PZ’s Birthday — with Gravitas.
Belated happy birthday to the big squid on the block of science blogging — see Bora’s (who else’s?) catalogue of those around the blogosphere who responded in a more timely fashion to the Dear Cephalopod’s numerologically significant planetary rotation.
Not much to add to the universal cheer for PZ Myers continued presence on earth, except a quibble. (What did you expect? This is a blog, for Spagetti Monster’s sake).
PZ, in his acknowledgment of the outpouring of blogolove, noted that his accomplishment was pretty ordinary:
Now I do have to remind you all, though, that we’re all aging at exactly the same rate (unless you have access to a spaceship that travels at a significant fraction of the speed of light), and all I’ve got is a head start on many of you…
But alas, PZ here makes an error common to the non physicist or non-mud-grubbing pedant. (as my last physics course was some 34 years ago, guess which category into which I fall.) He nods towards the special relativistic side of time dilation, but, (horrors!) he ignores the gravitational impact on the passage of time.
The effect is a consequence of the way Albert Einstein’s general theory of relativity. (See here, or here. Fwiw, I wrote and directed the animations accompanying the second essay. They’re more than ten years old, but I still like them). General relativity states that strong gravitational fields make clocks run slower than clocks in a weaker field — as in fact they do. An observer at sea level will, if she happens to have an atomic clock handy, observe time as passing more slowly than will her colleague flying in an atomic-clock equipped airplane. (More precisely — the airborne clock will be observed to have run slightly fast compared to the clock on the ground when the plane lands.)
The effect is small in most circumstances (not near a black hole, though!) — but significant enough to matter a great deal to the Global Positioning System. (Click on Clifford Will to see the relevant essay.) Left uncorrected, the seemingly small time dilation effect (a clock on a satellite in geosynchronous orbit orbiting medium earth orbit altitude used by the GPS system (20,200 kilometers or 12,552 miles) ticks 45 microseconds/day faster than a clock on earth) would, even when accounting for special relativity, which pushes slows the speedy satellites’ clocks by 7 microseconds/day, produce navigation errors of more than 10 kilometers a day. Will writes that failing to account for the effect would render the system useless for navigation in just two minutes.
All of which is to say that PZ, professing reason at about 1,138 ft above sea level, (give or take the height of his office building), is aging slightly slower than any colleague he might want to chaff at the University of Colorado, Boulder, altitude 5,430, but just a smidgeon faster than your faithful blogger, writing this in Boston (ish), maybe a hundred feet above high tide.
Use those microseconds wisely, I say.
Update: GPS satellite orbits corrected. Brain bubbles are my only excuse. Thanks to commenter Tom below for catching the error.
Image: Guercino, “Et in Arcadia Ego” c. 1628. Source Wikimedia Commons.
Tags: birthdays, Einstein, gravity, humor, PZ Myers, Time
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March 10, 2008 at 5:26 pm
GPS clocks are not, in fact, geosynchronous; their orbit is at about half of the required distance. Your values are correct for GPS.
March 10, 2008 at 6:30 pm
Corrected above. Thanks for the catch. Sometimes fingers type faster than brain thinks.