Hi everyone. In reading some popular textbooks I noticed that in (maybe) most of GR and SR we don't encounter situations where we can use wedge-product and differential forms. However, these things are presented to us in most of the textbooks. But... if most of the books present them, it means that they are important and useful, I think... So maybe what I've read on GR and SR is "nothing" compared to the existing material. Thus I would like to know some applications of the wedge-product and the differential forms. Can someone write down a list containing some topics where they are useful (or even essential)?