# Special Relativity

## If the Speed of Light were 100 miles/hour:

Everyone has heard that Einstein's Theory of Special Relativity makes strange and bizarre predictions about the way the Universe works, however, it is important to realize that if the speed of light were 100 miles/hour, relativity would be "common sense." Everyone (well, ok - almost everyone...) would know that every time you drive to Cincinnati and back you have to reset your watch to Batesville time when you return. Everyone would know that even though the map says that the trip from Batesville to the mall in Cincinnati and back is 100 miles, the odometer in the car registers only 88 miles (or so) for the trip. Everyone would know that if you stood on the overpass and watched the cars go down the interstate, the cars would appear shorter than normal. You might not know why these things happen, but the fact that they do happen would be a part of your everyday experience.

Everyone with scientific curiosity has heard of Einstein's Special Theory of Relativity and its bizarre and outlandish scenarios of slowing clocks, shrinking space ships, and twins that age at different rates. These things defy our intuition and "common sense." Surely, Special Relativity must be an extremely difficult subject to master! Well, this is not the case.

If you think about it, our intuition (or "common sense," if you prefer) is formed from our experience in the world. Our ideas about space, time, and motion are formed as we move about through space and time in our daily lives. Most people naturally expect that the notions about space, time, and motion that we acquire in this way are true, not just here and now, for us, but are true at all times, everywhere.

Special Relativity deals with objects that move with a speed that is at least a significant fraction of the speed of light, which is about 186,000 miles/second (3 x 108 m/s) - that's miles per second - not per hour, by the way! Nothing in our experience (except light, which seems to us to move instantaneously) moves anywhere near that fast! A commercial jet airliner flies at about 600 miles/hour (970 km/hr), which is about 0.00004% of the speed of light. A few lucky humans have flown in a Mach-3 jet fighter goes about 2200 miles/hour (3500 km/hr) - which is only about 0.0003% of the speed of light. About the fastest that any human has ever traveled is around 25,000 miles/hour (40,000 km/hr - during the Apollo missions to the Moon), which is only about 0.4% of the speed of light.

A graph of the relativistic time-dilation equation. The effects of Special Relativity lie well outside our common experience!

last update March 30, 2009 by JL Stanbrough