Ripples in Spacetime Are Passing Through You Right Now. This Mission Will Reveal Their Secrets.

It wasn’t a totally random thought—I was on my way to talk to an expert in these ripples, known as gravitational waves—but it seemed especially compelling looking out at the calm landscape spotlit by a bright winter sun. The view, almost like a high-contrast photo, somehow made it easier to imagine the invisible waves sweeping across the flatlined exurbs flanking the road between Amsterdam and the town of Noordwijk, home of the European Space Research and Technology Centre (ESTEC), which was my destination.

Gravitational waves are created by practically everything in the universe, from the ambient remnants of the Big Bang, to the explosive deaths of stars, to the mergers of giant galaxies. Even on the most boring days of your life, when you are laid-up sick in bed or handling the most tedious of workplace tasks, you can take comfort in the bizarre fact that you and everyone you know are being gently buffeted by these cosmic waves that warp the fabric of reality. 

The first detection of a gravitational wave, snagged in 2015, ushered in a major new era in astronomy that has exposed strange cosmic encounters—like mergers between distant black holes—that are invisible to light-based observatories. It also delivered on a century-old dream to capture the elusive waves, which were first proposed by Albert Einstein in his theory of general relativity. 

Now, scientists at the European Space Agency (ESA) are working toward a new milestone in the hunt for gravitational waves unlike anything that has come before. The mission, known as the Laser Interferometer Space Antenna (LISA), will be made up of three spacecraft flying in a synchronized formation that will stretch across millions of miles of deep space. 

If all goes to plan, this futuristic trio will be able to eavesdrop on epic cosmic events that have remained far out of reach for other detectors, such as encounters between supermassive black holes, while also probing unexplored realms of the universe. 

Located on the dune-rippled coast north of the Hague, ESTEC is ESA’s biggest site and its main space technology hub, billing itself as “Europe’s gateway to space.” Most of Europe’s extraterrestrial ambitions are born and nourished within this 100-acre complex, which contains several sophisticated testing facilities and harnesses the talents of some 2,500 scientists, engineers, and other space specialists.

After picking up my visitor’s badge, I was able to get a quick caffeine fix in ESTEC’s cafeteria, which sits in a historic building with some low-key “Jurassic Park Visitor Center” vibes, mostly due to its retro design and aura of scientific bustle. The site has a feline mascot, Micky the Space Cat, who often hangs around the sunlit reception area, but on that day he was off exploring the grounds. 

My guides led me down the center’s main corridor, a huge passageway adorned with images and replicas of ESA space missions, including a full-scale model of the agency’s experimental reentry vehicle. We met Jennrich in a conference room tucked along the corridor that was set up with a small-scale model of LISA Pathfinder spacecraft, a mission launched in 2015 and successfully tested out some of the technologies that will be used on LISA. 

As a researcher who has spent decades working on gravitational waves, Jennrich has seen his field move from the very fringes of the astronomical community into the center of mainstream attention. Now, with LISA, he is helping to push the field into an entirely new dimension that will expose some of the most mysterious, and consequential, interactions in our universe. 

Whereas existing gravitational wave detectors can hear the rumbles of “stellar-mass” black holes, which can be several dozen times the mass of the Sun, LISA will be able to tune into the hulking giants that sit at the center of galaxies, which can be millions or even billions of times more massive than the Sun. Understanding how these immense objects have coalesced over billions of years will shed light on the fundamental structure of the universe itself, and they are just one of LISA’s epic observational targets.

 “With LISA, we will see any supermassive black hole coalescence within the right mass range wherever they happen in the universe,” Jennrich told me. “They can't hide” because “the machine is so sensitive.” 

Indeed, the three spacecraft that make up LISA will be two and a half million kilometers apart, but will be able to measure distance changes on the order of picometers, a unit that is one trillionth the size of a meter.  

A main focus of LISA’s mission will be those supermassive black holes, or the “big boys” as Jennrich calls them. These objects are the dark hearts of galaxies, making them an essential piece of the universe’s fabric, but nobody is sure just how they grew to their enormous scales. 

LISA could help resolve this question by picking up the gravitational waves produced by seeds of supermassive black holes that existed in the early universe, less than a billion years after the Big Bang.  

