A Spacecraft Just Measured Movement Less Than the Width of an Atom

The successful results pave the way for a future mission that could detect low-frequency gravity waves

An artist’s conception of the LISA Pathfinder spacecraft in orbit at Lagrange Point 1. European Space Agency/ C. Carreau

A million miles from Earth, a European Space Agency satellite carrying two floating cubes of gold-platinum alloy showed that it's possible to measure motion on the scale of an atomic nucleus, which could reveal the nature of some of the most massive objects in the universe.

Called LISA Pathfinder, the spacecraft is the test bed for the Evolved Laser Interferometer Space Antenna (eLISA). The results appeared today in a paper in Physical Review Letters.

The eLISA mission will consist of three spacecraft orbiting the sun. One of the spacecraft will fire a laser towards the two others, describing an L shape 621,000 miles on a side. The lasers will measure the distance between test masses carried by the probes to within a few trillionths of a meter – smaller than atoms. The precise measurement will allow scientists to observe gravitational waves--disturbances that stretch space itself--which are a consequence of Einstein's general theory of relativity. That passing wave will change the length of one side of the L relative to the other, and let scientists see the actual curvature of space.

"Say if you had one mass in New York and one in Torino [Italy]," Stefano Vitale, professor of physics at the University of Trento in Italy and the principal investigator for LISA Pathfinder, tells Smithsonian.com. "They're both accelerating towards center of earth.  When a gravitational wave goes by they start to fall in slightly different directions." 

But tracking such tiny motions is difficult, said Fabio Favata, head of the coordination office of the ESA's Directorate of Science at a press conference announcing the results. That is why LISA Pathfinder was launched. "We decided that we should learn to walk before we can run," he said. "This is analogous to the Gemini project for Apollo… We have not only learned to walk but to jog pretty well."

Inside LISA Pathfinder, two 1.9-kilogram cubes of a gold-platinum alloy float exactly 14.8 inches apart. A laser beam is reflected off of each cube, and the superimposed lasers measure their motion relative to each other.

"We took the millions of kilometers of LISA and we shrunk it down into one spacecraft," said Paul McNamara, ESA project scientist for LISA Pathfinder. LISA Pathfinder is too small to measure gravitational waves, but it showed that the instruments could measure very small motions and that it's possible to build an environment with no disturbances from the outside environment.

The LISA Pathfinder showed it could pick up motion at the femtometer scale – one millionth of a billionth of a meter. That was orders of magnitude better than they had hoped, said Martin Hewitson, LISA Pathfinder senior scientist. "We wanted to see picometer scale motions," he said. A picometer is 1,000 times larger than a femtometer. "It's more than 100 times better than [observations] on the ground."

Gravitational waves have been detected before. Scientists working at the Laser Interferometer Gravitational Wave Observatory (LIGO) announced in February that they had found them. The waves were likely made by the collision of two black holes.

But LIGO is on Earth, which means it can't see the kinds of gravitational waves that might be produced by other phenomena. Earthquakes on the other side of the planet, passing trucks, and even thermal expansion of the equipment can drown out the signals LIGO seeks. Another factor is size. Any ground-based detector can only be so large; LIGO, which also describes an L-shape, is 2.5 miles on a side, and bounces the laser back and forth between the mirrors to get an effective length of 695 miles. That's big enough to efficiently see gravitational waves with frequencies measured from about 100 Hz to 1,000 Hz, said Shane Larson, a research associate professor at Northwestern University and one of the scientists who worked on LIGO. (When the LIGO team announced its discovery the lowest frequency "heard" was about 35 Hz). That translates to wavelengths of about 300,000 to 8.5 million meters. (Gravitational waves move at the speed of light). That means that besides colliding black holes, LIGO can listen to neutron stars as they spin or as pairs of them spiral into each other.  

eLISA, though, will be able to see gravitational waves that take many seconds to pass by – about 0.0001 to 1 Hz, which translates to gravitational waves as long as 3 billion kilometers.

Larson said that frequency range allows for the detection of objects and phenomena that LIGO can't match. "We could see neutron stars that are orbiting each other, but much earlier, before they get close to each other," he said. "Or white dwarf stars. White dwarfs will contact and merge but they will do so before LIGO can see them." eLISA, however, will pick them up.

Vitale added that eLISA will answer some fundamental questions about black holes and galactic centers. "We know that each galaxy has a black hole from hundreds of thousands to billions of solar masses," he said. "[eLISA] can see the collision of back holes of that size. We can also see a small black hole falling into a big black hole; that sends a signal that allows a sort of mapping of gravity field around black hole." The exact shape of those fields is an important open question in astrophysics. It might even show whether black holes actually have event horizons.

Larson said seeing the collisions of bigger black holes could also shed light on how the black holes at galactic centers got so large. "We see enormous black holes very early in the universe.  How do they get big that quickly? LISA can see these to edge of observable universe."

eLISA is planned for launch in 2034, and should start taking data within only a few months of launch. 

Get the latest Science stories in your inbox.