The aluminum hatches are the only clue to what lies beneath. Buried amid the corn and wheat fields of Fürstenfeldbruck, a sleepy monastery village 20 kilometers from Munich, Germany, is an inverted pyramid of concrete, steel pipes, and precision sensors, as deep as a three-story building. Last month, when lasers began coursing around the edges of the tetrahedron, Rotational Motions in Seismology (ROMY), as it is called, began its reign as the most sophisticated ring laser in the world, capable of sensing how Earth itself twists and turns.
"It's a structure that has never been built before," says Heiner Igel, a seismologist at Ludwig Maximilian University in Munich and the principal investigator for the €2.5 million machine. "It's something so special." What makes it singular is the finesse needed to keep the lasers stable and to detect tiny changes in their wavelengths.
In doing so, ROMY will measure minuscule changes in Earth's spin rate and spin axis. The speed and pace of those measurements promise to add an increment of precision to GPS navigation, and ROMY may even be able to detect a subtle effect predicted by Albert Einstein's theory of general relativity: the drag of the rotating planet on nearby spacetime, like a spoon turned in a pot of honey. ROMY also will be sensitive to the weak rotations that accompany earthquakes, long-ignored motions that contain clues to the interior structure of Earth. By showing the value of recording those motions, ROMY could pave the way for miniature sensors that could help oil and gas prospectors and even planetary scientists who want to listen for tremors on the moon and Mars.
Ring lasers are exquisite rotation sensors thanks to an effect that French physicist Georges Sagnac demonstrated in 1913. He split light into two beams that traveled in opposite directions around the mirrored perimeter of a spinning tabletop. When he recombined the light, he saw interference "fringes"—dark and bright bands indicating that the light waves in the two beams were out of phase. The beam moving in the direction of the spin had traveled slightly farther than its counterpart, causing the phase shift.
In the decades since, scientists put the Sagnac effect to work to track rotations. The principle underpins the laser and fiber optic gyroscopes that replaced finicky mechanical gyros in the 1970s and are now standard for navigation. The rotations they measure, like the turns and dives of a fighter jet, are fast and large. The idea of building a larger, more sensitive ring laser for geodesy—measuring Earth itself—didn't come around until the 1990s, when nearly perfect mirrors became available.
One of the first such lasers was C-II, a ring laser in the shape of a square with 1-meter arms, built in New Zealand in the mid-1990s and housed in a disused World War II bunker, where temperatures are stable. Whereas Sagnac shone light into his experiment from an external source, the C-II's ring itself generated laser beams, its cavities filled with a lasing medium of neon and helium gas. As before, a rotation lengthened one light path, but the effect on C-II was to stretch the wavelength of the laser resonating along that path, like the coils in a stretched spring. For the beam running in the opposite direction, the path and wavelength were squeezed. When the beams were interfered, their slightly clashing wavelengths caused the optical equivalent of the pulsing beats that piano tuners try to eliminate as they strike a note and a tuning fork at the same time. "You have beats because you're out of tune," Igel says. The beat frequency is a direct measure of the rotation that causes it, and C-II was able to measure Earth's rotation rate to one part in a million.
C-II also launched the career of Ulrich Schreiber, a laser physicist at the Technical University of Munich who led its design. Schreiber later worked on ring lasers in New Zealand, California, Germany, and Italy. "He is the lord of the rings," says Jacopo Belfi, a physicist at the National Institute for Nuclear Physics in Pisa, Italy, who works on GINGERino, a 3.6-meter square ring laser that is a forerunner to GINGER, a 6-meter, octahedral ring laser planned for Italy's Gran Sasso underground lab.
Having won funding from the European Research Council, Igel offered Schreiber his biggest challenge: designing ROMY. With its 12-meter arms, ROMY is more sensitive than previous ring lasers, capable of sensing Earth's spin to better than one part per billion. And instead of one square ring, it has four triangular ones. Three of them are required to pin down rotations in any direction, and the fourth adds redundancy. Construction began in March 2016 and finished 6 months later.
Last month, engineers achieved first light in all four rings at the same time—a sign that the geometry of the tetrahedron is precise enough to keep all the lasers resonating properly. "It's everything or nothing," Igel says. "Every time the red [laser] light is visible, people are screaming, really excited." The team is now working on interfering the lasers and measuring the Sagnac effect. They hope to present their first proof-of-principle measurements next week at a meeting of the European Geosciences Union in Vienna.
Eventually, ROMY scientists will monitor changes in the length of the day and the position of the poles. Neither is as fixed as you might think, varying by milliseconds and centimeters each day. The sun and moon tug on the planet, while the drift of continents, changes in ocean currents, and the rebounding of the crust since the retreat of ice age glaciers all shift mass around, altering Earth's moment of inertia and therefore its spin. Even hurricanes and earthquakes can give a tiny nudge this way or that.
Earth's little twitches have practical consequences. Precisely targeting a rocket, whether it is destined for Mars or geostationary orbit, requires taking them into account. And the data from GPS satellites—which businesses and consumers the world over use—would drift into irrelevance within weeks if their exact positions in relation to Earth's surface were not constantly corrected.
