LISA and Its Pathfinder Karsten Danzmann for the LISA Pathfinder Team and the Elisa Consortium on Astronomical Scales, Gravity Is the Engine of the Universe

FOCUS | COMMENTARY

for full contact with the differential equations between the instruments and the spacecraft being commissioned and inter-calibrated. that describe reconnection. One of the four command and data handling subsystem, All systems are working well as we enter a spacecraft is placed in the desired orbit and which implements a mass memory module few weeks of eclipse season, during which the other three are maneuvered relative to and burst data system with storage sufficient commissioning will stand down, followed it, using periodic thruster firings. Position for 4–5 days of data. Only a fraction of the by several more weeks of activations, inter- is measured by a navigator system that is full-resolution data can be telemetered. calibrations and preparations of the ground the first to use the global positioning system The high-resolution data is first reduced to system for full operation and arrival of the (GPS) from outside the GPS constellation. survey data products that are sent from the MMS constellation in the dayside region of The MMS orbits are coordinated with those four spacecraft to the ground. Then events interest for its first reconnection studies. ❐ of the NASA THEMIS (Time History of of interest are identified, and finally the Events and Macroscale Interactions during spacecraft are told which full-resolution data Thomas Earle Moore is at NASA’s Goddard Space Substorms) mission, which currently should be sent. The download, processing Flight Center, Greenbelt, Maryland 20771, USA. measures conditions on the opposite side of and upload process includes automated James L Burch is at Southwest Research Institute, the magnetosphere with a more extended steps that can be overridden or modified by San Antonio, Texas 78238, USA. Roy B. Torbert constellation of three spacecraft. ground personnel. is at the University of New Hampshire, Durham, Each MMS spacecraft carries an identical The MMS mission is a significant step New Hampshire 03824, USA. instrument complement that includes suites towards understanding when, where and how e‑mail: [email protected] for plasmas, energetic particles and fields, explosively magnetic reconnection occurs. References as well as two active spacecraft potential Every bit of experience and skill gained in 1. Priest, E. & Forbes, T. Magnetic Reconnection: MHD Theory and control devices and a central instrument data previous missions has been integrated into Applications (Cambridge Univ. Press, 2000). processor. The control devices neutralize the MMS science payload, and we expect 2. Burch, J. L. & Drake, J. F. Am. Sci. 97, 392 (2009). 3. Daughton, W. et al. Nature Phys. 7, 539–542 (2011). the electrical potential of the spacecraft, to learn a huge amount about this universal 4. Moore, T. E. et al. J. Atmos. Solar. Terr. Phys. 99, 32–40 (2013). which not only allows measurement of process as the mission begins to operate in 5. Burch, J. L. et al. Space Sci. Revs. http://doi.org/6c4 (2015). low-energy ions and electrons but also September. At the time of writing, a smooth 6. Fuselier, S. A. et al. Space Sci. Revs. http://doi.org/5pw (2014). eliminates spurious electric fields that can launch has placed the spacecraft in the Acknowledgements contaminate double-probe measurements. specified orbits, nearly all of the instruments This work was supported by the NASA Magnetospheric The data processor provides the interface have been activated, checked out, and are MultiScale mission and its partners. LISA and its pathfinder Karsten Danzmann for the LISA Pathfinder Team and the eLISA Consortium On astronomical scales, gravity is the engine of the Universe. The launch of LISA Pathfinder this year to prepare the technology to detect gravitational waves will help us ‘listen’ to the whole Universe.

he past century has seen enormous black holes across all stages of galaxy of low-frequency gravitational waves progress in our understanding of the evolution, and at the same time constrain using laser interferometry between free- TUniverse. We know the life cycles any deviation from the Kerr metric of flying test masses shielded by drag-free of stars and the structure of galaxies, have general relativity. spacecraft at the corners of an equilateral seen the remnants of the Big Bang, and triangle with million-kilometre arm lengths have a general understanding of how the Making it happen in heliocentric orbit some ten million Universe evolved. We have gone remarkably The European Space Agency (ESA) recently kilometres from the Earth. far using electromagnetic radiation as our selected ‘The Gravitational Universe’ as observational tool. However, gravity is the science theme for its third large space The fantastic science the engine behind many of the processes mission, L3, with a foreseen launch date in LISA will be the first ever mission to allow in the Universe, and much of its action 2034. The reference mission for this theme the study of the entire Universe with is dark. Opening a gravitational window is a large space-based laser-interferometric gravitational waves. LISA is an all-sky on the Universe will let us go further gravitational wave observatory, known as monitor and will offer a wide view of a than any alternative. Gravity has its own the Laser Interferometer Space Antenna, or dynamic cosmos using gravitational waves messenger: gravitational waves, ripples LISA. It has been under intense study for as unique messengers. It will give us the in the fabric of spacetime. They travel the past two decades both by the ESA and closest ever view of the early processes essentially undisturbed and let us peer deep the US space agency NASA. Since 2011, the at TeV energies, provide observations into the formation of the first seed black ESA has been proceeding alone, with studies of guaranteed sources in the form of holes, exploring redshifts as large as z ~ 20, that are continuing under the name eLISA verification binaries in the Milky Way, prior to the epoch of cosmic re-ionization. (evolving LISA)1,2, but it is expected that and enable us to probe the entire Exquisite and unprecedented measurements NASA will be back in the boat soon. Universe, from its smallest scales around of black hole masses and spins will make In this Commentary, I use the name singularities and black holes, all the way to it possible to trace the history of massive LISA as the trademark for an observatory cosmological dimensions.

