Albert Einstein kept changing his mind about the fate of the universe. As a young man, he had — like most other people at the time — believed the universe was static, a fixed size. By the end of his life, Einstein accepted the Big Bang, and had adopted the (now standard) view that the universe will keep growing outward, continuing to expand forever, its lights growing fainter until, eventually, they all fade to an endless inky black. But in between, in the early 1930s, he endorsed a rather different idea: that the universe goes through cosmic cycles.

In a cyclical model, our universe began with a Big Bang and continues to expand — but at some point it will stop. It will then contract, reversing its direction of travel, until eventually all matter collapses into a single point: a Big Crunch. But that is not simply the end of the universe: it is also a new starting point— it triggers another Big Bang, and another universe blooms. This pattern repeats, and repeats. Einstein wasn’t the first to suggest this idea using modern physics. It had been proposed by Alexander Friedmann, a Russian cosmologist whose work was fundamental for establishing the idea of the Big Bang. Friedmann had already been contemplating the physics of cosmic cycles when he died from an unlucky accident: some years earlier on his honeymoon, he had caught typhoid fever from an infected pear bought at a railway station — and in 1925 the infection took over his body.

Recurrence — the idea that the universe will die and be reborn, time and time again — might have been new to physics, but it is not a new idea. Globally, it is found in a number of religions, including Hinduism and Buddhism. It was also the subject of philosophical speculation in the premodern world. In particular, in the works of Greek and Roman philosophers, attempts were made to distinguish between different kinds of recurrence. There is the first kind we have already met through modern physics: the idea that the universe as a whole will die and be reborn. But this idea as it appears in Einstein and Friedmann does not make any claim that particular events, objects, or people will recur. What the physicists were interested in was a reshuffling of the cosmos.

A second type of recurrence is reincarnation, a concept common to many religions. Here the focus is not the world as a whole, but the human or animal soul: each soul is reincarnated, living varying lives in sequences that are determined or governed by some cosmic balance. This can be found alongside an idea of cosmic recurrence, as in Hinduism and Buddhism, but is also consistent with a belief in an eternal world (such as in Jainism or some indigenous Aboriginal religions) or with a stable created world (such as in Sikhism, certain branches of Judaism, and in some of the traditional religions indigenous to the Americas, including that of the Inupiaq people of Alaska). But a third, more extreme idea, is that of exact recurrence — the whole world dies and begins again, and with it every event happens again the same way, in the exact same sequence, forever. Unlike the abstract cosmic rebirth of the physicists, both reincarnation and strict recurrence offer us a reason for the randomness of life’s fortunes, a way to believe that things happen not out of pure chance, but because something from a previous cycle provides a template for the present. Recurrence offers us a bigger picture, a greater web in which to link the vastness of Friedmann’s astrophysical research with the feather-lightness of the chance that killed him.

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Greco-Roman writers treated the idea of eternal recurrence in a variety of ways. In a loose sense, many ancient writers observed that there are elements of circularity built into any idea of time measurement. Eudemus, a pupil of Aristotle, recognised this wider context, and put it clearly: “sameness is spoken of in different ways, and it does seem that a time [period] the same in kind recurs, e.g. summer, winter, and the other seasons.”¹ Aristotle himself said that time was “a sort of circle”.² Plato had called it “the moving image of eternity”, clearly linking time to the circular movements of the heavens.³ Early Greek texts on the weather and seasons elaborate this idea of a cycle of time. Hesiod’s Works and Days, for example, presents an almanac of recurring signs for the farmer, while the long-established festival calendars of the Greek city states were often inscribed and displayed, providing a visual clue to the sameness of each changing year.

Greek thinkers also theorised recurrence in the sense of reincarnation. One of the important features of Pythagorean thought is the idea of metempsychosis, or the transmigration of souls. Unfortunately, Pythagoreanism is particularly difficult to reconstruct: much was unwritten, and what was written has in large part been lost. After its first flush of popularity in the pre-Classical Greek world, centuries of often contradictory stories accreted around the cult figure of Pythagoras, silting up the historical record. (Later interpretations of Pythagoreanism are, of course, interesting in their own right as entryways into the teeming philosophical world of Late Antiquity, but they tell us very little about what Greeks of the sixth century BCE really thought). Pythagoreanism’s doctrines shaped a number of the genuinely secretive mystery cults of the Greek world, including Orphism and the Eleusinian mysteries, whose ideas about reincarnation are mostly lost to time. Reincarnation is, therefore, one of those topics on which the Greek texts whisper rather than shout.

