samedi 13 février 2016

Who was (were) the first scientist(s) to [fore]see {the direct detection of} genuine gravitational waves?

Henri Poincaré?
...j’ai d’abord été conduit à supposer que la propagation de la gravitation n’est pas instantanée, mais se fait avec la vitesse de la lumière. Cela semble en contradiction avec un résultat obtenu par Laplace qui annonce que cette propagation est, sinon instantanée, du moins beaucoup plus rapide que celle de la lumière. Mais, en réalité, la question que s’était posée Laplace diffère considérablement de celle dont nous nous occupons ici. Pour Laplace, l’introduction d’une vitesse finie de propagation était la seule modification qu’il apportait à la loi de Newton. Ici, au contraire, cette modification est accompagnée de plusieurs autres ; il est donc possible, et il arrive en effet, qu’il se produisent entre elles une compensation partielle.
Quand nous parlerons donc de la position ou de la vitesse du corps attirant, il s’agira de cette position ou de cette vitesse à l’instant où l’onde gravifique est partie de ce corps ; quand nous parlerons ou de la vitesse du corps attiré, il s’agira de cette position ou de cette vitesse à l’instant où ce corps attiré a été atteint par l’onde gravifique émanée de l’autre corps ; il est clair que le premier instant est antérieur au second.
Note de M. H. Poincaré. 5 Juin1905

Albert Einstein?
In 1918 Einstein published the paper ÜBER GRAVITATIONSWELLEN [1] in which, for the first time, the effect of gravitational waves was calculated, resulting in his famous “quadrupole formula” (QF). Einstein was forced to this publication due to a serious error in his 1916 paper [2], where he had developed the linear approximation (“weak- field”) scheme to solve the field equations of general relativity (GR). In analogy to electrodynamics, where accelerated charges emit electromagnetic waves, the linearized theory creates gravitational waves, propagating with the speed of light in the (background) Minkowski space-time. A major difference: Instead of a dipole moment, now a quadrupole moment is needed. Thus sources of gravitational waves are objects like a “rotating dumbbell”, e. g. realized by a binary star system. As there was no chance for detecting gravitational waves, due to their extreme weakness of the order (v/c)5, the theory advanced slow in the first decades. The existence of gravitational waves was always a matter of controversy. Curiously Einstein himself was not convinced in 1936. In a paper with Nathan Rosen he came tothe conclusion, that gravitational waves do not exist! Curiously too is the story of its publication. Einstein’s manuscript, titled DO GRAVITATIONAL WAVES EXIST?, was rejected by the “Physical Review”. In an angry reply he withdrawed the paper, to appear later in the “Journal of the Franklin Institute” (choosing a less provoking headline [3]). To clear the situation, various approximation schemes were developed. One of the first, introduced by Einstein, Infeld and Hoffmann in 1938 [4], led to the famous EIH equations. This “post-Newtonian” treatment describes slow moving bodies in a weak field (“bounded systems”). In the EIH approximation there is no radiation up to the order (v/c)4 , the energy remains constant. The QF appears in the next order, as demonstrated by Hu in 1947 [5]. What’s about fast moving particles? This problem had to wait until the early 1960’s, when the Lorentz-invariant perturbation methods (“fast-motion approximation”), describing “unbounded systems”, were developed. The question of an analogy to the QF (“radiation damping”) was strongly discussed. In 1975 a major boost was caused by the discovery of the binary pulsar PSR 1913 + 16 by Hulse and Taylor [6]. Over the next years their data showed a decrease of the period of revolution – as predicted by the QF! But this (indirect) proof – in the “bounded” case – did not stop the controversy: On the contrary, the fight gets even stronger. The different approximation formalisms were criticized by Ehlers, Havas and others [7]. The basic difficulties are: (1) In contrast to electrodynamics, the equations of motion in GR are not a separate part of the theory, but already inherent in the field equations. (2) GR is an essential non-linear theory. Any approximation must treat these facts carefully. After a phase of clarification, introducing new methods (e. g. asymptotic field conditions, post-linear approximations), the believe in gravitational waves, and especially in Einstein’s quadrupole formula, is now stronger than ever – eventually visible in expensive terrestrial and space experiments.

//Update February 20 2016:
There is an interesting presentation of Daniel Kennefick providing more information about the circumstances surrounding the 1936 Einstein and Rosen withdrawal of their article about the nonexistence of gravitational waves and the first encounter of the father of general relativity with anonymous peer review!

