In the previous article, I wrote about this recurring belief by Star Wars fans that Han Solo made a mistake when he boasted that the Millenium Falcon made the Kessel Run in less than twelve parsecs. Many people have claimed that since a parsec is a unit of distance, this is not an appropriate way of gauging the speed of a spaceship. And I showed how the theory of relativity argues that a parsec is a unit of time, and therefore Han was right.

This time I will give a second argument, also based on the theory of relativity, which could explain why the speed of a spaceship can be measured in distance. (I do however feel the need to add a disclaimer that, although I have now done two articles on Star Wars in the space of an afternoon, I do not mean this as a criticism of fandom or of amateur science. I merely hope to use this wonderful series of movies to elucidate some of the finer points of modern physics.)

According to the theory of relativity, distances decrease as speed increases. So much so that a beam of light, which travels of course at the speed of light, behaves as though the entire length of its journey - even if that journey crosses the entire visible Universe - is in fact a single point in space and time. At the speed of light, everything reduces to a single point.

And that means that if you have a standard distance that can be used for calibration and comparison - such as the distance between two known stars or perhaps the mythical Kessel Run - then you can in fact measure speed by the apparent distance as measured by the spaceship. If I know that two stars are 20 parsecs apart when measured by an observer who is not moving relative to the stars, then being told that another observer measures it to be only 12 parsecs immediately tells me that he is traveling at 80% of the speed of light, or 240,000 km/s. 

And since it is easier to say 12 parsecs than to say 240,000 km/s, it would make sense that a pilot would measure speeds in parsecs - if they have a standard distance such as the Kessel run to compare it to.

And so once more, not only did Han shoot first, but in knowing how to measure speeds -  Han was right!