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In Physics, what is the Principle of Relativity?

By Alan Rankin
Updated May 21, 2024
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The principle of relativity holds that the laws of physics will function the same way in similar conditions, regardless of an observer’s location or speed. The principle of relativity should not be confused with the theories of general or special relativity, although those theories use the principle as their basis. Those theories were developed in the 20th century; the principle of relativity was understood much earlier and illustrated by Galileo in a famous example known as “Galileo’s ship.” Einstein’s application of the principle of relativity to light led to his groundbreaking relativity theories.

For centuries, science was constrained by the Ptolemaic model of the universe, in which all stars and planetary bodies were believed to orbit the Earth. Copernicus realized in the 1500s that the sun was a more likely central body, but this belief was opposed by religious and scientific authorities. They argued that if the Earth were in motion, this would create effects that humans could observe. For example, an object dropped from a building would land somewhere west of the building, because the planet had rotated eastward during the time the object was falling.

Galileo, writing in 1632, refuted this argument with the eloquent thought-experiment “Galileo’s ship.” In this example, people traveling smooth seas on a fast ship would not be able to tell if the ship was in motion or at rest if they were enclosed in a windowless cabin. Any objects in the cabin, including flying insects, fish in a bowl, and a thrown ball, would move the same no matter what the ship’s external motion. In other words, their motion would be relative to their environment, not to external factors. The same principle applies to the Earth, which is why people are not knocked over by the force of the planet’s rotation.

Sir Isaac Newton, working later that same century, applied the principle of relativity to other planetary bodies and the mechanics of motion in general. This helped him to form his own theories, which became the basis for much of modern science. Over the centuries, the progression of science has generally been away from the comforting idea that there is some stable, unchanging reference point from which all things can be measured. Instead, science has repeatedly proven that there is no “fixed” reference point; everything must be measured as relative to something else.

Even in the early 20th century, many scientists believed space was filled with a stable medium called “aether.” Einstein and other scientists, however, realized that the principle of relativity applied to all the laws of physics, leading to famous relativity theories. The essence of these theories is that matter, energy, time and even space itself are not constants but can change in the right conditions. The speed of light, Einstein realized, was the only universal constant that could be used to measure and confirm these theories. The classical model of Galileo’s ship has sometimes been applied to spaceships to illustrate the principle, in which the motion of an object in space can only be measured in relation to other objects.

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