Detection of gravitational waves
Weightlessness and Grace

Thought experiment…

Imagine that during one of those traditional Saturday evening dinners, you politely ask your neighbour about how they spend their days. You receive the following response:

« I am working on the design of an instrument that will experimentally demonstrate one of the consequences predicted in 1915 by Einstein’s theory, the aptly named ‘general relativity’. Work began more than 20 years ago and will provide answers after 2035. »

Intrigued, realising that you’ve stumbled upon the physicist of the evening, you dare to continue your questioning... « This instrument will measure local variations in spacetime caused by the propagation of gravitational waves (between 1 and 10⁻⁴ Hz), highly elusive entities so difficult to detect that their existence was long doubted. Moreover, while not overturning Newtonian mechanics and its concept of universal gravitation, they relegate it to the status of an approximation. »

Faced with your bewilderment, this extraterrestrial-like individual adds: « A triangle with sides measuring 2.5 million km, whose vertices are three coordinated satellites, will orbit the Sun heliocentrically along the same trajectory as the Earth but offset by an angle of 20°, or 50 million km. It is a redundant Michelson interferometer, adapted for the experimental conditions of space and, more importantly, to what it aims to detect. The three satellites will exchange precisely calibrated laser beams to measure variations in their mutual distances on the order of a picometre (a trillionth of a millimetre, comparable to the scale of atoms). These distance variations, in the absence of any other influence or cause, will indicate a modification of spacetime due to the presence of gravitational waves produced by the merging of two black holes, whose existence is also hypothesised. These black holes, or extinguished stars of unimaginable density, can reach up to ten million times the size of our sun. They reside hundreds of thousands of light-years away from the tiny triangle tasked with tracking the picometric effects of the gravitational waves they are believed to emit. »

At this precise moment, you either burst into laughter, convinced you’ve fallen victim to a mischievous hidden camera, or you reach for a third triple cognac. In a final Baudelairean survival reflex to escape the weightlessness creeping over you, you solemnly murmur the closing lines of The Voyage: « To plunge to the depths of the abyss—Hell or Heaven, what does it matter? Into the Unknown, to find something new! »

The stumbling blocks always seem to come from the same point

But no, we are not dreaming. This is neither a thought experiment nor a farce. It is entirely real and international teams of hundreds of engineers and scientists have been working on it for over a decade, including the ARTEMIS² laboratory at the Côte d’Azur Observatory. The operational phase is set to begin in 2035.

Since the 17th century, Newton’s apple has never ceased to fall, erecting barriers against which our understanding of fundamental physics continues to collide. My God! To what distant shores must we travel for the law of universal gravitation to finally yield its secrets? Such a simple concept, yet one that has dictated the evolution of the universe since a supposed and mysterious Big Bang.

In 1915, Einstein propelled physics forward by theorising, with remarkable intuition, that what he called spacetime deforms in the presence of massive objects. Thus, a body A, in the absence of any external influence, which should simply follow a straight-line trajectory, will instead begin to orbit another, more massive body B, because the latter creates a distortion in space, much like a depression in which A will either fall or endlessly revolve. Even photons, those ultimate champions of straight-line travel, are bent by massive objects.

Black holes, insolently heavy and dense, mercilessly attract any photons venturing too close. These ravenous light-swallowers appear black, as no photon can escape their grasp. We infer their presence rather than truly seeing them.

In terms of waves, to go further you first have to go closer

This mental image of a bowl shape but whose effects are undeniably observed in the movements of celestial bodies implies the existence of so-called gravitational waves, capable of distorting space like ripples on water, propagating at the speed of light³.

Since the early 1990s, researchers have been engaged in a serious scientific courtship with these elusive waves. Fortunately, being on the same wavelength, three of them⁴ were awarded the Nobel Prize in 2017 following groundbreaking experiments conducted on Earth using the LIGO interferometer⁵.

Indeed, an enhanced version of the Michelson interferometer (its inventor in the late 19th century) enables the detection of these minuscule spatial variations caused by these mischievous and secretive waves.

So, gravitational waves exist and some have encountered them here on Earth. It is both tempting and logical, building on the knowledge already gained, to continue hunting for them, not only to better describe them but also to locate their most violent sources, namely black holes in the process of merging, which indirectly teach us about the primordial universe. These wild investigations also fuel other debates, such as the existence of dark matter and the nature and origin of this universe in which we are its tiny inhabitants.

Why would we do better in space than what we've already achieved on Earth?

