String
theory, the multiverse and other ideas of modern physics are
potentially untestable. At a historic meeting in Munich, scientists and
philosophers asked: should we trust them anyway?
Physicists George Ellis (center) and Joe Silk (right) at Ludwig Maximilian University in Munich on Dec. 7.
Physicists
typically think they “need philosophers and historians of science like
birds need ornithologists,” the Nobel laureate David Gross told a roomful of philosophers, historians and physicists 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.
The crisis, as Ellis and Silk tell it, is the wildly speculative
nature of modern physics theories, which they 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
— appear convinced of their ideas on the grounds that they are
beautiful or logically compelling, despite the impossibility of testing
them. Ellis and Silk accused these theorists of “moving the goalposts”
of science and blurring the line between physics and pseudoscience. “The
imprimatur of science should be awarded only to a theory that is
testable,” Ellis and Silk wrote, thereby disqualifying most of the leading 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 philosopher whose 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, a
researcher at LMU Munich, answered Ellis and Silk’s battle cry and
assembled far-flung scholars anchoring all sides of the argument for the
high-profile event last week.
Gross, a supporter of string theory who won the 2004 Nobel Prize in
physics for his work on the force that glues atoms together, kicked off
the workshop by asserting that the problem lies not with physicists but
with a “fact of nature” — one that we have been approaching inevitably
for four centuries.
The dogged pursuit of a fundamental theory governing all forces of
nature requires physicists to inspect the universe more and more closely
— to examine, for instance, the atoms within matter, the protons and
neutrons within those atoms, and the quarks within those protons and
neutrons. But this zooming in demands evermore energy, and the
difficulty and cost of building new machines increases exponentially
relative to the energy requirement, Gross said. “It hasn’t been a
problem so much for the last 400 years, where we’ve gone from
centimeters to millionths of a millionth of a millionth of a centimeter”
— the current resolving power of the Large Hadron Collider (LHC) in
Switzerland, 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 minds of theorists have wandered far beyond
the tiniest observable distances and highest possible energies. Strong
clues indicate that the truly fundamental constituents of the universe
lie at a distance scale 10 million billion times smaller than the
resolving power of the LHC. This is the domain of nature that string
theory, a candidate “theory of everything,” attempts to describe. But
it’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: No telescope will ever manage to peer past our
universe’s cosmic horizon and glimpse the other universes posited by the
multiverse hypothesis. Yet modern theories of cosmology lead logically
to the 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 best secrets, the conclusion is the same:
Theory has detached itself from experiment. The objects of theoretical
speculation are now too far away, too small, too energetic or too far in
the past to reach or 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 that can 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 the most 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 or the other way around. Over the course of
these pressing yet timeless discussions, a degree of consensus took
shape.
Rules of the Game
Throughout history, the rules of science have been written on the
fly, only to be revised to fit evolving 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 from empirical observation. In
other words, a theory must be proved experimentally to enter the book of
knowledge.
But what requirements must an untested theory meet to be considered
scientific? Theorists guide the scientific enterprise by dreaming up the
ideas to be put to the test and then interpreting the experimental
results; what keeps theorists within the bounds of science?
Today, most physicists judge the soundness of a theory by using the
Austrian-British philosopher Karl Popper’s rule of thumb. In the 1930s,
Popper drew a line between science and nonscience in comparing the work
of Albert Einstein with that of Sigmund Freud. Einstein’s theory of
general relativity, which cast the force of gravity as curves in space
and time, made risky predictions — ones that, if they hadn’t succeeded
so brilliantly, would have failed miserably, falsifying the theory. But
Freudian psychoanalysis was slippery: Any fault of your mother’s could
be worked into your diagnosis. The theory wasn’t falsifiable, and so,
Popper decided, it wasn’t science.
