The scientific method
is the process by which scientists, collectively and over time, endeavor to
construct an accurate (that is, reliable, consistent and non-arbitrary) representation
of the world.
Recognizing that personal and cultural beliefs influence
both our perceptions and our interpretations of natural phenomena, we aim
through the use of standard procedures and criteria to minimize those
influences when developing a theory. As a famous scientist once said,
"Smart people (like smart lawyers) can come up with very good explanations
for mistaken points of view." In summary, the scientific method attempts
to minimize the influence of bias or prejudice in the experimenter when testing
an hypothesis or a theory.
1. Observation and description of a phenomenon or group of phenomena.
2. Formulation of an hypothesis to explain the
phenomena. In physics, the hypothesis often takes the form of a causal
mechanism or a mathematical relation.
3. Use of the hypothesis to predict the existence of other phenomena, or to
predict quantitatively the results of new observations.
4. Performance of experimental tests of the predictions by several
independent experimenters and properly performed experiments.
If the experiments bear out the hypothesis it may come to be regarded as a
theory or law of nature (more on the concepts of hypothesis, model, theory and
law below). If the experiments do not bear out the hypothesis, it must be
rejected or modified. What is key in the description of the scientific method
just given is the predictive power (the ability to get more out of the theory
than you put in; see Barrow, 1991) of the hypothesis or theory, as tested by
experiment. It is often said in science that theories can never be proved, only disproved. There is always the possibility that
a new observation or a new experiment will conflict with a long-standing
theory.
As just stated, experimental tests may lead either to the
confirmation of the hypothesis, or to the ruling out of the hypothesis.
The scientific method requires that an hypothesis be
ruled out or modified if its predictions are clearly and repeatedly
incompatible with experimental tests. Further, no matter how elegant a theory
is, its predictions must agree with experimental results if we are to believe
that it is a valid description of nature. In physics, as in every experimental
science, "experiment is supreme" and experimental verification of hypothetical
predictions is absolutely necessary. Experiments may test the theory directly
(for example, the observation of a new particle) or may test for consequences
derived from the theory using mathematics and logic (the rate of a radioactive
decay process requiring the existence of the new particle). Note that the
necessity of experiment also implies that a theory must be testable. Theories
which cannot be tested, because, for instance, they have no observable
ramifications (such as, a particle whose characteristics make it unobservable),
do not qualify as scientific theories.
If the predictions of a long-standing theory are found to be in disagreement
with new experimental results, the theory may be discarded as a description of
reality, but it may continue to be applicable within a limited range of
measurable parameters. For example, the laws of classical mechanics (
We are all familiar with theories which had to be discarded in the face of
experimental evidence. In the field of astronomy, the earth-centered
description of the planetary orbits was overthrown by the Copernican system, in
which the sun was placed at the center of a series of concentric, circular
planetary orbits. Later, this theory was modified, as measurements of the
planets motions were found to be compatible with elliptical, not circular,
orbits, and still later planetary motion was found to be derivable from
Error in experiments have several sources. First,
there is error intrinsic to instruments of measurement. Because this type of
error has equal probability of producing a measurement higher or lower
numerically than the "true" value, it is called random error. Second,
there is non-random or systematic error, due to factors which bias the result
in one direction. No measurement, and therefore no experiment, can be perfectly
precise. At the same time, in science we have standard ways of estimating and
in some cases reducing errors. Thus it is important to determine the accuracy
of a particular measurement and, when stating quantitative results, to quote
the measurement error. A measurement without a quoted error is meaningless. The
comparison between experiment and theory is made within the context of
experimental errors. Scientists ask, how many standard
deviations are the results from the theoretical prediction? Have all sources of
systematic and random errors been properly estimated? This is discussed in more
detail in the appendix on Error Analysis and in Statistics Lab 1.
As stated earlier, the scientific method attempts to minimize the influence
of the scientist's bias on the outcome of an experiment. That is, when testing an hypothesis or a theory, the scientist may have a
preference for one outcome or another, and it is important that this preference
not bias the results or their interpretation. The most fundamental error is to
mistake the hypothesis for an explanation of a phenomenon, without performing
experimental tests. Sometimes "common sense" and "logic"
tempt us into believing that no test is needed. There are numerous examples of
this, dating from the Greek philosophers to the present day.
Another common mistake is to ignore or rule out data which do not support
the hypothesis. Ideally, the experimenter is open to the possibility that the
hypothesis is correct or incorrect. Sometimes, however, a scientist may have a
strong belief that the hypothesis is true (or false), or feels internal or
external pressure to get a specific result. In that case, there may be a
psychological tendency to find "something wrong", such as systematic
effects, with data which do not support the scientist's expectations, while
data which do agree with those expectations may not be checked as carefully.
The lesson is that all data must be handled in the same way.
