Oh, it's just a theory.

Theory is a good example of a word which is misused every day by most of us. The trouble arises because the word has several meanings. Here is how Collin's Dictionary of English defines it :

1. a system of rules, procedures, and assumptions used to produce a result.
2. abstract knowledge or reasoning.
3. a speculative or conjectural view or idea: I have a theory about that.
4. an ideal or hypothetical situation (esp. in the phrase in theory).
5. a set of hypotheses related by logical or mathematical arguments to explain and predict a wide variety of connected phenomena in general terms: the theory of relativity.
6. a nontechnical name for hypothesis (sense 1).
[16th century: from Late Latin theoria, from Greek: a sight, from theorein to gaze upon]

Dictionaries record and elucidate the usage of words, but they are poor guides to scientific concepts. Moreover, of these six definitions the non-scientist tends to confuse definitions 3 and 5.

The words used in science are assumption, hypothesis, theory and axioms. So what is the difference between them?

For the scientist an assumption is an assertion about reality which is unproven.

A   hypothesis is an assumption put forward as a starting point for further research from known facts.

A   theory is a hypothesis that has been confirmed or established by observation or experiment and is accepted by the scientific community as accounting for known facts.

An   axiom is a self-evident truth; for example the line and the point are axiomatic in Euclidean geometry and do not require any proof and all proofs in Eucledian geometry are derived from them. Axiomatic that is until they were undermined by Gaussian coordinates.

Modern scientists are suspicious of axioms and prefer theories. In assessing theories, what they are interested in is the status of a theory, the amount and quality of the evidence which supports it. Where a theory is supported by good evidence it is accepted until a new hypothesis is put forward and confirmed by independent observation and experiment.

It is the results of observation and experiments that leads to the framing of a hypothesis, which is tested in turn by further experiments in a concerted attempt to disprove it. Only when a hypothesis has survived every test to disprove it is it elevated to the status of a theory. A theory is the explanation which best fits the available evidence.

Let's consider Einstein's Theory of Relativity, perhaps being less controversial than Darwin's Theory of Evolution. I do not know of any physicist who does not fully accept Einstein's two theories of relativity. In honour of Einstein's 70th birthday 25 scientists contributed to a celebratory book including a whole batch of Noble Prize winners from Niels Bohr to Wolfgang Pauli.

The theory of relativity has survived every one of the many searching experimental tests to which it has been subjected during the last nine decades. Its status today is such that, if an experiment is inconsistent with relativity, physicists everywhere would conclude that there must be something wrong with the experiment.
           (Fundamentals of Physics, 4th edition, 1993, by Halliday, Rasnick, and Walker, page 1,106.)

To repeat, a theory is the explanation which best fits the available evidence after exhaustive testing.

Peter Ghiringhelli, BA(Hons), MA
January 2001

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