What is the value of G?

October 28, 2014 by Jennifer Lauren Lee

NIST has taken part in a new push to address a persistent and growing problem in physics: the value of G. The Newtonian constant of gravitation, used to calculate the attractive force of gravity between objects, is more than 300 years old. But although scientists have been trying to measure its value for centuries, G is still only known to 3 significant figures. By contrast, other constants have been measured with much greater precision; the mass of the electron in kilograms, for example, is known to about 8 digits.i

Worse yet, the more experiments researchers conduct to pin down the , the more their results diverge.

On October 9-10, 2014, several dozen scientists from around the world gathered at NIST to consider their options.

"We're all here because we have a problem with G – and I mean, boy, do we have a problem with G," said Carl Williams, Chief of PML's Quantum Measurement Division, to the assembled group on the first morning of the meeting. "This has become one of the serious issues that physics needs to address."

The gravitational constant is familiarly known as "big G" to distinguish it from "little g," the acceleration due to the Earth's gravity.ii Despite its name, big G is tiny – about 6.67 x 10-11 m3 kg-1 s-2 – and comparatively feeble, roughly a trillion trillion trillion times weaker than the electromagnetic force responsible for affixing souvenir magnets to refrigerators. And its weakness makes it difficult to measure.

Experimentalists have used a variety of approaches – swinging pendulums, masses in freefall, balance beams, and torsion balances that measure the torque or rotation of wires supporting masses that are attracted to other masses. But a plot of all the results from the past 15 years reveals a relatively wide spread in values ranging from about 6.67 x 10-11 m3 kg-1 s-2.

Furthermore, CODATA – the International Council for Science Committee on Data for Science and Technology, which analyzes the results of individual experiments and provides an internationally accepted sets of values for – has had to increase the uncertainty on its latest recommendation for a value of G due to the divergence of the experiments.iii

At the NIST workshop, the 53 participants agreed unanimously that something should be done. They recommended that one or more organizations establish annual or biannual meetings focused specifically on the campaign to determine big G's value with greater accuracy, and they supported the idea of focusing on new approaches to the measurement, such as the atomic interferometry setup used in a recent experiment involving laser-cooled rubidium atoms.iv

The main culprit in these discrepancies is suspected to be systematic uncertainties in the measurements, and much of the discussion focused on reducing noise. One way to address this problem, participants felt, is for different teams to conduct independent experiments using the same set of apparatus. Two groups with particularly deviant results offered their equipment during the meeting, pending discussions with the teams that will reuse the resources.

Workshop attendees expressed moderate interest in forming a consortium, an organization that would centralize the process of finding consensus. A potential benefit of a consortium would be providing NIST and other National Measurement Institutes (NMIs) with a means of contributing support, for example in the form of precision length metrology services, to members.

"Clearly, there is no right answer for how to move forward," Williams said. "But there is international support around resolving the big G controversy, and so it's a great time for us in that regard."

Explore further: The constants they are a changin': NIST posts latest adjustments to fundamental figures

More information: i The mass of an electron is 9.109 382 91(40) x 10-31 kg, where the number in parentheses indicates uncertainty in the final two digits.

ii Calculating the gravitational attraction between two objects requires taking the product of two masses and dividing by the square of the distance between them, then multiplying that value by G. The equation is F=Gm1m2/r2.

iii CODATA's latest set, released in 2010, recommended a value for G of 6.673 84(80) x 10-11 m3 kg-1 s-2 compared to its previous result from 2006 of 6.674 28(67) x 10-11 m3 kg-1 s-2. The values in parentheses indicate standard uncertainty (based on standard deviation), in this case plus or minus 0.000 80 x 10-11 m3 kg-1 s-2 and plus or minus 0.000 67 x 10-11 m3 kg-1 s-2 respectively.

iv In this experiment, researchers pushed two clouds of cold rubidium atoms into a vacuum chamber with laser light. The atoms accelerated differently depending on the placement of high-density masses (tungsten weights totaling about 500 kg) arranged in various configurations. Differences in acceleration due to the atoms' gravitational attraction to the tungsten masses could be picked up in the clouds' interference pattern. G. Rosi, F. Sorrentino, L. Cacciapuoti, M. Prevedelli and G.M. Tino. Precision measurement of the Newtonian gravitational constant using cold atoms. Nature. Vol. 510. 518–521. June 26, 2014. DOI: 10.1038/nature13433

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31 comments

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HannesAlfven
1 / 5 (7) Oct 28, 2014
Or, more simply, G changes.
Nik_2213
3.3 / 5 (3) Oct 28, 2014
Extraordinary claims require extraordinary evidence; the effect on GPS system would be noticeable, surely ??

