Over the last few decades “multiverse theories” have become increasingly fashionable within a relatively small—but publicly vocal—group of theoretical physicists. This group specialises in foundational problems in cosmology, particle physics, and quantum mechanics. These theories are advertised as science’s answer to much that we can’t otherwise explain about the universe we inhabit, the elementary particles we have discovered in it, and the reasons for our own existence.

It’s “theories” plural because the multiverse is used in various ways to fill gaps in our current understanding. Cosmological multiverse theories “explain” why the initial conditions that prevailed at the Big Bang origin of our universe, and the physical constants and laws which shaped its subsequent evolution, appear so exquisitely fine-tuned to allow for the possibility of life. The idea is that there’s nothing particularly special about our “Goldilocks” universe: it is simply one of a (possibly infinite) number of universes, all with different initial conditions, constants, and laws. Most will be inhospitable, but it should come as no surprise to find ourselves in a universe which isn’t.

Multiverse arguments are also used by some string theorists to “explain” why they’re unable to identify a specific formulation of the theory (among 10500 different possibilities) that uniquely describes the elementary particles in our universe and the forces that act between them. Their answer is that these possibilities form a “landscape” or a continuum—a multiverse—and once again it’s no surprise to find ourselves in the one structure hospitable to life, whatever that may be. Job done.

Finally, current theories of particle physics are based on a version of quantum mechanics, which has a reputation for not being understandable. Among the great variety of different possible interpretations of quantum theory, the “many worlds” interpretation attempts to understand it by introducing the notion of parallel universes, all equally real. One leading theorist has declared that such a “quantum multiverse” is as real as dinosaurs once were, and we should just get over it.

This is all very well, but the multiverse is also extremely controversial, with some scientists publicly voicing the concerns of many by arguing that it is entirely metaphysical. It is really no more than an idea bandied about as a handy way to fill some admittedly significant explanatory gaps in our current theories. In this sense it is used much as the mechanical philosophers of the 17th and 18th centuries (such as Isaac Newton) used God. “Multiverse theory” isn’t actually a theory. It doesn’t come with a recognised and accepted set of mathematical equations that can be used to perform calculations and make predictions, and so it offers no prospect of connecting in any meaningful way with empirical data, now or in the future. These theories can’t be tested, and so they can be neither proved nor disproved. It is argued that, like God or astrology, they do not belong in science.

The multiverse doesn’t have same status as familiar, well-established scientific theories such as relativity and quantum mechanics. But in their public pronouncements, multiverse theorists often fail to make this clear. The public is then misled to believe that this stuff is accepted science, a situation that must surely be ultimately damaging to the public perception and acceptance of its authority. More worryingly, some multiverse theorists appear keen to redefine what science is. They want to junk the scientific method, weakening the all-important connection between theory and empirical facts, pushing the demand for evidence firmly into the background. This might suit their personal agendas, but it pushes us inexorably towards the oxymoron that is “post-empirical science.”

These are serious issues. Whilst the scientific community seems caught in two minds on what to do about them, a few voices have nevertheless been raised in protest. In an article published in the scientific journal Nature in late 2014, cosmologist George Ellis and astrophysicist Joe Silk called on the community to defend the integrity of physics against this onslaught from untestable metaphysical speculation.

Here’s the problem. The multiverse is undoubtedly catchy. It appeals to a generation that has grown up with (among other things) the Marvel Cinematic Universe; an audience fascinated by the notion that the multiverse of Doctor Strange might actually be true. The multiverse is cool. Stick multiverse in the title or in the headline of an article and it is more likely to capture attention and invite that all-important click, or purchase. (A trick that Prospect will no doubt pull with this very article).

The first victim is context. For example, in a recent post to this blog, Philip Ball reports on a paper published by a team of astrophysicists in the “Monthly Notices of the Royal Astronomical Society.” Ball reports on this paper with an accuracy entirely consistent with his reputation as one of Britain’s best science writers, but it so happens that the paper is not about multiverse theory. And yet “multiverse theory” appears in the title. On publication, Prospect magazine tweeted a link with a caption that refers to “new ‘multiverse’ calculations,” implying that this is all about a real scientific multiverse theory, with the capacity to perform calculations (and, by association, make predictions) when multiverse theory actually can’t do any of this. This kind of thing helps to compound the deception, normalising multiverse theory as validated science.

Okay, but does any of this really matter? The answer depends on which side of the debate you stand. I firmly believe that consumers of popular science want to be entertained but they are principally interested in what real, empirically-based science has to say. I’m sure they’re also interested in flights of fancy from the deep thinkers working at the frontiers of our understanding. But they deserve to know what constitutes accepted science and what is really metaphysics peddled as science.

So, how can we all help to defend the integrity of physics? I believe the answer is quite straightforward. It’s easy to imagine that science is somehow different from other forms of human endeavour, such as politics. Those reporting on the politics of the day rightly refuse to relate the opinions of politicians without attempting to balance them with the contrasting opinions of others. In the era of “fake news” most consumers are much more likely to be sceptical about what they’re being told. And yet reporters and consumers will often take the pronouncements of scientists at face value, without challenge, particularly when the scientists concerned hold professorships at prestigious institutions.

The front line of defence is to acknowledge that the multiverse is controversial. If we can accept this, it becomes easier to question and if necessary challenge those who seek to promote it. Doing this doesn’t require a PhD in physical cosmology or particle physics. I think we just need to be mindful of a quotation from Albert Einstein:

“Time and again the passion for understanding has led to the illusion that man is able to comprehend the objective world rationally by pure thought without any empirical foundations—in short, by metaphysics.”

Try to gain a balanced perspective. The multiverse theorists hold one view, but it’s not hard to find other theorists who hold contrasting views. And be wary of reports that take the multiverse out of context and advertise it as the “latest scientific thinking.” Be sceptical. Keep some salt handy, and be ready to take a large pinch.

Jim Baggott’s next book, Quantum Space: Loop Quantum Gravity and the Search for the Structure of Space, Time, and the Universe, will be published by Oxford University Press in November 2018