Physics Lesson 18.1.3 - The Postulates

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Welcome to our Physics lesson on The Postulates, this is the third lesson of our suite of physics lessons covering the topic of Relativity. Galilean Transformations. Einstein's Postulates and Newton's Laws, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

The Postulates

Regardless of the work done by Galileo Galilei and other scientists in the field of relativity, it is Einstein the person who is credited for putting the concept of relativity on scientific basis. Initially, Einstein formulated the Special Theory of Relativity, which is valid only for inertial frames of reference (in which the Newton's Laws of Motion can be applied, as explained earlier) and later, he generalized his findings by formulating the General Theory of Relativity, which is also valid for non-inertial frames that undergo gravitational acceleration (this is a complex theory which goes beyond the scope of this section).

Sometimes, findings and discoveries in science are difficult to be made in a straightforward way, through direct measurements and experiments. Therefore, indirect methods based on assumptions are often used in such cases to prove a certain theory. There are some basic concepts that are taken as true without proof and then, the new theory is built based on them. Such concepts are known as "postulates". In other words, a postulate is a thing suggested or assumed as true as the basis for reasoning, discussion, or belief.

Einstein based his Special Theory of Relativity upon two postulated:

1. The laws of physics are the same and can be expressed in their simplest form in all inertial frames of reference (we discussed this point earlier in this tutorial). This is known as the relativity postulate. The laws of physics mentioned in this postulate include only those that satisfy this postulate.
2. The speed of light in vacuum has the same value c in all directions and in all inertial reference frames. This is known as the speed of light postulate.

In other words, the second postulate implies that in universe, there is an ultimate speed c, which is the same in all directions and in all inertial frames of reference. Light is the only known thing that travels at this ultimate speed. No entity that carries energy or information can exceed this limit. Moreover, no particle that has mass can actually reach the ultimate speed c, no matter how much or for how long that particle is accelerated.

Precise measurements using modern devices have given the value c = 299,792,458 km/s for the speed of light in vacuum. We often round this value to 300,000,000 km/s or 3 × 10^-8 m/s. However, in this chapter (section) we will use the exact value to describe the speed of light in vacuum.

Both postulates have been tested many times but they have resulted always true. The existence of a limit to the speed of accelerated electrons was shown in an experiment carried out in 1964 by W. Bertozzi, who accelerated electrons to various measured speeds and through an independent method, he measured their kinetic energies. He found that as the force on a very fast electron is increased, the electron's measured kinetic energy increases toward very large values but its speed does not increase appreciably. The graph showing the relationship between the speeds of electrons and their kinetic energy is shown below.

You have reached the end of Physics lesson 18.1.3 The Postulates. There are 5 lessons in this physics tutorial covering Relativity. Galilean Transformations. Einstein's Postulates and Newton's Laws, you can access all the lessons from this tutorial below.

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