[time-nuts] UTC and leap seconds

Hal Murray hmurray at megapathdsl.net
Fri Jun 11 12:04:17 EDT 2010


iovane at inwind.it said:
> I was wondering, why we assume that  Earth's rotation is slowing down,
> instead  that clocks are speeding up? 

The quick answer is that there is a mechanism that explains why the Earth is 
slowing down: tidal effects.  There is no corresponding way to explain why 
atomic clocks are speeding up.


How many different timekeeping mechanisms are there that are accurate enough 
to notice changes in the Earth's rotation?

Wikipedia says 2 ms/100 years and that it was noticed by Halley in 1695 and 
confirmed by Dunthorne in 1749.  I assume they were using the Earth's orbit 
around the sun as their reference clock.
  http://en.wikipedia.org/wiki/Tidal_acceleration
I think that's 6 parts in 1E12 so it takes a pretty good clock.  (I'm 
frequently amazed by how good the old astronomers were.)

The Wikipedia article discusses geological evidence: silt layers from 620 
million years ago give a calibration on the Earth's rotation and the Moon's 
orbital period.
  http://en.wikipedia.org/wiki/Tidal_acceleration#Historical_evidence



> (Speculative hint: We accept that the universe is expanding. Might this
> affect  the fine structure of matter, including cesium atoms? Is there any
> adverse  proof? What is easier to think? a) the expansion of the universe
> doesn't affect  at all the properties of matter. b) it might.). 

It's clearly easier to think that the expansion of the universe doesn't 
effect things.  Sure, it might, and I expect a few far-out theoreticians are 
working on things like that.  But nobody has proposed a mechanism yet, nor an 
experiment that would measure it.  Science is all about measuring.  Yes, it's 
fun to wave your hands about crazy ideas, but who cares if you can't come up 
with something to measure?

I'm not really a physics wizard.  There might be some gravity terms or 
experiments I've missed.  But the universe isn't expanding very fast (at 
least not in the local region) so any effect will probably be very hard to 
measure.  Maybe dark energy will distort time.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.







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