[ntp:questions] Re: Extracted fom comp.risks - What Time Is It?
davids at webmaster.com
Mon Aug 18 19:26:49 UTC 2003
"H. Peter Anvin" <hpa at zytor.com> wrote in message
news:bhpo89$k70$1 at cesium.transmeta.com...
> I think the big question is how do you *synchronize* these various
That's definitely the harder part.
> An atomic clock tells you what time it is in one particular location.
> Any kind you move a clock you have to deal with time dilation issues,
> and when you communicate its time using signals you have propagation
You can either ignore the time dilation issues or correct for them if
needed. It used to be pretty common to fly a synchronized atomic clock from
one place to another to synchronize the atomic clock at the destination. GPS
has pretty much replaced this (painful) process.
> To make things even more complex, the speed of time actually
> varies with the strength of gravity and therefore varies with
> altitude, latitude and geological composition, although I *believe*
> this is currently below the threshold of measurability (time dilation
> due to moving an atomic clock is, however, very much measurable and
> can amount to tens or even hundreds of nanoseconds on a
> round-the-world journey.)
I don't believe anyone corrects for gravitational effects. Time dilation
definitely requires compensation when you have to fly an atomic clock from a
standards lab to your radiotelescope observatory.
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