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

Towards Physics of Internal Observers: Exploring the Roles of External and Internal Observers