After enjoying the latest Star Wars movie, I happened to overhear a number of uber-fans making jokes about what they perceived to be inconsistencies and mistakes in the original movies. And while that is fine, and is something I enjoy chatting about as well, there is one point that keeps getting re-hashed and has unfortunately become a symbol of film errors. 

In A New Hope, Han Solo boasts about the speed of his spaceship by stating that it made the Kessel Run in than twelve parsecs. And ever since then people have been pointing out that a parsec is a unit of distance, and not time. George Lucas has tried to justify this by claiming that to avoid obstacles spaceships must take a circuitous route, and therefore it is the length of the route that Han plotted that he is boasting about. 

But what these critics do not realize is that Han was right, and they are not. The parsec is a unit of distance AND time.

Long, long ago, in 1905 to be precise, Einstein created the special theory of relativity. And soon after, Minkowski showed that it was equivalent to a Universe which contained not three but four dimensions. Time is itself a dimension, no different than space, except that we perceive it only in slices. Our minds can only comprehend one moment of time, but time itself is a dimension stretching from the Big Bang right through to the end of the Universe. And as such time and space have the same units.

In fact relativity makes the use of different units for time even more difficult, because the length of time that passes depends on the motion of the person measuring it. So a trip that lasts one year for a space traveler might appear to last ten years for someone left behind. Similarly, the distance the person travels might be ten lightyears for one person, but only one lightyear for the other. But if space and time are considered as a single spacetime, then the four-dimensional distance traveled is the same for all observers. Using different units for space and time in a relativistic spacetime is as absurd as deciding on Earth to measure forward-backward distances as miles, and left-right distances in kilometers!

So why do we use 'seconds' or 'hours' or 'years'? Because they have been ingrained in our society, and they are convenient. It would be awkward to say to someone "I will be there in 300,000 kilometers from now", but it would be a valid measure of time. 

And in fact, if you look at almost any serious research paper in a professional science journal, space and time are given the same units of measure. It is common practice to use natural units in research, in which the speed of light is set equal to 1 and in which time and space have the same units. 

And that means that as society evolves and as the language of modern science filters through to common usage, it will not be strange to hear people talk of time in units of meters, kilometers, or even parsecs. 

And so in this case, Star Wars fans are wrong, and Han Solo was right!