And have you checked to see if divergent experiments were running at the same time ??
HannesAlfven
2 / 5 (4) Oct 28, 2014
Re: "the effect on GPS system would be noticeable, surely ??"

Why is that, exactly? The changes are minute.

Also, you might want to check this link below, from one of the people who helped create the GPS system, Tom van Flandern:

http://metaresear...vity.asp
Returners
1 / 5 (6) Oct 28, 2014
Relativity:

"It is equally correct to say that the station moves past the train..."

Thus for experiments done in different parts of the world, if you look at their tangential velocity, they will be perhaps opposite one another. In Relativity, distances and masses change with Relative Motion, therefore each group's reference mass and reference distances will be different than the other groups, and they will obtain different measures of G if they assume their reference masses and distances are the same....assuming Special and General Relativity are correct in this regard.

Alternatively, perhaps Einstein's Relativity is wrong, and G varies slightly with Relative motion instead?
HannesAlfven
1 / 5 (5) Oct 28, 2014
It's interesting to me that nobody looks at the most impactful discoveries which occurred since Relativity became the law of the land.

It's not as though nothing of importance has happened since then. After all, we started sending probes into space in the 50's ...
Uncle Ira
4.1 / 5 (9) Oct 28, 2014
Hooyeei, Ol Ira-Skippy can put the Hanes-Skippy on the ignore. And guess you what? It worked.

Now that the nice peoples at physorg put that up for us to use, maybe some of those people who can not resist reading my stuffs and complain I'm taking up too much of their time can just push the ignore button and quit whining.

Returnering-Skippy, sorry Cher, it looks like you will be talking to the wall a lot from here on out. Don't let your psychologist-Skippys catch you doing much of that, otherwise they might be taking you back to the Mandeville or the Jackson for awhile, if you know what I mean.
Masleyko
3 / 5 (1) Oct 28, 2014
we can measure time much more preciselly ... up to 16 decimal places...
we can get the time difference ... between here and semisync orbit.
so much for GPS ... but why we cannot calculate force from it? Maybe not knowing exact mass of earth.. could anyone tell me why ?
movementiseternal
Oct 29, 2014
This comment has been removed by a moderator.
movementiseternal
Oct 29, 2014
This comment has been removed by a moderator.
Bob Osaka
3 / 5 (1) Oct 29, 2014
Ah, the arrogance of tiny, weak humans to call big "G" tiny and weak. Seems to be doing a good job of holding the universe together thus far.
Gravity as observed is a monopole. The repulsive force of gravity has not been directly observed and the best known theory which encompasses it is cosmological inflation. The force flinging all of the matter in the universe in all directions quite possibly is the illusive, repulsive gravity.
Were one to attempt precise measurements of the electromagnetic force with knowledge of only either the positive or negative poles accuracy would be nearly impossible.
Once the opposite pole of gravitational force is discovered, people will say "Now that's repulsive."
"No, it's the most beautiful discovery ever." Then we'll measure up.
Jeppe
Oct 29, 2014
This comment has been removed by a moderator.
Jeppe
Oct 29, 2014
This comment has been removed by a moderator.
swordsman
1 / 5 (6) Oct 29, 2014
I have shown that gravity is an electromagnetic force that is produced by electric charges. In order to determine the true gravitational constant, it must be done at the atom level. My calculations for the force between two hydrogen atoms exceed the force calculations based on the gravitational constant slightly. Note that the gravitational constant value has changed slightly over the decades since Planck first derived it. A main problem, in this respect, is that we are too close to the surface of the earth to measure it accurately here.

Refs: "The Secret of Gravity" (1997), "Secrets of the Atom" (1999), "The Birth of an Atom - How Matter is Formed in the Universe" (2010).
antialias_physorg
5 / 5 (10) Oct 29, 2014
Hooyeei, Ol Ira-Skippy can put the Hanes-Skippy on the ignore. And guess you what? It worked.

Thanks for point this out...never noticed that they added the ignore function. Yes: reading experience at physorg is going to improve immensely.
Uncle Ira
4 / 5 (8) Oct 29, 2014
Hooyeei, Ol Ira-Skippy can put the Hanes-Skippy on the ignore. And guess you what? It worked.

Thanks for point this out...never noticed that they added the ignore function. Yes: reading experience at physorg is going to improve immensely.


They just added that to my thing about three days ago. I'm glad they don't put on a limit on how many peoples you can use it on. But I still don't put it on for the extra silly couyons, because if you put it on someone you don't get to give them the bad karma points. If you can, I haven't figured out how to do that yet.
antialias_physorg
5 / 5 (11) Oct 29, 2014
They just added that to my thing about three days ago.

