Multiverse

• 1. Introduction

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  The multiverse is the hypothetical set of all universes. Not just our own observable cosmos, but every universe that could exist, bundled into one vast theoretical construct. The idea raises an immediate and vertiginous question: if other universes are out there, how would we ever know?

  In Dublin in 1952, Erwin Schrödinger gave a lecture in which he jocularly warned his audience that what he was about to say might seem lunatic. His equations, he told them, seemed to describe several different histories. And those histories were not alternatives. They all really happened simultaneously. That kind of thinking sits at the heart of multiverse theory.

  The concept is not new. It stretches back to ancient Greece, runs through medieval philosophy, resurfaces in the Renaissance, and erupts into modern physics in the twentieth century. Along the way it has attracted some of the most eminent names in science, and some of the most pointed critics. At stake is a question that is almost embarrassingly large: does everything that could exist actually exist? What follows is the story of how that question evolved from ancient speculation into a live debate inside theoretical physics.

• 2. Ancient Roots

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  The pre-Socratic philosopher Anaximander, writing in the sixth century BCE, may have been the first to suggest infinite worlds, though historians debate whether he meant co-existing worlds or successive ones. The matter is genuinely unsettled, and no one claims to resolve it.

  The first figures to whom historians can definitively attach the idea of innumerable worlds are the Ancient Greek Atomists. Leucippus and Democritus argued for it in the fifth century BCE. Epicurus, who lived from 341 to 270 BCE, carried the tradition forward. The Roman Epicurean Lucretius took it into Latin in the first century BCE. These thinkers were not speculating idly. Their conception of matter as fundamental particles moving through void made multiple worlds a logical consequence.

  In the third century BCE, the philosopher Chrysippus added a different wrinkle: the world, he argued, eternally expired and regenerated. That is not exactly a multiverse of co-existing universes, but it does imply multiple universes strung across time. In the Renaissance, Giordano Bruno, born in 1548 and executed in 1600, revived the concept and expressed it as infinite worlds. The American philosopher and psychologist William James used the word "multiverse" in 1895, though not in a cosmological sense. That same year, the modern scientific debate about the multiverse began in earnest during the exchange between Boltzmann and Zermelo.

• 3. Mapping the Multiverse

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  Max Tegmark, a cosmologist, has proposed a four-level taxonomy of possible multiverses. Each level encompasses the previous ones and adds a new layer of strangeness.

  Level I is the most conservative. Cosmic inflation predicts an infinite ergodic universe. An infinite universe must contain an infinite number of Hubble volumes, all governed by the same physical laws. Most will differ in the distribution of matter. But with infinitely many of them, some will eventually match our own configuration exactly. Tegmark estimates that an identical volume to ours should be roughly 10 to the power of 10 to the power of 115 meters away.

  Level II moves into more radical territory. In eternal inflation theory, the universe as a whole keeps stretching forever, but certain regions stop expanding and form distinct bubbles, like gas pockets in a loaf of rising bread. Different bubbles can undergo different spontaneous symmetry breaking, producing different physical constants. John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory also fall here.

  Level III draws on Hugh Everett III's many-worlds interpretation of quantum mechanics. Quantum mechanics holds that certain observations cannot be predicted absolutely; each possible outcome has a probability. Everett proposed that every possible outcome corresponds to a different real world. Tegmark writes that the only difference between Level I and Level III is where a person's doppelgangers live: in Level I they inhabit another region of three-dimensional space, while in Level III they occupy another quantum branch in infinite-dimensional Hilbert space.

  Level IV is Tegmark's own hypothesis: the ultimate ensemble, in which every mathematically possible universe is equally real. He argues that this implies any conceivable parallel universe theory can be described at Level IV, and that it subsumes all other ensembles, bringing closure to the hierarchy of multiverses. Juergen Schmidhuber pushes back on this, arguing that the set of mathematical structures is not even well-defined.

  Brian Greene, a theoretical physicist and string theorist, has separately catalogued nine types of multiverses: quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate. The brane version, for instance, postulates that our universe exists on a membrane floating in a higher dimension, where other membranes carry other universes, colliding every few trillion years to produce new Big Bangs. That picture requires extra spatial dimensions and falls under the string theory umbrella.

