Editing Hollow Earth (section)

=== Variations ===

In "[[A Culture of Conspiracy]]", Political scientist [[Michael Barkun]] draws a distinction between the terms ''hollow earth'' and ''inner earth'', to differentiate materials that conceive the majority of the interior of the planet to be hollow, from those that view it as solid but [[honeycomb]]ed with interconnected spaces.<ref>{{cite book |last1=Michael Barkun |title=[[A Culture of Conspiracy]] |date=2003 |publisher=[[University of California Press]] |isbn=0-520-23805-2 |page=207 |edition=1st |author1-link=Michael Barkun }}</ref><ref>{{cite book |last1=Kafton-Minkel |first1=Walter |title=Subterranean Worlds: 100,000 Years of Dragons, Dwarfs, the Dead, Lost Races & UFOs from Inside the Earth |date=1989 |publisher=[[Loompanics]] |location=[[Port Townsend]] |isbn=9781559500159 |pages=44–55 |url=https://shwetathanki.files.wordpress.com/2020/05/subterranean-worlds-walter-kafton-minkel-100000-years-of-dragons-dwarfs-the-dead-lost-races-ufos-from-inside-the-earth.pdf}}</ref><ref>{{cite book |last1=Radner, Radne |first1=Daisie, Michael |title=Science and Unreason |date=1982 |publisher=[[Cengage Group|Wadsworth]] |location=[[Belmont, California]] |isbn=9780534011536 |pages=48–50}}</ref>

==== Concave Hollow Earths ====
[[File:Concave hollow Earths.svg|thumb|200px|An example of a concave hollow Earth. Humans live on the interior, with the universe in the center.]]

Instead of saying that humans live on the exterior surface of a hollow planet, sometimes called a "convex" Hollow Earth hypothesis, it is hypothesized humans live on the ''interior'' surface. This has been called the "concave" Hollow Earth hypothesis or skycentrism.

[[Cyrus Teed]], a doctor from upstate New York, proposed such a concave Hollow Earth in 1869, calling his scheme "Cellular Cosmogony".<ref name="wired">{{cite magazine|url=https://www.wired.com/2014/07/fantastically-wrong-hollow-earth/ |title=Fantastically Wrong: The Legendary Scientist Who Swore Our Planet Is Hollow &#124; WIRED |magazine=Wired |publisher=wired.com|access-date=1 November 2015|date=2014-07-02 }}</ref> Teed founded a group called the [[Koreshan Unity]] based on this notion, which he called [[Koreshanity]]. The main colony survives as a preserved Florida state historic site, at [[Estero, Florida]], but all of Teed's followers have now died. Teed's followers claimed to have experimentally verified the concavity of the Earth's curvature, through surveys of the Florida coastline making use of "rectilineator" equipment.

Several 20th-century German writers, including [[Peter Bender]], Johannes Lang, Karl Neupert, and Fritz Braut, published works advocating the Hollow Earth hypothesis, or ''Hohlweltlehre''. It has even been reported, although apparently without historical documentation, that [[Adolf Hitler]] was influenced by concave Hollow Earth ideas and sent an expedition in an unsuccessful attempt to spy on the British fleet by pointing infrared cameras up at the sky.<ref>{{cite journal | last = Kuiper | first = Gerard. P. | author-link = Gerard Kuiper
| title = German Astronomy during the War | journal = Popular Astronomy
| volume = 54 | pages = 263–286
| date = June 1946
| bibcode = 1946PA.....54..263K
}} See pp. 277–78.</ref><ref>{{cite book |first=William |last=Yenne |chapter=Adolf Hitler and the Concave Earth Cult |title=Secret Weapons of World War II: The Techno-Military Breakthroughs That Changed History |location=New York |publisher=Berkley Books |year=2003 |pages=271–272 |isbn=978-0-425-18992-4 }}</ref>

The [[Egypt]]ian mathematician [[Mostafa A. Abdelkader|Mostafa Abdelkader]] wrote several scholarly papers working out a detailed mapping of the Concave Earth model.<ref>{{cite journal|first=M.|last=Abdelkader|title=A Geocosmos: Mapping Outer Space Into a Hollow Earth|issue=6|journal=Speculations in Science & Technology|pages=81–89|year=1983}}</ref><ref>''Notices of the American Mathematical Society,'' (Oct. 1981 and Feb. 1982).</ref> In his book ''On the Wild Side'' (1992), [[Martin Gardner]] discusses the Hollow Earth model articulated by Abdelkader. According to Gardner, this hypothesis posits that light rays travel in circular paths, and slow as they approach the center of the spherical star-filled cavern. No energy can reach the center of the cavern. A drill, Gardner says, would lengthen as it traveled away from the cavern and eventually pass through the "point at infinity" corresponding to the center of the Earth. Gardner notes that "most mathematicians believe that an inside-out universe, with properly adjusted physical laws, is empirically irrefutable". Gardner rejects the concave Hollow Earth hypothesis on the basis of [[Occam's razor]].<ref>''On the Wild Side'' (1992), Martin Gardner, pp. 18–19</ref>

Purportedly verifiable hypotheses of a Concave Hollow Earth need to be distinguished from a thought experiment which defines a [[Coordinates (elementary mathematics)|coordinate]] transformation such that the interior of the Earth becomes "exterior" and the exterior becomes "interior". (For example, in spherical coordinates, let radius ''r'' go to ''R''<sup>2</sup>/''r'' where ''R'' is the Earth's radius; see [[inversive geometry]].) The transformation entails corresponding changes to the forms of physical laws. This is not a hypothesis but an illustration of the fact that any description of the physical world can be equivalently expressed in more than one way.<ref name="gardnertransformation">On the Wild Side, 1992, [[Martin Gardner]].</ref>