Considerations of this kind led two people to take a new look at gyroscopic tests of general relativity shortly after the dawn of the space age. George E. Pugh and Leonard I. Schiff hit independently on the key ideas within months of each other. Pugh was stimulated by a talk given by Huseyin Yilmaz proposing a satellite test to distinguish his alternative theory of gravity from Einstein's, while Schiff was likely inspired at least in part by an advertisement for a new "Cryogenic Gyro Pugh's paper, published in a Pentagon memorandum in November , is now recognized as the birth of the concept of drag-free motion.
This is a critical element of the Gravity Probe B mission, whereby any one of the gyroscopes can be isolated from the rest of the experiment and protected from all non-inertial forces; the rest of the spacecraft is then made to "chase after" the reference gyro by means of helium boiloff vented through a revolutionary porous plug and specially designed thrusters.
See animation clip "Drag-Free Motion" below. The drag-free control system is only one of the innovations that made Gravity Probe B possible. The experiment depends on monitoring the precession of near-perfect gyroscopes relative to a fixed reference direction such as the line of sight to a distant guide star.
But how is one to find the spin axis of a perfectly spherical, perfectly homogeneous gyroscope suspended in vacuum? This is the "readout problem"; another, closely related problem is how to spin up such a gyroscope in the first place. Various possibilities were considered in the early days, until when Francis Everitt and William Fairbank hit on the idea of exploiting what had until then been a small but annoying source of unwanted torque in magnetically levitated gyroscopes.
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Spinning superconductors develop a magnetic moment, known as the London moment , which is proportional to spin speed and always aligned with the spin axis. If the rotors were levitated electrically instead of magnetically, this tiny effect could be used to tell where their spin axes were pointed. Measuring it would of course require magnetic shielding orders of magnitude beyond anything available in , another story in itself.
So sensitive are these devices that they register a change in spin-axis direction of 1 milliarcsec in five hours of integration time. See animation clip "London Moment Readout" below. These are only two pieces of an experiment so beautifully intricate that it is as much a work of art as it is science and technology. Many of its key features reflect a guiding principle of physics experimenters through the ages, namely to turn obstacles into opportunities. How, for instance, can one meaningfully compare the gyroscope spin-axis direction which is read out in volts with the position of the guide star which comes from an onboard telescope in radians?
The answer is to exploit nature's own calibration in the form of stellar aberration. What about the fact that the guide star has an unknown "proper motion" large enough to obscure the predicted relativity signal?
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This allows the experiment to be designed in a classic "double-blind" fashion; a separate team of astronomers uses VLBI Very Long-Baseline radio Interferometry to monitor the movements of the guide star itself , relative to even more distant quasars. Only at the conclusion of the experiment are the two sets of data to be compared; this helps to prevent the physicists from "finding what they want to see. The photograph above shows several of the early project leaders with a model of the spacecraft circa Dan Debra a propulsion expert , Fairbank the experimental low-temperature physicist par excellence , Everitt and Bob Cannon a gyroscope specialist.
Radio image of the giant jet in NGC Artist's depiction of a supermassive black hole. When Gravity Probe B was originally conceived, frame-dragging was seen as being of more theoretical than practical interest.
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To be sure, experimental confirmation of the Einstein i. Lense-Thirring prediction would place another independent constraint on alternative metric theories of gravity. However, frame-dragging is such a small effect in the solar system that experimental bounds it places on these parameters are not likely to be competitive with those from other tests. Confirmation of the Einstein i. Bardeen-Petterson effect.
Every Black Hole Contains a New Universe
This situation has changed dramatically since the s. Physicists now see frame-dragging as the gravitational analog of magnetism, and astrophysicists invoke this gravitomagnetic field as the engine and alignment mechanism for the vast and otherwise incomprehensible jets of gas and magnetic field ejected from quasars and galactic nuclei like the radio source NGC , as shown above left.
We know that these jets act as power sources for quasars and other strong extragalactic radio sources and that they are generated by compact, supermassive objects almost certainly black holes inside galactic nuclei, as illustrated above right. The megaparsec distance scale in the radio image above left implies that these compact objects are capable of holding the jet direction constant over timescale as long as ten million years.
Black holes can only do this by means of their gyroscopic spin, and they can only communicate the direction of that spin to the jet via their gravitomagnetic field H.
GP-B — Spacetime & Spin
Such a field will cause an accretion disk to precess around the black hole, and that precession combined with the disk's viscosity should drive the inner region of the disk into the hole's equatorial plane, resulting in only two preferred directions for the jets: the north and south poles of the black hole.
