Science & Technology

If You Drop a Feather and a Neutron Star in a Vacuum, Which Lands First?

Gravity is gravity is gravity. Here within Earth's atmosphere, a bowling ball will land long before a feather if you if you drop them simultaneously — but that's actually an exception to a vast cosmic rule. In fact, if you were to head out into the vacuum of space, you'd see that a feather drops at the same rate as a bowling ball. And a house. And, as astronomers recently confirmed, an ultra-dense, gravitationally hefty neutron star.

Light as a Feather, Stiff as a Ball

This question dates all the way back to the 16th century, when Galileo is said to have climbed the leaning tower of Pisa and dropped balls of different weights from the top. When they both landed at the same instant, he proved that gravity's effect on an object's acceleration is the same regardless of mass. (Whether he actually did that rather than just posing it as a thought experiment is a matter of debate.) Later, Isaac Newton proved this mathematically with the equivalence principle: In a uniform gravitational field, all objects fall with the same acceleration. Here on Earth, that's 9.8 meters (32.2 feet) per second squared.

Then why does a feather fall more slowly than a bowling ball if you or I do it? That's due to air resistance. It's the same reason baseball is played with a round ball instead of a boxy Rubik's cube: Certain shapes and materials produce more air resistance, or drag, than others, which slows them down. But remove all of the air, and whether it's a feather, a bowling ball, a baseball, or a Rubik's cube, it'll fall at the same rate. Physicist Brian Cox put this very concept to the test when he visited the world's largest vacuum chamber in 2014.

"Newton would say that the ball and the feather fall because there's a force pulling them down: gravity," Cox said in the BBC video about the test. "But Einstein imagined the scene very differently. The happiest thought of his life was this: The reason the bowling ball and the feather fall together is because they're not falling — they're standing still. There is no force acting on them at all." That's in reference to Einstein's version of the equivalence principle, which says that gravity is velocity and velocity is gravity. The acceleration of the feather, the acceleration of the bowling ball, and your weight on your own two feet are all a product of the same force.

Three's a Crowd

Okay, but what if the object is incredibly massive and incredibly dense? Isn't it possible for a really extreme object to have more gravity, beyond this whole gravity equals velocity thing? In fact, there are alternative theories that say just that. A neutron star is one of the densest objects in the universe — it forms when a star several times larger than the sun collapses into an object the size of a city. (And you thought packing for vacation was impressive). Those alternative theories predict that the amount of gravitational energy holding a neutron star together would make it fall differently than, say, a bowling ball.

In 2011, scientists at the National Science Foundation's Green Bank Telescope discovered the perfect place to test that idea: a triple-star system made up of a neutron star in a tight 1.6-day orbit with a white dwarf star, and the both of them in a wide 327-day orbit with another white dwarf star. Because a white dwarf is much less massive and dense than a neutron star, those alternative theories say that the two stars would orbit — which is really just a controlled fall — differently around the far white dwarf.

Luckily, this wasn't just any neutron star — it was a pulsar, which pulses electromagnetic waves with a consistency that rivals the most precise atomic clocks. "We can account for every single pulse of the neutron star since we began our observations," said principal author Anne Archibald of the University of Amsterdam and the Netherlands Institute for Radio Astronomy, in a statement. "We can tell its location to within a few hundred meters. That is a really precise track of where the neutron star has been and where it is going."

With a calculation that ended up being ten times more precise than the previous best test of gravity, the team proved that there's likely no difference in acceleration between the two stars. "If there is a difference, it is no more than three parts in a million," said co-author Nina Gusinskaia of the University of Amsterdam. Score another one for Einstein.

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Believe it or not, gravity is still one of science's most puzzling phenomena. Learn about the quest to understand it in "The Ascent of Gravity: The Quest to Understand the Force that Explains Everything" by Marcus Chown. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer July 17, 2018

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