In 1950, Immanuel Velikovsky published Worlds in Collision, a book which attempts to explain certain Biblical events by proposing a theory of the origin of the planet Venus, among other things. Twenty-nine years later, the astronomer Carl Sagan published a critique of Velikovsky's central claims.[Sagan, pp. 81-127]. Velikovsky asserts that around 1500 B.C. the planet Jupiter erupted and sent forth a huge comet which grazed Earth (perhaps twice grazed the earth--the text isn't clear), continued on and nearly collided with Mars which was in turn knocked out of orbit and sent flying toward Earth. Mars then also grazed Earth twice and continued on until it settled into its current orbit. The giant comet, meanwhile, ended up orbiting the Sun and became the planet Venus.
Velikovsky made the above claims in order to explain certain events in the Bible. The comet, he says, caused the plagues of flies and frogs on the Egyptians described in Exodus. The comet also caused the Nile to turn red, and produced earthquakes that leveled Egyptian (but not Hebrew) buildings. The comet also caused the Red Sea to part when the Israelites were being chased by the Egyptian army, allowing the former to escape. The comet also left a trail of hydrocarbons or carbohydrates (the text differs from place to place) in the sky, which fell on the desert for forty years, providing the wandering Jews with either bread or motor oil as their `manna' from heaven.
According to Velikovsky, the comet also caused the Earth to stop rotating (when Joshua said the sun stood still), assisting Joshua in battle. The movement of Mars accounts for the destruction of the Assyrian army by the Israelites. Then, somehow, the Earth began rotating again exactly as before.
One of the characteristics of a reasonable explanation is that it be a likely story. To be reasonable, it is not enough that an explanation simply be a possible account of phenomena. It has to be a likely account. To be likely, an account usually must be in accordance with current knowledge and beliefs, with the laws and principles of the field in which the explanation is made. An explanation of how two chemicals interact, for example, would be unreasonable if it violated basic principles in chemistry. Those principles, while not infallible, have not been developed lightly, but after generations of testing, observations, refutations, more testing, more observations, etc. To go against the established principles of a field puts a great burden of proof on the one who goes against those principles. This is true in all fields which have sets of established principles and laws. The novel theory, hypothesis, explanation, etc., which is inconsistent with already established principles and accepted theories, has the burden of proof. The proponent of the novel idea must provide very good reasons for rejecting established principles. This is not because the established views are considered infallible; it is because this is the only reasonable way to proceed. Even if the established theory is eventually shown to be false and the upstart theory eventually takes its place as current dogma, it would still have been unreasonable to have rejected the old theory and accepted the new one in the absence of any compelling reason to do so.
Some of Velikovsky's claims violate principles of Newtonian dynamics, laws of conservation of energy and angular momentum--all of which are rather firmly established in modern physics. Sagan refutes Velikovsky's claim that Jupiter ejected a comet which became Venus by examining the amount of kinetic energy needed for a body with the mass of Venus to escape from Jupiter's gravitational field. Sagan showed that the kinetic energy needed would heat the comet to several thousands of degrees. The `comet' never would have gotten off the launching pad; it would have melted! If the melted `comet' had been ejected into space, it would have been as a rain of "small dust particles and atoms, which does not describe the planet Venus particularly well."[Sagan, p. 97] Sagan also points out that escape from the gravitational field of Jupiter requires a velocity of at least 60 kilometers per second. But if the velocity is greater than 63 km/sec, the comet will be hurled out of our solar system. "There is only a narrow and therefore unlikely range of velocities consistent with Velikovsky's hypothesis."[Sagan, p. 98] Such energy is "equivalent to all the energy radiated by the sun to space in an entire year, and one hundred million times more powerful than the largest solar flare ever observed....We are asked to believe," says Sagan, "without any further evidence or discussion, an ejection event vastly more powerful than anything on the sun, which is a far more energetic object than Jupiter."[Sagan,m p. 98]
The essence of Velikovsky's unreasonableness lies in the fact that he does not provide evidence for his most extravagant claims, some of which turn out to be correct. He rejects physical laws without comment and in general offers no support for the plausibility of his theory. Of course, his scenario is logically possible, in the sense that it is not self-contradictory. It is also the work of a brilliant and educated man. To be plausible, Velikovsky's theory must provide some compelling reason for accepting it (other than the religious motivation to explain some events described in the Bible). But Velikovsky is silent on the important scientific matters Sagan brings up. His theory is farfetched and the burden of proof is therefore his; yet, Velikovsky offers no proof, only unsupported hypotheses, clever and brilliantly worked though they may be.
Finally, Velikovsky's work represents one frequently occurring mark of pseudoscientific thinkers: they are often motivated by an overriding commitment to religious dogma or mythology rather than to a commitment to understanding Nature, warts and all. Their theories are put forth to support unchangeable dogmas. Velikovsky's work demonstrates the extreme lengths to which some pseudoscientists will go in devoting their considerable talents to the service of religious dogma.
further reading
Richard Shand's Velikovsky page
Bauer, Henry H. Beyond Velikovsky, (University of Illinois Press: Urbana and Chicago, 1984).
Gardner, Martin. Fads and Fallacies in the Name of Science (New York: Dover Publications, Inc., 1957), ch. 3.
Goldsmith, Donald (Ed.) Scientists Confront Velikovsky. (Foreword by Isaac Asimov) (Cornell University Press, 1977).
Sagan, Carl. Broca's Brain (New York: Random House, 1979), ch. 7, "Venus and Dr. Velikovsky".