The desire to build such a tripod and send it into space is more than justified by the fact that interferometry experiments conducted in space will be far more reliable, as they will be free from all external influences. We need to be absolutely certain that we are measuring only gravitational deformations of space and nothing else, certainly not thermal expansion or cosmic ray effects... just a beautiful, unpolluted vacuum of space. To precisely position a system of three coordinated satellites in space and then reconstruct the branches of a laser interferometer, all fully automated and capable of taking precise picometric measurements and sending the results back to Earth, is no small feat. What we gain in experimental elegance, we lose in engineering complexity. A « Michelson » interferometer only needs two arms. LISA, however, is an equilateral triangle and simultaneously implements three interferometers, allowing for the possible failure of one of the three satellites without jeopardising the mission.

LISA, a three point plan with no gravity

Let’s summarise. Implementing LISA means launching three satellites with an Ariane rocket, then sending them on a one-and-a-half-year journey across 50 million km, and finally positioning them in a heliocentric orbit, arranged in an equilateral triangle with sides measuring 2.5 million km.

Next, the satellites will need to automatically release, in weightlessness, cubic masses of 4.5 cm per side made of an alloy of gold and platinum, with an almost zero thermal expansion coefficient, inside an ultra-high-vacuum chamber, free from any electromagnetic⁶ influence, so that they are only affected by gravity, that is, the very phenomenon we aim to measure. The design of the satellites and all the components of the interferometers (mirrors, optical benches, transponders, electronics, etc.) also adhere to a demanding set of specifications, requiring extreme precision, purity, and reliability...

Nicoleta shares an anecdote on this topic. The photodetectors of the optical benches, key components of a laser interferometer, must withstand the bombardment of solar protons. The ones for LISA were successfully tested at the Antoine Lacassagne Cancer Centre using the proton therapy machine, whose output could be calibrated to conduct this kind of test! Cancer research partnering with space engineering to detect spacetime distortions? In the world of LISA, nothing should surprise us.

Another challenge, and not a small one, is that LISA will measure the distances between these three masses in weightlessness. It is these masses that dictate the satellite’s position and trajectory, not the other way around. In July 2017 the LISA-Pathfinder programme successfully validated in July 2017, in space at a Lagrange point, the autonomous and synchronised operation of these incredibly delicate and precise systems.

Only then, and at last, will the lasers be activated, and the interferometric measurements will begin to assess the « gravitational » variations in space on the picometre scale between the three test masses, which are 2.5 million km apart. Following this, the mathematical work of digital filtering and the transmission of the acquired data to earth-based computers will commence, hopefully revealing the gravitational signature of those elusive distant black holes we have been tracking with such determination.

At the intersection of physics, space engineering and maths

The LISA mission will take at least another ten years before it yields its results. Will it be successful? Theoretically, yes, from the perspective of Newton’s apple, which will not become the apple of discord, potentially reconciling quantum physics and Einstein's theories around the concept of the graviton, the possible counterpart to the photon for gravitational waves, just as the photon is for light.

From the standpoint of ultra-precision technology, advancements in material science, ultra-sensitive optical benches, space construction, synchronised control of multiple spacecraft in a constellation, digital processing, and mathematical analysis of experimental results, the breakthroughs will be undeniable.

Advancing, again and again... As far as possible...

Spacetime, much more than a mere pastime, is irrefutably a vocation at this level of budget and commitment. Its nature and mystery push us to scientific and engineering frontiers that were scarcely imaginable just twenty years ago. The LISA project, in its effort to grasp the gravity of the universe and understand the universe of gravity, presents technological, experimental and theoretical challenges of rare magnitude.

Even though the destination remains distant, ultimately, the journey matters more. By allowing ourselves to explore the possibility of such fundamental laws, we discover just how much the invisible outweighs the visible in the universe that hosts us. This is good news, as « what is essential is invisible to the eye. »

1. LISA: Laser Interferometer Space Antenna

2.ARTEMIS: Astrophysique Relativiste, Théories, Expériences, Métrologie, Instrumentation, Signaux – UMR n° 7250

3. Newtonian mechanics assumes the instantaneity and permanence of gravity.

4. Rainer Weiss (USA), Barry Clark Barish (USA), and Kip Thorne (USA) were awarded the Nobel Prize in Physics in 2017.

5. LIGO: Laser Interferometer Gravitational-Wave Observatory

6. To prevent the measurements from being disrupted by an electric charge, the test masses are continuously discharged by the radiation from blue LEDs.