For years, a classic question posed to university physics students is how long it would take to fall to the centre of the Earth. Given that we can smash tiny particles into each other at really high speeds using magnets, you'd think this would be an easy problem to solve. But apparently not.
For a while, the answer to the question (as calculated by physics undergrads and their profs) has been 42 long, lonely minutes in freefall. But a new paper published in the American Journal of Physics challenges this assumption, saying it should take 38 minutes and 11 seconds instead.
The discrepancy is down to gravity. Specifically, the standard calculation assumes a constant density for the Earth -- meaning that the only change in gravity is due to the distance your falling man is from the centre. But, as Alexander Klotz points out in his paper, the Earth's density varies layer by layer. Using the Preliminary Reference Earth Model, based on seismic data, he was able to approximate the new figure, which takes into account the different densities as you fall.
Although this isn't exactly a crucial question for science to answer, it's a neat illustration that the field is always changing -- assumptions are challenged, new theories formed, and where necessary, college students get told everything they learned is wrong. [American Journal of Physics]