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Scientific Theories

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Theory vs. theory. Is a scientific theory different from the everyday use of the word theory?

The Big Bang Theory. The Theory of Gravity. The Plate Tectonic Theory. The Modern Atomic Theory. The Cell Theory. The Theory of Evolution by Natural Selection. These are all classic scientific theories. So, without a doubt, yes, a scientific theory is very different from the everyday use of the word theory. A scientific theory is accepted as a scientific truth, supported by evidence collected by many scientists. Lots of data has been collected to support the theory, and no data has been identified to prove the theory incorrect. That does not mean that evidence does not exist, it means that no one has been able to identify it.

Scientific Theories

Scientific theories are hypotheses which have stood up to repeated attempts at falsification and are thus supported by a great deal of data and evidence. Some well known biological theories include the theory of evolution by natural selection, the cell theory (the idea that all organisms are made of cells), and the germ theory of disease (the idea that certain microbes cause certain diseases). The scientific community holds that a greater amount of evidence supports these ideas than contradicts them, and so they are referred to as theories. In fact, no evidence has been identified to disprove these or other scientific theories.

In every day use, people often use the word theory to describe a guess or an opinion. For example, “I have a theory as to why the light bulb is not working.” When used in this common way, “theory” does not have to be based on facts. It does not have to be based on a true description of reality. This usage of the word theory often leads to a misconception that can be best summed up by the phrase, "It's not a fact, it's only a theory." In such everyday usage, the word is most similar to the term hypothesis.

Scientific theories are the equivalent of what in everyday speech we would refer to as facts . In principle, scientific theories are always subject to corrections or inclusion in another, wider theory. As a general rule for use of the term, theories tend to deal with broader sets of phenomena than do hypotheses, which usually deal with much more specific sets of phenomena or specific applications of a theory.

A video discussing the difference between a hypothesis and a theory can be viewed at http://www.youtube.com/watch?v=jdWMcMW54fA (6:39).

Constructing Theories

In time, a confirmed hypothesis may become part of a theory or may grow to become a theory itself. Scientific hypotheses may be mathematical models. Sometimes they can be statements, stating that some particular instance of the phenomenon under examination has some characteristic and causal explanations. These theories have the general form of universal statements, stating that every instance of the phenomenon has a particular characteristic.

A hypothesis may predict the outcome of an experiment in a laboratory or the observation of a natural phenomenon. A hypothesis should also be falsifiable, and one cannot regard a hypothesis or a theory as scientific if it does not lend itself to being falsified, even in the future. To meet the “falsifiable” requirement, it must at least in principle be possible to make an observation that would disprove the hypothesis. A falsifiable hypothesis can greatly simplify the process of testing to determine whether the hypothesis can be proven to be false. Scientific methods rely heavily on the falsifiability of hypotheses by experimentation and observation in order to answer questions. Philosopher Karl Popper suggested that all scientific theories should be falsifiable; otherwise they could not be tested by experiment.

A scientific theory must meet the following requirements:

  • it must be consistent with a pre-existing theory in that the pre-existing theory has been experimentally verified, though it may often show a pre-existing theory to be wrong in an exact sense,
  • it must be supported by many strands of evidence rather than a single foundation, ensuring that it is probably a good approximation, if not totally correct.

Also, a theory is generally only taken seriously if it:

  • allows for changes to be made as new data are discovered, rather than claiming absolute certainty,
  • is the most straight forward explanation, and makes the fewest assumptions about a phenomenon (commonly called “passing the Occam's razor test”).

This is true of such established theories as those of special relativity, general relativity, quantum mechanics, plate tectonics, and evolution. Theories considered scientific meet at least most, but ideally all, of these extra criteria.

In summary, to meet the status of a scientific theory, the theory must be falsifiable or testable. Examples of scientific theories in different areas of science include:

  • Astronomy : Big Bang Theory
  • Biology : Cell Theory; Theory of Evolution; Germ Theory of Disease
  • Chemistry : Atomic Theory; Kinetic Theory of Gases
  • Physics : General Relativity; Special Relativity; Theory of Relativity; Quantum Field Theory
  • Earth Science : Giant Impact Theory; Plate Tectonics

Unverifiable Theories

The term theory is sometimes stretched to refer to theoretical speculation which is currently unverifiable. Examples are string theory and various theories of everything. String theory is a model of physics, which predicts the existence of many more dimensions in the universe than the four dimensions that current science understands (length, width, height, and space-time). The theory of everything is a hypothetical theory in physics that fully explains and links together all known physical phenomena.

