J. Rogers, SE Ohio
A Deep-Space Oscillating Test Mass Experiment to
Determine
the Gravitational Constant G to Nine Significant
Figures
Abstract
The gravitational constant G remains the least precisely known
fundamental constant in physics, determined to only approximately five
significant figures after three centuries of effort. We identify the
structural cause: G and the mass M of any gravitating body large enough
to produce a measurable field are observationally inseparable. Every
astronomical measurement yields only the product GM. Cavendish-style
laboratory experiments have failed to converge across 300 years. We
propose a conceptually simple resolution: construct a toroidal mass of
precisely known composition in deep space, far from competing
gravitational sources, bore a hole through its center, and drop a test
mass through the hole. The test mass oscillates indefinitely under
gravity alone, with period determined entirely by the known surface mass
density of the toroid. An LED-based laser interferometer simultaneously
tracks oscillation period T, instantaneous acceleration a(t), and
distance r(t) from the center hole continuously throughout each
oscillation, providing three independent and overdetermined routes to G
from a single data stream. Because the mass is known from construction
rather than inferred from gravity, the GM degeneracy is broken for the
first time. The test mass is repeatedly lifted, settled, and released,
accumulating thousands of independent period measurements per run over a
mission lifetime exceeding one year on station. Tidal contamination from
the Sun at 3 AU is verified by calculation to be 1.1 × 10⁸ times smaller
than the measurement signal, and galactic tidal forces are 6.7 × 10⁷
times smaller. The deep space environment is not merely quiet — the rest
of the universe has gone effectively silent. All required technologies
are flight-proven. The mission requires no new physics and no new
engineering principles.
1. The Problem: GM is What Nature Exposes
Newton's law of gravitation is conventionally written as:
F = G M m / r²
and general relativity encodes gravitational geometry through terms
of the form GM/c²r. In both frameworks, G and M appear as a product.
This is not a mathematical convenience — it reflects a deep operational
fact: no measurement that relies on gravity to establish the behavior of
a gravitating body can separate G from M. The observable is always
GM.
NASA's operational practice makes this explicit. Planetary
ephemerides, spacecraft navigation, and GPS relativistic corrections all
use the gravitational parameter mu = GM, known for Earth to
approximately nine significant figures. The individual values of G and M
are not used. They cannot be, because the mass of any astronomically
significant body is inferred from its gravitational behavior, making any
G determination from astronomical sources tautological. If we knew the
mass of the Earth independently, we would know G to identical precision.
We do not, and the circularity is exact and complete.
G is not philosophically uncertain. It has an exact value. Nature
knows it precisely. The uncertainty is entirely on the measurement side,
and the measurement side has a specific, identifiable structural problem
that has gone unresolved for three centuries.
2. The Failure of Laboratory Methods
The Cavendish torsion balance, introduced in 1798, was designed to
escape this circularity by using laboratory-scale masses whose weight
could be determined independently through mechanical means. After 300
years of refinement across dozens of independent experiments at leading
metrology institutes worldwide, the results do not converge.
Recent high-precision determinations of G disagree with each other by
40 to 50 parts per million, while individual experiments claim
uncertainties of 10 to 20 parts per million. The discrepancy between
results exceeds the claimed precision by a factor of two to five. CODATA
periodically widens the accepted uncertainty interval to accommodate the
spread.
The fundamental difficulty is the signal-to-noise environment.
Gravity is the weakest force. Laboratory-scale masses produce
gravitational forces at or below the level of seismic noise, thermal
expansion, electrostatic coupling, and the gravitational influence of
nearby structures. No experimental design has succeeded in isolating the
gravitational signal cleanly from this noise floor at the required
precision. Three hundred years is sufficient to conclude this is not a
solvable engineering problem in the terrestrial environment.
3. The Consequence: A Blurred Axis
The inability to determine G precisely propagates directly into every
dimensionless ratio that crosses the gravitational-electromagnetic
interface. The Planck mass is defined as:
m_P = sqrt(hbar * c / G)
and inherits the full uncertainty of G. The fine structure constant
alpha is known to twelve significant figures. The ratio m_p/m_P — proton
mass to Planck mass — is a fundamental dimensionless number that any
unified theory must address. It is known to only five significant
figures, not because of any difficulty on the electromagnetic side, but
entirely because G limits the determination of m_P.
