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A Fight for the Soul of ScienceString theory, the multiverse and other ideas of modern physics are potentially untestable. At ahistoric meeting in Munich, scientists and philosophers asked: should we trust them anyway?

By Natalie Wolchover

Physicists George Ellis (center) and Joe Silk (right) at Ludwig Maximilian University in Munich onDec. 7.

Physicists typically think they “need philosophers and historians of science like birds needornithologists,” the Nobel laureate David Gross told a roomful of philosophers, historians andphysicists last week in Munich, Germany, paraphrasing Richard Feynman.

But desperate times call for desperate measures.

Fundamental physics faces a problem, Gross explained — one dire enough to call for outsiders’perspectives. “I’m not sure that we don’t need each other at this point in time,” he said.

It was the opening session of a three-day workshop, held in a Romanesque-style lecture hall atLudwig Maximilian University (LMU Munich) one year after George Ellis and Joe Silk, two white-haired physicists now sitting in the front row, called for such a conference in an incendiary opinion

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piece in Nature. One hundred attendees had descended on a land with a celebrated tradition in bothphysics and the philosophy of science to wage what Ellis and Silk declared a “battle for the heartand soul of physics.”

The crisis, as Ellis and Silk tell it, is the wildly speculative nature of modern physics theories, whichthey say reflects a dangerous departure from the scientific method. Many of today’s theorists —chief among them the proponents of string theory and the multiverse hypothesis — appearconvinced of their ideas on the grounds that they are beautiful or logically compelling, despite theimpossibility of testing them. Ellis and Silk accused these theorists of “moving the goalposts” ofscience and blurring the line between physics and pseudoscience. “The imprimatur of science shouldbe awarded only to a theory that is testable,” Ellis and Silk wrote, thereby disqualifying most of theleading theories of the past 40 years. “Only then can we defend science from attack.”

They were reacting, in part, to the controversial ideas of Richard Dawid, an Austrian philosopherwhose 2013 book String Theory and the Scientific Method identified three kinds of “non-empirical”evidence that Dawid says can help build trust in scientific theories absent empirical data. Dawid, aresearcher at LMU Munich, answered Ellis and Silk’s battle cry and assembled far-flung scholarsanchoring all sides of the argument for the high-profile event last week.

David Gross, a theoretical physicist at the University of California, Santa Barbara.

Gross, a supporter of string theory who won the 2004 Nobel Prize in physics for his work on theforce that glues atoms together, kicked off the workshop by asserting that the problem lies not withphysicists but with a “fact of nature” — one that we have been approaching inevitably for fourcenturies.

The dogged pursuit of a fundamental theory governing all forces of nature requires physicists toinspect the universe more and more closely — to examine, for instance, the atoms within matter, the

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protons and neutrons within those atoms, and the quarks within those protons and neutrons. Butthis zooming in demands evermore energy, and the difficulty and cost of building new machinesincreases exponentially relative to the energy requirement, Gross said. “It hasn’t been a problem somuch for the last 400 years, where we’ve gone from centimeters to millionths of a millionth of amillionth of a centimeter” — the current resolving power of the Large Hadron Collider (LHC) inSwitzerland, he said. “We’ve gone very far, but this energy-squared is killing us.”

As we approach the practical limits of our ability to probe nature’s underlying principles, the mindsof theorists have wandered far beyond the tiniest observable distances and highest possibleenergies. Strong clues indicate that the truly fundamental constituents of the universe lie at adistance scale 10 million billion times smaller than the resolving power of the LHC. This is thedomain of nature that string theory, a candidate “theory of everything,” attempts to describe. Butit’s a domain that no one has the faintest idea how to access.

The problem also hampers physicists’ quest to understand the universe on a cosmic scale: Notelescope will ever manage to peer past our universe’s cosmic horizon and glimpse the otheruniverses posited by the multiverse hypothesis. Yet modern theories of cosmology lead logically tothe possibility that our universe is just one of many.

Whether the fault lies with theorists for getting carried away, or with nature, for burying its bestsecrets, the conclusion is the same: Theory has detached itself from experiment. The objects oftheoretical speculation are now too far away, too small, too energetic or too far in the past to reachor rule out with our earthly instruments. So, what is to be done? As Ellis and Silk wrote, “Physicists,philosophers and other scientists should hammer out a new narrative for the scientific method thatcan deal with the scope of modern physics.”