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the frontiers of science will be when LISA up to around 100 solar masses (M◉) and Box 1 | LISA Pathfinder mission facts. operates. For example, massive progress measured the population statistics. It is can be expected in transient astronomy. possible that the third-generation Einstein Objective. Demonstrate the most Telescopes like the Large Synoptic Survey Telescope will have come into operation by important LISA technologies in space Telescope (LSST)3 and the Square Kilometre 20307, further extending the volume of space and validate a complete noise model. Array (SKA)4 are likely to identify new in which these signals can be detected. At systems that flare up irregularly or only the other end of the mass spectrum, PTAs8 Method. Drag-free operation at Lagrange once, and there is a good chance that some will have detected a stochastic background point L1, featuring laser interferometry of these will be associated with gravitational due to many overlapping signals from between two free-flying test masses in wave signals. As another example, extremely supermassive black hole binaries with one satellite. large telescopes — the European Extremely masses over 109 M◉, and they may have Large Telescope (EELT), Thirty Meter identified a few individual merger events. Launch. End of 2015. Telescope (TMT) and Giant Magellan The background will help determine the Telescope (GMT) — and large space mass function of supermassive black holes at Duration. Six months for nominal mission. telescopes — the James Webb Space the high-mass end, but it will not constrain Telescope and Athena — will be observing the mass function for the much more Orbit. Halo orbit around Earth–Sun (proto-)galaxies at unprecedentedly high common 106 M◉ black holes that inhabit Lagrange point L1. redshifts, at which LISA will simultaneously the centres of typical galaxies and that are observe individual merging black accessible through LISA observations. Instrument suite. Laser interferometer, hole systems. Besides making high-sensitivity free-flying test masses, gravitational observations of individual systems, LISA reference sensor, drag-free operation and Gravitational wave science by 2030 will give us the chance to characterize the micro-Newton thrusters. By 2030, gravitational wave astronomy population statistics of black holes in the will be well established through ground- centres of galaxies, of intermediate-mass Team. ESA, France, Germany, Italy, based observations operating at 10 Hz and black holes and of the early black holes that NASA, Netherlands, Spain, Switzerland above, and pulsar timing arrays (PTAs) at eventually grew into the supermassive holes and the UK. nHz frequencies. The huge frequency gap we see today. between them will be completely unexplored until LISA is launched. LISA and fundamental science in 2030 Scientific landscape of 2030 The ground-based network of advanced One of the signature goals of the LISA Naturally, science is not predictable, and the interferometric detectors — three LIGO mission is to test gravitation theory, and it most interesting discoveries between now detectors, VIRGO5 and the Kamioka seems unlikely that any other method will and 2030 will be the ones we cannot predict! Gravitational Wave Detector (KAGRA)6 — achieve the sensitivity of LISA to deviations But planned projects already hint at where will have observed in-spiralling binaries of strong-field gravity by 2030. Unlike ground-based instruments, LISA will have sufficient sensitivity to allow us to detect small deviations from the general theory of relativity, and possibly to recognize unexpected signals that could indicate new phenomena. By observing the long-duration waveforms from extreme mass ratio inspiral (EMRI) events, LISA will allow us to map with exquisite accuracy the geometry of supermassive black holes, and will detect or limit extra scalar gravity-type fields. X-ray and other electromagnetic observatories may measure the spins of a number of black holes, but LISA’s ability to follow EMRI events and merge signals through to the formation of the final horizon will not have been duplicated, nor will its ability to identify naked singularities or other exotic objects (such as boson stars or gravastars), if they exist. By 2030 we will know much more about the large-scale Universe: in particular, about the nature of dark energy from the upcoming optical/infrared surveys dedicated

ESA/ASTRIUM UK © ESA/ASTRIUM to probing large-scale structure. However, only detections of gravitational waves can Figure 1 | LISA Pathfinder. The image shows the LISA Pathfinder satellite in preparation. The payload probe the period in the evolution of the is now completed and we expect that by July integration will be finished and final environmental tests Universe ranging from reheating after will be underway in preparation for shipment to the launch site in Kourou, French Guiana, for a launch inflation until Big Bang nucleosynthesis. at the end of 2015. LISA can thus gather information about the

