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Dropping Atoms from the Tower of Pisa


If you ever climb to the top of the Leaning Tower of Pisa, you might be tempted to drop something back down. Please don’t do it, unless you are a sixteenth-century scientific genius, like Galileo Galilei. (Or if you happen to have 100 euros, and I’m right underneath you. Just don’t let the guard see us.)

We don’t know if Galileo actually did drop anything from the Leaning Tower of Pisa. But he certainly fantasised something like that. Like in the picture above.

Galileo wanted to show something called the Universality of Free Fall, aka the "Equivalence Principle". It says if you drop two things from the same height, then, regardless of how much they weigh or what they're made of, they'll hit the ground at the same time. More technically put, "gravity accelerates all objects at the same rate, regardless of their mass or composition."

That's right. You can drop two of the same plastic bags, one full of rock candy and the other full of gummy bears, and they'll hit the ground at the exact same time. Or a cannonball and a Styrofoam ball. Or a water balloon, and a bigger water balloon.

(But you’re probably thinking, what if one of them has a parachute. Good thinking! The object with the parachute will certainly fall slower. But that’s wind resistance – the gravitational acceleration is the same.)

The Equivalence Principle is a principle, not a theory. That means it’s not something you can prove. You can only observe it over and over again. For hundreds of years, scientists have been dropping all kinds of things, from all kinds of heights, to double- and triple- and gadzoople-check the Equivalence Principle. And gravity always accelerated them the same. No one ever measured a difference.

But scientists are curious people, and they still want to know just exactly how exactly exact the Equivalence Principle is. Some of them guess that, actually, chemical composition does have an very small effect on gravitational acceleration. So NASA scientists have an experiment where they shoot lasers at the moon, where they hit mirrors that astronauts put up there 30 years ago, and measure how long the lasers take to reflect back to earth.

Ribidum, Max-Planck-Gesellschaft

And scientists also want to know if the Equivalence Principle holds for everything. That’s where Dr. Sebastian Fray comes in. He’s at the Max Planck Institute for Quantum Optics in Garching, Germany. And his team has been measuring if the Equivalence Principle doesn’t just hold for candy, water balloons, and 100 euro bills, but for little strange things like atoms as well.

It turns out, it does! Hundreds of years after Galileo, the Equivalence Principle still holds – even for atoms. And it has big implications for the way we see the universe. Click below to hear Dr. Fray explain why.

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