The Dawn of the Post-Naturalness Era

Gian Francesco Giudice is a theoretical physicist at CERN (where my brother Andrew did research); the research activity of Giudice mainly deals with the formulation of new theories that extend our present knowledge of the particle world toward even smaller distances. He is also studying how these theories can be applied to cosmology in order to describe the early stages of the history of our universe. His most notable results are in the areas of supersymmetry, extra dimensions, electroweak physics, collider physics, dark matter, and leptogenesis. Giudice is clear in the Dawn of the Post-Naturalness Era (In an imaginary conversation with Guido Altarelli), he still holds that “naturalness is a well-defined concept.” Dr Sabine Hossenfelder believes this is wrong, or rather if you make naturalness well-defined, it becomes meaningless. Dr Sabine Hossenfelder's argument goes as follows. Naturalness in quantum field theories – ie, theories of the type of the standard model of particle physics – means that a theory at low energies does not sensitively depend on the choice of parameters at high energies. People say this means that “the high-energy physics decouples.” However that changing the parameters of a theory is not a physical process - the parameters are whatever they are. The processes that are physically possible at high energies decouple whenever effective field theories work, pretty much by definition of what it means to have an effective theory. However this is not the decoupling that naturalness relies on. To quantify naturalness you move around between theories in an abstract theory space. This is very similar to moving around in the landscape of the multiverse. Indeed, it is probably not a coincidence that both ideas became popular around the same time, in the mid 1990s. So if you want to quantify how sensitively a theory at low energy depends on the choice of parameters at high energies, you first have to define the probability for making such choices. This means you need a probability distribution on theory space. It’s the exact same problem you also have for inflation and in the multiverse. Most papers on naturalness, have the probability distribution left unspecified which implicitly means one chooses a uniform distribution over an interval of about length 1. The typical justification for this is that once you factor out all dimensionful parameters, you should only have numbers of order 1 left. It is with this assumption that naturalness becomes meaningless because you have now simply postulated that numbers of order 1 are better than other numbers. You can read Gian's paper here: https://arxiv.org/pdf/1710.07663.pdf