For a scientific theory to be valid it must be verified experimentally. Many parts of the string theory are currently untestable due to the large amount of energy that would be needed to carry out the necessary experiments as well as the high cost of conducting these experiments. Therefore string theory may not be tested in the foreseeable future. Some scientists have asked if it even deserves to be called a scientific theory because it is not falsifiable.

Superseded Theories

A superseded, or obsolete, scientific theory is a theory that was once commonly accepted, but for whatever reason is no longer considered the most complete description of reality by mainstream science. It can also mean a falsifiable theory which has been shown to be false. Giraffes, shown in Figure below , are often used in the explanation of Lamarck's superseded theory of evolution. In Lamarckism, a giraffe is able to lengthen its neck over the course of its life, for example by stretching to reach higher leaves. That giraffe will then have offspring with longer necks. The theory has been superseded by the understanding of natural selection on populations of organisms as the main means of evolution (Darwin's theory of evolution by natural selection), not physical changes to a single organism over its lifetime.

Superseded theories like Lamarck’s theory of evolution are theories that are now considered obsolete and have been replaced by newer theories that have more evidence to support them; in Lamarck’s case, his theory was replaced by Darwin’s theory of evolution and natural selection, which will be further discussed in additional concepts.

Scientific Laws

Scientific laws are similar to scientific theories in that they are principles which can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. Usually scientific laws refer to rules for how nature will behave under certain conditions. Scientific theories are more overarching explanations of how nature works and why it exhibits certain characteristics.

A physical law or law of nature is a scientific generalization based on a sufficiently large number of empirical observations that it is taken as fully verified.

Isaac Newton's law of gravitation is a famous example of an established law that was later found not to be universal—it does not hold in experiments involving motion at speeds close to the speed of light or in close proximity of strong gravitational fields. Outside these conditions, Newton's laws remain an excellent model of motion and gravity.

Scientists never claim absolute knowledge of nature or the behavior of the subject of the field of study. A scientific theory is always open to falsification, if new evidence is presented. Even the most basic and fundamental theories may turn out to be imperfect if new observations are inconsistent with them. Critical to this process is making every relevant part of research publicly available. This allows peer review of published results, and it also allows ongoing reviews, repetition of experiments and observations by many different researchers. Only by meeting these expectations can it be determined how reliable the experimental results are for possible use by others.

Vocabulary

  • law of nature : A scientific generalization based on a sufficiently large number of empirical observations that it is taken as fully verified; physical law.
  • physical law : A scientific generalization based on a sufficiently large number of empirical observations that it is taken as fully verified; law of nature.
  • scientific law : A principle which can be used to predict the behavior of the natural world.
  • scientific theory : A hypothesis which has stood up to repeated attempts at falsification; supported by a great deal of data and evidence.
  • superseded theory : A theory that was once commonly accepted, but is no longer considered the most complete description of reality by mainstream science.

Summary

  • Scientific theories are hypotheses which have stood up to repeated attempts at falsification and are thus supported by much data and evidence.
  • Scientific laws are similar to scientific theories in that they are principles which can be used to predict the behavior of the natural world.
  • A scientific theory must be supported by many strands of evidence rather than a single foundation, ensuring that it is probably a good approximation, if not totally correct.
  • A superseded scientific theory is a theory that was once commonly accepted, but is no longer considered the most complete description of reality by mainstream science.

Practice

Use this resource to answer the questions that follow.

  1. Describe scientific theories and laws. Give examples.
  2. What is meant by the following statement: Theories can never be proven absolutely…

Review

  1. Identify two features that a theory must have, to qualify as a scientific theory.
  2. Give an example of a superseded theory.
  3. What is meant by the following statement: A hypothesis should be falsifiable.
  4. Distinguish between a scientific theory and a scientific law.
  5. Give examples of scientific theory from a variety of scientific fields.

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