Gravitomagnetisches Feld nun erstmals im Laborexperiment nachgewiesen?
| Scientific Paper | |
|---|---|
| Title | Gravitomagnetisches Feld nun erstmals im Laborexperiment nachgewiesen? |
| Read in full | Link to paper |
| Author(s) | Felix Scholkmann |
| Keywords | Gravitoelectromagnetism, Gravity |
| Published | 2007 |
| No. of pages | 16 |
Read the full paper here
Abstract
Österreichische Forscher um den Physiker Dr. Martin Tajmar berichteten kürzlich, dass sie mit großer Wahrscheinlichkeit erstmals im Laborexperiment mit einem rotierenden Supraleiter eine von der Allgemeinen Relativitätstheorie vorhergesagte Gravitationskraft (gravitomagnetische Kraft) nachweisen konnten. Dabei stellte sich zudem heraus, dass die gemessene Kraft wesentlich größer als erwartet war. Der vorliegende Artikel geht den Fragen nach, was unter einer gravitomagnetischen Kraft zu verstehen ist und welche Bedeutung gravitomagnetische Kräfte für die moderne Physik und Technik haben könnten.
Overview
This is a German-language review article, dated 10 March 2007 and written by Felix Scholkmann and Thomas Ganka while they were physics students at FH Isny. It was prompted by the announcement from Martin Tajmar's group at ARC Seibersdorf research GmbH in Austria that a gravitomagnetic field had, for the first time, been detected in a laboratory rather than inferred from satellite orbits. The paper has three parts: a tutorial on what a gravitomagnetic field is and where the idea came from, a description of the Seibersdorf rotating-superconductor experiment and its results, and an outlook section on what the result might mean for gravitational theory and for propulsion.
The authors do not propose a theory of their own. Their departure from the textbook account is one of emphasis rather than of principle: they take seriously a class of experimental claim — anomalous gravitational behaviour of superconductors — that mainstream gravitational physics has largely set aside, and they note that the Seibersdorf signal is not a confirmation of General Relativity at all but an anomaly, since the measured field was reported to be some "100 Millionen Billionen Mal größer" than the general-relativistic prediction. They close by suggesting that classical field-theoretic and fluid-mechanical accounts of Gravity may be the more promising route to explaining it — a distinctly dissident recommendation in a paper otherwise written in the idiom of orthodox relativity.
The argument
Gravitoelectromagnetism as an analogy
The exposition begins from the standard electrodynamic pairing: a charge at rest produces an electrostatic E field, a moving charge produces a magnetic H field. Classical mechanics supplies only half of the corresponding pair for mass — a mass at rest produces a gravitational g field — and says nothing about what a moving mass produces. The authors pose the question directly: "Hat I. Newton eine zweite Art von Gravitationskraft übersehen?"
They then trace the history of the missing second field. Maxwell speculated about it in 1865; Holzmüller and Tisserand around 1870 supposed that the Sun's motion induces a magnetic-like component of the gravitational field affecting planetary orbits; Oliver Heaviside in 1893, asking how energy propagates in a gravitational field, concluded that a gravitational Poynting vector must exist. Einstein raised the possibility in 1913, de Sitter joined Holzmüller and Tisserand in 1916, and in 1918 Thirring and Lense showed that general relativity does require a rotating mass to drag the orbits of surrounding bodies (the Thirring–Lense effect). Robert Forward completed the programme in 1963 by linearising Einstein's field equation into a set with the same structure as Maxwell's Equations.
The Maxwell–Einstein equations
The paper sets the two systems side by side. Against Maxwell's
∇·E = ρ/ε0, ∇·B = 0, ∇×E = −∂B/∂t, ∇×B = μ0ρv + (1/c2)∂E/∂t
they place the gravitational analogues
∇·g = −ρm/εg, ∇·C = 0, ∇×g = −∂C/∂t, ∇×C = μgρmv + (1/c2)∂g/∂t
with g the gravitoelectric (ordinary Newtonian) field, C the gravitomagnetic induction, ρm the mass density, and the gravitomagnetic induction constant μg = 4πγ/c2. The field K is related to C by C = μgK, exactly as B = μ0H. The Lorentz Force on a moving charge in an H field then has a gravitomagnetic counterpart, and Coulomb's Law its gravitoelectric counterpart.
Two supporting arguments are cited. Jefimenko pointed out in 1992 that a magnetic aspect of gravitation is required if momentum and energy conservation are to hold without restriction in all mechanical processes; Behera showed in 2006 that a gravitomagnetic field is derivable even from Newtonian mechanics. The authors make a terminological point they clearly care about: the effect "hat nichts mit Magnetismus zu tun" — nothing to do with magnetism — and they argue the term ought to be gravitodynamisch rather than gravitomagnetisch, keeping the older word only because it is established. Known effects the framework covers are listed as the Thirring–Lense effect, the Sagnac Effect, the Schiff (frame-dragging) effect and the de Sitter effect.
