We are scientists from the LISA Pathfinder Collaboration, and we have demonstrated the technology to detect low-frequency gravitational waves from space. Ask us anything!

As an undergraduate physics student with a driving interest in space and space exploration, I look to the work done by projects such as LISA as a source of inspiration and hope that it really is possible to be involved in research that pushed the boundaries of our understanding of the Universe.
How did the team members get to their position at the forefront, and what recommendations do you have for those of us at the starting line?
LIGO could make an estimate of the location in the sky of the object they were observing due to the two axis of measurement, from what I've understood, LISA has only one axis of measurement. Does this mean that you've got a telescope that you can't aim? Is there any way to compensate for this, or any ideas for a future project that could?

To answer your first question, I was lucky enough to do my undergraduate studies at the University of Glasgow in Scotland. I was studying astronomy and physics, but had not settled on a field. But in Glasgow there is a large group who have worked on Gravitational Wave research for many years, and I was luck enough to do a summer research project in that group. I was utterly captivated by the blend of instrument science, technology and astrophysics and cosmology. It seemed to me like I could study all the things I'm interested in by working on Gravitational Waves, and I was hooked. I did my PhD in the Glasgow group, moved on to working on the GEO600 Gravitational Wave detector in Hannover, and from there to the LISA Pathfinder mission, with the promise of building and 'hearing' the Universe through the ears of LISA!

For the second question, LISA can estimate the position of sources in the sky, to around 1 arc second for the 'brightest' sources. LISA has a response equivalent to 2 Michelson interferometers like those used in the LIGO detectors. Just like the LIGO detectors, we can't, however, 'aim' or 'point' the detectors, but rather we 'hear' all directions in the sky all the time, just like an omnidirectional microphone. However, because the LISA 'microphone' is moving in its orbit around the Sun, we can determine the change in the signal over time via doppler shifts in the signal, just as you can locate a police car as it passes by, even with your eyes closed. Also, in LIGO and in LISA, we can time the 'arrival' of the signal in the two detectors (two LIGO detectors on ground, two equivalent Michelson detectors in LISA), which gives us a triangulation of the source position.

M. H.

Can you give us a quick overview of what these gravitational waves are and what are the implications of their discovery?

Gravitational waves are ripples in the fabric of spacetime. In Einstein's theory of general relativity, gravity is perceived as curvature in spacetime (think of a trampoline canvas as an analogy of spacetime - when you place a heavy object at the centre of the trampoline, the canvas curves into a funnel shape: the heavier the object the more deep the funnel - this is similar to the way gravity affects spacetime). Now assume you have two objects orbiting each other on the trampoline....the funnels move, and the trampoline surface oscillates. This is the same principle of gravitational waves propagating through the Universe.

What are the implications: One source of gravitational waves comes from the most powerful events in the Universe - the merger of black holes. When supermassive black holes merge, they produce enormous amounts of energy in gravitational waves, allowing us to observe them throughout the entire Universe, all the way back to the earliest stars and Galaxies. Using gravitational waves we can start to build a a picture of how the large scale structure of the Universe came to be.

Put simply: gravitational waves gives us a new sense on how to observe the universe. [PMN]

Hi! Congratulations for smashing your performance requirements! As a European this makes me proud too :-) But, my question - given this stunning accuracy, do you think there is an opportunity to extract interesting physics from the mission? New constraints on general relativity for example? I know gravitational waves can't be seen at this scale, but you still possess the most precise accelerometer in the world now! Thanks!

Thank you. The goal of the mission is to understand the physics of a free floating test body (in our case, a gold platinum cube). So the main reason for the mission is to understand new physics.

In terms of GR, we will not place new constraints with the LPF mission. However, future gravitational wave detectors will place very stringent constraints on strong-field GR, using the so-called Extreme Mass Ratio Inspirals (EMRIs). These are objects where a supermassive black hole captures, and eventually merges with a small black hole. The chaotic orbit of the small black hole as it inspirals into the larger body, allows us to map the gravitational field of the supermassive body, and hence test GR in this (as of yet, un-tested) regime.