• 4. Searching for Signals

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  Around 2010, Stephen M. Feeney and colleagues analyzed data from the Wilkinson Microwave Anisotropy Probe and claimed to find evidence suggesting this universe had collided with other universes in the distant past. The claim attracted attention. A more thorough analysis of data from both the WMAP and the Planck satellite, which has a resolution three times higher than WMAP, found no statistically significant evidence of any such bubble universe collision. Nor was there any sign of gravitational pull from other universes on our own.

  In 2015, the astrophysicist Dr. Ranga-Ram Chary analyzed the cosmic radiation spectrum and found a signal 4,500 times brighter than it should have been, based on the number of protons and electrons scientists believe existed in the very early universe. The signal, an emission line arising from the formation of atoms during the era of recombination, is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. Chary noted that there is a 30% chance the signal is noise rather than a genuine detection. One possible explanation is that a parallel universe dumped some of its matter particles into ours during recombination, producing more atoms, more photons, and a greatly enhanced signature line. Another explanation is that the signal reflects incoming light from distant galaxies, or even clouds of dust surrounding our own galaxy. The question remains open.

• 5. The Case Against

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  Paul Davies, author and cosmologist, made the case against multiverse hypotheses in a 2003 opinion piece in the New York Times titled "A Brief History of the Multiverse". He argued that the concept is non-scientific.

  George Ellis, writing in August 2011, provided a sustained criticism. He accepts that the multiverse is thought to exist far beyond the cosmological horizon, and he acknowledges that it is theorized to be so far away that no evidence may ever be found. His core objection is about method: he argues that observational testing is at the core of science and should not be abandoned. Some theorists, he notes, do not consider the lack of empirical testability a serious problem. Ellis disagrees with that position. He also argues that the multiverse is a metaphysical idea that ultimately leaves the deepest questions about existence unresolved, because those questions cannot be settled by empirical science.

  Storager, Ellis, and Kircher point out that in a true multiverse theory the universes are completely disjoint; nothing that happens in one is causally linked to what happens in another. That causal disconnection, they argue, places the multiverse beyond any scientific support.

  Philosopher Philip Goff argues that inferring a multiverse to explain the apparent fine-tuning of our universe is an example of what he calls the Inverse Gambler's Fallacy. In May 2020, the astrophysicist Ethan Siegel wrote in a Forbes blog post that parallel universes would have to remain a science fiction dream for the time being, given the state of the evidence. Scientific American contributor John Horgan has argued that multiverse theories are bad for science.

• 6. Fine-Tuning and the Anthropic Principle

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  Our universe looks, in certain respects, like it was set up to permit life. The physical constants and laws appear to occupy a narrow range that allows matter to form, stars to ignite, and planets to persist long enough for life to emerge. Explaining that apparent fine-tuning without invoking intelligent design is one of the main reasons serious scientists take the multiverse seriously.

  The argument runs as follows. If a large or even infinite number of universes exist, each with different physical laws or different fundamental constants, then some tiny fraction of them will happen to have the combination required for life. The weak anthropic principle states that we, as conscious beings, would only exist in one of those few universes that were finely tuned. The fine-tuning requires no special explanation once you accept that most universes do not support life and we can only observe the one we are in.

  An early version of this reasoning appears in Arthur Schopenhauer's 1844 work "Von der Nichtigkeit und dem Leiden des Lebens". He argued that our world must be the worst of all possible worlds, because if it were significantly worse in any respect it could not continue to exist. The logic is structurally similar: we observe the world we observe because it is the one that permits observation.

  The application of Occam's razor to this debate divides proponents and critics. Critics say that positing an almost infinite number of unobservable universes to explain our own violates the principle of parsimony. Proponents, including Tegmark, counter that in terms of Kolmogorov complexity, the multiverse is actually simpler than a single idiosyncratic universe. Modal realism, the philosophical position associated with David Lewis, holds that all possible worlds exist and are as real as the world we inhabit. That position dovetails with the anthropic argument, though it was developed independently in philosophy rather than physics.