  In both quantum mechanics and relativity theory, the concept of the observer
plays a critical role. However, there is no consensus on the definition of
observer in these theories. Following Einstein's thought experiments, one could
ask: What would it look like to sit inside a photon or to be a photon? And what
type of observer could represent this more global perspective of the photon's
interior? To address these questions, we introduce the concepts of internal and
external observers with a focus on their relationship in quantum theory and
relativity theory. The internal observer, associated with the internal
observables super-algebra, glues the external interactions.
  Drawing inspiration from the advancements in abstract algebraic topology, we
propose mathematical representation of the internal observer. We also outline
principles for ensuring the consistency of observers in terms of information
theory. It becomes evident, through the analysis of the introduced hierarchy of
observers, that entanglement is a primitive of space-time causal relationships.
While external observers must abide by the relativistic causality linked with
the no-signaling principle in quantum mechanics, the internal observer is
inherently non-local and may be acausal. However, its consistency is maintained
through the formulation of the self-consistency principle.
  One of the goals of this paper is to construct the representation of the
internal observer from the local external algebra of observables, which can be
associated with external observers. Additionally, we demonstrate how the
concepts of internal and external observers can be applied in the fields of
quantum information theory, algebraic quantum field theory, and loop quantum
gravity. The concept of internal observer seems to be also fundamental for
further development of quantum gravity.
