The Solar System

CHAPTER 5 | GRAVITY 89

Particles move according to three rules of motion and attract

each other with a force called gravity. Th ese motions are predict-

able, and that makes the universe a vast machine based on a few

simple rules. It is complex only in that it contains a vast number

Principia changed astronomy by ushering in a new age. No

longer did people have to appeal to the whim of the gods to

explain things in the heavens. No longer did they speculate on

why the planets wander across the sky. After Principia astrono-

mers understood that the motions of the

heavenly bodies are governed by simple, uni-

versal rules that describe the motions of

everything from orbiting planets to falling

apples. Suddenly the universe was under-

standable in simple terms, and astronomers

could accurately predict future planetary

motions (How Do We Know? 5-2).

Principia also changed science in gen-

eral. Th e works of Copernicus and Kepler

had been mathematical, but no book before

had so clearly demonstrated the power of

mathematics as a language of precision.

Newton’s arguments in Principia were so

powerful an illustration of the quantitative

study of nature that scientists around the

world adopted mathematics as their most

powerful tool.

Also, Principia changed the way people

thought about nature. Newton showed that

the rules that govern the universe are simple.

The So-Called Scientifi c Method

How are the predictions of a hypothesis

useful in science? (Note the careful use of

language here, to distinguish a tentative

hypothesis from a theory that is well con-

fi rmed.) Scientifi c hypotheses face in two direc-

tions. They look back into the past and explain

phenomena previously observed. For example,

Newton’s laws of motion and gravity explained

observations of the movements of the planets

made over many centuries. But hypotheses also

look forward in that they make predictions about

what you should fi nd as you explore further. For

example, Newton’s laws allowed astronomers

to calculate the orbits of comets, predict their

return, and eventually understand their origin.

Scientifi c predictions are important in two

ways. First, if a prediction of a hypothesis is

confi rmed, scientists gain confi dence that the

hypothesis is a true description of nature. But

predictions are important for a second reason.

They can point the way to unexplored avenues

of knowledge.

Particle physics is a fi eld in which pre-

dictions have played a key role in directing

research. In the early 1970s, physicists

proposed a hypothesis about the fundamen-

tal forces and particles in atoms called the

Standard Model. This hypothesis explained

what scientists had already observed in experi-

ments, but it also predicted the existence of

particles that hadn’t yet been observed. To

test the hypothesis, scientists focused their

efforts on building more and more powerful

particle accelerators in the hopes of detecting

the predicted particles.

A number of these particles have since

been discovered, and they do match the

characteristics predicted by the Standard

Model, further confi rming the hypothesis. One

predicted particle, the Higgs boson, has not

yet been found, as of this writing, but an even

larger accelerator may allow its detection. Will

the Higgs boson be found, or will someone

come up with a better hypothesis? This is only

one of many cliff-hangers in modern science.

You learned in an earlier chapter that a

hypothesis that has passed many tests and

made successful predictions can “graduate” to

Physicists build huge accelerators to search for

sub-atomic particles predicted by their theories.

(Brookhaven National Laboratory)

being considered a theory. As you read about

any scientifi c hypothesis or theory, think

about both what it can explain and what it

can predict.

5-2 Testing a Hypothesis by Prediction

■ Figure 5-9

Newton, working from the discoveries of Galileo and Kepler, derived three laws of motion and the

principle of mutual gravitation. He and some of his discoveries were honored on this old English

pound note. Notice the diagram of orbital motion in the background and the open copy of Principia

in Newton’s hands.