LISA Pathfinder – in space to feel gravity

ESA goes to space to feel it quiver as gravitational waves rush by

Thursday 11 February 2016: The LIGO team announces the detection of gravitational waves as ripples in space, stretching and compressing it as they pass. The ripples were created in one of nature’s most violent events: the merger of two black holes. A century after Einstein announced his general theory of relativity, humanity’s quest for supporting evidence has perhaps never been more fierce.

Interferometers as LIGO try to detect gravitational waves on Earth, the misorbited GALILEO GNSS satellites’ incredible precise clocks are measuring time go slower every time they come nearer to Earth in their elliptical orbits, and ESA is preparing a grand space version of LIGO with its LISA Pathfinder, which now has a Lissajous orbit around L1. Over the course of the next two days, LISA Pathfinder will execute its mission to prepare for an experiment designed to feel the curves and bends of spacetime. In this article, we will explore how ESA is going to put gravity to the test.

Gravity – a force acting at a distance?

The launch coincided with the 100–year celebration of the publication of Einstein’s theory of general relativity, and that is not by chance. LISA Pathfinder holds two golden cubes that will be released to float freely in space, with the spacecraft itself aligning it as best it can around them. By following their motion, it is putting a fundamental principle from Einstein’s theory to the test – that the contents of space determines how the cubes will move.

Here on Earth, we are more accustomed to this Einsteinian principle through our notion of gravity. We say that gravity is a force that pulls us down, accelerating us downwards continuously, keeping our feet sturdy on the ground. For Newton, it was the force that supposedly snapped an apple off a branch, directing it downwards, where his head happened to be.

This force is said to have inspired him to describe this phenomenon using the concepts of a force coming from massive, heavy Earth, acting instantaneously at a distance on the apple. And, since the apple also had mass, it would also pull on Earth, just as much as Earth did on it. Obviously, the Earth is more difficult to move than the apple. The apple falls.

But, forces are abstract concepts. Were the Earth and the apple really pulling on each other without being in touch? If we instead try to imagine that the mass of Earth—and the apple—bends space and time, spacetime, in the same way as a bowling ball would bend the sheet of a trampoline, then we’re closer to Einstein’s description of gravity.

In this picture, the motion of the apple would be dictated by the bends Earth has made in the trampoline’s sheet. The apple was sliding downwards, there was no spooky force pulling on it. The astronomer Arthur Eddington observed this effect during a solar eclipse in 1919, seeing stars shifting their apparent location on the sky when their light was passing near the obscured Sun. The path of the starlight was bent by the mass of the Sun.

To Einstein, gravity was only a geometrical effect. The presence of masses and energy in space and time also bends it. Obstacles in spacetime, like Newton’s sorry head for the apple that hit it, or the Earth for us who have our feet planted on it, are obstacles that prevent us from floating freely along the bends and curves of spacetime.

The apple, before it hit the head, felt no such obstacles. It was in free fall, going downwards in the large pit Earth has caused in the trampoline sheet we imagine spacetime to be. Had we painted a curve, tracing the apple as it fell freely through space, it would become a curve that general relativists and mathematicians call a geodesic. And geodesics are straight lines in spacetime. The starlight Eddington saw gracing the Sun was travelling straight ahead all the time.

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Flying through space along geodesic curves

ISS Commander Kelly and his crew as well as the two golden cubes aboard LISA Pathfinder are in free fall, floating along geodesic curves. When Felix Baumgartner stepped out to make his stratospheric jump, he probably felt quite a few things, but gravity was not one of them. How could LISA Pathfinder then even imagine to do gravitational experiments?

The trick is to see if the motion of the cubes are the way Einstein want them to be. When it releases its two golden cubes to float freely, following geodesic curves, it will have near-total oversight of all the disturbances that could affect the motion of the spacecraft. It was originally planned to use electric propulsion for micro-attitude control, but instead uses cold gas micro-Newton thrusters to counteract the nudges caused by interplanetary particles and plasma, most of them having been spit out by the Sun. The freely floating cubes themselves are affected by thermal motion of particles inside the spacecraft, internal temperature differences and electrostatic effects, to mention a few.