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state of the Universe at much earlier epochs wave detectors like LISA are inherently between the test masses, a discharge system, than those directly probed by any other all-sky monitors: always on and having a diagnostic package, and a drag-free and cosmological observation. a nearly 4π field of view. They naturally attitude control system (DFACS). Two The information contained in complement other surveys and monitoring different micro-propulsion systems will be gravitational waves from the early Universe instruments operating at the same time, carried on board the LISA Pathfinder: a is complementary to that accessible by like LSST, SKA, neutrino detectors, gamma- set of cold-gas thrusters provided by ESA particle accelerators. The presence of a ray and X-ray monitors. The massive black and a set of colloid thrusters provided by first-order phase transition at the TeV scale, hole mergers detected by LISA out to NASA. The latter is part of the ST7 DRS the presence of cosmic (super-)strings in modest redshifts z( = 5–10) could well be (disturbance reduction system) payload, the Universe, the properties of low-energy visible to SKA and LSST as transients in the which will provide an independent test inflationary reheating, even the nature of same region of the sky. The identification of DFACS using the European LTP as the the quantum vacuum state before inflation of 5 to 10 counterparts during a two-year reference sensor. began, are some of the fundamental issues LISA mission would not be surprising. The 2015 LISA Pathfinder flight that will still be open in 2030, and to These might then be followed up by large- provides a final verification and in-orbit which the LISA mission might provide collection-area telescopes like TMT, GMT commissioning of these systems and some answers. and EELT, providing an unprecedented most of the LISA metrology capability. view of the conditions around two merging After a successful LISA Pathfinder flight, LISA and cosmology in 2030 massive black holes. the worldwide scientific community will By 2030 observations may well have be poised to begin this revolutionary constrained the supermassive black Technology status new science. ❐ hole mass spectrum from a few times All critical technologies for LISA have 1010 M◉, or even higher, down to around been under intense development for more Karsten Danzmann is at Leibniz Universität Hannover 107 M◉, but probably not into the main than 15 years, and today all are available and the Max Planck Institute for Gravitational LISA range of 104–106 M◉, especially at in Europe and almost all in the US. The Physics, Callinstr. 38, D-30167 Hannover, Germany. z > 2. LISA observations will fill this gap interferometry with million-kilometre The complete list of authors and contributors is and also provide a check on selection arms cannot be directly tested on the available at http://elisascience.org/authors and effects and other systematics of the ground, but it is being studied through http://elisascience.org/contributors. electromagnetic observations. By being scaled experiments and simulations. All e-mail: [email protected] able to measure the masses and spins of other technologies are being tested on the massive black holes as a function of redshift precursor mission LISA Pathfinder, to be References out to z = 20, LISA will allow us to greatly launched at the end of 2015 (Box 1). The 1. Jennrich, O. et al. NGO: Revealing a Hidden Universe: Opening a New Chapter of Discovery (ESA, 2012); http://sci.esa.int/cosmic- improve models of how supermassive black LISA Pathfinder satellite (Fig. 1) carries the vision/NGO_YB.pdf holes grow so quickly, so as to be in place LISA Technology package (LTP) provided 2. Amaro-Seoane, P. et al. Doing Science with eLISA: Astrophysics at z ~ 7. by European member states. The LTP and Cosmology in the Millihertz Regime (eLISA, 2012); http://elisascience.org/whitepaper is a complete system resembling a LISA 3. Merloni, A. et al. Preprint at http://arxiv.org/abs/1209.3114 (2012). LISA and massive black holes in 2030 arm shortened to 38 cm, it employs two 4. Aharonian, F. et al. Preprint at http://arxiv.org/abs/1301.4124 (2013). LISA has extraordinary sensitivity to gravitational reference sensors containing 5. Aasi, J. et al. Preprint at http://arxiv.org/abs/1304.0670 (2013). 6. Somiya, K. Class. Quantum Grav. 29, 124007 (2012). massive black holes in the mass-range of the free-falling test masses, an optical 7. Punturo, M. et al. Class. Quantum Grav. 27, 194002 (2010). most galactic-core black holes. Gravitational metrology system for laser interferometry 8. Hobbs, G. et al. Class. Quantum Grav. 27, 084013 (2010). Ψ in the sky Kai Bongs, Michael Holynski and Yeshpal Singh Quantum technologies, including quantum sensors, quantum communication and quantum metrology, represent a growing industry. Out in space, such technologies can revolutionize the way we communicate and observe our planet.

ost of modern technology — where single-particle control enables us to budget of €1.5 billion with over 7,000 primarily transistors and lasers — is harness more advanced aspects of quantum active researchers in this area worldwide. Mbased on our understanding of mechanics, such as superposition and A particular push towards technology the quantum rules governing energy levels entanglement. The new quantum technologies is the UK Quantum Technologies and electronic transport in conductors include quantum communication, quantum Program investment of €370 million over and semiconductors. This ‘Quantum 1.0’ computing, quantum imaging, quantum 2014–2019 as a first step in implementing a technology relies on many-body quantum metrology, quantum sensors and quantum pioneering Quantum Technologies Strategy effects without the need for control at simulation. Extrapolating from the economic (http://go.nature.com/sZafBA). the single-particle level. We are now on impact of Quantum 1.0, expectations are Some Quantum 2.0 developments the verge of a Quantum 2.0 revolution, high, driving an estimated global research already offer substantial benefits over

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Correction In the Commentary ‘LISA and its pathfinder’ (Nature Physics 11, 613–615; 2015), in Box 1 Italy and Spain were mistakenly omitted from the list of mission team members. This error has been corrected in the online versions 20 August 2015.

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