Special Relativity
From our studies of kinematics we know how the velocity of an object depends on the reference frame of the person observing it, this is called Galilean Relativity. For example, if you are running at five miles per hour and you throw a ball forward at three miles per hour, a person at rest observes the ball to be moving at eight miles per hour.
James Clerk Maxwell’s famous equations that govern electricity and magnetism claim that the speed of light is three times ten to the eight meters per second and is the same in every reference frame. The problem was if Galilean Relativity was correct, then the speed of light would have to be different in different reference frames. If the speed of light was the same in every reference frame, then Galilean Relativity had to be wrong. Experiments undertaken to measure differences in the speed of light for different reference frames did not measure any differences. Scientists did not understand how this could happen. To put this in more tangible terms, this would be like measuring your speed when you are swimming with the current or against it and finding that your speed is the same both ways. The velocity addition method did not work for light. No matter the speed at which the observer was moving, the observer saw light moving at the same speed as three times ten to the eight meters per second.
Einstein saw this contradiction and came up with a way to resolve it, but to resolve it, he had to think about the problem in a very different way. Einstein had to be willing to consider that perhaps time is not the same for all observers and that space is not absolute. This made Einstein’s relativity profoundly different and totally original.
View this video clip on the Theory of Special Relativity from Discovery EducationTM streaming to learn more about Einstein's relativity.
Special Relativity
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This video, E = mc2, from Discovery EducationTM streaming shows how the kinetic energy equation must also be modified for objects traveling close to the speed of light. Einstein’s equation shows that the kinetic energy of an object is equal to it’s total energy minus its rest energy. The subtraction of m c squared indicates that there is a rest energy associated with a mass. Since mass is multiplied by c squared, which is nine times ten to the sixteenth power, a small amount of mass has a fairly large rest energy.
Mass Energy Equivalence
Kinetic Energy = Total Energy - Rest Energy
Before Einstein, conservation of energy and conservation of mass were thought of as two separate laws of physics. After Einstein, the two laws have become unified into one law of conservation of mass and energy.
It is important to remember that the Lorentz Factor is only significant when speeds are close to the speed of light. Ordinary particles do not travel at large enough speeds for the Lorentz Factor to be larger than one.
Special Relativity Review
In this interactivity, apply your knowledge of special relativity. After selecting each answer, click submit. Click the player to get started.