Special and General Principle of Relativity
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The Special and General Principles of Relativity
Summary:
The foundation of previous discussions has been the special principle of relativity, which highlights the relativity of all uniform motion. Let’s delve into its meaning.
Keywords: theory of relativity, relativity, Einstein, general relativity, special theory of relativity
Article:
By Albert Einstein
The foundation of our discussion has been the special principle of relativity, emphasizing the relativity of all uniform motion. Let’s analyze its meaning.
Motion, in essence, should always be viewed as relative. Using the familiar example of a train and an embankment, we can understand motion in two valid ways:
1. The train moves relative to the embankment.
2. The embankment moves relative to the train.
Here, the train or the embankment acts as the reference point. For describing motion, it doesn’t matter which reference we choose. This basic fact should not be confused with the broader "principle of relativity," which forms the core of our studies.
Our principle asserts that if we derive the laws of nature using:
1. The embankment as the reference point.
2. The train as the reference point.
The laws, such as mechanics or light propagation, remain identical in both cases. This means neither reference point is special or unique. This idea is not self-evident and requires experiential validation for accuracy.
So far, we haven’t claimed the equivalence of all reference bodies for formulating natural laws. Initially, we assumed a reference body (K) where Galilean laws apply: A free particle moves in a straight line uniformly. The laws of nature should be simplest with respect to K. Bodies like K1 moving uniformly relative to K are considered Galilean reference bodies. We assumed the principle of relativity applies only to these bodies, not to those moving differently. This is called the special principle of relativity.
In contrast, the "general principle of relativity" suggests all reference bodies (K, K1, etc.) are equivalent for describing natural phenomena. However, this definition will be refined later.
While the special principle is justified, seeking a general principle tempts any curious mind. Yet, a thought experiment suggests challenges to this pursuit. Imagine a train moving uniformly; its passenger feels no motion, so they might consider the train at rest and the embankment moving. The special principle supports this view.
However, if the train slows abruptly, the passenger feels a jolt. The mechanical behavior differs from uniform motion, suggesting the same mechanical laws can’t apply to non-uniform motion as they do to uniform motion. The Galilean law doesn’t apply to the non-uniform train, leading to the notion that non-uniform motion holds a sort of absolute reality, contradicting the general principle of relativity. But this idea is soon challenged.
To learn more, visit [Effortless Physics Lessons](http://www.EffortlessPhysicsLessons.com/relativity/).
Stephan Bourget, Physicist
Effortless Physics Lessons
[Effortless Physics Lessons](http://www.EffortlessPhysicsLessons.com)
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