Ask Ethan: Is Zero Gravity Really A Thing? (Synopsis)

"It was a strange lightness, a drifting feeling. Zero gravity. I understood that everything that once seemed solid and immovable might just float away." -Lisa Unger

If you want to experience zero gravity, all you need to do is go to space, turn your rocket off and feel that weightless sensation, right? Except, from up there in Earth’s orbit, the gravitational force on you is still almost as great as it is on Earth’s surface! You still accelerate down towards the center of the Earth, not quite at 9.8 m/s^2, but not by much less.

Astronauts, and fruit, aboard the International Space Station. Note that gravity isn't turned off, but that everything -- including the spacecraft -- is uniformly accelerated, resulting in a zero-g experience. Public domain image.

There’s no place in the Universe where you can go to hide from the long-range force of gravity; its effects reach everywhere. So how, then, can we claim to truly know what it’s like in a zero-gravity environment? Because zero gravity isn’t about turning gravity off; it’s about the forces we do (and don’t) experience on our bodies.

The identical behavior of a ball falling to the floor in an accelerated rocket (left) and on Earth (right) is a demonstration of Einstein's equivalence principle. Image credit: Wikimedia Commons user Markus Poessel, retouched by Pbroks13.

According to Einstein, free-fall and zero gravity are actually equivalent, although that’s far from obvious. Here’s the science behind why on this week’s Ask Ethan!