Historical research guide · 0591 · 2007
Measurement of Gravitomagnetic and Acceleration Fields around Rotating Superconductors
Martin Tajmar · Florin Plesescu · Bernhard Seifert · Klaus Marhold
In one minute
This is Martin Tajmar’s progress report on the hardest measurement in chapter 11. His group at ARC Seibersdorf spun a niobium ring in liquid helium and asked whether a superconductor drags spacetime the way a spinning mass does, only enormously harder. Two things make this paper the one to read rather than the 2006 first announcement. First, the sensors were rearranged into what Tajmar, Florin Plesescu, Bernhard Seifert and Klaus Marhold call a curl configuration, in which every accelerometer has a mirror partner and the pair’s difference cancels tilts, torques and vibration offsets. That raised the signal-to-noise ratio to about 15 to 1 and, importantly, it corrected their own earlier number downward — the single-sensor peaks had been over-read. Second, they added laser gyroscopes, which respond to rotation rate rather than to acceleration and are almost immune to vibration. Both instruments tracked the Cooper-pair density; a reference sensor on the same frame saw nothing.
Connection
Chapter 11 asks whether coherent quantum matter can amplify frame dragging into something you could use, and this is the most carefully instrumented attempt to catch it, by the group that made the claim. It belongs to chapter 1 just as strongly, as a worked example of how the evidence ladder is supposed to run: the authors publish their error budget, correct their own earlier figure downward when a better configuration disagrees with it, and end by asking other laboratories to try. That is where the question still sits — the effect has not been established at this size by an independent group, and the two follow-up runs on record are R. D. Graham, R. Hurst, R. Thirkettle, C. H. Rowe and P. Butler’s search in a lead superconductor, Physica C 468, 383 (2008), and Tajmar’s own higher-resolution niobium, liquid-helium and superfluid-helium evaluation, Superconductor Science and Technology 24, 125011 (2011). The 2022 Torino review at /library/stm-c909553daa sets the whole programme in context.
From the source
Linearising Einstein’s equations with a non-zero cosmological constant gives Proca-like equations for a massive spin-1 graviton, and if the graviton mass depends on the local density of matter — the condition for the dark energy density measured by WMAP to be constant across the universe — then the cosmological constant becomes something testable on a laboratory bench rather than only across the sky.Introduction, opening paragraphs
A rotating superconductor should carry a gravitomagnetic London moment measurable outside the material, equal to twice the angular velocity scaled by the ratio of Cooper-pair mass density to bulk density — for niobium about 3.9 times ten to the minus sixth multiplied by the angular velocity at zero kelvin — and angularly accelerating the ring should induce an azimuthal gravitational field opposing its cause, a gravitational Lenz law.Introduction, Equations 1 and 2
In the curl configuration, where each tangential accelerometer is differenced against its mirror partner so that only a closed-loop field survives, a niobium ring gave an in-ring coupling factor of minus 2.26 plus or minus 0.3 times ten to the minus eighth in units of Earth gravity per radian per second squared while superconducting, against minus 1.24 plus or minus 1 times ten to the minus ninth while normal conducting, with a signal-to-noise ratio of about 15 to 1.Accelerometer Measurements, Curl Sensor Configuration, Figure 8
Signal-averaging more than 100 profiles in one-kelvin intervals gives an in-ring coupling factor extrapolated to zero kelvin of minus 14.4 plus or minus 2.8 times ten to the minus ninth, following the temperature dependence of the Cooper-pair density prediction multiplied by two — and this figure corrects the group’s earlier single-sensor numbers, which over-predicted the effect because the accelerometer’s tilting plates oscillate and exaggerated the peaks.Accelerometer Measurements, Figure 9 and the paragraph following it
Preliminary laser-gyroscope runs, which respond to angular velocity rather than acceleration and are insensitive to vibration, showed an above-ring signal tracking the Cooper-pair density and dropping exactly when the ring crossed its critical temperature, while a reference gyroscope on the same mechanical structure showed nothing within a noise level of 3 times ten to the minus fifth radians per second — which rules out a mechanical torque on the sensor chamber as the cause.Gyroscope Measurements, Figure 11
The authors work through each alternative cause in turn — mechanical drag through the helium gas, magnetic offsets, which would need at least 100 millitesla against a measured field change below 1 microtesla, and acoustic noise from boiling helium, which does not reverse with the sense of rotation while the observed effect does — conclude that the gravitomagnetic London moment is the most probable explanation, fitting their prediction within a factor of two, and state the settling condition themselves: the result must be investigated and verified in other laboratories, with different sensors, before any facility-induced effect can be ruled out.Discussion and Conclusion of Results
The way in
https://doi.org/10.1063/1.2437552How to cite it
Martin Tajmar, Florin Plesescu, Bernhard Seifert, Klaus Marhold (2007) Measurement of Gravitomagnetic and Acceleration Fields around Rotating Superconductors doi:10.1063/1.2437552