*By David H. Levy with Roy L. Bishop*

Gravity is one of the most fundamental things in physics. Everything and everyone has gravity. The more massive something is, the more gravity it has. When you jump into the air, Earth’s gravity brings you back down. What you cannot see while you are in the air is that your gravity brings Earth towards you just a wee little bit, off-setting the extra push away from you that your feet gave Earth when you jumped.

Isaac Newton presented the first ever mathematical description of gravity in 1687. I admit that I know nothing about gravitation, except that it is all around me. I do recall the myth that Newton was sitting under a tree when an apple fell on his head. Supposedly, he then formulated his law of gravity. Did the apple actually fall on his head? I doubt it. But at his childhood home in the village of Woolsthorpe, England, he probably did witness an apple fall from a tree.

During the last half of the nineteenth century physicists realized that Newton’s theory of gravity did not accurately describe the orbit of Mercury, the planet closest to the Sun. Mercury’s elongated orbit precesses slightly faster than Newton’s theory predicts. Several unsuccessful attempts were made to account for this discrepancy.

Newton’s theory, which assumes that gravity is a force, held sway for more than two centuries, until superseded by Albert Einstein’s General Theory of Relativity in 1915. A decade earlier, Einstein realized that mass and energy are two aspects of one thing, and that space and time are interrelated, a blended spacetime. With General Relativity, Einstein treated gravity not as a force, but as the geometry of spacetime. The geometry of spacetime is curved by the mass-energy of matter, and the curvature instructs matter how to move.

Now comes the hard part. When Roy Bishop, emeritus professor of Physics at Acadia University, pointed out to me that gravitation is geometry, and not a force at all, I didn’t believe him at first. But Dr. Bishop is the most brilliant person I have even had the privilege of knowing. Recently he described gravity this way, and he is right:

“Einstein spent several years in an eventual successful attempt to include gravity in a modified description of spacetime. Early in his progress toward that goal Einstein had what he called the happiest thought of his life — that if a person were to fall off the roof of a house, while falling she would not feel a force of gravity. Before she falls, she feels the force of the roof supporting her. When her fall comes to its abrupt halt she feels the ground pushing against her. If she cannot feel a force of gravity while she is falling, why pretend that she felt a force of gravity when the roof supported her before she fell, or that she feels a force of gravity when she is lying on the ground?

“When thinking about the falling lady, Einstein had the fantastic insight that perhaps gravity never was a force. By late in 1915 he had that insight in elegant mathematical form such that the resulting theory, General Relativity, can be used to make precise predictions concerning gravitation.”

Einstein was elated when, on November 18, 1915, he found that his General Theory of Relativity predicted the measured precession of Mercury’s orbit. According to his friend and biographer Abraham Pais: “This discovery was, I believe, by far the strongest emotional experience in Einstein’s scientific life, perhaps in all his life.” Pais then continues with five words that crystallize that profound experience: “Nature had spoken to him.” After several years of work, on that day Einstein knew that he was the only person on Earth who understood gravity!

Today, there are thousands of people who understand gravity. Roy is one of them. Most of us, including me, are not one of them. But reading it described so well is one of the pleasures we can feel as we try to appreciate the wonderful cosmos in which we live. Not only does General Relativity correctly predict the precession of Mercury’s orbit, but it is essential to the programs used in the GPS navigation system, and it describes the gravitational waves (ripples in the geometry of spacetime) generated by two coalescing black holes, directly detected 100 years after 1915 by LIGO, the Laser Interferometer Gravitational-Wave Observatory.

*David and Wendee Levy live in Vail. Together they run the Jarnac Observatory.*