Unraveling the Spectrum of the Open String

Abstract

We construct a large portion of the massive spectrum of the open bosonic string using light-cone quantization, providing explicit oscillator realizations for individual single-particle states as well as for full Regge trajectories. We show how combinations of transverse oscillators organize into irreducible SO(25) representations, and provide an algorithm for constructing them level by level. We then develop a general method to "climb" the spectrum-adding oscillators in a controlled way that generates entire Regge trajectories from a finite set of seed states. Remarkably, the coefficients determining each state's oscillator composition depend on the level in a simple way, allowing closed-form expressions for infinitely many states. Beyond individual trajectories, we explore internal regularities of the spectrum and establish relations among families of trajectories, extending the concept of a Regge trajectory to more general constructions. Our results expose a highly ordered and recursive structure underlying the open-string spectrum, suggesting that its massive excitations form an algorithmically constructible network. The framework presented here lays the groundwork for computing three-point amplitudes of arbitrary massive states, the essential building blocks of string interactions, which we tackle in upcoming work.