A Skeptic’s Guide to Passover

Scientific explanations for the parting of the Red Sea, the 10 plagues, and the burning bush.

Moses at the top of Mt. Horeb with Aaron and Hur

For thousands of years, skeptics and believers alike have debated whether the events described in the Passover story—the parting of the Red Sea, the 10 plagues, and the burning bush—actually took place. Roman Jewish historian Josephus Flavius speculated that the parting of the Red Sea “might be of God’s will or of natural origin. Let everyone believe at his own discretion.”The skeptic’s skeptic, Sigmund Freud, called the Passover story “a pious myth,” contending that Moses was a rebellious Egyptian prince who worshiped the sun god Aton and made up the Jewish religion as a political ploy. * In more recent times, scientific explanations of the Passover story range from formula-laden academic papers like “Modeling the Hydrodynamic Situation of the Exodus” to more popular inquiries such as Cambridge materials scientist Colin Humphreys’ The Miracles of Exodus. Whether or not you subscribe to these theories, they beat listening to your little cousin sing the “Four Questions.”

As anyone who has seen The Ten Commandments can attest, the parting of the Red Sea is one of, if not the most, climactic moments in the Passover story. As Exodus describes it:

And Moses stretched out his hand over the sea; and the Lord caused the sea to go back by a strong east wind all that night, and made the sea dry land, and the waters were divided. And the children of Israel went into the midst of the sea upon the dry ground; and the waters were a wall unto them on their right hand, and on their left.

Accepting the biblical account as a “possible ‘qualitative’ description of an event,” Florida State oceanographer Doron Nof set out to investigate whether the parting of the Red Sea is “plausible from a physical point of view.” Using a common phenomenon called wind set-down effect, he found that “a northwesterly wind of 20 m/s blowing for 10-14 h is sufficient to cause a sea level drop of about 2.5m.” Such a drop in sea level, Nof speculates, might have exposed an underwater ridge, which the Israelites crossed as if it were dry land. Although the event is plausible, Nof estimated that the likelihood of such a storm occurring in that particular place and time of year is less than once every 2,400 years.

While scientists agree that wind set-down effect could have caused the Red Sea to part as described in the Bible, most biblical scholars and archeologists insist that the Israelites’ crossing did not take place at the Red Sea at all. The original Hebrew (yam suph), they contend, should be translated as Sea of Reeds, not Red Sea. So where’s the Sea of Reeds? It depends whom you ask. In the somewhat specious History Channel documentary Exodus Decoded, Simcha Jacobovici (aka the Naked Archaeologist) places the Israelites’ crossing in the Bitter Lakes, a reedy marshland north of the Gulf of Suez that was subsumed during the construction of the Suez Canal. For his part, Walking the Bible author Bruce Feiler concludes that the Sea of Reeds is Lake Timsah, located halfway between Port Said and Suez. But The Miracles of Exodus author Humphreys argues that while the translation of “the Red Sea” may be incorrect, the Sea of Reeds nevertheless refers to the Red Sea, concluding that “there can be little doubt that the Red Sea crossing was made possible by wind setdown at the head of the Gulf of Aqaba.”

Before he parted whatever sea it was he parted, the Bible describes Moses and his brother Aaron delivering 10 plagues on the people of Egypt. The Nile turns to blood, all the fish die, frogs are brought forth abundantly, and so on. Drawing on theology, Egyptology, and biology, epidemiologist John Marr developed a “domino theory” to explain each of the 10 plagues in order. Marr believes the plagues were a series of natural disasters and diseases triggered by a bloom of water-borne organisms called dinoflagellates. The dinoflagellates turned the Nile red and killed the frog-eating fish, which in turn caused a population explosion among frogs. The tainted water eventually killed the frogs, causing lice and flies to run rampant, which lead to a number of animal diseases (including African horse sickness) and an outbreak of boils (fancy glanders). This reign of disaster and disease continued through hail, locusts (Schistocerca gregaria, to be precise), and sandstorms until the death of the firstborn sons, which Marr thinks was caused by grain infected with mycotoxins. Others, building on Marr’s domino theory, argue that the plagues were triggered by the eruption of the Greek island of Santorini, causing a string of disasters such as those that occurred at Lake Nyos, Cameroon, in 1986.

Although not quite as impressive as the plagues or the parting of the Red Sea, Moses’ encounter with the burning bush is a pivotal moment in the Passover story and has, for a long time, been the source of much scientific speculation. As the story goes, God speaks to Moses from a burning bush and tells him, “I am come down to deliver [the Israelites] out of the hand of the Egyptians.” Most scientific explanations of the story focus not on the voice of God but on the description of the bush: “[T]he bush burned with fire, and the bush was not consumed.” Humphreys believes the bush continued to burn because of a natural gas or volcanic vent underneath it. Others have pointed to the work of Norwegian physicist Dag Kristian Dysthe and his article on the subsurface combustion of organic material in Mali, saying the bush could have combusted spontaneously.

As for the voice of God, Hebrew University psychology professor Benny Shanon proposes that Moses was tripping at the time on a hallucinogenic substance similar to ayahuasca. Shanon argues further that the presentation of the Ten Commandments might have been a mass hallucination. “The thunder, lightning and blaring of a trumpet which the Book of Exodus says emanated from Mount Sinai could just have been the imaginings of a people in an altered state of awareness.”

By speculating that the voice of God is a hallucination, Shanon, like Freud before him, is attempting to cast doubt on the foundations of monotheism. But not all the explanations of the Passover story are motivated by such ardent secularism. In The Miracles of Exodus, Humphreys writes that “a natural explanation of the events of the Exodus doesn’t to my mind make them any less miraculous. … What made certain events miraculous was their timing.”

Correction, April 8, 2009: This article originally misidentified the Egyptian sun god Aton as Akhenaton. (Return  to the corrected sentence.)