Researchers use new method to calculate gravitational constant

Jun 19, 2014 by Bob Yirka report
The big picture for big G. Published results of measurements of the gravitational constant, G, over the past 32 years. Credit: Nature (2014) doi:10.1038/nature13433

(Phys.org) —A team of researchers in Europe has come up with a new way to determine the gravitational constant G. Rather than relying on using torque based techniques to measure gravitational pull, the researchers instead attempted to measure the attraction between a cloud of cold rubidium atoms and tungsten weights. They came up with a value for G of 6.67191(99) x 10−11 m3 kg−1 s−2. In their paper published in the journal Nature, the team describes their technique in great detail, and suggest it can be used to further refine a value for G. Stephan Schlamminger offers a News & Views piece describing the new technique and what it might mean, in the same journal issue.

It was Isaac Newton who first came up with the law describing how gravity pulls everything together, though he wasn't able to come up with an actual constant value. That was left to Henry Cavendish—he measured the torque that resulted from the gravitational attraction of weights attached to a rotating balance. Subsequent attempts have used roughly the same strategy with better precision. Unfortunately, as more precision is introduced the more differences researchers see in their results compared to what others find—there are just too many other attractive forces interfering. To overcome some of those problems, the researchers working on this new effort took an entirely new approach.

The team set up a cylinder containing tungsten weights surrounding a container that held a cloud of rubidium atoms. The apparatus was set up in such a way as to allow the researchers to pull the atoms up against Earth's gravity, or down to allow them to accelerate. The researchers used a laser light to split the cloud into two areas which were then pushed into a vacuum chamber. The two different cloud groups would rise to different heights before being pulled to the bottom of the chamber, creating an interference pattern—measurements of trajectory alterations in the interference pattern caused by the tungsten weights was used to determine G. The team ran measurements using multiple devices and with the weights in different positions to minimize noise from other sources.

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Researcher Guglielmo Tino on a new way to measure Big G. Credit: Nature.

Currently it's not clear if it's a good or bad thing that the new value for G differs from what others have found using traditional methods. Further research will help—the team notes that their technique offers promise of further refinement which could bring an even more precise value for G.

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More information: Precision measurement of the Newtonian gravitational constant using cold atoms, Nature (2014) DOI: 10.1038/nature13433

Abstract
About 300 experiments have tried to determine the value of the Newtonian gravitational constant, G, so far, but large discrepancies in the results have made it impossible to know its value precisely. The weakness of the gravitational interaction and the impossibility of shielding the effects of gravity make it very difficult to measure G while keeping systematic effects under control. Most previous experiments performed were based on the torsion pendulum or torsion balance scheme as in the experiment by Cavendish in 1798, and in all cases macroscopic masses were used. Here we report the precise determination of G using laser-cooled atoms and quantum interferometry. We obtain the value G = 6.67191(99) × 10−11 m3 kg−1 s−2 with a relative uncertainty of 150 parts per million (the combined standard uncertainty is given in parentheses). Our value differs by 1.5 combined standard deviations from the current recommended value of the Committee on Data for Science and Technology. A conceptually different experiment such as ours helps to identify the systematic errors that have proved elusive in previous experiments, thus improving the confidence in the value of G. There is no definitive relationship between G and the other fundamental constants, and there is no theoretical prediction for its value, against which to test experimental results. Improving the precision with which we know G has not only a pure metrological interest, but is also important because of the key role that G has in theories of gravitation, cosmology, particle physics and astrophysics and in geophysical models.

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User comments : 20

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Egleton
2.4 / 5 (7) Jun 19, 2014
Who said Big G is a constant?
Torbjorn_Larsson_OM
5 / 5 (8) Jun 19, 2014
Both our basal theories and cosmology says G is constant. General relativity holds, with G fixed, within a vast volume of space and time through our cosmology observations.
julianpenrod
1 / 5 (8) Jun 19, 2014
A number of trends are apparent here. If you look at this and other charts of the obtained values of G, not only are the actual average values of G from each experiment different, which can be expected, but the overall error ranges are increasingly placing calculations outside the average value range from all past experiments, and, in addition, measurement error ranges are increasingly failing to overlap each other.
Maybe there's a key here. Remember, gravity is seen as inconsistent with quantum measurements because gravity depends on distance and, supposedly, distance is not obtainable with high precision on the quantum level. The same seems to be happening here, experiments with high errors, not using quantum interactions, agree, while quantum related experiments don't agree with each other! Maybe no fine measurement can derive G! Maybe the same, large error experiment, performed many, many, many times will produce a value!
CrossMan
5 / 5 (10) Jun 19, 2014
Or maybe "The weakness of the gravitational interaction and the impossibility of shielding the effects of gravity make it very difficult to measure G while keeping systematic effects under control."
julianpenrod
1.2 / 5 (9) Jun 19, 2014
What CrossMan, and whoever decided to give them a rating of "5", do not demonstrate enough understanding of the world to realize is that, if the measurement attempts are performed by qualified individuals, the error terms applied already take "systematic effects" into account! If they provide small errors, that is because, supposedly, all influences have been accounted for and amount to no greater an effect than that indicated! And, even then, both the values of G disagree, the error terms do not overlap, even within a reasonable range! The supposedly more imprecise measurements from the past fall within a general region, but the modern, supposedly more accurate values, don't! The evidence is that, in those ever more precise measurements, even with all sources of error accounted for, the value of G is straying from the value universally derived from less precise tests!
supamark23
5 / 5 (11) Jun 19, 2014
Julian, what you don't understand is that gravity knows no bounds - you are attracted gravitationally to everything else in the universe, so it can be a bit hard to measure when you cannot shield the experiment from the gravitational effects of every other bit of mass in the universe.
Uncle Ira
3.7 / 5 (7) Jun 19, 2014
Julian, what you don't understand is that gravity knows no bounds - you are attracted gravitationally to everything else in the universe, so it can be a bit hard to measure when you cannot shield the experiment from the gravitational effects of every other bit of mass in the universe.


