New-born strings are tensionless

TL;DR

This paper introduces a novel physical origin for tensionless strings, showing they emerge only at their birth within finite-lifetime string dynamics in a causal diamond setting.

Contribution

It formulates string dynamics in finite regions, revealing tensionless strings arise at the moment of their creation, a departure from traditional eternal string models.

Findings

Tensionless strings appear only at the moment of their birth.

Finite-lifetime string dynamics in a causal diamond are formulated.

A new tensionless string phase with Carrollian structure is identified.

Abstract

We report a physical origin of tensionless strings, obtained by formulating string dynamics in finite-lifetime settings for the first time. Departing from the conventional paradigm of eternal strings, we construct an inertial worldsheet confined to a finite region of two-dimensional Minkowski spacetime, known as a causal diamond. This reveals a striking result: tensionless strings arise only at the moment of their birth. The ultra-shrinking limit of the diamond worldsheet realizes this birth configuration, thereby uncovering a new tensionless string phase characterized by a global, ultra-local Carrollian structure.