Deep space experiment could measure the gravitational constant with nearly 1,000 times improvement in accuracy (Update)

May 17, 2016 by Lisa Zyga, Phys.org report

In the proposed experimental setup, a host spacecraft (right) shines a femtosecond laser pulse onto a retroreflector moving in the tunnel of a sphere (left). The period of the retroreflector’s harmonic motion provides information on the value of G. Credit: Feldman et al. ©2016 IOP Publishing
(Phys.org)—Scientists have proposed an experiment that could measure the value of Newton's gravitational constant, G, from deep space instead of an Earth-based laboratory. The researchers predict that the deep space experiment could estimate G with an improvement in precision of nearly three orders of magnitude, since it would avoid the influence of Earth's gravity.

The researchers, Michael Feldman et al., have published a paper on the proposed experiment in a recent issue of Classical and Quantum Gravity.

Uncertainty with Big G

Newton's gravitational constant, G, determines the strength of the gravitational force between any two objects anywhere in the universe. Over the past century, a dozen or so Earth-based experiments have used torsion balances, atom interferometers, and other tools to measure the value of G to be approximately 6.67408 x 10-11, with an uncertainty of 4.7 × 10−5.

Although this may sound precise, it is not very precise at all compared to many other physical constants, which have uncertainties that are many orders of magnitude smaller than this. In recent years, the large variations in the measured values of G have caused scientists to question if G is truly constant at all. (Currently, the overwhelming consensus is that G is constant, and that the variations are due to large systematic measurement errors.)

"G is currently the least well known of all the , which is embarrassing," Feldman told Phys.org. "A more precise number, and the possibility that G could vary with time, location, or the type of matter involved, could link to improvements in Einstein's general relativity, including ."

One of the main reasons that G is so difficult to measure accurately is that experiments must account for the influence of Earth's gravity, g (sometimes called "little g" in contrast to "big G"). Little g is the acceleration due to gravity specifically on Earth, where it has a constant value of approximately 9.8 m/s2. Elsewhere in the universe, this value changes, since it depends on the Earth's mass and the distance between the Earth and another object. However, the value of big G does not depend on these factors, and so it remains the same everywhere in the universe.

Deep space lab

In the new paper, the researchers suggest that the best way to avoid the effects of Earth's gravity on measurements of G is to perform the experiment in deep space, which refers to space outside our solar system.

The scientists propose to launch their apparatus into deep space, likely by "piggybacking" on a major mission. Out there, where the gravity of planets and stars would be negligible, the host spacecraft would release a spherical object that has a 1-cm-wide tunnel through its center. Then (this would likely be the most difficult part), the host spacecraft—which is constantly spinning the whole time—would eject a much smaller oscillating object into the tunnel in the sphere at just the right angle and speed so that the object would move back and forth through the tunnel, without bouncing off the walls.

The host apparatus would continually shine femtosecond laser pulses on the object as it oscillates in the tunnel, and the object (a retroreflector) would reflect these pulses back to the host spacecraft. These pulses would provide data on the period of the object's harmonic motion, which is directly dependent on the value of G. The data would then be sent back to Earth via radio communication for interpretation.

If everything goes as expected, the researchers' simulations showed that this experiment could measure G with an uncertainty of 6.3 x 10-8, which is nearly three orders of magnitude more precise than the current best measurement.

Even though the experiment wouldn't have to deal with the Earth's gravity, it would still have to contend with other, smaller non-gravitational accelerations that would also affect the retroreflector's motion. These influences include solar radiation pressure, solar tidal effects, cosmic rays, and the momentum from the . Some of these effects could be dealt with through careful design—for example, the sphere could be shielded from solar radiation pressure by positioning it in the shadow of the host spacecraft. But the researchers explain that any acceleration greater than 10-17 m/s2 must be modeled and accounted for when interpreting the data.

Why measure G?

The National Science Foundation in the US recently issued a solicitation for new approaches for measuring G (Ideas Lab: Measuring "Big G" Challenge). The NSF webpage says that measuring a more precise value of G will benefit many fields of physics and metrology, such as understanding the Casimir effect, improving the spring constants that are used to calibrate atomic force microscopy cantilevers, and understanding intermolecular forces in DNA. A precise value of G might also be used to test proposed theories that unify gravity with quantum electrodynamics.

Explore further: Why do measurements of the gravitational constant vary so much?

