I’ve had the mixed pleasure of spending a fair amount of time experiencing what is usually called “weightlessness”. I say it is a mixed pleasure because while the sensation of weightlessness is amazing and so different from our everyday experience of the world, I have experienced it on parabolic airplane flights which have the unhappy side effect in a segment of the population of inducing nausea and vomiting. I am in that unlucky segment. The body does adapt, and my last flight was puke-free. Other names used to describe the state of weightlessness are zero-g, no gravity, microgravity, and freefall. The latter is the only one that is truly accurate.
As an astronomer, gravity is the force that most concerns me professionally, and it is also the force that most of us have the most direct intuitive relationship with in our daily lives. And yet the relationship between gravity and freefall or “weightlessness” seems to be as elusive to most people as the sensation itself. Whether I am lecturing to a university astronomy class, speaking to a group of elementary school kids, or giving a public lecture to educated professionals, I always try to demonstrate the amazing insight of Isaac Newton about gravity: the same force that makes the Moon orbit the Earth is responsible for apples falling to the ground. While it is easy to understand those words, their implications for how the solar system works and for “weightlessness” usually remain abstract or obscure. Working against us is not just our daily experience (and, one could reasonably argue, millions of years of evolution), but also the language we use to describe gravity and its presumed absence.
Here is my standard gravity stump speech. For these purposes we do not need to stray into the exotic terrain of warped space-time and Einstein’s general relativity. Our sensation of gravity here on planet Earth comes not from the force of gravity exerted on us by the Earth, but by the competition between that force and all the stuff that gets in the way of it. If you are sitting now, you feel your weight because the chair is stopping you from falling to the floor. The actual sensation of weight I feel right now is due to pressure of a chair seat against the backs of my legs, the pressure of the floor against the bottom of my feet, and the pressure throughout my body produced by the weight of head on neck, torso on lower back, and so forth. So there are two ways to get rid of that pressure: get rid of the Earth, or get rid of the chair. If the chair beneath you were instantly snatched away, you would fall to the floor. And in that split second you would not feel the pressure of the chair on your backside. That sensation of weight would be gone, even though the Earth’s gravity is still very much present.
How about the weight of your head on your neck, etc? Galileo’s famous experiment at the tower of Pisa gives us the answer. Here again, though we may be familiar with the facts of the experiment, the implications are difficult to internalize: gravity makes everything fall at the same speed, whether it be a feather or a hammer, a head or a body. We (and centuries of thinkers between Aristotle and Galileo) have a hard time with this because air does a better job of slowing a feather than it does of slowing a hammer, so, in fact, the feather does fall slower. But if you get rid of the air (easy enough in a small lab experiment), they all fall at exactly the same rate. So when that chair is snatched away, all parts of your body will fall toward the floor at exactly the same rate. There will be no pressure of any part pushing up against any other part. And since that pressure is what we experience as weight, its absence gives us, in that brief period before slamming into the floor, “weightlessness.”
And yet we are still experiencing the Earth’s gravitational pull. In fact, in physics the term “weight” refers not to the pressure we feel from the chair, but simply the force of gravity acting on an object. Removing the chair does nothing to alter that force. It removes instead what is called the “normal force” of the chair that exactly cancels the force of gravity acting on our bodies. The rigid structure of the chair exerts an upward force on our bodies that keeps us from moving down due to the force of gravity. One might then consider the sensation we experience when the chair disappears not to be weightlessness, but normallessness.
I don’t think that will catch on.
We usually associate “weightlessness” with the image of astronauts “floating” inside a spaceship. This gives the impression of motionlessness (I’m going to see how many words I can add “lessness” to). However, it is the very large motion of these astronauts that makes them “weightless”. They are in a spaceship that is falling toward the Earth. There is no chair holding it up. And because the spaceship and the astronaut (like the hammer and the feather) fall toward the Earth at the same rate, the astronaut does not move relative to the spaceship. She appears to float inside it, yet there is nothing holding her up. Both she and the spaceship are falling freely toward the center of the Earth. Happily, they will not hit the Earth because previously, rockets accelerated the spaceship to such a high speed that by the time it has fallen the distance needed to hit the Earth, it has zipped over so much of the Earth that the curvature of the Earth has made the surface that much further away from the spaceship again. Here, then, is the similarity between the apple and the Moon that Newton recognized: the Moon is falling toward the Earth, but because of its great speed, it keeps missing the Earth.
So the only connection between space (as in “outer space”) and weightlessness is that getting above the atmosphere is the easiest way to fall for a very long time without running into something. But the exact same thing happens (for a very short time) when you snatch the chair out from under someone. So, “weightlessness” can be achieved by finding a way to fall for an extended period of time without any slowing due to air friction or, preferably, uncomfortably hard landings. Parabolic airplane flights accomplish this by flying the same path that an object falling toward the Earth would follow if there were no atmosphere. Because this is easily calculated, pilots can fly planes on such paths. While they do so, everything inside the plane follows the path than an object falling toward the Earth would follow if there were no atmosphere. So the airplane seat is falling as fast as you are, and it therefore doesn’t push up on you. Your arms are falling as fast as your shoulders, so they do not pull down on you either. You experience “weightlessness” because you are falling freely very quickly. The pilots make sure to achieve crashlessness (okay, that’s a stretch) on the flight by having the plane pull up before it heads toward the Earth too quickly. When it does, your body wants to head toward the Earth quickly, but the plane is rudely interrupting that fall and exerts a pressure against you that is much greater than normal. We thus feel heavy or excessive weight.
In fact, you are, when “weightless” accelerating at 1-g, where g is 9.8 meters per second per second. Right now, sitting on a chair in a normal terrestrial environment, your acceleration is zero-g. Weightlessness is really motion at 1-g, and not zero-g. The net force acting on us when we feel heavy is zero, while the net force acting on us when we feel weightless is equal to the local force due to gravity.