Saturday, July 28, 2007


The most spherical object ever made... used for the gyroscopes in NASA's Gravity Probe B. Launched in April 2004, Gravity Probe B tests two effects predicted by Einstein's theory: the geodetic effect and the frame-dragging (see here for a brief intro).

In order for Gravity Probe B to measure these tiny effects, it must use a gyroscope that is nearly perfect—one that will not wobble or drift more than 10-12 degrees per hour while it is spinning.

"A nearly-perfect gyroscope must be nearly perfect in two ways: sphericity and homogeneity. Every point on its surface must be exactly the same distance from the center (a perfect sphere), and its structure must be identical from one side to the other [...]

After years of research and development, Gravity Probe B produced just such a gyroscope. It is a 1.5-inch sphere of fused quartz, polished and “lapped” to within a few atomic layers of perfect sphericity. A scan of its surface shows that only .01 microns separate the highest point from the lowest point. Transform the gyroscope into the size of the Earth and its highest mountains and deepest ocean trenches would be a mere eight feet from sea level!"



  1. Now wave your hands around it BEE
    Look into the gazing ball, and tell me what you see ...

    Gravity is it Real
    Is it particulate reality

  2. Beeautiful Sun filled day here, hope its headed your way

  3. Dear Quasar:

    Thanks :-) Unfortunately it looks rather rainy here (a typical German summer I'm afraid). What I see in that sphere? R2 completed at infinity is what comes into my mind. What do you see?

    Have a great day,


  4. Hi,

    "to only a few atomic layers away from perfect spherity"

    "0,1 micron (0,0001mm)between highest and lowest"

    And they couldn't do better??!

    How thick are your atomic layers Bee?




  5. I looked itup, and a typical Quartz crystal has a lattice spacing of about 0.5 nm so they polished it to an accuracy of about 200 atomic layers. Which compared to the 76 million atomic layers the whole thing has is certainly, "a few".

  6. BTW, if you never want to complain about rain again, spend a year in England, after that everywhere else will seem just perfect to you ;)

  7. Black hole on my desktop.

    I am wondering how they verify that the drift is only 10^-12 degrees per hour.

  8. Wow, look up Benjamin Lange, writing about this.

    "In addition if a nearby reference star is chosen, it is sensitive enough to possibly detect earth-sized planets."

    Simply follow a star with a telescope coupled to this gyroscope and the gyroscope will detect the wobble of the star 'cause of orbiting planets!!!!!

  9. June 2007 mission update
    "known as 'misalignment torques'

    Somebody didn't tell them about patch potentials and static tribocharging of dielectric surfaces in a dry gas stream. Blessed are the paranoid for they will have made backups.

  10. soooo shiny...

    must pick up and play with...

  11. Due to the first effect Uncle Al mentioned, the preliminary results announced for Gravity Probe B last April were a disappointment, with no measurement of frame dragging. Fortunately the system is redundant (4 identical gyros) and they say it will be possible to model and subtract most of the unexpected electromagnetic perturbations.

    A frame-dragging measurement is expected by year's end, hopefully approaching the experiment's intended accuracy. Where Michelson-Morley found a null for ether dragging, I'm predicting GP-B will find the same for frame dragging.

  12. Dear Bee,

    thank you for reminding us of this!

    By the way, I remember that attempts to provide new standards for the Avogadro constant and the mass of a "kilogram" also involve nearly-perfect spheres, made of silicon, in this case - see this link from the PTB, the German equivalent of NIST.


    concerning deviations from the perfect sphere, it is 0.01 micrometer, or 10 nanometer - with the lattice parameter of quartz mentioned by fh, that is consistent with the less than "40 atomic layers from perfect" mentioned in the documentation Bee has linked to.


    there was no frame dragging measured? I thought they had something preliminary "as expected"?

    Best, stefan

  13. Oooooh, smooth reflective spheres! They also function perfectly as an abstract photographer's device.

  14. Hi Stephan,

    As Bee noted, Gravity Probe B measures two effects: A geodetic precession, where the gyros precess north-south, and frame-dragging, where they move east-west. The first is orders-of-magnitude larger and requires much less precision to be detected.

    So far, the only result announced has been confirmation of the geodetic effect. (Already confirmed to comparable accuracy by observations of the Moon's orbit.)

    At the April APS meeting, there was a poster showing "glimpses" of frame dragging based on data from gyro #3. It's here.

    However, the data from other gyros was different. The principle investigator, Francis Everitt, said that preliminary analysis of #3 should not be taken seriously as a possible indicator of frame-dragging.

  15. I should add the quotation marks around "glimpses" are not mine, but were on the original poster.



  16. Hi Kris,

    thank you for these explanations, and the link! So what is seen in the data so far is the de Sitter precession, which is not so spectacular ...


  17. Hi Stephan,

    It's true the geodetic and de Sitter effects are different names for the same thing. The other precession GP-B is measuring can be called the Lense-Thirring effect, gravitomagnetism, or frame-dragging, depending on how you feel that day!

    Best, Kris

  18. Oops. Sorry for misspelling your name!

  19. Hi Kris,

    thanks again for your comments, and...

    Sorry for misspelling your name!

    Keine Ursache :-) Happens quite often, even here in Germany.

  20. I also misspelled "principal investigator." But maybe you could say Francis Everitt really is a "principle investigator."

  21. The Cosmic Microwave Background varies by one part in 10^5. That is equivalent to Earth's altitude varying by 63 meters. One could argue that the cosmos forms a nearly perfect sphere.

  22. The New Scientist has a short note about the "perfect spheres" of silicon used in metrology. It says the National Measurement Institute of Australia in its attempt to define the standard of the kilogramm by "counting" the number of silicon atoms in a sphere procduces spheres [..] round to within about 50 nm and the mean diameter of the NMI sphere has been measured to about 2 nm. (If the sphere were as big as the Earth its diameter would be known to about 200 mm, and the highest mountain would be less than 5 m high.)

  23. Hi Amara:

    What a nice photo! I was totally fascinated by the 'Cloud Gate' in Chicago, see e.g. here. Best,


  24. Dear Bee, And to go to infinity.. (!), another version of the Cloud Gate, this time by Anders Sandberg.


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