Gặp gỡ LISA, Trạm quan sát sóng hấp dẫn của tương lai

In about 11 years, one of humankind’s most ambitious missions is set to launch into space. Decades in the making, the Laser Interferometer Space Antenna, or LISA, could revolutionize our understanding of the universe through its detections of gravitational waves. This is your in-the-weeds walkthrough of the science that will make this intrepid project possible.

The Hubble Space Telescope redefined our view of the universe, and the newly launched Webb Space Telescope is now doing the same. An ambitious, unprecedented space telescope, set to launch next decade, will continue in this tradition, but it will do so in ways never before imagined, with the ability to detect phenomena like gravitational waves—ripples in spacetime that offer a new window into the universe’s most mysterious events.

Gravitational waves and why they matter

Our universe is rife with gravitational waves—almost imperceptible ripples in spacetime generated by the movements of the universe’s most massive objects, neutron stars and black holes. Gravitational waves travel at light speed, but don’t get it twisted: they aren’t light. But like gravitational fields around massive objects, the waves warp light, revealing their presence to only the most attentive scientists—with the most sensitive equipment.

In 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations announced the first detections of the subtle waves, which stretch and squeeze the fabric of the universe as they emanate from their ginormous sources. To date, the LIGO-Virgo-KAGRA detector network has made over 100 gravitational wave detections.

Gravitational waves offer up plenty of information about the systems that generate them, helping scientists revise their catalogues of the possible sizes, environments, and mechanics of black holes and neutron stars.

It’s been nearly 10 years since the first LIGO detections, which confirmed gravitational waves as feature of the universe, one that was famously predicted by physicist Albert Einstein a century before. But LISA was in the works years before those detections—it was first proved out on paper by the late Pete Bender, a physicist at JILA, over 30 years ago. But the hugely complex endeavor began taking shape in earnest in the late 1990s, and the mission  was given the formal go-ahead by ESA in January.

“LISA’s so complicated that, in the beginning when it was proposed, no one believed it was possible,” said Ewan Fitzsimons, a researcher at the University of Glasgow and principal investigator of the UK hardware contribution to LISA, in a video call with Gizmodo. Fitzsimons has been involved with LISA for 18 years, beginning with LISA Pathfinder and now working on the optical benches for the current mission. What are optical benches? We’ll get into that.

The science of spotting gravitational waves

Gravitational waves are not made up of photons of light, so they are invisible to telescopes like the one set up in your backyard and the multi-billion-dollar machines floating in space already. But light is what astronomers have got to work with. So how do they see these ripples in spacetime? Simple: precise measurements of laser beams.

“We are doing laser interferometry both in LIGO and in LISA,” said Ryan DeRosa, who has led the development of the LISA telescope and worked on the mission’s interferometry, in a call with Gizmodo. “That means we’re essentially using the wavelength of the laser as a ruler, to figure out if the length changed or not.”

In LIGO, these laser beams are contained in underground, miles-long tunnels, where they are shielded (to the best of scientists’ abilities) from the disruptive rumblings of passing trains, the wind, and even the grumbles of Earth itself. The laser beams are bounced around mirrors in the observatory. As gravitational waves pass through LIGO, the time it takes the laser light to bounce through the system informs physicists as to whether a gravitational wave event just passed through our cosmic neighborhood.

“But that only works up to the degree to which your ruler does not change,” DeRosa added. In other words, if the frequency of your laser deviates at all as the beams make their heavenly journey to and fro between the LISA spacecraft, the data you get from them is useless. The gravitational waves’ subtle interactions with LISA would get lost.

A footnote: laser interferometry isn’t the only way to spot gravitational waves. Pulsar timing arrays spot ripples with even longer wavelengths; these arrays track the timing of light flashes from rapidly spinning pulsars to determine when gravitational waves have hastened or hampered the transit of those photons.

Why put an observatory in space?

Soon after the first gravitational waves were detected, NASA and ESA launched the LISA Pathfinder, a proof-of-concept mission that tested out scientific components critical to LISA’s success. Specifically, the pathfinder contained two test masses to show that a near-perfect gravitational free fall was possible within the spacecraft, and could be precisely measured.

LISA will “operate in basically an Earth-like orbit. Each of the (three) spacecraft is in a similar orbit around the Sun than Earth is, but they’re all shifted behind the Earth,” said Ira Thorpe, a LISA project scientist, in a phone call with Gizmodo. “They’re all at slightly different inclinations and slightly different orbital phases, and you end up with this triangular constellation that is actually remarkably stable.”

Thorpe is working on LISA on behalf of NASA, though the mission is actually an ESA-led collaboration. Before LISA—indeed, before the LISA Pathfinder—Thorpe was involved in a first attempt at the LISA mission, also called LISA. “We like our brand vision,” Thorpe said.

There are two main technical challenges for a gravitational wave detector, Thorpe said. One is that you need at least two freely falling objects, meaning that the only force acting on those masses is gravity. The other challenge is to measure the distance between those objects, to measure the curvature of spacetime.

How is LISA’s science different from LIGO’s?

“You’re always chasing small numbers and you’ve got two options,” DeRosa said. “You can measure an extremely small change in length over a long length—that’s what LIGO does. Or you can measure a reasonably small length change over an enormous length—that’s what LISA does.”

LIGO’s arms are just (“just!”) 2.5 miles (4 kilometers) long. That is beyond puny—it’s downright microscopic compared to LISA, whose laser-beamed arms will each measure 1.55 million miles (2.5 million kilometers) in length. The Sun measures 864,000 miles (1.39 million kilometers) across, which means each of LISA’s arms will be longer than our star is wide.

