From Falling Apples to Black Holes

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From The Wall Street Journal:

Gravity has suddenly become a hot topic in science. In the past few years, gravitational waves have been detected for the first time: ripples in the fabric of space, coming from colliding black holes and neutron stars. Both Marcus Chown’s “The Ascent of Gravity” and A. Zee’s “On Gravity” mention those discoveries, but neither focuses on them. Rather, the books provide the background to our understanding of this fundamental force of nature—a force that is the weakest one known but is, because of its long range, the most important one in the universe at large.

The first person to appreciate the literally universal importance of gravity was Robert Hooke, who realized that every object in the universe attracts every other object. Hooke, a slightly older contemporary of Isaac Newton, was an experimenter and observer as well as a theorist. His insight about gravity came partly from telescopic observations of the moon.

Hooke noticed that lunar craters 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 that, when illuminated from the side, looked just like lunar craters.

Hooke speculated that the material thrown up from the center 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.” They seemed to be tugged toward the center by gravity, and so Hooke concluded the moon had “a gravitating principle as the Earth has.” This was published in 1665, when Newton was just completing his degree at Cambridge. Hooke went on to suggest that planets are held in orbit by an attractive gravitational force from the sun.

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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. The author somehow makes his discussion of bizarre phenomena (such as the way rotation actually distorts space) just about as intelligible and entertaining as the fantasy. He eschews equations, but provides clear explanation along with useful guides to further reading, and his book’s easy, conversational style likely took hard work to produce.

Link to the rest at The Wall Street Journal

Let it be known that, unlike technology (see the Telegraph post below), gravity has delivered on everything it promised. On the earth, only a very few items float around. Unless there are children around the house, most objects stay where you put them. Without gravity, scales would be useless and you would have to spend a lot of time hunting around to find almost anything useful.

Like gravity, time has delivered all it promised. In the memorable words of  Ray Cummings’ 1922 science fiction novel, The Girl in the Golden Atom, “Time is what keeps everything from happening at once.”