Unit 8: Special Relativity
The physical descriptions we have studied to this point were based on a notion of absolute space and time. A model for this point of view was that space is filled everywhere by a continuous medium called the ether. Light and other forms of electromagnetic radiation were waves in this ether, analogous to sound waves in air. All other phenomena were to be understood as various manifestations of Maxwell's electromagnetism, which was originally based on a mechanical model of ether. It seemed reasonable that the 19th Century 'theory of everything' could be tied down by measuring the 'elastic' properties of the ether.
Toward the end of the 1800s, however, this model became associated with more and more hastily patched cracks. The detailed history of the gradual realization that ether models were not quite right is complex and technical. However, there is one rather clear indication of trouble. In 1887, Albert Michelson and Edmund Morley of the Case Institute (now Case Western University) performed an experiment using an optical interferometer in which they compared the speed of light in two beams traveling at right angles to each other. If the speed of light relative to the ether was always the same, the measured speed of light would be larger or smaller depending on the direction the experiment was traveling through the ether. The motion of the Michelson-Morley experiment was provided by the rotation of the Earth on its axis and the orbital motion of the Earth around the Sun, as well as the absolute velocity (if any) of the Sun relative to the ether.
They expected to see both diurnal changes and yearly changes in the relative velocities of light in the two paths. True, the changes expected by classical ether theory were small (on the order of 0.01% of the velocity of light), but the Michelson-Morley interferometer was able to detect velocity changes about 6-7 times smaller. To the surprise of all, there were no changes whatever observed. This experiment was widely repeated, using constantly improving equipment - a new version of the experiment carried out in 2002 established that the velocity of light is constant to better than 1 part in 1015 - one of the most precise physical measurements ever accomplished.
The explanation of the Michelson-Morley null result was length contraction, as developed by Hendrik Lorentz and George Francis FitzGerald. Length contraction explained the Michelson-Morley result, the idea being that matter is held together by electromagnetic forces (true), and so the actual size of objects will change with motion through the ether (false). In the end, it was Albert Einstein's formulation of the theory of Special Relativity that gave us a consistent explanation of all such phenomena. His primary postulate was to accept that the speed of light and the laws of physics are constant in all reference frames – including reference frames that are in motion. Oddly, despite the fact that Einstein's theory completely explained the Michelson-Morley result, he took no motivation for his theory from that experiment.
Completing this unit should take you approximately 10 hours.
Upon successful completion of this unit, you will be able to:
- identify the postulates upon which the Special Theory of Relativity is based;
- solve problems involving time dilation and length contraction;
- explain the principle of equivalence as introduced in the General Theory of Relativity;
- compare and contrast the special and general theories of relativity;
- list the most significant consequences of Einstein's special and general theories of relativity; and
- explain the results of the Michelson-Morley experiment using the Special Theory of Relativity.
8.1: Introduction to Relativity
Watch this lecture.
This article introduces special relativity. You do not have to memorize every single fact. Strive to make sure that it makes sense. When you finish, briefly summarize the main points.
Watch this video to review the essential physics of relativity.
This applet demonstrates the expectation of Michelson and Morley that if the speed of light was constant through the ether (the speed of the swimmers is constant through the water) and the detection device is moving through the ether (the platform is moving through the water), then the travel time for the perpendicular light beams (the travel time for the two swimmers) would be different. Why does special relativity address the fact that the travel times in the Michelson-Morley were actually the same?
Watch this video, which discusses some of Einstein's work and interests outside of physics.
Watch this video and make sure that you understand the logic of its two conclusions.
This demonstration illustrates what happens when you add a series of velocities that, in Newtonian mechanics, would reach speeds faster than the speed of light. The demonstration illustrates why relative speeds can add up to nearly the speed of light but cannot exceed that speed.
8.2: Simultaneity, Time Dilation, and Length Contraction
Watch this lecture. Check your understanding by attempting problems 1, 3, and 5–7 from this problem set. Check your solutions here.
This page shows how time dilation and length contraction can explain certain apparent paradoxes in relativity. Be sure that you understand the logic used by Jack and by Jill to account for the observations.
This page shows two worked examples: a train moving through a tunnel and the twin paradox. The last example uses the Doppler effect to show a clever way to explain the difference in ages.
Explore this applet, which illustrates the paradox of a muon originating in the upper atmosphere striking the surface of the earth.
This demonstration illustrates how a set of events appears in two inertial frames moving with respect to one another.
Read "The Michelson-Morley Experiment" through "History of Special Relativity". These give a solid review of relativity.
8.3: The General Theory of Relativity
Read this tutorial.
Unit 8 Assessment
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.