Can I ask what the peak in the spectrum at ~7e-2 Hz is? Calibration signals between 1-> 30mHz into the suspension are mentioned, but this peak is too high frequency for that? Is it a signal injected into the drag-free loop or the laser power, or something else entirely?

Slightly tech question I know – feel free to answer with "read the paper" ;)

Also, congratulations in general on such a successful mission!

You should read the paper, but you won't find the answer there ;)

The truth is we are still investigating the source of that line. It is likely an electrical interference in the phase measurement system or on the modulated light we use to make the phase measurement. It's pretty likely to be a high frequency line aliased (or beat down) into our band, and therefore is difficult for us to diagnose with our routine low-frequency telemetry. Specific measurements are planned to investigate this using specialised telemetry. All that said, the line is very narrow in frequency, and does not disturb our main measurement.

Thanks for the good question!

M. H.

To what distance (z) would you expect to be able to detect GWs? Basically wondering if LISA will be able to see before "first light"

A spaceborne observatory would be able to detect gravitational waves from merging supermassive black holes up to redshift z~30, around the time of the formation of the first stars and galaxies, a few hundred million years after the Big Bang. But it could also see primordial gravitational waves, allowing us to probe much farther back in time, until the epoch of cosmic inflation, a tiny fraction of a second after the Big Bang. This depends on the instrument sensitivity but also on the physical details of inflation. – PMN

What does this mean for the full LISA mission? Is there any chance that could be accelerated?

LISA Pathfinder is a major milestone on the way to LISA, a full-scale space-based gravitational wave observatory. It tests a lot of the technology and measurement concepts we need to do Gravitational Wave observations from space. ESA is working hard on the other technologies needed for the full observatory, and personally I'm confident that the schedule can be accelerated. The world is ready to observe the Universe with Gravitational Waves, and LISA is a fantastic way to do it!

M. H.

More than a chance to accelerate the launch the full-scale observatory, we should look at it as an opportunity. The progress made over the last few years, underpinned by the results, gives Europe a unique opportunity to lead the field worldwide. CGM (ESA's LISA Pathinder project manager)

Scifi area/ far future question.
Could gravity wave detection be used to detect smaller objects with mass? Such as astroids, rouge planets, spaceships (even cloaked ones)?
Could a "gravity radar" be a thing on the future, for use in interstellar space exploration?

Although, in principle, all accelerating mass (even you waving your hands about) produces gravitational waves, it takes the extreme acceleration of really enormous masses to produce signals that are detectable by the kinds of detectors we can build today. The kind of signals we expect to see with a future Gravitational Wave observatory like LISA, are produced by pairs of blackholes orbiting each other, each with masses millions of times that of our Sun.

It's highly unlikely that in any kind of foreseeable future, detectors sensitive enough to see gravitational waves from asteroids can be built. However, with LISA, we would see the influence of passing asteroids on the gravitational field in the neighbourhood of the observatory.

M. H.

How can this mission helps us on earth? why using "platinum" cubes? And can their presence determine the shape of the universe, wether it is flat or not?

LISA Pathfinder (LPF) is a technology demonstration mission. It is designed to test the parts of a future space-based gravitational wave detector which cannot be tested the ground - it does not directly help us on earth in terms of the technology we are testing.

Our cubes are made form an alloy of Gold:Platinum - the reasons being two-fold: 1) Gold:Platinum has a very high density (20,000kg/m^3). This is very important as our little cubes (46mm on a side) have a mass of 2kg. If we consider Newton's 2nd Law (F=ma) then for a given (external) force, then the acceleration is lowest when the mass is largest. 2) The particular alloy has a very low susceptibilty to magnetic fields, which again reduces the noise of the test ass motion.

