So You Think You Know Gravity? Let Us Drop Some Knowledge

Gravity is just gravity, right? You slip on some stairs, take a tumble to the bottom, and everyone laughs. Hilarious, that’s a given, but that’s also not giving gravity its due. Gravity is all around us, an omnipresent force that keeps our feet on the ground or our bodies falling down the stairs, but there’s actually a whole lot more to this story.

Think about how you feel sitting right now. Kinda heavy, as your arms fall to your side? Great! Earth’s gravity is pulling you onto the planet’s surface, and everything’s in working order. Earth is also pulling on the moon, keeping it in orbit, and the moon is in turn pulling a bit on us, while the Sun is pulling on Earth, keeping us from flinging out into the cosmos.

But maybe a better way to approach gravity isn’t to think about being stuck on Earth, but about being weightless. This is the kind of thinking that helped Einstein revolutionize the field of physics.

Imagine yourself in an elevator that clearly failed its last safety inspection and is now in free fall. You begin floating, because you’re traveling at the same speed as the elevator. Drop an apple and it’ll float in front of you. You’re weightless, something known as the equivalence principle, what Einstein called his happiest thought. “What you’re really doing when you’re experiencing gravity isn’t being heavy in your chair, it’s falling weightlessly in the gravitational field,” says astrophysicist Janna Levin.

Now think about surviving that free fall, landing gracefully, and throwing the apple. It’ll drop to Earth in an arc—the faster you throw it, the longer the arc. So when things fall freely around a large object like our planet, they trace curved paths. Space-time itself, Einstein realized, is curved. “That intuition is so elegant and so beautiful, and just comes from these two simple thought experiments,” Levin says.

Building on those intuitions, physicists have since made discovery after discovery about the very fabric of existence. Thanks to Einstein, we can (theoretically) explore black holes, whose gravity is so strong, not even light can escape. We can think about approaching super-dense neutron stars, whose gravity would crush us into subatomic particles. And we can put humans into orbit aboard the ISS which, like a plummeting elevator, is forever falling toward Earth, allowing its inhabitants to float.

WIRED asked Levin to sit down with five people of varying levels of physics expertise, from a kid on up to a full-blown physicist, to talk about the force to end all forces. Check out the video above for a mind- and space-time-bending look at gravity.

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