More information: Michael R. Feldman et al. "Deep space experiment to measure G." Classical and Quantum Gravity. DOI: 10.1088/0264-9381/33/12/125013
Also at arXiv:1605.02126 [gr-qc]

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notalltheglitterisgold
1.4 / 5 (16) May 17, 2016
does the magnetism of the core, the make up of the core also the planet that is conducting the core magnate have any effect of Gforce? example global warming unfreezing hydrogen molecules there for producing more gas to transform the magnetic core momentum? also think about the pole flip (molecules changing in the atmosphere) will the Gforce change on earth when the poles flip? am I getting the whole concept of Gforce wrong? Thank you for your time.
compose
May 17, 2016
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compose
May 17, 2016
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shavera
4.8 / 5 (25) May 17, 2016
magnetism has exceedingly little to do with gravitation. I really can't understand anything else you posted since none of that really sounds like contemporary scientific theory. I'm not really aware of any hypotheses that global warming will affect the Earth's magnetic field.

"G-force" isn't really a scientific concept, but can be a useful one. It's just a measure of how strong a force is. Is the force equivalent to the force of gravitation at sea level? It's a 1-G force. Twice as strong: 2-G.

The "G" here isn't the same as the "G" in "G-force." At the very simplest explanation, it's just a way to tell us how strongly two masses will attract each other at some distance apart. More specifically, it's a constant that tells us how strongly energy changes measures of space and time around it, which indirectly leads to the effect we call gravity.
shavera
4.8 / 5 (19) May 17, 2016
compose: the same authors address the apparent fluctuations in 'G' measurements in an earlier paper: https://arxiv.org...6604.pdf Their conclusion is that it's largely to do with seismic trends on Earth, where all the experiments are currently held. This experiment is, perhaps, specifically designed to address that issue since it won't suffer from the same systematic error. It is specifically a response to this NSF proposal to do such an experiment http://www.nsf.go...520.htm.
compose
May 17, 2016
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compose
May 17, 2016
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compose
May 17, 2016
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antigoracle
1.4 / 5 (11) May 17, 2016
They should definitely design this to be a long term experiment so that perhaps we may shed some light on the following - http://phys.org/n...tml#nRlv
compose
May 17, 2016
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TechnoCreed
5 / 5 (10) May 17, 2016
does the magnetism of the core, the make up of the core also the planet that is conducting the core magnate have any effect of Gforce
If yes, then this effect must be very subtle - the magnetism at the surface of Sun is at least twenty-time higher than at the Earth surface (in sunspots even much higher) - yet the Sun remains https://www.thegu...-nature.

Thank you Zephir. I was not aware that the sun was so close to be a perfect sphere. Here is the link to the paper related to the Guardian article https://www.resea...iability
compose
May 17, 2016
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Whydening Gyre
5 / 5 (6) May 17, 2016

does the magnetism of the core, the make up of the core also the planet that is conducting the core magnate have any effect of Gforce

If yes, then this effect must be very subtle - the magnetism at the surface of Sun is at least twenty-time higher than at the Earth surface (in sunspots even much higher) - yet the Sun remains https://www.thegu...-nature.

Techno. That was good of you to say. I've found most of his input in this thread to be informative and thought-provoking...
And Shavera. Good of you to engage him so civilly.
And not a single water strider on a ripple in sight! :-)
compose
May 17, 2016
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adam_russell_9615
5 / 5 (6) May 17, 2016
Is there any way to derive the expected value of G from theory or is it only able to be found empirically?
compose
May 17, 2016
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Whydening Gyre
5 / 5 (7) May 17, 2016
This is all consequence of water strider/ducks ripples on background because the above effects aren't consistent with 4D general relativity. That is to say, the relativity considers that the space-time gains energy when being curved, it's even aware, that energy can be converted to mass by E=mc^2 formula - but massive space-time? Impossible...

I'm stickin' to Highly Improbable...
Whydening Gyre
5 / 5 (5) May 17, 2016
You can't calculate the numerical value of Newton's constant from the first principle because it is a dimensionful constant – it has units – so the numerical value depends on the magnitude of the units. And because e.g. the kilogram is defined as the mass of a platinum prototype, "pure calculation" can't know how large the kilogram is, which also means that it can't determine the numerical value of Newton's constant which depends on the definition of a kilogram. The best physical theory so far (Heim one) uses h (Planck's Constant), G (Gravitational constant), vacuum permittivity and permeability for calculations of mass of many particles.

