Gravity: The Law Without a Cause

In Chapter 7-7 of The Feynman Lectures on Physics, Richard Feynman probes a deceptively simple question—what is gravity? He begins by highlighting that Newton’s law of universal gravitation is profoundly successful in describing planetary motion yet remains silent on why gravity occurs—it doesn’t explain the mechanism, only the mathematics. Feynman muses that this abstraction is characteristic of physics: laws like energy conservation or mechanics provide accurate, predictive formulas without delving into the underlying machinery. He illustrates how speculative models such as the “particle wind” or push-gravity idea—where high-speed particles bombard Earth unevenly due to screening by the Sun—can reproduce the inverse-square law yet fail elsewhere, producing an unreal orbital drag that doesn’t align with observation.

To ground the discussion, Feynman recalls the extraordinary precision experiments of Loránd Eötvös, a Hungarian physicist working in the early 20th century. Eötvös designed an exquisitely sensitive torsion balance to compare the acceleration of different materials in Earth’s gravitational field. His aim was to test the equivalence principle—the idea that gravitational mass and inertial mass are exactly proportional. Even with the technology of 1909, he demonstrated that any difference between the two was smaller than one part in 100 million, an astonishing achievement for the time. Decades later, Robert H. Dicke and his colleagues at Princeton refined the method with modern materials, vacuum enclosures, and improved vibration isolation. By the mid-20th century, Dicke’s team had pushed the limits to show equivalence within one part in a billion, confirming with overwhelming precision that all substances fall at the same rate in a gravitational field, regardless of composition.

Feynman then touches on the tantalising similarity between gravity and electromagnetism: both forces follow an inverse-square law. Yet while electrical repulsion between two electrons vastly outweighs their gravitational attraction, the enormity of this ratio—on the order of 10^42—raises profound questions about the fundamental constants of nature. He speculates whether such enormous numbers might connect to cosmic scales, like the age of the universe expressed in “natural units,” but dismisses the idea that the gravitational constant G changes significantly with time. Even slight variation would imply that the early Earth was too hot for oceans or life to exist—conclusions that don’t fit our observations.

In wrapping up, Feynman emphasises that despite centuries of experimentation and elegant theory, gravity remains without a mechanistic explanation. And yet its laws, distilled into mathematical form, remain stunningly effective. The Eötvös and Dicke experiments stand as remarkable demonstrations of the precision with which we can test nature’s principles—affirming the perfect balance between inertia and weight, and leaving us with the enduring mystery of why the universe should work so exactly.