The Postulates of Special RelativityRocketObserverFirecrackerFigure 1Special relativity is a theory that predicts how events aremeasured with respect to various observers who can be in motionwith respect to the event. An event is just a physical happening, suchas the explosion of a firecracker or the ticking of a clock or a train'spassing a certain point. Events happen at a particular place and time,as measured by the people who observe them. Those people can bemoving with respect to the event, or they can be stationary withrespect to the event. For example, in Figure 1, a firecracker goes off,causing a flash of light. That's an event, and two observers watch it.One observer is stationary with respect to the firecracker, and the other observer is moving in a rocket - in astraight line at a constant speed with respect to the firecracker. Each observer carries his or her owncoordinate system, as the figure shows, and the observers each measure the event - its location and time -with respect to their individual coordinate systems and clocks. So each observer gets his or her own x, y, and zcoordinates for the event and his or her own time. Special relativity is all about relating the measurements thatthe two observers make. The coordinate systems and clocks that the observers carry with them are called theirreference frames, or their frames of reference. So why is it "special" relativity? Each observer's referenceframe is a special type of reference frame an inertial reference frame. An inertial reference frame is justone that's not accelerating. When Einstein came up with the theory of special relativity, he started with twopostulates, or assumptions, that the theory rests on. The postulates are: The relativity postulate which statesthat the laws of physics are the same in all inertial reference frames, The speed of light postulate whichstates that the speed of light in a vacuum (c) always has the same value in any inertial reference frame, nomatter how fast the observer and the light source are moving with respect to each other.The relativity postulateThe speed of light postulateoneThe relativity postulate tells you that oneinertial reference frame is as good asanother, and you can't tell them apartwith experiment. For example, if you're ininertial reference frame andsomeone else is in another, you can't tellthose frames apart through tests. So ifyou're in the rocket back in Figure 1, it'sjust as valid to say that the Earth ismoving toward you as it is to stand on theEarth and say that rocket is movingtoward you. That's what relativity is allabout.The speed of light (c = 3x108 m/s) is a constant, independentof the relative motion of the source. If you're in one car going5 meters per second and are approached by another going10 meters per second, you're moving at 15 meters persecond with respect to the other car. So if you're standing bythe side of a road and a car approaches you at 10 metersper second with its headlights on, wouldn't you measure thespeed of light from the headlights as c + 10 meters persecond? That's not how it works with the speed of light,however, or with speeds approaching the speed of light. Asthe postulate says, the speed of light from the headlights ofthe car coming toward you would be c, not c + 10 meters persecond.ACTIVITY 1: FACT or BLUF. Write FACT if the statement is correct and BLUFF if it is incorrect.1. Earth can be an inertial frame of reference.2. In special relativity, if a body is moving without an external force, then it'll continue to move in astraight line at constant speed - this is the inertial motion.3. The second postulate of special relativity states that the laws of physics are the same in all inertialreference frames.4. The speed of light in empty space has the same value for all observers regardless of theirvelocity.5. The first postulate argues that a spaceship, for example, cannot measure its speed relative toempty space, but only relative to other objects.

ACTIVITY 1: FACT or BLUF. Write FACT if the statement is correct and BLUFF if it is incorrect. 1. Earth can be an inertial frame of reference. 2. In special relativity, if a body is moving without an external force, then it'll continue to move in a straight line at constant speed - this is the inertial motion. 3. The second postulate of special relativity states that the laws of physics are the same in all inertial reference frames. 4. The speed of light in empty space has the same value for all observers regardless of their velocity. 5. The first postulate argues that a spaceship, for example, cannot measure its speed relative to empty space, but only relative to other objects.

ACTIVITY 1: FACT or BLUF. Write FACT if the statement is correct and BLUFF if it is incorrect. 1. Earth can be an inertial frame of reference. 2. In special relativity, if a body is moving without an external force, then it'll continue to move in a straight line at constant speed - this is the inertial motion. 3. The second postulate of special relativity states that the laws of physics are the same in all inertial reference frames. 4. The speed of light in empty space has the same value for all observers regardless of their velocity. 5. The first postulate argues that a spaceship, for example, cannot measure its speed relative to empty space, but only relative to other objects.