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Space Science

NASA Gravity Probe Launched 223

Posted by michael from the up-up-and-away dept.
ping pong writes "Forty-five years in the making and 24 hours late, NASA launched the $700 million satellite into orbit today to test Albert Einstein's general theory of relativity. The satellite, which was inserted into a polar orbit, will spend two months getting ready, then 16 months making measurements." NASA's mission news has more.
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NASA Gravity Probe Launched More

NASA Gravity Probe Launched

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  • And if his theory is proved wrong... (Score:5, Funny)

    by Anonymous Coward on Tuesday April 20, 2004 @05:45PM (#8922474)
    could this post be considered a relatively first post?

  • Cheap shot (Score:5, Funny)

    by platypibri ( 762478 ) <platypibri@@@gmail...com> on Tuesday April 20, 2004 @05:45PM (#8922478) Homepage Journal
    We fail to understand the gravity of this situation.

  • That's a lot of money to spend (Score:5, Informative)

    by mindless4210 ( 768563 ) * on Tuesday April 20, 2004 @05:46PM (#8922481) Homepage Journal
    It's a pretty fascinating experiment, although it seems like a lot of money to spend just for testing his theory. I think that recent missions to mars were a bit more interesting.

    Stanford has a great overview of the mission [stanford.edu]. It's in pdf format.
    • This is an experiment designed to test the correction due to General Relativity of the thomas precession of a tiny spinning sphere.

      The correction to the precession will be on the order of arcseconds (1/3600 of a degree) per year.

      There are some very good general relativists who have very severe reservations about this project. If they do detect a signal, I suspect it will be more of a testament to the power of experimental precision rather than a test of GR, which practically every serious physicist believes to be correct.

      It's also worth noting that if nothing is seen, it's more likely than not due to the difficulty of detecting such a small signal.
      • So if the general theory of relativity is correct or not does not really matter to the results of this experiment :)

      • Many of the reasons were previously discussed in the previous Slashdot story, "NASA Gravity Probe Set for Launch [slashdot.org]."

      • Re:Why has it been in the making for so long? (Score:5, Informative)

        by Stuntmonkey ( 557875 ) on Tuesday April 20, 2004 @06:38PM (#8923040)

        The correction to the precession will be on the order of arcseconds (1/3600 of a degree) per year.

        It's significantly smaller than that -- the precession due to frame dragging is predicted to be only 0.04 arcseconds over one year.

        And I agree that the physics community is 99% confident that the Lense-Thirring effect is real. However, I also think this is more because of the aesthetic beauty of the theory, rather than actual measurements. If it were a less fundamental theory being tested I would call it a waste of money, but for something as fundamental as GR I think a confirming direct experiement is justified.

        The real question is how many viable alternatives to GR are ruled out by this test, assuming it is successful. For example nearly all viable GR alternatives proposed have weak gravitational wave properties identical to GR, so detecting these waves provides little support for GR. I wonder if the Lense-Thirring frame-dragging effect is more discriminating.

        Of course, by far the more interesting case is if the effect is not observed. They seem to have many sigma of signal to noise here, so a null result would be pretty compelling.

        • Lense-Thirring effect exists (Score:2, Informative)

          by Anonymous Coward
          Of course, by far the more interesting case is if the effect is not observed. They seem to have many sigma of signal to noise here, so a null result would be pretty compelling.

          The Lense-Thirring effect has been observed: http://www.iop.org/EJ/abstract/0264-9381/17/12/30 9

          There is no null result. However Gravity Probe B will increase the accuracy of the measurements DRAMATICALLY. Progress in physics has always been made by:
          1. new ideas
          2. high accuracy measurements allowing to discriminate between those i
      • ... I suspect it will be more of a testament to the power of experimental precision rather than a test of GR, which practically every serious physicist believes to be correct.

        Err... I don't think so... every serious physicist strongly believes that relativity laws are an excellent model of the behaviour of the universe in the macroscopic scale.

        Science is inherently an asymptotic quest for the truth. Any serious scientist knows that.
      • which practically every serious physicist believes to be correct.

        You mean just like every serious physicist believed Newtonian mechanics to be correct till the early 20th century?
      • Practically every serious physicist knows that GR as currently stated and QM as currently stated are mutually contradictory in certain domains, and that thus one or both are in correct in the same sense Newton's laws were.