This phenomenon, known as the Bardeen-Petterson effect diagram at left , is widely believed to be the physical mechanism responsible for jet alignment. Blandford-Znajek effect. Kip Thorne discusses black holes in the context of GP-B. Gravitomagnetism is also thought to explain the generation of the astounding energy contained in these jets in the first place. The event horizon of the black hole acts like a "gravitomagnetic battery", driving currents around closed loops like that shown in the diagram at left: up the magnetic field lines from the horizon to a region where the magnetic field is weak, across the field lines there, and then back down the field lines to the horizon and through the "battery" where the gravitomagnetic potential of the black hole interacts with the tangential component of the ordinary magnetic field B to produce a drop in electric potential see Kip Thorne's contribution to Near Zero: New Frontiers of Physics , This phenomenon, known as the Blandford-Znajek mechanism, effectively draws on the immense gravitomagnetic, rotational energy of the supermassive black hole and converts it into an outgoing stream of ultra-relativistic charged particles.
Gravity Probe B has thus become a crucial test of the mechanism that powers the most violent explosions in the universe. Star trails at night. Here is a simple experiment that almost anyone can perform on a clear night: pirouette freely around while looking up at the stars. You will notice two things: one, that the stars seem to spin around in the sky, and two, that your arms are pulled upwards by centrifugal force. Are these phenomena connected in some physical way? Not according to Newton. For him, centrifugal force is a consequence of accelerating i.
It is, furthermore, a coincidence that the stars above us are at rest with respect to this same absolute space. We look upward, in effect, from two fundamentally different reference frames: one defined by our local sense of inertia, and the other defined by the global rest frame of the universe at large.
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Why should these two reference frames happen to coincide? Newton did not try to answer this question. We know that the concept of absolute space time is retained in general relativity, so we might have expected that the same coincidental alignment of our local inertial frame with that of the global matter distribution would carry over to Einstein's theory as well. Astonishingly, however, it does not. If general relativity is correct, then there are strong indications that our local "compass of inertia" has no choice but to be aligned with the rest of the universe — the two are linked by the frame-dragging effect.
These indications do not come from experiment, but from theoretical calculations similar to that performed by Lense and Thirring. The calculations show that general-relativistic frame-dragging goes over to "perfect dragging" when the dimensions of the large mass its size and density become cosmological. In this limit, the distribution of matter in the universe appears sufficient to define the inertial reference frame of observers within it.
For a particularly clear and simple explanation of how and why this happens, see The Unity of the Universe by Dennis Sciama. Had Mach lived 10 years longer, he could have predicted the existence of the extragalactic universe based on observations that the stars in the Milky Way rotate around a common center! Would the earth still bulge, if it were standing still and the universe were rotating around it? To put the cosmological significance of frame-dragging in concrete terms, imagine that the earth were standing still and that the rest of the universe were rotating around it : would its equator still bulge?
Newton would have said "No". According to standard textbook physics the equatorial bulge is due to the rotation of the earth with respect to absolute space. On the basis of Lense and Thirring's results, however, Einstein would have had to answer "Yes"! In this respect general relativity is indeed more relativistic than its predecessors: it does not matter whether we choose to regard the earth as rotating and the heavens fixed, or the other way around: the two situations are now dynamically, as well as kinematically equivalent.
Pfister sums up current thinking this way in Mach's Principle: From Newton's Bucket to Quantum Gravity : "Although Einstein's theory of gravity does not, despite its name 'general relativity,' yet fulfil Mach's postulate of a description of nature with only relative concepts, it is quite successful in providing an intimate connection between inertial properties and matter, at least in a class of not too unrealistic models for our universe. Perhaps against majority expectation, this connection is instantaneous in nature.
Singer, and R. Cosmology and gravitation. Nato advanced study institutes series: Series B, Physics; v. Gauge theories physics -Congresses.
Supergravity Cosmology Rotation And Gravitation And Spin Torsion
Supergravity- Congresses. Bergmann, Peter Gabriel. De Sabbata, Venzo.
North Atlantic Treaty Organization. Division of Scientific Affairs. N33 Our aim was to delineate the present status of the principal efforts directed toward this end, and to explore possible directions of work in the near future. Efforts to incorporate spin as a dynamic variable into the foundations of the theory of gravitation were poineered by E.
Cartan, whose contributions to this problem go back half a century. Accord- ing to A.