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150 120 90 60 30 0 30 60 90 The most recently developed scientific theory was refined
and expanded during the 1970s. This theory concerns the sur-
face of Earth, and it has changed our model of what Earth is
60
like. At first, the basic idea of today’s accepted theory was pure
and simple conjecture. The term conjecture usually means an
explanation or idea based on speculation, or one based on triv-
ial grounds without any real evidence. Scientists would look at
30
a map of Africa and South America, for example, and mull over
how the two continents look like pieces of a picture puzzle that
had moved apart (Fig ure 1.16). Any talk of moving continents
was considered conjecture, because it was not based on any-
0 thing acceptable as real evidence.
Many years after the early musings about moving conti-
nents, evidence was collected from deep-sea drilling rigs that the
ocean floor becomes progressively older toward the African and
South American continents. This was good enough evidence to
30
establish the “seafloor spreading hypothesis” that described the
two continents moving apart.
If a hypothesis survives much experimental testing and
leads, in turn, to the design of new experiments with the genera-
tion of new hypotheses that can be tested, you now have a work-
A
ing theory. A theory is defined as a broad working hypothesis
that is based on extensive experimental evidence. A scientific
theory tells you why something happens. For example, the plate
tectonic theory describes how the continents have moved apart,
just as pieces of a picture puzzle do. Is this the same idea that
was once considered conjecture? Sort of, but this time it is sup-
ported by experimental evidence.
The term scientific theory is reserved for historic schemes
of thought that have survived the test of detailed examina-
tion for long periods of time. The atomic theory, for example,
was developed in the late 1800s and has been the subject of
B
extensive investigation and experimentation over the last cen-
tury. The atomic theory and other scientific theories form
FIGURE 1.16 (A) Normal position of the continents on a
world map. (B) A sketch of South America and Africa, suggesting the framework of scientific thought and experimentation to-
that they once might have been joined together and subsequently day. Scientific theories point to new ideas about the behavior
separated by continental drift. of nature, and these ideas result in more experiments, more
data to collect, and more explanations to develop. All of this
can make standing waves only at certain distances from the may lead to a slight modification of an existing theory, a major
nucleus. Thus, the model of the atom changed from one that modification, or perhaps the creation of an entirely new theory.
views electrons as solid particles to one that views them as These activities are all part of the continuing attempt to satisfy
vibrations. our curiosity about nature.

SUMMARY

Physical science is a search for order in our physical surround- Measurement is a process that uses a well-defined and agreed-
ings. People have concepts, or mental images, about material objects upon referent to describe a standard unit. The unit is compared to the
and intangible events in their surroundings. Concepts are used for property being defined by an operation that determines the value of the
thinking and communicating. Concepts are based on properties, or unit by counting. Measurements are always reported with a number, or
attributes that describe a thing or event. Every property implies a value, and a name for the unit.
referent that describes the property. Referents are not always explicit, The two major systems of standard units are the English system and
and most communications require assumptions. Measurement the metric system. The English system uses standard units that were origi-
brings precision to descriptions by using numbers and standard nally based on human body parts, and the metric system uses standard
units for referents to communicate “exactly how much of exactly units based on referents found in nature. The metric system also uses a
what.” system of prefixes to express larger or smaller amounts of units. The metric

1-19 CHAPTER 1 What Is Science? 19