@ Supamark-Skippy. Thanks for that because I was wondering about that. I was just about to ask if all the stuffs might be making it hard to measure here if something over there was working on it. You saved me from asking and getting embarrassed like I did with my guess on the UV making new molecules talking with no-Skippy.. Good karma points for you Supamark-Skippy.
julianpenrod
1.6 / 5 (8) Jun 19, 2014
An exercise in the unethicality of "science" devotees.
If the universality of gravity interacts from all directions, then "error" terms are meaningless. Why don't CrossMan and supamark23 criticize the use of "error" terms?
In a uniformly distributed universe, any effects are distributed evenly. If there was a net effect of gravity in the universe, it would be obvious. Also, the earth is turning, so the experiments are not constantly affected in the same way.
And why didn't any universal effect influence past measurements? Those from before agree within an interval, the new ones fall outside the interval.
But the evident hate Mafia on PhysOrg will apparently still give me a 1" rating, even without reading what I say.
supamark23
5 / 5 (6) Jun 19, 2014
We are not in a uniformly distributed universe. If you look at the large scale structure of the visible universe, it looks a bit like cobwebs with dense bits (filaments) and less dense bits (voids). Also, the discrepancy between this measure and the accepted standard is actually quite small - they can both be rounded to 6.67 x 10^-11. It seems the problem here is you don't have the educational background to understand the articles.
Uncle Ira
3.4 / 5 (5) Jun 19, 2014
But the evident hate Mafia on PhysOrg will apparently still give me a 1" rating, even without reading what I say.


Well Julia-Skippy, I'm glad to oblige you with one karma point. I don't hate you. Ol Ira don't even know you. I did too read you what you say and that is why you get the bad karma points. I read you say the nice peoples at the physorg is mafia like the Really-Skippy says. That's why you get the one point. You should be glad they don't let me give the negative numbers for karma points. Now you can take your one karma rating and put it under that silly looking pointy cap you look so good in when it's on your head.
arom
1 / 5 (5) Jun 19, 2014
About 300 experiments have tried to determine the value of the Newtonian gravitational constant, G, so far, but large discrepancies in the results have made it impossible to know its value precisely…. There is no definitive relationship between G and the other fundamental constants, and there is no theoretical prediction for its value, against which to test experimental results…


Maybe understanding the working mechanism of gravity, i.e. how mass attract each other could help to answer mentioned problem …
http://www.vacuum...=7〈=en
julianpenrod
1.5 / 5 (6) Jun 20, 2014
In the aggregate, even a cobweb is approximately uniform. But, as I asked, and which the liars are avoiding addressing, if there is such an overal imbalance as to affect themeasuring of G, what directioin is it? If there is such an imbalance, it would have to be dragging everything along with it.
But, then, too, remember, the apparatus was rotating with the earth, revolving around the sun, its orientation constantly changing. No imbalance could so constantly alter results.
And, which the liars are avoiding addressing, the values in the past tended to fall within the same band, even to overlap each other. If there is such a tendency of the distribution of matter to throw off measurements, why was there such an agreement of measurements before, but, now, they are thrown off further than any other such measurements?
Native
1 / 5 (2) Jun 20, 2014
This is really an interesting experiment which can be of great cosmological value!

Here scientists mixes the fundamental forces in order to find the finite constant of the supposed fundamental gravity force.

They govern the atom clouds experiment chamber by magnetic fields.

They add electromagnetic energy in order to accelerate the atoms.

They measure the experiment by electromagnetic apparatus.

