The main parts of this book consist of three long articles that have previously been published by the Galilean Electrodynamics journal during three succeeding years as special summer issues. The subject is a new theory of ether that has been developed. This version of ether is compatible with special relativity theory (SRT), and thus it cannot be considered as an absolute frame of reference. One of the virtues of this book is that many of the novel predictions can be tested in high-tech laboratories; moreover, the book includes some new and extraordinary physical concepts that make the proposed claims rational and plausible. The words on the front page, "From an alternative to general relativity to the unified mass-charge equation," are not intended as a gossipy slogan, the likes of which sometimes appear in pseudoscientific media or science-fiction books; it is a true motto with a deep and physical basis. It is up to the reader, however, to judge whether or not the book really does what it sets out to do. The first chapter introduces a substantial number of Einstein's general relativity predictions, which are recalculated according to density theory (DT) to validate the theory for the reader. The second chapter uses interesting concepts, some of which present empirically bizarre results and are undisclosed as of yet; e.g., it is shown that static pressure can dilate time and increase mass; the light beam is affected in specific magnetic and electrical fields, and thus we can deduce that photons are not always electrically neutral; ether negative mass causes universal expansion; and finally, water can behave as a biological ether especially for deep sea creatures, as its hydrostatic pressure can affect biological longevity. In chapter three, The author tries to unify mass and electrical charges - one of the oldest and most disputatious physical problems - in the single equation of "mass + rotation = charge" for the very first time in the history of science. Additionally, an alternative to the dynamo theory is brought forth. After explaining each novel concept, the author uses some numerical examples with no complicated mathematical methods beyond common calculus. These concepts and examples make the book read as an argumentative dissident textbook that is suitable for bachelor students in science and engineering who want to nourish their desires to research beyond the realm of orthodoxy in frontier physics.