Gravity is a “fact” in the sense that there is no doubt that it is real and does many things. For example, gravity really does hold us to Earth, it really does make objects fall when you drop them, and it really does keep Earth and other planets in orbit around the Sun.

But why does gravity do these things? This is where a theory of gravity comes in, by which we mean a model that can explain the many roles of gravity and predict how gravity will act in other situations. Historically, it took a very long time for an adequate theory of gravity to be developed, and our theory of gravity may still be incomplete even today.

Ancient Ideas about Gravity

In ancient times, the concept of gravity meant nothing more than the tendency of objects to fall to the ground. Because the ancient Greeks recognized that the world is round, they also recognized that gravity acted in a direction toward the center of the Earth. In other words, gravity gave meaning to the notions of “up” and “down” (Figure 3.18).

Notice that this implies at least two major differences between the ancient idea of gravity and our modern ideas:

• The ancient idea of gravity applied only to objects on Earth. As far as we know, the idea that gravity might also affect the Moon, planets, or stars never occurred to anyone in the ancient world. They simply assumed that “the heavens” (which is what they called everything in the sky) operated by different rules than Earth.
• People of the ancient world never developed a clear understanding of exactly how gravity affects falling objects. For example, it was commonly assumed that heavier objects must fall faster than lighter-weight objects. This idea wasn’t clearly disproven until Galileo demonstrated that, aside from effects of air resistance, gravity makes all objects fall to the ground at the same rate (Figure 3.19).

Newton’s Theory of Gravity

Galileo’s discovery was an important step in understanding gravity, but it still did not give us a model of gravity, because it didn’t explain why objects of different weights should all fall at the same rate. Moreover, neither Galileo nor anyone else had yet recognized any connection between gravity and planetary motion.

The breakthrough in our understanding of gravity came from Isaac Newton a few decades later. By his own account, Newton experienced a moment of inspiration in 1666 when he saw an apple fall to the ground and suddenly realized that the gravity making the apple fall was the same force that held the Moon in orbit around Earth. (Contrary to a popular myth, Newton did not say that the apple hit him on the head.) But where did this insight come from? To understand it, we have to think back to Kepler’s model of planetary motion, which is embodied in his three laws of planetary motion.

Recall that Kepler had published his first two laws of planetary motion in 1610, and his third law in 1619, and that these laws were clearly successful at predicting planetary positions in the sky. However, for decades afterward, no one could explain why Kepler’s model worked so well, though many scientists recognized that the laws contained two key clues to whatever might explain planetary motion:

1. The first clue came from the fact that orbits were ellipses rather than perfect circles. Clearly, whatever caused planetary motion must make it possible for orbits to have these elliptical shapes.
2. The second clue came from the fact that planets farther from the Sun orbit at slower speeds (according to the precise mathematical formula of Kepler’s third law). This suggested that the Sun might be the source of whatever force held the planets in their orbits, and that this force weakened with distance, which would explain why more distant planets moved more slowly.

The second clue was particularly important, and many scientists tried to identify the force that might explain planetary motion. There were many guesses; for example, some scientists wondered whether the planets might be held in their orbits by some sort of magnetic force from the Sun.

From this standpoint, if Newton had simply said “I think it’s gravity,” it would likely have been seen as just another guess. But Newton didn’t stop there; instead, he put mathematics to work and didn’t announce his conclusion that the force was gravity until he had proved it beyond any reasonable doubt.

This work took a while, and also required him to formalize a set of laws describing motion (those that we call Newton’s laws of motion) and to invent a new type of mathematics, called calculus, to work out the details. He finally published his work on motion and gravity in 1687 — more than 20 years after his insight with the falling apple (Figure 3.20).

The Universal Law of Gravitation

Before we continue, it’s worth noting three important ideas that you should keep in mind about Newton’s universal law of gravitation:

• This law represents a model of gravity, because it can be used to calculate the strength of gravity in any situation and thereby to predict how objects will move as a result of gravity. This model has proven to be extremely successful, and it is this success that leads us to consider it a theory of gravity.
• Newton’s successful development of a theory of gravity represented the culmination of the Copernican revolution. Once Newton demonstrated that gravity explained why planets orbit the Sun as they do, there could be no further doubt that Earth really is a planet orbiting the Sun.
• His discovery that the same law of gravity applies to falling apples and to orbits in space provided the first evidence of the idea that the same laws of nature operate everywhere. In other words, Newton was the first to show that Earth and “the heavens” were not distinct realms, but all part of the same universe, governed by the same laws of nature.

Gravity Beyond Newton

Newton’s theory of gravity works so well that it might be tempting to think that it represents everything we need to know about gravity. But it doesn’t. In the centuries after Newton, scientists discovered that the predictions of Newton’s theory are not quite perfect, particularly in cases where gravity is very strong. As a result, scientists kept working to understand gravity better.

Today our best theory of gravity is a theory first developed by Albert Einstein (1879–1955), called the general theory of relativity. In other words, despite its odd-sounding name, “general relativity” (as it is usually called) is actually a theory of gravity. The reason that we consider general relativity to be a better theory is because when we measure with high enough precision, we find that Einstein’s general theory of relativity makes predictions that differ very slightly from Newton’s theory — and the observations agree better with Einstein’s theory. Moreover, in some cases, general relativity makes predictions that go far beyond the bounds of Newton’s theory. For example, general relativity predicts the existence of both black holes and of something called “gravitational waves,” both of which have now been observed to exist just as predicted (Figures 3.22 and 3.23).

Does this mean that Newton’s theory of gravity was “wrong”? Remember that Newton’s theory successfully explains nearly all observations of gravity in the universe, and it works so well that we can use it to plot the courses of spacecraft to the planets. Moreover, in all cases in which Newton’s theory works well, Einstein’s theory gives essentially the same answers. The differences in the predictions between the two theories are noticeable only with extremely precise measurements or in cases where gravity is unusually strong.

We therefore do not say that Newton’s theory was wrong, but rather that it is only an approximation to a more exact theory of gravity—Einstein’s general theory of relativity. Under most circumstances, the approximation is so good that we can barely tell the difference between the two theories of gravity, but in cases of strong gravity, Einstein’s theory works and Newton’s fails.

Summary: Gravity as Fact and Theory

We can now summarize the distinction between talking about gravity as a fact and as a theory:

• The fact of gravity is obvious in the observations we make of falling objects on Earth and of orbiting objects in space. There is no doubt that gravity really exists.
• A theory of gravity is a specific model of gravity that we use to make predictions that we then put to the test through observations or experiments.

Newton provided our first theory of gravity, and it still works extremely well in almost all cases. However, it is not quite perfect, and our best theory of gravity today is Einstein’s general theory of relativity. In the future, our theory of gravity may be further improved, but gravity remains a fact regardless of how we revise the theory.

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