Suspending test masses in terrestrial millihertz gravitational-wave detectors: a case study with a magnetic assisted torsion pendulum

TL;DR

This paper explores the technical challenges of suspending test masses in terrestrial millihertz gravitational-wave detectors, using a case study of a magnetically assisted torsion pendulum, and discusses potential solutions and limitations.

Contribution

It provides a detailed noise budget and analysis of suspension challenges for low-frequency terrestrial gravitational-wave detectors, highlighting the difficulties and potential mechanical alternatives.

Findings

Magnetic suspensions face significant noise and technical challenges.

Purely mechanical designs may be more feasible for millihertz detectors.

Developing millihertz suspensions is a highly demanding technological task.

Abstract

Current terrestrial gravitational-wave detectors operate at frequencies above 10 Hz. There is strong astrophysical motivation to construct low-frequency gravitational-wave detectors capable of observing 10 mHz - 10Hz signals. While space-based detectors provide one means of achieving this end, one may also consider terretrial detectors. However, there are numerous technological challenges. In particular, it is difficult to isolate test masses so that they are both seismically isolated and freely falling under the influence of gravity at millihertz frequencies. We investigate the challenges of low-frequency suspension in a hypothetical terrestrial detector. As a case study, we consider a Magnetically Assisted Gravitational-wave Pendulum Intorsion (MAGPI) suspension design. We construct a noise budget to estimate some of the required specifications. In doing so, we identify what are likely to be a number of generic limiting noise sources for terrestrial millihertz gravitational-wave suspension systems (as well as some peculiar to the MAGPI design). We highlight significant experimental challenges in order to argue that the development of millihertz suspensions will be a daunting task. Any system that relies on magnets faces even greater challenges. Entirely mechanical designs such as Zollner pendulums may provide the best path forward.