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  Einstein and Shiny Space BallsApr 16, 2011 1:14 AM PDT | url
 
Added 1 new A* page:My Sunday fairy tale comic "The Princess and the Giant" will update this Sunday, and if you haven't been keeping up with it, here's a preview of what you missed last weekend (and it's clickable and stuff to go right there)
 
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According to this AP news article, this past Monday was "the 50th anniversary of the first human spaceflight by Yuri Gagarin." The article has some interesting quotes from people involved with the old Russian space program, saying for instance that they weren't certain of humans could survive being in space, psychologically, and that the risk assessment for the mission's success was only 50%! It also mentions the controversy surrounding Gagarin's death in a test flight in 1968--rumors persist that it was arranged by the KGB "for his alleged opposition to the Communist regime."
 
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This Space.com article talks about supermassive black holes, as well as black holes in general, saying that a formula has been developed that can calculate when a black hole will reach "Kerr spacetime," which is supposedly a state where time is no longer passing. Sounds kind of interesting. The scientific paper this came from might be this one, of which I can understand very little. :P
 
And there isn't much on "Kerr spacetime" on Wikipedia, but there is an article on the "Kerr metric," which is sort of like the better-known Schwarzchild metric, used to calculate the event horizon of a black hole (the distance from the singularity inside of which light cannot escape), but it has to do with the ellipsoidal "ergosphere" around a *rotating* black hole, outside the event horizon, inside of which particles are forced to rotate due to the rotating black hole's warping of spacetime in rotational frame dragging, aka the Lense–Thirring effect.
 
Sounds fun, huh? Turns out there was already an experiment done to verify that this effect, predicted by General Relativity, actually exists: NASA and Stanford University's Gravity Probe B went into a polar Earth orbit in 2004, and spent 16 months measuring the effect of the Earth's gravity on four precisely engineered gyroscopes mounted on the probe--at the time they were made, the ping-pong-sized fuzed quartz gyroscopes on GP-B were the most perfect spheres ever made by humans! They were coated with a thin layer of the superconducting metal niobium, and suspended in magnetic fields; the spin induced on them by the Earth's gravity could be read by monitoring the magnetic field of their niobium coating. Neat-o. Here's one without the coating, refracting a photo of Einstein (oh those jokers at NASA!):
 
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image by NASA (source)
 
This NASA graphic in a BBC article (can I steal that, since NASA made it? hm not sure :P) illustrates what was expected would happen: the gravitational effect of the Earth's spin would gradually cause the gyroscopes to rotate away from their starting alignment.
 
A camera on the Delta II rocket that took the probe into orbit got a photo of it up there:
 
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image by NASA (source)
 
The findings apparently confirmed the geodetic effect--that is, an alteration of the orientation of an orbiting body around a mass, due to the mass' gravitational effect on spacetime--to an accuracy of better than 0.5%, meaning the data from the experiment was only 0.5% off of what general relativity predicted would happen. Another triumph for experimental space science and general relativity!
 
However, the frame-dragging effect--due to the rotation of the gravitating mass, not just its non-rotating gravitational (geodetic) effect--is "170 times smaller" than the geodetic effect, so it's much harder to measure, and the results at that level of accuracy were muddied by a non-uniformity of the niobium coating on the gyroscopes, which gave them an "electrostatic axis," causing them to rotate just ever so slightly, "of a magnitude similar to the expected frame dragging effect." So they had to figure out just what effect that had on the results, and calculate it out before they could see what effect frame-dragging might have had.
 
This page on Stanford's site shows the measured rotation of the gyros as nifty gif animations, and later, over here, in November 2009 (convenient since their final funding, secured Saudi Arabia's science and technology complex after NASA's funding ran out in 2008, was due to run out in December 2009) they announced that they'd confirmed the existence of the frame-dragging effect predicted by Einstein's theory of general relativity, within 14% accuracy of the predicted values. (Wikipedia editors, maybe you should update your article! :D)
 
So...maybe good! Frame-dragging experiments have also been run by the Mars Global Surveyor, and even before that, by the LAGEOS ("Laser Geodynamics Satellites"), two Earth-orbiting probes launched in 1976 and 1992 "to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth," both of which are still in service! The LAGEOS look really neat:
 
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image by NASA (source)
 
And the reason why they're still in service--and for practical purposes may still be for another 8.4 million years, at which point they'll have drifted back down into the atmosphere--is because they don't have any electronics or mechanisms in them at all: they're just 411 kg, 60 cm, aluminum-covered brass spheres. Because they're so simple, and their mass-to-area ratio is so high (they're dense!), their orbit is very predictable, and they're "the most precise position references available." You fire a laser beam up to them, measure the time it takes to reflect back, and that tells you your exact position to a very high degree of accuracy; from that, you can model the movement of the Earth's crust, and so forth.
 
But anyway the frame-dragging results of both the MGS and LAGEOS experiments are apparently highly contested. Due to the irregularity in the niobium coating of the gyros, GP-B's results may be somewhat contested as well (?), so I suppose we're still waiting for a really really good experimental confirmation of frame-dragging.
 
(Boy I really only meant to link that Space.com article--darn Internet makin' me look stuff up! :P)
 
 
 
 
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