It's Fluid

30 June 2023
Gwen Weerts

If, like me, you get impatient waiting for water to boil, imagine the monk-like patience needed by Andrew White, the current custodian of the world’s longest-running experiment: the pitch drop.

The pitch drop experiment was started in 1927 by Professor Thomas Parnell to demonstrate the viscosity of tar pitch, which feels solid and brittle at room temperature, but in actuality flows, albeit very slowly. In the 96 years since the experiment began, a drop of pitch has separated exactly nine times, most recently in 2014. But no one has ever seen the drop fall—not even John Mainstone, the experiment’s longest custodian, who looked after it for 52 years at the University of Queensland. He died in 2013, one year before the most recent drop. Anyone who wants to experience the riveting action of flowing pitch can join hundreds of thousands of other pitch-curious people and watch a livestream at

That the pitch drop livestream receives so many hits each year indicates that I’m not the only person who finds it captivating—is that because it’s so simple, or because it’s so slow? In physics, where undergrads are now encouraged to learn quantum principles, and where speed can be measured in attoseconds, the pitch drop experiment feels like a meditative inhale.

The pitch drop experiment at the University of Queensland, with 9-volt battery for size comparison.

Not all fluid science is so simple, especially where optics is concerned. This issue of Photonics Focus includes two exciting histories about seminal developments in underwater imaging: Dimitri Rebikoff, inventor of the first waterproof flash unit, which enabled Jacques Cousteau’s early Oscar-winning underwater cinematography; and the underwater imaging advances that enabled the CIA’s top-secret Project Azorian to locate and raise from the seabed part of a Soviet submarine in the 1970s. But the intersection of optics and water are not limited to historical advancements: Today, photonic tools are being used to identify some of the ocean’s most pressing problems, like toxic microplastics.

This issue’s emphasis on cooling fluids feels quenching, as it’s now mid-summer in the Northern Hemisphere, and record heat and wildfire smoke are keeping people confined indoors, where air conditioners make both more tolerable.

In fact, due to the addition of air conditioning at the University of Queensland in the 1990s, the pitch drop underwent a significant change: the duration between pitch drops lengthened from an average of 8 years to 13, which means we can expect the next drop to fall sometime around 2026. I imagine the Tenth Watch website will experience a surge of traffic that year, and hopefully, maybe this time, at least one person will see it happen.



 Gwen Weerts, Editor-in-chief


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