In science, an ''experimentum crucis'' ( English: crucial experiment or critical experiment) is an

experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...

capable of decisively determining whether or not a particular

hypothesis A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educated guess o ...

theory A theory is a systematic and rational form of abstract thinking about a phenomenon, or the conclusions derived from such thinking. It involves contemplative and logical reasoning, often supported by processes such as observation, experimentation, ...

is superior to all others whose acceptance is currently widespread in the scientific community. In particular, such an experiment—if true—must typically be able to produce a result that rules out all other hypotheses or theories, 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), those hypotheses and theories are ''proven false'' but the experimenter's hypothesis ''is not ruled out''. An opposite view, rejecting the decisive value of the ''experimentum crucis'' in choosing one hypothesis or theory over its rivals, is the

Duhem–Quine thesis In philosophy of science, the Duhem–Quine thesis, also called the Duhem–Quine problem, says that unambiguous falsifications of a scientific hypothesis are impossible, because an empirical test of the hypothesis requires one or more back ...

History

Francis Bacon Francis Bacon, 1st Viscount St Alban (; 22 January 1561 – 9 April 1626) was an English philosopher and statesman who served as Attorney General and Lord Chancellor of England under King James I. Bacon argued for the importance of nat ...

in his ''

Novum Organum The ''Novum Organum'', fully ''Novum Organum, sive Indicia Vera de Interpretatione Naturae'' ("New organon, or true directions concerning the interpretation of nature") or ''Instaurationis Magnae, Pars II'' ("Part II of The Great Instauratio ...

'' first described the concept of a situation in which one theory but not others would hold true, using the name ''instantia crucis'' ("crucial instance"). 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 Robert Hooke (; 18 July 16353 March 1703) was an English polymath who was active as a physicist ("natural philosopher"), astronomer, geologist, meteorologist, and architect. He is credited as one of the first scientists to investigate living ...

and then famously used by

Isaac Newton Sir Isaac Newton () was an English polymath active as a mathematician, physicist, astronomer, alchemist, theologian, and author. Newton was a key figure in the Scientific Revolution and the Age of Enlightenment, Enlightenment that followed ...

and

Robert Boyle Robert Boyle (; 25 January 1627 – 31 December 1691) was an Anglo-Irish natural philosopher, chemist, physicist, Alchemy, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the foun ...

. The production of such an experiment 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.

Examples

was the first person to hail an experiment as ''experimentum crucis'' when he referred to the famous mercury barometer experiment on Puy-de-Dome in 1648. This experiment settled the question: Was there some natural resistance to the creation of an apparently empty space at the top of the tube, or was the height of the mercury determined solely by the weight of the air? In his ''

Philosophiæ Naturalis Principia Mathematica (English: ''The Mathematical Principles of Natural Philosophy''), often referred to as simply the (), is a book by Isaac Newton that expounds Newton's laws of motion and his law of universal gravitation. The ''Principia'' is written in Lati ...

'',

(1687) presents a disproof of Descartes' vortex theory of the motion of the planets. In his ''

Opticks ''Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light'' is a collection of three books by Isaac Newton that was published in English language, English in 1704 (a scholarly Latin translation appeared in 1706). ...

'', 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

index of refraction In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

. A 19th-century example was the prediction by Poisson, based on Fresnel's mathematical analysis, that the

wave theory of light In physics, physical optics, or wave optics, is the branch of optics that studies interference, diffraction, polarization, and other phenomena for which the ray approximation of geometric optics is not valid. This usage tends not to include effec ...

predicted a bright spot in the center of the shadow of a perfectly circular object, a result that could not be explained by the (then current) particle theory of light. An experiment by

François Arago Dominique François Jean Arago (), known simply as François Arago (; Catalan: , ; 26 February 17862 October 1853), was a French mathematician, physicist, astronomer, freemason, supporter of the Carbonari revolutionaries and politician. Early l ...

showed the existence of this effect, now called the Arago spot, or "Poisson's bright spot", which led to the acceptance of the wave theory. A famous example in the 20th century of an ''experimentum crucis'' was the expedition led by

Arthur Eddington Sir Arthur Stanley Eddington, (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lu ...

to Principe Island in

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

in 1919 to record the positions of stars around the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

during a

solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...