“We will immediately know what scenarios were more likely and what was the prevalent cause of growth,” Jennrich said. “That is of tremendous interest for the people who are looking into the structure formation of the universe. That is difficult to assess through optical astronomy, simply because you're so far away.”

In addition to probing the evolution of supermassive black holes, LISA will be able to listen to the hum of tens of thousands of compact objects, such as black holes and neutron stars, that are stuck in close orbits with one another. It will be able to pick up the sound of merging objects much earlier than ground-based detectors, expanding our understanding of how these unions occur. It may even be able to hear an ancient stochastic echo, known as the gravitational wave background, that could open a new window into the very early eras of the universe.

“If I wave my hand, that hand emits gravitational waves,” Jennrich explained. “A bus that passes by, or turns around the corner emits gravitational waves—tiny, non-measurable amounts in this case. What you really need are enormously big masses and you need to measure extremely small effects.” 

There’s little doubt that Einstein would have been delighted to have been proved wrong about our capacity to snag these tiny effects. It took a century of overcoming naysayers and enduring painstaking trial-and-error, but the first gravitational wave was finally captured in 2015 at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. The historic breakthrough marked a new era of “multimessenger astronomy,” a term that describes observations from very different signal sources, and earned the Nobel Prize in Physics in 2017. 

Unlike light, gravitational waves can travel through the universe without getting warped, obscured, or swallowed by massive objects like black holes or galaxy clusters. As a result, these abundant waves are valuable messengers from dark and distant reaches of the universe that are beyond the range of light-based telescopes.  

While they are commonly described as “ripples in spacetime,” Jennrich thinks gravitational waves are most easily understood through their physical effect on the universe. 

“What I sometimes tell people is, ‘let's concentrate on what gravitational waves do’” which is “change distances,” he said. For instance, if you were out to dinner with friends and a gravitational wave passed through the party, the distances between you and your friends would shorten and lengthen ever so slightly. 

We don’t notice all of these ambient waves because their oscillations are so small. Even waves produced by massive objects, such as merging black holes or colliding neutron stars, produce ripples with wavelengths that are far smaller than the width of a proton.

LISA will be like an ultra-super-sized version of these ground detectors that covers an area 10 times bigger than the Moon’s orbit around Earth. The idea is to launch three spacecraft that will each carry two lasers that can shoot pulses across the vast distances between the nodes of the constellation. In this way, the trio can detect waves that pass through its immense baselines as it trails Earth’s orbit around the Sun.  

“LISA is a mission that is designed to detect gravitational waves from space,” Jennrich said. “To measure them, we will have three satellites in orbit and we will continuously measure the distance between the three satellites with laser links. When a gravitational wave passes by, it will change the distance by a very small amount. We can measure and register that change, and send the data to the ground, to scientists, to analyze the data to determine what was the source of the gravitational wave.” 

The enormous scale of the mission presents numerous technical challenges that are expected to take more than a decade to overcome, which is why launch is expected no earlier than the mid-2030s. However, the success of LISA Pathfinder in demonstrating the feasibility of this wild concept is a source of optimism for the team.  

“I think it’s fair to say that LISA Pathfinder over-fulfilled our expectations quite a bit,” Jennrich said. “The performance was about, by a factor of 10, better than we designed it and expected it to be. We are very happy and I think the results give us all confidence that the same technology that we will use on LISA will work flawlessly.”

With that in mind, let’s imagine the moment when LISA does finally reach the launchpad in a little more than a decade, if all goes to plan. Once the trio has spread out into its enormous constellation and finished its tests, it will tune into the kind of low-frequency hums of the universe that have eluded scientists for centuries. 

Besides probing black holes and the structure of the universe, LISA’s most exciting discoveries may be those that cannot be predicted today. Like a giant net in outer space, the gargantuan constellation is likely to catch waves from all kinds of sources that are beyond our current imagination.

“All but one of the signals that LIGO saw are things that we didn't know existed,” Jennrich said. “I’m reasonably sure that we will see something that we don't expect.”

“Even if we don't find anything unexpected, if we just stick to the stuff we already know is out there, to me, it's already mind-boggling,” he concluded.