They shield the experiment chamber from gravitational Solar and Lunar influence but completely ignores their conscious use and influence from the electromagnetic fundamental forces which in fact are crucial for getting the experiment going in the first hand.

They think they adds matter to the atoms by charging laser energy, but in fact, they just add volume.

They think this is a constant measurement of gravitation where it in fact is a pure measurement of electromagnetical influence and has nothing to do with gravity in traditional thinking.

To be continued.
Native
1 / 5 (2) Jun 20, 2014
Continued
The scientists in this experiment simply confuses electromagnetical influence for gravitational influence and in this strange way the scientist struck the real truth:

"Gravitation" is not a matter of matter alone but more so a matter of energy governed by the fundamental electromagnetic forces and their qualitative dynamics. Forces which are much stronger than the supposed "gravitational force".

So: Now we are indeed going somewhere fast in modern cosmology – if the scientists discover and recognized what they accidentally have stumbled over in this experiment.
Tachyon8491
5 / 5 (1) Jun 20, 2014
"...There is no definitive relationship between G and the other fundamental constants..." That may well be true but is because we have not been able to quantify and model relationships which must exist between these dynamics. Theoretical physics needs to make more headway here.
twalsh
not rated yet Jun 20, 2014
A basic; but, perhaps irreverent question: 'is ALL mass a generator of gravity'? All mass is affected by gravity; but, do all particles that have mass generate gravity? Could gravity be a physical process and, therefore, G may not be continuously a constant? A study that has me thinking this is:
http://phys.org/n...ark.html

Do the stream concentrations, mentioned above, contain elements or hydrogen isotopes above neutral hydrogen and in concentrations that might indicate gravitational attraction missing from the stream itself? Does gravitational generation stop at and below neutral Hydrogen? Strange thinking, I know...when I asked this elsewhere, quick deletes follow with what garbage responses - still, not so sure. Do we know with certainty how initial fusion in the early universe started?
otero
Jun 20, 2014
This comment has been removed by a moderator.
Reg Mundy
1 / 5 (1) Jun 22, 2014
"there is no theoretical prediction for its value, against which to test experimental results"

Well, there is, actually. In expansion theory, the apparent increase in the wavelength to light over time (if we are not in an expanding universe) provides a mechanism for the expansion of a photon in terms of increase per second of our time. As matter is equivalent to energy (E=mc2) then the average number of photons (or elementary particles in terms of photon components) can be ascertained, thus the expansion per gram of matter per second which, if the matter is in the form of a sphere, results in an increase in radius per second, i.e. an expansion producing an acceleration of approximately 10 meters/sec/sec on a sphere of Earth mass. This calculation is described in "The Situation of Gravity" as evidence for an effect Newton called gravity in a steady-state universe.
twalsh
not rated yet Jul 13, 2014
The http link (although interesting on its own) I miscoppied in my entry above is:

...csmonitor.com/Science/2013/0508/Colossal-hydrogen-bridge-between-galaxies-could-be-fuel-line-for-new-stars/(page)/2

This article refers to a 'Nature' journal study. From the article, if there is no Hydrogen accretion from either Galexy then, maybe, it is possible to ask if Neutral Hydrogen even generates a gravitational force?
If Neutral Hydrogen does not exert an atom to atom gravitational attraction, then isn't primordial Helium critical to first star fusion? That G is, unlike Pi, not refined by a century of experiments, then G may not be a true constant; but rather, on earth(?), a stable variable? If the numerous experiments to measure G were repeated over time and/or place, do the repeated individual experiments produce identical results? If not, is there any relationship? Has this articles experiment been repeated elsewhere? If so, what were the results?
DoieaS
1 / 5 (1) Jul 13, 2014
This study didn't calculate the gravity constant - it just added another method into toolbox of gravity constant measurements. And it didn't remove the controversy about fluctuating values of gravity constant, on the contrary - it increased its uncertainty instead.

IMO the recent changes in gravity constant are caused with clouds of dark matter pervading the solar system. Note that the maximum of gravity constant change in 2002 coincides with reversal of the trend of global warming. The physicists should be more observant to their experimental noise - especially when their own theories expect such a situation. The easy way how to verify this hypothesis is to follow the "missing weight" of iridium kilogram prototype - it should repair itself gradually together with global warming...
twalsh
not rated yet Jul 13, 2014
Imagination should not demean valuable and productive theories; neither should valuable and productive theories demean imagination. Just don't take imagination too seriously and no theory, no matter how productive, is gospel. Dirac's large number hypothesis proposed a variable value for G; a value that varies continuously over time. Perhaps the variance of G over time, is not continuous. Could G be the result of an unseen physical process? If so, QM and gravity might have a common connection? ...just imagination, of course. The article in my first post does raise questions about dark matter and galaxy halos.