Can we determine the shape of the Universe: No. [PMN]

Where will the full LISA be deployed? I'm guessing trace gas pressure in LEO is still measurable

The three satellites of LISA will be in heliocentric orbits, which together form a triangular constellation which 'cart-wheels' around the Sun. Only far away from Earth is the environment quiet enough to observe Gravitational Waves at the level we want.

M. H.

Nice results!

Even things like the gravitational acceleration of propellant mass are relevant, so I was wondering: Could a LPF-like mission improve measurements of the gravitational constant by tracking test masses? Terrestrial measurements show some odd discrepancies, so a space-based test would be interesting.

An interesting question! This is something we have been studying internally to see at what level we can say something about the gravitational constant using measurements made with LISA Pathfinder. For various reasons, we are not able to make a measurement of Big G competitive with those on ground, but it still may be interesting to do the best we can, as the measurement would be very different from those made on ground. Anyway, this is still being studied, and if there is something interesting to learn, we will pursue it.

M. H.

With this, are we able to determine more information about what happens near a black hole even horizon?

Thank you for taking the time to do this AMA.

With LISA Pathfinder, the distance between the two free-falling test masses is too short to allow us to observe gravitational waves. But with LISA, the full-scale observatory, where the test masses are millions of km apart, we will be able to see the orbits of compact objects around super-massive black holes in the centres of galaxies. This will allow us to 'map' out the space-time around these black holes and study what happens to these objects as they approach and cross the event horizon of the central black hole.

M. H.

First of all, I feel like I must congratulate the LISA Pathfinder team for developing such a clever experiment and for the precision with which this first mission was executed. A relative acceleration lower than 1 part in ten millionths of a billionth of Earth's gravity is simply unimaginable.

So my question is concerning the nature of gravity. Simply asked, what is it? As a third year mech. engineering undergraduate, I've learned much about it from Kepler's to Newton's Laws concerning gravity all the way to Einstein's theory of relativity. But what is the true nature of gravity? Is it a particle? Is space-time an actual physical "fabric" that permeates all of space? Is it truly a fundamental force or simply just the phenomenon of light and matter following the curve of space-item? The idea of gravitational waves alone leads me to assume that it must be some physical fabric or field that permeates spaces but I'm unsure.

So what is the true nature of gravity?

Thanks and good luck to the LISA Pathfinder team!

Only addressing your congratulations paragraph: thank you! For LPF to succeed many people (thousands) with different backgrounds had to share the vision and the goal, and work as a team. Not unlike other space missions, but maybe to a larger extent, scientists and engineers had to work together and understand each other's viewpoint. We're all proud to be part of this! - CGM

Congrats to the ESA and LPF science & tech team, great and somewhat unexpected results:

1) Considering the results, are LISA funds secured, at least partially, and will it require more than 1 bn euros to build the detector

2) Are you going to revise the sensitivity projections for LISA considering that LPF almost reached them

3) What is the chance of LISA detecting the gravity waves produced by interactions between primordial black holes of the first eons of Universe's existence, which are now a matter of heated debate in cosmology and astrophysics ?

An answer to 2):

The measurements we have made with LISA Pathfinder already allow us to improve our physical modelling of such systems, and it was always the aim to use the results to help us design the best LISA we can. So yes, we will be folding all the new information into the design of LISA as we move forward.

M. H.

What types of gravitational waves do you expect LISA to be able to find? Not sure what is characteristic of each frequency range ...

The frequency band that LISA will observe is rich with signals from different kinds of astrophysical and cosmological sources. We will observe the signals from merging black holes at the centres of galaxies, the complex waveforms produced small, stellar mass black holes falling into the supermassive black holes in the centres of galaxies, the almost sinusoidal signals from compact binary systems in our galaxies, and possibly the stochastic ripples from the Big Bang itself. And of course, all those systems we have not yet predicted!

M. H.

How many detectors would be needed to predict solar events like coronal mass elections or solar flares?

