# Understanding Temporal Dilation: The Time Vs. Velocity Equation - Part 4

Updated on September 25, 2009

If it had been possible for Suzie to look far into space into Sarah's fast-moving spaceship, it would appear as if Sarah were living her life in slow motion, at half the usual speed. Likewise, if it was possible for Sarah to look on towards the people on earth as she was zipping around the galaxy, everything would appear to be moving at a high speed frenzy, or twice the normal speed.

But to both Suzie and Sarah, it would seem as if their time was the "right" time. Not only would a watch or clock run slower in the moving ship, but Sarah's biological clock would also run slow. So while to Suzie it would seem as though Sarah had found the magical fountain of youth, they would have just as much "time" to live. If Suzie could read 100 books in her life span, Sarah could also read 100 books in her lifespan. It is just that to a stationary observer, Sarah would appear to be doing her reading in slow motion since Sarah's whole world is actually going in slow motion due to the high speeds.

These remarkable consequences are all based on the fact that the speed of light is constant. The closer that a person gets to the speed of light, the slower that time goes for them, and the faster the world around them seems to be moving. At 99.5 percent the speed of light, the world around Sarah would be moving nearly ten times faster than her own world. If she were to travel at this speed for twenty years, then return home, 200 years would have passed on earth. In a sense, she would have traveled into the future.

Following the rules of relativity further, if something could move faster than the speed of light, then it would actually go backwards in time. The faster an object moves the slower that the object experiences time, until at the speed of light time stands still. After the speed of light, time moves backward. This brings to mind the famous physics limerick:

There was a young lady named Bright,
Whose speed was far faster than light.
She set out one day,
In a relative way,
And returned home the previous night.

Although the limerick describes what Einstein predicted would happen if the speed of light could be surpassed, it is impossible for an object with any mass to actually go the speed of light, let alone exceed it. One hundred eighty-six thousand miles per second is enshrined in physics lore as the universal speed limit. This is due to the fact that the closer an object gets to the speed of light, the more energy it takes to further accelerate the object. It is always possible to get closer to the speed of light, but it would take an infinite energy source to actually accelerate to that speed. Since light has no mass, it is the lone thing in the universe that can travel at the speed of light.

Continued In: Understanding Temporal Dilation: The Time Vs. Velocity Equation - Part 5

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