GRTensor is a very handy Maple package for doing calculations in General Relativity. Given a specific metric, it’s great at calculating the components of tensors. You can define your own tensors or use the predefined ones. You can even define your own metrics.

The install instructions in the Readme file are fairly good if you’re a Windows or Linux user. As is often the case, however, there are one or two catches when installing on Mac OS X. With that in mind, the following instructions should help with the installation:

1. Download GRTensor from from their website to your Desktop. For any recent version (since 6) of Maple and Mac OSX, you want the version “GRTensorII 1.79 for maple 6, 7, 8 (32 and 64 bit machines PCs Macs and UNIX boxes)(with metric files, examples, html help system)”
2. If it doesn’t get unzipped automatically, double click the grii-1.79.tar.gz file on the desktop and it should create a folder called grii
3. Open the Terminal application and change to the grii directory with the command:

   cd Desktop/grii

4. Still in the terminal, run the GRTensor installer as root:

   sudo ./Install.unix

5. Again in the terminal, copy the mapleinitfile to your home directory:

   cp /usr/local/grii/mapleinit.sample ~/.mapleinit

6. Quit Maple if it’s running and start it up again.
7. When you want to load grtensor, just type grtw(); into Maple and hit enter. You can load metrics with qload(schw); for Schwarzschild for example.
8. You can now safely delete the grii directory and file from your desktop.

If you have problems with loading the metric, have a look at this article and hopefully that will fix them.