Fact check: Laser beam tests done over water are skewed by refraction, don't prove Earth is flat

Isabella Fertel, USA TODAY
·5 min read
3

The claim: Laser tests show bodies of water are level, which proves the Earth is flat

Scientists have ample evidence to prove the Earth is a sphere. But some social media users claim to have devised an experiment showing the earth is flat.

The post shows a green laser beam projected over a body of water. It also includes a diagram showing how the distance between a straight laser beam and the water below should increase as the beam travels further away from its source as a result of the curve of the Earth.

“Earth is flat. Water at rest does not deviate from level,” claims a Nov. 27 Instagram post (direct link, archive link).

But the claim is false.

Scientists say that the post’s logic is flawed and that laser tests in uncontrolled conditions are not a reliable way to prove the Earth's shape. That’s because environmental factors can cause a laser beam to bend, or refract – especially over water. Furthermore, scientists said that experiments utilizing laser beams in controlled conditions, and on a larger scale than the diagram in the post, actually prove the Earth is curved.

USA TODAY reached out to the user who shared the post for comment.

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Laser tests in uncontrolled conditions unreliable due to refraction

The post's logic is fundamentally flawed, according to Oliver Kreylos, a project scientist at University of California, Davis’ Department of Earth and Planetary Sciences.

"’Level’ does not mean ‘flat.’ Level specifically means ‘at a right angle to the direction of down,’” Kreylos said in an email. “On a spherical planet, the direction of down always points – close to – the center of the sphere."

Scientists agree that these kinds of uncontrolled laser tests – especially when conducted over water – are not reliable ways to prove the Earth is flat.

“Lasers, or light in general, behave in odd ways that counter our intuitive understanding that light always travels in straight lines,” Kreylos said.

Various environmental conditions can distort and bend, or refract, the laser beam, according to physicist Jason Steffen.

“Unless you have uniform air composition, uniform humidity, uniform temperature and uniform pressure across the whole length of the instrument, then the laser beam will wiggle,” said Steffen, an associate professor of physics at the University of Nevada, Las Vegas. “When you're over water, the humidity is a bit higher, and so the index of refraction is higher, and it would cause light to bend towards the surface of the Earth.”

Refraction can bend the laser along the Earth’s surface to make it appear that the globe’s radius is much larger than it actually is, or even that the Earth is flat, according to Kreylos.

“If the weather conditions are just right, refraction is so strong in a very thin layer of air above a water surface that a laser beam could in theory travel all the way around the globe,” Kreylos said.

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Scale of laser test is too small to show the Earth’s curve

Furthermore, scientists say a laser test at the scale described in the diagram, even in controlled conditions, would not be large enough to prove the Earth is flat.

That’s because the Earth's curvature only drops by about 8 inches per mile, according to David Brown, a professor of physics at North Carolina State University. The diagram in the post describes a laser test over less than a half-mile.

Steffen agreed and said the social media users were thinking "too small."

"If their detectors were spaced every 200 km instead of only 200 meters, they would see the effect that they are showing," said Steffen.

When scientists utilize laser beams over long distances for other purposes, they need to account for the curve of the Earth.

For example, the Laser Interferometer Gravitational-Wave Observatory is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves. The experiment comprises two perpendicular four-kilometer-long vacuum tubes with laser beams running through them.

Because the tubes are so long, the Earth's curvature was a complicating factor when installing them in order for the lasers to function properly, according to the experiment’s website.

“To ensure a perfectly level beam path, the Earth’s curvature was countered by GPS-assisted earth-moving and high-precision concrete work,” the website states. “It wasn’t enough for (the experiment's) civil engineers to smooth a level path and assemble each arm’s tubes in a straight line.”

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Our rating: False

Based on our research, we rate FALSE the claim that laser tests show bodies of water are level, which proves the Earth is flat. Scientists say the claim is built on flawed logic. Scientists agree that laser beam tests in uncontrolled conditions, especially over water, are not a reliable means to prove the Earth's shape due to atmospheric refraction. Furthermore, scientists say the test outlined in the post’s diagram is not at a large enough scale to show the Earth’s curve.

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This article originally appeared on USA TODAY: Fact check: False claim laser tests prove Earth is flat