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 mapleinit file 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.

Unfortunately, getting it working on Mac OS X is a bit of a pain. This is due to a bug in GRTensor that has gone unfixed for quite a while. Fortunately, there is a straightforwarde solution.The bug is triggered whenever you try to load a metric, which is usually the first thing you would do in a GRTensor session. When you do so, you are presented with the following error:

> qload(schw2);
Warning: grOptionMetricPath has not been assigned.
Error, (in qload) Could not find metric schw2

This error is telling us that GRTensor isn’t able to find the schw metric, even though we know it is present in the metrics directory. The problem seems to arise from the way GRTensor deals with path separators on the Mac. Historically, Mac OS used the colon as a path separator but, since OSX, it uses the more unix-like slash. GRTensor appears to be trying to use the old, pre-OSX colon system, so it mistakenly searches for metrics:schw, which it obviously can’t find.

The solution is fairly straightforward – since GRTensor is looking for the file metrics:schw.mpl, all we need to do is make sure our file is named as such and GRTensor will be able to find it (Note that the file must also reside in the directory GRTensor is installed in, not the metrics subdirectory). This rule is also valid for any other metric files you create – their names must all be prefixed with metrics:. Unfortunately, OSX is a bit awkward about naming things with colons in them, so it’s usually easiest to rename the files using the command line.

Now, quite a few metrics come bundled with GRTensor, so renaming them all manually is quite time consuming. It’s easiest to use a simple bash script such as this one to do it automatically. Just download this script and place it in the directory where GRTensor is installed (probably /usr/local/grii/). Make sure it’s executable and run it. On the command line, you could do so using the commands (assuming you downloaded the file to your desktop and GRTensor is in the default install location of /usr/local/grii/):

$ sudo cp ~/Desktop/fixmetrics.sh /usr/local/grii/
$ cd /usr/local/grii/
$ chmod +x fixmetrics.sh
$ ./fixmetrics.sh

You should now be able to load all your metrics in GRTensor without any problems:

> qload(schw2);
Calculated ds for schw (0.000000 sec.)
                         Default spacetime = schw
                         For the schw spacetime:
                               Coordinates
                                  x(up)
                           a
                         x   = [r, theta, phi, t]
                               Line element
          2
  2    d r       2         2     2           2       2    /     2 m\     2
ds  = ------- + r   d theta   + r  sin(theta)   d phi   + |-1 + ---|  d t
          2 m                                             \      r /
      1 - ---
           r
            The Schwarzschild metric in curvature coordinates