of contravariant/covariant formulas

GR2e – electromagnetism : classical formulas; special relativity formulas; general relativity formulas

GR2f – general relativity, take two : differences from special relativity; affine parameters; proper acceleration; a recap of

parallel transport and geodesics; *an in-depth
as-clear-as-I-can-make-it look at the stress-energy-momentum tensor *

*(including my own standard-physics explanation of why
“pressure” draws matter and energy towards it)*; energy

conditions; more on the Weyl tensor; geodesic deviation; a discussion of the pitfalls when converting line elements

to metrics; *the
meaning of off-diagonal terms in the metric (which is rarely discussed
anywhere!)*

I have in the back of my mind an
idea for a third set of documents which would include enough information so
that you could actually __work problems__ in general relativity, but that
probably won’t happen for another year or two. Some of the GR2 files reference these
non-existent GR3 files.

DISCLAIMER #2 : while I have copied text and figures from other people’s web sites to create these notes, much of it has been heavily edited, rearranged and merged, and some of it is entirely my own. If you find any of your own material here that you would like me to reference, please let me know, I would be glad to do so (since these were originally intended as my own notes, I didn’t keep track of what came from where). If you would like me to remove your material, I will, but please realize that if I used it that means that I thought it was exceptionally clear and helpful.

As I continued to look into general
relativity, I discovered an amazing thing : Einstein's field equation is a __simplification__!
The equation, as complicated as it is, ignores something called
"torsion" (which is what happens when you turn a screwdriver or
wrench). Including torsion adds a second, equally-as-complicated, equation to
the mix!