Historical research guide · 0280 · 2006
Experimental Detection of the Gravitomagnetic London Moment
M. Tajmar · F. Plesescu · K. Marhold · C. J. de Matos
In one minute
Spin a superconductor and it makes a magnetic field. That is the London moment, and it is textbook physics. Martin Tajmar, Florin Plesescu, Klaus Marhold and Clovis de Matos asked whether it also makes a gravitational one. Their reason was a puzzle already in the literature: Tate’s precision measurement of the Cooper-pair mass in niobium came out heavier than theory predicts, by about eighty parts in a million. Tajmar and de Matos had proposed that a gravitational twin of the London moment — a frame-dragging field, the same kind of effect Gravity Probe B later measured around the Earth — would account for the difference exactly. So they built the test: a niobium ring in liquid helium, spun to 6500 revolutions per minute inside a cryostat bedded in 1500 kilograms of sand, read by accelerometers sealed in an evacuated Faraday cage that never touches the moving parts. Accelerate the ring while it is superconducting and the sensors register about 100 microgravities the other way. Warm it past its critical temperature and the signal is gone.
Connection
Chapter 11 is the superconductor thread, and this is its most carefully instrumented single experiment — the one that tries to catch a laboratory frame-dragging field with the mechanical noise engineered out. It also belongs to chapter 1, because Tajmar’s own later and better-controlled runs pushed the signal back toward the noise, which makes this the site’s cleanest worked example of how a strong claim is meant to be tested by the person who made it.
From the source
A rotating superconductor produces a magnetic field proportional to its angular velocity — the London moment, with the field set by the ratio of Cooper-pair mass to Cooper-pair charge — and measuring that field against the rotation rate is how the mass of a Cooper pair is obtained in the laboratory.Section 1, Introduction, Equation 1
In the highest-precision measurement of its kind, Tate and colleagues found the Cooper-pair mass in niobium to be 1.000084 plus or minus 0.000021 in units of twice the electron mass, against a theoretical prediction of 0.999992 — an anomaly discussed in the literature for years without an accepted resolution.Section 1, Introduction, References 5 to 10
In the weak-field form of general relativity a changing gravitomagnetic field induces a gravitational field, the gravitational counterpart of Faraday’s law, so angularly accelerating a ring that carries such a field should produce a non-Newtonian acceleration along the azimuthal direction rather than the ordinary radial one.Section 2, Measurement Concept, Equations 5 and 6
The authors had earlier proposed that a gravitomagnetic London moment accompanies the ordinary one, and that a field of about 1.84 times ten to the minus four multiplied by the angular velocity would account for Tate’s excess mass — a value thirty orders of magnitude above what classical general relativity predicts for laboratory-scale matter, and traceable in their model to a non-zero graviton mass inside the superconductor set by the ratio of Cooper-pair density to bulk density.Section 1, Introduction, Equations 2 to 4, References 11 to 15
Applying about 1500 radians per second squared to a superconducting niobium ring in liquid helium produced a tangential acceleration of roughly 100 microgravities outside the ring and opposite in sense to the applied acceleration — a gravitational analogue of Lenz’s law — with a coupling factor near minus 6.6 times ten to the minus eight seconds squared, nearly 200 peaks correlating with the applied acceleration above 0.96, the above-ring sensors reading 77 to 90 percent of the in-ring sensors, lead giving 84 percent of niobium as the density ratio predicts, and no effect at all above the critical temperature or for the two high-temperature ceramics.Section 4C, Testing with Liquid Helium; Table 2; Figures 4 to 6
The authors state their own settling condition plainly: the signal-to-noise ratio is 3.3, the coupling factor is about thirteen times larger than their theory predicts, and the result stands or falls on independent replication by other groups — while the same apparatus saw no vertical change within plus or minus 5 microgravities, placing an upper limit of less than 0.0005 percent on any weight change above a rotating superconductor.Section 5, Conclusions
The way in
https://arxiv.org/abs/gr-qc/0603033How to cite it
M. Tajmar, F. Plesescu, K. Marhold, C. J. de Matos (2006) Experimental Detection of the Gravitomagnetic London Moment arXiv:gr-qc/0603033