Useful feature. (Except that you can ignore yourself, which seems kind of a bug). Certainly infintely better than the Report button and waiting for mods to do something about the cranks.
because if you put it on someone you don't get to give them the bad karma points.

They crave attention. So not noticing them is probably more effective than handing out bad karma. But yeah - It's fun on occasion.
Uncle Ira
3.9 / 5 (7) Oct 29, 2014
But yeah - It's fun on occasion.


It is the big fun with the Really-Skippy during his troll/mod/gang/bot/mafia hunts. He is so silly I would never put him on the ignore list.
Scroofinator
1 / 5 (3) Oct 29, 2014
My calculations for the force between two hydrogen atoms exceed the force calculations based on the gravitational constant slightly


My guess is that the gravitational force between two H atoms would be slightly greater than expected. Is that what your calculations have shown?

Gravity is just the manifestation of EM acting in every direction instantaneously.
Uncle Ira
3.9 / 5 (7) Oct 29, 2014
I have shown that gravity is an electromagnetic force that is produced by electric charges.


Are you that guy? Well how you are Skippy? Most peoples would be ashamed to show their face after showing the gravity is an electromagnetic force.
Uncle Ira
3.9 / 5 (7) Oct 29, 2014
Note that the gravitational constant value has changed slightly over the decades since Planck first derived it..


Well golly gee Skippy, that is where your ciphering went wrong, you are using the wrong numbers. Planck-Skippy didn't derive a gravitational constant. Ol Cavendish-Skippy was the one who starting making numbers for that.

Now aren't you embarrassed that it was Ira-Skippy who knew that and you didn't? I know I would be. But I was reading a book about the Cavendish-Skippys, all of them, the family I mean, last week
HannesAlfven
1 / 5 (4) Oct 29, 2014
Re: "Now that the nice peoples at physorg put that up for us to use, maybe some of those people who can not resist reading my stuffs and complain I'm taking up too much of their time can just push the ignore button and quit whining."

I encourage everybody who wants to exist within a bubble of their own making to click the ignore button. That way, we can all talk past one another, instead of with each other.
tritace
Nov 02, 2014
This comment has been removed by a moderator.
Scroofinator
not rated yet Nov 02, 2014
Are you sure neutrons contribute gravitationally? Have we ever tested the gravitational force between neutron-neutron? I know we have to use them for mass gravitational calculations, but they have always been a part of atoms as a whole.

While we're on the subject, why do we still not understand what causes the mass defects in measured vs calculated atomic weights?
Captain Stumpy
5 / 5 (3) Nov 02, 2014
the vacuum density is arbitrary value in AWT
@jeppe/ZEPHIR
that is because aw/daw (which is a philosophy, NOT a theory) is a false premiss and a failed conjecture which has been proven to a very high degree of accuracy with experimental evidence:
http://exphy.uni-...2009.pdf

the only reason that anyone would still believe in it is if they were ignorant backwards acolytes pushing a religion that is proven false the world over

They crave attention. So not noticing them is probably more effective than handing out bad karma.
@AA_P
@Ira
yeah... but then who will make sure that real science bets posted and that the idiots don't flood the site with aw/daw or eu Pseudoscience?

that is the only reason i am not using it
except on rc

pseudoscience must be stopped
you can report them & skip the ignore button too
Uncle Ira
3.7 / 5 (3) Nov 02, 2014
that is the only reason i am not using it
except on rc


How you are Captain-Skippy? Where has the Really-Skippy been lately? Maybe he getting the inpatient treatment of his mental conditions lately.

Last time I heard from him was when he was interfering with my studying time for my radio license. I took the test and passed it good so I have more time to give to him since he sounded like he was missing our discussing of the not science stuffs.
rfw
not rated yet Nov 04, 2014
Perhaps G changes because the molten masses of material in the Earth's core beneath us shift around?
NOM
5 / 5 (4) Nov 04, 2014
you can report them & skip the ignore button too
Except it appears that the mods have an "ignore report" button
erson
Nov 05, 2014
This comment has been removed by a moderator.
gstillwell
1 / 5 (2) Dec 21, 2014
Depending on source and sample mass composition and room temperature where the experiment is performed and data collected will determine G. It will vary by these parameters along with distance from the equator, height from sea level and location of lab i.e. if experiment room (lab) is near a large attractor mass such as a body of water or mountain; height of room in building i.e. basement - ground floor versus up on a higher floor and temperature of surrounding obscure bulk mass such as a stainless steal platform or cement floor.
imido
Dec 21, 2014
This comment has been removed by a moderator.
imido
Dec 21, 2014
This comment has been removed by a moderator.

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