The Second Postulate of the Special Theory of Relativity

The second postulate of the special theory of relativity is very interesting and gives some unexpected results because it plays with frames of references. The postulate is: The speed of light is measured as constant in all frames of reference(If you are unsure as to what frames of references are, go back to a few posts, and I have explained it). Basically it is very similar to the first one, just that it comes out in a different form. If the laws of physics apply equally to all frames of reference, then light (electromagnetic radiation) must travel at the same speed regardless of the frame. This is required for the laws of electrodynamics to apply equally for all frames.

Think about it...it is a very strange and odd one. Here is one fact you can derive from the postulate: Regardless of whether you are flying in an airplane or sitting on the couch, the speed of light would measure the same to you in both situations. The reason that is unexpected is because most physical objects that we deal with in the world add their speeds together. Consider this, car with a driver and passenger is approaching you at a speed of 50 miles/hour. The passenger pulls out a slingshot and shoots a rock 20 miles/hour at you. If you measured the speed of the rock, you would expect it to be traveling at 70 miles/hour (the speed of the car plus the speed of the rock from the slingshot). That is, in fact, what happens. If the driver measured the speed of the rock, he would only measure 20 miles/hour, since he is already moving at 50 miles/hour with the car. Now if that same car is approaching you at 50 miles/hour and the driver turns on the headlights, something different happens? Since the speed of light is known to be 186,000 miles/second, and if we convert that into an hour we get 669,600,000 miles/hour. Coommon sense tells us that the car's speed plus the headlight beam speed gives a total of 669,600,050 miles/hour (50 miles/hour + 669,600,000 miles/hour). The actual speed would measure 669,600,000 miles/hour, exactly the speed of light. To understand why this happens, we must look at our notion of speed.

Speed as we all know is the distance traveled in a given amount of time. For example, if you travel 60 miles in one hour, your speed is 60 miles per hour. We can easily change our speed by pressing or releasing the accelerator in the car. In order for the speed of light to be constant, even if the light is "launched" from a moving object, only two things can be happening. Either something about our notion of distance and/or something about our notion of time must be out of line. If we look at what really happens it turns out, that both are lot of line. Remember, speed is distance divided by time. Think about it, and you will know what I am saying.

That is enough to think about for the time being. I am going to introducing this very gently, just to make sure our gray matter does not get into a twist!!