Fact check: Gravity allows cities on opposite sides of the world to both face 'up'

The claim: The Earth is flat because cities cannot be upside-down

Flat Earth theory, the erroneous idea that the Earth is flat, continues to persist online despite overwhelming evidence to the contrary.

Some proponents of this model believe that the Earth cannot be a globe because that would mean some areas of the Earth are upside-down.

Fact check roundup: Debunking the flawed science behind flat Earth claims

That group includes the user behind a Nov. 22 Facebook post (direct link, archive link) that features a photo of an upside-down city. It was shared more than 100 times in less than three weeks.

"This is scientifically Impossiball," reads text included in the post.

This argument is wrongheaded.

There is no true "up" or "down" in space. Gravity pulls objects toward the core of the Earth, creating the sensation of being upright, physicists say. The photo in the post has been distorted.

USA TODAY reached out to social media users who shared the post for comment.

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Space doesn't have an 'up' and 'down'

There is no true up or down in space, David Brown, a physics professor at North Carolina State University, said in an email to USA TODAY.

"Near Earth – or any other planet or star – 'true up' is always the direction opposite the gravitational force," he said. "That is, 'true down' is always toward the center of Earth."

This means that Earth, like any sphere, does not have a fixed top or bottom. Instead, people experience the sensations of "up" and "down" in reference to the core of the Earth.

"Gravity pulls us toward the surface of Earth," said Brown. "So wherever you are on the spherical Earth, you are attracted towards the center of Earth. That is just as true in Antarctica, or Australia, as it is in North America."

This is why, regardless of the Earth's position during rotation, buildings remain on the ground and water stays in the oceans, said Jason Steffen, a mathematician and physicist at the University of Nevada.

If the Earth was flat, Brown said, people would also feel a gravitational pull toward the center of that disk. This would mean people on the "edge" of the flat Earth would fall sideways.

"This is one of very many ways we know Earth is not flat," Brown said.

The photo in the post has also been manipulated, said Siwei Lyu, a digital forensic expert at the University of Buffalo. It shows the Earth curving sharply on the sides of the image even though the scale only includes a single city.

"We should not see the early warping to that effect with that altitude," he said.

Our rating: False

Based on our research, we rate FALSE the claim that the Earth is flat because cities cannot be upside-down. Gravity pulls objects to the center, no matter where people or cities are located on Earth. There is no true "up" or "down" in space.

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This article originally appeared on USA TODAY: Fact check: Gravity pulls objects toward the center of the Earth