SCIENTIFIC LAWS AND THEORIES
Submitted to Georgia Science Teacher
Every year we teach students science methods and skills as they
do lab activities, experiments and projects. One of the primary
goals of this effort is to help children see science not as a
collection of facts but rather as a process of constructing
understanding of the world around us. Children need to learn how
we know the concepts we teach them and why we believe phenomena
occur as they do.
Students learn how historical theories have been discredited by
new information and evidence gained from well-designed
experiments. Examples of discredited theories include spontaneous
generation, Lamark's theory of inherited traits and Aristotle's
impetus theory. Students learn about competing theories and how
the search for evidence helped to substantiate one theory rather
than another. Examples include geocentric v heliocentric solar
systems and the particle and wave theories of light. We encourage
them to examine the evidence for each theory they study. With the
historical sequence of atomic theories, students learn that
theories are limited by the available information and measurement
techniques. They may be modified and refined as new information
becomes available. Often the outdated theories remain useful
approximations of the more complete theories that replace them.
For instance the Bohr model of the atom, with electrons arranged
in shells orbiting the positively charged nucleus, is still
useful for beginning chemists because it is less abstract and
complex than the wave mechanical model and quantum theory that
has replaced it. Newton's Law of Universal Gravitational is
adequate to get astronauts to the moon even though it is
inadequate to describe motion at relativistic speeds.
This "science" skill is integrated into many everyday activities.
As we read newspaper articles, assess claims made by politicians
and salesmen, and hear trial court cases, we look for the
evidence on which the claims are based. Every year students
willingly accept the concept that science is a human endeavor.
Our understanding is constructed by people interpreting current
information. Our theories will change as our base of information
expands: some theories will be clarified, some will be modified,
some will be rejected.
However, every year there comes a topic when individual students
abruptly change their reasoning modus operandi. Because they do
not accept a particular theory, they are unwilling to evaluate or
discuss the evidence. Usually (but not always) the topic is one
where the student perceives a conflict between religious beliefs
and scientific theories. Typical topics include Big Bang theory
of the beginning of the universe, geologic history of the age of
the earth, astronomy and theories of stellar evolution, chemistry
and theories of nucleosynthesis, and fossils and evolutionary
theory. Sometimes the conflict arises from rigid belief in
social, cultural or environmental "truths." This may occur in
classroom discussions of greenhouse effect, acid rain and the
ozone hole. For these topics, whether the students agree with the
prevailing theory or not, I would like them to use their
reasoning skills in examining and evaluating the evidence for and
against the theories rather than automatically reject the
evidence along with the theory.
Often, when one of these teaching blocks occurs, a students will
say to me "But it's only a theory." As I probe this response
further, the student is likely to suggest that there isn't enough
evidence or enough agreement among scientists for it to reach the
status of being a scientific law. This response underscores a
fundamental communication problem: We have different definitions
of laws and theories.
Last summer (1995), as part of the process of evaluating
textbooks for adoption, I examined how different texts defined
"law" and "theory." I was surprised by the ambiguous, confusing
and contradictory definitions used in many texts. Consider the
different definitions given in two Physical Science texts.
Definition 1: A theory is the most logical explanation of
events that occur in nature. Once a scientific theory
has been proposed, it may be tested over and over
again. If test results do not agree, the theory may be
changed or even rejected. When a scientific theory has
been tested many times and is generally accepted as
true, scientists may call it a law. But even laws can
be changed as a result of future observations and
experiments. (Prentice Hall 1988, p7)
Definition 2: In science a law is a summary of many experimental
results and observations. A law only describes what happens,
not why it happens. ... An explanation of why things work
the way they do is called a theory, or a model. A theory
explains the results of many different kinds of experiments,
observations, and occurrences. (Holt 1994, p9)
Clear definitions and understandings of these concepts are
vitally needed, starting in the elementary grades where students
are constructing their vocabulary and understanding of the
processes of science which they will use for the rest of their
lives. Unfortunately the best and clearest definitions are
usually found in the upper level texts. Here is a good example.
A law is a description of a relationship in nature that
manifests itself in recurring patterns of events. It
can be a prescription for how things change or it might
be a statement of how things remain invariant to
change... Theory is the explanation of phenomena in
terms of more basic natural processes and
relationships. To explain phenomena, we draw on
intuition and imagination and guess at what is
happening. We propose hypotheses, and leap beyond what
we know to what might be. A construct of definitions,
hypotheses, and laws that explains some observed order
in nature is the essence of theory. (Hecht, p2)
In other words, a law is a description of observations and a
theory is an explanation of observed phenomena. The explanation
(theory) includes the description (law). Theories encompass and
transcend laws: They are not unsubstantiated precursors to laws.
A good example of the difference is provided by Newton's law of
gravity, which describes and quantifies how objects fall as a
result of gravitational forces. We can use it to predict the
trajectory of projectiles in sports and artillery exercises and
to navigate space craft through the solar system. However,
Newton's theory does not explain what causes things to fall or
explain what is a gravitational force. Some aspects of these
explanations were provided by Einstein's General Theory of
Relativity, but scientists still have no theory to explain why
the universal gravitational constant is universal or why it is
the size it has been measured. Our theory of gravity is still
incomplete.
At the beginning of the year in my 10th-grade Physical Science
classes, many of the students who are willing to describe laws
and theories say that theories become laws as they are repeatedly
tested. Because I think that this part of the vocabulary is
essential to discussions of theories throughout the year and
because I have been teaching from a text (Prentice Hall Physical
Science) that uses similar incorrect definitions (see above), I
devote considerable effort to redefining the terms using other
textbooks, discussing why the differences are important, and
giving students lots of practice in distinguishing between laws
and theories. Subsequently, when I test students on the
differences, a significant number of the students resort to the
textbook definition. When I mark it wrong, some students protest
that their answer was the same as the text. This underscores the
importance of insuring that the textbooks we select for use in
our classrooms have as few significant conceptual errors as
possible.
At the end of the semester we review the law-theory distinction.
By that time, very few students tell me that a theory becomes a
law after further testing. However, a significant number of
students produce a more sophisticated variation of that notion by
saying that laws are more correct, certain or "truthful" than
theories. This demonstrates that they haven't understood the
fundamental distinction. The difference in certainty is a
consequence rather than a cause of the differences in
definitions. Any description (law) will be inherently more
certain than an explanation (theory).
Paul Hewitt uses a very effective approach in addressing
student's perceptions of a conflict between science and religion
in his text Conceptual Physics. He says "Science is about cosmic
order. Religion is about cosmic purpose."
When we study the nature of light later in this book, we
will treat light as a wave, then as a particle. To the
person who knows just a little physics, waves and
particles are contradictory; light must be one or the
other and a choice must be made between them. But to
the enlightened physicist, waves and particles
complement each other and provide a deeper
understanding of light. In a similar way, we don't have
to choose between science and religion. We can embrace
science and religion if we truly understand that they
address different facets of the human experience. (Paul
Hewitt, p6)
Footnote: Prentice Hall have corrected their definitions of laws
and theories (among other corrections) in their Physical Science
textbook that will be available for adoption in Georgia schools.
References
Cuevas, M. & W. Lamb. (1994) Physical Science. Austin, TX: Holt,
Rinehart & Winston.
Hecht, E. (1994) Physics: Algebra/Trig. Pacific Grove, CA:
Brooks/Cole.
Hewitt, P. (1992) Conceptual Physics. Menlo Park, CA: Addison
Wesley.
Hurd, D. et al. (1988) Physical Science. Englewood Cliffs, NJ:
Prentice Hall.