The Hollow Earth hypothesis proposes that
the planet Earth is either entirely hollow or otherwise contains a substantial
interior space. The hypothesis has been shown to be wrong by observational
evidence, as well as by the modern understanding of planet formation; the
scientific community has dismissed the notion since at least the late 18th
century.
The concept of a hollow Earth still recurs
in folklore and as the premise for subterranean fiction, a subgenre of
adventure fiction. It is also featured in some present-day pseudoscientific and
conspiracy theories.
Concave hollow Earths
Instead of saying that humans live on the
outside surface of a hollow planet, sometimes called a "convex"
hollow-Earth hypothesis, some have claimed that our universe itself lies in the
interior of a hollow world, calling this a "concave" hollow-Earth
hypothesis. The surface of the Earth, according to such a view, might resemble
the interior shell of a sphere.
Purportedly verifiable hypotheses of a
"concave hollow Earth" need to be distinguished from a thought
experiment which defines a 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 R²/r where R is the Earth's radius.) 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.
Cyrus Teed, a doctor from upstate New York,
proposed such a concave hollow Earth in 1869, calling his scheme "Cellular
Cosmogony". 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 twentieth-century German writers,
including Peter Bender, Johannes Lang, Karl Neupert, and Fritz Braun, 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 (Wagner, 1999).
The Egyptian mathematician Mostafa
Abdelkader wrote several scholarly papers working out a detailed mapping of the
concave Earth model.
In one chapter of 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, which
corresponds to no point a finite distance away from Earth in the widely
accepted scientific cosmology. 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 in the widely accepted
scientific cosmology. Supposedly no experiment can distinguish between the two
cosmologies.
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.
Contrary evidence
Seismic
The picture of the structure of the earth
that has been arrived at through the study of seismic waves is quite different
from the hollow earth theory. The Earth's interior is made up of layers of
molten rock and various elements, in a mantle and core.
Gravity
Another set of scientific arguments against
a hollow Earth or any hollow planet comes from gravity. Massive objects tend to
clump together gravitationally, creating non-hollow spherical objects we call
stars and planets. The solid sphere is the best way in which to minimize the
gravitational potential energy of a physical object; having hollowness is
unfavorable in the energetic sense. In addition, ordinary matter is not strong
enough to support a hollow shape of planetary size against the force of
gravity; a planet-sized hollow shell with the known, observed thickness of the
Earth's crust, would not be able to achieve hydrostatic equilibrium with its
own mass and would collapse.
Someone on the inside of a hollow Earth
would not experience a significant outward pull and could not easily stand on
the inner surface; rather, the theory of gravity implies that a person on the
inside would be nearly weightless. This was first shown by Newton, whose shell
theorem mathematically predicts a gravitational force (from the shell) of zero
everywhere inside a spherically symmetric hollow shell of matter, regardless of
the shell's thickness. A tiny gravitational force would arise from the fact
that the Earth does not have a perfectly symmetrical spherical shape, as well
as forces from other bodies such as the Moon. The centrifugal force from the
Earth's rotation would pull a person (on the inner surface) outwards if the
person was traveling at the same velocity as the Earth's interior and was in
contact with the ground on the interior, but even the maximum centrifugal force
at the equator is only 1/300 of ordinary Earth gravity.
The mass of the planet also indicates that
the hollow Earth hypothesis is unfeasible. Should the Earth be largely hollow,
its mass would be much lower and thus its gravity on the outer surface would be
much lower than it is.
Direct observation
The
deepest hole drilled to date is the SG-3 borehole which is 12.3 km (7.6 mi)
deep, part of the Soviet Kola Superdeep Borehole project; thus, visual
knowledge of the Earth's structure extends that far.
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