Is history repeating itself?

Alexander Vilenkin

The idea that history repeats itself is a commonplace notion, often expressed through sayings such as man is the only animal that trips twice over the same stone. However, that is not what we are addressing here, as such repetitions are always partial—events never recur in exactly the same way. Here, we will discuss whether it is possible for history to repeat itself in such way that events unfold exactly as they did before.

In principle, there are two ways history could repeat itself:

·         In time: According to theories of cyclical time—accepted by various civilizations—time repeats itself. Greek astronomers like Hipparchus and Ptolemy calculated that the cycle of the precession of the equinoxes (the wobble of the Earth's axis) lasted about 36,000 years (one degree every 100 years). Believing that the position of the stars determined human destiny, some concluded that every 36,000 years—when the stars returned to the same positions—history would repeat itself exactly, and the same people would be born again. Although Saint Augustine rejected this theory in The City of God, some Scholastics revived it, and the Church was forced to condemn their ideas as heretical. I discussed cyclical time in two previous blog posts: Cyclic Time and Linear Time and Is Time Infinite?

Second-generation planets

Betelgeuse, a red giant

Theories regarding planet formation have evolved over time. In the early decades of the 20th century, it was assumed that the planets of the solar system were formed when a passing star came close to the Sun and, through gravitational attraction, stripped away material that subsequently coalesced into planets. Given that such close encounters between stars must be extremely rare, this theory implied that the existence of planets around a star would be a highly uncommon phenomenon.

Current theory differs. It is now believed that when a cloud of gas and dust contracts to form a star, a portion of the cloud remains outside the protostar, forming a disk that flattens around the system's equator. Gradually, the gas and dust concentrate and condense around larger nuclei, eventually giving rise to planets. This theory explains why nearly all planets in the solar system orbit the Sun in roughly the same plane. It suggests that the existence of planets should be commonplace; indeed, the discovery of thousands of planets around nearby stars lends strong support to this theory.

A city on the Moon?

Lunar transport vehicle, film 2001

Building a lunar colony is a favorite theme in science fiction. I would cite one of my favorite stories featuring such a colony: The Menace from Earth by Robert A. Heinlein (1957). A lunar colony also appears in the novel and film 2001: A Space Odyssey by Arthur C. Clarke.

Recently, with the return of humans to the Moon (still in its early stages, as astronauts have only orbited the Moon), there has been renewed discussion about the possibility of building a permanent lunar facility. There is even talk of establishing a city, with a population of over 100,000.

A piece in Physics World—referencing another article published in Frontiers in Space Technologies—addresses this issue by examining the needs of a lunar colony and the potential cost of maintaining it.

A planet, or a satellite?

ESO/M Kornmesser

On July 23, the journal Nature published an article announcing the discovery of what it called an “exosatellite,” although in my opinion, this is a wordplay with the meaning of the word satellite. Let's look at the concepts involved:

·         In our solar system, there is a central star, the Sun, around which eight planets, several smaller planets, and many thousands of asteroids and comets revolve. In addition, almost all the planets have satellites, much smaller than the planets. The largest in proportion to its planet is the Moon, whose mass is 81 times less than that of Earth. There are four satellites larger than the Moon: three of Jupiter (Ganymede, Callisto, and Io) and one of Saturn (Titan). Many exoplanets have been discovered in systems of this type.

Ontological proof of God’s existence

Elizabeth Anscombe 

In 1078, Saint Anselm of Canterbury proposed a proof of God’s existence in his work Proslogion, which has given rise to various versions that attempted to improve it (usually without success) and to attempts to demolish it. Even today, it continues to provoke the attention of philosophers and corrections to its translation. This is Saint Anselm's argument, originally written in Latin, as it is usually translated:

i.      That than which nothing greater can be conceived at any rate exists in the mind of the fool who says there is no such thing.

ii.   If that than which nothing greater can be conceived is only in a mind, it can be thought to exist in reality too, which is greater.

iii. Therefore if that than which nothing greater can be conceived is only in a mind, it is not that than which nothing greater can be conceived.

iv.  But it is a contradiction to say that something than which nothing greater can be conceived is something than which a greater can be conceived.

v.    Therefore that than which nothing greater can be conceived exists in reality as well as in the mind.

The Origin of the Universe and the Immortality of the Soul

Aristotle

The philosophical and scientific works of Aristotle were lost to Western Europe for centuries and were rediscovered through Arabic translations. Their rediscovery gave rise to a Western European philosophical movement, Scholasticism, with such prominent figures as Albertus Magnus, Thomas Aquinas, Duns Scotus, William of Ockham, Jean Buridan, Albert of Saxony, Nicholas Oresme, Gregory of Rimini, and many others.

I will recall here a curious debate that took place among the Scholastics between the 12th and 14th centuries, which may seem pointless to us today, but which was quite important in its time. For those who wish to learn all the details of the debate, I recommend Pierre Duhem's book, Le Système du Monde, volume VII.

The debate originated from the detection of an incompatibility between two statements due to Aristotle:

Time and Motion

Pierre Duhem

Time and motion are intimately related, because there can be no motion without time. In fact, Aristotle understood it the other way around, thinking that if there were no motion, there would be no time. He defined time in terms of motion, as the number of changes in terms of before and after. And the basic motion, upon which all others depend, would be the rotation of the sphere of the fixed stars.

·         Saint Augustine, in his Confessions, dedicated Book XI to the philosophical study of time. He rejects Aristotle's theory, denying that time depends on the existence of motion, and doubts that time exists outside our minds, although his famous quote reveals his uncertainty about time: What is time? If no one asks me, I know; if I want to explain it to a questioner, I don’t know.