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The most precise test of General Relativity ever conducted just happened in space
Einstein wins again.
In 2018, Dwayne Johnson took his particular brand of action hero to the world’s tallest fictional skyscraper in the appropriately named action film Skyscraper. The building in question stands an astonishing 1,060 meters tall, overshadowing the world’s tallest real building, the Burj Khalifa, which comes in at 830 meters. At one point Johnson’s character, Will Sawyer, finds himself clinging for his life from the top of the building with plenty of opportunity to plummet to his death.
Of course, he’s the hero, so he survives. But what if he hadn’t? How quickly would a Rock fall to the Earth? And does it matter if the Rock in question is more or less massive? We’ve begun from an unrealistic yet utterly popcorn-worthy premise and stumbled into a very old question of physics.
Human beings have a surprisingly difficult time understanding how gravity works. Despite interacting with it every day, close examination reveals that things don’t operate quite the way we might assume. There’s an intuition that heavier objects fall more quickly than light ones, but that’s not entirely true. The belief is partly due to the fact that heavy objects often do fall more rapidly than lighter ones. If you drop a brick and a piece of paper at the same time, you’re destined to see the brick hit the ground while the paper is still gently drifting to a graceful landing. That has less to do with gravity, though, and more to do with wind resistance. Place those same objects in a vacuum and you’ll see them plummet to the ground like they’re both made of lead.
We’ve known about this since at least Galileo, when the famed scientist allegedly dropped two metal spheres of different masses from the leaning tower of Pisa and watched them hit the ground at the same time. That story is probably apocryphal, but the result isn’t. It was a few centuries later, when Einstein came around, that we finally had an explanation for why this happens.
Among the many world-changing ideas contained within General Relativity, was something called the Weak Equivalence Principle. Put simply, it states that there is no difference between feeling a gravitational force or accelerating at an equivalent rate. Put another way, we can think of gravity as a sort of acceleration. It’s as if the ground is coming toward us at a constant 9.8 meters per second. It doesn’t matter if you’re a 10-ton steel ball or a feather, the ground is coming for you at the same speed. So says Einstein. If the Weak Equivalence Principle is correct, then objects of different mass should fall toward a source of gravity as if they were no different from one another.
CNES, France’s space agency, designed an experiment to test this theory at a level of precision not seen before. And they did it in space. The satellite, dubbed MICROSCOPE contains two cylinders made of different materials and with different masses. Inside the orbiting satellite the objects are in a constant state of freefall, plummeting toward the Earth but never reaching it.
They are held in place using electrostatic accelerometers. That serves to protect the satellite from the payloads banging around, but it also provides an opportunity to precisely measure their movements. By measuring how much energy is required to hold each object in place, scientists on the ground can determine their acceleration of each object with two orders of magnitude more accuracy than ever before.
After traveling tens of millions of kilometers around the Earth, the measurements were compared, and no differences were found. If the two objects experienced any difference in the force of gravity, it was less than one out of ten to the fifteenth power.
As always, science is a work in progress, but these new experiments confirm that Weak Equivalence holds up to an incredibly high degree. If there’s any funny business going on, gravitationally speaking, we’re going to have to look harder. In the meantime, you can rest easy in the knowledge that the ground is always beneath your feet, or it will be very soon.