        Me, I'm hoping we find a divergence with the GR expectation. Some inexplicable data will hopefully inspire a future Nobel Prize winner into making sense of the contradictions and get us a unified theory that can be tested.
      • If they do detect a signal, I suspect it will be more of a testament to the power of experimental precision rather than a test of GR, which practically every serious physicist believes to be correct.

        That's the whole point - if they prove the effect, then we no longer have to 'believe' it to be correct :)

        I know, I know - it'll still only be a theory, but it'll have yet more experimental evidence confirming it, which is a good thing(TM).

        It's also worth noting that if nothing is seen, it's more likely tha

    • Re:That's a lot of money to spend (Score:5, Insightful)

      by I_Love_Pocky! ( 751171 ) on Tuesday April 20, 2004 @05:51PM (#8922551)
      Wouldn't our understanding of something as fundamental as general relativity far out weigh any kind of understanding we could gain from Mars? Even if life is found on Mars, it does little to solidify our understanding of the fundamental forces of the universe.

      Gravity is one of the most important, and least understood forces in the physical world. Mars is just a big rock in orbit around our tiny little sun. Going there is a cooler project that this, but the information garnered from such a mission seems to be less important than what this mission is set to show.
      • The question isn't whether the experiment should be done, but should it be done *now*. Is it worth 700M to do it *now* compared to 70M in 50 years when it would be cheaper to get high precision instruments and to put things into space.

      • Re:That's a lot of money to spend (Score:5, Interesting)

        by Anonymous Coward on Tuesday April 20, 2004 @07:08PM (#8923300)
        Wouldn't our understanding of something as fundamental as general relativity far out weigh any kind of understanding we could gain from Mars? Not necessarily. More has been learned from putting this experiment together than will be learned from its results - it is a fine-scale test of one of the most successful theories of all time. Going to Mars, though expensive and such, could be a big deal if only as a proof of concept. Look at how much was learned going to the Moon. Going to Mars is orders of magnitude more complex - interplanetary radiation, the technological and social advances that will let a crew live together for months, getting people there and back ... if we can get to Mars and back we can go anywhere in our Solar System with trivial amounts of scaling. And, if we do find life on Mars (or elsewhere), that is potentially the most important discovery, um, ever. It tells us things about the fundamental nature of the universe too - and, short of an experiment either detecting gravity waves or detecting an overall curvature to spacetime, there isn't that much experiments will tell us about gravity right at the moment.
      • Insightful? That's like the classic Seinfeld bit about the bio-engineers that design seedless watermelon instead of using their time to come up with cures for cancer or AIDS. It's about interests. Sure, the fundamentals of the Universe is a very interesting topic, and I'd love to see String Theory proven as much as the next guy, but one person's value of importance may differ from another person. Neither person is wrong, they're simply different.

    • Re:That's a lot of money to spend (Score:5, Insightful)

      by Rosco P. Coltrane ( 209368 ) on Tuesday April 20, 2004 @05:54PM (#8922589)
      although it seems like a lot of money to spend just for testing his theory

      I think it's really a cheap experiment, considering the importance of the results. If there's something wrong or incomplete in Einstein's theory, we (as in humanity) should know about it, firstly because it's the human nature to try to know more all the time, and secondly because it could be very important in practical terms: you wouldn't want to take a plunge off a cliff with your SUV because your GPS receiver had a slight error, would you?

      This is theorical science and experimentation at its best. The price is really cheap to advance mankind's knowledge. Compare this to the weekly cost of certain recent military activities that probably won't bring back much to mankind anytime soon...
      • I agree that this is a cool experiment and that it's important in terms of understanding special relativity. But considering that this equipment is the most sensitive made yet and still might not be sensitive enough to detect what it's measuring. Why do you think a GPS satellite is going to care?
        • oops... make that general relativity, not special relativity....

        • Re:That's a lot of money to spend (Score:2, Interesting)

          by Anonymous Coward

          But considering that this equipment is the most sensitive made yet and still might not be sensitive enough to detect what it's measuring. Why do you think a GPS satellite is going to care?

          It turns out that gravity's second-order effects on time are much easier to measure than its second-order effects on motion. Gravitational redshift makes clocks on the Earth's surface run slow compared to clocks far away, to the tune of about 40 microseconds a day. GPS relies on timing differences between signals from

      • Please explain the downside of SUVs plunging off cliffs...

