Last week’s Observer carried a debate over the status of string theory by a theoretical physicist, Michael Duff,  and a science journalist, James Baggott.  Mostly, they talk past each other.   There is much in what they say that could provoke comment, but since time is short,  I will only comment on one statement.
Duff’s final contribution includes these words:

Finally, you offer no credible alternative. If you don’t like string theory the answer is simple: come up with a better one. “

This is plain wrong for several reasons.  First, we would have no scientific progress at all if critics of scientific theories first had to develop an alternative theory before they could advance their criticisms.   Indeed, public voicing of criticisms of a theory is one of the key motivations for other scientists to look for alternatives in the first place.  So Duff has the horse and the cart backwards here.  
Secondly, “come up with a better one“?   “better“?     What means “better“?  Duff has missed precisely the main point of the critics of string theory!  We have no way of knowing – not even in principle, let alone in practice – whether string theory is any good or not, nor whether it accurately describes reality.  We have no experimental evidence by which to assess it, and most likely (since it posits and models alleged additional dimensions of spacetime that are inaccessible to us) not ever any way to obtain such empirical evidence.    As I have argued before, theology has more empirical support – the personal spiritual experiences of religious believers and practitioners – than does string theory.    So, suppose we did come up with an alternative theory to string theory:  how then could we tell which theory was the better of the two?   
Pure mathematicians, like theologians, don’t use empirical evidence as a criterion for evaluating theories.  Instead, they use subjective criteria such as beauty, elegance, and self-coherence.   There is nothing at all wrong with this.  But such criteria ain’t science, which by its nature is a social activity.