The idea of a sphere-world was constructed by Henri Poincaré who, while pursuing his argument for

conventionalism Conventionalism is the philosophical attitude that fundamental principles of a certain kind are grounded on (explicit or implicit) agreements in society, rather than on external reality. Unspoken rules play a key role in the philosophy's structur ...

(see

philosophy of space and time Philosophy of space and time is the branch of philosophy concerned with the issues surrounding the ontology and epistemology of space and time. While such ideas have been central to philosophy from its inception, the philosophy of space and time wa ...

), offered a

thought experiment A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences. History The ancient Greek ''deiknymi'' (), or thought experiment, "was the most anc ...

about a

sphere A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is th ...

with strange properties.

The concept

Poincaré asks us to imagine a sphere of radius ''R''. The temperature of the sphere decreases from its maximum at the center to absolute zero at its extremity such that a body’s temperature at a distance ''r'' from the center is proportional to

R^2-r^2

. In addition, all bodies have the same coefficient of dilatation so every body shrinks and expands in similar proportion as they move about the sphere. To finish the story, Poincaré states that the index of refraction will also vary with the distance ''r'', in

inverse proportion In mathematics, two sequences of numbers, often experimental data, are proportional or directly proportional if their corresponding elements have a constant ratio, which is called the coefficient of proportionality or proportionality constan ...

R^2-r^2

. How will this world look to inhabitants of this sphere? In many ways it will look ''normal''. Bodies will remain intact upon transfer from place to place, as well as seeming to remain the same size (the Spherians would shrink along with them). The geometry, on the other hand, would seem quite different. Supposing the inhabitants were to view rods believed to be rigid, or measure

distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...

with light rays. They would find that a geodesic is not a straight line, and that the ratio of a circle’s circumference to its radius is greater than

2\pi

. These inhabitants would in fact determine that their universe is not ruled by

Euclidean geometry Euclidean geometry is a mathematical system attributed to ancient Greek mathematician Euclid, which he described in his textbook on geometry: the '' Elements''. Euclid's approach consists in assuming a small set of intuitively appealing axioms ...

, but instead by

hyperbolic geometry In mathematics, hyperbolic geometry (also called Lobachevskian geometry or Bolyai–Lobachevskian geometry) is a non-Euclidean geometry. The parallel postulate of Euclidean geometry is replaced with: :For any given line ''R'' and point ''P ...

Commentary

This

is discussed in

Roberto Torretti Roberto Torretti (February 15, 1930 - November 12, 2022) was a Chilean philosopher, author and academic who is internationally renowned for his contributions to the history of philosophy, philosophy of physics and philosophy of mathematics. Bio ...

's book ''Philosophy of Geometry from Riemann to Poincaré'' and in Jeremy Gray's article "Epistemology of Geometry" in the Stanford Encyclopedia of Philosophy. This sphere-world is also described in Ian Stewart's book ''

Flatterland ''Flatterland'' is a 2001 book written by mathematician and science popularizer Ian Stewart about non-Euclidean geometry. It was written as a sequel to ''Flatland'', an 1884 novel that discussed different dimensions. Plot summary Almost 100 ...

'' (chapter 10, Platterland).

The concept

Commentary

References

See also