Yeah, but...
Isn't the gravitational constant derived from Newton, that heim requires to use his method, in the first place?
axemaster
4.8 / 5 (8) May 17, 2016
I have to admit, I have a hard time believing that this experiment is even possible to carry out... It really sounds too difficult.
compose
May 18, 2016
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Kedas
4.4 / 5 (5) May 18, 2016
It would be so cool if it turns out NOT to be a constant, 3 more digits can show that :)
because that would be very interesting for anti-gravity.
compose
May 18, 2016
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indio007
1 / 5 (5) May 18, 2016
Maybe the Cavendish experiment was crap and that's the reason for the discrepancies?
compose
May 18, 2016
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Gustav
5 / 5 (8) May 18, 2016
I know for sure that G changes with time and often. It is gigantic early in the morning. When I get out of bed, I nearly fold under my weight. Eventually, it gets weaker and weaker to finally settle on something approaching normalcy, but only after I have had my coffee.
Tektrix
5 / 5 (7) May 18, 2016
@Gustav- Gravity is increasing in direct proportion to my age. ;)
Whydening Gyre
5 / 5 (8) May 18, 2016
Maybe the Cavendish experiment was crap and that's the reason for the discrepancies?

Just read about it. Amazingly accurate considering the experimental setup he had. Even more amazing was the elegant math that interpreted its actions....
barakn
4.6 / 5 (11) May 18, 2016
Why does it take the mods so long to identify and remove Zephir's sockpuppets?
antigoracle
1.7 / 5 (11) May 18, 2016
This is why we need more women in science; to help us find the G spot.
Da Schneib
5 / 5 (6) May 18, 2016
Is there any way to derive the expected value of G from theory or is it only able to be found empirically?
Great question.

It is an empirical value in both Newtonian TUG and Einsteinian GRT. If we had a quantum gravity theory we might be able to get a theoretical value. Note "might;" it's possible to imagine quantum gravity that still had the gravitational constant (or some constant it's dependent upon) be an empirical value, too. Remember the 23 free parameters of the Standard Model.
Whydening Gyre
4.5 / 5 (8) May 18, 2016
Is there any way to derive the expected value of G from theory or is it only able to be found empirically?
Great question.

It is an empirical value in both Newtonian TUG and Einsteinian GRT. If we had a quantum gravity theory we might be able to get a theoretical value.

Are you intimating it might be variable according to mass variations in any given locality locality?
Da Schneib
5 / 5 (7) May 19, 2016
I have to admit, I have a hard time believing that this experiment is even possible to carry out... It really sounds too difficult.
It's easier in space, but it's still very exacting. I haven't looked at the paper yet so I haven't seen the exact method of establishing the harmonic motion of the object in the tunnel; I expect it's electromagnetic.

More later if the article isn't paywalled or is on arXiv.
Da Schneib
5 / 5 (7) May 19, 2016
Is there any way to derive the expected value of G from theory or is it only able to be found empirically?
Great question.

It is an empirical value in both Newtonian TUG and Einsteinian GRT. If we had a quantum gravity theory we might be able to get a theoretical value.

Are you intimating it might be variable according to mass variations in any given locality locality?

No, but the measurements might. Too many variables; these guys are over a masscon in the mantle, those guys are over a batholith, them guys over there have a lot of mountains close by. Each one is at a different altitude, and that's relative to sea level which is different in different places because the Earth is pear-shaped. Then there are systematic errors due to things like air currents. Somebody forgets to subtract the gravity of the Moon or daily errors from the gravity of the Sun creep in. Out in space you don't have to worry about a lot of these things.
BongThePuffin
May 19, 2016
This comment has been removed by a moderator.
Whydening Gyre
4.2 / 5 (10) May 19, 2016
Too bad... Zephir actually had some interesting input in this thread....
But, like the Terminator - He'll be back...:-)
Ryan1981
5 / 5 (2) May 19, 2016
I was wondering, is there not a way we can use the data from the LHC in some way to estimate G? Or are the forces there too small to be of any use?
antialias_physorg
3.9 / 5 (7) May 19, 2016
I was wondering, is there not a way we can use the data from the LHC in some way to estimate G

Erm...Whut? Please look up what G is and then what the LHC does. Then you can likely answer this for yourself.
torbjorn_b_g_larsson
4.7 / 5 (3) May 19, 2016
A quick peek in the arxiv paper show me that "deep space" isn't even outside our planet system, far less outside our vast solar system. They mean > 25 AU while Neptune has a semi-major axis of 30 AU. [ https://en.wikipe.../Neptune ]

@not: No, the concept of gravity as force (for weakly curved spacetime) isn't wrong. Magnetism is included as all energies as soon as you use general relativity as your description of gravity.
torbjorn_b_g_larsson
5 / 5 (3) May 19, 2016
@adam: "Is there any way to derive the expected value of G from theory or is it only able to be found empirically?"