But the philosophical schools that developed after Plato’s time took a more precise and pedantic approach. “I will talk, staff in hand, to you sitting like this, and everything else will be alike.” With these words, Eudemus summarised the view of a group of Stoic philosophers that all the events in the world would exactly recur, again and again. As he details, some Pythagoreans seem to have believed not only in reincarnation, but in exact recurrence: that things would be numerically the same, with the relative order of events exactly mapped between one iteration of the universe and the next. It was an idea reportedly taken up by Chrysippus (ca. 279–206 BCE), the most renowned of all the Stoic philosophers, although other Stoics disagreed. The Stoic version of recurrence had a terrifying additional element to it: the ekpyrosis or great conflagration. Stoics argued that ekpyrosis was a process in which the entire universe periodically goes up in flames and a new world emerges from its ashes like a phoenix — in fact, a lot like the Big Crunch. Many Stoic views are available to us only in later summaries. Hippolytus, a Christian writer of the second century CE, describes the Stoic doctrine, capturing the way in which the passing away of the old and the birth of the new are intertwined:

[The Stoics] expect that there will be a conflagration and a purification of this world, some say entirely, others say in part, and they . . . call the destruction and the subsequent generation of another [world] from it a purification.⁴

Writing around the same time as Hippolytus, Alexander of Aphrodisias, a pagan commentator on Aristotle, elaborates on the Chrysippean view:

[Some Stoics] held that after the conflagration all the same things come to be again in the world numerically, so that even the same peculiarly qualified individual as before exists and comes to be again in that world.⁵

Some Stoic philosophers explain the theory behind this periodic conflagration with a mysterious astronomical concept: the Great Year. The Great Year appears as a framing device in The Cradle of Life, the 2003 Tomb Raider sequel film starring Angelina Jolie: in a hidden ice cavern, a giant model of the solar system ticks down to the time when the orbits of all nine planets will align. In the ancient world, there were only five known planets (Saturn, Jupiter, Venus, Mars, Mercury) plus the two luminaries (the Sun and the Moon). If you started a timer at the moment that they were all lined up, their extremely different orbits mean that it would then take approximately 36,000 years before they returned to the same perfect line. The term “Great Year” was used to describe this particular alignment, but it was also used to refer to the time interval that it would take for any particular arrangement of the planets in the sky to occur again. Plato describes it as the “perfect number of time” in the Timaeus, a work which revolves around demonstrating the perfect harmony of the cosmos.⁶ Cicero, more practically-minded, says the following:

On the diverse motions of the planets the mathematicians have based what they call the Great Year, which is completed when the sun, moon and five planets having all finished their courses have returned to the same positions relative to one another. The length of this period is hotly debated, but it must necessarily be a fixed and definite time.⁷

The Great Year carries in it some idea of a celestial reset, hence its association with ekpyrosis. Although it’s tempting to picture the Stoic ekpyrosis like the death of the dinosaurs in Disney’s Fantasia, with streams drying up and creatures’ scorched flesh falling from their bones, most Stoics were actually rather sober about the whole thing. While the ever-dramatic Seneca luridly imagines cities “swallowed up in yawning chasms” and the inhabited world “cover[ered] with floods”, Marcus Aurelius is more representative of the tradition as a whole: he calmly suggests that everything “will evaporate . . . or it will be scattered”.⁸ The time period involved, 36,000 years, is large enough to be basically non-threatening: just as we are able to dismiss without too much fear the idea of the universe eventually collapsing in a heat death in billions of years’ time, the ancients were also reassured by the enormity of the Great Year.

This Stoic idea of cosmic cycles — of something like a general refreshing of the universe — became part of the common currency of ancient thought. But for those who wanted to get into detail, recurrence had its conceptual problems. Particularly tricky was the strict form: the exact recurrence that some of the Stoics and Pythagoreans seem to have endorsed. Various philosophers weighed in on these difficulties: first and most fundamentally, if everything recurs exactly, does that not also mean time itself recurs? In that case, does everything actually happen more than once in any meaningful way? If we can only distinguish two otherwise identical events by saying that one happens before the other, that suggests we need an outside observer or a continuous measure of time in order to distinguish sameness and difference. These are exactly grounds on which Eudemus objects to the idea of strict recurrence:

For when the motion is one and the same, and similarly there are many things which are the same, their before and after is one and the same, and hence so is their number. So everything is the same, which means that the time is as well.

