The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or mathematician to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time for free here.
Imagine passing through a mysterious portal and encountering a version of yourself as you would have wished to be. Instead of taking up that tedious job at the accountancy firm, you followed your dreams and became a rock musician. Amazingly, you were invited on a popular music competition show and impressed all the judges. Your first video went viral and now you are internationally famous. A simple doorway to a parallel branch of reality revealed that truth.
Many people think about the multiverse in terms of such personal reveries. In stories, films, and television series, depictions of parallel universes draw upon our natural curiosity about roads not taken. Each of us has but one life. Decisions we make could be beneficial or catastrophic. Often, we don’t know the ramifications until far down the line. Witnessing the alternative paths people’s lives didn’t follow, therefore, is a common fantasy.
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For example, in Sliding Doors, we see how a woman catching or missing a train affects her relationships and career. In Everything Everywhere All at Once, the protagonist meets variations of herself that have incredible talents that she doesn’t possess, such as singing, cooking and martial arts. Many recent Marvel films show alternative versions of superheroes with distinct attributes. Unfulfilled aspirations, such depictions seem to suggest, are breathing elsewhere in the ether.
While it is fun to imagine all of the unrealised alternatives in our lives, that’s not how science views the multiverse. Rather, scientific notions have little to do with our personal decisions and all to do with consistency in physicists’ vision of the cosmos. Researchers’ constructs are far from frivolous. On the contrary, advocates have embraced them seriously – and in some cases reluctantly – as a way of resolving conundrums in subjects like quantum measurement theory and cosmology.
Scientific interest in multiverse ideas stems back to the 1970s, after key articles by respected physicist Bryce DeWitt introduced the work of a little-known researcher, Hugh Everett III, to the public. Everett’s 1957 PhD thesis on quantum measurement theory suggested that, during the process of quantum measurement, an observer’s conscious awareness splits to encompass all possible outcomes rather than just one. It had gained little traction until DeWitt dubbed it the “many-universes” approach. Later, he and others in the community rechristened it the many-worlds interpretation (MWI) of quantum mechanics – a catchy name that undoubtedly helped bring it to prominence.
Briefly, the MWI is an alternative to the orthodox quantum approach, known as the Copenhagen interpretation. According to that theory, named after the city where quantum giant Niels Bohr held reign, in any subatomic measurement, an observer triggers the “collapse” of a quantum state from a superposition of possibilities down to a single outcome. It thereby establishes a dichotomy between quantum systems – essentially black boxes with mysterious innards – and human actions, which are open, clear and resolute (at least from a physics point of view).
Why should people play a role in subatomic workings? After all, aren’t they simply made of atoms themselves? And what did nature do before humans? In positing such a powerful role for our species, the Copenhagen interpretation certainly raises profound questions.
Not all prominent modern physicists were on board with the Copenhagen interpretation. Erwin Schrödinger and Albert Einstein each mocked the idea of observer-triggered collapse, calling for alternatives. In a well-known thought experiment, Schrödinger envisioned a cat, a radioactive sample, a Geiger counter, a hammer and a flask of poison, all encased in a closed box. If the sample decayed – with 50-50 odds during a certain time interval – the counter would activate the hammer to break the flask, release its deadly contents and kill the cat. If not, the feline would live. However, according to quantum orthodoxy, the sample would remain in a superposition of decayed and not decayed until an observer opened the box. Similarly, the cat would persist in a zombielike blend as well, until the moment of unveiling. Naturally, Schrödinger didn’t believe that would really happen. Rather, he was pointing out the absurdity of human-induced collapse.
Similarly, in one of his final lectures, held at Princeton University in 1954, Einstein asked the silly question of whether or not a mouse could conduct a quantum observation. It is likely that Everett was in the audience. He subsequently became convinced that the human element must be removed from quantum measurement theory. Aage Petersen, one of Bohr’s assistants, came to Princeton the following year and was drawn into heated arguments with Everett and others about the topic. Everett contended that human experience must be part of a universal quantum state that includes every physically realisable alternative in the cosmos. During quantum measurement, rather than a particle’s state collapsing, any observers’ conscious awareness must encompass the full range of outcomes. That means that their sense of identity would split into multiple possibilities, each recording a different result.
