Laplace Theorizes That the Solar System Originated from a Cloud of Gas

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Laplace Theorizes That the Solar System Originated from a Cloud of Gas

Overview

In Exposition du système du monde (Exposition of the System of the World) (1796), the French astronomer Marquis Pierre Simon de Laplace (1749-1827) briefly stated his "nebular hypothesis" that the Sun, planets, and their moons began as a whirling cloud of gas. This hypothesis sparked controversy among theologians and politicians as well as astronomers and physicists.

Background

After the pioneer astronomers Nicolaus Copernicus (1473-1543), Galileo Galilei (1564-1642), and Johannes Kepler (1571-1630) put forth their respective heliocentric (sun-centered) theories of the relationships among celestial bodies, Christianity was hard pressed to defend its traditional geocentric (earth-centered) cosmology. In the sixteenth and seventeenth centuries, the church suppressed heliocentric astronomy. Many scientists became disenchanted with religion. By the late seventeenth and early eighteenth centuries, astronomers were arguing publicly among themselves for and against the immanence of God, i.e., the presence and activity of God in the world.

Accepting heliocentrism did not entail disbelief in either God or Christianity. Nevertheless, some adjustments in the content of Christian doctrine became necessary because of heliocentrism. It prompted a Christian theological debate that lasted about two centuries. In the light of the new cosmology, some thinkers even came to question the reality of God.

By the beginning of the eighteenth century, atheism had become such a viable alternative that even some astronomers were engaged in arguing for the reality of God. Among them were Isaac Newton (1642-1727) and William Derham (1657-1735), an astronomer, physicist, entomologist, and Anglican priest who published several passionate defenses of God's immanence. Throughout history, relatively few great physicists and astronomers have been atheists. Albert Einstein (1879-1955) insisted that God is real and does not "play dice with the universe."

The eighteenth-century debate more clearly defined three possible stances: theism, the belief that God created the world and remains active in it; deism, the belief that God created the world but then left it alone; and atheism, which asserts that there is no God. Theists, concerned with God's immanence, sometimes accused deists of atheism. Atheists, convinced that there is no place for God at all, sometimes accused deists of theism. By the late eighteenth century, deism was dominant among intellectuals, so that the argument was no longer primarily between theism and deism, but between deism and atheism. It was in this theologically skeptical environment that Laplace arose.

The two greatest astronomers of Revolutionary and Napoleonic France were Laplace and his rival, Joseph Jérôme Le Français de Lalande (1732-1807), director of the Paris Observatory. Both were atheists. Lalande is reported to have said that because he had searched throughout the heavens with his telescope and found no God, therefore there is no God. He popularized his science and took his case for atheism to the public. Laplace thought Lalande was foolish to involve the common people in matters of astronomy. Laplace preferred the company of other scientists, intellectuals, and political leaders. He is supposed to have told Napoleon privately that he did not mention God in his Traité de mécanique céleste (Treatise on Celestial Mechanics) (1799-1825) because there was no need of that hypothesis.

A cogent, scientific, non-theological explanation of the origin of the solar system was not possible until scientists had absorbed the full import of Newton's law of universal gravitation and three laws of motion. Newton published these four laws in 1687 in Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy), one of the most important books of all time. Newtonian physics was also a key part of Laplace's environment.

Laplace attacked the traditional "design argument" for the reality of God. The locus classicus of this argument is the "Fifth Way" of Thomas Aquinas. It claims that God can be known by contemplating the natural phenomena of the world and reasoning that the world must have been designed by an intelligent being in order for these phenomena to interact the way they do. Both theism and deism are compatible with the design argument. According to the former, God continues actively in time as creator and sustainer after the primordial design and creation; but according to the latter, God designs and creates the world once, before time, then, like an architect whose commission is complete, abandons it to its fate.

Not considering any role for God at all, either as architect or as creator, Laplace suggested a possible origin of the solar system. He argued that as a hot nebula cooled, it would require a proportionately faster rate of rotation in order to conserve its angular momentum, and the increased centrifugal force resulting from this increased speed would throw out material which would eventually condense into planets. This suggestion is known as the "nebular hypothesis." Laplace did not speculate to the ultimate origin of the materials in the nebula.

