Let’s be honest: while the recently announced proof of the existence of gravitational waves was undeniably an enormous, exciting moment for science, it didn’t really accomplish much all on its own. After all, if it had turned out that gravitational waves don’t exist, then there would be quite a few scientific careers invalidated, and decades of well-supported research thrown into disarray. It’s important that scientists have proven the existence of gravitational waves — but not in the least bit surprising. For the study of these ephemeral ripples in space-time to truly revolutionize science, the readings they produce will need to be applied in some novel way — and that quest just got a whole lot closer.
The visionaries at the ESA’s LISA program have reported very promising results for their pilot project, LISA Pathfinder, which is itself an enormously impressive mega-project. Its readings not only direct the engineering of the final LISA satellites, but prove that launching them is even worth the expense. To prove that LISA is indeed a viable space-based gravitational wave detector, the LISA Pathfinder had to prove that mankind can put multiple objects into simultaneous, near-flawless free fall around the Sun. And that is what the ESA has done.
The two test weights that produced this result have been found to be virtually motionless with respect to one another, despite that they are both free-floating within their spacecraft, and that they are hurtling around the Sun at incredible speed. This means they are both in “freefall,” or affected only by the gravitational fields around them. Since they’re so physically close to one another, this also means they’re subjected to the exact same gravitational fields. So, their paths never (or virtually never) meet or diverge, and they endlessly fly around the Sun with not the slightest deviation in the distance between them.
Why does this matter? Because if you can trust that two objects are only being influenced by gravity, and then they begin acting differently, then you can say that they are each in a different gravitational situation — which implies that a gravitational wave has just passed by! LIGO, the instrument that found black hole gravitational waves, spans North America — a few thousand kilometers of distance separate its laser interferometer instruments, and provide the readings. This limits the physical size of the waves it can detect, since a wave more than a few thousand kilometers “thick” could pass over both detectors without producing a noticeably different reading between them.
But put the detectors a million kilometers apart and you can suddenly look at whole new types of waves — and the only place to find a million kilometers of space is… space. Each of LISA’s satellites will function like one of LIGO’s installations, providing readings that are only really meaningful in reference to one another.
This is where gravitational wave science could really go crazy. LIGO and similar instruments could (and will) look deep into the hearts of black holes, no small or trivial feat by any means, but LISA’s larger physical size should allow it to watch the interaction of so-called supermassive black holes at the center of galaxies, granting insight into the mysterious core of the Milky Way and the collision of billion-star collections of mass.
On the other hand, space-based gravitational wave detectors also have the potential to look at “primordial” gravitational waves, those that originate from the Big Bang itself. Could we have been looking for signals of the birth of the universe with entirely the wrong sense — looking for light with our eyes when we should have been listening for gravity with our ears?
LISA Pathfinder’s results have exceeded expectations in proving that modern space tech is capable of tackling that challenge. The weights in the Pathfinder experiment barely deviated from one another as they flew, showing a “relative acceleration lower than ten millionths of a billionth of Earth’s gravity.” That means it’s putting its test weights into orbit so stable that their deviation is a factor five lower than required by the LISA gravitational wave-finding mission.
They achieved this amazing result through a number of approaches, but the first was to place Pathfinder in the Lagrange Point #1. A Lagrange Point is any point between two massive objects where the pull exerted between the two of them is equal; any object affected only by gravity at the Lagrange Point will orbit with the same distance relative to both objects. So, by placing Pathfinder in the L1 point, they could have it orbit the Sun while keeping it a uniform distance from the Earth.
The Lagrange point offers the perfect low-turbulence environment for a gravitational wave detector, so this is the point from which the team launched their free fall experiment. Small trajectory adjustments from the thrusters allowed them their shield the internal weights from exterior forces — bringing the weights into perfect free fall alignment with each other and the walls of the craft. If they couldn’t do this, then the detectors of the LISA experiment could not be made to produce reliable-enough results for useful comparison.
LISA Pathfinder is doing more at the L1 point than just monitoring how well we can make cubes float, however. It also tested a form of laser interferometry that uses a wavelength of light that cannot be used on Earth, and found that it produced noise levels two orders of magnitude lower than required for LISA to work.
LISA itself is still a fair ways out — the launch date is still a full 18 years away. But with these readings in hand the ESA astronomers behind the mission can create far more reliable and specific designs, and truly get started designing and building the world’s first instrument capable of detecting the residual shockwaves of some of the universe’s most violence events.
And, as we learn more about the gravitational effects of dark matter, we may find that it can be studied via the tiny deviations in flight path of two objects placed a million kilometers apart. This sort of work also begs the question — when physical distance becomes the metric for an instrument’s sensitivity, how big, and how accurate, might we one day be able to go?
Now read: Here’s why we don’t have light-based computing just yet