I read the following statement in the introduction to an article:
Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the positivity of the total gravitational energy, both at spatial and null infinity. It is precisely its positivity that makes this notion not only important (because of its theoretical significance), but also a useful tool in the everyday practice of working relativists.
Since I haven't been involved in GR for about 30yrs, I've missed this. Could someone briefly explain how these results are stated and give some references if possible.