For example, in Everett’s interpretation of Schrödinger’s conundrum, in one branch of reality, the cat would live and the researcher would rejoice. In the other, the cat would die and the researcher would mourn. The universal quantum state would encompass both possibilities at once. However, neither observer would know about their near replica.
As the editor of a conference proceedings, DeWitt encountered Everett’s thesis after Everett’s supervisor John Wheeler submitted it. At first DeWitt thought that the idea of the branching of consciousness was ridiculous because he couldn’t feel it happen. However, after Everett rebutted that we don’t feel Earth’s rotation either, DeWitt realised that he was outmanoeuvred and responded “Touché!” Ironically, DeWitt would become the concept’s greatest populariser.
One brilliant young thinker who was fascinated by the MWI was Brandon Carter. While studying at Cambridge he came to learn about the concept of parallel universes. Encouraged by Wheeler, Stephen Hawking and others, he decided to draw upon it as a way of understanding why the observable universe has particular properties that are friendly to the formation of planets hospitable to life, such as Earth. In an ensemble of assorted universes – what we now call a multiverse – perhaps only a tiny fraction happen to have features conducive to the eventual emergence of conscious entities such as humans. Others might expand so quickly that the formation of stable, self-gravitating structures such as stars would be impossible, or, alternatively, so slowly that they’d lack the impetus to continue and be fated to collapse. Yet, without intelligent beings living in those bleak alternatives, no one would be around to complain about residing there. Thus, in what Carter called the “anthropic principle”, our mere presence would narrow down the options.
The notion that the big bang wasn’t unique, that it occurred in multiple parallel universes, was made even more cogent in the mid-1980s with cosmologist Andrei Linde’s introduction of a theory called “eternal inflation”. Inflation is the idea that conditions in the very early universe led to an ultra-rapid era of explosive expansion that smoothed out the observable universe. It explains why the number of galaxies, the background temperature and other aspects of space are roughly equal today in all sky directions. Linde demonstrated how the factors that produced our region of the cosmos would be relatively simple to replicate elsewhere, leading to an endless sea of bubble universes. Such reasoning practically mandated a multiverse, rather than a solitary expansion.
Yet another impetus for multiverse thinking in science had to do with the question of the cosmological constant – a kind of stabilising term included in some versions of Einstein’s general theory of relativity. By the turn of the 21st century, cosmologists were faced with a dilemma. Measurements of the growth of the universe showed that its pace was picking up, suggesting a small but non-zero cosmological constant to engender such late-stage acceleration. On the other hand, models of the vacuum of space that took into account its energy due to quantum effects implied an extremely large cosmological constant. The discrepancy led a number of physicists, including Nobel laureate Steven Weinberg, to conclude that our universe must be an improbable outlier with an extremely small cosmological constant, housed in an array of spaces with far greater values. The anthropic principle would narrow that multiverse down to the subset in which planets might form – namely, the universes with puny, rather than mighty, cosmological constant effects.
None of those scientific multiverse models, we see, bears the slightest resemblance to the cinematic depictions. A Marvel or DC epic about a failed universe – accelerating beyond all limits and housing absolutely no stars, planets or living beings – would be exceedingly boring. Even the experiences of the near-replica researchers in the MWI, if they somehow managed to meet up despite the theory asserting the impossibility of doing so, would make a rather prosaic film, as their key difference would be alternative recollections of the result of a quantum measurement. Given the huge gap between cultural perceptions and physical conceptions of the multiverse, no wonder some deride the scientific concept, calling it science fiction, without delving into its maths and its justification. With regard to the science, keeping an open mind is a good idea. After all, black holes were once dismissed as science fiction too.