    • For the long duration it took to complete the project, I don't think it was *that* much money per year really. I don't think $15 million / year is much in the big picture.

    • Re:That's a lot of money to spend (Score:5, Insightful)

      by Stuntmonkey ( 557875 ) on Tuesday April 20, 2004 @06:01PM (#8922660)

      it seems like a lot of money to spend just for testing his theory. I think that recent missions to mars were a bit more interesting.

      That boils down to less than $3 per American, spread out over the last 40 years.

      To prove conclusively (or not) our most fundamental theory of gravity, space, and time.

      Man, you are a cheapskate.

      • Did you take into account inflation and the time-value of money? If not, the people at the beginning of those 40 years got screwed!!
      • I'm unclear about what exactly is being tested here. I was under the impression that satellites with atomic clocks had already confirmed relativistic effects on time [lsu.edu]:

        At the time of launch of the first NTS-2 satellite (June 1977), which contained the first Cesium clock to be placed in orbit, there were some who doubted that relativistic effects were real. A frequency synthesizer was built into the satellite clock system so that after launch, if in fact the rate of the clock in its final orbit was that pre

        • Well that test proved that time is warped, this test is to test frame dragging, which I guess in simple terms is testing to see if the space/gravity/time is spun around like a tornado or whirlpool, except the visual those two things give is somewhat inaccurate. Frame dragging involves too many dimensions for most to visualize it, but hopefully you get the idea.

          Regards,
          Steve
      • Even better if you're not in the US, which makes it free! Seriously, they should do more of this!

  • Probe componentry (Score:5, Funny)

    by Rosco P. Coltrane ( 209368 ) on Tuesday April 20, 2004 @05:46PM (#8922491)
    The experiment uses three key components: a spinning sphere, a telescope and a star.

    One of these components can't be had from Sharper Image : can you guess which?

  • A Great Man (Score:5, Insightful)

    by osewa77 ( 603622 ) <naijasms@gma[ ]com ['il.' in gap]> on Tuesday April 20, 2004 @05:46PM (#8922494) Homepage
    The greatest men are those who keep shaking up the world even after they are long gone. Albert Einstein wasn't a businessman, or a soldier, but look how much research and spending has been affected by his findings. Kudos!

  • Perfect Quartz Spheres (Score:5, Interesting)

    by RobertB-DC ( 622190 ) * on Tuesday April 20, 2004 @05:46PM (#8922497) Homepage Journal
    The most facinating tidbit from the NASA article is the absoutely beyond perfect Niobium-coated Quartz spheres at the heart of the ultra-precise gyroscopes.

    A quick Google found this link [stanford.edu] with more cool details, including:

    * The 1.5-inch diameter rotors are within 40 atomic layers (0.3 millionths of an inch) of a perfect sphere.

    * "Electrical sphericity" must be held to parts in ten million.

    * Each rotor spins inside a quartz housing with clearances to the rotor of barely one thousandth of an inch.

    * To lift the rotor on earth takes 1,000V. In space, only a fraction of a volt is needed.

    * In 1,000 years the gyroscope should barely lose 1% of its starting speed.

    * To isolate the gyroscope from the Earth's magnetic field, it will be shrouded in four layers of lead balloons, plus an outer shield of iron.

    Plus these cool facts (and a ton more), there are steampunk-styled drawings of the manufacturing process.

    Seems like NASA could make some money selling the rejects (you know there are plenty) as the ultimate shooters [akronmarbles.com]!
    • One complaint I have about this is, although the engineering is incredibly fancy, and advances were probably made on many fronts, this was shelved for 45 years for a reason -- a ridiculously small effect is expected to be observed.

      Compare the expected General Relativistic correction to the Newtonian contribution and you'll see why, the GR contribution is about 3-4 orders of magnitude smaller.

      Case in point, it took hundreds of years of observations of mercury to determine its orbit precessed by 5599 arcsec
      • I have serious reservations about whether a 16 month experiment will observe what it's designed to observe.

