== Classification of e/m fields == 

 Hi, (esp. CH). I want to create an article called classification of electromagnetic fields. My reason for doing so is that there are nice analogies between the e/m and grav. fields both physically and mathematically and that the proposed article will clarify this to some extent. The approach used will have to be looked at: is the spinor approach better (it's mathematically elegant, but is it physically clearer ?), or perhaps just crunching out the classification using tensors ? Certainly, the importance of (principal) null dirctions should be stressed (and maybe even repeated principal null directions), as these appear to be important in the classification (both e/m and grav.). Need to think about this.