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# Physics/Gravity

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QUESTION: Is there a special tool one could use to measure gravity in specific places on earth?

ANSWER: Yes there are such tools, and they're incredibly precise.  These devices have been around for decades, in a couple of different forms.  Just google "gravimeter" (not to be confused with "gravitometer," which might refer to fluid density).  There are also satellites deployed (and more planned) that are mapping the entire Earth's gravitational field repeatedly.  http://www.csr.utexas.edu/grace/gallery/gravity/  I had one at the University of Texas Permian Basin (oil country, so huge geology resources) that had a uranium test mass inside it.  It was decades old, but accurate to within one inch of altitude change.  When I said they were sensitive, I wasn't kidding...

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QUESTION: What do they usually cost? A friend and I are interest in time dilation and may need to know roughly the amount of gravity at different elevations. We are amateurs just playing around.

Answer
If all you need is "roughly," then just calculate it.  Or maybe rent one, I googled "gravity meter" and it autocompleted with several things (including "rental").  http://www.gravitymeter-repair.com/Rentals/tabid/57/Default.aspx  Easy enough, you don't need to own it to make a measurement.  You don't need to know it to the microGal level for that matter (1 Gal = 1 cm/s^2).  What you really need is an atomic clock.  Those are pricey, but not as bad as you might think to fix up an old one.  You may be interested in this story: http://leapsecond.com/great2005/tour/  As to sourcing them, you get those ones that aren't actual atomic clocks if you google, but set themselves based on radio signals from an actual atomic clock.  That doesn't work, since that clock stays in place.  Maybe you can ask the guy who did this where he got his atomic clocks, there's contact info on that website.  You may also want to see this cool article: http://www.eetimes.com/document.asp?doc_id=1258348

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#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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