Numerical computation confirms the scaling precisely. Across the
current uncertainty range of G — approximately 15 parts per million —
the natural unit scale derived from hbar, c, and G shifts by 7.5 parts
per million, following the theoretical scaling X proportional to
G^(-1/2) exactly. The internal self-consistency within any fixed value
of G is maintained to floating-point precision (~10^-16). The geometry
is exact. The uncertainty is entirely in our measurement access to
G.
G is a unit conversion factor — the bridge between our
independently-defined kilogram and the natural geometry of gravity. Like
c, it would be 1 by construction if our mass unit had been defined
gravitationally. The constants G and c are not deep truths about nature.
They are artifacts of defining length, time, and mass independently
using different physical processes. The scandal is not that G is
uncertain. The scandal is that we have treated a units alignment problem
as a fundamental mystery for three centuries.
Any theory proposing an exact relationship between m_p/m_P and alpha
cannot be tested better than five significant figures. The relationship
may be exact, numerically sitting in plain sight, and we cannot resolve
it. This is structurally identical to how epicyclic astronomy concealed
elliptical orbits for over a millennium. Epicycles made accurate
predictions. The system was internally consistent. And buried inside
that predictive success was exact geometry the representation could not
expose. GM is our epicycle. The exact dimensionless ratios connecting
gravity to electromagnetism are hidden inside a perfectly predictive
framework that cannot expose them.
4. The Proposed Experiment
4.1 Core Concept
The GM degeneracy is broken by one structural change: use a
gravitating body whose mass is known from construction, not from its
gravitational behavior. A body assembled from measured components in a
zero-gravity environment has a mass determined by the sum of its parts,
each weighed through the metrological chain anchored to the SI kilogram,
independently of gravity. This mass M is known before the gravitational
measurement begins.
A flat toroidal disk — a large washer — with a hole bored through its
center axis is the chosen geometry. A test mass m is dropped through the
hole. It oscillates back and forth through the disk under gravity alone.
The period of this oscillation depends only on the surface mass density
sigma of the disk, which is known from construction. The measurement of
G reduces to a measurement of oscillation period, acceleration, and
distance — all tracked simultaneously by a single LED
interferometer.
4.2 Physics of the Toroidal Oscillator
For a uniform infinite plane of surface mass density sigma, the
gravitational acceleration is constant on both sides:
g = 2 * pi * G * sigma
independent of distance from the surface. A test mass released from
rest at height h above the disk undergoes simple harmonic motion with
period:
T = 2 * pi * sqrt(h / (2 * pi * G * sigma))
The critical feature is that period is independent of amplitude. As
the oscillation slowly damps, T remains constant. Every cycle from first
to last gives the same measurement of G. The experiment does not degrade
as energy dissipates. The LED interferometer simultaneously tracks three
independent observables throughout every oscillation:
— T: oscillation period, from precise timing of successive zero
crossings at the disk plane
— a(t): instantaneous gravitational acceleration, from the second
time derivative of interferometric position
— r(t): distance from the center of the hole at every instant,
providing continuous geometry verification and confirming the mass
distribution model
These three observables are overdetermined for a single unknown G.
Their mutual consistency throughout each oscillation provides direct
internal systematic error estimation with no additional apparatus. Any
unmodeled perturbation shows up as inconsistency between the three
routes to G before it corrupts the result.
4.3 Washer Construction: Batteries as Known Mass
The toroidal disk is constructed from cylindrical battery cells
packed into a toroidal form and encased in a precision metal shell. The
batteries serve a dual purpose: they are the power source for the
mission and they constitute the primary known mass. No dead-weight
ballast is carried. Every kilogram of structural mass is simultaneously
delivering power.
The metal shell provides a uniform, precisely characterized outer
geometry. Shell thickness is known. Battery cell geometry and individual
mass are measured before assembly. Total mass M is the sum of precisely
accounted components, all measured on the ground before launch. The
surface mass density sigma = M / pi(R_outer^2 - R_inner^2) is known to
the precision of the pre-launch mass accounting.
Battery discharge does not meaningfully change the mass. The chemical
energy released is accounted for by E = mc^2 at a level far below the
measurement floor. Mass M is effectively constant throughout the mission
lifetime, and any slow drift is trackable from power consumption
telemetry.
4.4 LED Interferometer
The measurement instrument is an LED-based Michelson interferometer.
