“Scientific method: Defend the integrity of physics” by George Ellis and Joe Silk,” Nature, open access:
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.
Earlier this year, championing the multiverse and the many-worlds hypothesis, Carroll dismissed Popper’s falsifiability criterion as a “blunt instrument” (see go.nature.com/nuj39z). He offered two other requirements: a scientific theory should be “definite” and “empirical”. By definite, Carroll means that the theory says “something clear and unambiguous about how reality functions”. By empirical, he agrees with the customary definition that a theory should be judged a success or failure by its ability to explain the data.
He argues that inaccessible domains can have a “dramatic effect” in our cosmic back-yard, explaining why the cosmological constant is so small in the part we see. But in multiverse theory, that explanation could be given no matter what astronomers observe. All possible combinations of cosmological parameters would exist somewhere, and the theory has many variables that can be tweaked. Other theories, such as unimodular gravity, a modified version of Einstein’s general theory of relativity, can also explain why the cosmological constant is not huge.
Some people have devised forms of multiverse theory that are susceptible to tests: physicist Leonard Susskind’s version can be falsified if negative spatial curvature of the Universe is ever demonstrated. But such a finding would prove nothing about the many other versions. Fundamentally, the multiverse explanation relies on string theory, which is as yet unverified, and on speculative mechanisms for realizing different physics in different sister universes. It is not, in our opinion, robust, let alone testable.
No wonder some would like to abandon testability for elegance, and reality for fairy tales.
Unfortunately, the plea ends on a somewhat tinny note,
The imprimatur of science should be awarded only to a theory that is testable. Only then can we defend science from attack.
Guys, listen (yes, you George Ellis and you Joe Silk, it is you we are looking at): The problem really isn’t attacks from outside. Quit fooling yourselves.
The problem is entirely within. If physicists want to join the many and various advocates of self-expression who do not depend on rigorous examination of evidence to validate their assertions, that is a choice physicists make.
No one forces that choice on physicists. But they are free to make it.
It sounds as though some of your colleagues have been making just such choices, and defending their choices by asking for exemption from traditional standards. It’s your profession’s call to determine whether their wishes/demands can be accommodated simply to prop up whatever rickety theoretical structures they have built.
But if your profession does choose to accommodate, two things:
1. Physics becomes just another player in a culture war, with no more genuinely respectable claims for attention than the demands we hear daily from grievance warriors that their version of events be accepted without cavil as Truth. You could find yourselves currying favour with politicians, as an identity group, for your version of nature versus that of magical thinking. Is that really what you want?
2. If so, just remember, no one did that to you. You did it to yourselves.
See also: The bill arrives for cosmology’s free lunch
Hat tip: Peter Woit