Here's a very interesting video of the 2012 Isaac Asimov Memorial Debate on whether neutrinos can travel faster than the speed of light. One of my favorite scientists, Dr. Neil DeGrasse Tyson (the fellow whom Sheldon Cooper blamed for having Pluto downgraded from a planet to a mere ball of ice) does a great job of moderating and keeps the discussion both lively and understandable for public school graduates like me.
Part of the discussion focuses on Einstein's Theories of Relativity, both Special and General. The General Theory of Relativity states that time moves faster in low gravitational fields. This is known as gravitational time dilation. Starting about the 30 minute mark the discussion turns to how the atomic clocks on board the US Global Positioning System (GPS) satellites are intentionally 'slowed' to compensate for the changes in the progression of time in lowered gravitational fields. One of the panel members, Dr. Christopher Hegarty of the MITRE corporation, comments on how tests have shown that if the clocks on the GPS satellites are not intentionally slowed then the signal accuracy based on the uncompensated clock will drop from a few dozen feet to about 11 kilometers in just one day!
Dr. Hegarty also comments about how some of the time compensation computations are actually handled by the GPS receiver software.
So remember folks, every time you fire up your GPS (or even just use your smartphone to find the nearest Starbucks) you are helping to verify the Theory of General Relativity. Go Einstein!
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