# Is the gravitational force G constant **URL:** \ **Category:** Factual Questions\ **Created:** [September 17, 2013, 12:30pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043 "2013-09-17T12:30:42Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 12:30pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/1 "2013-09-17T12:30:42Z")
Yesterday’s [Fermilab newsletter](http://www.fnal.gov/pub/today/archive/archive_2013/today13-09-16.html) referenced [this article](http://www.newscientist.com/article/dn24180#.UjNQ4LzrGPa) in New Scientist. > [@](#): > > Did gravity, the force that pins us to Earth’s surface and holds stars together, just shift? Maybe, just maybe. The latest measurement of _G_, the so-called constant that puts a figure on the gravitational attraction between two objects, has come up higher than the current official value. > Measurements of _G_ are [notoriously unreliable](http://www.newscientist.com/article/mg21028091.500-notorious-big-g-the-struggle-to-pin-down-gravity.html), so the constant is in permanent flux and the official value is an average. However, the recent deviation is particularly puzzling, as it is at once starkly different to the official value and yet very similar to a measurement made back in 2001, not what you would expect if the discrepancy was due to random experimental errors. > It’s possible that both experiments suffer from a hidden, persistent error, but the result is also prompting serious consideration of a weirder possibility: that _G_ itself can change. That’s a pretty radical option, but if correct, it would take us a step closer to tackling one very big mystery – [dark energy](http://www.newscientist.com/article/mg20727672.400-general-relativity-the-dark-universe.html), the unknown entity accelerating the expansion of the universe. More at link ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 17, 2013, 12:39pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/2 "2013-09-17T12:39:39Z")
From the New Scientist article > [@](#): > > Now a team led by Terry Quinn of the International Bureau of Weights and Measures (BIPM) in Paris, France, and Clive Speake of the University of Birmingham, UK, has measured G using two methods… The resulting value for G is 240 parts per million bigger than the official one, set in 2010. > > The figure alone is not the weird part - one recent measurement came up 290 ppm below today’s official value. … it is just 21 ppm off the value Quinn’s team got using the same set-ups in 2001. So same setup produces similar results. I’m calling experimental error until 2 or 3 other teams reproduce the same thing. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 12:47pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/3 "2013-09-17T12:47:36Z")
Yes. I did read the whole article too. Can’t say I really followed what they were getting at, but then that’s why this is in GQ. Also I would point out that the Fermilab newsletter is written by the people who run one of the world’s premier facilities for particle physics research. Even with the Tevatron gone, they still have one of the most powerful accelerators I believe - although I don’t actually know what the rating of the main injector is. I do know that it’s more than a half mile in diameter - if that gives you any idea though. So for them to reference an article, you should probably take it seriously. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 17, 2013, 12:55pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/4 "2013-09-17T12:55:07Z")
Who said I didn’t? It’s interesting but then again so was superluminal neutrinos and I was equally skeptical about that. Besides any article coming from the IBWM dealing with G is going to get mentioned in an institutional newsletter. That says nothing about the validity of the claim. Take the Fermi newsletter article about[faster than light neutrinos](http://www.fnal.gov/pub/today/archive/archive_2011/today11-10-28.html) from October 2011. You’ll note that neutrinos are still not faster than light. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 1:07pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/5 "2013-09-17T13:07:44Z")
Your statement was: “I’m calling experimental error until 2 or 3 other teams reproduce the same thing.” That sounds a lot like prejudging things to me. And comparing this to the neutrino situation is completely bogus. That was ONE experiment. Here we have hundreds using a variety of methods. So the experiments have already been done. Maybe not precisely the same experiments that are the subject of the article, but G has been measured countless times, so the analogy is completely inapposite. As for Fermilab referencing the neutrino fiasco, that’s all they did - reference it. And that was completely appropriate - as is this. Neither reference was an endorsement but only an indication that these are things that are of scientific significance, which they are/were - or at least in the neutrino case, would certainly have been were it not merely and issue of bad measurement - which seems to be very unlikely in this case given the vast number of experiments already done. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [September 17, 2013, 2:07pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/6 "2013-09-17T14:07:37Z")
> [@](#): > > Measurements of G are notoriously unreliable, so the constant is in permanent flux and the official value is an average. So variables won’t, and constants aren’t-gotcha. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 17, 2013, 2:27pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/7 "2013-09-17T14:27:57Z")
> [@deltasigma](#): > > Your statement was: “I’m calling experimental error until 2 or 3 other teams reproduce the same thing.” That sounds a lot like prejudging things to me. > > And comparing this to the neutrino situation is completely bogus. That was ONE experiment. Here we have hundreds using a variety of methods. So the experiments have already been done. Maybe not precisely the same experiments that are the subject of the article, but G has been measured countless times, so the analogy is completely inapposite. > > As for Fermilab referencing the neutrino fiasco, that’s all they did - reference it. And that was completely appropriate - as is this. Neither reference was an endorsement but only an indication that these are things that are of scientific significance, which they are/were - or at least in the neutrino case, would certainly have been were it not merely and issue of bad measurement - which seems to be very unlikely in this case given the vast number of experiments already done. You continue to misunderstand me. First off the neutrino experiment was not a fiasco at all. It was an experiment repeated over and over again by a single team that resulted in consistently faster than light velocities. They worked to eliminate every possible variable and then released their findings to the wider community to investigate. Turns out others noticed a systemic issue. I’d call that an shining example of personal integrity and validation of the scientific method. In this instance we have fundamental constant and a team that has 2 values both on the high end of that constant. So I’m intrigued but my default assumption is experimental error. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 2:40pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/8 "2013-09-17T14:40:42Z")
