Einstein's theory of General Relativity has been around for 93 years, and it just keeps hanging in there. With advances in technology has come the ability to put the theory under some scrutiny. Recently, taking advantage of a unique cosmic coincidence, as well as a pretty darn good telescope, astronomers looked at [...]

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No - specifically, the gravitational wave experiments from the LIGO class of telescopes were expected to detect gravitational waves in our galaxy by now, with somewhere between one and fifty percent confidence. Each day of none-detection does not disprove GR; but neither does GR get a passing grade until gravitational waves are detected.

Also, the Gravity B probe failed to detect GR frame-dragging, and as of now, the APOLLO Lunar ranging GR testing is not consistent with predictions. In both cases this is thought to be due to systemic errors, but what if it isn't? We also had the Pioneer 7 GR test failure (thought to be due to solar atmospheric effects), the Cassini GR test (that was announced and then withdrawn).

GR has not passed every test, but neither has it failed - but in many cases where the results have been different that expected (including, by the way, GPS satellite calibrations), the error margins have been greater-than-expected. This does not mean GR has failed any tests, but if every time the results are 'wrong' the error margins are widened, how will we ever know?

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jwj

If you always believe what you already know, you can't learn anything - Liz

Jerry, if your point was that testing our current gold standard theory of gravitation remains a priority, however many tests it has passed to date, then I agree. If your point was that the UT article didn't provide a thorough review of all the available evidence, granted, although of course it clearly was not intended to do that.

Here are some recent reviews of the available evidence:

• Slava Turyshev, Experimental Tests of General Relativity, Annu. Rev. Nucl. Part. Sci. 58 (2008): 207-248,
• Orfeu Bertolami, Jorge Paramos, Slava G. Turyshev, General Theory of Relativity: Will it survive the next decade?, Lasers, Clocks, and Drag-Free: Technologies for Future Exploration in Space and Tests of Gravity, H. Dittus, C. Laemmerzahl, S. Turyshev, eds., Springer , 2006,
• Clifford M. Will, The Confrontation between General Relativity and Experiment, Living Rev. Relativity 9, (2006): 3,
• Clifford M. Will, Was Einstein Right? Testing Relativity at the Centenary, in 100 Years of Relativity: Spacetime Structure - Einstein and Beyond, Abhay Ashtekar, ed., World Scientific, 2005

That description is potentially misleading: the problem was the GP-B apparently failed to test the frame-dragging effect at all, because some unanticipated subtle but conventional physics effects appear to have spoiled the data. The GP-B team tried to eliminate the unanticipated sources of error, but tragically, a recent review concluded that the experiment cannot be saved. This is very unfortunate, but science is very hard, and some experiments fail. So astrophysicists will simply have to try again.

You should also have mentioned that there are some (slightly controversial) claims to have confirmed frame-dragging by other means.

Same comment: science is very hard, and some experiments fail. You should be happy that scientists recognize failure when they see it.

Citation? Does your source contradict this review?

Juergen Mueller, James G. Williams, Slava G. Turyshev, Lunar Laser Ranging Contributions to Relativity and Geodesy, op cit.

C'mon. These are tiny effects, and it is very difficult to eliminate conventional effects which are typically much larger. This is particularly true when researchers try to "piggyback" on data acquired during a mission primiraly designed for another task, since such data typically don't yield a clean test of the effect of interest to gravitation physics researchers.

In particular, the GPS system is neither designed nor operated as a test of gravitation theories.

This too seems misleading to me. Leading researchers continue to state their expectation that gravitational wave astronomy will soon become a reality. In the worst case, if the interferometric detector design (or analysis) turned out to be horribly flawed, there are any number of (more or less implausible to be sure) alternative design proposals out there. It's a judgement call, but I'm still inclined to give the LIGO/VIRGO teams more time.

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Yes, I'm very interested in what Jerry means by LLR data not being consistent with GR predictions. There are some papers by Nordtvedt that say that the motion of the moon is within *2cm* of GR predictions, and further that frame-dragging/gravitomagnetic effects are confirmed by that to high confidence (I remember somebody referred to these Nordtvedt papers as "we don't need no stinking Gravity Probe-B). I don't know if that's "slightly controvesial" or not. Basically, when Nordtvedt and certainly Will say something, I'm pretty much going to believe it. IIRC, Kopiekin of the (in)famous "speed of gravity" test with Jupiter criticized that, but I took that as mostly sour grapes since they criticized and disproved his speed of gravity claim.

One frame dragging claim I would consider *very* controversial was one made by Iorio that he pulled frame dragging out of some Mars probe orbital data, which was just nuts. Iorio was always pretty good I thought (he had some papers saying the motion of the outer planets ruled out a lot of alternative gravity theories, such as Moffet and Brownstein's STVG), but I think he got in some sort of food fight with some others and that was the result.

ETA: http://arxiv.org/abs/gr-qc/0702028

and http://arxiv.org/abs/0705.0513 for a reply to Kopeikin

and here's an earlier one:

http://arxiv.org/abs/gr-qc/0301024

IOW, solar system motion and GR fit like a hand in a glove.