By keeping track of the spacecraft’s motion, these disturbances can be removed from the tracked motion of the cubes as they follow geodesics through spacetime. The distance between the cubes, a mere 38 cm, is precisely monitored using laser interferometry. Here, a laser beam travels between the cubes before going through a slit, creating a diffraction pattern where any minuscule changes in the distance between the cubes as they float freely through space will become quite evident as changes in the diffraction pattern.

In the experiment LISA Pathfinder prepares for, eLISA, three cubes will be placed inside three separate spacecraft that themselves are separated by a million kilometres, a distance that also will be monitored using laser interferometry. This experiment will be different mainly in size, sensitivity and obviously the number of cubes, but the key principles and technologies are the same. Those are about to be tested by LISA Pathfinder.

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Ripples in the trampoline fabric

For eLISA, the sensitivity would be high enough to measure distance contractions and elongations in spacetime caused by passing gravitational waves. These are waves of energy that is radiated away from massive systems that looses quadropole moment, not as light, but as disturbances in space. The quadropole moment is the main physical quantity that can be lost as gravitational radiation, as there cannot be any losses from changes in a system’s mass (monopole moment) or position (dipole moment).

With eLISA, ESA hopes to surpass ground-based detectors like VIRGO and LIGO in detectable frequency ranges and sensitivity. The rather frenetic rumours that have circulated recently, hinting that LIGO might possibly, or possibly not, have detected some gravitational waves, were finally confirmed Thursday 11 February. When the LIGO team announced the detection of a significant signal in a press conference watched by scientists around the globe, it marked the remarkable success of Einstein’s general theory of relativity.

My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!! Exciting.
— Lawrence M. Krauss (@LKrauss1) January 11, 2016

Ladies and gentlemen, we have detected gravitational waves. We did it!
— David Reitze, executive director of LIGO, February 11, 2016

The eLISA setup where there are three freely floating cubes is necessary to precisely detect and describe gravitational waves. A wave would propagate in one direction, as any normal wave would do, but the ripples will not simply go up and down, but in two dimensions that neither are in the same direction as that of the wave. With three cubes, you have two directions that, if you are lucky, aligns with the directions that the wave oscillates.

The ripples are manifested through squeezes and pulls in distances. The distances between two eLISA cubes along one direction (say, the horizontal direction) would be squeezed together, becoming shorter, whereas the space between two cubes would be pulled in the other direction (the vertical direction), increasing the distance.

This bigger sister of LISA Pathfinder should then be able to feel these oscillations in spacetime. LIGO has confirmed that they exist, now the waves have to be put to some good, astronomical use. Using gravitational waves to see the Universe, rather than electromagnetic waves, a whole new world opens up. 

With eLISA, we would be able to quite precisely pinpoint the creation of the first black holes, their masses, their spin and their little known interplay with the creation and growth of galaxies. And all this is happening at a time when the Universe was an infant, only a few hundred million years old. This epoch in the cosmic history is virtually impossible to observe otherwise, as light emanating from that time would be hindered by the vast amount of neutral, light-scattering gas that filled most of the Universe back then.

Gravitational waves have been indirectly observed as loss of energy from two stars that are spiraling increasingly closer to each other, a discovery leading to the 1993 Nobel prize in Physics. These would form the control sample for eLISA.

The European Space Agency’s LISA Pathfinder signals a bright, ehrm, gravitational future for humanity’s continuing quest to understand and observe the Universe. It could also be a hint that the future in precision astronomy lies in free fall—in space.

[Sources]

• eLISAscience.org – describing LISA Pathfinder’s mission as well as the upcoming eLISA mission.

The following articles give in-depth information on the payloads as well as the scientific experiment:

• Clear overview:
  • Danzmann, Karsten, LISA Pathfinder Team, and eLISA Consortium. “LISA and its pathfinder.” Nature Physics 11 (2015): 613–615-613–615.
• Very clear description of the physics that will be tested:
  • Armano, M et al. “The LISA Pathfinder Mission.” 10th International LISA Symposium (LISAX) (2010).
• Description of the forces acting on the spacecraft and the test masses:
  • Antonucci, F et al. “From laboratory experiments to LISA Pathfinder: achieving LISA geodesic motion.” Classical and Quantum Gravity 28.9 (2011): 094002.
• Describes payload and expected performance (behind paywall):
  • Antonucci, F et al. “The LISA Pathfinder mission.” Classical and Quantum Gravity 29.12 (2012): 124014

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