A narrow-bandwidth LED with a bandpass filter provides sufficient
coherence length for path differences involved in tracking oscillation
amplitudes of order one meter. Power consumption is in the milliwatt
range. No laser cooling, no frequency stabilization, and no moving
optical components are required.
At 3 AU from the Sun the sky is dark. There is no solar background to
filter against. Starlight provides negligible interferometric noise. The
deep space environment is here an advantage: the LED signal completely
dominates the detector with no competing background illumination.
The test mass carries a retroreflector. The interferometer arm length
changes as the test mass oscillates, producing fringes that encode
position continuously. Zero crossings at the disk plane are timed to
atomic clock precision. The combination of T, a(t), and r(t) as
continuous functions throughout each oscillation delivers G from three
independent routes simultaneously from a single passive optical
system.
4.5 Operational Procedure
The test mass is lifted to a specified height above the hole center
and held. The LED interferometer confirms the test mass is at rest —
zero velocity, stable position — before release. The actuator releases.
The test mass falls through the hole, decelerates on the far side,
returns, and oscillates. The interferometer tracks every cycle
continuously.
The oscillation runs until amplitude has decayed to near the noise
floor or lateral drift approaches the hole wall. The actuator catches
the test mass, lifts it back to the starting height, and waits for the
interferometer to confirm stillness. A new run begins. This cycle
repeats throughout the on-station mission phase.
Each run provides thousands of independent period measurements. Each
lift-settle-release cycle is independently characterized. Run-to-run
consistency is a direct systematic error check. The experiment is not a
one-shot measurement — it is the same experiment performed thousands of
times with the same apparatus in a zero-noise environment, accumulating
statistics continuously for over a year.
4.6 Mission Architecture
The washer payload is delivered to the target location by a
conventional booster. On arrival the booster releases the washer and
fires away to a minimum separation distance of 100 kilometers. At this
separation the booster's gravitational influence on the test mass
oscillation is negligible, and the booster acts as a radio relay —
receiving low-power data from the washer and forwarding it to Earth at
full deep space communication power. The booster requires no further
maneuvering and recedes on a diverging trajectory.
Following separation, the washer is left to settle. Vibration from
separation, thermal distortion as the system reaches radiative
equilibrium, and any residual rotation are all monitored by the LED
interferometer and allowed to damp naturally. This settling phase may
take weeks to months. Formal measurement runs do not begin until the
interferometer confirms the system is at rest to the required precision.
The settling period characterizes the system in detail and verifies the
mass distribution model against the measured gravitational field
geometry before any G determination is attempted.
The target location is at or beyond 3 AU from the Sun. The solar
tidal gradient across the experimental apparatus at this distance is
verified by calculation (Section 5) to be more than 100 million times
smaller than the measurement signal. The precise location is determined
by a standard orbital mechanics trade between mission delta-V and
acceptable tidal contamination. Minimum on-station measurement duration
is one year.
4.7 Power Budget
The LED interferometer operates in the milliwatt range. The test mass
actuator draws power only during brief lift and release operations. The
onboard computer handles data logging and interferometer control at low
clock rates. The radio transmitter to the booster relay at 100
kilometers requires trivial power at that range. Total payload power
budget is estimated at 20 to 30 watts continuous. The toroidal battery
mass, sized for the known mass requirement of the experiment, provides
this power for the required mission lifetime with margin.
5. Tidal Contamination Analysis
The primary concern for any deep space gravitational experiment is
contamination from external tidal forces. We calculate the tidal
acceleration from all significant sources across the 1-meter scale of
the apparatus and compare to the measurement signal.
5.1 Signal Strength
For a toroidal mass M = 5000 kg with a test mass at distance r = 1
meter from the center hole, the gravitational acceleration constituting
the measurement signal is:
a_signal = G*M / r^2 = (6.674e-11)(5000) / (1)^2 = 3.34e-7
m/s^2
This is the reference against which all tidal contaminations are
compared.
5.2 Solar Tidal Force
The tidal acceleration from the Sun across an apparatus of length
delta_r is:
a_tidal = 2 * G * M_sun / R^3 * delta_r
where R is the heliocentric distance. At 1 AU this gives 7.93 × 10⁻¹⁴
m/s² across 1 meter — already 4.2 million times smaller than the signal.
The tidal force scales as 1/R³, so at 3 AU it drops by a further factor
of 27:
a_tidal,Sun (3 AU) = 2.94e-15 m/s^2
This is 113 million times smaller than the measurement signal. As a
fraction of the signal it represents 8.8 parts per billion — well below
the 1 part per billion threshold required for nine significant figures
in G.
