More than three years ago Scientific American ran a feature article by Bousso and Polchinski promoting the then new idea of The String Theory Landscape. Now that this pseudo-science has become well-entrenched in the physics community, this month’s issue of the magazine has a feature article on The Great Cosmic Roller-Coaster Ride, describing how
one of the emerging themes of 21st-century cosmology is that the known universe, the sum of all we can see, may just be a tiny region in the full extent of space
and claiming that this is “stimulating a thorough rethinking of the early universe in terms of string theory.” There is quite a bit of defensiveness about string theory in the article, where it is described as the “leading candidate for the foundational laws of nature”. The authors note that “String theory has received some unfavorable press of late”, and characterize criticism of the theory as due to the fact that it “has yet to be tested experimentally”, ignoring the fact that much of the criticism is about string theory’s inherent untestability. Not only has it not been tested yet, but no one has any idea how to test it ever. They admit as much when it comes to predictions about particle physics:
string theory has disappointed because it has not yet been possible to test it experimentally, despite more than 20 years of continued investigation. It has proved hard to find a smoking gun – a prediction that, when tested, would decisively tell us whether or not the world is made of strings. Even the Large Hadron Collider (LHC) – which is now nearing completion near CERN , the European laboratory for particle physics near Geneva – may not be powerful enough.
At the same time, they imply that the answer to string theory’s problems is that it will produce testable predictions about cosmology. They describe work of their own and other people attempting to use as an inflaton field positions of branes or moduli parameters describing positions in the Landscape. What predictions do they see coming out of this?
I don’t think it does much for the public understanding of science or increases respect for scientists when they decide to go to the public in this way, promoting extremely speculative and complex ideas that lack not only a glimmer of experimental evidence, but also any plausible idea about how they can be tested.
Update: There’s a new review of string cosmology up on the arXiv tonight. The authors contradict the SciAm article’s claim about whether these models can accomodate observed effects of gravitational waves:
As an example, in many – but not necessarily all – string inflation models, the primordial tensor signal is very small.
giving examples of models with detectable gravitational waves (see here).
While the article as a whole is pretty much unadulterated hype for string cosmology, it ends on a downbeat:
Despite these promising signs, it remains to be seen whether this endeavor will lead to genuine contact between experiment and Planck-scale physics. In many scenarios, inflation is described by a well-controlled, albeit fine-tuned, effective field theory Lagrangian, and inflation lasts long enough to obscure all evidence of a pre-inflationary stage. If we live in such a universe, cosmological observations can, at best, teach us about the nature of the inflaton, but will provide few clues about more fundamental physics, except perhaps through the enduring mystery of dark energy.