## Hafele–Keating Experiment

The Hafele–Keating experiment was a test of time dilation. Four atomic clocks were flown around Earth, following and against Earth’s rotation, then compared to those remaining on the ground. The results were consistent with relativity.

The theory of relativity can be counterintuitive & difficult to understand. The Hafele–Keating experiment demonstrates a direct measurement of time dilation, a specific aspect of relativity, using a practical concept in our everyday lives.

## Free Fall

Objects fall down because of gravity. Flat-Earthers deny gravity & wrongly claim they fall down because of density. For a demonstration, we can ask any flat-Earthers the following simple physics problem:

“A rigid object is released at the height h above the ground, what is the object’s velocity right before it hits the ground?”

Flat-Earthers will not be able to solve this free-fall problem without using the gravitational acceleration g=9.8 m/s². Some will call it by a different name, showing that it is just a “branding” problem. Practically all the time, they will not involve density to solve it, if they can solve it at all.

## Quote Mined Einstein Statement

In 1922, Albert Einstein gave the speech “How I Created the Theory of Relativity” at Kyoto University. Flat-Earthers took a sentence fragment from his speech and quoted him out of context. It became a hoax as if Einstein believed a stationary Earth.

By reading the entire paragraph to understand the context, it is clear that Einstein did not mean Earth is stationary. He understands that Earth is in motion around the Sun.

## Theories of Gravity

Gravity is a natural phenomenon by which all things with mass or energy are brought toward one another. Today, gravity is explained by two theories: Newton’s law of universal gravitation and Einstein’s general relativity.

Flat-Earthers treat the existence of the two theories as a supposed conflict, and they use it as “evidence” of wrongdoing. In reality, these are two separate theories that explain the phenomenon of gravity. Einstein’s is more accurate yet more complex. In contrast, Newton’s is simpler but less accurate. Despite having been superseded, Newton’s gravity continues to be used as an excellent approximation of the effect of gravity in most applications.

## Allais Effect

The Allais effect is the alleged anomaly of pendulums or gravimeters, which is purportedly observed during a solar eclipse. The effect has not been able to be consistently measured and was most likely caused by experimental error.

Flat-Earthers use the Allais effect to dismiss the result of Foucault’s pendulum. In reality, the Allais effect is still not conclusive. Even if it is true, it only occurs during an eclipse and does not rule out that the pendulum’s precession is due to Earth’s rotation. Not to mention that there is plenty of other evidence that Earth is in motion.

## Sagnac Effect

The Sagnac effect occurs to a pair of light beams traveling in a circuit in the opposite direction. If the circuit itself is rotating, then the beams will complete the circuit in different duration. The effect was first demonstrated by Georges Sagnac in 1913.

After discovering this effect, Sagnac incorrectly used the results as proof of the aether’s existence, and today’s flat-Earthers use it as “evidence” of stationary Earth. In reality, Sagnac was not aware that the effect is consistent with Einstein’s theory of relativity. And if the effect really proved aether, then optical gyroscopes are impossible.

The Sagnac effect is used as the basis of optical gyroscopes, including fiber-optic gyroscopes and ring-laser gyroscopes. In an optical gyroscope, a pair of light beams are made to travel in a circuit in the opposite direction. Because light travels at a constant speed, irrespective of the transmitter’s speed, then if the gyroscope is rotated, a beam of light will complete the circuit sooner than the other. From the difference, we can measure the rotating motion of the gyroscope.

## “What is Gravity”

In a StarTalk episode, Neil deGrasse Tyson was given the question “what is gravity?” He quipped “we have no idea” before elaborating more. Flat-Earthers quoted it out of context to discredit Neil and gravity, completely disregarding what Neil explained afterward.

In reality, Neil did provide a very comprehensive answer to the question “what is gravity” but flat-Earthers only gave an emphasis to the “we have no idea” part of his answer, and skipped the rest of his answer.

## Airy’s Failure

Airy’s Failure was an experiment performed in 1871 in which Sir George Biddell Airy failed to confirm the aether theory by measuring stellar aberration. The phenomenon of stellar aberration, in itself, was the first direct evidence of Earth’s motion around the sun.

Flat-Earthers claim that the “failure” in “Airy’s Failure” is the failure of proving the motion of Earth. In reality, the experiment aimed to confirm the aether theory. The “failure” was in proving the aether theory. The experiment relied on the already accepted knowledge that Earth is in orbit around the sun.

## Fictitious Force

A fictitious force is a force that appears to act on every object when the frame of reference is accelerating. The term “fictitious” does not mean the force does not exist, but it is not an actual force that arises from an interaction between objects.

Flat-Earthers noticed gravity is considered a fictitious force and use the fact to “prove” gravity does not exist. In reality, we can feel a fictitious force. It is merely a term we use for forces that arise due to an accelerating frame of reference.

## Einstein’s Thought Experiments and Daydreaming

Albert Einstein used thought experiments as a tool for formulating his theories. His theories provided predictions that can be tested in observation and experiments. He did not perform the observation and experiments himself. Others did and confirmed his theories.

Some flat-Earthers dismiss Einstein’s theories —especially those about gravity— on the basis that they are founded on thought experiments and daydreaming. They are wrong. His theories have been repeatedly confirmed by various experiments and observation performed by many others, even to this day.

## Classical vs Modern Physics

Classical physics —like Newton’s law of motion, and Newton’s law of universal gravitation— has been superseded by Special Relativity, General Relativity, and Quantum Mechanics. But it doesn’t mean classical physics is wrong. Classical physics is still applicable when the parameters approach common, terrestrial values we use every day.

Pseudoscientists —including flat-Earthers— often misunderstood this and take the fact that classical physics is still being taught in schools as ‘proof’ of deception and the failure of modern science to explain the world. They are wrong.

## The Hypothesized Graviton and Flat-Earthers’ Zeal to Disprove Gravity

Some flat-Earthers mentioned the fact that graviton —the mediating particle of gravitation— is ‘not yet observed’. They took this fact as ‘proof’ that gravity does not exist.

Often they would triumphantly show us screenshots of the table of fundamental particles they took from Wikipedia and other sources. In the ‘gravitation’ column, they would point out its mediating particle has not been observed yet. And they would conclude that scientists are aware gravity does not exist, and we all have been lied to.

Obviously, they are wrong on all accounts.

## The Aether Experiments are Never Evidence of Flat and Stationary Earth

Aether was the hypothetical material that fills the region of space. It was assumed to be the medium that allows light and gravity to propagate in space. Throughout the late 1800s and early 1900s, some experiments were carried out to prove if the aether exists.

Flat-Earthers (and geocentrists alike) often use the results of these experiments to support their case that the Earth is stationary. But they are wrong. These experiments were conducted to prove if the Aether theory, or if one of its competing hypotheses —like the Special Relativity— better explains reality.