When you naively quantize gravity you find you can do that for weak fields. That is summed up in Core Theory, "the Feynman path-integral formulation of an amplitude for going from one field configuration to another one, in the effective field theory consisting of Einstein's general theory of relativity plus the Standard Model of particle physics", the theory behind everyday physics. [ ]http://www.preposterousuniverse.com/blog/2015/09/29/core-theory-t-shirts/

But you also find that without a quantum gravity theory for high energies we have to use empirical fits for gravity field theory parameters, which then goes into G of the general relativity theory. [ https://golem.ph....639.html ]
Da Schneib
5 / 5 (3) May 19, 2016
@not: No, the concept of gravity as force (for weakly curved spacetime) isn't wrong. Magnetism is included as all energies as soon as you use general relativity as your description of gravity.
Errr, actually this is misleading. GRT only accounts for gravity; it does so as curvature of space. Electromagnetism and the weak and color forces aren't included. You'd need more terms for them than are present in the EFE. I don't think @not will be able to make much of your description here.

GRT does include a magnetic field that's attracting an object that does not move; this is potential energy and thus goes into the first and second terms of the EFE. But that's pretty esoteric for someone who's perhaps a bit unclear on the difference between gravity and magnetism.
torbjorn_b_g_larsson
5 / 5 (3) May 22, 2016
@not: No, the concept of gravity as force (for weakly curved spacetime) isn't wrong. Magnetism is included as all energies as soon as you use general relativity as your description of gravity.
Errr, actually this is misleading. GRT only accounts for gravity; it does so as curvature of space. Electromagnetism and the weak and color forces aren't included.


Maybe "included as all energies" was a bit cryptic. The stress-energy tensor makes the inclusion, and "tells spacetime how to curve". In the linearized EFE, but not in the Newtonian gravity approximation, a slowly changing stress-energy (small velocities) can accommodate some magnetic fields, I think. (But I haven't studied GR, so I can't estimate the effects.)
lupus
5 / 5 (1) May 23, 2016
Have I misunderstood this. The uncertainty of 4.7 × 10−5 seems to be about 700,000 times greater than G itself which is 6.67408 x 10-11
TechnoCreed
5 / 5 (3) May 23, 2016
@lupus
They mean a relative uncertainty. The absolute uncertainty is 4.7 × 10−5 * 6.67408 x 10-11.

Right on

In consise form 6.674 08(31) x 10-11 m³ / kg s²
It means that the true value of G is somewhere between 6.67377 x 10-11 m3 / kg s² and 6.67439 x 10-11 m³ / kg s² http://physics.ni...Value?bg
Da Schneib
5 / 5 (3) May 23, 2016
@lupus
They mean a relative uncertainty. The absolute uncertainty is 4.7 × 10−5 * 6.67408 x 10-11.


In consise form 6.674 08(31) x 10-11 m³ / kg s²


Yep. A little bit of calculation shows the absolute uncertainty would be 3.1 x 10^-15. Exactly what @Techno said.
JackBidnik
2.3 / 5 (3) May 24, 2016
There is an interesting corollary to the relation that I posted on Leonard Susskind's blog on March 11,2015, where I showed
that the same equation can describe both the gravitational force on planets, and the electrostatic force between proton and electron in the Bohr hydrogen atom.

The equation is: F =Gs* M*m/r ^ 2, where

Gs = (1/ (M+m))*r*Vo^2*c^4/(Vo^2 +c^2)^2

and where Vo is orbital velocity, c is speed of light, and Gs is what I call Special G.

The surprising corollary is that this equation also applies to stars in orbit about galaxies,
thus indicating there is no need to postulate dark matter, as the equation dispels the problem of
insufficient force using the Newtonian constant.
Check last page of Susskind's blog for more info.
radiohead_769
not rated yet Jun 10, 2016
Apart from G, have permittivity and permeability of free space been measured in deep space? Anybody has a reference?

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