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I have a question about the gravity probe B experiment.

According to this site:

The measurements they made confirm Einsteins theory. What I'm wondering is:

  1. Did they use a control in the experiment? That is did they put gyroscopes in some place where space time was not supposed to be twisted and get a measurement that was not deflected?
  2. If they didn't measure a negative result too, how does this the experiment prove the theory correct?
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Well, it certainly didn't prove the theory was correct, but it did provide evidence supporting the theory. Sorry to be pedantic, but I think that's a distinction worth making (throughout science, not here in particular). To be specific, general relativity predicted that the gyroscope should precess at a certain rate, and the experiment confirmed that prediction. As far as controls are concerned, I'm sure that they tested the gyroscope in different circumstances (such as before launch), but I don't know details. – Ted Bunn May 21 '11 at 0:11
Ted makes a good and very important point. Nowhere in the article does it say that any theory was proven correct. In fact, it doesn't even say that the measurements "confirm" the theory. the only use of that word is "Researchers confirmed these points at a press conference..." i.e. the researchers verbally confirmed the expected result that the measurements taken by the probe match the predictions of GR. – David Z May 21 '11 at 2:10
Dear David, you have diluted what has happened and what has been shown in such a way that 1) one loses the idea why the experiment would be paid for in the first place, and 2) one isn't sure what you wanted to say. Were those the goals of your remarks? ;-) It's surely another confirmation of GR in the regime where we expected it to be correct, anyway, but with every test that is independent of the previous ones, at least partially, our confidence in GR increases. – Luboš Motl May 22 '11 at 16:20
@Ted Bunn: This idea, "falsificationalism" is a really dumb idea of philosophers. It is no different to verify an idea than to falsify its negation. The distinction between the two processes is one of those things that appeals to people for bad reasons. – Ron Maimon Dec 29 '11 at 14:15

Since the other answer is just questioning the errors with a belief statement,

we thus believe the measurement errors of both the geodetic effect and the frame-dragging effect should be close to each other.

one should point out that science is not about beliefs but about measurements.

The functional dependence of the two quantities whose errors differ by a factor of 2.5 could easily explain the difference if we had the formulae.From the linked announcement:

"We measured a geodetic precession of 6.600 plus or minus 0.017 arcseconds and a frame dragging effect of 0.039 plus or minus 0.007 arcseconds," says Everitt.

For readers who are not experts in relativity: Geodetic precession is the amount of wobble caused by the static mass of the Earth (the dimple in spacetime) and the frame dragging effect is the amount of wobble caused by the spin of the Earth (the twist in spacetime). Both values are in precise accord with Einstein's predictions.

We see that the values themselves have a large difference and the geodetic precession is measured much more accurately than the frame dragging .

> The gyroscopes will certainly have been tested that they showed no deflection when in lab conditions. There are extensive descriptions of the construction of the gyroscopes and the experiment.

From the question:

If they didn't measure a negative result too, how does this the experiment prove the theory correct?

I am answering this to make clear that a theory can never be proven correct. It can only be proven wrong. A theory is continually tested by new data and one can say "the new data confirm the predictions of the theory", but confirmation is not proof. Even one good measurement that contradicts a theory falsifies it, i.e. proves that it is wrong at some part of its domain of definition, if the measurement is repeatable.

Measurement errors are a part of what defines a domain of definition. The domain of definition has to be examined and the limits found. Example: Newtonian mechanics and General Relativity, where Newtonian mechanics are a limiting case of General relativity, valid for specific domains.

Any alternate theories could be toyed with within the errors, but that does not mean that the confirmation of GR is false because there are experimental errors

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A Question on Gravity Probe B Experiment Results

Dr. Hao Shi


The final results of the Gravity Probe B (GP-B) experiment for testing general relativity (GR) theory published on May 4, 2011 by NASA is somewhat controversial. Since GP-B scientific sensors have symmetric property about the satellite spin axis, we thus believe the measurement errors of both the geodetic effect and the frame-dragging effect should be close to each other. However, in the published results, the former is 2.5 times of the latter, which has not been explained by GP-B final report and thus shows that some physics is probably still missing or inadequately addressed in processing experimental data.

Quoted from J. of Beijing University of Technology 2011-10

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Here is a link to the journal (although I haven't been able to retrieve the article) – Qmechanic Jan 28 '12 at 15:46

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