5.3 Galactic Tidal Force
The local galactic tidal acceleration is known from stellar dynamics
and pulsar timing studies to be approximately 5 × 10⁻¹⁵ m/s² per meter
of apparatus length. For the 1-meter scale of this experiment:
a_tidal,Galaxy = 5.00e-15 m/s^2
This is 67 million times smaller than the signal — comparable to the
solar tidal and equally negligible.
5.4 Planetary Tidal Forces
Jupiter, the most massive planet, presents a tidal acceleration
across 1 meter of approximately 1.18 × 10⁻¹⁸ m/s² at a conservative
minimum separation of 4 AU. This is 280 billion times smaller than the
signal and requires no further consideration.
5.5 Summary
Table 1 summarizes all tidal contamination sources. The worst-case
external contamination — the galactic tidal force — is 67 million times
smaller than the measurement signal. At 3 AU, the rest of the universe
has gone effectively silent. This is not a marginal improvement over the
terrestrial environment. It is a qualitative change in what measurement
is possible.
| Source |
Tidal Acceleration
(m/s²) |
Ratio to Signal |
Orders of Magnitude Below
Signal |
| Toroid 5000 kg at 1 m (Signal) |
3.34 × 10⁻⁷ |
1 (reference) |
— |
| Sun at 1 AU across 1 m |
7.93 × 10⁻¹⁴ |
4.2 × 10⁶ × smaller |
6.6 |
| Sun at 3 AU across 1 m |
2.94 × 10⁻¹⁵ |
1.1 × 10⁸ × smaller |
8.1 |
| Milky Way galaxy across 1 m |
5.00 × 10⁻¹⁵ |
6.7 × 10⁷ × smaller |
7.8 |
| Jupiter at 4 AU separation across 1
m |
1.18 × 10⁻¹⁸ |
2.8 × 10¹¹ × smaller |
11.4 |
Table 1. Tidal contamination at 3 AU compared to measurement
signal (M = 5000 kg, r = 1 m, delta_r = 1 m).
The solar tidal force at 3 AU, as a fraction of signal, is 8.8 parts
per billion. For reference, nine significant figures of precision in G
requires controlling systematics to 1 part per billion. The solar tidal
is below this threshold by a factor of nearly 9. For experiments
requiring fewer than nine figures of precision, 3 AU provides ample
margin. For the full nine-figure target, an orbit at 4 AU reduces solar
tidal contamination by a further factor of 2.4, comfortably below the
threshold.
6. Statistical Power of Repeated Oscillation
The fundamental advantage of this experiment over every previous G
determination is statistical accumulation. A single period measurement T
has some uncertainty epsilon from timing precision and environmental
noise. After N independent cycles, the uncertainty on the mean period is
epsilon / sqrt(N).
In a one-year on-station mission with oscillation periods of order
minutes, the number of measurable cycles is of order tens of thousands
per run and millions across the full mission. The statistical reduction
factor sqrt(N) is of order 1000. Random errors that would limit a single
measurement to five significant figures are beaten down to nine or more
by accumulation alone.
No ground-based experiment has ever had this. Cavendish apparatus
yields one measurement per configuration. Resets are slow and noisy. N
never gets large. The noise floor never drops because statistics never
accumulate. Here N is limited only by mission lifetime. The experiment
improves continuously as long as the apparatus operates.
The three simultaneous observables — T, a(t), and r(t) — provide
independent routes to G from the same data stream. Their mutual
consistency serves as a continuous systematic error monitor throughout
the mission. Any unmodeled perturbation that would corrupt one
observable will show up as inconsistency among all three before it
biases the G determination.
7. Scientific Return
A determination of G to nine significant figures immediately
propagates precision improvement through every dimensionless ratio in
physics that involves gravity. The Planck mass m_P = sqrt(hbar*c/G)
becomes known to nine figures. The ratio m_p/m_P — proton mass to Planck
mass — sharpens from five to nine significant figures with no additional
measurement on the electromagnetic side.
The gap between five and nine significant figures is where proposed
exact relationships between m_p/m_P and alpha either are confirmed or
are falsified. A correct unified theory would predict this ratio exactly
as a function of the fine structure constant and other dimensionless
electromagnetic parameters. Such predictions are currently untestable
beyond five figures. This experiment makes them testable to nine.
