Pièrre Simon Laplace: The nebular hypothesis
The history of ideas
Pierre Simon Laplace is famous for his concept that the solar system formed from a spinning cloud of gas. His theory is the combined result of a man’s philosophy, religion, and skills as an observer of the skies. It is important in looking at the Nebular Hypothesis that we understand something of the background of Laplace.
In 1749 Pierre was born to a middle class family in Beaumont-en-Auge, France. During his schooling he first made his mark by the pious views expressed in his theological essays. His immense intellect can be seen by a brief look at some of his achievements.
His academic career commenced at age 18, in 1767, with his appointment as Professor of Mathematics at the Ecole Militaire in Paris. His early scientific work was done in cooperation with Lavoisier. Together they demonstrated that the amount of heat required to decompose a compound into its elements, is equal to the heat given off when the compound is formed from its elements. Their work formed a basis for the future science of Thermochemistry.
In France during the 1770’s and 80’s, the complex and varied systems of measurements were notorious for both hampering trade and for endless fights in court. To end this confusion, the French Academy of Science appointed a committee to ‘standardize all weights and measures’. Laplace was chosen as a member.
It was decided that the new system should be ‘decimal’. Distance units, such as the meter, were chosen and represented fractions of the earth’s circumference. In the words of the committee this standard ‘was not arbitrary nor peculiar to any people in the world’, a thought that was intended to express the progressiveness of France after the revolution.
On September 10, 1799 the kilogram and the meter were adopted as standard. Laplace and Delambre ordered a medal struck which was inscribed ‘for all time and for all people’, an inscription portraying the type of humanism that had begun to dominate Laplace’s thinking, i.e. our work will last forever! The first volume of his influential Traité de la Mécanique Céleste, was published in 1799. Four more volumes appeared in the next 26 years.
Laplace spent much time working in astronomy. He was impressed by the remarkable order that existed in the solar system, especially the shape of the paths the planets traveled in space (near circular), and the existence of the planets in nearly one plane.
He studied in great detail the motions of the planets and attempted to explain why the solar system was stable. He proposed that as long as the solar system was isolated and the sun did not change radically, then the solar system was so stable that it should exist indefinitely. This stability was used to refute Newton’s belief that a Divine Force kept the solar system intact.
His involvement in astronomy led Laplace to speculate on the origin of the solar system, with the Nebular Hypothesis first appearing in his book Exposition of a World System published in 1796. Despite the pious attitudes expressed in his early days, Laplace had by this time, reached the conclusion that the stability so obvious in the solar system, would best be accounted for by a process of evolutionary chaos. Laplace had now become one of France’s foremost Monday to Saturday atheists, in spite of his believing attendance at mass every Sunday.
His theory is based largely on the observation that all then known planets revolved around the sun in the one direction. Laplace suggested that:
- The sun was originally a giant cloud of gas or nebulae that rotated evenly.
- The gas contracted due to cooling and gravity.
- This forced the gas to rotate faster, just as an ice skater rotates faster when his extended arms are drawn onto his chest.
- This faster rotation would throw off a rim of gas, which following cooling, would condense into a planet.
- This process would he repeated several times to produce all the planets.
- The asteroids between Mars and Jupiter were caused by rings which failed to condense properly.
- The remaining gas ball left in the center became the sun.
It is interesting to note that, apart from the Biblical account of creation this theory on the origin of the solar system has been adhered to longer than any other. It still has a wide acceptance, and it is currently being promoted in modified form by theorists such as Prentice (Australian National University).
Laplace’s work has had many criticisms, the more serious of which are listed below.
- It is now known that not all planets move in the same way. At the time of Laplace, Pluto and Neptune were unknown, and both of these planets rotate from E to W. All other planets rotate from W to E. This difference cannot be explained by a theory which produces all planets from a gas cloud rotating in one direction only.
- Laplace assumed the original cloud existed and was spinning. He did not attempt to explain where it came from and how it got to be that way.
- All known physics indicates that a cloud of gas in space will expand and not contract.
- The Scottish physicist, Maxwell (1831-1879) demonstrated that even a fluid ring in space would not condense into space but form a ring, such as around Saturn, or a belt of planetoids as in the asteroid belt.
- Studies on the angular momentum of the sun and the planets shows that 98% of this energy is involved in the movement of the planets. According to Laplace’s theory, most of the angular momentum should still be in the sun. This should have resulted from the fact that as the ball of gas contracted, the energy of motion was tied up in a smaller volume. The ball spun faster, flinging rings of matter from the outside. These rings, however, were only small in mass compared to the gas ball, and therefore would have taken only small amounts of energy from the gas.
- Laplace’s theory predicts that the sun should be spinning once every few hours, but it spins only once in approximately every 25 earth days.
- All planets formed from Laplace’s gas cloud lie in the plane of the sun’s equator, however several planets lie at angles to the sun’s plane.
- The major objection to this theory is best illustrated by a conversation Laplace had with Napoleon:
Emperor Napoléon Bonaparte (the man who commissioned the invention of margarine) inquired of Laplace after reading his theory, ‘Where does God fit into your system?’ Laplace replied, ‘Sire, I have no need for that hypothesis’.
The Nebular hypothesis represents the outworking of a man of great intellect who carefully studied and observed the evidence through eyes that were tied to a form of practical atheism. To Laplace, theology and science were independent forms of knowledge, and science was the better way of knowing.
Laplace’s comments to Napoleon were not a conclusion that God was not necessary, nor do they represent a belief that God did not exist. They represented the starting point around which he built his theories: God was simply irrelevant to the everyday world of matter and energy.