        As with CCD imagers, longer integration times with this instrument translate to higher and higher signal-to-noise ratios in the final data.
    • Whoever said a lead balloon would never float?
    • From this link; [freerepublic.com]

      I worked as a consultant for the company that was awarded the contract for working on the zerodur glass block that made up the housing for the gyros. They brought us in to try and teach machinists optical fabrication. The tolerances needed for this thing were unbelievable, extremely tough even for a master optician. They manufactured 3 housing blocks, one of them was destroyed during the rough machining process, and an optician trainee who was attempting to polish one of the precision lands

  • Scientists crossing fingers, pacing (Score:5, Funny)

    by Neil Blender ( 555885 ) <neilblender@gmail.com> on Tuesday April 20, 2004 @05:46PM (#8922499)
    "E had just better equal MC squared...E had just better equal MC squared..."

  • alarm bells (Score:3, Funny)

    by rokzy ( 687636 ) on Tuesday April 20, 2004 @05:48PM (#8922513)
    "The experiment uses three key components: a spinning sphere..."

    ding ding ding ding ding!

    is this going to be like Event Horizon where the probe travels to Hell and back and then kills most of us?
  • Was this at all offset by any other governments? Seems kind of pricy for research that will freely be shared worldwide, though from the representations Americans get in the news, you have to wonder if they can really trust the data gathered from something built by products of one of the supposedly worst education systems in the world :P
    • if they can really trust the data gathered from something built by products of one of the supposedly worst education systems in the world :P

      Hey, but at least it is the most expensive...
    • There's always the issue of prestige and technological supremacy. When you can do somethign no one else can technologically, it's more likly other governments/researchers will defer to you on the subject. Your influence increases and your ability to make certain claims increases. It's like civilization, achievement = prestige = influence.

    • Re:The question is who funded it? (Score:2, Funny)

      by Anonymous Coward
      Who said we're going to share the information? The secrets of relativity are ours, and the rest of the world can just go on thinking that e=mc^2! Suckaz!

    • Re:The question is who funded it? (Score:5, Insightful)

      by Long-EZ ( 755920 ) on Tuesday April 20, 2004 @06:03PM (#8922679)
      I really don't think the financial analysis is the correct one. I'm fairly sure the US will derive enough benefit to justify the cost, although the benefit is admitedly difficult to quantify and is amortized over the rest of our specie's existence. Does it matter if the rest of the world gets a free ride? They do pure science too, and we benefit. Science is a collaborative effort. This isn't some billion dollar defense department project seeking a military advantage over a perceived adversary. This is about scientific discovery and learning things that have never been known. In my cynicism concerning politics, I sometimes forget to be optimistic about the science.
    • I wonder why people continue to buy into this bit of "worst educational system in the world". Sure, test results suck (mostly because of economic disparities if you look at the data), but the real world results (at least *so far* :-) seem to indicate that it's not too shabby.

      Ok, at least the post-secondary education system isn't too shabby. The primary (and even secondary) education systems in the US are designed to turn most people into the mindless drones they were always destined to be anyway. It seems

  • Einstein... (Score:4, Interesting)

    by PeaceTank ( 758859 ) on Tuesday April 20, 2004 @05:52PM (#8922563)
    I think that Einstein would turn over in his grave if he knew that we were spending 700 million dollars to test one of his theories. Remember, this was the man that came up with some of the most complicated theories in modern physics, and he did it in his head. He used 'geddonken' experiments, and however useful it may be to 'prove' his theories, one has to wonder what he would think...

    • Click here [google.com]

    • Re:Einstein... (Score:4, Informative)

      by BlueUnderwear ( 73957 ) on Tuesday April 20, 2004 @06:13PM (#8922770)
      I think that Einstein would turn over in his grave

      Nope, he was cremated. However, his brain could be spinning in its jar [guardian.co.uk]

      Remember, this was the man that came up with some of the most complicated theories in modern physics,

      ... except that he plagiarized Dirac's works [physicsweb.org]...

      He used 'geddonken' experiments,

      Gedankenexperimente, i.e. though experiments.

      • ... except that he plagiarized Dirac's works...

        Many people had a mathematical understanding of Special Relativity waay before Einstein. You take the rules for Electricity and Magnetism and you immediately see that they don't work right when you add velocities like with Newtonian Mechanics. Newtonian Mechanics has Gallilean relativity. That's right, there was relativity before Einstein.

        It is a straight forward exercise to see that Maxwell's Equations (for E&M) have Lorentzian relativity. That's right

      • Re:Einstein... (Score:2, Informative)

        by www www www ( 763043 )

        ... except that he plagiarized Dirac's works...