That doesn’t mean ground-based detectors like those managed by the LIGO-Virgo-KAGRA Collaboration aren’t useful. They will detect different sorts of events. Higher frequency gravitational waves correspond to sources of lower mass, while lower frequency waves are generated by much larger things, like supermassive black holes. LISA will collect data on a lower frequency band than LIGO, revealing gravitational wave sources we simply couldn’t see using earthbound machines.

Unlike LIGO, with LISA “we don’t have to deal with the limitations of being on the planet,” DeRosa said. That means a couple things. For one, it means all the pesky sources of noise that can disrupt Earth-based observations won’t matter to LISA. Once the mission is in orbit, spinning behind Earth like a giant trawling net for black holes, it’s a fairly hands-off enterprise.

That’s partly by design. As DeRosa points out, to ship LISA off with any more servicing components than those which are absolutely necessary just adds more payload for a rocket, and more vectors for failure. It is better for LISA to be pared down to the fundamental systems necessary for the mission objective, a fairly ubiquitous philosophy when it comes to spaceflight.

However, that doesn’t mean LISA’s experience in orbit will be rainbows and butterflies. Even at its most peaceful, space is a harsh and unrelenting environment.

As the LISA spacecraft cartwheels in Earth’s tow, the constellation “breathes a little bit” annually, Thorpe said. Earth’s gravity tugs slightly more on whichever spacecraft is closest to it as they rotate, throwing the spacecraft out of alignment. However, the slow drift of the spacecraft won’t interfere with the team’s ability to make gravitational wave measurements, which by-and-large happen on minute-to-hours timescales.

LISA isn’t a telescope—it’s a ‘beam expander’

Remember how LISA is slowly drifting, and how that won’t affect the team’s ability to make gravitational wave measurements? Well, in part that’s because LISA has a telescope system, a crucial mechanism for getting the laser beams to haul their photonic asses the million-mile distance through space. As the spacecraft drift, the telescope adjusts to aim the laser beams towards their target. But that mechanism only has so much range, Thorpe said.

“Eventually the distortions in the constellation—over something like a decade—get big enough that we run out of room on that adjustment mechanism,” Thorpe said. “So that’s actually what sets the lifetime of LISA, ultimately.”

“If you shoot a laser beam in space, it does not stay the same size,” DeRosa said. “It gets bigger and bigger and bigger as it propagates along just due to diffraction.” In other words, as the laser moves away from its source, its power weakens. The LISA telescopes fix that issue—they blow up the radius of the laser beam by several hundred times its size, so that by the time the diffracted beam arrives at the other LISA spacecraft, it delivers a good number of photons along the arms.

“We call it a telescope, but it’s probably more accurate to think of it as a beam expander,” Fitzsimons said. Putting the laser beam through the system increases the number of photons per unit area on the far side of the laser, maximizing the light transferred between the spacecraft.

The optical benches provide “a reference plane for all of these measurements and the telescope itself,” DeRosa said. In that way, it’s not just the wavelength of the laser that acts as a ruler. The optical bench is what the team is measuring against, making it a ruler too. “Both of them are effectively your ruler, and if either one is not performing, then you don’t have a measurement,” Fitzsimons said.

What will LISA see exactly?

LISA will be able to detect gravitational wave sources that Earth-based interferometers simply cannot: sources with longer wave periods, like compact objects ensnared by supermassive black holes and the supermassive binaries at the hearts of galaxies.

LISA will also be able to spot merging white dwarfs in our Milky Way, merging intermediate-mass black holes (of which the universe is famously absent, at least as far as astronomers can tell), and perhaps hitherto unknown exotic objects.

Theory begets observation and vice versa; when LIGO spotted gravitational waves, not only did it validate Einstein, it also provided a new proving ground for more advanced ideas about the makings of the universe. LISA will reveal much more about the compact objects that litter our universe, and around which life revolves. The Milky Way galaxy has a black hole about four million times the mass of the Sun at its heart. Many of the supermassive black holes LISA will study will be much larger than that (on the scale of 10⁴ to 10⁷ times the Sun’s mass).

The biggest challenges are yet to come

LISA is a $1.6 billion project decades in the making. Now, teams at ESA and NASA are building the actual hardware that will be sent to space. “The biggest challenge with LISA is knowing that it works, because so much of it is not testable on the ground,” Fitzsimons said. “One of the hallmarks of spacecraft engineering is that, apart from very select cases, once it’s up there you can’t fix it.” In other words, the team has one chance to get things right.

“This is a space-based interferometer,” DeRosa said, “and usually to get into space, you have to deal with a rocket. And the rocket’s got launch loads, and shocks, and big thermal swings. And my whole telescope is made of glass.”

That is a very literal statement. Metal swells and shrinks with temperature fluctuations, the very slightest of which will disrupt LISA’s measurements. That’s why the team is using plenty of glass in the telescope’s construction; while glass is brittle, it’s also strong and is a useful material for when LISA is spinning through space. Getting it up there intact will prove to be a trickier endeavor.

The spacecrafts’ optical benches are being assembled by a specialized robotic integration system to fasten the optical elements to the base plate using hydroxide catalysis bonding, with picometer-level precision. Most of the bench is made of glass and ceramic, and the bonding technique “basically grows glass between the optic and the base plate,” Fitzsimons said. The team is building 10 of the benches, including a couple of prototypes and two spares, “in case someone drops one.”

We’re still years away from LISA launching, but this massive undertaking is the marquee project-of-the-century for deciphering one of the cornerstones of astrophysics: black holes and the ways they shape spacetime.