The vertiginous video also offers an opportunity to consider theories posited by two of the giants of science
https://youtu.be/Wfoy_OvNDvw While on a spacewalk outside the International Space Station over Mexico, NASA astronaut Randy Bresnik captured this spectacular, vertiginous video with a GoPro camera. I spotted it in a NASA Tweet yesterday, and when I watched it, I really did have the sensation that this would be as close as I'll ever come to experiencing free-falling around the Earth. (Short of a virtual reality video, that is.) Bresnik shot the video awhile ago — on Oct. 20, 2017, while on one of three spacewalks during his mission totaling more than 20 hours. So this isn't exactly breaking news. But I figured that there would be others who had't seen it until now. I also got to thinking that it offered an opportunity to talk about the phenomenon of free-falling around the Earth — in other words, orbiting. Let's start with a simple 'what if' scenario: Imagine Earth's gravity suddenly disappearing while Bresnik was on his spacewalk. I think you can easily picture what would have happened: Both he and the space station itself would have shot off in a straight line out into space. But of course, our planet's gravitational field continued to pull on both, causing them to fall — right toward the Earth. But as they did, Earth's curved surface fell away at the same rate. So instead of falling to the ground, they continued falling around the Earth. And this is, of course, is what it means to be in orbit. I've always taken it for granted that an astronaut, a small tethered tool he or she may be working with, and the space station itself, would free-fall like this at the very same rate and therefore stick together. But if you stop to think about it for a moment, that's not necessarily intuitive. You could imagine that the much more massive space station (on the ground it would weigh in at about 925,000 pounds) would fall much faster than a far more diminutive astronaut. For millennia, people subscribed to the common sense notion that heavier objects do fall faster than lighter ones. In fact, Aristotle himself believed that objects fall at a speed proportional to their weight. But then Galileo Galilei came along and upset the apple cart, observing that bigger pieces of fruit fell to the ground at the same rate as smaller ones. Okay, I'm being silly. But in the 1500s, Galileo proposed what has come to be called the "equivalence principle": The rate at which falling objects drop is independent of how much they weigh. Galileo first arrived at this idea through a thought experiment that he outlined in his book "On Motion." And then, as we all learned in elementary school (or should have, at any rate!), he allegedly tested it by dropping objects of different weights from the Leaning Tower of Pisa. That story is apocryphal. Maybe it happened, maybe not. But Galileo really did test his theory by rolling objects of different weights down inclined planes. And sure enough, he observed that they all fell at the same rate. In 1971, Apollo 15 astronaut Dave Scott famously gave the equivalence principle another test — on live TV during a walk on the Moon. https://youtu.be/oYEgdZ3iEKA As you'll see when you watch the video above, he held out a geologic hammer and a feather and dropped them at the same time. They were essentially in a vacuum, which meant there was no air resistance to affect the experiment. Even though I knew the outcome before watching the video, it still gave me a thrill: The hammer and the feather hit the Moon dirt at precisely the same moment. Flash forward to 2016. A French satellite called MICROSCOPE was tasked with carrying out a far more precise experiment while orbiting Earth. It involved concentric cylindrical shells a few centimeters long but of different masses. Since both objects were in orbit with the spacecraft, they were free falling around the Earth. If Galileo was right, then the concentric shells should fall at exactly the same rate under gravity. And actually, another epochal theory was being tested too: Einstein's general theory of relativity. It also dictated that the two objects should fall at the same rate despite their different masses. Over the course of more than 1,500 orbits around Earth, extremely precise detectors checked to see whether there were any deviations in the rate at which the cylinders fell. The result? According to a story in the journal Science:
. . . no discrepancy in the acceleration of two small test masses to about one part in 100 trillion (10^14).That’s more than 10 times better than the most sensitive ground-based experiments, which look for disparities in the response of weights to Earth’s spin.
It is the most precise confirmation yet of the equivalence principle.