Facts of the Matter

Richard Brill

Gravity establishes a natural system of orientation. It's what makes up "up" and down "down," despite the efforts of some to defy it.

Gravity responsible for
much more than we realize

"I offer this work as the mathematical principles of philosophy, for the whole burden of philosophy seems to consist in this -- from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena."
-- Sir Isaac Newton
"The Mathematical Principles of Natural Philosophy"

Many of us would be happy if we could change the law of gravity to allow for a lighter sentence for our culinary violations. Alas, the mutual attraction between us and the earth is not variable, despite the extra weight that a tough day at work or the passing years seems to place on us regardless of the numbers on the bathroom scale.

Everybody knows that gravity is that which holds us to Earth and makes things that we drop fall.

But gravitation is much more than the obvious and is fundamental to so many other things that we take for granted.

The very long list of things that gravity is responsible for begins with the universe itself. Without gravity the lumps of gas in the primordial universe could not have coalesced into stars, nor could galaxies of billions of stars have organized themselves into the spheres and spirals.

Without gravity there would be no planets, galaxies, stars or sunshine.

Gravity draws the atoms necessary to make a star or a planet in the first place and keeps a star from blowing outward once pressure and temperature are sufficiently high for nuclear fusion to begin. Planets grow by sweeping rocks and debris out of the vicinity of its orbit like a huge vacuum cleaner. Come to think of it, a vacuum cleaner wouldn't work without air pressure, which is another result of gravity.

Gravity holds the thin veil of atmosphere in place, keeps water in the ocean basins, allows bubbles to rise and clouds to float, tells plant roots which direction to grow and causes water to flow in streams and rivers.

Gravity establishes a natural system of orientation of which we are largely unaware. Ask any astronaut how difficult it is to find things when there is no "up" or "down." Imagine trying to find your way around the house if there were no way to tell which was floor, which was ceiling and which were walls.

There was no concept of gravitation until late in the 17th century. It is said that Sir Isaac Newton discovered gravity, but that is not entirely true. What he discovered was a law of gravitation that described what it does, not explaining what it is or how it does what it does.

Gravity existed before Newton reluctantly published the book that made him famous and which is considered by many to be the greatest intellectual achievement of all time.

The book, "Mathematical Principles of Natural Philosophy," contained Newton's concisely stated principles of science; precise definitions of mass, space and time; his three laws of motion; the law of gravitation; methods, calculations and applications of geometrical and mathematical analysis; explanations of gravitationally related phenomena such as the tides, the motion of the planets and satellites; how to launch artificial satellites; and the equatorial bulges of planets, just to name a few.

"The Principia" marked the beginning of modern science. In addition to being a seminal text in the way it outlined methods of mathematical analysis and problem-solving, the complementary use of inductive and deductive logic, and the importance of observation as arbiter of reality, it established the science of physics and contributed heavily to the 21st-century Zeitgeist.

Newton demonstrated exceptional insight and conceptual leaps at several stages in the development and presentation of his work, the solution to each problem serving as a theorem for solving the next. When it was all done, he had tied everything together like a spider's web, prepared to catch anything attempting to pass through it, each part relying on and communicating to the whole.

Perhaps the greatest insight, and the one "occasioned by the fall of an apple," was that falling objects and planets were governed by the same principle. The nature of these two phenomena (falling objects and planetary motion) was being considered separately by other thinkers of his time, but no one besides Newton made the connection.

Others had speculated that gravity might emanate from an object like light from a candle, growing dimmer at a rate that depends on the second power of the distance, a relationship known as inverse square. Although it is a logical and aesthetically appealing metaphor, no one could think of a way to verify that it is a good model for understanding planetary motion.

Solving one problem after another with insight, inference, logic and mathematical methods of his own invention, including vector algebra and differential calculus, Newton eventually demonstrated that an inverse-square force would produce the elliptical orbits of the planets.

Having been successful in this geometrical, Euclidian proof, he hypothesized a new and previously unknown force, gravitation, then used the hypothesis to formulate a clever way to verify it using the orbit of the moon and the acceleration of Earth's gravity.

One conceptual leap was his invoking the action-reaction law, his own axiomatic third law of motion, which states that every force is mutually exerted on by two distinct objects. He deemed the laws to be universal, or else they were not good laws, so they had to apply to the planets as well as earthly objects.

