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Gravity Probe B Webcast: Mission and Science Q&A
Question and Answer Board

Junichi from Niihama-City
Would the gravitational waves by ripples in time-space cause the gyroscope of Gravity Probe B to indicate the minute changes?
That's an interesting question. Gravity Probe B is actually not a gravity wave experiment and it's really not sensitive enough in that manner to do gravity wave detection. Indirectly, of course, gravity waves do affect everything in the universe, but GP-B can't really measure that effect with any precision.

There is actually a research theme in the Office of Space Science called Beyond Einstein that has a series of missions, and one of those missions is called Lisa, and its purpose is to detect gravity waves. In order to do that, you have to have a different kind of very special instrumentation that can detect gravity waves. That means being able to understand the differences between spacecraft on a very precise level.
Rich from Elk Grove, Illinois
Simple question: Why is this important to the scientific community and everyone else?
There are a couple of reasons why we want to do Gravity Probe B, really. One of the reasons is that general relativity, in contrast to popular belief, is actually a fairly weakly tested theory. It's a foundational theory but not well tested, so we have to test it. It is a counter-intuitive and strange theory in some ways, so certainly scientists are compelled to test it. The feature we are measuring with GP-B, the frame dragging effect, has never been measured or tested before.

I think the second reason is that we know that cosmological models -- models that describe how the universe operates -- will eventually break down. Newton's model broke down, and scientists believe that Einstein's model will break down at some point, because it's not reconciled with quantum mechanics.

So it's possible that Gravity Probe B will find the place where general relativity does break down.
Lucas from Waldo
Will this probe help you understand the inner workings of a Black Hole?
Gravity Probe B is not a black hole experiment per se, but being able to understand a black hole depends on certain aspects of general relativity. So this is really foundational work that helps us to understand the nature of gravity and whether or not general relativity is a valid theory in all of its aspects. That understanding does help us to understand black holes in an indirect way.
Shigemi from Tsukiyono
How to divide the effectiveness of general and special theory of relativity. (Revolution's effect)
I think the questioner is asking about what are the differences between the two theories. Gravity Probe B is an experiment that has to do with general relativity. Special relativity had to do with reference frames, and it had some important and very famous consequences, like the equivalence of mass and energy equals MC squared, and things like time dilation and other kinds of strange things that had to do with frames of reference...Also, the speed of light as a constant limit in the universe. General relativity is really a theory about gravitation. That's what led Einstein from the special theory to the general theory: there were incompatibilities between special relativity and the Newtonian model for gravity. So that's how you break the two out, and I think in its simplest terms, general relativity is really a gravitational experiment.
Jim from Ontario
Will this test help me understand what gravity is?
Well, absolutely. As I discussed earlier, this experiment is measuring two aspects of general relativity related to gravitation. One is called geodetic procession and the other is called frame dragging. Geodetic procession has been measured before, but never this precisely. This will improve that measurement by some factor. The frame dragging effect, which is this counter-intuitive notion that somehow space and time are dragged around with the rotating mass has never been quantitatively measured before. So it will certainly help us to understand gravity in a very detailed way in these subtle aspects.
How were the pingpong-sized balls of quartz polished (to a perfect sphere)? By laser beam? How?
These spheres we use, these gyroscope rotors about an inch and a quarter -- about the size of a ping pong ball -- they are nearly perfect. We believe they are the most perfectly round spheres ever manufactured. They are not completely perfect but they almost are. If you looked at the imperfections in those spheres and you blew it up to the scale of the Earth, the distance between the deepest ocean and the highest mountain would be only a few feet.

So they're very finely machined and polished, but we didn't use lasers or anything exotic like that. We actually used kind of standard machining techniques, and the way we got the roundness and the flatness on that surface had to do with the polishing technique. We had to invent a special piece of equipment that uses a cam and a rotor type of system to manipulate this rotor, and then we had a special slurry, which is kind of a polishing compound we used to polish it, and we literally polished these things for weeks and for months to make them as round as they turned out to be.
Luis from Long Beach
How can you know measuring changes in the paths of the spheres won't be significantly affected by the observation required for measurement?
I think Luis' question is probably relating to the Heisenberg Uncertainty Principle, which says that any time you observe an experiment, you affect it in some way. That's true, but that affect really only matters on the quantum mechanical level. In other words, if you get to the atomic or subatomic scale, then Heisenberg's principal can come into play.

In the case of Gravity Probe B, the real issue that we had was trying to make sure that so-called classical disturbance torques, the Newtonian torques, are absolutely minimized or eliminated, because the relativity effects that we’re trying to see are so subtle and so small that we really have to get rid of any other systemic noises.

That's the kind of issue we had to deal with and Heisenberg doesn’t really play into that significantly here.
Colin from Dayton
Could this kind of experiment be performed on the ISS?
Actually, it could not. In principle, what you need for Gravity Probe B is a very quiet platform, and the ISS does give you a microgravity type of environment (in other words, a very low-gravity environment), but GP-B requires an even quieter environment. Probably three or four orders of magnitude, in other words 1,000 or 10,000 times less noisy or less motion oriented than the space station is. So you really couldn't do it on the space station, even though that platform is suitable for other kinds of high-precision instrumentation.