Experimentum crucis

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In the sciences, an experimentum crucis (English: crucial experiment or critical experiment) is an experiment capable of decisively determining whether or not a particular hypothesis or theory is superior to all other competing hypotheses or theories. In particular, such an experiment must typically be able to produce a result that rules out all other hypotheses or theories if true, thereby demonstrating that under the conditions of the experiment (i.e., under the same external circumstances and for the same "input variables" within the experiment), the competing hypotheses and theories are proven false, but the experimenter's hypothesis is not ruled out. Francis Bacon in his Novum Organum first described the concept of a situation in which one theory but not others would hold true, using the name instantia crucis; the phrase experimentum crucis, denoting the deliberate creation of such a situation for the purpose of testing the rival theories, was later coined by Robert Hooke and then famously used by Isaac Newton.

The production of such an experiment, if possible, is considered necessary for a particular hypothesis or theory to be considered an established part of the body of scientific knowledge. It is not unusual in the history of science for theories to be developed fully before producing a critical experiment. A given theory which is in accordance with known experiment but which has not yet produced a critical experiment is typically considered worthy of exploration in order to discover such an experimental test.

Some sciences are not fully amenable to an experimentum crucis, for example stellar evolution, for the simple reasons that it would take too long to run the experiment and the apparatus would be too large. Another example is field-testing testing global warming climate models, which would require additional Earths. In these cases, evidence is built up by other means and applied against the theory.

For one famous example, in his Opticks, Newton describes an optical experimentum crucis in the First Book, Part I, Proposition II, Theorem II, Experiment 6, to prove that sunlight consists of rays that differ in their wavelength, by decomposing the sunlight into its differnt colors. He did this by passing the ray of sunshine through a prism, showing that the different colors bend at different angles due to the dispersion of the prism material's index of refraction.

Isaac Newton performing his crucial prism experiment - the 'experimentum crucis' - in his Woolsthorpe Manor bedroom. Acrylic painting by Sascha Grusche (17 Dec 2015)

Poor examples

A famous example in the 20th century of an attempted experimentum crucis was the expedition led by Arthur Eddington to Principe Island in Africa in 1919 to record the positions of stars around the Sun during a solar eclipse. The observation of star positions were said to confirm predictions of gravitational lensing made by Albert Einstein in the general theory of relativity published in 1915. Eddington's observations were considered to be the first solid evidence in favor of Einstein's theory. Later analysis by others of Eddington's data reduction methods, his photographic plates and film emulsions, his camera mount and steering sytem, his lensing, together with the bad weather that day, show that he had faked his conclusion "when he cherry-picked among his observations of an eclipse".[1] [2]

Had the experiment, or more precisely the observation, been successful, it would not have qualified as exclusive as there are other theories of starlight bending.

Exclusion

In some cases, a proposed theory can account for existing anomalous experimental results for which no other existing theory can furnish an explanation. An example would be the ability of the quantum hypothesis, proposed by Max Planck in 1900, to account for the observed black-body spectrum, an experimental result that the existing classical Rayleigh–Jeans law could not predict. Such cases are not considered strong enough to fully establish a new theory, however, and in the case of quantum mechanics, it took the confirmation of the theory through new predictions for the theory to gain full acceptance.

Alternate views

For an opposite view putting into question the decisive value of the experimentum crucis in choosing one hypothesis or theory over its rival see Pierre Duhem. Duhem's name is given to the under-determination or Duhem–Quine thesis, which holds that for any given set of observations there is an innumerably large number of explanations. It is, in essence, the same as Hume's critique of induction: all three variants point at the fact that empirical evidence cannot force the choice of a theory or its revision. Possible alternatives to induction are Duhem's instrumentalism and Popper's thesis that we learn from falsification.

As popular as the Duhem–Quine thesis may be in the philosophy of science, in reality Pierre Duhem and Willard Van Orman Quine stated very different theses. Pierre Duhem believed that experimental theory in physics is fundamentally different from fields like physiology and certain branches of chemistry. Also Duhem's conception of theoretical group has its limits, since not all concepts are connected to each other logically. He did not include at all a priori disciplines such as logic and mathematics within these theoretical groups in physics which can be tested experimentally. Quine, on the other hand, conceived this theoretical group as a unit of a whole human knowledge. To Quine, even mathematics and logic must be revised in light of recalcitrant experience, a thesis that Duhem never held.

A quote of Duhem on physics:

A theory of physics is not an explanation. It is a system of mathematical propositions, deduced from a small number of principles, which have for their aim to represent as simply, as completely and as exactly as possible, a group of experimental laws.[3]

See also

Notes

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