[ExI] speed of light at the speed of light

John Clark johnkclark at gmail.com
Fri Aug 23 20:23:50 UTC 2013


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.

 John K Clark
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