Another common mistake arises from the failure to estimate quantitatively
systematic errors (and all errors). There are many examples of discoveries
which were missed by experimenters whose data contained a new phenomenon, but
who explained it away as a systematic background. Conversely, there are many
examples of alleged "new discoveries" which later proved to be due to
systematic errors not accounted for by the "discoverers."
In a field where there is active experimentation and open
communication among members of the scientific community, the biases of
individuals or groups may cancel out, because experimental tests are repeated
by different scientists who may have different biases. In addition, different
types of experimental setups have different sources of systematic errors. Over
a period spanning a variety of experimental tests (usually at least several
years), a consensus develops in the community as to which experimental results
have stood the test of time.
In physics and other science disciplines, the words "hypothesis,"
"model," "theory" and "law" have different
connotations in relation to the stage of acceptance or knowledge about a group
of phenomena.
An hypothesis is a limited statement
regarding cause and effect in specific situations; it also refers to our state
of knowledge before experimental work has been performed and perhaps even
before new phenomena have been predicted. To take an example from daily life,
suppose you discover that your car will not start. You may say, "My car
does not start because the battery is low." This is your first hypothesis.
You may then check whether the lights were left on, or if the engine makes a
particular sound when you turn the ignition key. You might actually check the
voltage across the terminals of the battery. If you discover that the battery is
not low, you might attempt another hypothesis ("The starter is
broken"; "This is really not my car.")
The word model is reserved for situations when it is known that the
hypothesis has at least limited validity. A
often-cited example of this is the Bohr model of the atom, in which, in an
analogy to the solar system, the electrons are described has moving in circular
orbits around the nucleus. This is not an accurate depiction of what an atom
"looks like," but the model succeeds in mathematically representing
the energies (but not the correct angular momenta) of
the quantum states of the electron in the simplest case, the hydrogen atom.
Another example is Hook's Law (which should be called Hook's principle, or
Hook's model), which states that the force exerted by a mass attached to a
spring is proportional to the amount the spring is stretched. We know that this
principle is only valid for small amounts of stretching. The "law"
fails when the spring is stretched beyond its elastic limit (it can break).
This principle, however, leads to the prediction of simple harmonic motion,
and, as a model of the behavior of a spring, has been versatile in an
extremely broad range of applications.
A scientific theory or law represents an
hypothesis, or a group of related hypotheses, which has been confirmed through
repeated experimental tests. Theories in physics are often formulated in terms
of a few concepts and equations, which are identified with "laws of
nature," suggesting their universal applicability. Accepted scientific
theories and laws become part of our understanding of the universe and the
basis for exploring less well-understood areas of knowledge. Theories are not
easily discarded; new discoveries are first assumed to fit into the existing
theoretical framework. It is only when, after repeated experimental tests, the
new phenomenon cannot be accommodated that scientists seriously question the
theory and attempt to modify it. The validity that we attach to scientific
theories as representing realities of the physical world is to be contrasted
with the facile invalidation implied by the expression, "It's only a
theory." For example, it is unlikely that a person will step off a tall
building on the assumption that they will not fall, because "Gravity is
only a theory."
Changes in scientific thought and theories occur, of course, sometimes
revolutionizing our view of the world (Kuhn, 1962). Again, the key force for
change is the scientific method, and its emphasis on experiment.
While the scientific method is necessary in developing scientific knowledge,
it is also useful in everyday problem-solving. What do you do when your
telephone doesn't work? Is the problem in the hand set, the cabling inside your
house, the hookup outside, or in the workings of the phone company? The process
you might go through to solve this problem could involve scientific thinking,
and the results might contradict your initial expectations.
Like any good scientist, you may question the range of situations (outside
of science) in which the scientific method may be applied. From what has been
stated above, we determine that the scientific method works best in situations
where one can isolate the phenomenon of interest, by eliminating or accounting
for extraneous factors, and where one can repeatedly test the system under
study after making limited, controlled changes in it.
There are, of course, circumstances when one cannot isolate the phenomena or
when one cannot repeat the measurement over and over again. In such cases the
results may depend in part on the history of a situation. This often occurs in
social interactions between people. For example, when a lawyer makes arguments
in front of a jury in court, she or he cannot try other approaches by repeating
the trial over and over again in front of the same jury. In a new trial, the
jury composition will be different. Even the same jury hearing a new set of
arguments cannot be expected to forget what they heard before.
The scientific method is intricately associated with science, the process of
human inquiry that pervades the modern era on many levels. While the method
appears simple and logical in description, there is perhaps no more complex
question than that of knowing how we come to know things. In this introduction,
we have emphasized that the scientific method distinguishes science from other
forms of explanation because of its requirement of systematic experimentation.
We have also tried to point out some of the criteria and practices developed by
scientists to reduce the influence of individual or social bias on scientific
findings.