Late in the 19th century, the major tenets of classical physics were well established. The second postulate upon which Einstein based his theory of special relativity deals with the speed of light. Direct link: If you are reading this book online it will open in a window directly below. Here is a nice video by a high school student explaining this postulate. Another outcome of this postulate is the famous equation E = mc 2. We shall find that the definitions of relativistic momentum and energy must be altered to fit. The laws of physics include only those that satisfy this postulate. The laws of physics are the same and can be stated in their simplest form in all inertial frames of reference.Īs with many fundamental statements, there is more to this postulate than meets the eye. Newton’s first law, the law of inertia, holds exactly in such a frame. The simplest frames of reference are those that are not accelerated and are not rotating.

For example, a car’s motion is measured relative to its starting point or the road it is moving over, a projectile’s motion is measured relative to the surface it was launched from, and a planet’s orbit is measured relative to the star it is orbiting around. All velocities are measured relative to some frame of reference. The first postulate upon which Einstein based the theory of special relativity relates to reference frames.

With two deceptively simple postulates and a careful consideration of how measurements are made, he produced the theory of special relativity. If we are careful to choose the correct postulates, then our theory will follow and will be verified by experiment.Įinstein essentially did the theoretical aspect of this method for relativity. Physics is the same way with the exception that all parts must describe nature.

Each part is based on a set of postulates, and you can always connect the parts by applying those postulates. Trigonometry is reliable because of the certainty that one part always flows from another in a logical way. Each time you perform the same calculation, you know that the answer will be the same. Have you ever used the Pythagorean Theorem and gotten a wrong answer? Probably not, unless you made a mistake in either your algebra or your arithmetic. Special relativity resembles trigonometry in that both are reliable because they are based on postulates that flow one from another in a logical way.

Describe one way the speed of light can be changed.įigure 1.

Explain what an inertial frame of reference is.

State and explain both of Einstein’s postulates.

Einstein special theory of relativity