-Richard

Ditto; Iorio continues to write weekly eprints defending his viewpoint. Unfortunately, this unseemly arXiv dispute has already erupted into a Wikipedia edit war, so we probably need to help the moderators avoid this controversy from disrupting BAUT. (The Kopeikin controversy has also caused recurrent problems at WP.)

...sigh...

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Oh my lord. I just looked at the frame dragging Wiki discussion page. I had forgotten exactly what was going on, but I remember it. Iorio was in a fight with Ciufolini who was trying to find frame-dragging in LAGEOS data, I think.

If I remember, Iorio said their error was much greater than they said, all the while he was ridiculously underestimating the error in the Mars probe thing.

Food fight, indeed.

-Richard

Yeah, it makes you cringe, doesn't it?

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

This seems to be what has Jerry excited about LLR:

http://en.wikipedia.org/wiki/Apache_...eration#Status

I note Nordtvedt is in this APOLLO group. This is a paper from the horses' mouths:

http://arxiv.org/abs/0710.0890

The moon's motion is known to the 2cm level, which is just absolutely fraktacularly amazing (a little BSG lingo, there) and its motion agrees with GR predictions in that range.

But now they're trying to improve the precision to down to the mm level, which means any early differences are on that mm level. Jerry is making a mountain out of a millimeter.

Now, when they get it worked out, if Nordtvedt et all come out with a paper that says they are confident that the Moon's motion disagrees with GR predictions at mm levels to high confidence, then we've got something. But that will be a *long* way off, if ever.


-Richard

I'm not trying to be hostile or blow anybody any atitude but I learn more from the replies to Jerry's posts than if he hadn't posted. Almost like a professional sock puppet as per definition two, (or three).

As you have never done me any harm Jerry, I'm sorry about the implied insult.

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"The beauty of that discussion of averages is that you don't have to be an expert in Apollo or in photography in order to see where this time study "analysis" breaks down. You just have to be, well...not an idiot." -JayUtah

Good find, publius!

The sentence which leapt out at me was:

Thanks, Don, for the compliment! Er... someone who knows BAUT tells me Jerry gets teased a lot around here...

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Here's the APOLLO web site, authored by Tom Murphy, APOLLO project director:

http://physics.ucsd.edu/~tmurphy/apollo/apollo.html

The Wiki discussion page is short and simply says that Tom Murphy gave permission to use that page and the team's papers to create the Wiki article. So the "apparently well-informed wikipedian" (user Lou Scheffer) is using that.

We see the purpose of APOLLO is to probe gravity to even greater precision, plotting the motion of the moon to an unbelievable 1mm precision. As the project page and papers indicate, quantum gravity notions (as well as many alternate theories of gravity) predict some violation of the Equivalence Principle (EP) in various forms in some regimes. These will be very, very tiny (IOW, if GR is not right, the replacement theory of gravity must very close to it in all regimes we've probed), hence the need for incredible precision.

An example of a supposed EP violation was the recent hubub over some apparent time delay in receiving very high energy photons (TeV, IIRC) relative to lower energy photons from some distant source. IIRC, the delay would be a minute or so relative to billions of years of light travel time.

If so, that is an EP violation -- the TeV photons follow an every so slightly different path through space-time than lower energy photons.

So yes, this is the mission of APOLLO -- probe gravity to even more precision. It is way, way, way, way too early to draw any conclusions based on their first runs.

I want to readers here to understand we're talking mm levels relative to the moon's distance from the earth. That's just amazing. The way Jerry worded that made it sound like there was some big difference in the moon's trajectory vs GR predictions, ie the moon zigged when GR predicted it would zag.


-Richard

Teased or corrected?

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"The beauty of that discussion of averages is that you don't have to be an expert in Apollo or in photography in order to see where this time study "analysis" breaks down. You just have to be, well...not an idiot." -JayUtah

Never insulted when others are critically analysing the papers and observational data. This is always appreciated.

The only line in the article I took exception to, is the statement that GR has "passed every test". In a paper referenced by Chris, Turyshev uses slightly different, but meaningful wording (my bold):

http://arxiv.org/PS_cache/arxiv/pdf/...806.1731v2.pdf
'Surviving' is not quite the same as passing, and Turyshev goes on to explain why:
When we try to improve our understanding, we hope to be able to better predict what will happen when we try to push our understanding to the next level. This is proving to be very difficult, as the Gravity B Probe team underscores (my bold):

http://einstein.stanford.edu/highlights/status1.html

Like the Lunar Ranging experiments, the GP-B probe team ran into unexpected complications. GP-B probe funding was cut, in part, because, as the senior review committee wrote, "the goals of GP-B have already been fulfilled by other measurements".

I don't think this is true. I think 'survived every test' is a better description than 'passed every test'. Whether or not the observational failures are a result of a flawed theory, or flawed observations remains to be seen.

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jwj

If you always believe what you already know, you can't learn anything - Liz