Emergent rainbow spacetimes: Two pedagogical examples

TL;DR

This paper presents two concrete examples of energy-dependent rainbow geometries, illustrating their mathematical consistency and physical relevance, and suggesting ways to explore emergent spacetime concepts beyond quantum gravity.

Contribution

It introduces two pedagogical models—acoustic spacetimes with dispersion and a reinterpretation of Newton's law—that exemplify energy-dependent geometries, clarifying their physical and mathematical validity.

Findings

Rainbow geometries are mathematically consistent in specific models.

Energy-dependent conformally flat manifolds can model classical mechanics.

These examples serve as templates for more complex emergent spacetime theories.

Abstract

There is a possibility that spacetime itself is ultimately an emergent phenomenon, a near-universal "low-energy long-distance approximation", similar to the way in which fluid mechanics is the near-universal low-energy long-distance approximation to quantum molecular dynamics. If so, then direct attempts to quantize spacetime are misguided - at least as far as fundamental physics is concerned. Based on this and other considerations, there has recently been a surge of interest in the notion of energy-dependent and momentum-dependent "rainbow'' geometries. In the present article I will not discuss these exotic ideas in any detail, instead I will present two specific and concrete examples of situations where an energy-dependent "rainbow'' geometry makes perfectly good mathematical and physical sense. These simple examples will then serve as templates suggesting ways of proceeding in situations where the underlying physics may be more complex. The specific models I will deal with are (1) acoustic spacetimes in the presence of nontrivial dispersion, and (2) a mathematical reinterpretation of Newton's second law for a non-relativistic conservative force, which is well-known to be equivalent to the differential geometry of an energy-dependent conformally flat three-manifold. These two models make it clear that there is nothing wrong with the concept of an energy-dependent "rainbow'' geometry per se. Whatever problems may arise in the implementation of any specific quantum-gravity-inspired proposal for an energy-dependent spacetime are related to deeper questions regarding the compatibility of that specific proposal with experimental reality.