It's not difficult to spot the tourists at the Metro stations in Washington, DC. Besides their casual clothes and relaxed gaits, visitors to the US capital usually buy paper tickets from vending machines. Many of us who live in DC and its surrounding suburbs carry a contact-less stored-value card called SmarTrip.
Like London's Oyster card and Hong Kong's Octopus card, Washington's SmarTrip keeps track of its owner's trips and subtracts the appropriate fares from the total stored on the card. Users replenish the card's funds at vending machines, which take cash or credit.
Two weeks ago, my wife, Jan, forgot to remove her SmarTrip card from a trouser pocket. When I did the household laundry, I unwittingly washed the card. Given that my Seiko watch had survived the same treatment 10 years ago, I was optimistic that her SmarTrip card would also continue to work.
I was wrong. When Jan tried to use the newly clean and apparently undamaged card, she got an error message at the first ticket gate she approached. The station manager tested the card and confirmed that it was irreparably broken.
SmarTrip cards seem robust. What caused Jan's to break?
Unlike credit cards, SmarTrip cards do not store information on a magnetic strip. But even if they did, the strip's Curie temperature—the temperature above which the strip's magnetic domains become randomly oriented—was not likely to have been exceeded in the warm wash cycle that I used.
Could the detergent have damaged the card? Possibly, but the three layers that make up each card are fused together tightly. What's more, Londoner Frank Swain has demonstrated, on YouTube, that it takes a chemical far harsher than diluted laundry detergent, such as nail-polish remover, to soften and dissolve an Oyster card's plastic layers.
SmarTrip cards store and update value on a microchip embedded in the card's middle layer. Also embedded in the card are wires that loop continuously along the periphery of the card and serve as an antenna. When the user holds the card over the coaster-sized reader at the ticket gate, radio waves emitted by the reader induce an electric current in the antenna, which powers the chip. The same antenna transmits information back to the reader.
The innards of a SmarTrip card as revealed by Tom Lee in a story he wrote for the website DCist.
For Physics Today's July 2003 issue, I wrote a news story about a flexible conductor that Stéphanie Lacour and her colleagues developed at Princeton University. In reporting the story, I learned that thin, flat metal wires of the kind that form a SmarTrip's antenna will rupture at strains of just a few percent.
At the end of the first stage of the washing cycle, Jan's SmarTrip card was pressed against the washing machine's rotating drum as warm soapy water was expelled from the clothes. The card became bent. Could the strain associated with the curvature have broken the antenna and, along with it, the card itself?
The card is about a millimeter thick. The drum of my washing machine has an inside radius of 25 cm. The strain induced in the card when it acquired the same radius of curvature is therefore around 1%, which might have sufficed to break the antenna.
Unlike the problems I encountered in my physics exams, problems in the real world don't have clear, predetermined answers. Although can't be sure that I identified what really broke Jan's SmarTrip card, I enjoyed the modest challenge of speculating on the cause. Without realizing it at first, I had tackled what physics teachers call an open-ended problem.
Out of curiosity, I googled the phrase "open-ended physics problems." The top hit was to the webpage of Alex Alemi, a physics graduate student at Cornell University. Alemi has posted 223 questions that Pyotr Kapitsa devised in the late 1940s to test students at the Moscow Institute of Physics and Technology.
The second hit was more recent. In 2010 the Scottish Qualifications Authority, which administers the nation's school-level examinations, published a handbook for teachers. To quote the introduction:
Open-ended questions are designed to encourage a full and meaningful answer using the student’s knowledge of physics. Such a question therefore allows a student the opportunity to demonstrate a deeper understanding of physics principles than can be demonstrated by familiar quantitative-type problems.
Kapitsa would approve.