ESA Science & Technology - Publication Archive

In the months since the publication of the first results, the noise performance of LISA Pathfinder has improved because of reduced Brownian noise due to the continued decrease in pressure around the test masses, from a better correction of noninertial effects, and from a better calibration of the electrostatic force actuation. In addition, the availability of numerous long noise measurement runs, during which no perturbation is purposely applied to the test masses, has allowed the measurement of noise with good statistics down to 20  μHz. The Letter presents the measured differential acceleration noise figure, which is at (1.74±0.01)  fm s^-2/√Hz above 2 mHz and (6±1)×10  fm s^-2/√Hz at 20  μHz, and discusses the physical sources for the measured noise. This performance provides an experimental benchmark demonstrating the ability to realize the low-frequency science potential of the LISA mission, recently selected by the European Space Agency.

DOI: https://doi.org/10.1103/PhysRevLett.120.061101
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

This issue of Spatium features an article by Professor Karsten Danzmann of the Max Planck Institute for Gravitation Physics and the Institute for Gravitation Physics of the Leibniz Universität, Hannover. It is devoted to gravitational waves, their sources and the marvellous technologies required for observing them. Over and above, this issue renders homage to Albert Einstein and the scientists of his time, who laid the cornerstone for our understanding of gravitational waves.

Spontaneous collapse models are phenomological theories formulated to address major difficulties in macroscopic quantum mechanics. We place significant bounds on the parameters of the leading collapse models, the continuous spontaneous localization (CSL) model, and the Diosi-Penrose (DP) model, by using LISA Pathfinder's measurement, at a record accuracy, of the relative acceleration noise between two free-falling macroscopic test masses. In particular, we bound the CSL collapse rate to be at most (2.96±0.12)×10^-8  s^–1. This competitive bound explores a new frequency regime, 0.7 to 20 mHz, and overlaps with the lower bound 10^-8±2  s^-1 proposed by Adler in order for the CSL collapse noise to be substantial enough to explain the phenomenology of quantum measurement. Moreover, we bound the regularization cutoff scale used in the DP model to prevent divergences to be at least 40.1±0.5  fm, which is larger than the size of any nucleus. Thus, we rule out the DP model if the cutoff is the size of a fundamental particle.

We report on electrostatic measurements made on board the European Space Agency mission LISA Pathfinder. Detailed measurements of the charge-induced electrostatic forces exerted on free-falling test masses (TMs) inside the capacitive gravitational reference sensor are the first made in a relevant environment for a space-based gravitational wave detector. Employing a combination of charge control and electric-field compensation, we show that the level of charge-induced acceleration noise on a single TM can be maintained at a level close to 1.0  fm s^-2 Hz^-1/2 across the 0.1–100 mHz frequency band that is crucial to an observatory such as the Laser Interferometer Space Antenna (LISA). Using dedicated measurements that detect these effects in the differential acceleration between the two test masses, we resolve the stochastic nature of the TM charge buildup due to interplanetary cosmic rays and the TM charge-to-force coupling through stray electric fields in the sensor. All our measurements are in good agreement with predictions based on a relatively simple electrostatic model of the LISA Pathfinder instrument.

Wave function collapse models postulate a fundamental breakdown of the quantum superposition principle at the macroscale. Therefore, experimental tests of collapse models are also fundamental tests of quantum mechanics. Here, we compute the upper bounds on the collapse parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder, and AURIGA. We consider the most widely used collapse model, the continuous spontaneous localization (CSL) model. We show that these experiments exclude a huge portion of the CSL parameter space, the strongest bound being set by the recently launched space mission LISA Pathfinder. We also rule out a proposal for quantum-gravity-induced decoherence.

Published online 7 June 2016

We report the first results of the LISA Pathfinder in-flight experiment. The results demonstrate that two free-falling reference test masses, such as those needed for a space-based gravitational wave observatory like LISA, can be put in free fall with a relative acceleration noise with a square root of the power spectral density of 5.2±0.1  fm s^-2/√Hz, or (0.54±0.01)×10^-15  g/√Hz, with g the standard gravity, for frequencies between 0.7 and 20 mHz. This value is lower than the LISA Pathfinder requirement by more than a factor 5 and within a factor 1.25 of the requirement for the LISA mission, and is compatible with Brownian noise from viscous damping due to the residual gas surrounding the test masses. Above 60 mHz the acceleration noise is dominated by interferometer displacement readout noise at a level of (34.8±0.3)  fm/√Hz, about 2 orders of magnitude better than requirements. At f≤0.5  mHz we observe a low-frequency tail that stays below 12  fm s^-2/√Hz down to 0.1 mHz. This performance would allow for a space-based gravitational wave observatory with a sensitivity close to what was originally foreseen for LISA.

 
 This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI (10.1103/PhysRevLett.116.231101)

ESA's LISA Pathfinder will help to open up a completely new observational window into the 'gravitational Universe', proving new technologies needed to measure gravitational waves in space. This article, by Claudia Mignone, explains how.

This media kit contains background information of use to journalists and reporters covering the LISA Pathfinder mission.

Topics covered:
Why LISA Pathfinder?
Mission at a glance
A challenging build
What LISA Pathfinder is doing and how
Paving the way for gravitational-wave observatories in space
100 years of general relativity
LISA Pathfinder in the context of great physics experiments

There are also contact details for members of the LISA Pathfinder team and press officers for the agencies and institutes involved.

