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Is the Earth in a Vortex of Space-Time?
da6d writes "Apparently, we'll soon know for sure.... NASA has announced in an article that 'A NASA/Stanford physics experiment called Gravity Probe B (GP-B) recently finished a year of gathering science data in Earth orbit. The results, which will take another year to analyze, should reveal the shape of space-time around Earth--and, possibly, the vortex.'" More from the article: "If Earth were stationary, that would be the end of the story. But Earth is not stationary. Our planet spins, and the spin should twist the dimple, slightly, pulling it around into a 4-dimensional swirl. This is what GP-B went to space to check."
Actually, it doesn't have to do with universal reference frames in the sense you mean. In Newtonian mechanics, there is a limited set of preferred reference frames within which Newtonian physics is valid - the inertial reference frames, or, casually speaking, the ones moving at a constant velocity - none of which is a "Universal" or better reference frame than any other. But even in Einstein's model, which incorporates accelerated reference frames in the same framework as inertial, there are still "preferred" reference frames; non-rotating ones. ROTATING reference frames lead to unambigious differences, both in Newtonian and Einsteinian models. While sloppily written, the article means that it is the ROTATION of the Earth's reference frame that leads to different predicted results, not the TRANSLATIONAL motion. Not all reference frames are created equal.
Behind the Gravity Probe B is here and here . It is a fascinating read, esp. about the gyroscopes.
"The four gyro rotors are made of fused quartz, fabricated to an extreme level of material homogeneity and then ground to the near-absolute sphericity (Figure 1). The spheres are round to within 40 atomic layers, which is proportionally equivalent to an Earth-sized sphere with surface height variations of only 16 feet...."
"It's one thing to have a virtually perfect gyro rotor, but that alone does not provide the necessary performance for this experiment......The electric fields center the rotors to a few millionths of an inch. They did not perform the spinning up electrically, however. Instead, they directed a precise stream of helium gas, traveling at nearly Mach 1, at the rotors. It takes about half an hour for the rotor to reach full speed, and it loses less than 1% of this speed over 1000 years in the super-vacuum of the cavity."
The "vortex" is a analogy. The relativity formulae predict a behaviour of the universe that this probe is trying to observe. If "vortex" makes you dizzy then think corriolis effect or centrifugal forces, both of which are bogus simplifications humans use to describe messy bits of Newtonian mechanics. So, think of the vortex as a simplified mathematical/physics construction to describe some horrifically complicated equations.
Oh, and don't try this experiment in Kansas. Relativity is only a theory, after all.
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The idea of relativity is that no frame of reference is "special". Working this out for frames that differ by a constant velocity is pretty straightforward, but the situation with respect to rotation isn't so straightforward. If you spin yourself around you will quickly find that there does seem to be a special frame that doesn't make you dizy, which we call the non-rotating frame. To know that you are spinning, you don't appear to have to measure your rotation relative to anything else.
Einstein had the idea that really, rotation is relative, too, and this apparently special non-rotating frame is really just the frame in which you are not spinning relative to the other bodies in your region of space time. In other words, seen from a different region of the universe, maybe our region of the universe is spinning furiously, but we don't notice it because all the bodies near us are all spinning furiously together.
When you work out the math in the context of general relativity, the implication is that, near a big spinning body, for you to feel like you are not spinning you actually needs to be spinning slightly relative to what would fell like not spinning far away from the body. The effect is called frame-dragging. This experiment tried to measure the frame-dragging effect of the Earth on some extremely precise gyroscopes in orbiting satelites.
There's no reason *not* to be confused by the article. It's a pretty subtle phenomenon, described in an astoundingly sloppy writeup. Hard to believe it took three people to write something which is neither complete nor coherent, and which doesn't even give you enough key words to search for more information.
The Gravity Probe B homepage has a far better introduction to the experiment. (Go to "classroom" -> "story of GPB" for a concise intro.)
http://einstein.stanford.edu/
In short, general relativity predicts that a massive rotating object (like the earth) distorts the space around it in such a way that nearby objects that are locally at rest are actually rotating slightly when compared to distant stars. (Locally at rest means that, for example, if you put some guy in a box with any measurement apparatus he could imagine, his measurements would show that the box isn't rotating.) This doesn't happen in Newtonian physics, and Gravity Probe B should be able to measure it and compare it to what one predicts using GR.
The effect is usually called "frame dragging," or the "Lense-Thirring Effect."
It's not even so much relativity at all; these principles were understood before 1900. If you're in a spinning reference frame, things work differently. Spinning your reference frame is not the same as moving it at a constant velocity -- while you can't tell how fast you are moving in the absolute sense, you can tell precisely how fast you are spinning.
GP is needlessly complicated. All he needs to say is that spinning reference frames are not on the same footing as non-spinning ones. Stand on a merry-go-round with a toddler and film him trying to walk around and you'll see that there's definitely a difference between spinning frames and stationary ones. Also, you'll see a toddler fall over, which is always funny.
xkcd.com - a webcomic of mathematics, love, and language.
Laser ring gyros verify the Sagnac effect in relativity, not the Lense-Thirring frame dragging effect. There have been some crude verifications of the latter so far (by the LAGEOS laser ranging satellites for Earth, and some astrophysical observations of accretion disks for black holes), but Gravity Probe B will be the first direct, precision test of the phenomenon.
The experiment isn't so much to test if earth is in a so called "space-time vortex" (a nicely hyped up, futuristic word, btw), but to verify (and test) the effects of frame dragging. Or, so said the research project's leaders some time ago when they began the mission. Books like "The Fabric of the Cosmos", "The Elegant Universe" (both by Brian Greene) and "Three Roads to Quantum Gravity" (by Lee Smolin) mention this experiment, and do a somewhat better job of explaining what frame dragging is, and its implications for physics.
No, this is the bizarre thing that special relativity tells us. You may be moving at 0.999c relative to me- but if I shine light from my flashlight, it will move at 1.0c in both directions relative to both of us. Light does not move at c in an absolute frame of reference. Light move at c in ANY "inertial" (non-rotating) reference frame. You are agreeing with Newton and Galileo, not Einsten.
This is precisely what general relativity says.