Book Review: Reinventing Gravity by John Moffat

Some books that deserved to be read multiple times. This is one of three books I first read a few months ago, and I just revisited them all to find that they are still as good as I remembered them. See also my reviews of Wetware: A Computer in Every Living Cell and In Search of Time.

Gravity is one of those things that we typically take for granted. Most of us consider ourselves to be at least passingly familiar with the work of Newton and Einstein, but I for one was surprised to discover just how little I know. This book is jam-packed with enough tidbits of trivia to keep even me amused (and that's saying something).

Reinventing Gravity is a wonderful book. The author, respected Physicist John Moffat, doesn’t assume that the reader has any form of expert knowledge. Instead, he starts by walking us step-by-step through the various theories of gravity, from Aristotle to the present day.

Along the way we grow into an appreciation of a different way of looking at things. For example, most of us are aware that in his Philosophiæ Naturalis Principia Mathematica, published in 1687, Sir Isaac Newton described his theory of Universal Gravitation.

At first this theory seemed to completely describe the motions of the planets and the stars and led to the idea of a "clockwork universe". One very interesting aspect to all of this occurred when astronomers began to realize that there was a problem with regard to the "anomalous precession of the perihelion of the planet Mercury" (which is the clever way of saying that Mercury wasn't orbiting the Sun as expected).

The folks of the time absolutely believed in the theory of Newtonian gravity, so they looked for an explanation in this context. The idea they came up with was that there was an – as yet undiscovered – planet (which they called Vulcan) in orbit between the Sun and Mercury. Based on this proposal, many folks devoted huge amounts of effort and ingenuity trying to find a planet that we now know does not exist.

Then Albert Einstein came along with his theory of General Relativity. Amongst other things, this accurately predicted the orbit of Mercury without the need to introduce a "fudge factor" in the form of a non-existent planet.

For close to 100 years, General Relativity has been accepted by the majority of folks as fully describing gravity. But once again there's a problem. Astronomers have discovered that the stars at the edges of rotating galaxies are travelling much faster than they should be... so fast that they should fly off into space... but they don't.

In order to address this, folks have come up with the concept of Dark Matter. The idea in a nutshell is that Dark Matter is something we can't "see" or "taste" or anything like that... except through its gravitational interactions (the posh way to say this is that "Dark Matter is hypothetical 'stuff' that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter").

Doesn’t this seem a little strange to you. It certainly does to me. The idea is that we are so accepting that General Relativity fully defines gravity that when we make observations that don’t fit we simply invent some invisible matter to make everything work. And don’t even get me started about other galaxies like NGC 4736 which seem to lack Dark Matter.

And so we come to the author's MOG (Modified Gravity) theory, which addresses this conundrum. Even more exciting, MOG also eliminates the need for the singularities at the centre of black holes (which solves lots of problems) along with the singularity at the beginning of creation (no more "big bang" per se).

Of course this is still just a theory, but the author also describes some observations and measurements we [the human race] could make that should resolve the issue as to which gravitational theory is correct. The really cool thing is that we should have the technological capability to make these measurements in the not-so-distant future (perhaps in the next few years).

I, for one, can’t wait!