Pitch-drop experiments: science's long wait
Maggie Koerth-Baker reports on the strange science of an experimental result decades in the making.
It was late on a Thursday night last July when John Mainstone first heard the news: The pitch had dropped, and it had been caught on camera.
He’d spent more than half his life waiting for this moment. In 1961, on his second day as a professor of physics at Australia’s University of Queensland, Mainstone had stumbled upon a strange little experiment—more of a demonstration, really—which proposed to show how a seemingly solid material could flow like a liquid. It consisted of a glass funnel, poised on a tripod made of some dark, polished wood, above a simple glass beaker, the whole thing shrouded by a bell jar. The funnel was filled with a glassy, black material. Solid stuff, it would shatter into bits if you threw it at the floor. But it could also drip—very very slowly. Dangling from the end of the funnel was a hard, shiny black globule. More of the black stuff—again all solid—lined the bottom of the beaker.
The stuff was pitch, a distillation product of coal that’s also known as asphalt. Over the decades to come, Mainstone would see five drops fall from the funnel to the beaker. Or, rather, he would see the build up to those drops, and their aftermath. In more than 50 years, he’d never actually been able to watch as the drop of pitch separated itself from the contents of the funnel and fell. He’d tried live stakeouts. He’d tried video cameras. Both options had failed. But now, finally, in 2013, he sat in the dark playing and replaying a time-lapse clip that showed a long strand of pitch stretching and pulling itself apart like some kind of demonic taffy.
There was only one catch: It wasn’t John Mainstone’s pitch.
In the last seven years, the University of Queensland pitch drop experiment has become semi-famous, earning an 2005 IgNobel Award and a feature on NPR’s Radiolab. Begun in 1927, the pitch drop is listed by The Guinness Book of World Records as the world’s longest continuously running laboratory experiment. When news spread on July 18th of 2013 that a falling drop of pitch had been captured on video, many people assumed the longest experiment had finally reached its conclusion.
But that’s not the case.
The video we all watched—the video Mainstone watched over and over that July night—shows a completely different pitch drop experiment, from Trinity College in Dublin, Ireland. Meanwhile, in Queensland, the drop of pitch was still dangling. It finally fell last month, on April 17, 2014. But, while the Queensland pitch drop has finally been caught on film, John Mainstone didn't see it. He died on August 23rd, following a stroke at the age of 78.
John Mainstone had never heard of Shane Bergin, but Bergin knew about him. Like many of us, Bergin had seen news stories about the University of Queensland pitch drop experiment and its longtime caretaker. What made Bergin different than the average consumer of science media was that he knew about a second pitch drop.
A graduate of Trinity College Dublin, Bergin had returned to the school as a professor in 2012. It’s an old institution with a rich sense of its own history. Founded in 1592, Trinity College is the sort of place where collections of curiosities just sort of multiply. The physics department is “almost a museum to Victorian science”, Bergin told me. At its heart is the century-old Schrödinger lecture theatre. The room has tiers of benches that look like church pews stenciled with seat numbers. At the front is a long, rectangular wood podium, a set of sliding chalkboards covered in drifts of white dust, and a set of tall, glass-fronted cabinets filled with eccentric apparati—a treasure trove of silver wheels, brass fittings, and dark tubes.
Bergin, and many other people in the Trinity College physics department, knew that one of those items was a pitch-drop experiment. Like the one in Queensland, it was begun as a classroom demonstration. In fact, while we call them experiments, neither pitch drop really counts as “serious science”. Instead, they’re more like teaching tools, meant to give us a simplified picture of something complicated—in this case, the properties of a viscoelastic material.
The key here is how a material reacts to the physical forces of stress — i.e., what does it do when you bend it, hit it, drop it, stretch it, or generally just abuse the poor thing? Solids put up a lot of resistance to stress … to a point. They resist and resist and then suddenly fall apart. Think of the way you can bang your hand on the side of clay pot, and it holds. But the same pot will shatter if you drop it from up high enough or throw it with enough force. Liquids, on the other hand, don’t resist stress much at all. If you stick your hand into a bowl of water, it doesn’t force you back. It just bends around you.
Viscoelastic materials are substances that do a little of both. You can’t stick your hand through a solid block of pitch, and you can break it with a hammer. But that same pitch will still reform itself, bend and flex, under the right kinds of stress. The result is a material that looks and feels solid. But, over time and under the influence of gravity, it will flow like the goop it secretly is, stretching and stretching and stretching … until it suddenly remembers that it’s supposed to kind of be a solid … and promptly snaps.
The demonstration of this effect that’s set up at Trinity College is a little less elegant in appearance than the model at the University of Queensland. It’s just a funnel held over a beaker by a clamp on a pole. The date of its birth, OCT. 1944, is printed in block letters on the side of the funnel. Younger than the Queensland pitch drop, the Trinity College demonstration is also much less well-documented. Nobody is even sure exactly who started it. And, unlike the Queensland example, it’s never had a dedicated caretaker. Instead, curious students and professors noted when the pitch was about to calve, but otherwise largely ignored it. The funnel gathered dust. “News sources in Ireland had written full articles [about the Queensland pitch drop] without mentioning we had one,” Bergin said. “I guess we’d never told anyone about it.”
