There are differing definitions of fringe science. By one definition (see below) it may be valid science but not considered mainstream in one way or another; by another broader definition fringe science is generally viewed as a negative description involving what is believed to be a questionable scientific approach which may be considered too speculative, or questionable concerning: mainstream postulates, theory, hypothesis, explanations, methods, interpretations, conclusions, etc.
- Fringe science is scientific inquiry in an established field of study that departs significantly from mainstream or orthodox theories, and is classified in the "fringes" of a credible mainstream academic discipline.
- Three classifications of scientific ideas have been identified (center, frontier, fringe) with mainstream scientists typically regarding fringe concepts as highly speculative or even strongly refuted. However, according to Rosenthal "Accepted science may merge into frontier science, which in turn may merge into more far-out ideas, or fringe science. Really wild ideas may be considered beyond the fringe, or pseudoscientific." 
- A particular concept that was once accepted by the mainstream scientific community can become fringe science because of a later evaluation of previously supportive research. For example the idea that focal infections of the tonsils or teeth were a primary cause of systemic disease was once considered medical fact, but is now dismissed for lack of evidence. Conversely, fringe science can include novel proposals and interpretations that initially have only a few supporters and much opposition. Some theories developed on the fringes (for example, continental drift, existence of Troy, heliocentrism, the Norse colonization of the Americas, and Big Bang Theory) have become mainstream because of the discovery of supportive evidence.
- Fringe science covers everything from novel hypotheses that can be tested via the scientific method to wild ad hoc theories and "New Age mumbo jumbo" with the dominance of the latter resulting in the tendency to dismiss all fringe science as the domain of pseudoscientists, hobbyists, or quacks. Other terms used for the portions of fringe science that lack scientific integrity are pathological science, voodoo science, and cargo cult science. Junk science is a term typically used in the political arena to describe ideas that proponents erroneously, for political reasons, dubiously or even fraudulently claim scientific backing for.
In the philosophy of science, the question of where to properly draw a boundary between science and non-science, when the objective actually is objectivity, is called the demarcation problem. Compounding this issue is that proponents of some fringe theories use both proper scientific evidence and outlandish claims to support their arguments.
Fringe science is used to describe unusual theories and models of discovery. Those who develop such fringe science ideas may work within the scientific method, but their results are not accepted by the mainstream community. Usually the evidence provided by supporters of a fringe science is believed only by a minority and rejected by most experts. Fringe science may be advocated by a scientist who has a degree of recognition by the larger scientific community (typically through the publication of peer reviewed studies by the scientist), but this is not always the case. While most fringe science views are ignored or rejected, through careful use of the scientific method, including falsificationism, the scientific community has come to accept some ideas from fringe sciences. One example of such is plate tectonics, an idea that had its origin as a fringe science of continental drift, and was held in a negative opinion for decades. It is noted that:
- The confusion between science and pseudoscience, between honest scientific error and genuine scientific discovery, is not new, and it is a permanent feature of the scientific landscape [...] Acceptance of new science can come slowly.
The phrase fringe science can be considered pejorative. For example, Lyell D. Henry, Jr. wrote, "'fringe science' [is] a term also suggesting kookiness." Such characterization is perhaps inspired by the eccentric behavior of many researchers on the fringe of science (colloquially and with considerable historical precedent known as mad scientists). The categorical boundary between fringe science and pseudoscience can be disputed. The connotations of fringe science are that the enterprise is still rational, but an unlikely avenue for future results. Fringe science may not be a part of the scientific consensus for a variety of reasons, including incomplete or contradictory evidence.
Some historical ideas that are considered refuted by mainstream science include:
- Wilhelm Reich's work with orgone, a physical energy he claimed to have discovered, contributed to his alienation from the psychiatric community and eventually to his jailing. At the time and continuing today, other scientists and skeptics disputed Reich's claims that he had scientific evidence for the existence of orgone. Nevertheless, dedicated amateurs and a few fringe researchers continue to believe that Reich was correct.
- Focal infection theory as a primary cause of systemic disease rapidly became accepted by mainstream dentistry and medicine after World War I, largely on the basis of what later turned out to be fundamentally flawed studies providing evidence to support the theory. As a result millions of people were subjected to needless dental extractions and surgeries. While certain mainstream study continues on certain aspects of FIT, the original approach and science of FIT started falling out of favor in the 1930s and was relegated to the fringe of oral medicine by the late 1950s.
- Clovis First theory: The idea that the Clovis was the first culture in North America was long regarded as mainstream until mounting evidence of pre-Clovis occupation of the Americas discredited it.
