# Introduction

Mass and gravitation are explained by association of three simple and basic observations:

**Volumes with/without mass**

Matter is made of five different kinds of volumes, some have mass such as protons, others do not such as orbitals of atoms. Therefore, since*m = f*in some cases, it becomes possible to replace mass by volume in particular situations. This is the case of Einstein Field Equations (EFE). These equations can be re-written using the variable_{(x,y,z)}*"Volume"*instead of*"Mass"*, in perfect agrement with the multiple experimentations conducted since 1919.

**Attraction vs. pressure force**

During the last century, some physicists suggested that gravitation could be an external pressure force on objects instead of an attraction. This suggestion failed because some explanations were missing: What is the origin of this pressure? What does "object" mean? ... Nevertheless, this idea is full of good sense and has been taken up again here in accordance with the latest developments in physics.**General Relativity (GR)**

A thorough study of General Relativity (GR) shows that the curvature of spacetime is not concave as we think but convex. This study also confirms the previous observation, i.e. the force exerted by spacetime on object is not an attractive force but a pressure force.

These three observations do not solve the puzzles of physics when taken separately, but taken together, they
explain with great simplicity the curvature of spacetime, mass, and gravitation,

This website also gives you solutions to several enigmas of Modern Physics such as: *"In E=mc ^{2},
how mass can be converted in energy?"* or

*"How time can expand away from a massive object?"*

Note: The basic laws of the universe are very simple, but their formulation is often complex. For example, a child of 6 years old intuitively knows that beating a drum makes noise. However, the mathematical description of the surface waves requires the knowledge of Bessel Functions. The presented theory follows the same rule. The principle of mass and gravity here proposed is very simple, even "too simple to be credible", but the mathematical formulation is complex (Einstein Field Equations).