The Kilo in Crisis

Here's a recent story from the Los Angeles Times.

It tells about how the various standard kilograms around the world are no longer standard. When these kilogram weights were checked against the standard kilogram in Paris (made of platinum-iridium), they no longer weighed the same.

Here is a link to a picture of the standard kilogram. The kilogram is the last physical standard that is still defined in reference to a physical object. Originally, the kilogram was defined as the mass of a cube of water with sides equal to 10 centimeters. No one knows why the standard kilograms are losing weight, but the loss has led to a search for alternative standards. One approach is to construct a very sensitive magnetic balance, another is to actually count the atoms of a particular element.

All other standard units are defined in other ways, without reference to physical objects. The National Institute of Standards and Technology (NIST) defines the other standards. CLICK HERE to see all of their definitions and the historical context that led to the definitions. The NIST's definitions of the meter, the second, and the candela are reproduced below.

"The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second." Originally, the meter was defined as 1/10,000,000 of a quarter of the earth’s circumference.

"The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom." Originally, the second was defined as 1/86,400 of the mean solar day.

"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." Originally, the candela was the light given off by a standard candle (candela is Latin for candle).

One of the fundamentals of science is measurement. As sciences became more and more developed, the need for precision became more acute. So, redefining the kilogram is a real crisis is science.