Last Friday, the 26th General Conference on Weights and Measures voted to replace “Le Grand K”, the 129 year old physical prototype of the kilogram, with a new measure based on physical constants. The new definition, which comes in to force next spring, is now fixed by defining the Plank constant as exactly 6.626 070 15 x 10-34 Joule seconds.
The kilogram was the last SI unit based on a physical prototype (which means that until next year, Le Grand K is, by definition, exactly one kilogram). Other prototypes, such as the physical meter, have been long since retired. This redefinition is a triumph of high-precision experimental physics. The problem with physical prototypes for units is that they can change. Le Grand K, for example, has been losing mass, to the tune of about 50 micrograms (5 parts in 100 million), possibly because the identical cylinders it is compared to have gotten dirty from being handled more often.
Farewell Le Grand K! Thank you for your service!
This month’s Physics Today has a great article by Roberto Zenit and Javier Rodríguez-Rodríguez about the physics that governs bubbly drinks. It is relatively nontechnical (aside from a few equations which can be safely ignored).
The fluid mechanics of bubbly drinks
Roberto Zenit and Javier Rodríguez-Rodríguez
Physics Today 71 11 44 (2018)
A few months ago I signed up for Skype a Scientist, a service that connects scientists with classrooms around the world so students have a chance to meet a real scientist. Today I had my first session with a 7th grade IB class in Bangkok, Thailand. It was a lot of fun! I introduced myself and my field and talked a little bit about what it’s like being a scientist, then I answered questions from the students for the remaining time. There were all sorts of questions from “What challenges did you overcome to become a computational physicist?” to “Is the Earth’s magnetic field changing?”
If you’re a fellow scientist or a teacher who wants to skype a scientist you can sign up on their website: https://www.skypeascientist.com/ The commitment is small (you can sign up to do just one session) and there’s no need to prepare a lecture. I had a blast and I’m looking forward to more skype sessions!