The Famous Twin Paradox

Maybe you have heard about the fantastic story of twin brothers who part ways on their twentieth birthday.  Let's call them Billy and Timmy.  Billy stays on Earth, while Timmy travels at 99.9% the speed of light on his fancy space ship.  Timmy returns to Earth on Billy's fortieth birthday after having explored the universe.  Timmy is visibly younger than Billy.  In fact, he has only been travelling for 326 days as far as he is concerned.  He is yet to turn 21 - he is not legal to consume alcohol in the United States though his ID says he is middle-aged.

Why the time discrepancy?  It has everything to do with speed.  Special relativity, as theorized by Einstein in 1905 and confirmed countless times by modern experiments, says that the faster we move, the slower time passes for us.  This is not science fiction; this is science.

The time ratio, also known as the Lorenz factor, which gives a ratio of the passage of time between stationary and moving observers, is given as follows...

 

NASA Aims for Faster than Light Space Travel

Perhaps you have heard that NASA has recently set its sights on building a spacecraft that can traverse space at a rate greater than 300,000 km/s - the speed of light.  The final product may well arrive a century from now, but at first glance, the very prospect of a spacecraft exceeding the speed of light seems to violate special relativity.  One of the first things we learn when studying relativistic physics is that 300,000 km/s is a cosmic speed limit.

Before investigating this apparent violation of physical law, let us examine what a faster than light speed spacecraft really means in the context of current space travel standards.