(see Eddington experiment). The observation of star positions confirmed predictions of

gravitational lensing A gravitational lens is matter, such as a galaxy cluster, cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's Ge ...

made by

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

in the

general theory of relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physi ...

published in 1915. Eddington's observations were considered to be the first solid evidence in favor of Einstein's theory. 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 Max Karl Ernst Ludwig Planck (; ; 23 April 1858 – 4 October 1947) was a German Theoretical physics, theoretical physicist whose discovery of energy quantum, quanta won him the Nobel Prize in Physics in 1918. Planck made many substantial con ...

in 1900, to account for the observed black-body spectrum, an experimental result that the existing classical

Rayleigh–Jeans law In physics, the Rayleigh–Jeans law is an approximation to the spectral radiance of electromagnetic radiation as a function of wavelength from a black body at a given temperature through classical arguments. For wavelength ''λ'', it is B_\l ...

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.

DNA, experimentum crucis

:''See §Context for crucial experiment in the discovery of the §structure of DNA, and §List of experiments in biology'' In the discovery of the significance of the structure of DNA, the fact that DNA was a double helix enabled the discoverers, Francis Crick and James Watson, to suggest that one strand of the double helix could serve as the template for the second strand, as the second strand was being duplicated. This explained the ''secret of life'', how the structure of DNA could serve as the

mechanism Mechanism may refer to: *Mechanism (economics), a set of rules for a game designed to achieve a certain outcome **Mechanism design, the study of such mechanisms *Mechanism (engineering), rigid bodies connected by joints in order to accomplish a ...

for the

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

(the

genetic code Genetic code is a set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished ...

), in which four

nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

s serve to encode the sequence of

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s needed to catalyze the production of macromolecules in the cell, and which led to its application in

synthetic biology Synthetic biology (SynBio) is a multidisciplinary field of science that focuses on living systems and organisms. It applies engineering principles to develop new biological parts, devices, and systems or to redesign existing systems found in nat ...

, in

genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of Genetic engineering techniques, technologies used to change the genet ...

, in

forensics Forensic science combines principles of law and science to investigate criminal activity. Through crime scene investigations and laboratory analysis, forensic scientists are able to link suspects to evidence. An example is determining the time and ...

genetic testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...

genomics Genomics is an interdisciplinary field of molecular biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, ...

and

pharmaceutical Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy ( pharmacotherapy) is an important part of the ...

s, among other industries.

Tanis fossil site

In the 21st century, the discovery of the Tanis fossil site, a killing field in the

Hell Creek formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...

of North Dakota, proved that the K-T boundary (now known as the KPg, or the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event, also known as the K–T extinction, was the extinction event, mass extinction of three-quarters of the plant and animal species on Earth approximately 66 million years ago. The event cau ...

) was the same event (the

Chicxulub impact The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). I ...

) which killed off the

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

s. This

impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...

was previously hypothesized from the global existence of

iridium Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group, is considered the second-densest naturally occurring metal (after osmium) with a density ...

deposits (a rare element on Earth). In this case, the existence of a microtektite layer raining down upon the multiple intermixed species (including a

Triceratops ''Triceratops'' ( ; ) is a genus of Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous Period (geology), period, about 68 to 66 million years ago on the island ...

) which were found at the site (the Tanis Konservat-Lagerstätte) served as the conclusive witness, as cited in ''Science Daily''.University of California - Berkeley: (29 March 2019) 66-million-year-old deathbed linked to dinosaur-killing meteor
/ref> Based on the dating of the Tanis, the event occurred 65.76 million years ago (± 0.15 My).

Theory of Experimentum Crucis

There's an emerging scholarship extending understanding and evaluation of experiments that fit into this category. J. A. Lohne tracks the development of the idea from

's 1620 Instantie Crucis through the various

prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

experiments and discussions of 1722. An early indicator of a theory of Experimentum Crucis appears in

John Locke John Locke (; 29 August 1632 (Old Style and New Style dates, O.S.) – 28 October 1704 (Old Style and New Style dates, O.S.)) was an English philosopher and physician, widely regarded as one of the most influential of the Enlightenment thi ...

's Doctrine of Abstraction. Lorne Falkenstein, reviewing Van Cleve expands the discussion of Experimentum crucis to the more general philosophical realm of

Property dualism Property dualism describes a category of positions in the philosophy of mind which hold that, although the world is composed of just one kind of Substance theory, substance—Materialism, the physical kind—there exist two distinct kinds of pro ...

Notes