Critics accuse string theory and the multiverse hypothesis, as well as cosmic inflation
— the leading theory of how the universe began — of falling on the
wrong side of Popper’s line of demarcation. To borrow the title of the
Columbia University physicist Peter Woit’s 2006 book on string theory,
these ideas 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 philosophy of science. Massimo Pigliucci,
a philosopher at the Graduate Center of the City University of New
York, 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 ad nauseam —
and yet it isn’t science. Physicists’ preoccupation with Popper “is
really something that needs to stop,” Pigliucci said. “We need to talk
about current philosophy of science. We don’t talk about something that
was current 50 years ago.”
Nowadays, as several philosophers at the workshop said, Popperian
falsificationism has been supplanted by Bayesian confirmation theory, or
Bayesianism, a modern framework based on the 18th-century probability
theory of the English statistician and minister Thomas Bayes.
Bayesianism allows for the fact that modern scientific theories
typically make claims far beyond what can be directly 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 up or down between 0 and 100
percent as new information becomes available. “The Bayesian framework is
much more flexible” than Popper’s theory, said Stephan Hartmann, a Bayesian philosopher 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 my
life!’”
Another advantage of Bayesianism, Hartmann said, is that it is
enabling philosophers like Dawid to figure out “how this non-empirical
evidence fits in, or can be fit in.”
Another Kind of Evidence
Dawid, who is 49, mild-mannered and smiley with floppy brown hair,
started his career as a theoretical physicist. In the late 1990s, during
a stint at the University of California, Berkeley, a hub of
string-theory research, Dawid became fascinated by how confident many
string theorists seemed to 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
thinking about it than the canonical understanding?”
String theory says that elementary particles have dimensionality when
viewed close-up, appearing as wiggling 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 the strings in this higher-dimensional
space give rise to the spectrum of particles that make up the observable
world. In particular, one of the vibrational modes fits the profile of
the “graviton” — the hypothetical particle associated with the force of
gravity. Thus, string theory unifies gravity, now described by Einstein’s theory of general relativity, with the rest of particle physics.
Laetitia Vancon for Quanta Magazine
Video: Richard Dawid, a physicist-turned-philosopher at Ludwig Maximilian University in Munich.
However string theory, which has its roots in ideas developed in the
late 1960s, has made no testable predictions about the observable
universe. To understand why so many researchers trust it anyway, Dawid
signed up for some classes in philosophy of science, and upon
discovering how little study had been devoted to the phenomenon, he
switched fields.
In the early 2000s, he identified three non-empirical arguments that
generate trust in string theory among its proponents. First, there
appears to be only one version of string theory capable of achieving
unification in a consistent way (though it has many different
mathematical representations); furthermore, no other “theory of
everything” capable of unifying all the fundamental forces has been
found, despite immense effort. (A rival approach called loop quantum
gravity describes gravity at the quantum scale, but makes no attempt to
unify it with the other forces.) This “no-alternatives” argument,
colloquially known as “string theory is the only game in town,”
boosts theorists’ confidence that few or no other possible unifications
of the four fundamental forces 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 theory incorporating all known fundamental
particles and forces (apart from gravity) in a single mathematical
structure — and the Standard Model also had no alternatives during its
formative years. This “meta-inductive” argument, as Dawid calls it,
buttresses the no-alternatives argument by showing that it has worked
before in similar contexts, countering the possibility that physicists
simply 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.
The third non-empirical argument is that string theory has
unexpectedly delivered explanations for several 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, presented several
examples of these “unexpected explanatory interconnections,” as Dawid
has termed them, in a paper
read in Munich in his absence. String theory explains the entropy of
black holes, for example, and, in a surprising discovery that has caused
a surge of research in the past 15 years, is mathematically translatable into a theory of particles, such as the theory describing the nuclei of atoms.
Polchinski concludes that, considering how far away we are from the
exceptionally fine grain of nature’s fundamental distance scale, we
should count ourselves lucky: “String theory exists, and we have found
it.” (Polchinski also used Dawid’s non-empirical arguments to calculate
the Bayesian odds that the multiverse exists as 94 percent — a value
that has been ridiculed by the Internet’s vocal multiverse critics.)