Currently, the best measurements of those variables come from a system called very-long-baseline interferometry (VLBI), which uses radio dishes spaced across Earth to stare at quasars—brilliant beacons in the distant universe that occasionally flicker. By clocking when widely spaced dishes record a change in brightness, geodesists can calculate the planet's spin rate and its axis. But the system requires dozens of observatories to give up valuable astronomy time, and for the best timing comparisons, hard drives have to be shipped overnight from remote locales to supercomputer centers. It can take days to turn observations into a published measurement.
ROMY will try to match the precision of VLBI—and outdo it in speed. In theory, ROMY could monitor Earth's spin rate and axis constantly, updating measurements in real time, says Lucia Plank, a geodesist at the University of Tasmania in Hobart, Australia, who helps provide the VLBI service. "The advantage of ROMY is you have an instantaneous result," Plank says, though she adds that the VLBI technique, being more stable, is unlikely to go away anytime soon.
Whereas VLBI measures Earth's rotation with respect to markers billions of light-years away, ROMY measures it right at the surface—and the difference could be telling. That's because Einstein's frame-dragging effect, in which the gravity of Earth's rotating mass warps and twists nearby spacetime, should cause an infinitesimal shift in the rotation rate as measured close to Earth. It's the same test that was done, famously and expensively, by Gravity Probe B, a $750 million NASA mission that put gyroscopes on a satellite and measured the frame-dragging. Belfi says that doing it again, from the ground, is worthwhile. "In physics this is not a trivial result," says Belfi, who wants to use GINGER to do the test if ROMY cannot.
Being so new, ROMY is plagued by experimental drift. The structure is still settling in the soft sediments of Fürstenfeldbruck. Unlike other ring lasers, which were fixed to blocks of Zerodur—a ceramic resistant to temperature changes—ROMY's steel tubes flex with the temperature swings of day and night. It also is prone to shifting after rains saturate the ground. Igel eventually wants to eliminate those drifts by putting small motors behind each of ROMY's mirrors to make tiny adjustments to the rings in real time. But he is keen to embrace one type of fast-moving "drift": earthquake shaking.
In the past, seismologists have measured only translation—the displacement of the ground along any of the three cardinal axes. But seismic waves also drive tilt motions, which rotate points without shifting their positions. Traditional seismometers could not measure tilt motions, but theory suggested, reassuringly, that they are small enough to ignore. As Charles Richter, the seismologist who developed the famous magnitude scale for earthquakes, wrote in 1958, "such rotations are negligible."
"But they are there," Igel says. Indeed, experiments in recent years have suggested that the motions can actually be large. Soft soils can amplify them to 10% or more of the magnitude of translational motions. Engineers have been designing buildings only for translational shaking, but they should take tilts into account as well, says John Evans, a seismologist with the U.S. Geological Survey in Santa Cruz, California. "It's best to know what [shaking] actually goes into a building to make its response within tolerable limits."
Measurements of tilt also could pay dividends for earth science. Traditional seismometers can misclassify tilting as translational motion—a problem especially acute for ocean bottom sensors that sit on soft muds, Evans says. By measuring tilt directly, researchers could limit such "data contamination." Tilt measurements also might sharpen 3D models of the interiors of volcanoes, where swelling magmas create tremors with larger-than-normal rotations, Igel says. "If you do not take into account these tilt motions, your model might be wrong," he says.
ROMY should help earth scientists explore this new seismological frontier—if only by showing that it exists. Soon after the team turned on its first triangular ring, it sensed rotations from the magnitude-6.6 Norcia earthquake in Italy last October.
Eventually, scientists will want to get closer to the source. "You cannot move ROMY," says Frédéric Guattari, head of seismic rotation sensors at iXBlue, a navigation sensor company in Paris. "Now, we need a portable device." The answer from iXBlue is a compact sensor that relies not on lasers but on a fiber optic loop 5 kilometers long, wound into a coil just 20 centimeters across. The device sends photons in opposite directions through the loop, interferes them, and tracks phase shifts to detect rotations. Guattari has already placed prototypes astride the Stromboli volcano and in the Florence cathedral.
At up to €50,000 each, the sensors will be much more expensive than a traditional seismometer, but Guattari says they will ultimately offer a cheaper way to map the subsurface. Typically, geoscientists search for oil and gas traps deep in Earth by laying out dozens or even hundreds of sensors in an array. The array listens for the echoes of seismic waves—generated by distant earthquakes or small explosions detonated nearby—as they bounce off subsurface structure. But by measuring rotation as well as translation, seismologists can get not only the displacement of earthquake waves but also their velocities, which are a powerful probe of subsurface structure. "You can do a lot more with this point measurement," Igel says.
Technology from iXBlue might allow the oil and gas industry to get by with fewer sensors. It also could prove useful in situations when deploying even one sensor is challenging—such as on missions to other planets. Evans predicts that tilt sensors could flourish. "I think we're going to see slow adoption," he says. "In 20 years they could be standard."
But Igel and Schreiber hope that it won't be just the small fry that proliferate—they also want ROMY to spawn offspring. With multiple large ring lasers scattered around the globe, geodetic measurements could be coordinated, calibrated, and checked against one another to create a richer and more precise picture of our planet's twists and turns. Plank, though loyal to VLBI, says she shares the hope that Germany's great ring won't reign alone. "The ultimate goal would be to have more of these around the globe."