        That statement is plain stupid. Dirac finished school in 1918 [st-and.ac.uk]. Einstein published the Special Relativity in 1905 and the General Relativity in 1915. Can you back up your statement?! Beside, it is well known that Dirac was a great admirer of General Relativity, considering the Einstein equations the most beautiful in physics. That is the reason Dirac chose GR as a research topic in 1923 as a young student in Cambridge ...

        The Dirac generation might have had

  • Slow (Score:5, Funny)

    by Mateito ( 746185 ) on Tuesday April 20, 2004 @05:52PM (#8922570) Homepage
    > will spend two months getting ready

    Sounds like my girlfriend.

  • An experiment in inertia? (Score:5, Informative)

    by David Hume ( 200499 ) on Tuesday April 20, 2004 @05:52PM (#8922577) Homepage

    That is, inertia in big science funding?

    In 1995, the GP-B was described as the "only experiment ever devised to test [the existence of frame-dragging] [sfsu.edu]."

    However, in 1997 NASA announced that it had successfully tested frame dragging [nasa.gov]. See also here [scienceweb.org].

    • They had evidence supporting this effect from a black hole. GP-B is designed to provide much more conclusive measurements from earth herself.

      A black hole is a pretty extreme example of this, too, and such behavior around the singularity is more likely to have alternative explanations than said behavior around earth.

  • Yay! Hoorah for science, wooo!! (Score:4, Insightful)

    by Toxygen ( 738180 ) on Tuesday April 20, 2004 @05:54PM (#8922592) Journal
    All quoting aside, I wonder what will or would happen if the theory of relativity turns out to be nothing but bunk. It wouldn't be the first time our scientists knew something, even if it were based partly on observation. I'm no physicist, but I know Einstein's made assumptions that haven't been proven wrong or right, for example the speed of light in a vaccum is the fastest attainable speed in the universe. Just because we haven't doesn't it doesn't. And what about the unexplainable increase in velocity of the voyager probe as it neared the edge of the solar system? When I read that article, I remember thinking "wouldn't it be great if I was alive to see such a monumental discovery, along the lines of 'the earth ain't flat no more'?" I think it'd be so cool (ok, interesting) if this experiment means we need to rewrite our laws of gravity.
    • Well... disproving this one aspect of the theory would not invalidate the remainder of the theory, which has been verified experiementally numerous times.

      Newton's Laws of Motion didn't become "bunk" all of a sudden when Einstein (and later QM) discovered holes in it.

      The speed of light bit is actually really well tested. It really does take lots more energy to continue speeding things up near light speed, and the trend of that is completely consistent with it taking an infinite amount of energy to get a

      • I don't understand how time dilation really works. I've seen examples of it, but as far as I can see it appears they all state that time appears to slow down for a fast-moving object relative to a stationary observer. From my admittedly layman's point of view, it seems to me that time slowing down would basically be seen along the same lines as the doppler effect on a moving sound source. I know it may be apples and oranges, but sound and light both travel in waves, correct? I hate making myself sound l

        • Re:about time travel (Score:3, Informative)

          by drik00 ( 526104 )
          You seem to misunderstand...time DOES slow down the faster that you're traveling. Einstein's famous twins paradox, for example. Imagine twins born at the same time, one is put on the on a spacecraft travelling through space at near-light speed (since C is unachieveable), and the other child grows up on earth normally. When the earth-twin is 30 years old, his space-twin returns, and is only 3 days old.

          -J
      • Additionally, time dilation is well demonstrated, and it definitely would allow the creation of time machines (something I morally object to :-) if faster than light travel were possible.

        Sheesh. Let me paraphrase our 100 year old counterparts: "the atomic decay we call 'radiaton' is very well demonstrated. If this so called 'fission' device could be built, it would definitly destroy the entire planet."

        Our theories simply do not work if somehow we can move faster than light. A more likely explanation t
    • General relativity may surely be proven wrong by the probe's results, but this will not turn it to be "nothing but bunk". So far it successfully passed all tests, which makes it at least a very good approximation (within our current measurement limits).

  • Better understanding of gravity (Score:5, Insightful)

    by Outosync ( 214525 ) on Tuesday April 20, 2004 @05:54PM (#8922596)
    Gravity is a force that effects everything in our universe (and in theory some other universes :P )

    It's a force we can calculate for and predict but we still aren't completely sure HOW it works. So whether this mission proves or disproves Einstein's theories we should at least get data that will help bring us a step closer to understanding a significant force in the universe.