Every GM product for solar system bodies simultaneously becomes a
precise mass determination. The mass of the Earth, the Moon, Mars, and
Jupiter — all known to nine figures immediately by dividing their known
GM by the newly precise G. This is a complete remeasurement of solar
system masses at no additional observational cost.
8. Cost and Comparison
The cumulative cost of Cavendish-style G determinations over the past
century, across major metrology institutes in multiple countries, has
been substantial. No convergence has been achieved. Three hundred years
of investment has produced not precision improvement but a widening
recognition that the terrestrial environment is fundamentally the wrong
place to do this experiment.
The proposed mission is less technically complex than many current
planetary science missions. It requires no landing, no sample return, no
complex in-situ chemistry, no precise pointing at a distant astronomical
target. It requires transporting a known mass to deep space, releasing a
booster, and operating an LED interferometer and test mass actuator for
one or more years. The payload has no moving parts except the test mass
actuator. The primary instrument draws milliwatts.
The mission fits within the cost envelope of an ESA Medium-class or
NASA Discovery-class science mission. The scientific return — resolving
a 300-year measurement failure and opening the
gravitational-electromagnetic interface to genuine precision tests — is
disproportionate to the engineering investment. A formal feasibility
study is the appropriate immediate next step.
9. Conclusion
G has an exact value. The universe does not have error bars. The
uncertainty in G is entirely on the measurement side and has a specific
identifiable cause: we have never had independent access to the mass of
a body large enough to produce a measurable gravitational field. Every
previous approach either uses astronomical bodies whose masses are
inferred from gravity, or uses laboratory masses too small to overcome
the terrestrial noise floor.
The proposed experiment resolves this by construction. A toroidal
battery mass of precisely known composition is placed at 3 AU from the
Sun. A test mass oscillates through its central hole under gravity
alone. An LED interferometer tracks period, acceleration, and distance
simultaneously through thousands of oscillations over a mission lifetime
exceeding one year. The mass is known before the gravitational
measurement begins. The GM degeneracy is broken.
Tidal contamination from all external sources — Sun, galaxy, planets
— is verified to be between 67 million and 280 billion times smaller
than the measurement signal. The deep space environment does not merely
reduce noise. It eliminates it.
The result is G to nine significant figures, the Planck jacobians to nine
figures, and every dimensionless ratio at the
gravitational-electromagnetic interface sharpened by four to five
significant figures. Proposed exact relationships between m_p/m_P and
alpha become directly testable as a ratio against kinematics for the first time. No new physics is required. No
new engineering principles are required. The only reason this has not
been done is that it falls between the institutional mandates of
metrology and deep space science. That gap should be closed.
References
[1] CODATA 2018 recommended values of the fundamental physical
constants. Rev. Mod. Phys. 93, 025010 (2021).
[2] Cavendish, H. Experiments to determine the density of the Earth.
Phil. Trans. R. Soc. London 88, 469-526 (1798).
[3] Gillies, G.T. The Newtonian gravitational constant: recent
measurements and related studies. Rep. Prog. Phys. 60, 151 (1997).
[4] Rothleitner, C. & Schlamminger, S. Measurements of the
Newtonian constant of gravitation. Rev. Sci. Instrum. 88, 111101
(2017).
[5] Quinn, T. et al. Improved determination of G using two methods.
Phys. Rev. Lett. 111, 101102 (2013).
[6] Rosi, G. et al. Precision measurement of the Newtonian
gravitational constant using cold atoms. Nature 510, 518-521 (2014).
[7] Armano, M. et al. Sub-Femto-g Free Fall for Space-Based
Gravitational Wave Observatories: LISA Pathfinder Results. Phys. Rev.
Lett. 116, 231101 (2016).
[8] Folkner, W.M. et al. The planetary and lunar ephemeris DE 430 and
DE 431. Interplanet. Netw. Prog. Rep. 196, 1-81 (2014).
[9] Mohr, P.J., Newell, D.B. & Taylor, B.N. CODATA recommended
values of the fundamental physical constants: 2014. Rev. Mod. Phys. 88,
035009 (2016).
[10] Duff, M.J. How fundamental are fundamental constants? Contemp.
Phys. 56, 35-47 (2015).
[11] Iorio, L. Galactic tidal effects on the Oort Cloud and the outer
solar system. MNRAS 443, 2523-2534 (2014).