“The issue in confronting the next step,” said Gross, “is not one of ideology but strategy: What is themost useful way of doing science?”

Over three mild winter days, scholars grappled with the meaning of theory, confirmation and truth;how science works; and whether, in this day and age, philosophy should guide research in physics orthe other way around. Over the course of these pressing yet timeless discussions, a degree ofconsensus took shape.

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Rules of the GameThroughout history, the rules of science have been written on the fly, only to be revised to fitevolving circumstances. The ancients believed they could reason their way toward scientific truth.Then, in the 17th century, Isaac Newton ignited modern science by breaking with this “rationalist”philosophy, adopting instead the “empiricist” view that scientific knowledge derives only fromempirical observation. In other words, a theory must be proved experimentally to enter the book ofknowledge.

But what requirements must an untested theory meet to be considered scientific? Theorists guidethe scientific enterprise by dreaming up the ideas to be put to the test and then interpreting theexperimental results; what keeps theorists within the bounds of science?

Today, most physicists judge the soundness of a theory by using the Austrian-British philosopherKarl Popper’s rule of thumb. In the 1930s, Popper drew a line between science and nonscience incomparing the work of Albert Einstein with that of Sigmund Freud. Einstein’s theory of generalrelativity, which cast the force of gravity as curves in space and time, made risky predictions — onesthat, if they hadn’t succeeded so brilliantly, would have failed miserably, falsifying the theory. ButFreudian psychoanalysis was slippery: Any fault of your mother’s could be worked into yourdiagnosis. The theory wasn’t falsifiable, and so, Popper decided, it wasn’t science.

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Paul Teller (by window), a philosopher and professor emeritus at the University of California, Davis.

Critics accuse string theory and the multiverse hypothesis, as well as cosmic inflation — the leadingtheory of how the universe began — of falling on the wrong side of Popper’s line of demarcation. Toborrow the title of the Columbia University physicist Peter Woit’s 2006 book on string theory, theseideas are “not even wrong,” say critics. In their editorial, Ellis and Silk invoked the spirit of Popper:“A theory must be falsifiable to be scientific.”

But, as many in Munich were surprised to learn, falsificationism is no longer the reigning philosophyof science. Massimo Pigliucci, a philosopher at the Graduate Center of the City University of NewYork, pointed out that falsifiability is woefully inadequate as a separator of science and nonscience,as Popper himself recognized. Astrology, for instance, is falsifiable — indeed, it has been falsified adnauseam — and yet it isn’t science. Physicists’ preoccupation with Popper “is really something thatneeds to stop,” Pigliucci said. “We need to talk about current philosophy of science. We don’t talkabout something that was current 50 years ago.”

Nowadays, as several philosophers at the workshop said, Popperian falsificationism has beensupplanted by Bayesian confirmation theory, or Bayesianism, a modern framework based on the18th-century probability theory of the English statistician and minister Thomas Bayes. Bayesianismallows for the fact that modern scientific theories typically make claims far beyond what can bedirectly observed — no one has ever seen an atom — and so today’s theories often resist a falsified-unfalsified dichotomy. Instead, trust in a theory often falls somewhere along a continuum, sliding upor down between 0 and 100 percent as new information becomes available. “The Bayesianframework is much more flexible” than Popper’s theory, said Stephan Hartmann, a Bayesianphilosopher at LMU. “It also connects nicely to the psychology of reasoning.”

Gross concurred, saying that, upon learning about Bayesian confirmation theory from Dawid’s book,he felt “somewhat like the Molière character who said, ‘Oh my God, I’ve been talking prose all mylife!’”

Another advantage of Bayesianism, Hartmann said, is that it is enabling philosophers like Dawid tofigure out “how this non-empirical evidence fits in, or can be fit in.”

Another Kind of EvidenceDawid, who is 49, mild-mannered and smiley with floppy brown hair, started his career as atheoretical physicist. In the late 1990s, during a stint at the University of California, Berkeley, a hubof string-theory research, Dawid became fascinated by how confident many string theorists seemedto be that they were on the right track, despite string theory’s complete lack of empirical support.“Why do they trust the theory?” he recalls wondering. “Do they have different ways of thinkingabout it than the canonical understanding?”

