How Special Relativity Works


In the headlight example, the distance that you are using in your measurement is not the same as the distance that the light is using. This is a very difficult concept to grasp, but it is true. When an object (with mass) is in motion, its measured length shrinks in the direction of its motion. If the object reaches the speed of light, its measured length shrinks to nothing. Only a person that is in a different frame of reference from the object would be able to detect the shrinking - as far as the object is concerned, in its frame of reference, its size remains the same. This phenomenon is referred to as "length contraction". It means, for example, that as your car approaches the speed of light, the length of the car measured by a stationary observer would be smaller than if the car was measured as it stood still. Look at Fig 2 and Fig 3 below.

In Fig 2 the car is stopped at the stop sign. In Fig 3 the same car is moving past you. You will readily notice that the moving car in the figure is shorter than the stopped car. Note that the car would only be shorter in the direction it is traveling, its height and width are not affected - only its length. Length contraction only affects the length in the direction you are traveling. Imagine that you are running super fast toward an open door. From your perspective, the distance from the front of the door opening to the back of the door opening would decrease. From the doors perspective the width of your body - the distance from your chest to your back - would decrease.


Scientists feel that they have actually proved this notion of length contraction. Therefore, in reality, all objects are perceived to shorten in the direction they are traveling, if they are viewed by someone who is not in motion with them. If you are in a moving car and measure the length of the armrest, you will never notice the change regardless of how fast you are going, because your tape measure would also be shortened from the motion.

In our lives we do not ever perceive length contraction because we move at speeds that are very small with respect to the speed of light. The change is too small for us to notice. Remember the speed of light is 669,600,000 miles/hour or 186,400 miles/sec, so it is easy to see why our everyday speeds are negligible.

We'll look at length contraction in the next section.