Ray Trace is grayed out in Revit Architect 2013

Perhaps you have noticed that in Autodesk Revit Architecture 2013 with the Graphic Display Options popup (from the Visual Style icon) the Ray Trace option is grayed out with a 3D model loaded like this:

 

For an explanation, see the Known Issues PDF located here.

Here is the important context from the PDF. 

Ray Trace Rendering functionality is disabled in 32-bit versions of Revit due to performance degradations. To use Ray Trace Rendering functionality, you need to install Revit on a 64-bit operating system.