You’ve probably heard this sort of claim before: Einstein made a prediction, usually with general relativity, and we’re just now getting around to testing it. It’s a slightly misleading idea, since general relativity has to do with the relationships between all objects in the universe, between time and behavior — of course we keep having to test different implications in all sorts of different contexts, since it affects literally everything. But one upcoming test of Einstein’s relativity won’t just be important to theoreticians. It will directly inform the next-generation search for gravitational waves.
The effort was recently launched aboard a French satellite called Microscope, housing two painstakingly fabricated weights, one made of titanium and the other of platinum-rhodium. The idea is to put these weights into perfect — and I do mean perfect — free fall. They are set floating within the satellite as it orbits the Earth, their positions measured to incredible accuracy. If the two test masses don’t diverge in the slightest, then it means two things: Einstein was right about objects in perfect free fall and, perhaps more importantly, we have the ability to put objects into perfect free fall.
In fact, this test of our ability to make spheres of metal move due to gravity and nothing else is a much smaller version of similar tests still to come. The European Space Agency (ESA) recently launched the LISA Pathfinder mission, which aims to determine much the same thing by larger means. This mission will use heavier weights, made of a gold-platinum alloy, and set them on a longer orbit between the planets, not just around ours.
Regardless of the mass of the weights or the size of the orbit, however, they are both designed to create the same thing: “geodesic motion,” or motion due to gravity alone. In theory, a satellite’s geodesic is a straight line through spacetime, and it’s just the curvature of that spacetime itself that results in the observed, circular motion.
Why might we want to be able to put objects into flawless free fall? Well, as the name of LISA Pathfinder suggests, we have a pretty fascinating reason to hope we can do it: LISA. The Laser Interferometer Space Antenna is aimed at finding gravitational waves like those that rocked physics a few months ago — but LISA is hunting even bigger game, not the clash of black holes, but the clash of super massive black holes. This basically means the collision of two galactic cores during meeting of galaxies, and LISA could let us observe such events directly.
It can only do this if we are adept enough at making weights float perfectly. By getting three weight-floating satellites into orbit around the Sun (not the Earth) and forming them into a triangle a million-kilometers to a side, the ESA can make sure that any disturbances are due to a disturbance in gravity, not any static attribute of our solar system — AKA, a gravitational wave.
Though this is being framed as a “test” of Einstein’s relativity, it’s really intended as a test of our engineers and their abilities. If they cannot put the weights into free fall, that might mean there’s a fault in Einstein’s famous theory, but more likely it means that there’s a flaw in the free fall rig itself. Only if the rig is confirmed (and confirmed and confirmed and confirmed) would we consider doubting relativity.
But don’t worry, these geodesic experiments are more than important enough to justify their existence, even if they are unlikely to overturn physics as we know it.
Now read: What are gravitational waves, and where does physics go from here now that we’ve found them?