Gravity Probe B v.s. Sumatra Quake

[backspace]

Forgive me if this has been answered elsewhere, but is it possible that the effects of the Sumatra quake may have screwed up some of the data / configuration for Gravity Probe B?

 

[bobvanx]

Hmmm.<br /><br />Since we hypothesize that a nuetron star quake would radiate gravity waves, and they'd be detectable across a vast distance, perhaps a nearby earthquake would be detectable?<br /><br />Gravity Probe B isn't measuring gravity waves. It's measuring the curvature of space-time where the reference frame is being dragged by the rotation of a massive object.<br /><br />Since the Earth's rotation was disturbed minutely (if at all) I doubt the quake itself makes a bit of difference.<br /><br /><i>However</i><br /><br />The Earth's gravity field is "lumpy." There's a wide, shallow place over the Indian Ocean, which causes satellites to shift in their orbits. The shape of this region has changed (again, very slightly) from the movement of the landmasses. The built-in capability of the satellite to remove all external acceleration forces (except the ones they are trying to measure) will compensate just fine.<br /><br /><i>And</i><br /><br />If something interesting did pop out of the data, it would be a wonderful, exciting thing, since it would tell us we have more to learn.

 

[backspace]

Well, if the gyroscopes are as precise as they say they are, and the speed of the Earth's rotation increased, it would stand that the frame dragging around the earth would have picked up speed as well, and that change would shift the way the instruments needed to be calibrated, non?

 

[mrmorris]

From what I've read of GPB -- determining if frame-dragging really happens will be close to the threshold of sensitivity for the instruments. <b>If</b> the Sumatra quake increased the rotation of the Earth -- it did so at the very threshold of detection, and any changes to the frame-dragging will be <b>extremely</b> small.<br /><br />It's unlikely in the extreme that GPB then will notice or be affected by a tiny change in an effect that it can barely even determine the presence or absence of.

 

[bobvanx]

mrmorris,<br /><br />That's my understand as well. Those very precise gyros are hard-pressed to detect the much larger effect of a 24-hour rotation of a large mass. Since that rotation was changed less than 20microseconds, if at all, and as Maddad so enthusiastically points out in his arguments that the effect is transient, I'll bet it's a non-issue for Gravity Probe B.<br /><br />Here, there are<br /><br />86400000000 microseconds in a day, and the effect of the quake was less than 20 microseconds.<br /><br />Gravity probe B is trying to measure a change in its gyro's orientation that swamps that ratio.

 

[meteo]

"that the effect is transient" for rotation right<br /><br />I think from a geological perspective yes, but for several years I'm not sure.<br /><br />http://www.ecgs.lu/pdf/jlg92/JLG92_Gross.pdf (page 16)<br />I think this shows that the "earthquake effect" changes in jumps, caused by other earthquakes. Regardless there are atmospheric and ocean and Chandler cycles of polar motion so I'd think this was in mind when the probe was launched.<br /><br />From the USGS<br />http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html<br /><br />Answer: Richard Gross at JPL has modeled the coseismic effect on the Earth's rotation of the December 26 earthquake in Indonesia by using the PREM model for the elastic properties of the Earth and the Harvard centroid-moment tensor solution for the source properties of the earthquake. The result is:<br /><br />change in length of day: -2.676 microseconds<br />polar motion excitation X : -0.670 milliarcseconds<br />polar motion excitation Y: 0.475 milliarcseconds<br /><br />A lot less than 20microseconds<br /><br />http://www.oso.chalmers.se/~hgs/ESST/2003/Additional_material/Oceans/spacelod_mei.jpg<br />Note that the graph is in miliseconds! The effect the earthquake had on rotation is really insignificant.