INTRODUCTION

Since my first interaction with the Kerr metric, early in 1967, when Dennis Sciama suggested to me that I work on it, I was fascinated by the magic of that solution to reduce whatever mathematical expression to simple terms, and by the richness of the information it provided. After nearly 25 years of intense investigation of the Kerr metric carried out by almost all the relativists around the world, new properties continue to be discussed and perhaps deep information about the very nature of gravity is still to be brought to light.

There are basic questions about gravity which, in my opinion, still need to be answered. Some (and perhaps the most obvious ones) are:

1. i) - Why do the properties of a physical system, like energy and momentum, bend the background geometry?

2. ii) - How are energy and momentum actually transferred to the background geometry, leading to a non zero curvature?

3. iii) - To what extent does energy and momentum of the background geometry contribute to these same properties of a physical system?

Answering these types of question is what I mean by going to the roots of gravity. Evidently, central to this issue is the concept of energy in general, for which we require, at the classical level at least, the fulfillment of the energy conditions.