The principle of relativity states that the laws of physics work the same regardless of an observer’s position or speed. Galileo’s ship experiment illustrated this in the 1600s. Newton applied the principle to planetary bodies and motion, leading to his own theories. Einstein’s application of the principle to light led to his groundbreaking theories of relativity. The principle applies to all laws of physics, and the speed of light is the only universal constant.
The principle of relativity holds that the laws of physics will work the same under similar conditions, regardless of an observer’s position or speed. The principle of relativity should not be confused with theories of general or special relativity, although such theories use the principle as a basis. These theories were developed in the 20th century; the principle of relativity had been 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 theories of relativity.
For centuries, science has been constrained by the Ptolemaic model of the universe, in which it was believed that all stars and planetary bodies 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, it would create effects that humans could observe. For example, an object that fell off a building would have landed somewhere west of the building, because the planet had rotated to the east during the object’s fall.
Galileo, writing in 1632, refuted this thesis with the eloquent thought experiment “Galileo’s Ship”. In this example, people traveling through calm seas in a fast vessel would be unable to tell whether the vessel was moving or stationary if they were enclosed in a windowless cabin. Any object in the cabin, including flying insects, fish in a bowl, and a thrown ball, would move the same regardless of 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 don’t get swept up in the force of the planet’s rotation.
Sir Isaac Newton, working later in the same century, applied the principle of relativity to other planetary bodies and to the mechanics of motion in general. This helped him form his own theories, which have become the basis for much of modern science. Over the centuries, the progression of science has generally been far from the comforting idea that there is a stable, unchanging point of reference by which all things can be measured. Science, on the other hand, has repeatedly demonstrated that there is no “fixed” point of reference; everything has to be measured in relation to something else.
Even in the early 20th century, many scientists believed that space was filled with a stable medium called “ether.” Einstein and other scientists, however, realized that the principle of relativity applied to all laws of physics, leading to famous theories of relativity. The essence of these theories is that matter, energy, time and even space itself are not constant but can change under the right conditions. The speed of light, Einstein realized, was the only universal constant that could be used to measure and confirm these theories. Galileo’s classic ship model has sometimes been applied to spaceships to illustrate the principle that the motion of an object in space can only be measured in relation to other objects.
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