Coronal Mass Ejections (CME) and flares, although very large and powerful events in terms of the solar system, are actually very weak in terms of the gravity and gravitational waves. No gravitational wave detector will be able to measure the gravitational wave from an CME.

However, even with LISA Pathfinder (and any future GW observatory), we can detect CMEs, however not from their gravity. CMEs are essentially matter ejected from the sun. When this matter hits LISA Pathfinder, it leads to a build up of the electrical charge of the test masses. We are sensitive to electrical charge, so can see very clearly when we are hit by a CME! [PMN]

Congratulations! I'm about to start my PhD working with the GW-EM follow up, so advancements like these are inspirational. Do you guys know if it is possible to detect GWs inside the orbit of the two masses? Are there any models for what kind of radiation a trinary system would produce?

Thanks! If you're asking if we know how the gravitational waves would look like in between two orbiting black holes, then I can tell you that people that calculate gravitational waves numerically get the field everywhere, including the place you are alluding to. I don't know exactly about three-body systems, but I don't see why those could not be calculated numerically -- SV

Given these fantastic results, what do you think the biggest technical challenge will be for LISA?

LISA will be the first constellation of (three) spacecraft flying around the sun at a distance from each other longer than one million km. And yet, they have to communicate with each other and with the ground to synchronise their operation. This sounds like a challenge to me. - CGM

1. Given the sensitivity of LISA Pathfinder and that of eLISA, how do we plan to distinguish numerous gravitational wave signals and detect the source for each of them?
2. How has been the performance of micro-thrusters? Have they been tested yet?
3. What are the requirements or constraints taken into account for designing trajectory for such missions?
4. Can you extrapolate these results with respect to formation flying?
5. Can eLISA mission also give more insight into Space Environment?

On the micro-thrusters, they have been working seamlessly since day one. There are two branches (main and redundant, that is 6+6 micro-thrusters), and so far we always used the main branch. Each thruster operates at about 10 micro-Newton pushing against the sun at an angle. The sun (the solar pressure) is effectively pushing the spacecraft towards the earth with a force of 28 micro-Newton. Small numbers. - CGM

Congratulations to ESA on the results from LPF! As a Ph.D. student in Aerospace Engineering, I would like to know about the future research prospects in eLISA and similar missions, specifically with respect to Guidance, Navigation and control? What is the primary research focus to achieve performance requirements for Disturbance Reduction System and maintain the necessary stability of the spacecrafts of eLISA.

We had to be careful with testing the controllers for robustness considering there are thousands of parameters and many combinations of worst cases and/or margins in the parameters. While we tested as best we could, we found it impossible to test all combinations. There will be other issues with constellation dynamics that will need being looked upon. - CGM

First of all congratulations! Those are amazing results! My question: has LISA already been greenlighted and can you keep the team together to build it?

First of all, thanks for the congratulations! A future space-based observatory like LISA is currently planned as ESA's 3rd large mission. From the scientific point of view, LISA Pathfinder gives us a green light in the sense that we are confident we understand the physics of free-falling test masses well enough to fly LISA. Given these results, and the exciting detection from LIGO, my view is that there is a push within ESA to advance the development of the future observatory. Obviously we will make every effort to keep the team together and to maintain the knowledge and enthusiasm that exists in the LISA and LISA Pathfinder teams.

M. H.

What will these findings mean? And can we use gravity as a power source?

The performance of LISA Pathfinder give us the confidence to go ahead and build a large scale space-borne gravitational wave detector. The critical technologies, which cannot be tested on ground, have now been demonstrated in a flight environment.

Any accelerating body (arms, legs, supermassive black holes!) create gravitational waves (GW), However, only the biggest objects of the universe create GW which have sufficient amplitude to be detected by our (very) sensitive detectors. We cannot use these results to allow us to use gravitational waves, or gravity, as a power source. [PMN]