Some of this difficulty, as the philosophers realised, is to do with language: what does it mean to claim something happens again or earlier or later in a series of cyclical sequences that occur outside time? Aristotle used this linguistic problem as a way of demarcating between two different types of time. Sometimes we use the term “time” (chronos), he argued, when really we mean the scale we’re using to measure change — this is the sense of time we are employing when we say that it took Zeno four minutes to run the race, or that Seneca was in the bath for an hour. But the other type of time is the abstract concept, the thing that he sees as being outside of us, which marches ever onward. As he says:

Time is the measurement of circular motion [of the heavens], and is itself measured by a circular motion [of the heavens]. . . there is nothing else to be seen in what is measured except for the measure.⁹

One response to the conceptual problem Aristotle raises was to deny that everything would be exactly the same in a recurring universe — to avoid the question of whether time starts again by positing that there are minutely different universes potentially (even if not practically) distinguishable against a continuous backdrop of ongoing time. This cop-out route seems to have been taken by some of the Stoics. The Christian philosopher Origen of Alexandria (180–250 CE) reports that: “to avoid supposing that Socrates will live again, [some Stoics] say that it will be someone indistinguishable from Socrates, who will marry someone indistinguishable from Xanthippe, and will be accused by men indistinguishable from Anytus and Meletus.”¹⁰

But Origen explained this idea in order to reject it: for him, any kind of recurrence was incompatible with Christian doctrine, in which time had a distinct sense of direction. Most early Christians felt the same. Primarily, they were deeply uncomfortable with any idea of recurrence that included reincarnation: Christians instead believed that a person’s soul would go to heaven (or to hell, which became increasingly dominant in Christian thought as the Middle Ages wore on). The millenarian movement that was so popular in the early Church also supported the idea of the bodily resurrection of all Christians at the time of Jesus’ Second Coming. Reincarnation — where the soul travels through many bodies — was fundamentally incompatible with the idea of resurrection, which kept body and soul connected, even after death. Christian literature of the first few centuries abounds in half-comic, half-ghastly thought experiments about the mechanics of this final universal sorting and reckoning, including a fascination with how the bodies of cannibals and their victims are arranged in heaven.

But more strict ideas of recurrence were also unacceptable to Christians: it was totally incompatible with Christian doctrine to suggest that the Creation could be repeated, or that the Second Coming would in truth be one of infinitely many appearances of Christ. In fact, this strong Christian commitment toward the fixed linear structure of time represents an enormous shift in Western thought. Pre-Christian pagans and Jews had articulated a mix of ideas about cyclical and linear time, but in general nearly all ancient Mediterranean cultures had a profound orientation toward the past. Christianity changed that: it was now important to understand our lives on earth in the context of the afterlife, and, beyond it all, structuring it all, God’s final judgement. Time was linear, but chance was also never random — the world ran under the watchful eye of an omniscient God. Ideas about recurrence and reincarnation would fall out of favour in the West for over 1500 years.

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Fifty years before Einstein embraced the idea of cosmic cycles, Nietzsche argued for a return to ideas of recurrence. God was dead and with him went the afterlife and its promised judgement of a lifetime of Christian morality. Nietzsche was worried that with this loss, solipsism could sneak in: for him, it was intolerable to think of life as arbitrary or ephemeral. Returning to recurrence was a way to put his own spin on the pre-Christian search for the meaning of life. Nietzsche used Thus Spake Zarathustra (1883–85) to argue that instead of making us feel depressed and trapped, eternal recurrence should be seen as a call to action. He uses recurrence in the strict sense: the exact repetition of the same sequence of events forever. When he first raises the idea in The Gay Science (1882), he imagines a demon explaining this fatalistic doctrine:

You will have to live once more and innumerable times more; and there will be nothing new in [your life], but every pain and every joy and every thought and sigh and everything unutterably small or great in your life will have to return to you, all in the same succession and sequence.¹¹

Our first reaction to this disclosure, suggests Nietzsche, would be a kind of debilitating horror. But it might then present us with an opportunity for transformation, a challenge to live in such a way as to make time’s recurrence a gift. It is not the ephemerality of life that forces us to live wisely, argues Nietzsche, but the idea of it repeating forever. If you know that you must re-live again and again, your every choice becomes loaded with meaning: you are called on to live totally — fully, joyously, and virtuously. For those who are not crushed by the proposition, recurrence is not a curse but an endless blessing.