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# 15: Special Relativity

The phrase “special” relativity deals with the transformations between reference frames that are moving with respect to each other at constant relative velocities. Reference frames that are accelerating or rotating or moving in any manner other than at constant speed in a straight line are included as part of general relativity and are not considered in this chapter.

• 15.1: Introduction
• 15.2: Preparation
Detailed discussion of the exact definitions of the units of time, distance and speed is part of the subject of metrology. That is an important and interesting subject, but it is only marginally relevant to the topic of relativity, and consequently, having quoted the exact value of the speed of light, we leave further discussion of metrology here.
• 15.3: Preparation
• 15.4: Speed is Relative - The Fundamental Postulate of Special Relativity
• 15.5: The Lorentz Transformations
It is impossible to determine the speed of motion of a uniformly-moving reference frame by any means whatever, whether by a mechanical or electrical or indeed any experiment performed entirely or partially within that frame, or even by reference to another frame
• 15.6: But This Defies Common Sense
• 15.7: The Lorentz Transformation as a Rotation
• 15.8: Timelike and Spacelike 4-Vectors
A “light-year” is a unit of distance used when describing astronomical distances to the layperson, and it is also useful in describing some aspects of relativity theory. It is the distance travelled by light in a year.
• 15.9: The FitzGerald-Lorentz Contraction
• 15.10: Time Dilation
The interval s between two events is clearly independent of the orientation any reference frames, and is the same when referred to two reference frames that may be inclined to each other. But the components of the vector joining two events, or their projections on to the time axis or a space axis are not at all expected to be equal.
During the late 1950s and early 1960s there was great controversy over a problem known as the “Twins Paradox”. The controversy was not confined to within scientific circles, but was argued, by scientists and others, in the newspapers, magazines and many serious journals. . The paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more.
• 15.12: A, B and C
A , B and C were three characters in the Canadian humorist Stephen Leacock’s essay on The Human Element in Mathematics. “ A , B and C are employed to dig a ditch. A can dig as much in one hour as B can dig in two...”
• 15.13: Simultaneity
If the time interval referred to one reference frame can be different when referred to another reference frame (and since time interval is merely one component of a four-vector, the magnitude of the component surely depends on the orientation in four space of the four axes) this raises the possibility that there might be a time interval of zero relative to one frame (i.e. two events are simultaneous) but are not simultaneous relative to another.
• 15.14: Order of Events, Causality and the Transmission of Information
Maybe it is even possible that if one event precedes another in one reference frame, in another reference frame the other precedes the one. In other words, the order of occurrence of events may be different in two frames. This indeed can be the case, and Minkowski diagrams can help us to see why and in what circumstances.
• 15.15: Derivatives
We’ll pause here and establish a few derivatives just for reference and in case we need them later.
• 15.17: Aberration of Light
• 15.18: Doppler Effect
It is well known that the formula for the Doppler effect in sound is different according to whether it is the source or the observer that is in motion. So I shall try to explain why, physically, there is a difference. Then, when you have thoroughly understood that observer in motion is an entirely different situation from source in motion, and the formulas must be different, we shall look at the Doppler effect in light, and we’ll return to square one when we find that the formulas for source in
• 15.19: The Transverse and Oblique Doppler Effects
• 15.20: Acceleration
• 15.21: Mass
• 15.22: Momentum
• 15.23: Some Mathematical Results
• 15.24: Kinetic Energy
• 15.25: Addition of Kinetic Energies
• 15.26: Energy and Mass
• 15.27: Energy and Momentum
• 15.28: Units
• 15.29: Force
Force is defined as rate of change of momentum, and we wish to find the transformation between forces referred to frames in uniform relative motion such that this relation holds on all such frames.
• 15.2: The Speed of Light
• 15.30: Electromagnetism

Thumbnail: Light cone. Image used with permission (Public Domain; Sakurambo).