Introduction: Connecting Your Learning
Motion is always measured with respect to a frame of reference. Inertial frames of reference are frames of reference that are not accelerating. The idea of relative motion is useful in comparing airspeed with ground speed for an airplane or the speed of a boat relative to the water with the speed of the boat relative to the shoreline. Therefore, it was natural to assume light also had differing speeds depending on the frame of reference. In the late 1800s, physicists assumed light was a wave and needed a medium in which to travel. This assumed medium was referred to as ether and was attached to space to provide the ultimate frame of reference upon which everything else could be measured. An apparatus was constructed (an interferometer) that was designed to measure the time difference between the speed of light traveling with the ether compared to light traveling across the ether .
The purpose of this laboratory activity is to gain familiarity with the terms and phrases used in relativistic physics and to compare relative motion scenarios at everyday speeds with motion near the speed of light (c).
Resources and Assignments
Focusing Your Learning
By the end of this lesson, you should be able to:
1. Deﬁne and use the terms and phrases associated with relativity such as: frames of reference, simultaneity, space time, time dilation, and length contraction.
2. Discuss the Twin Paradox as it relates to special relativity.
3. Discuss how the Correspondence Principle relates to special relativity.
In the experimental apparatus below, light is projected toward the half-silvered mirror, and the light beams are split. Part of the light goes straight ahead in the direction of Mirror 2. The other part of the light is reﬂected by the half-silvered mirror and travels to Mirror 1. The light then returns to the observer. The observer sees the recomposed light as an interference pattern. Fringes in the interference pattern would suggest there is a difference between the speeds of the two paths of light due to the relative motion caused by the ether.
Skipping a few pages of algebra and several tedious calculations, it can be shown that fringes in the interference pattern could have been easily observed if there was a velocity difference in the light beams. However, no fringes were observed, and the conclusion was reached that the speed of light is the same for all observers, regardless of the motion of the observer or the motion of the source.
This became known as one of the postulates of the Special Theory of Relativity. The ﬁrst postulate states that the laws of physics are the same in all inertial (non-accelerating) frames of reference.
This result prompted one of the greatest physics mysteries in the early 1900s.
Most "experiments" dealing with relativity are "thought experiments" due to the high speeds involved and the cost of the equipment necessary to collect such precise measurements. This lab activity will allow you to put your creative thought into the mysteries of special relativity. Make sure to use your own words, and do not simply copy from a book or Web site.
What follows is a series of "thought experiments." Based on what you have learned from the text and other reading, offer an explanation for the scenarios given.
1. Person A is standing in