An experiment that has been running in a glass case at the University of Queensland since 1927 will turn 100 next year, making it what is generally described as the longest-running science experiment in the world.
It was set up by Thomas Parnell, the university's first professor of physics, to demonstrate the unusual properties of pitch, a tar-like material then in wide use for waterproofing. Parnell heated a batch, sealed it into a glass funnel and left it to cool.
In 1930, three years later, the bottom of the funnel was cut off. The pitch, which at room temperature can be shattered with a hammer, began to flow.
It has been flowing ever since, at a rate that is difficult to convey in ordinary units. Professor Andrew White, the experiment's third custodian, describes the pitch as moving at about an atom a second, more than ten times slower than continental drift. He compares the material to toothpaste, except billions of times more viscous.
In the 96 years since the funnel was opened, nine drops have fallen. Not one of them has been seen by anybody.
The record of near misses is unusually complete. Professor John Mainstone, the second custodian, sat with the funnel through an entire weekend in 1979; the drop fell after he went home for a shower, and had landed by the time he returned on the Monday morning. In 1988 a drop fell during Expo in Brisbane, in the interval when the people watching it had gone to get a drink.
By 2000 the plan was to catch a drop on camera. A blackout dealt with that. Air conditioning installed in the 1980s had meanwhile slowed the pitch further and lengthened the gaps between drops, which did not help anybody's shift planning.
The ninth drop, in 2014, managed to go wrong in a new way. The beaker beneath the funnel was by then so full of earlier pitch that the drop never actually fell. It made contact with the pile below and then spent roughly eight years slowly tipping over.
A fresh and empty beaker is now in place, and White expects the tenth drop within the next couple of years. He has made his requirements clear. "I'd love to be able to witness it in person, but I would settle for just a really good recording," he said.
It is worth being precise about the state of the evidence. The experiment was built to show that pitch is a fluid, and it has shown this nine times. On each occasion the demonstration consisted of the pitch no longer being where it had previously been.
This newspaper regards that as a methodological landmark. Nine results have been obtained without a single observation, which makes the pitch drop the most economical experiment in the physical sciences: it requires a funnel, a beaker and nobody at all.
The mechanism is now reasonably well characterised. A drop falls when the last person watching it leaves to wash, to drink, or to attend to the power supply. In 2014 the pitch declined to fall even then, which our analysts read as the apparatus moving to a more defensive posture.
The Department of Unnecessary Research notes that replication is the remaining obstacle. A second funnel, prepared today, would deliver its first confirming drop at some point in the 2030s, and a full nine-drop series by roughly 2120, assuming nobody installs air conditioning.
The tenth drop is therefore the most heavily anticipated event in Australian physics, and the preparations are complete. The beaker is empty, the camera is ready, and the only variable left in the experiment is the professor's bladder.