The author spends a lot of time telling us the reason why a paradigm shift for our thoughts on gravity is necessary. Most of it is about how a lot of the data doesn't agree with the theory anymore. Also, it is difficult to observe a lot of the things predicted in the theory and to accommodate it, it is necessary to invent things like Dark Energy and Dark Matter, which can't really be observed.

Thus, the author wants to simplify the current paradigm of gravity. To do this he develops a modified gravity theory or MOG.

So most of the book is the history of gravity theories starting Aristotle and going on to the Cosmological ideas of Claudius Ptolemy and finally getting on to Copernicus and Newton and Kepler. Of course it was found that some things did not follow in this theory, so they thought another planet existed that was between the Sun and Mercury, but was not found.

Then along comes Einstein with his General Theory of Relativity which had proof, but was contested. This is mostly due to the fact that combining it with the Special Theory of Relativity causes the equations to break down, and that it was hard to view gravitational lensing with the technology at the time. However, his equations seemed to fit the data so they were accepted.

Then he finally gets into his MOG. Not much else to say about it.

I liked the book, I thought it was pretty good. I am not sure of the current state of Physics though since this book is not exactly cutting edge, and I was wondering if a lot of these issues are still issues. In any case, if I had the time I would read it again. ( )

Dr. Moffat, a mainstream cosmologist with impeccable credentials, is also professor emeritus of physics in the University of Toronto and a resident affiliate member of the Perimeter Institute for Theoretical Physics in Toronto. He has authored many professional papers, has worked with many cosmologists, and even discussed cosmology with Albert Einstein. He points out many inconsistencies in the current theory about gravity and suggests his own thoughts on how to correct those inconsistencies. His suggestion is not a totally new hypothesis but a modification in the understanding of gravity. His book is written for the layperson and is easy to understand, but he provides detailed end notes for readers more versed in cosmology.

One of the many items that impressed me is Moffat’s description of a singularity, like a black hole. I had already picked up a definition from the media that a singularity is a situation where all known laws of physics no longer apply. I always wondered about that. Moffat describes a singularity as a theoretical situation arrived at mathematically where the mathematical numbers become extremely large, too large to calculate, so large that if a computer were used to make the calculations, the computer would crash. Cosmologists are in the habit of describing the cosmos through mathematics because mathematical computations can be precise enough to predict hitherto unknown behavior in the cosmos (and in physical nature), behavior that can subsequently be searched for and authenticated by observation. This is how science progresses.

The present understanding of what holds the cosmos together is a combination of Newton’s theory of gravity, Einstein’s relativity, and quantum mechanics. All three theories have been proven reliable, but attempts to unify all three into a single Grand Unified Theory have failed. These theories of cosmology propose four basic forces in nature: gravity, electromagnetism, weak nuclear force, and strong nuclear force. Moffat’s work indicates that there is another force to consider.

There is much new information obtained from telescopes orbiting the Earth. Distant galaxies show evidence that they are rotating too fast for the galaxies to hold together. If current understanding of gravity is the only force holding these galaxies together, they would have spun apart long ago. There simply isn’t enough mass within those galaxies to counteract the centrifugal force of the outermost stars orbiting at observed speeds. To make the current understanding of gravity sufficient to counteract observed centrifugal forces, cosmologists have presumed that there must be unseen dark matter within the galaxies. Recent observations also indicate that the expansion of the universe is accelerating, implying that there is an unknown force causing the acceleration. Cosmologists propose that, in addition to dark matter, there must also exist dark energy, a proposed undetected energy that serves to explain the acceleration. Dark matter and dark energy have never been observed even though there have been very expensive experiments performed to detect both.

Moffat spent many years studying whether gravity really is a force of uniform and constant strength throughout the universe or might it be a variable-strength force, especially outside the solar system. He proposes a modified gravity hypothesis that adds a fifth force to the four already recognized. The fifth force can be thought of as a “gravitational degree of freedom: part of the overall geometry or warping of space-time.” Newton’s gravity constant would now no longer be a constant but a variable force in time and space. This variable force of gravity, together with the new force, strengthens the pull of gravity in far away galaxies and in clusters of galaxies.

Moffat relates that mathematical equations based on the addition of his new force can explain all current and past observations without singularities or the need to propose dark matter or dark energy. In addition, his hypothesis holds that, when a star collapses under its own gravity, it does not form a black hole (a singularity where there is a reversal of time and space at the “event horizon”). A collapsed star forms, instead, a very massive and very small object that he calls a “grey star.”

Moffat has described a very difficult subject, usually understood only through mathematics. He describes his position in plain English easily understood by the average reader. This is a very informative and interesting book written by someone who has the credentials to know what he is talking about. “Reinventing Gravity” may well be the book that will introduce you to the new frontier on cosmology. ( )

No crackpot, Moffat was long in Toronto U's physics department and is now with the Perimeter Institute. His Modified Gravity theory (MOG), an extension of general relativity, involves a varying G and a fifth force mediated by "phions." It does away with the universe's postulated dark matter. It also leads him to question the conventional understanding of dark energy, black holes, and the Big Bang. Great stuff, but I would bet he is wrong. (In not being a fan of string theory, though, he may be right.)