Naturalness versus the anthropic principle

The cosmological constant problem (CCP) is one of the most pressing problems in physics. It has eluded traditional approaches based on symmetries or dynamics. In contrast, the anthropic principle has scored a significant success in accounting for both the smallness of the cosmological constant (CC) and the proximity of the vacuum and matter energies in our universe [1]. Once we accept the anthropic principle as a legitimate approach for solving the CCP, it is natural to ask whether it might be applicable to other problems that can also be addressed with traditional methods. In this case, nature would have the interesting dilemma of choosing between an anthropic and a normal solution. An example is the gauge hierarchy problem (GHP). Like the CCP, it is a naturalness problem characterized by a small dimensionless number. Unlike the CCP, it can be solved with traditional symmetries, such as low-energy supersymmetry. As we will argue later, the GHP can also be addressed via anthropic arguments. So does nature choose the supersymmetric or the anthropic solution to the GHP? This question is far from academic, since the answer will be revealed experimentally by 2007 at the Large Hadron Collider (LHC).

The rise of naturalness as a principle for physics in the late 1970s led to the apparent need for a natural solution to the GHP and has convinced the majority of particle physicists that the LHC will discover either supersymmetry or another ‘natural’ theory that solves GHP. So if the LHC discovers nothing beyond the Standard Model, it will be a surprise. In our opinion, such a (non-)discovery would significantly strengthen the case for the anthropic principle and would cause a shift away from the usual naturalness-driven paradigm of attempting to understand the parameters of the Standard Model via symmetries or string theory. Instead, the nature of the dynamics that leads to the multiverse, and consequently provides a home for the anthropic principle, will become a primary question of physics. We now turn to this question.