# Documentation

## General Relativity

These examples are using Einstein's General Relativity to calculate geodesics in curved space-time.

### Prerequisites

There are no prerequisites for this example.

### Create a New Folder and Copy Relevant Files

The following code will create a folder in your current working folder (pwd). The new folder will only contain the files that are relevant for this example. If you do not want to affect the current folder (or if you cannot generate files in this folder), you should change your working folder.

### Run Command: Create a New Folder and Copy Relevant Files

```coderdemo_setup('coderdemo_gr'); ```

### Example: Computing the Precession of the Planet Mercury

This example computes the precession of the planet Mercury numerically. The precession is a slight rotation of the elliptical orbit around the sun. Analytically, using the equations of general relativity the value is extremely small, an extra 43" (arc seconds) per century. An arc second is 1/3600th of one degree (counting 360 degrees for a complete revolution.) Even though the extra precession is extremely small it matches exactly with observation. Pure Newtonian mechanics (if we choose to ignore all the other planets of our solar system) predicts no precession.

This application is using Euler's method with variable time step where the major time step is .5 seconds. We reduce the time step as we approach one complete revolution. The precession is computed as the planet is reaching its maximum distance from the sun for which we compute its relative angle to the coordinate axis.

### Generate the MEX Function: Precession of the Planet Mercury

Generate a MEX function using the command `codegen` followed by the name of the MATLAB file to compile.

```codegen gr_mercury_precession ```

The 'gr_mercury_precession' function calls other MATLAB functions, but you need to specify only the entry-point function when calling 'codegen'.

By default, 'codegen' generates a MEX function named 'gr_mercury_precession_mex' in the current folder. This allows you to test the MATLAB code and MEX function and compare the results.

### Run the MEX Function: Precession of the Planet Mercury

Run the generated MEX function.

```gr_mercury_precession_mex ```
```Progress: 5% Progress: 10% Progress: 15% Progress: 20% Progress: 25% Progress: 30% Progress: 35% Progress: 40% Progress: 45% Progress: 50% Progress: 55% Progress: 60% Progress: 65% Progress: 70% Progress: 75% Progress: 80% Progress: 85% Progress: 90% Progress: 95% Progress: 100% precession: 0.10468" (0 years 87.87009 days) => 43.481"/century ```

### Example: Ray-tracing a Black Hole

Einstein's equations of motion in general relativity can handle any object at any speed, so let's apply it to photons that travel with the speed of light. In this configuration we have a black hole in front of a background image. To make the effect more visible, we increase the mass of the black hole to astronomical proportions as well as the background image. In this way we can study the effects of gravitational lensing; the background image becomes distorted by the curved space-time produced by the black hole.

### Generate a MEX Function: Ray-tracing a Black Hole

```codegen gr_raytrace ```

### Run the MEX Function: Ray-tracing a Black Hole

Ray-tracing the picture takes a minute or two on a 2 GHz x86 machine. On your screen, you see the original picture (the Vittorio Emanuele Mall in Milano, Italy) and, to the right, the rendered image of the same picture with a black hole in front of it.

```gr_raytrace_mex('mall.jpg'); ```
```Progress: 5% Progress: 10% Progress: 15% Progress: 20% Progress: 25% Progress: 30% Progress: 35% Progress: 40% Progress: 45% Progress: 50% Progress: 55% Progress: 60% Progress: 65% Progress: 70% Progress: 75% Progress: 80% Progress: 85% Progress: 90% Progress: 95% Progress: 100% ```

### Cleanup

Remove files and return to original folder

### Run Command: Cleanup

```cleanup ```
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