Predictive power
The predictive power of a scientific theory
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...

 refers to its ability to generate testable
Testability, a property applying to an empirical hypothesis, involves two components: the logical property that is variously described as contingency, defeasibility, or falsifiability, which means that counterexamples to the hypothesis are logically possible, and the practical feasibility of...

 predictions. Theories with strong predictive power are highly valued, because the predictions can often encourage the falsification
Falsifiability or refutability of an assertion, hypothesis or theory is the logical possibility that it can be contradicted by an observation or the outcome of a physical experiment...

 of the theory. The concept of predictive power differs from explanatory
Explanatory power
Explanatory power is the ability of a theory to effectively explain the subject matter it pertains to. One theory is sometimes said to have more explanatory power than another theory about the same subject matter if it offers greater predictive power...

 and descriptive power (where phenomena that are already known are retrospectively explained by a given theory) in that it allows a prospective test of theoretical understanding.

Scientific ideas that do not confer any predictive power are considered at best "conjecture
A conjecture is a proposition that is unproven but is thought to be true and has not been disproven. Karl Popper pioneered the use of the term "conjecture" in scientific philosophy. Conjecture is contrasted by hypothesis , which is a testable statement based on accepted grounds...

s", or at worst "pseudoscience
Pseudoscience is a claim, belief, or practice which is presented as scientific, but which does not adhere to a valid scientific method, lacks supporting evidence or plausibility, cannot be reliably tested, or otherwise lacks scientific status...

". Because they cannot be tested or falsified in any way, there is no way to determine whether they are true or false, and so they do not gain the status of "scientific theory".

Theories whose "predictive power" presupposes technologies that are not currently possible constitute something of a grey area. For example, certain aspects of string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

 have been labeled as predictive, but only through the use of machines that have not yet been built and in some cases may never be possible. Whether or not this sort of theory can or should be considered truly predictive is a matter of scientific and philosophical debate.


A classic example of the predictive power of a theory is the Discovery of Neptune
Discovery of Neptune
Neptune was mathematically predicted before it was directly observed. With a prediction by Urbain Le Verrier, telescopic observations confirming the existence of a major planet were made on the night of September 23, 1846, and into the early morning of the 24th, at the Berlin Observatory, by...

 as a result of predictions made by mathematicians John Couch Adams
John Couch Adams
John Couch Adams was a British mathematician and astronomer. Adams was born in Laneast, near Launceston, Cornwall, and died in Cambridge. The Cornish name Couch is pronounced "cooch"....

 and Urbain Le Verrier, based on Newton's theory of gravity.

Other examples of predictive power of theories or models include Dmitri Mendeleev
Dmitri Mendeleev
Dmitri Ivanovich Mendeleev , was a Russian chemist and inventor. He is credited as being the creator of the first version of the periodic table of elements...

's use of his periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

 to predict previously undiscovered chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s and their properties (though largely correct, he misjudged the relative atomic masses of tellurium and iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

), and Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

's use of his knowledge of evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 by natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 to predict that because there existed a plant (Angraecum
The genus Angraecum, abbreviated as Angcm in horticultural trade, common name Angrek or Comet Orchid, contains about 220 species, some of them among most magnificent of all orchids. They are quite varied vegetatively and florally and are adapted to dry tropical woodland habitat and have quite...

) with a long spur in its flowers, a complementary animal with a 30 cm proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...

 must also exist to feed on and pollinate it (twenty years after his death Xanthopan morgani
Xanthopan morgani
Xanthopan morgani, or Morgan's Sphinx, is a very large hawk moth from West Africa and Madagascar. It is the sole member of its genus, and little is known of the biology, though the adults have been found to visit orchids .In January 1862 while researching insect pollination of orchids, Charles...

, a form of hawk moth, was found which did just that).

Another example of predictive power is the prediction of Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's General Theory of Relativity that the path of light would bend in the presence of a strong gravitational field. This was experimentally verified by an expedition to Sobral
-People:* José María Sobral , Argentine military scientist and Antarctic explorer* Leila Sobral , Brazilian basketball player* Marta Sobral , Brazilian female basketball player...

 in Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

 and the Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 island of Príncipe
Príncipe is the northern and smaller of the two major islands of the country of São Tomé and Príncipe lying off the west coast of Africa. It has an area of 136 km² and a population of approximately 5,000. The island is a heavily eroded volcano over three million years old, surrounded by other...

 to measure star positions during the solar eclipse of May 29, 1919, when observations made by the astrophysicist Arthur Eddington seemed to confirm Einstein's predictions. Although the measurements have been criticized by some as utilizing flawed methodology, modern reanalysis of the data suggests that Eddington's analysis of the data was accurate. Later, more precise measurements taken by radio
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

 confirmed the predictions to a high degree of accuracy.


The predictive power of a theory is closely related to applications.

General relativity not only predicts the bending of light, but also predicts several other phenomena. Recently, the calculation of proper time
Proper time
In relativity, proper time is the elapsed time between two events as measured by a clock that passes through both events. The proper time depends not only on the events but also on the motion of the clock between the events. An accelerated clock will measure a smaller elapsed time between two...

 of satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s has been a successfully-measured prediction, now incorporated into the method used to calculate positions via GPS.

If a theory has no predictive power, it cannot be used for applications.
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