I was reading an article in Natural Science (I think) talking about the experiments being preformed by Gravity Probe B (GP B). It went to explain the history of the project and the experiments performed by GP A back in the 70s. One of the experiments had to do with two synchronized atomic clocks. When the probe was recovered, the clock on GP A showed that it tracked time at a different rate than the one left on Earth. Now according to the article, it said that resulting difference in time tracking was caused by the difference in gravity experienced by the two clocks, thus proving a portion of special relativity.

I was under the impression that the difference in time was caused by a slight time dilation caused by the clock on the probe traveling at a greater relative speed to the clock on the ground. Am I wrong in this assumption?

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Both the speed and the gravitational potential affect time - I gather from descriptions that for GP-A the latter was more significant than the former.

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Here, see this Stanford website NASA news release about the gravity effect on the atomic clock:

LINK

It only mentions the gravity related clock rate change. Atomic clocks speed up in weak gravity and slow down in strong gravity. This is basic electrodynamics. This article says nothing about a “relative motion” related clock rate change.

Actually, the GP-A was designed to test gravitational time dilation, not relative time dilation. The test used three channels to compare the time shifts, which allowed them to isolate the gravitational time dilation from the relative time dilation.

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Hi Tensor,

Could you explain that please and provide some links about it? Are you saying that since the 1972 Hafele-Keating flying clocks experiment, NASA has found a way to insulate an atomic clock against the effects of “relative motion” time dilation? Can you explain how they do it?

Sam5, Tensor said isolate, not insulate.

Hi Tensor,

Can you explain to us how NASA “isolates” the gravitational time dilation from the relative time dilation?

Free willy!!!! =D> adn alien was here =D> =D> =D>

good question

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Sorry for the delay Sam5. I was out of town and just got back today. I had a portable with me, but somehow missed this one (also, the portable doesn't have any of my refereces on it). Here's the link that describes the Gravity Probe A experiment, and how the it was isolated.

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Hmm, on page two the second term of equation #1, looks to me like they just subtracted what they called the SR effect. That’s the same thing Hafele and Keating did. Is that what you mean by “isolation”?

Yep. That effect was measured two ways, validated and then subtracted out.

Not quite. Hafele and Keating calculated both effects and then compared their calculations with the observed differrences in the clocks. In Gravity Probe A, the SR effect was measured and then subtracted out of the spaceborn maser before the comparison with the earthbound maser. Different methods.

Well, if you have two effects and measure one and subtract that one out, that does isolate the effect you are trying to measure.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

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In your post above, you said, “Actually, the GP-A was designed to test gravitational time dilation, not relative time dilation. The test used three channels to compare the time shifts, which allowed them to isolate the gravitational time dilation from the relative time dilation.”

This suggests they used some fancy “isolation” method. You didn’t say anything about just “subtracting” the other numbers.

Anyway, the link I gave doesn’t say anything about it either. It doesn’t mention any SR effect.

link

And why did you feel compelled to link me to that complicated paper when all you had to do is say, “They subtracted it”?

Did you not think I could find the subtraction in that paper?

Why? Because that is they way you read it? They wanted to measure Gravitational Time Dilation. They couldn't just measure the difference between the maser in flight with the maser on the ground because of the time dilation due to motion would have been included. The have to isolate the Gravitational effect. Which they did.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

[quote="Sam5"]Well, when you said:

I thought you wanted a link, so I provided one.

I thought the link was self explanatory, which is seems it was. They explained the method better than I could.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

I was referring to links regarding what you said about Gravity Probe A. You gave me a link to a paper about a 1980 rocket test.

The paper was published in 1980. It refered to the launch of the Gravity Probe A experiment, which was conducted in 1976.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Thanks. When did they start calling the 1976 Scout rocket test “Gravity Probe A”? The books I have that mention it don’t call it Gravity Probe A.