Whenever the physicist and aerospace engineer Theodore von Kármán (1881–1963) visited Lake Constance – then on the western fringes of the Austro-Hungarian empire – he’d head down to the lakeshore to watch the seagulls. Von Kármán understood aerodynamics so well that, standing on the dock, he was able to use food to guide the birds so that they stalled while in flight. It was clever stuff, but then von Kármán is far from the only physicist to spend vacation toying with physics.

That, at least, is what I learned after asking Physics World readers what they do while supposedly on vacation. In fact, no-one admitted to simply lolling around. Giancarlo Franzese of the University of Barcelona spoke for most respondents when he described his guilt at having to tell family members that the long August break was “the only opportunity to focus on research and write the papers that have been waiting for months to be completed”.

Lee Jones, an accelerator physicist from Daresbury in the UK, takes his backlog of 3–5 months’ worth of indispensable magazines – including Physics World – on holiday to binge-read while basking in the sunshine. He can’t do this at home, for “basking-in-the-sun opportunities in the UK are notably lacking”. Jones then returns with lists of interesting points, features, websites and contacts to follow up. “My holiday actually generates homework!” 

David Wolfe, who now lives in London but used to be at the University of New Mexico, remarked that “holiday” in the US means the likes of the fourth of July or Labor Day, while in the UK it is equivalent to what Americans call a two-week “vacation”. The amount of free time also varies by country. In parts of Europe, Wolfe noted, there is at least a month’s holiday time. It runs from 14 July until mid-to-late August in France – and longer in Germany. 

Time for problems

Ed Cracknell, a physicist in the nuclear industry, noted that age and marital status are variables. “I recall spending Christmas holidays improving my calculational methods so that I could work more efficiently on my return,” he says. Now, with a family, “I wouldn’t dream of doing that!” Many respondents, though, dreamed big.  

Tom McLeish, a soft-matter physicist at the University of York, admits that he occasionally “works” during holidays, but seeks out holiday-specific challenges. In Pembrokeshire last summer, for example, McLeish tackled the problem of why wind-excited, short-wavelength gravity waves don’t propagate into the wakes behind boats. The correct answer, he believes, is that the vorticity in the wakes suppresses the surface gravity waves.

“I tried at odd moments in the garden of the little cottage we were hiring,” McLeish explained, “to develop an effective medium theory of fluid with the vorticity in it to show that gravity waves would be damped below a critical wavelength.” Sadly for him – and for physics for that matter –the need to ferry his family down to the beach kept interrupting McLeish just as he was about to solve the “smooth-wake” problem.

Another coastal problem he pondered is breathing while snorkelling. If we could genetically modify our haemoglobin to cycle all four oxygens rather than one – while at the same time maximizing our metabolic efficiency – how long could you theoretically dive for with a single breath?

Farther from the coast, McLeish watched a hot-air balloon rise. “Does the differential adiabatic lapse rate between the interior and exterior of the balloon,” he wondered, “increase or decrease lift as it rises?” This buoyancy-stability problem involves factors such as demands on the burner and estimates of heat loss.

On clear summer nights, McLeish has also found himself estimating the height of the atmosphere from the duration of twilight and the Sun’s position, or calculating how high a satellite has to orbit overhead to be sunlit at any time of night. Or, if you are at a holiday campsite, looking up at the Perseid meteor showers, and a meteor falls towards the horizon, over what point on the globe did it finish its burn?

While on a flight, McLeish and a colleague once discussed the G-forces passengers would experience if the plane hit the ocean after an uncontrolled dive. Don’t make the mistake he did, however, and discuss it out loud while in flight. A safer question involves going on a spacewalk with a friend while on a SpaceX flight and unfortunately getting untethered from each other and from the ship. Floating two metres apart with nothing to propel you, how long would it take to bump into each other under your own mutual gravitational attraction? McLeish insists we have to guess the answer, then work it out.  

The critical point

But you don’t have to wait for summer holidays to think of physics questions. I’m reminded of Johannes Kepler’s humorously written 1611 book On the Six-Cornered Snowflake, in which he describes snowflakes falling onto his coat while crossing a bridge in Prague. The incident inspired him to wonder why snowflakes always display a six-fold symmetry, rather than, say, five or seven. In the end Kepler realizes he cannot solve the puzzle, leaving it to chemists.

Snow is an inexhaustible source of natural wonder and scientific riches

Or take the new book Snowflake Science by Kenneth Libbrecht, a physicist at the California Institute of Technology. It has sections on everything from aerodynamics, kinetics and imaging to crystallography, optics and “identity” (one chapter is called “No two alike?”). Isn’t it amazing that the science of snow has continued for over 400 years? Like rainbows, geysers, the aurora borealis, glaciers, how wonderful that snow is an inexhaustible source of natural wonder and scientific riches.  

Surely Physics World readers are active and alert enough to pick out more. Send me your ideas and I’ll devote a future column to your natural wonders – which I’ll, of course, write during my next vacation.