Relatively recent fringe sciences include:
- Aubrey de Grey, featured in a 2006 60 Minutes special report, is working on advanced studies in human longevity, dubbed "Strategies for Engineered Negligible Senescence" (SENS). Many mainstream scientists believe that his research, especially de Grey's view on the importance of nuclear (epi)mutations and his purported timeline for antiaging therapeutics, constitutes "fringe science".
- Technology Review controversy: In an article released in a 2006 issue of the magazine Technology Review (part of a larger series), it was written that "SENS De Grey's hypothesis is highly speculative. Many of its proposals have not been reproduced, nor could they be reproduced with today's scientific knowledge and technology. Echoing Myhrvold, we might charitably say that de Grey's proposals exist in a kind of antechamber of science, where they wait (possibly in vain) for independent verification. SENS does not compel the assent of many knowledgeable scientists; but neither is it demonstrably wrong".
- A nuclear fusion reaction called cold fusion occurring near room temperature and pressure was reported by chemists Martin Fleischmann and Stanley Pons in March 1989. Numerous research efforts at the time were unable to replicate these results. Subsequently, a number of scientists with a variety of credentials have worked on the problem or participated in international conferences on cold fusion. In 2004, the United States Department of Energy decided to take another look at cold fusion to determine whether their policies towards the subject should be altered because of new experimental evidence, and commissioned a panel on cold fusion.
- The theory of abiogenic petroleum origin holds that natural petroleum was formed from deep carbon deposits, perhaps dating to the formation of the Earth. The ubiquity of hydrocarbons in the solar system is taken as evidence that there may be a great deal more petroleum on Earth than commonly thought, and that petroleum may originate from carbon-bearing fluids that migrate upward from the mantle. Abiogenic hypotheses saw a revival in the last half of the twentieth century by Russian and Ukrainian scientists, and more interest has been generated in the West after the publication by Thomas Gold in 1999 of The Deep Hot Biosphere. Gold's version of the hypothesis is partly based on the existence of a biosphere composed of thermophile bacteria in the Earth's crust, which may explain the existence of certain biomarkers in extracted petroleum.
Responding to fringe science
Michael W. Friedlander suggests some guidelines for responding to fringe science, which he argues is a more difficult problem to handle, "at least procedurally", than scientific misconduct. His suggested methods include impeccable accuracy, checking cited sources, not overstating orthodox science, thorough understanding of the Wegener continental drift example, examples of orthodox science investigating radical proposals, and prepared examples of errors from fringe scientists.
Though there are examples of mainstream scientists supporting maverick ideas within their own discipline of expertise, fringe science theories and ideas are often advanced by individuals either without a traditional academic science background, or by researchers outside the mainstream discipline, although the history of science shows that scientific progress is often marked by interdisciplinary and multicultural interaction. Friedlander suggests that fringe science is necessary for mainstream science "not to atrophy", as scientists must evaluate the plausibility of each new fringe claim and certain fringe discoveries "will later graduate into the ranks of accepted" while others "will never receive confirmation". The general public has difficulty distinguishing between "science and its imitators", and in some cases a "yearning to believe or a generalized suspicion of experts is a very potent incentive to accepting pseudoscientific claims".
Towards the end of the 20th century, religiously inspired critics[who?], such as Answers in Genesis, began to cite fringe science theories with limited support. The goal was frequently to classify as "controversial" entire fields of scientific inquiry (notably paleo-anthropology, human sexuality, evolution, geology, and paleontology) that contradicted literal or fundamentalist interpretation of various sacred texts. Describing ongoing debate and research within these fields as evidence of fundamental weaknesses or flaws, these critics argued that "controversies" left open a window for the plausibility of divine intervention and intelligent design. As Donald E. Simanek asserts, "Too often speculative and tentative hypotheses of cutting edge science are treated as if they were scientific truths, and so accepted by a public eager for answers," ignorant of the fact that "As science progresses from ignorance to understanding it must pass through a transitionary phase of confusion and uncertainty." The media also play a role in the creation and propagation of the view that certain fields of science are "controversial". In "Optimising public understanding of science: A comparative perspective" by Jan Nolin et al., the authors claim, "From a media perspective it is evident that controversial science sells, not only because of its dramatic value but also since it is often connected to high-stake societal issues."
- List of topics characterized as pseudoscience
- Obsolete scientific theory
- Paradigm shift
- Philosophy of science
- Sociology of scientific knowledge (SSK)
- Scientific misconduct
- Science and technology studies (STS)
- Fringe theory
- The Structure of Scientific Revolutions
- Journal of Scientific Exploration
- Fringe, a US series revolving around fringe science
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