We have frequently criticized some of the crazy ideas emerging from modern cosmology: notions like the multiverse, inflation, Everett’s "many-worlds" scenario, and other concoctions that try to escape the overwhelming evidence for design in the universe (namely, cosmic fine-tuning and the stringent requirements for habitability). Now, two leading big-bang cosmologists are joining us in the criticism, in a very high level venue, but for quite different reasons. Here’s what George Ellis and Joe Silk say in Nature ("Scientific Method: Defend the Integrity of Physics"):

This year, debates in physics circles took a worrying turn. Faced with difficulties in applying fundamental theories to the observed Universe, some researchers called for a change in how theoretical physics is done. They began to argue — explicitly — that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical. We disagree. As the philosopher of science Karl Popper argued: a theory must be falsifiable to be scientific. (Emphasis added.)

In particular, they chide the string theorists, multiverse advocates, inflation theorists, and purveyors of the "many worlds" interpretation of quantum mechanics. Speaking of inflation, it’s interesting that Science Magazine listed the downfall of the BICEP2 results on inflation as one of the leading "Breakdowns" (as opposed to "Breakthroughs") of the year.

Whether Ellis and Silk’s definitions of science and the scientific method are adequate is beside the point (the definition of science is a vexed question, testability is vague, and falsifiability has its flaws). What worries them is something else:

The issue of testability has been lurking for a decade. String theory and multiverse theory have been criticized in popular books and articles, including some by one of us (G.E.). In March, theorist Paul Steinhardt wrote in this journal that the theory of inflationary cosmology is no longer scientific because it is so flexible that it can accommodate any observational result. Theorist and philosopher Richard Dawid and cosmologist Sean Carroll have countered those criticisms with a philosophical case to weaken the testability requirement for fundamental physics.

We applaud the fact that Dawid, Carroll and other physicists have brought the problem out into the open. But the drastic step that they are advocating needs careful debate. This battle for the heart and soul of physics is opening up at a time when scientific results — in topics from climate change to the theory of evolution — are being questioned by some politicians and religious fundamentalists. Potential damage to public confidence in science and to the nature of fundamental physics needs to be contained by deeper dialogue between scientists and philosophers.

Ah, it’s those "religious fundamentalists" again. The label means, approximately, "anybody who is not a materialist." It’s like what astronomers call metals: "anything that is not hydrogen or helium." If we let those "fundamentalists" use the same tactics Carroll and Dawid are advocating, people might come to question climate change or (God forbid!) the Darwinian theory of evolution.

Such a case must be made in formal philosophical terms. A conference should be convened next year to take the first steps. People from both sides of the testability debate must be involved….

The imprimatur of science should be awarded only to a theory that is testable. Only then can we defend science from attack.

What’s ironic is that their advice has all the trappings of a religious council. "Science" is under attack from heretics within and without. It needs to convene its college of cardinals. The council needs to apply more stringent guidelines for use of its imprimatur. It needs to stop issuing the imprimatur to lazy priests, or else the heretics will claim they deserve it, too.

Ellis and Silk do a great job of dismantling the pretensions of modern theoretical physicists and cosmologists who have loosened their grip on observational evidence. Readers will enjoy the critiques of string theory and multiverse notions, but may be surprised by their critique of Bayesian reasoning as a substitute for evidence:

Citing Bayesian analysis, a statistical method for inferring the likelihood that an explanation fits a set of facts, Dawid equates confirmation with the increase of the probability that a theory is true or viable. But that increase of probability can be purely theoretical. Because "no-one has found a good alternative" and "theories without alternatives tended to be viable in the past", he reasons that string theory should be taken to be valid.

In our opinion, this is moving the goalposts. Instead of belief in a scientific theory increasing when observational evidence arises to support it, he suggests that theoretical discoveries bolster belief. But conclusions arising logically from mathematics need not apply to the real world…. The idea that preconceived truths about the world can be inferred beyond established facts (inductivism) was overturned by Popper and other twentieth-century philosophers.

They don’t name the "other twentieth-century philosophers," but to be sure, the world has moved beyond Popper, notable as his contributions were (some commenters at the end of the article point this out). We are even past Quine, Kuhn, Feyerabend, Foucault, van Fraasen, and numerous other philosophers who arose from the ashes of logical positivism.

Currently, there exists a nebulous consensus called "scientific realism," wherein Big Science insiders help themselves to notions of unobservable reality and testability. In this new consensus, academia, like a church council, dictates what gets the "science" imprimatur. It’s an internecine dispute within this consensus that Ellis and Silk are describing. Only those blessed by the cardinals (or mandarins, as Phillip Johnson calls them) are allowed to speak in the officially approved missives, the journals.

The current situation is not friendly to mavericks; thus the epithets "religious fundamentalists," "politicians" and "pseudoscientists" aimed at outsiders. It is, however, interesting to listen in. Look what they say about multiverse theory:

The multiverse is motivated by a puzzle: why fundamental constants of nature, such as the fine-structure constant that characterizes the strength of electromagnetic interactions between particles and the cosmological constant associated with the acceleration of the expansion of the Universe, have values that lie in the small range that allows life to exist. Multiverse theory claims that there are billions of unobservable sister universes out there in which all possible values of these constants can occur. So somewhere there will be a bio-friendly universe like ours, however improbable that is.

That sounds like a lead-up to The Privileged Planet, or to Privileged Species. Ellis and Silk go on to describe how one of the constants, the cosmological constant, is 120 orders of magnitude off from the predicted theoretical value. So are they ready to consider scientific theories that embrace this evidence, like intelligent design? No, but one thing they do know: multiverse theory is not scientific.

Billions of universes — and of galaxies and copies of each of us — accumulate with no possibility of communication between them or of testing their reality. But if a duplicate self exists in every multiverse domain and there are infinitely many, which is the real ‘me’ that I experience now? Is any version of oneself preferred over any other? How could ‘I’ ever know what the ‘true’ nature of reality is if one self favours the multiverse and another does not?

That’s a logical statement, not an observational one. By this, we see that they are not against using logical inference to eliminate unworthy theories. Isn’t that what Meyer does in his books, using "inference to the best explanation"? Isn’t that what Dembski does to eliminate the chance hypothesis?

In the end, Ellis and Silk never explain cosmic fine-tuning. They just preach that science needs to stick to the old-time religion: respect for observable, testable evidence.

The consequences of overclaiming the significance of certain theories are profound — the scientific method is at stake (see go.nature.com/hh7mm6 [a link to Alan Sokal’s critique of postmodernism]). To state that a theory is so good that its existence supplants the need for data and testing in our opinion risks misleading students and the public as to how science should be done and could open the door for pseudoscientists to claim that their ideas meet similar requirements.

What to do about it? Physicists, philosophers and other scientists should hammer out a new narrative for the scientific method that can deal with the scope of modern physics. In our view, the issue boils down to clarifying one question: what potential observational or experimental evidence is there that would persuade you that the theory is wrong and lead you to abandoning it? If there is none, it is not a scientific theory.

But if peers with equal credentials but without the imprimatur (say, a Dembski, Meyer, or Berlinski) can ask that question of materialism, why shouldn’t they get a hearing? Ask the materialists: "What potential observational or experimental evidence is there that would persuade you that the theory is wrong and lead you to abandoning it?"

ID advocates have proposed ways to falsify intelligent design (for example, see here and in Dembski’s book The Design Revolution, p. 282). Now it’s the materialists’ turn. If they don’t have an answer, then by Ellis and Silk’s own standards, it is not a scientific position.

Image credit: "Hubble Sweeps a Messy Star Factory," European Space Agency via NASA.