Updated in June 2016 to reflect the detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and to account for operational milestones of LISA Pathfinder.

Albert Einstein's general theory of relativity predicted the existence of ripples in the fabric of space–time called gravitational waves, but so far no experiment has been able to detect them directly. Space offers many advantages in this search, and ESA's LISA Pathfinder mission is a technology demonstrator that will pave the way for future spaceborne gravitational-wave observatories by testing the necessary instrumentation for the first time in that environment.

Table of contents:

• A quest for silence
• In the realm of gravity
• The gravitational Universe
• How does it work?
• Building LISA Pathfinder
• Launch
• A physics laboratory in space
• An international enterprise

eLISA is a space mission designed to measure gravitational radiation over a frequency range of 0.1-100 mHz (European Space Agency LISA Assessment Study Report 2011). It uses laser interferometry to measure changes of order 10 pm per square root Hertz in the separation of inertial test masses housed in spacecraft separated by 1 million km. LISA Pathfinder (LPF) is a technology demonstrator mission that will test the key eLISA technologies of inertial test masses monitored by laser interferometry in a drag-free spacecraft. The optical bench that provides the interferometry for LPF must meet a number of stringent requirements: the optical path must be stable at the few per square root Hertz level; it must direct the optical beams onto the inertial masses with an accuracy of better than ±25 µm, and it must be robust enough not only to survive launch vibrations but to achieve full performance after launch. In this paper we describe the construction and testing of the flight optical bench for LISA Pathfinder that meets all the design requirements.

LISA Pathfinder, formerly known as SMART-2, is the second of the European Space Agency's Small Missions for Advance Research and Technology, and is designed to pave the way for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission, by testing the core assumption of gravitational wave detection and general relativity: that free particles follow geodesics. The new technologies to be demonstrated in a space environment include: inertial sensors, high precision laser interferometry to free floating mirrors, and micro-Newton proportional thrusters. LISA Pathfinder will be launched on a dedicated launch vehicle in late 2011 into a low Earth orbit. By a transfer trajectory, the sciencecraft will enter its final orbit around the first Sun-Earth Lagrange point. First science results are expected approximately 3 months thereafter. Here, we give an overview of the mission including the technologies being demonstrated.

LISA Pathfinder (formerly known as SMART-2) is an ESA mission designed to pave the way for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission by testing in-flight the critical technologies required for spaceborne gravitational wave detection; it will put two test masses in a nearperfect gravitational free fall, and control and measure their motion with an unprecedented accuracy. This is achieved through technology comprising inertial sensors, high-precision laser metrology, drag-free control and an ultraprecise micro-Newton propulsion system. The LISA Pathfinder mission is now in Phase C/Dthe Implementation Phase, and is due to be launched in 2010, with results on the performance of the system being available within 6 months thereafter.

LISA Pathfinder (LPF) is a science and technology demonstrator planned by the European Space Agency in view of the LISA mission. As a scientific payload, the LISA Technology Package on board LPF will be the most precise geodesics explorer flown as of today, both in terms of displacement and acceleration sensitivity. The challenges embodied by LPF make it a unique mission, paving the way towards the space-borne detection of gravitational waves with LISA. This paper summarizes the basics of LPF, and the progress made in preparing its effective implementation in flight. We hereby give an overview of the experiment philosophy and assumptions to carry on the measurement. We report on the mission plan and hardware design advances and on the progress on detailing measurements and operations. Some light will be shed on the related data processing algorithms. In particular, we show how to single out the acceleration noise from the spacecraft motion perturbations, how to account for dynamical deformation parameters distorting the measurement reference and how to decouple the actuation noise via parabolic free flight.

An overview of the LISA Pathfinder mission written by Paul McNamara (LISA Pathfinder Project Scientist).

An introduction to the LISA Pathfinder mission written by Giuseppe Racca (LPF Project Manager) and Paul McNamara (LPF Project Scientist).

ESA's report to the 37th COSPAR meeting (13-20 July 2008) covers the missions of the Science Programme of ESA. This section contains the report on the LISA Pathfinder mission.

The project science team has revisited the science case for LISA Pathfinder and produced this document on the scientific and technological goals of the mission. Abstract: LISA Pathfinder is an experiment to demonstrate Einstein's geodesic motion in space more than two orders of magnitude better than any past, present, or planned experiment, except for LISA. The concept that a particle falling under the influence of gravity alone follows a geodesic in spacetime is at the foundation of general relativity, our best model of gravitation, yet. LISA Pathfinder's experiment concept is to prove geodesic motion by tracking two test-masses nominally in free-fall through laser interferometry with picometre distance resolution. LISA Pathfinder will show that the relative parasitic acceleration between the masses, at frequencies around 1 mHz, is at least two orders of magnitude smaller than the value demonstrated so far or to be demonstrated by any planned mission. LISA Pathfinder hardware has been designed to be transferred directly to LISA. However, it is obvious that many other possibilities are opened by the results of LISA Pathfinder. LISA Pathfinder is a mission both in general relativity and in precision metrology and will open the ground for an entirely new generation of missions not just in general relativity, but in fundamental physics at large and in Earth observation.