Many of you have probably read (or at least seen the movie of) The Hitchhiker’s Guide to the Galaxy. Even if you haven’t, you might be familiar with a certain part of the story that is frequently referenced among fans — that when asked for the answer to the ultimate question of Life, The Universe, and Everything, the supercomputer called “Deep Thought” finally responded with “42″.

I was watching an episode of the History Channel miniseries The Universe this evening, and this particular episode was concerning gravity and its role in the universe at large. Most of the concepts covered in the show were nothing new to me, but I found one particular scenario very intriguing: what would happen if you bored a hole through the Earth straight to the other side, vacuum-sealed it (to remove friction), and dropped something through? How long would it take to reach the other side, considering that it should free-fall until it reaches the core, at which point gravity will start pulling it back and it will slow down?

As it turns out, the most interesting part of this scenario is that by picking any two points on the Earth — New York and LA, Boston and London, Cairo and Anchorage, whatever you want — and boring a hole straight from one to the other, the amount of time required by such a “gravity train” will always be exactly the same, due to the angle of the hole and thus the varying degree to which Earth’s gravity will be able to affect the object. How much time, you ask?

42 minutes.

No matter what two points on the planet you choose, no matter if they’re on opposite sides of the globe or are very near each other, the time to get from one to the other driven purely by gravity will always be 42 minutes. As soon as I heard that, I had to wonder if it was Douglas Adams’ inspiration for the answer to the Ultimate Question asked of Deep Thought. If not, it’s quite a coincidence — and still a really cool physics fact.

More on the 42 minute “gravity train” concept on Damn Interesting, Wikipedia (which references a mathematical proof from my own Purdue University), and a 1966 TIME Magazine article.