> [@Grey](#): > > You continue to misunderstand me. > > First off the neutrino experiment was not a fiasco at all. Maybe I am misunderstanding you but I’m quite sure that you’re one of the few people who doesn’t see that as a fiasco. As for the values, the one reported by the BIPM was higher but another was lower, so I don’t know which 2 you’re talking about. > [@](#): > > Now a team led by [Terry Quinn](http://www.bipm.org/) of the International Bureau of Weights and Measures (BIPM) in Paris, France, and Clive Speake of the University of Birmingham, UK, has measured _G_ using two methods: a modern version of the Cavendish experiment and one that relies on electrostatics. The resulting value for _G_ is 240 parts per million bigger than the official one, set in 2010. > **Violets in springtime** > > ``` > The figure alone is not the weird part - one recent measurement came up 290 ppm below today's official value. The strange thing about the latest one is that it is just 21 ppm off the value Quinn's team got using the same set-ups in 2001. Since the team took care this time around to remove every source of error that might have been at play back then, you would not expect the two results to be identical. > > ``` And the article itself concedes that it may be experimental error. As was noted in an earlier quote, the official estimate of G is an estimate, so that’s not very remarkable. This is why I’m posing the question here. I don’t need someone to tell me what is already in the article. I’m looking for additional background. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 17, 2013, 3:25pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/9 "2013-09-17T15:25:07Z")
So your question was what exactly? Is G constant? The answer is yes, as far as we know though the measuring of the value is difficult and recently a team has a larger value that needs to be corroborated by other teams. Or was your question "Is there evidence of changing G when we look back in time?, “Is there evidence of changing G is the orbital decay rate of neutron star pairs?” or maybe “Is there a consistent growth in the experimental value of G over time?” ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 17, 2013, 3:28pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/10 "2013-09-17T15:28:59Z")
The short answer is that it is always, always, always, always experimental error until a dozen separate teams in disparate places using different methods provide a new answer. That has not happened. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 17, 2013, 3:40pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/11 "2013-09-17T15:40:30Z")
It’s good science, and it’s worth keeping an eye on, but it’s almost certainly experimental error of some sort or another. Not only do the vast majority of experiments all (roughly, to within known random error) agree with each other (and disagree with this result), but there are also other lines of research that show that, whatever the value of G is, it’s constant or very close to it. If G varies with time, then the orbits of planets would vary with time, too, as would things like the luminosity of stars. Worse, the effects are in the same direction, so you’d get both the Earth being closer to the Sun and the Sun being hotter, at the same time (or vice-versa, of course, if the change is the other way). There are a variety of ways to tell what the temperature of the Earth was in the past, and they’ve never been even close to consistent with what varying-G models would predict (you tend to get things like the Earth being above the boiling point of water a billion years in the past, which is very blatantly not true). ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 3:43pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/12 "2013-09-17T15:43:16Z")
> [@Grey](#): > > So your question was what exactly? Is G constant? The answer is yes, as far as we know though the measuring of the value is difficult and recently a team has a larger value that needs to be corroborated by other teams. > > Or was your question "Is there evidence of changing G when we look back in time?, “Is there evidence of changing G is the orbital decay rate of neutron star pairs?” or maybe “Is there a consistent growth in the experimental value of G over time?” > [@Exapno\_Mapcase](#): > > The short answer is that it is always, always, always, always experimental error until a dozen separate teams in disparate places using different methods provide a new answer. > > That has not happened. My question is the question in the article and I’m sorry neither of you are able to understand it. 1. Is it possible that G is not really constant? 2. Why has it been so difficult to measure more precisely? 3. What are the implications for either 3a. a force that independently oscillates in intensity or 3b. changes intensity due to a fifth force or gauge field. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 3:47pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/13 "2013-09-17T15:47:31Z")
> [@Chronos](#): > > It’s good science, and it’s worth keeping an eye on, but it’s almost certainly experimental error of some sort or another. Not only do the vast majority of experiments all (roughly, to within known random error) agree with each other (and disagree with this result), but there are also other lines of research that show that, whatever the value of G is, it’s constant or very close to it. > > If G varies with time, then the orbits of planets would vary with time, too, as would things like the luminosity of stars. Worse, the effects are in the same direction, so you’d get both the Earth being closer to the Sun and the Sun being hotter, at the same time (or vice-versa, of course, if the change is the other way). There are a variety of ways to tell what the temperature of the Earth was in the past, and they’ve never been even close to consistent with what varying-G models would predict (you tend to get things like the Earth being above the boiling point of water a billion years in the past, which is very blatantly not true). If it oscillates but only by parts per million, how would that be true. Oscillations, by their very nature eventually cancel out, don’t they? So if they’re very small and average to zero, I’m having a hard time understanding how they would affect things on a cosmological scale. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 17, 2013, 4:12pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/14 "2013-09-17T16:12:23Z")
