Looking at it up close it almost seems like a laboratory joke left there by mistake: a glass funnel, a black mass, a container underneath, a patience that no sane human being would be able to guarantee without getting distracted at least seven hundred times. And instead that dark, compact stuff, almost hostile to the eye, is one of the most famous experiments in modern physics. It’s called the pitch drop experiment and has been going on since 1927 at the University of Queensland, Australia. Since 1930, when the stem of the funnel was cut, only nine drops have fallen. The tithe is still taking shape, with that irritating calm of things that have never asked our permission to exist.
Pitch, in this case, must be understood without being deceived by the eye. At room temperature it appears solid, can break if hit, has the look of a block of tar determined to remain a block forever. However, it belongs to the family of extremely viscous fluids. Bitumen, asphalt, certain substances derived from tar play this joke: they seem still, then physics arrives with its old lady’s pace and reminds you that movement exists even when no one can bear its pace. The experiment was set up by physicist Thomas Parnell, who heated pitch, poured it into a sealed funnel and let it cool for three years. In 1930 the stem was opened. From there the descent began, if you can call something that is measured in decades that way.
The first drop fell in December 1938. The second in February 1947. For a long period the rhythm seemed almost regular, one drop every seven, eight, nine years. The sixth arrived in April 1979, the seventh in July 1988, the eighth in November 2000. Then the time expanded even further. The ninth drop touched the previous one in April 2014, after more than thirteen years. For an experiment born as a university classroom demonstration, not bad: it had to explain to students that “solid” and “liquid” are not as convenient categories as they seem in books, and it ended up becoming a small global monument to slowness.
Guinness World Records recognizes it as the longest running laboratory experiment still in progress. The most beautiful fact, because it is almost offensive in its disproportion, concerns the viscosity: from the drop of pitch it was possible to show that this substance can be approximately 100 billion times more viscous than water. There is still enough material left in the same funnel to carry us through another century. There is something comical and at the same time very serious in this idea: we change phones, houses, jobs, governments, obsessions, and she remains there, hanging from a funnel, doing her job with an almost rude slowness.
The tenth drop is slow in arriving
The slowdown of recent decades tells an important part of the story. The drop of pitch does not descend into a perfectly controlled machine, sealed from the world, protected from every environmental whim. It is in a display case, exposed to changes in temperature and building conditions. Precisely this makes the experiment less clean, perhaps, but also more alive. After the seventh drop, the building was renovated and the air conditioning installed in the 1980s would cool and stabilize the environment. For a substance like this even a few degrees can be enough. The pitch becomes more viscous, the pressure of the material remaining in the funnel decreases, the separation becomes slower.
Physics here doesn’t need special effects. Viscosity measures how much a fluid resists flowing. The water runs away as soon as it can. Honey already offers a certain dignity. Pitch seems to have signed a pact with geological time. Yet it flows. It drops by tiny fractions, it lengthens, it thins, it forms a drop that at a certain point should break off. The problem lies in that “at a certain point”. For human beings it is a banal phrase. For the drop of pitch it could mean a whole slice of biography.
The observation also had an almost tragicomic flavour. The experiment is also famous because the drops, in one way or another, mocked those who wanted to watch them fall. Thomas Parnell died without seeing the result at the exact moment. John Mainstone, second custodian of the experiment since 1961, followed it for over half a century and remained indifferent. A drop fell when he was absent. Another while he wandered off during an exhibition. In 2000 the webcam was ready, pointed at the eighth drop, almost as if to say: this time we’re here. An ill-timed blackout erased the scene.
Mainstone died in 2013, eight months before the ninth drop reached its tipping point. Custody passed to Andrew White, who found himself faced with a very real problem: the ninth drop was falling towards the previous eight collected in the glass under the funnel. If it had merged with that mass, it could have altered the formation of future drops. White then decided to replace the container. During the operation the structure moved and the filament of the ninth drop separated completely. Here too, the perfect scene got out of hand. The pitch had reached rock bottom, in a technical and almost theatrical sense, without really granting that clear gesture that everyone was waiting for.
In 2005 Parnell and Mainstone also received an Ig Nobel for physics, the prize dedicated to research that makes you smile and then think. The experiment fully deserved it: a pitch funnel that takes decades to drip seems like a university corridor joke, then forces you to put categories, time scales, patience and perception in order.
The pitch also flows elsewhere, with the same habit of humiliating human haste
The Australian experiment is the most famous, but it is not alone. At Trinity College in Dublin, Ireland, a similar demonstration was started in 1944. There too the pitch seemed solid, even there its task was to display an exasperated fluidity. In July 2013, however, something happened that Queensland had always missed: a drop was finally recorded falling. The scholars calculated a viscosity for that pitch equal to 2×10⁷ pascal second, about two million times that of honey. For those who live off notifications, such a number almost seems like a personal provocation.
In Aberystwyth, Wales, there is an even older experiment, started on 23 April 1914. It is perhaps one of the longest-running ever, even if it has a notable narrative flaw: it has not yet produced a drop. The National Museum of Scotland instead preserves a demonstration that could date back to 1902; in that case the historical data are less continuous, and this uncertainty makes it difficult to compare it with the Australian case. The difference lies here: Queensland has a monitored story, a sequence of drops, a continuity that allows us to talk about a laboratory record that is still active.
All this might seem like a collection of academic eccentricities, good material to intrigue students during a guided tour. Instead, the drop of pitch continues to work because it challenges one of our most tenacious beliefs: the idea that what doesn’t change before our eyes is still. Physics, with this experiment, does something very simple and very unpleasant. It takes away the privilege of the human scale. A substance can move without giving us satisfaction. A process can be real even if no one can see it happening. A demonstration can span generations and remain, almost obstinately, still incomplete.
In 2027, one hundred years will have passed since Parnell poured pitch into the funnel. The real flow, the one that began with the cutting of the stem, will be one hundred years old in 2030. They are two different anniversaries, and it is worth keeping them separate. The first concerns the initial gesture, almost domestic in its simplicity: heat, pour, let cool. The second concerns the initiation of movement, gravity beginning to work with the ferocious calm of inevitable things. Today the tenth drop remains suspended. He’s in no hurry. He doesn’t have to publish updates, he doesn’t have to close a loop, he doesn’t have to prove anything to anyone. It’s just going down. And that’s enough for now.
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