String theory says that elementary particles have dimensionality when viewed close-up, appearing aswiggling loops (or “strings”) and membranes at nature’s highest zoom level. According to the theory,extra dimensions also materialize in the fabric of space itself. The different vibrational modes of thestrings in this higher-dimensional space give rise to the spectrum of particles that make up theobservable world. In particular, one of the vibrational modes fits the profile of the “graviton” — thehypothetical particle associated with the force of gravity. Thus, string theory unifies gravity, nowdescribed by Einstein’s theory of general relativity, with the rest of particle physics.

However string theory, which has its roots in ideas developed in the late 1960s, has made notestable predictions about the observable universe. To understand why so many researchers trust it

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anyway, Dawid signed up for some classes in philosophy of science, and upon discovering how littlestudy had been devoted to the phenomenon, he switched fields.

In the early 2000s, he identified three non-empirical arguments that generate trust in string theoryamong its proponents. First, there appears to be only one version of string theory capable ofachieving unification in a consistent way (though it has many different mathematicalrepresentations); furthermore, no other “theory of everything” capable of unifying all thefundamental forces has been found, despite immense effort. (A rival approach called loop quantumgravity describes gravity at the quantum scale, but makes no attempt to unify it with the otherforces.) This “no-alternatives” argument, colloquially known as “string theory is the only game intown,” boosts theorists’ confidence that few or no other possible unifications of the four fundamentalforces exist, making it more likely that string theory is the right approach.

Second, string theory grew out of the Standard Model — the accepted, empirically validated theoryincorporating all known fundamental particles and forces (apart from gravity) in a singlemathematical structure — and the Standard Model also had no alternatives during its formativeyears. This “meta-inductive” argument, as Dawid calls it, buttresses the no-alternatives argument byshowing that it has worked before in similar contexts, countering the possibility that physicistssimply aren’t clever enough to find the alternatives that exist.

Emily Fuhrman for Quanta Magazine, with text by Natalie Wolchover and art direction by Olena Shmahalo.

Click on the interactive to start. Learn more about this map.

The third non-empirical argument is that string theory has unexpectedly delivered explanations forseveral other theoretical problems aside from the unification problem it was intended to address.The staunch string theorist Joe Polchinski of the University of California, Santa Barbara, presentedseveral examples of these “unexpected explanatory interconnections,” as Dawid has termed them, ina paper read in Munich in his absence. String theory explains the entropy of black holes, forexample, and, in a surprising discovery that has caused a surge of research in the past 15 years, ismathematically translatable into a theory of particles, such as the theory describing the nuclei ofatoms.

Polchinski concludes that, considering how far away we are from the exceptionally fine grain ofnature’s fundamental distance scale, we should count ourselves lucky: “String theory exists, and wehave found it.” (Polchinski also used Dawid’s non-empirical arguments to calculate the Bayesianodds that the multiverse exists as 94 percent — a value that has been ridiculed by the Internet’svocal multiverse critics.)

One concern with including non-empirical arguments in Bayesian confirmation theory, Dawidacknowledged in his talk, is “that it opens the floodgates to abandoning all scientific principles.” Onecan come up with all kinds of non-empirical virtues when arguing in favor of a pet idea. “Clearly therisk is there, and clearly one has to be careful about this kind of reasoning,” Dawid said. “Butacknowledging that non-empirical confirmation is part of science, and has been part of science forquite some time, provides a better basis for having that discussion than pretending that it wasn’tthere, and only implicitly using it, and then saying I haven’t done it. Once it’s out in the open, onecan discuss the pros and cons of those arguments within a specific context.”

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The Munich Debate

The trash heap of history is littered with beautiful theories. The Danish historian of cosmology HelgeKragh, who detailed a number of these failures in his 2011 book, Higher Speculations, spoke inMunich about the 19th-century vortex theory of atoms. This “Victorian theory of everything,”developed by the Scots Peter Tait and Lord Kelvin, postulated that atoms are microscopic vortexes inthe ether, the fluid medium that was believed at the time to fill space. Hydrogen, oxygen and allother atoms were, deep down, just different types of vortical knots. At first, the theory “seemed to behighly promising,” Kragh said. “People were fascinated by the richness of the mathematics, whichcould keep mathematicians busy for centuries, as was said at the time.” Alas, atoms are notvortexes, the ether does not exist, and theoretical beauty is not always truth.