    I'm really exicited to see the results in 2 years :)

    • We already have a better understanding of gravity. (Score:4, Informative)

      by I'm a racist. ( 631537 ) on Tuesday April 20, 2004 @06:20PM (#8922850) Homepage Journal
      In all honesty, this probe won't tell us anything we don't already know. At the time the idea was proposed, it was useful. Since then, we've made more precise measurements of gravity and observed relativistic effects.

      The only way this probe will really teach us anything (outside of the engineering that went into its construction) is if it fails, spectacularly. Sadly, those "eureka" moments don't happen very often, and I wouldn't hold out much hope for one here. Then again, the Hipparcos data has caused some debate, while its mission was somewhat routine (although highly precise).

      We already know that relativity is wrong (in the same sense that classical mechanics is wrong). This experiment is not designed to figure out exactly how relativity is wrong, rather it is designed to tell us if relativity is wrong at all. Since we already know the answer to that question, it isn't very helpful.

      I'm not blaming the guys that worked on this project. There were political/financial/logistical issues that made this launch 20+ years too late to be useful. The PhDs awarded during this project are good, they did some nice work, most notably in materials science and fabrication, but other areas as well. It's just not very meaningful in the areas of physics/cosomology.

      Oh well, that's what happens when science is a slave to beauracracy.
      • We already know that relativity is wrong (in the same sense that classical mechanics is wrong).

        I've never heard anything about relativity being proven wrong, please explain further.
      • In what sense would you claim that relativity is "wrong"? Just to clarify (and for other readers :), is it that you believe there is a bigger picture theory that encompasses GR AND quantum mechanics, etc? (In the same way that GR encompasses Newtonian mechanics) This is sort of theory (quantum gravity!) is needed to explain the inner workings of a black hole, for example, and the beginning of the universe, earlier that 10^{-43} seconds.

        But the purpose of this project is to determine whether all the pred

      • I agree with everything that you have said, factually.

        However, there is much room for a similar line of research to be done in the future that is very much like this.

        Outerspace gravity experiments can do soooo much more than Earth based ones. Things like detecting gravitational waves at different energy ranges, !detecting the Axion!, and probing gravity at small ranges to detect the compactified dimensions of string theory.

        Hopefully this can pave the way for the real stuff.

  • It was actually launched last week (Score:5, Funny)

    by DJStealth ( 103231 ) on Tuesday April 20, 2004 @06:00PM (#8922657)
    What most people don't know is that it was actually launched last week.

    Its experiments of relativity caused it to move close to the speed of light forcing the effects of time dilation to make it appear as if it was delayed 24 hours, when in reality it was launched long before its scheduled date.

  • Let's not forget (Score:3, Insightful)

    by sibdib ( 772980 ) on Tuesday April 20, 2004 @06:05PM (#8922698)
    That the hope of theoretical physicists is to unite gravity with the other forces, understanding the why and how of divergance, and hopefully uniting quantum dynamics with general relativity (properly fund NASA!, GWB) creating one theory to explain them all.

    Needless to say, much will need to be discovered even after a successful GP-B mission.

  • Probes (Score:2, Funny)

    by wramsdel ( 463149 )
    Jeez, first the 9/11 probe, now this. Does governmental inquest know no bounds?!

  • this is depressing. (Score:3, Funny)

    by kin_korn_karn ( 466864 ) on Tuesday April 20, 2004 @06:17PM (#8922811) Homepage
    This story is depressing. Gravity brings me down.

  • Wow! (Score:5, Funny)

    by jonfromspace ( 179394 ) <jonwilkins@@@gmail...com> on Tuesday April 20, 2004 @06:26PM (#8922912)
    The satellite, which was inserted into a polar orbit, will spend two months getting ready, then 16 months making measurements.


    Sounds like my Girlfrind when we go shopping...

  • Isn't frame dragging a forgone conclusion? (Score:3, Insightful)

    by synaptik ( 125 ) on Tuesday April 20, 2004 @06:28PM (#8922927) Homepage
    There's a question I wanted to ask the last time this probe was discussed on slashdot, but alas I discovered the discussion too late to be assured a viable discussion.

    Is the presence of frame dragging a forgone conclusion, given that (a) gravity waves do not travel instantaneously, and (b) the moon is able to maintain a stable orbit around the earth, even though the earth itself is in motion?

