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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."
Live long and vortexed?
"If Earth were stationary, that would be the end of the story. But Earth is not stationary."
Are you on crack!? The earth is stationary. It is the sun that's moving.
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.
So, your problem with the story is that it's too geeky? Which part of "News for Nerds" is opaque to you?
f -External-Forces Greek chap, but humanity has learned the lack of wisdom of that approach several times over.
Proving things that are suspected to be true is the meat-and-bones of science. After all, they might turn out to not be true. Idle speculation may have been good enough for Aristotle the Things-Will-Stop-Moving-Without-the-Application-o
My other body is also not wearing any.
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."
I think it's interesting - general relativity makes some very hard to verify but specific predictions. Many competing theories to it over the last 50 years have made predicitions that have, one by one, turned out to be false. Rotational frame dragging is (I think?) one of the last unverified ones. According to Newtonian gravitation & mechanics, the rotation or non-rotation of the earth should not affect an orbiting satellite a whit (ignoring "complications" like variable atmospheric drag based on rotation rate, different shape of earth at different rotation rates, etc.), or put more abstractly, the rotation of an axially symmetric mass distribution should not have anything to do with its gravitational field. General relatitivity does not agree with Newtonian mechanics here, which brings up yet another interesting question:
Is there a difference between rotating reference frames and non-rotating reference frames because of the universe of matter around them, or is it self-generated? In other words, if we "removed" the entire universe except the rotating Earth, would rotation still have meaning? Could we still do an experiment and detect its rotation, or is that an artifact of the universe of matter around it that would vanish when it did? As far as I understand general relativity (and IANAP), it does not make a hypothesis one way or the other. Is the question meta-physical? Or is there some clever way to set up an experiment to actually tell?
Sigh - sometimes, I wish I was a physicist!
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."