On day 3 of HowTheLightGetsIn at Hay, an extraordinary panel of theoretical physicists debated the reality of the multiverse. Nobel prize-winner Roger Penrose, string theory pioneer Michio Kaku, and quantum gravity researcher Sabine Hossenfelder, argued over whether the idea that our universe is one of many can be taken to be a scientific claim, backed up by observational evidence, or whether it’s merely a popular fiction.

Our universe isn’t the only one. It’s one of many, an infinite number of other universes, some of which are very similar to our own with only minute differences, others in which space has more than three dimensions, and others still in which the laws of physics themselves are completely different. Sounds like a crackpot theory, something someone came up with when high on drugs. In fact, it’s a serious proposal by theoretical physicists and goes by the name the multiverse. But does the fact that it’s a theory put forward by scientists automatically make it scientific? Is there really observational evidence for it? Could there ever be? And if not, where does that leave the theory? Is it delegated back to crackpot fiction, or do we simply have to stretch our idea of what counts as a scientific theory?

An extraordinary panel of theoretical physicists including Noble prize winner Roger Penrose, string theory pioneer Michio Kaku, and foundations of physics researcher Sabine Hossenfelder, came together on the third day of HowTheLightGetsIn to debate the question of whether the theory of the multiverse is really based on evidence, or mere speculation. Whether it’s science or fiction. As usual with these big questions, it turned out there were no clear-cut answers.

The festival site of HowTheLightGetsIn at Hay, 2022

Michio Kaku kicked off the debate arguing that the multiverse is in fact the best way to explain our most successful scientific theory: quantum mechanics.  The entire global economy is based on the ability of electrons to be in a state of superposition, in two or more states at the same time.  Semiconductors which make up computers and mobile phones, run on the principles of quantum mechanics. So, if the multiverse is the best way to make sense of quantum mechanics, then surely the multiverse must be real?

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It’s a false dichotomy to say that a theory is either science or comic book material.

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Sabine Hossenfelder remained unconvinced. For her the multiverse is a result of physicists ending up believing that the elegance of the mathematics of certain theories means the theories are real. That’s not to say, though, that theories like that of the multiverse are mere fiction. It’s a false dichotomy to say that a theory is either science or comic book material (the multiverse has famously featured in marvel comics). Even if at this point in time it’s merely speculation, it could be fruitful speculation, leading to future scientific developments. It might even become possible to experimentally test the multiverse theory sometime in the future. But for the meantime, calling it a scientific theory is misleading.

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For Penrose it’s not enough to try and accommodate quantum mechanics as it is today within a greater theory.

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Roger Penrose started out by reminding the audience that while quantum mechanics is indeed a very successful theory, it’s also a theory that is in tension with itself. One of the physicists who defines quantum mechanics, Erwin Schrödinger, already pointed it out in his famous thought experiment involving a cat that, according to the theory, could be both alive and dead at the same time, until someone checked on her.  Quantum mechanics, it seemed, allows for particles to be in a state of superposition, occupying more than one mutually exclusive states at the same time. Until, that is, a measurement takes place that leads to the collapse of wavefunction into one definite state. So for Penrose it’s not enough to try and accommodate quantum mechanics as it is today within a greater theory – the way people usually think of bridging quantum mechanics and Einstein’s general theory of relativity. What needs to happen is for us to find a way to fix quantum mechanics, to resolve this deep question that’s been haunting the theory since its early days: the measurement problem. But it didn’t seem plausible to him that the theory of the multiverse was how one could solve this fundamental problem.

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Hossenfelder warned the audience that physicists often try to conceal that their pet theory isn’t in fact testable.

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The question then turned less on whether the multiverse theory could account for quantum mechanics, and whether it’s a theory that’s testable. Hossenfelder didn’t think so. But she also warned the audience that physicists often try to conceal that their pet theory isn’t in fact testable (or that the observations are actually in conflict with them). She outlined a three strategies that people usually follow:

1) They claim that the theory is testable in principle, but not in practice, for example it requires a particle accelerator the size of the Milky Way.

2) They follow a bait and switch technique that involves making a prediction that concerns only a very small part of the theory. When that prediction gets falsified by observation, the move is to claim that the falsification only affects that small part of the theory, not the theory as a whole.