The experimental record before Seibersdorf
Because the field is so weak, laboratory tests were abandoned in favour of satellite orbit analysis. The LAGEOS programme reported in 1998 an effect 110 ± 20 % of the predicted size; a repeat with laser ranging accurate to ±1 cm gave 99 ± 5 % agreement, published in autumn 2004. Gravity Probe B, launched April 2004 at a cost of 700 million dollars, was expected to show a precession of 40.9 milliarcseconds per year and its result was still awaited when the article was written.
The Seibersdorf rotating-superconductor experiment
Tate, Cabrera, Felch and Anderson had reported in 1989–90 that the measured Cooper-pair mass in a rotating niobium superconductor did not match the theoretical value, an anomaly that resisted explanation. Tajmar and de Matos argued in 2002 that the discrepancy would follow if a rotating superconductor generated a strong gravitomagnetic field, and that coherent matter should produce a far larger field than incoherent matter. The predicted field strength was K = 2ω(ρ′m/ρm), where ρ′m is the mass density of the Cooper pairs and ρm the bulk density of the superconductor.
The experiment used ring-shaped superconductors of niobium, lead, BSCCO and YBCO, rotated at cryogenic temperature, with accelerometers inside, outside and above the ring and two gyroscopes at different heights. The detection logic is indirect: a non-stationary gravitomagnetic field should induce a gravitoelectric field, which the accelerometers register as an acceleration. The chamber was fixed to the ceiling by steel beams and loaded with 1.5 t of sandbags against vibration.
After more than 250 runs over three years, the results announced in March 2006 were: no effect above the noise for BSCCO and YBCO, as predicted from their low Cooper-pair density; but for niobium, once below Tc = 9.4 K, a signal of 100 μg on the sensors at an angular acceleration of 1500 rad/s2; and for lead, below Tc = 7.2 K, a signal 84 % as large. The gyroscope directly above the ring deflected while the more distant one did not, consistent with a field falling off with height. The signal appeared specifically when the rotation rate was changing — a discontinuity in angular acceleration — not during steady rotation.
Outlook
The authors survey the neighbouring literature on superconductors and gravity and are candid about it. Podkletnov's reported 0.05–2.1 % weight loss above a rotating YBCO disc has not been successfully replicated: attempts by Li et al. under NASA Marshall, by Woods et al. at Sheffield with BAE Systems funding and with Podkletnov consulting, by Hathaway et al. in 2001, and by Robertson, Litchford, Thomson and Peters, all returned null results, and Unnikrishnan's 1996 analysis raised doubts about the original measurements. Tajmar's own group also failed to reproduce claims that superconductors lose weight at Tc or under ELF irradiation.
For the future they suggest that the decisive variable is the coherent quantum state of the material, that further amplification mechanisms may exist, and that propulsion is the obvious application — while quoting Bertolami and Tajmar's own analysis that no revolution should be expected. They call for new theoretical approaches, particularly field-theoretic and fluid-mechanical accounts of gravity, and note that Reginald T Cahill had recently derived the Thirring–Lense effect from a fluid-mechanical model, predicting both a stronger-than-expected effect in Gravity Probe B and a variation of the precession with orbital position.
Assessment
The article's real merit is as a careful, honest piece of science journalism for physicists. It does something rare in this literature: it distinguishes sharply between an effect that is well confirmed (orbital frame-dragging, at the 99 ± 5 % level from LAGEOS), an effect that is claimed but unreplicated (Podkletnov), and an effect that is newly reported and not yet independently checked (Seibersdorf). It does not conflate them. The list of failed Podkletnov replications is given in full, with names and sponsors, by authors who plainly would have preferred a positive result. The insistence that "gravitomagnetic" is a misleading name, and that "gravitodynamic" would be more accurate, is a good pedagogical point that the mainstream literature would do well to adopt.
The weaknesses are largely those of the source material rather than of the review, but they matter. The central claim is presented as a vindication of general relativity while the reported magnitude — twenty orders of magnitude above the GR prediction — is in fact a flat contradiction of it. The paper notices this and calls it "sensationell" rather than treating it as the problem it is: a discrepancy of that size is far more naturally read as an uncontrolled systematic than as a real gravitational signal. The authors offer no independent assessment of the possible systematics in the Seibersdorf apparatus — thermal contraction of the rotor during the transition through Tc, mechanical coupling of angular acceleration into the accelerometer mounts, magnetic interaction with the sensors at the moment of flux expulsion — even though the signal appears precisely at the two moments (crossing Tc, and changing rotation rate) when such systematics would be largest. The prediction K = 2ω(ρ′m/ρm) is quoted, not derived, and the claim that BSCCO and YBCO should show nothing while niobium and lead should show something is presented as a successful prediction without any figure for how large the predicted niobium effect actually was.
The subsequent history is not kind to the paper's optimism. Gravity Probe B, whose result the authors awaited "mit großer Spannung," published in 2011 a frame-dragging measurement consistent with general relativity to about 19 %, with no sign of the anomalously enhanced effect Cahill's model required and no orbit-dependent variation of the precession. Read today, the article stands as a well-documented snapshot of a live anomaly at the moment when it looked most promising, and as an honest record of how the surrounding replication attempts had already gone.