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From: kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England))
Newsgroups: sci.physics,sci.philosophy.tech,sci.research,sci.misc,misc.misc
Subject: Re: Einstein's relativity and daily life
Message-ID: <13663@bu-cs.BU.EDU>
Date: Tue, 29-Sep-87 13:57:14 EDT
Article-I.D.: bu-cs.13663
Posted: Tue Sep 29 13:57:14 1987
Date-Received: Wed, 30-Sep-87 07:21:26 EDT
References: <391@nikhefh.UUCP>
Reply-To: kwe@bu-it.bu.edu (Kent England)
Organization: Boston Univ. Information Tech. Dept.
Lines: 44
Keywords: relativity usefulness
Summary: a navigation system first used relativistic corrections
Xref: mnetor sci.physics:2327 sci.philosophy.tech:494 sci.research:232 sci.misc:510 misc.misc:1910

In article <391@nikhefh.UUCP> jona@nikhefh.UUCP (Jona Oberski) writes:
>
>What would YOU answer to the following:
>
>Question: Which things - common instruments, consumergoods, etc. - would not
>exist if we would not have Einstein's Theory of Relativity: "ETR"?
>

Very interesting.  The first "practical application" of relativistic
effects in a "product" was in a Defense Dept navigation system called
the "Global Positioning System (GPS)".  GPS is currently in service to
the US military in "super-accurate" mode and to civilians in coarse
mode, like shippers and surveyors, who can pay for a GPS receiver
which was $50k three years ago when I last worked on navigation
systems.

GPS uses a constellation of satellites traveling well-tracked routes
and the Doppler effect to let the receiver determine precisely where
it is and how fast it is going.  GPS is quite an advance in
satellite-based nav systems.

So where does relativity come in?  Well, the satellites are 'up there'
a hundred miles where the gravity/acceleration field is different.
GPS is so accurate that they need a relativistic correction to the
satellite clocks (cesium, I believe) to keep them in sync with
terrestrial clocks.  First instance, I believe.

It's been a while since I thought 'nav'.  It would be interesting if
anyone out there knew more details of the relativistic correction.
Unlikely, I guess, unless there is some other refugee from the Stars
Wars universe on this internet.

So if you are a sailor or you are out on the high seas looking for oil
or sunken treasure, know that Einstein had much to do with how well
you know where you are in that big pool of water.
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