The reference International Prototype Kilogram (IPK), a cylindrical shaped piece of platinum-iridium alloy that is stored in Sèvres, France, just outside of Paris, has mysteriously shrunk by 50 micrograms. The kilogram remains the only unit of measure based on a physical artifact instead of fundamental physical properties that can be reproduced in different laboratories.

Scientists are unsure what has caused the main prototype to become lighter, although the fact that it has been cleaned more frequently than the other “sister” official prototypes suggests that it is losing mass through the cleaning process, which cannot be perfect because it is physical (not frictionless) in nature and the prototype is stored in air (not in a vacuum). Interestingly, the weight loss is equivalent to the weight of an average fingerprint.

Currently, there are proposed future definitions for the kilogram that will render the IPK obsolete. If you are a total geek, you’ll thoroughly enjoy the proposals which vary from counting atoms of silicon and gold, to the weight of a huge number of electrons in a vacuum, to measuring the mass of an object with electricity passing through it. The IPK’s days are numbered.

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