Laplace was not the first thinker to propose a nebular hypothesis. In 1755 the German philosopher Immanuel Kant (1724-1804) published Allgemeine Naturgeschichte und Theorie des Himmels (General Natural History and Theory of the Heavens) in which he suggested that the gravitational forces in a slowly rotating nebula would gradually flatten it and create within it several denser clouds of gas which would separately compact themselves into distinct spheres, the Sun, and planets.

Kant's astronomical speculations may have been influenced by the Swedish theologian Emanuel Swedenborg (1688-1772), who in 1734 asserted that the solar system originated as a rapidly rotating nebula condensing and coalescing into the Sun and planets. Swedenborg claimed that this revelation came to him in a seance, not as the result of scientific inquiry. Neither Swedenborg nor Kant could say what started the gas cloud rotating in the first place. Gravity alone is not sufficient to do that.

Laplace derived his nebular hypothesis in part from the work of William Herschel (1738-1822) on the origin of stars. He was probably unaware of Kant's cosmology when he theorized about the solar system in the 1790s. Even though Kant and Laplace never collaborated, and even though there are significant differences between their respective versions of the nebular hypothesis, it is sometimes called the "Kant-Laplace hypothesis."

Impact

The Swedenborgian and Kantian versions of the nebular hypothesis were completely disproved in the twentieth century. The Laplacean version was still workable in 2000, and despite frequent refutations, it has just as frequently been revived and refined. The Hubble telescope has found evidence in the Orion Nebula, the Eagle Nebula, and other nebulae that supports it. When it is in favor, science progresses by deliberating it; when it is out of favor, its discredit provides fodder for creationists.

James Clerk Maxwell (1831-1879) and Sir James Hopwood Jeans (1877-1946) criticized Laplace's hypothesis on the grounds that the material thrown out by centrifugal force would not have enough mass to generate enough gravity to pull the material together into planets. In support of this criticism, Moulton noted that the Sun has 99.9% of the mass of the solar system while the planets have 99% of its angular momentum. Angular momentum measures the intensity of the motion of a rotating body. Moulton calculated that the Sun would have to rotate over 100 times faster than it actually does in order to conserve the angular momentum demanded by the nebular hypothesis. Therefore, he reasoned, the hypothesis must be false.

The nebular hypothesis was the first coherent nontheological explanation proposed for the origin of the solar system. Several others, some ephemeral, some with lasting importance, arose in the eighteenth century. The most important of these was the "collision hypothesis," proposed in 1778 by Count Georges Louis Leclerc de Buffon (1707-1788) and revived in various forms in the early twentieth century by Thomas Chrowder Chamberlin (1843-1928), Forest Ray Moulton (1872-1952), Jeans, Sir Harold Jeffreys (1891-1989), and others. It stated that the solar system could have arisen from the collision or near miss of two stars or a star and a comet, with the smaller debris cooling into planets and the larger debris becoming the Sun.

In 1900 Chamberlin and Moulton theorized that small meteors or particles of solid matter, not gas, pulled off from the Sun by a passing star or comet would eventually accumulate into planets by their mutual gravitational attraction. This was the "planetesimal hypothesis." In 1917 Jeans and Jeffreys proposed their "tidal hypothesis" that the gravity of a massive celestial body passing close to the Sun would pull out from the Sun a long stream of gaseous beads, which would subsequently cool into planets. Both the planetesimal and tidal hypotheses were attacked on the grounds that the material pulled off would be so hot that it would more likely disperse into space than coalesce into planets.

In 1944 Carl Friedrich von Weizsäcker (b. 1912), disappointed in both the planetesimal and the tidal hypotheses, resurrected Laplace's nebular hypothesis. Weizsäcker argued that the core of a nebula would collapse to form a sun while the outlying gases, being more turbulent because of the collapse of the core, would form eddies that would eventually condense into planets. He also tried to explain the origins of the nebulae themselves. Drawing upon the "big bang" theory of the origin of the universe that Georges Lemaître (1894-1966) introduced in the early 1930s, Weizsäcker claimed that expanding gases from the primeval explosion would randomly clump together and that the gravity within these clumps would condense them into nebulae.

Many subsequent theories of the origin of the solar system, such as the "cloud hypothesis" proposed in 1948 by Fred Whipple (1906- ) and the "protoplanet hypothesis" proposed in 1951 by Gerald P. Kuiper (1905-1973), depend to significant extents upon Weizsäcker's revival of Laplace's theory.

ERIC V.D. LUFT