Getting to Grips with Gravity

The original version of a double review for the Wall Street journal:

The Ascent of Gravity

Marcus Chown


On Gravity

A. Zee



John Gribbin


Gravity has become a hot topic in science, with the discovery of gravitational waves, ripples in the fabric of space coming from colliding black holes and neutron stars.  Both The Ascent of Gravity and On Gravity mention those discoveries, but neither book focuses on them.  Rather, they provide the background to our understanding of this fundamental force of nature, a force which is the weakest one known but which paradoxically, because of its long range, is the most important one in the Universe at large.

The first person to appreciatee the literally universal importance of gravity was Robert Hooke, who realised that gravity is a universal force possessed by every object in the Universe, which attracts every other object.  Hooke, a slightly older contemporary of Isaac Newton, was both an experimenter and observer, and a theorist.  His insight about gravity came partly from his telescopic observations of the Moon.  He studied lunar craters, and noticed that they are formed of nearly circular walls, around a shallow depression.  They looked, in his words “as if the substance in the middle had been digg’d up, and thrown on either side.”  So he carried out experiments, dropping bullets onto a mixture of water and pipe-clay, making miniature craters which, when illuminated from the side by a candle, looked just like lunar craters.  He realised that the material thrown up from the centre of the craters of the Moon was pulled back down by the Moon’s own gravity, independent of the Earth’s gravity.  He pointed out that apart from small irregularities like craters, the Moon is very round, so that “the outermost bounds. . . are equidistant from the Center of gravitation”, tugged towards the center by gravity, and concluding that it had “a gravitating principle as the Earth has.”  This was published in 1665, when Newton was just completing his degree at the University of Cambridge.  Hooke went on to suggest that planets are held in orbit by an attractive gravitational force from the Sun.

The two books considered here both fill in what has become known about gravity since Hooke’s day, but they are very different, both in approach and style.  Marcus Chown is a science writer, and a very good one.  He favours the historical approach, starting with Newton’s work on gravity and taking us through Albert Einstein’s contribution to the mysterious world beyond Einstein where physicists hope to find a theory that will explain gravity and quantum physics in one package.  He eschews equations, but provides clear explanation with a useful guide to further reading at the end of each chapter.  The result feels easy and natural, like the author talking to you, although I suspect it took a lot of hard work to produce that effect.

By contrast, A. Zee (who only uses the initial) is a professor of physics who has previously written an epic tome on gravity, and is now trying to “bridge the gap between popular books and textbooks.”  He is only partially successful.  Some of his attempts to be “popular” seem forced, as with sentences such as “Ah, the glory days of trial and error experimental physics!”, and the logical structure of his arguments is sometimes faulty, as when (in a book about gravity!) he tells us that “just about the only commonplace example of a force acting without  contact is the refrigerator magnet.”  He does provide equations, and diagrams, and is on secure footing there.  But the sloppiness of his writing is highlighted by comparing his mention of the myth that Galileo dropped weights from the Leaning Tower of Pisa with Chown’s.  Chown correctly identifies this as a legend; Zee presents it as a fact “we all learned in school”.  Maybe we did learn the story there, but it is definitely legend, not fact.

A particularly delightful feature of The Ascent of Gravity is the inclusion of several fictional vignettes in which the author imagines how the big ideas came to his protagonists – for example, a story of the young Einstein walking out with his girlfriend Marie Winteler under a moonlit sky, and having a sudden insight about the way light travels across space.  Fantasy, but fun – and no real surprise that it should work so well, since Chown is also a successful writer of science fiction (on some of which, long ago, I collaborated with him).  Chown’s great achievement is to make his discussion of such bizarre phenomena as the way rotation distorts space just about as intelligible and entertaining as the fantasy.

Zee’s great achievement is to provide the clearest explanation I have seen of the physical principle known as “action”, which among other things explains why light travels in straight lines – or, more accurately, why light travels along the path that takes least time.  Action is arguably the most powerful tool in the physicist’s box of tricks.  In Einstein’s own formulation of the general theory of relativity he required a set of ten equations to explain the interaction between matter and spacetime; but the whole thing can be described much more simply in terms of a single action.  I was also particularly pleased to see Zee emphasising the point that Einstein did not prove that Newton was incorrect.  Newton’s version of physics is perfectly adequate for things moving much more slowly than light in weak gravitational fields, and Einstein’s version includes Newtonian physics within itself.  The famous headline in the London Times of 7 November 1919 proclaiming “Newtonian Ideas Overthrown” was just plain wrong.  Science does not progress by revolutions, but by building, brick by brick, on what has gone before.

The latest brick in the edifice is, of course, the discovery of gravitational waves, and it is unfortunate that these books are unable to give much space to this.  The Ascent of Gravity was written a little earlier than Zee’s book, and gives the discovery only passing mention.  On Gravity was written, the author tells us, after the first detection was announced, but even so gives it a rather cursory mention.  I was baffled by the fact that although Zee mentions plans for a gravitational wave detector to be built in India, he does not mention the one already built in Italy (and a curious footnote suggests that he is unaware of its existence).  If you do want the full story of gravitational wave research, it is covered by Marcia Bartusiak in her excellent book Einstein’s Unfinished Symphony.

If you are looking for a good read and a chance to absorb painlessly some ideas about the force that controls the Universe, Marcus Chown is the man for you.  If you think you already know a little bit about the topic, and are not afraid of a few equations, then On Gravity will take you deeper; if you are very brave, the Appendix will explain the meaning of curved spacetime.  If I had a magic wand, I would wave it to put Zee’s diagrams into Chown’s book, and get the best of both worlds.




John Gribbin is a Visiting Fellow in astronomy at the University of Sussex and author of Out of the Shadow of a Giant (Yale UP).