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Redefining the kilogram
In a vault on the outskirts of Paris sits a small platinum-iridium cylinder that has the unique property that its mass can neither rise nor fall. What makes this lump of metal so special is that it is the "official" kilogram. So whether the cylinder gets "heavier" by picking up dust or "lighter" as a result of being cleaned, it always has a mass of 1 kg. Ian Robinson describes two new experiments that aim to sweep away this unsatisfactory state of affairs - by redefining the kilogram so that its value cannot change with time. One experiment defines it in terms of a certain number of carbon atoms, while the other involves matching the pull of gravity on an object with the electromagnetic force on it from a current-carrying wire. Unfortunately, agreeing upon a new definition is still "many years" away and would also mean having to redefine the units of energy (the Joule) and force (the Newton).

Also in this issue:
Gravity hunt goes underground
US science adviser takes on his critics
Hubble successor shifts to the red
Challenges for a new Europe
US threat to European journals
Beating the diffraction limit
Tales from the transneptunian sea
Atoms and light get intimate
Lasers feel the Z pinch
Dark energy: the latest findings

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Contact: David Reid
david.reid@iop.org
44-20-7470-4815
Institute of Physics
3-May-2004