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Quote from: MikeS on 21/01/2012 13:57:18The most accurate clock is the speed of light in a vacuum. Presumably you mean the distance light travels (speed requires knowledge of time), but then you're back to the problem of distance, so it's really a pretty lousy clock.

The most accurate clock is the speed of light in a vacuum.

Quote from: CliffordK on 21/01/2012 20:04:55as well as precisely knowing either distance, or time, the remaining factor can be calculated.But you don't know the distance precisely which is why light does not make a very good clock. Atomic clocks avoid this problem by using some sort of atomic activity instead.

as well as precisely knowing either distance, or time, the remaining factor can be calculated.

We already have clocks that (just about) run at measurably different rates on the top floor of the building, compared to the ground floor.Any proper clock will do the same. What's the point of using some exotic alloy?Surely it's easier to use something that isn't so critically dependent on the physical size of something?For example, the wavelength of microwaves emitted or absorbed by some bunch of atoms is practically independent of the size of the container they are in.The effect of gravity can be accounted for by using atoms that are in free fall at the time of the measurement.http://en.wikipedia.org/wiki/Atomic_fountain

"If one could find a material that did not expand with relativity"What are you going to measure it with?You need to compare it to the definition of the metre.That definition is expressed in terms of the speed of light.

That definition is expressed in terms of the speed of light.Light is affected by relativity.

"The hyperfine transition state clocks are considered accurate to 15 decimal points, but all fail to remain synchronized if moved more than a few feet apart, "Not if you move them slowly enough and keep them at the same height.