Gravity is weird. It is the oldest of the four fundamental forces that we have been able to describe and yet what it is remains mysterious. When Isaac Newton first introduced his theory of gravity and the idea that objects with mass attract each other, he was accused by some critics of introducing a form of mysticism into science by postulating non-contact forces that could act instantaneously over empty space.
Einstein’s theory of general relativity brought in more sophisticated ideas by replacing instantaneous action-at-a-distance between two masses by saying that one mass distorts that space around it and that the distortion spreads through space at the speed of light and that the second mass responds accordingly when that distortion reaches it, thus removing the instantaneous action-at-a-distance problem. His Principle of Equivalence also showed that we cannot distinguish between being in a uniform gravitational field and being subjected to a constant acceleration. When we are in free fall, we are effectively weightless. The catch is that at some point, very quickly, we hit the ground.
Gravity is a ubiquitous force. We cannot shield ourselves from it. All this makes it hard for Earthbound people like us to imagine what life might be like in the absence of gravity. Now with space travel, we see astronauts in space stations in gravity-free situations. It should be noted that that he Earth’s gravitational field at the orbital height of the space station is about 90% of what we feel on Earth. But because they are in free-fall as the station orbits the Earth, they are effectively in zero-gravity (or more accurately microgravity) fields for a long time as long as they are in orbit. This has given us some idea of what life might be like in such an environment but there are still surprises. Part of the surprise is due to the fact that many forces that on Earth are small compared to the Earth’s gravitational field and are swamped by it, become significant when in zero gravity but many people do not realize this.
Take for example, a recent story about a video of Chinese astronauts (they refer to them as taikonauts) that had a glass of water. Since many people expect that in zero gravity water must float in the air in the shape of a sphere, this raised suspicions that the video had not been shot in space. But they are wrong because they ignored how important adhesive forces become in the absence of gravity.
News footage from a 2021 science lecture by Tiangong’s Shenzhou-13 crew and of the crew’s return to Earth plays throughout the compilation. The water glass is highlighted in a circle that has been edited into the footage.
“How did they get the water into the glass?” text in the video asks. “And how is it not floating out of the glass?”
One Instagram post featuring the compilation suggests this is proof that the footage wasn’t actually taken in space: “How could they get away with such a massive deception? Wouldn’t other countries call us out?” The post had received more than 36,500 likes by Friday.
But the footage is not proof of deception: There’s a basic scientific phenomenon that explains the water’s behavior, an expert told The Associated Press.
“Water molecules like to stick to glass and also to other water molecules more than they like to disperse in the air,” Jordan Bimm, a postdoctoral researcher and space historian at the University of Chicago, told the AP. “So if there is no external force, water remains in ‘clumps’ in the weightless environment, and in this case inside the glass.”
He added that surface tension — a property of a liquid’s surface that helps define its shape and allows it to resist external forces — “also works to help maintain the static shape and presents the illusion of how water would act on the ground.”
The idea that the Chinese space agency has faked these videos is as preposterous as the idea that the Apollo Moon landing was faked, this one further fueled by the idea that other countries are incapable of having a successful space program. You can see a video here of the Chinese taikonauts carefully putting water into a glass container that is held to the table by adhesive strips.
To see how these other forces become important in the absence of gravity, see this video below of what happens when an astronaut wrings out a wet washcloth.
You can also see how they brew coffee.
What we call ‘intuition’ is based on experience. When we think of situations that are so far removed from our own experience, such as in zero gravity, we should be very wary of jumping to conclusions about how things should behave.