[ExI] speed of light at the speed of light

[Mike Dougherty]

On Fri, Aug 23, 2013 at 4:23 PM, John Clark <johnkclark at gmail.com> wrote:

> On Fri, Aug 23, 2013 at 3:38 PM, Mike Dougherty <msd001 at gmail.com> wrote:
>
>> > I'm happier with relativity being a best-so-far explanation* that seems
>> to work well-enough for the people who use it.  However, it doesn't help
>> much for driving to the grocery store
>>
>
> Actually not just relativity but General Relativity helps a lot if you're
> driving to the grocery store if you use GPS. Because the GPS satellites are
> moving fast Special Relativity says the atomic clocks on the satellites
> will fall behind clocks on the ground by 7 microseconds per day; but
> General relativity says that because those satellites are further from the
> Earth's core than you are and in a shallower gravity well the atomic clocks
> on the satellites will gain on the clocks on the ground by 45 microseconds
> per day. So when viewed from the surface the clocks in the satellites are
> ticking faster by 38 microseconds (45-7=38). Without correcting for
> relativity the error in GPS would grow by about 6 miles PER DAY and you'd
> never find the grocery store.
>

Ah touche.  I don't use GPS to find the grocery store though, I still have
some onboard CPU & memory left.
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