    My college physics were limited to 2 semesters, but I do recall discussions of a velocity component to gravity. To use more severe example than the earth and moon:

    Pretend, for simplicity's sake, that the earth's orbit is circular, and is exactly 8 light-minutes in radius. By the time gravity waves reach the earth from the sun, 8 minutes have transpired, and the sun is certainly no longer in the same spatial position that it was 8 minutes prior. This means that earth is no longer orbitting what it "thought" it was orbitting (if you'll excuse the tongue-in-cheek anthropomorphization.) The only two ways I've ever heard of accounting for this are:
    (a) gravity waves are not limited by C, and in fact gravity's effect is felt instantaneously
    (b) there is a velocity component to the effect of gravity, that takes into account the speed and direction of travel of the object(s) involved.

    I think (a) is pretty much out of favor, right? If so, that leaves (b). Thus, velocity matters... regardless of whether that happens to be linear or angular velocity.

    Since rotation is angular velocity... does this not imply that frame dragging exists?

    I'm definitely interested in replies from Physics whizzes on this one... it's bugged me for a while now.
    • Alright, disclaimer first:
      Just a grad student, still learning stuff, apologies ahead of time if it's wrong.

      Attempt at an answer:
      "Frame-dragging", as I understand it, goes all the way back to an old theory of the aether, that the aether is all around us, but is dragged by masses so that some oddball features of special relativity is explained. I'm not sure how this applies to the problem here, so maybe people use frame-dragging to refer to something else.

      This part, though, how gravity works, is easier. Einstein's theory relies upon the stress-energy tensor. All forms of energy, including energy due to angular momentum and relative motions, are included in this. Binary pulsars precess and their orbits evolve in time, as do their rotation rates, as energy is radiated away gravitationally. There is definitely a contribution to gravity due to what you call "velocity components". Gravitational signals only propagate at c, so don't worry.

      You can look at my first 2 posts on this topic if you like, but basically GR predicts that there will be a precession of this little spinning sphere that's very small and hopefully detectable. If we don't detect it, it's probably due to the difficulty of the experiment, not to the failure of GR.
    • The video's from PBS's Nova - The Elegant Universe [pbs.org] - Newton's Embarassing Secret explains this.
  • Please use the links on the left to brows thought the image categories. Fantastic!!!
  • a spinning sphere, a telescope and a star Well, if one of these would have been left aside, the production costs could have been used to make a whole movie (2hrs) on the Moon, .. if there remains any land not already sold : )

  • Isn't this just NASA spin? (Score:3, Insightful)

    by Anonymous Coward on Tuesday April 20, 2004 @08:11PM (#8923817)
    Not that I think the science isn't valid enough for NASA to afford this (they've obviously got money to burn) but isn't NASA trying this on as a means to validate their science budget from which they feed?

    The manned spaceflight missions have always had the justification that understanding the effects of zero gravity on humans over extended periods was sufficient to secure funding from the NSF and others but zero-G on humans has been tried and tested over the past 40 odd years and is no longer considered of interest to fundamental science.

    The timing seems to indicate that NASA wants to show it can carry out fundamental science experiments even if the results aren't relevant to modern questions in fundamental physics. They even go so far as duplicate well accepted results in a field that has progressed well beyond the best precision of GP-B.
    • If you bring the vector of politics into this research, the results are almost guaranteed to be unexpected.

      If the experiment goes as planned and detects measurable precession, the specific data could be promoted as a valuable piece of military inteligence to make aiming devices, tracking weapons, and positioning systems just a bit more accurate than anyone else on earth (One press release did say that the results weren't being publicly released; just the general findings). Which of course leads to more fu

    • Terribly sorry, but it was an easy shot.
  • I'm glad to see that gravity is a field that attracts all the finest scientific minds.

  • We read too much sci-fi (Score:2, Interesting)

    by ogma ( 755652 )
    From the article A Near-Perfect Gyroscope [stanford.edu] provided by another poster:

    "Mechanically, the 1.5-inch diameter rotors are within 40 atomic layers (0.3 millionths of an inch) of a perfect sphere, rounder than anything within many light-years distance from us....Only neutron stars are rounder."

    Now I know that here on slashdot such things as neutron stars are always only a synapse or two away from our collective consciousness, but I have to say that reading those words sent a shiver up my spine. A sentence th

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