Core Concepts

The absence of spacetime supersymmetry in string theory leads to the emergence of tadpole diagrams, which have dramatic consequences for the physics and require a complete change of perspective.

Abstract

The content discusses the spacetime aspects of non-supersymmetric string theories. It starts by emphasizing the importance of conformal invariance as the defining feature of string theory, which leads to the emergence of spacetime geometry and gravity. The author then introduces the one-loop vacuum amplitudes, which are efficient tools for determining the complete perturbative spectrum of string theories.

The main focus of the work is the three known ten-dimensional tachyon-free string theories that do not have spacetime supersymmetry: the SO(16)×SO(16) heterotic string, the USp(32) orientifold of type IIB string theory, and the type 0A and 0B orientifolds. These non-supersymmetric string theories are the central topic of the thesis, and the author explores their implications for the physics, particularly the consequences of the absence of supersymmetry.

To Another Language

from source content

arxiv.org

Stats

The one-loop vacuum amplitude of the SO(16)×SO(16) heterotic string is negative, indicating a net excess of massless fermions over massless bosons.
The one-loop vacuum amplitude of the USp(32) orientifold differs from the type I string theory by a sign change in the Möbius strip contribution.

Quotes

"Conformal invariance is the defining feature of string theory. The sum over two-dimensional geometries, which characterizes the worldsheet formulation, can be understood only when conformal invariance holds."
"Apples fall, and we currently lack a satisfactory understanding and a consistent mathematical framework for the mechanism by which they fall: gravity. Explicitly, the missing tile is the absence of spacetime supersymmetry in a quantum theory of gravity."

Key Insights Distilled From

by Salvatore Ra... at **arxiv.org** 10-01-2024

Deeper Inquiries

The study of non-supersymmetric string theories provides a unique framework for exploring the fundamental aspects of our universe, which appears to lack spacetime supersymmetry. By examining the gravitational consequences of tadpole potentials and the vacuum solutions in these theories, researchers can gain insights into the stability and dynamics of our universe. Non-supersymmetric string theories, such as the SO(16)×SO(16) heterotic string and the orientifold models, reveal how the absence of supersymmetry leads to significant modifications in the behavior of string-derived vacua.
These insights can be particularly relevant for understanding phenomena such as the emergence of scalar fields and their potential implications for cosmology. For instance, the scalar tadpole potentials that arise in non-supersymmetric settings can be linked to the dynamics of the early universe, potentially influencing inflationary models and the evolution of the cosmological constant. Furthermore, the exploration of brane-like solutions in these theories can shed light on the role of localized sources and their interactions, which may parallel the behavior of matter and energy in our universe. Thus, the insights from non-supersymmetric string theories can help bridge the gap between theoretical frameworks and observable cosmological phenomena.

The absence of spacetime supersymmetry in string theory has profound implications for our understanding of quantum gravity. Supersymmetry is often viewed as a stabilizing principle that mitigates the effects of quantum corrections, leading to a more controlled and predictable theory of gravity. In non-supersymmetric string theories, however, the emergence of tadpole diagrams and the associated scalar potentials indicate that quantum corrections can become significant, potentially destabilizing the vacuum and leading to a breakdown of the theory.
This instability raises critical questions about the nature of quantum gravity in a non-supersymmetric context. It suggests that our current understanding of gravitational interactions may need to be revised, as the absence of supersymmetry could lead to unexpected phenomena, such as the emergence of new degrees of freedom or modifications to the effective field theories that describe gravity at low energies. Additionally, the study of non-supersymmetric string theories may provide insights into the nature of singularities and the behavior of spacetime under extreme conditions, which are central issues in the quest for a complete theory of quantum gravity.

Yes, there are intriguing connections between the tadpole potentials that arise in non-supersymmetric string theories and the cosmological constant problem. In string theory, tadpole potentials are associated with the presence of certain fields in the vacuum, which can lead to non-trivial contributions to the vacuum energy. The cosmological constant problem, which refers to the discrepancy between the observed value of the cosmological constant and theoretical predictions from quantum field theory, can be viewed through the lens of these tadpole contributions.
In non-supersymmetric string theories, the presence of scalar tadpole potentials can lead to a vacuum energy that is not only non-zero but also potentially negative, as indicated by the negative torus amplitude in models like the SO(16)×SO(16) string. This situation mirrors the challenges faced in addressing the cosmological constant problem, where the effective vacuum energy density must be finely tuned to match the observed value. The insights gained from studying tadpole potentials in non-supersymmetric contexts may provide new avenues for understanding how vacuum energy behaves and how it could be reconciled with the observed dynamics of our universe. Thus, exploring these connections could lead to a deeper understanding of both string theory and cosmological phenomena.

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