That changed in Spring of 2013, when Bergin stuck a webcam in the cabinet and aimed it at the pitch drop. It was, essentially, a whim. He knew a drop was close to falling and, as a big supporter of science outreach, he knew that some average folks would be interested in seeing the split. He just didn’t anticipate how many. The drop dripped on July 11th. When the story broke in Nature News a week later, it brought 1.5 million people to the Trinity College website.
When I spoke to John Mainstone on August 6, 2013, he told me that he’d never really thought of the pitch drop experiment at The University of Queensland as his. Instead, it belonged to Thomas Parnell, the professor who originally set up the demonstration and a man who Mainstone clearly admired. When the pitch drop won the IgNobel Award in 2005, Mainstone said he almost declined to accept it. Not because he thought the IgNobels — which honor ridiculous-sounding-but-thought-provoking scientific achievement — were too silly, but because he felt that it would be inappropriate to take the glory for himself. Parnell was the innovator. Mainstone was just the custodian.
But Mainstone was also clearly devoted to the pitch drop. At the end of his life, he was doing 15 interviews a week, talking to journalists from all over the world. He talked about the pitch drop like it was a favorite pet, one with a bit of an ornery streak and “a mind of its own”, and he was proud of the community it had built up around itself — as everyone from Ph.D.s to housewives tuned in to watch its progress on the live video feed. He promised to protect it from the “Philistines” who might otherwise happily shut it up in a closet or chuck it out with the trash. Marc Abrahams, the editor of the Annals of Improbable Research, which gives out the IgNobel Awards, says the email conversations he had with Mainstone about who should get the award for the pitch drop don't exactly match with Mainstone's memory. Instead of initially rejecting the honor, Mainstone was part of the conversation, helping the Annals of Improbable Research decide to give the IgNobel to both Parnell and himself. Mainstone understood the important role he played in keeping the pitch drop alive.
In fact, he even seemed concerned about the welfare of the Trinity College pitch drop. “I was a bit horrified to see them pick it up off a shelf and just carry it around the room,” he said. Mostly, though, the discovery of another pitch drop and the fact that it had been caught on camera before his seemed to leave John Mainstone more perplexed than anything. For one thing, he didn’t understand why no one had told him about it before — despite the fact that he’d actually visited Trinity College on at least one occasion. “It seemed to come out of nowhere,” he said.
But the other source of Mainstone’s confusion was more practical. After all that time he spent watching the video from Trinity College, he had come away convinced that their pitch drop was somehow different. It had to do with the way the drop had fallen. In the video, the strand of pitch stretches and lowers itself all the way down to the point that it comes into contact with the beaker below. Then it snaps, and the drop tips to one side and comes to rest against the beaker’s edge. While nobody had seen the moment of separation happen at Queensland, the drips from that pitch drop never seemed to exhibit the kind of extreme stretching you can see in the Trinity College video. It was unfamiliar enough that Mainstone suspected the two different pitch drops might actually be operating under different definitions of the word “pitch”.
Pitch actually refers to several different things. It could be the black ooze left behind when you burn wood in an enclosed system. It could be the similar ooze that’s left when you do the same thing with coal. There are also places in the world where you get natural, liquid-looking seeps of hydrocarbons — like the La Brea Tar Pits — and this stuff is also sometimes called pitch. All of these things are viscoelastic materials, but they aren’t chemically identical and you wouldn’t expect them to behave exactly the same. Mainstone wasn’t sure what kind of pitch was being used in Queensland, but he thought it was coal tar pitch and wondered whether the stuff in Ireland weren’t something else.
Shane Bergin thinks otherwise. “Ours looks very much like a teardrop and theirs is more spherical,” he said. “I can’t account for that.” He also pointed to lines that formed on the surface of the Trinity College pitch as it stretched. Those lines were a sign that the drop was accelerating towards the beaker, which is also something that doesn’t seem to happen in Queensland. But Bergin thinks both experiments are using coal tar. Instead, he suspects that the differences might have something to do with the ambient temperature. Neither pitch drop is climate controlled and, frankly, it’s a lot warmer in Australia than in Ireland. Heat can have a big effect on the behavior of a viscoelastic material.
John Mainstone is dead, but the pitch drop experiment isn’t over yet. And it might never be “over”. That’s the thing about scientific curiosities, as opposed to discrete experiments. Curiosities keep going as long as there is something to be curious about, and there’s more that both Mainstone and Bergin want to know. They’d only spoken briefly since the Trinity College pitch drop was caught on camera, but Mainstone had plans to set up a longer discussion and even collaborate. Meanwhile, Bergin and his colleagues are talking about setting up a third pitch drop. They want to see how the drops might be different if they had a longer way to fall, or if the whole apparatus was kept in a different environment.
