Broadly then, in the red corner we have string theory, which is our current best stab at unifying the four forces of nature, but which despite three and a half decades of research is still speculative. Some physicists claim that despite all the progress it has made, it is now reaching something of a crisis because it hasn’t delivered on its early promise. Indeed, it can be argued that it is not yet even a proper scientific theory, since it has not made any testable predictions.