Except sometimes it is. Rationalism guided Einstein toward his theory of relativity, which hebelieved in wholeheartedly on rational grounds before it was ever tested. “I hold it true that purethought can grasp reality, as the ancients dreamed,” Einstein said in 1933, years after his theory hadbeen confirmed by observations of starlight bending around the sun.

The question for the philosophers is: Without experiments, is there any way to distinguish betweenthe non-empirical virtues of vortex theory and those of Einstein’s theory? Can we ever really trust atheory on non-empirical grounds?

In discussions on the third afternoon of the workshop, the LMU philosopher Radin Dardashtiasserted that Dawid’s philosophy specifically aims to pinpoint which non-empirical arguments shouldcarry weight, allowing scientists to “make an assessment that is not based on simplicity, which is notbased on beauty.” Dawidian assessment is meant to be more objective than these measures,Dardashti explained — and more revealing of a theory’s true promise.

Gross said Dawid has “described beautifully” the strategies physicists use “to gain confidence in a

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speculation, a new idea, a new theory.”

“You mean confidence that it’s true?” asked Peter Achinstein, an 80-year-old philosopher andhistorian of science at Johns Hopkins University. “Confidence that it’s useful? confidence that …”

“Let’s give an operational definition of confidence: I will continue to work on it,” Gross said.

“That’s pretty low,” Achinstein said.

“Not for science,” Gross said. “That’s the question that matters.”

Kragh pointed out that even Popper saw value in the kind of thinking that motivates string theoriststoday. Popper called speculation that did not yield testable predictions “metaphysics,” but heconsidered such activity worthwhile, since it might become testable in the future. This was true ofatomic theory, which many 19th-century physicists feared would never be empirically confirmed.“Popper was not a naive Popperian,” Kragh said. “If a theory is not falsifiable,” Kragh said,channeling Popper, “it should not be given up. We have to wait.”

But several workshop participants raised qualms about Bayesian confirmation theory, and aboutDawid’s non-empirical arguments in particular.

Carlo Rovelli, a proponent of loop quantum gravity (string theory’s rival) who is based at Aix-Marseille University in France, objected that Bayesian confirmation theory does not allow for animportant distinction that exists in science between theories that scientists are certain about andthose that are still being tested. The Bayesian “confirmation” that atoms exist is essentially 100percent, as a result of countless experiments. But Rovelli says that the degree of confirmation ofatomic theory shouldn’t even be measured in the same units as that of string theory. String theory isnot, say, 10 percent as confirmed as atomic theory; the two have different statuses entirely. “Theproblem with Dawid’s ‘non-empirical confirmation’ is that it muddles the point,” Rovelli said. “And ofcourse some string theorists are happy of muddling it this way, because they can then say that stringtheory is ‘confirmed,’ equivocating.”

The German physicist Sabine Hossenfelder, in her talk, argued that progress in fundamental physicsvery often comes from abandoning cherished prejudices (such as, perhaps, the assumption that theforces of nature must be unified). Echoing this point, Rovelli said “Dawid’s idea of non-empiricalconfirmation [forms] an obstacle to this possibility of progress, because it bases our credence on ourown previous credences.” It “takes away one of the tools — maybe the soul itself — of scientificthinking,” he continued, “which is ‘do not trust your own thinking.’”

The Munich proceedings will be compiled and published, probably as a book, in 2017. As for whatwas accomplished, one important outcome, according to Ellis, was an acknowledgment byparticipating string theorists that the theory is not “confirmed” in the sense of being verified. “DavidGross made his position clear: Dawid’s criteria are good for justifying working on the theory, not forsaying the theory is validated in a non-empirical way,” Ellis wrote in an email. “That seems to me agood position — and explicitly stating that is progress.”

In considering how theorists should proceed, many attendees expressed the view that work on stringtheory and other as-yet-untestable ideas should continue. “Keep speculating,” Achinstein wrote in anemail after the workshop, but “give your motivation for speculating, give your explanations, butadmit that they are only possible explanations.”

“Maybe someday things will change,” Achinstein added, “and the speculations will become testable;and maybe not, maybe never.” We may never know for sure the way the universe works at all

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distances and all times, “but perhaps you can narrow the live possibilities to just a few,” he said. “Ithink that would be some progress.”

This article was reprinted on TheAtlantic.com.