But that doesn’t really answer the question of how this is going to be interpreted by the public and, in its own small way, by history. Who is going to be remembered here? Will it be Mainstone, who served as the face of pitch drop experiments for decades, or Bergin, who captured the first video clip? How will Mainstone’s death affect all of this, now that the Queensland pitch drop has a new caretaker?
The pitch drop experiments are demonstrations, but they aren’t just demonstrations of physics, they’re also demonstrations of a larger issue — what happens when multiple scientists converge on the same discovery, and how we, the general public, think about scientific history.
The truth is, many of the things that we think about as having a single discoverer actually involved the work of many more people. Some basic concepts — including both calculus and evolution — were even discovered more-or-less independently by two separate people. There’s no single way that these sort of turf wars play out, said Robert Iliffe, a professor of science history at the University of Sussex. Much of it depends on the personalities of the specific scientists involved. When Gottfried Leibniz published his first treatise on calculus in 1684 it touched off a slow-motion flame war as no less a public genius than Isaac Newton claimed to have been working on the same thing since 1666. Newton hadn’t published on it, and wouldn’t until 1693, but it was there in his letters and notebooks and, at one point, Newton accused Leibniz of plagiarising calculus — specifically, he thought Leibniz had managed to reverse-engineer the whole thing from some notes in a letter they’d exchanged.
At the other end of spectrum, Iliffe said, you’ve got Charles Darwin and Alfred Russel Wallace, who both came to the conclusion that species could change over time. Darwin had been working on the idea longer, but, like Newton, hadn’t published anything. “It’s Wallace’s appearance that actually prompts Darwin to publish in 1859,” Iliffe told me. “Otherwise, who knows. He might have just gone on collecting evidence until he died.” Unlike Leibnitz and Newton, though, there was no academic deathmatch. Darwin and Wallace published together and, later, Darwin made sure Wallace received a pension from the British government in recognition of his work.
Different as these stories are, though, they’re linked, and linked in a way that also connects them to the story of the pitch drop experiments. It’s never as simple as “who got to the idea first”, Iliffe said. That’s because, regardless of that, the people involved are almost always thinking about the idea in very different ways. They came to it from different perspectives. Their discoveries highlight different issues. They applied the idea to different problems.
Sometimes, that fact can have a huge influence on who ends up remembered as “the” discoverer. Alfred Russel Wallace, for instance, thought of species change as something that happened because of environmental changes, but he completely discounted the power of sexual selection. That perspective had a lot to do with why he ultimately rejected the idea that humans might still be evolving, Iliffe said. People had used the tools of civilization to remove themselves from nature and now nature couldn’t act upon us. Charles Darwin, obviously, took a different tack. He ended up spending the next few decades of his life looking at human evolution, while Wallace — who was partly motivated to investigate evolution because it was, at the time, kind of a fringe idea — went on to study topics like spiritualism, psychic research, and socialism. “In a lot of ways, his career is much more interesting than Darwin’s,” Iliffe said.
For the pitch drop experiment, those differences manifest most in how the two different institutions approached the thing. At Queensland, John Mainstone had long been dedicated to rigorous documentation, regular monitoring, and continuous filming. In the 1970s, when scientists published research showing that human blood was a viscoelastic material, Mainstone realized just how important it was to understand what these materials did and how they worked. Although the pitch drop was “just” a demonstration, he treated it like an experiment in many ways and it influenced how he thought about the world — and how he presented the pitch drop to other people.
At Trinity College, the focus has been more internal. The pitch drop demonstration was a source of community spirit, a nifty thing for scientists to watch together. Instead of leading to broad outreach, it’s lead to cross-department collaborations that might not have happened otherwise. Right now, Shane Bergin, whose real research is focused on nanomaterials, has begun to work with experts in foams, joining forces to study surface tension and viscosity in ways that were inspired by a shared interest in the pitch drop.
There’s also another factor that affects how we ought to think about scientific discovery. The truth is, no matter who get the credit for an idea, they’re ultimately building on the ideas of others — without A, B, and C, you’d never get to F. Look into the history of any great discovery deeply enough, Iliffe said, and it starts to become difficult to tease out awards we often take for granted. Who was “the first person to …”, what is “the longest running …”? It depends a lot on how you frame things.
And that’s true for the pitch drop, as well. Trinity College will go down as the first place to film a drop of falling pitch. The University of Queensland has their Guinness listing. John Mainstone will probably remain the personality that most people think of when they think of pitch drop experiments. He, in turn, would have preferred you give credit to Thomas Parnell. But buried under all of that complicated history is an even older experiment that likely inspired both Parnell and whoever set up the Trinity College pitch drop.
Go to the Hunterian Museum in Glasgow, Scotland, and you can find a wooden pedestal, shaped like a miniature water slide — a trough that dips over two steep inclines. Instead of water, though, the trough is filled with hard, black pitch. It feels solid, but you can see the stretch marks and sag lines that show how the forces of gravity have pulled it down the slopes. At the bottom, it overflows the base, a pseudo-liquid, frozen to our eyes, but not to time. Described as an artificial glacier, this demonstration was originally set up by William Thompson, Lord Kelvin, in 1887.
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