Weak Gravity Conjecture From Low Energy Observers' Perspective
  The Weak Gravity Conjecture (WGC) was proposed to constrain Effective Field
Theories (EFTs) with Abelian gauge symmetry coupled to gravity. In this
article, I study the WGC from low energy observers' perspective, and revisit
the issue of to what extent the WGC actually constrains EFTs. For this purpose,
for a given EFT, I introduce associated idealized low energy observers who only
have access to the energy scale below the UV cut-off scale of the EFT. In the
framework of EFT, there is a clear difference between the particles lighter
than the UV cut-off scale and the particles which are heavier than the UV
cut-off scale, as the lighter particles can be created below the UV cut-off
scale while the heavier particles are not. This difference implies that the
knowledge of the low energy observers on the stable heavy particles can be
limited, as the availability of the stable heavy particles is determined by the
environment prepared by some UV theory unknown to the low energy observers. The
limitation of the knowledge of the low energy observers regarding the stable
heavy particles whose mass is above the UV cut-off scale of the EFT leads to
the limitation of the WGC for constraining EFTs. To illustrate these points in
an example, I analyze a model proposed by Saraswat arXiv:1608.06951 which
respects the WGC at high energy, but which may appear to violate the WGC for
the low energy observers. Implications of the analysis to the bottom-up model
buildings using EFTs are discussed.
