Keeping Time with a Mercury Atom

Roland Piquepaille writes "The National Institute of Standards and Technology (NIST) has announced that a new experimental atomic clock based on a single mercury atom is now at least five times more precise than NIST-F1, the U.S. standard clock. This mercury atomic clock 'would neither gain nor lose a second in about 400 million years' while it would take 'only' 70 million years to NIST-F1, based on a 'fountain' of cesium atoms, to gain or lose a second. But even if this new kind of optical atomic clock is more accurate than cesium microwave clocks, it will take a while before such a design can be accepted as an international standard. A ZDNet summary contains pictures and more details about the world's most precise clock."

Comments

Re:How accurate is accurate enough?

[rwwh]

Scientific American once had a nice paper about time. I remember these two facts:

• At an accuracy of 10^-17, the earths gravity makes that two identical clocks, one of which is 5cm higher up than the other one, will start deviating from each other (i.e. time really IS different 5 cm up, at this accuracy)
• At an accuracy for 10^-17, relativistic effects start playing a role at walking speeds (i.e. time really IS different at walking speed than at rest, at this accuracy).

I think 5cm and 5km/hour are reasonable usability limits, hence an accuracy of better than 1:10^17 would not make much sense to me.