# Bei's Study Notes

### General Relativity 01Last updated: 2017-05-01 22:09:22 PDT.

Reference: UCI OpenCourseWare - GR 02 (Playlist)

# Reminders of Special Relativity

Special Relativity is a thoery of translating coordinates in planar spacetime, which has a constant measure .

## Lorentz Transformation

Any two inertial reference frames can be related by a Lorentz Transformation (or Poincaré transformation). Consider a photon is shot from a point , then we can move the origin of the two frame to that point. Then we can rotate the frames so that their relative speed lays on the axis. The location of the photon in these two frames can be defined as and , then the photon's motion forms a straight line in each frames of reference. Since the speed of light is constant, we have these relations:

In Newtonian transformion, frame of references are related by Galilean transformation:

Thus we have the notion of "absolute" time flow. This does not satisfy . Lorentz transformation multiplies the time coordinate by a factor , which depends on :

Pluggin it into :

In general, the Lorentz transformation relates two coordinates in different reference frames by

has a important property:

where . It is similar to rotation in Euclidean space. It is usually seen as rotation in Minkovsky space.

From now on, we set .

Define proper time that

Also,

, so the proper time in another frame is

General form of boost-only Lorentz transformations:

If we define , we then have a unified form:

We can check if this obeys the identity:

## Some 4-vectors

### 4-force F^\alpha

Define 4-force in that

1. If the particle is momentarily at rest, then , then the 4-force is

2. Then apply Lorentz boost

### 4-momentum p^\alpha

It can be defined as

then we get:

Another way to generalize momentum: the momentum at rest is , applying Lorentz boost:

When , :

This should become the clasicall limit where , therefore .

## Scalars, Vectors, and Tensors

Contravariant vectors transform by Lorentz transformation; covariant vectors transform by inverse Lorentz transformation:

where

To prove this is indeed inverse Lorentz transformation:

Derivative with respect to a contravariant vector is covariant.