> [@deltasigma](#): > > My question is the question in the article and I’m sorry neither of you are able to understand it. 1. Is it possible that G is not really constant? > 2. Why has it been so difficult to measure more precisely? > 3. What are the implications for either > 3a. a force that independently oscillates in intensity or > 3b. changes intensity due to a fifth force or gauge field. 1. We don’t think so and there’s no evidence that it is 2. Because gravity is so incredibly weak that testing it it very difficult 3. See Chronos’ points 3a/b - Maybe but I don’t know of any models that posit an oscillating constant in anything and again gravity is the weakest force - why would we notice variations in it rather than the others? Especially considering how accurate our measurements are of those. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 17, 2013, 4:18pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/15 "2013-09-17T16:18:33Z")
If it oscillates, then you wouldn’t get secular effects like the Earth boiling, but you’d expect to see some sort of resonance with systems with the same period. Just talking solar system tests, I’d expect to see a band of distances from the Sun with a suspiciously low object density, since objects at that distance would be knocked out by the resonance. Or maybe it’d have to be a galactic test instead of a solar system one, depending on the period. Granted, it’s possible that there is such a band and we just haven’t noticed it yet. Alternately, it might vary around a central value, but in a non-periodic way. But that’s a lot harder to explain what underlying mechanism would cause that to happen. > [@](#): > > 1. Why has it been so difficult to measure more precisely? Oh, that’s an easy question to answer. G is small enough that the forces between any objects you can fit in an Earthbound laboratory are extremely weak, so you have to very precisely account for all of the other extremely weak forces in your experiment. A little excess static charge, or a little air resistance from a vacuum chamber that’s not quite good enough, can ruin the whole measurement, as well as any number of other possible error sources. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 4:18pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/16 "2013-09-17T16:18:49Z")
> [@Grey](#): > > 3a/b - Maybe but I don’t know of any models that posit an oscillating constant in anything and again gravity is the weakest force - why would we notice variations in it rather than the others? Especially considering how accurate our measurements are of those. Thanks anyway. > [@](#): > > An oscillating _G_ could be evidence for a particular theory that relates dark energy to a fifth, hypothetical fundamental force, in addition to the four we know – gravity, electromagnetism, and the two nuclear forces. This force might also cause the strength of gravity to oscillate, says Padilla. “This result is indeed very intriguing.” ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 4:23pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/17 "2013-09-17T16:23:36Z")
> [@Chronos](#): > > If it oscillates, then you wouldn’t get secular effects like the Earth boiling, but you’d expect to see some sort of resonance with systems with the same period. Just talking solar system tests, I’d expect to see a band of distances from the Sun with a suspiciously low object density, since objects at that distance would be knocked out by the resonance. Or maybe it’d have to be a galactic test instead of a solar system one, depending on the period. Granted, it’s possible that there is such a band and we just haven’t noticed it yet. > > Alternately, it might vary around a central value, but in a non-periodic way. But that’s a lot harder to explain what underlying mechanism would cause that to happen. Yes, but you’re assuming universal oscillation and I don’t think that’s the assumption in the article. I got the impression that the variations they’re looking for are local. If the oscillations are local, those considerations wouldn’t appertain. > [@Chronos](#): > > Oh, that’s an easy question to answer. G is small enough that the forces between any objects you can fit in an Earthbound laboratory are extremely weak, so you have to very precisely account for all of the other extremely weak forces in your experiment. A little excess static charge, or a little air resistance from a vacuum chamber that’s not quite good enough, can ruin the whole measurement, as well as any number of other possible error sources. Yeah, that’s what I figured. But even so, with Josephson junction devices, interferometers etc., it seems to me that we ought to be capable of sufficient precision that this shouldn’t be such an issue. But I’ll take your word for it. Thanks. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 4:29pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/18 "2013-09-17T16:29:48Z")
never mind. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [September 17, 2013, 4:32pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/19 "2013-09-17T16:32:50Z")
> [@deltasigma](#): > > Thanks anyway. No problem. Still, betting on hypothetical maybe forces vs. experimental error seems to be a poor choice. We’ll see once a few more differing teams run the experiment. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [September 17, 2013, 4:37pm UTC](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043/20 "2013-09-17T16:37:27Z")
> [@Grey](#): > > No problem. Still, betting on hypothetical maybe forces vs. experimental error seems to be a poor choice. We’ll see once a few more differing teams run the experiment. I’m not “betting” on anything. I respect the people at Fermilab and pay attention when they point me to something. I find this possibility intriguing. If you don’t, that’s fine. [Next page](https://boards.straightdope.com/t/is-the-gravitational-force-g-constant/669043.md?page=2)