3) They point to some small parts of the theory that are in fact testable (even if other parts of it are not) and claim that the whole theory is therefore testable.

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Michio Kaku argued instead that string theory, a theory that involves the multiverse, makes predictions that are  testable not just in principle, but in practice.

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Michio Kaku argued instead that string theory, a theory that involves the multiverse, makes predictions that are very much testable not just in principle, but in practice. He suggested four areas in which thinks experimental evidence in favour of string theory can be found.

1) Look for dark matter, since string theory predicts that dark matter is probably the "photino" or a higher vibration of the string. A particle detector may be able to detect dark matter colliding with a proton, and that will allow us to compare the results of this collision with the prediction of string theory.

2) Launch NASA’s LISA a gravity wave detector, into orbit. There is a small probability that LISA may detect radiation emitted a trillionth of a second after the instant of the Big Bang. If that’s the case, then we can "run the video tape backwards" using string theory’s predictions, and calculate what the radiation before the Big Bang must have been. In doing this, we might be able to experimentally detect the presence of an umbilical cord connecting our baby universe with a mother universe.

3) Look for deviations from Newton's inverse square law in the laboratory. If Newton's inverse square law becomes an inverse cube law at short distances, then this signals the presences of a higher dimensional universe.

4) Look for deviations from the Standard Model. Recently, a tiny deviation in the muon magnetic moment indicates that maybe there is a higher theory beyond the Standard Model, perhaps string theory.

Penrose dismissed string theory as fundamentally mistaken since, to begin with, it predicts the wrong number of dimensions for our universe. Unlike some, like Kaku, who are willing to accept that perhaps we just can’t detect the other dimensions predicted by string theory, Penrose saw is as a fundamental flaw in the theory. He restated that the measurement problem of quantum mechanics, understanding how a quantum system goes from being in a state of superposition to one, definite state, is indeed a serious problem that needs an answer. But that answer doesn’t lie in string theory either. 

Sir Roger Penrose at HowTheLightGetsIn, Hay 2022

So if there is such disagreement among professional physicists when it comes to the multiverse, what explains its mass appeal outside of the academy?

Hossenfelder conceded that it’s a fascinating idea. But we shouldn’t confuse fascinating ideas with science.

For Kaku this idea of other worlds and parallel universes is ancient, signifying a kind of universal attraction. We also find it in fiction, for example Lewis Carrol’s Alice in Wonderland. For Kaku the looking glass is really a metaphor for a wormhole connecting different universes. But it’s perhaps poetry that gives the answer to why people seem so taken by the idea.  Robert Frost’s poem The Road not Taken confirms this sense of wonder about what our lives would have been like if they were only slightly different, if we had taken some different decisions. For Kaku the multiverse gives a kind of solace that these alternative versions of ourselves exist somewhere out there, in different universes, a result of the quantum wave functions having collapsed differently.

It’s a lovely thought. But is it science? And should scientists entertain theories that seem untethered from observational evidence currently available? Despite Penrose having been dismissive of the idea of the multiverse he answered that question affirmatively. Even if some proposed theories don’t have experiential support, they could be worth thinking about. If a theory, for example, is simpler than others, and is consistent with current observations, even if it makes no new predictions that are testable, it would be worth considering. But the multiverse, for Penrose, is not such a theory.

It seems that the multiverse, then, occupies a no-man's land between science and fiction. Without any observational evidence supporting it, but with enough potential to spark future scientific research and discovery. Perhaps its popularity outside the academy is what's most telling about it. For non-scientists, the scientific status of the theory might not be what's the most pressing issue. And my hunch is that people's fascination also goes well beyond it simply being a interesting mental exercise. The power of the multiverse is its ability to lighten the gravity of our actions, allowing us to take solace in the thought that there are an infinite number of universes, in which there are an infinite number of different (some better, some worse) paths we have all taken, whether in our personal lives, or collectively as a species. The idea of some super- Copernican revolution - removing not only planet Earth from the centre of the world, but our entire universe - makes our daily dramas seem even less important in the grand scheme of things. Some would argue the nature of that insight verges on the religious, a long way away from the scientific method.