Previous philosophers from Aristotle to Galileo thought of weight as an intrinsic property of an object alone and did not consider what, if any, effect its distance from Earth might have on it.

Aristotle believed weight was a result of an object's desire to attain its proper and rightful place in the structure of the sublunar realm (his word for everything inside the orbit of the moon) according to the proportion of the four essential elements that he thought comprised every object.

He saw falling motion as a consequence of an imponderable cosmological purpose to achieve a state of cosmic perfection: four concentric spheres of pure earth, water, air and fire to complement the already perfect quintessence of the heavenly realm about which we could never have knowledge.

With the rise and fall of the Roman Empire, much of the ancient knowledge of the Greek philosophers was lost or hidden until various works were rediscovered early in the second millennium. Aristotle remained the authority on just about everything, including those things that he understood the least. Motion was one of them.

Two thousand years after Aristotle's death, Galileo would conclude, after years of carefully designed experimentation, that objects fell according to natural law regardless of their composition and any attendant desires that they might have to attain spiritual perfection and harmony.

The capstone to Newton's verification of the inverse-square problem came more than half a century after Galileo's experiments when he reasoned that there was a way to verify the inverse-square nature of gravity.

Having first presented a convincing argument that gravity "acts" from the center of objects, he then noted that the surface of the earth is roughly one-sixtieth (1/60) of the way from the center of the earth to the center of the moon, so the effect of Earth's gravity on the moon's orbit should be about one-thirty-six-hundredth (1/3600) as much (60 to the second power is 3,600).

His working hypothesis was that Earth's gravity reduced by the factor of 1/3600 would be just the right amount to keep the moon in its orbit.

Newton used the best estimates of the radius of Earth and the distance to the moon that were available at the time. After some further mathematical machinations to reckon the acceleration of an object in circular motion, including the moon, and approximating the moon's orbit as circular, he did some relatively simple calculations and found them to "agree pretty nearly."

The two calculated values for the moon were not close by today's experimental standards; they differed by about 15 percent. But it was the first demonstrated relationship between falling motion and orbital motion, and it was the rigor, cross-linking and interrelationships of Newton's mathematical proofs, correlations and predictions as much it was his "solving" the moon's orbit that convinced others of its efficacy.

One thing that troubled Newton, and everyone since, was that gravity managed to act through millions of miles of empty space, and no one had any good answer as to how it did that.

Even though it did not answer all questions about gravitation, Newton's model stood alone until early in the 20th century, having been verified time and again in multiple experiments and observations. Then Einstein visualized gravity in a completely different manner in formulating his general theory of relativity.

Today we see gravitation from a somewhat different perspective, thanks to Einstein's brilliant consideration of warped space-time. In our modern theories of gravitation, matter tells space how to distort, and distorted space tells matter how to move.

It is the weakly warped nature of space-time that is responsible for many gravitational effects that Newton's law of gravitation, remarkable as it was, could not explain. Although warped space-time forms the conceptual paradigm for studying gravity today, the nature of the interaction between space, time and matter is far from being understood, and physicists still hope for the day when gravity can be reconciled with the other fundamental forces of nature and quantum mechanics into the Grand Unification Theory.

On April 20, NASA launched a satellite into a 400-mile-high polar orbit containing 4 1/2-inch diameter gyroscopes (the most perfectly precise objects ever made) that will measure how Earth warps space and time and how Earth's rotation drags space-time around with it like batter in a mixer. The project represents more than 20 years of planning and 40 years of design and engineering.

The Gravity Probe B satellite is part of a joint experiment with Stanford University that will be the best test yet of our current understand of gravitation.

Stanford maintains a Web site at with details on every aspect of the project, including links to many other sites about gravitation, including its cultural history and discussions about the multitude of ways that gravity affects us intellectually, spiritually, emotionally and technologically.

If all of this information is too heavy, don't let its weight get you down. Instead, relax, take a load off and reflect on the brilliance of scientists such as Newton and Einstein as further examples of remarkable capabilities that exist in the human genome.

While you're at it, glance around and appreciate all of the things that are here only by the grace of gravity, whatever it is.

Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at


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