One concern with including non-empirical arguments in Bayesian
confirmation theory, Dawid acknowledged in his talk, is “that it opens
the floodgates to abandoning all scientific principles.” One can come up
with all kinds of non-empirical virtues when arguing in favor of a pet
idea. “Clearly the risk is there, and clearly one has to be careful
about this kind of reasoning,” Dawid said. “But acknowledging that
non-empirical confirmation is part of science, and has been part of
science for quite some time, provides a better basis for having that
discussion than pretending that it wasn’t there, and only implicitly
using it, and then saying I haven’t done it. Once it’s out in the open,
one can discuss the pros and cons of those arguments within a specific
context.”
The Munich Debate
The trash heap of history is littered with beautiful theories. The Danish historian of cosmology Helge Kragh, who detailed a number of these failures in his 2011 book, Higher Speculations,
spoke in Munich 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 in the
ether, the fluid medium that was believed at the time to fill space.
Hydrogen, oxygen and all other atoms were, deep down, just different
types of vortical knots. At first, the theory “seemed to be highly
promising,” Kragh said. “People were fascinated by the richness of the
mathematics, which could keep mathematicians busy for centuries, as was
said at the time.” Alas, atoms are not vortexes, 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 he believed in wholeheartedly on rational grounds
before it was ever tested. “I hold it true that pure thought can grasp
reality, as the ancients dreamed,” Einstein said in 1933, years after
his theory had been confirmed by observations of starlight bending
around the sun.
The question for the philosophers is: Without experiments, is there
any way to distinguish between the non-empirical virtues of vortex
theory and those of Einstein’s theory? Can we ever really trust a theory
on non-empirical grounds?
In discussions on the third afternoon of the workshop, the LMU
philosopher Radin Dardashti asserted that Dawid’s philosophy
specifically aims to pinpoint which non-empirical arguments should carry
weight, allowing scientists to “make an assessment that is not based on
simplicity, which is not based 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 speculation, a new idea, a new
theory.”
“You mean confidence that it’s true?” asked Peter Achinstein,
an 80-year-old philosopher and historian 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 theorists today. Popper called speculation that
did not yield testable predictions “metaphysics,” but he considered such
activity worthwhile, since it might become testable in the future. This
was true of atomic 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 about Dawid’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 an important distinction that
exists in science between theories that scientists are certain about and
those that are still being tested. The Bayesian “confirmation” that
atoms exist is essentially 100 percent, as a result of countless
experiments. But Rovelli says that the degree of confirmation of atomic
theory shouldn’t even be measured in the same units as that of string
theory. String theory is not, say, 10 percent as confirmed as atomic
theory; the two have different statuses entirely. “The problem with
Dawid’s ‘non-empirical confirmation’ is that it muddles the point,”
Rovelli said. “And of course some string theorists are happy of muddling
it this way, because they can then say that string theory is
‘confirmed,’ equivocating.”
The German physicist Sabine Hossenfelder,
in her talk, argued that progress in fundamental physics very often
comes from abandoning cherished prejudices (such as, perhaps, the
assumption that the forces of nature must be unified). Echoing this
point, Rovelli said “Dawid’s idea of non-empirical confirmation [forms]
an obstacle to this possibility of progress, because it bases our
credence on our own previous credences.” It “takes away one of the tools
— maybe the soul itself — of scientific thinking,” 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 what was accomplished, one important outcome,
according to Ellis, was an acknowledgment by participating string
theorists that the theory is not “confirmed” in the sense of being
verified. “David Gross made his position clear: Dawid’s criteria are
good for justifying working on the theory, not for saying the theory is
validated in a non-empirical way,” Ellis wrote in an email. “That seems
to me a good position — and explicitly stating that is progress.”
In considering how theorists should proceed, many attendees expressed
the view that work on string theory and other as-yet-untestable ideas
should continue. “Keep speculating,” Achinstein wrote in an email after
the workshop, but “give your motivation for speculating, give your
explanations, but admit 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 distances and all
times, “but perhaps you can narrow the live possibilities to just a
few,” he said. “I think that would be some progress.” This article was reprinted on TheAtlantic.com.
