The special theory of relativity is mainly founded on two assumptions (postulates) that Einstein made about the nature of the universe. The first can be stated like this:

You may be thinking to yourself, "Huh?! What is an inertial frame of reference?" As you may know already, Albert Einstein had a knack for using seemingly complex phrases to describe concepts which are surprisingly easy to understand. In this case, all he is saying is that whether an object is standing still or moving in a straight line at a constant speed, all of the physical laws of nature governing that object will be the same. For example, if you are standing out on a pitcher's mound throwing a ball up in the air, what will happen? Why, gravity will pull it back down toward the earth and you will catch it your glove (as long as you know how to catch a baseball, that is). Now, let's say you are sitting in the first-class section on a 747 jumbo jet. You have nothing better to do so you continue to toss that ball up in the air and wait for gravity to pull it back down so you can catch it. Since you are paying so much attention to this little activity, you have probably forgotten to take into account that the plane you are sitting in is flying at a speed of several hundred miles per hour. However, you don't have to worry about the ball flying backwards and getting lodged in the oral cavity of the guy sitting behind you, do you? No, because from your point of view and that of every other object inside the plane, you are sitting still. So, if you are not moving, how can the ball fly backwards if you throw it straight up into the air above you? The answer -- it can't . . . and it won't, not as long as no other forces interfere with gravity and the ball's upward motion. The reason is simple. All of the objects on the plane which are not moving around horizontally within the plane itself are moving at the same speed as the plane. Therefore, they are all in the same frame of reference and all of the laws of physics will be the same for each of them. You may question, however, the validity of this example. How can you compare a person sitting in a moving plane to a person standing still on Earth? Well, here's something you may have overlooked: the Earth is just like the plane, only on a much larger scale. It too is a moving object, much faster than the plane as well. If you are standing at the equator, you are actually moving at a speed of over 1600 km per hour as the Earth rotates on its axis, not to mention that you are also moving through space at 29 km per second as the Earth revolves around the sun. Pretty fast, isn't it? Yet, you don't stop to contemplate this fact with every step that you take, because you don't need to do so when everything else around you is moving in the same frame of reference.

Let's now take this one step further. Suppose two people - we'll call them Bob and Tom for simplicity's sake - decide to do a little experimenting. They take their trusty baseball and head to the nearest train station. Bob stands on a flatbed train car and Tom looks on as an observer. According to what we have just shown above in the previous example, if Bob throws the ball forward at a speed of 50 ft/s, the ball will be 50 feet in front of him after one second has gone by . . . as long as the train beneath him is standing still or moving in a straight line at a constant speed. Now, if the train was moving at a speed of 60 ft/s, it would be 60 feet in front of Tom one second after it passes him. Assuming that Bob doesn't become train-sick, if he throws the ball at 50 ft/s the exact moment he passes Tom, the ball will be 50 ft in front of him and 110 feet in front of Tom one second after the ball was thrown. How is that possible, you ask? Well, since the train with Bob on it will be 60 feet in front of Tom after a second has gone by and the ball will be another 50 feet in front of Bob at the same time, you just add those two distances to arrive at the result of 110 feet between Tom and the ball. In other words, from Tom's point of view the ball is traveling at 110 ft/s.

In summary:

• The velocity of the ball relative to Bob is 50 ft/s.
• The velocity of Bob relative to Tom is 60 ft/s.
• The velocity of the ball relative to Tom is 110 ft/s.

This situation is an example of what is sometimes referred to as "Galilean relativity". The logic behind it is simple and reasonable enough that is hard to imagine how it could possibly be wrong. However, Einstein found a slight flaw concerning light.

• The Second Postulate of The Special Theory
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