Joseph Weber?
There appears to have been little interest in the experimental detection of gravitational radiation for fortyfive years after their prediction. However in the late 1950s this changed with Joseph Weber of the University of Maryland suggesting the design of some relatively simple apparatus for their detection [8, 9]. This apparatus in its later stages consisted of an aluminium bar of mass approximately one ton with piezoelectric transducers bonded around its centre line. The bar was suspended from anti-vibration mountings in a vacuum tank. By means of the amplified electrical signals from the transducers Weber monitored the amplitude of oscillation of the fundamental mode of the bar. A gravitational wave signal of suitable strength would be expected to change the amplitude or phase of the oscillations in the bar. In the 1969/70 period Weber operated two such systems one at the University of Maryland and one at the Argonne National Laboratory and observed coincident excitations of the bars at a rate of one event per day [10, 11]. These events he claimed to be gravitational wave signals. 
However other experiments – at Moscow State University [12], Yorktown heights [13], Rochester [14], Bell Labs [15], Munich [16] and Glasgow [17] - failed to confirm Weber's detections... Several years of lively debate about the interpretation of Weber's results followed, the outcome being a somewhat predictable standoff between Weber and the rest of the community. An analysis of detector sensitivity of the Weber bar design suggested that the sensitivity was approximately 10-16 for millisecond pulses. However an event rate of one per day resulting from events at the centre of the galaxy - as claimed by Weber - corresponded to a very high loss of energy, and thus mass, from the galaxy, so high in fact that changes in the position of the outermost stars should have been visible due to a reduction in gravitational force towards the galactic centre [18]. A solution suggested for this – beaming of the energy in a narrow cone so that each detected event implied much less overall energy loss - was discussed by many authors but did not receive wide acceptance.
(Submitted on 4 Jan 2005)

M. E. Gertsenshtein and V. I Putsovoit?
Almost as soon as Weber had begun work on the first gravitational wave detector or the resonant-mass style, the idea arose to use interferometry to sense the motions induced by a gravitational wave. Weber and a student, Robert Forward, considered the idea in 1964. We will discuss below how Forward later went about implementing the idea. But the first discussion of the idea is actually due to two Soviet physicists, M.E. Gertsenshtein and V.I. Pustovoit. They wrote in 1962 a criticism of Weber’s 1960 Physical Review article, claiming (incorrectly) that resonant gravitational wave detectors would be very insensitive. Then, they make a remarkable statement justified only by intuition, that “Since the reception of gravitational waves is a relativistic effect, one should expect that the use of an ultrarelativistic body — light — can lead to a more effective indication of the field of the gravitational wave.” 
Gertsenshtein and Pustovoit followed up this imaginative leap by noting that a Michelson interferometer has the appropriate symmetry to be sensitive to the strain pattern produced by gravitational waves. They give a simple and clear derivation of the arm length difference caused by a wave of amplitude h. Next, they note that L.L. Bernshtein had with ordinary light measured a path length differences of 10-11 cm in a 1sec integration time. The newly invented laser, they claim, would “make it possible to decrease this factor by at least three orders of magnitude.” (The concept of shot noise never appears explicitly here, so it is not clear what power levels are being anticipated.) They assume that one might make an interferometer with arm length of 10 m, thus leading to a sensitivity estimate of 10-14Hz for “ordinary” light, or as good as 10--17Hz for a laser-illuminated interferometer. This, Gertsenshtein and Pustovoit claim, is 107 to 1010 times better (it isn’t clear whether they mean in amplitude or in power) than what would be possible with Weber-style detector. Putting aside their unjustified pessimism about resonant-mass detectors, their arguments about interferometric sensing are right on the mark, even conservative. 
For improvements beyond the quoted level, they make suggestions that are somewhat misguided. They say that observation time could be lengthened beyond 1 sec, which would be obvious for some sources (such as “monochromatic sinusoidal signals” or signals of long period) and hopeless for short bursts. Their other suggestion is to use “known methods for the separation of a weak signal from the noise background”; this suggestion is curious because known methods appear to be already built into their estimates that are referenced to a specific observing time. The other lack that is obvious in hindsight is any mention of mechanical noise sources. Still, the gist of the idea of interferometric detection of gravitational waves is clearly present, as is a demonstration that the idea can have interesting sensitivity.
For a variety of reasons, not least of which must have been the fact that it was written too early (before Weber’s work had progressed beyond design studies), the proposal of Gertsenshtein and Pustovoit had little influence. The activity that began the by-now flourishing field of interferometric gravitational wave detection started independently in the West. In fact, it began semi-independently at several places in the United States at around the same time. The roots of this work can be seen in a pair of papers, written in 1971-2, by two teams linked in an unusual collaboration that is acknowledged in the bodies of the papers, although not in the author lists. The first to be published was that of the Hughes Research Lab team, whose most committed member was Robert L. Forward, the former Weber student mentioned above. Later to appear, and not in a refereed journal, was the work of Rainer Weiss, an MIT physicist who had spent an influential postdoctoral stint with Robert H. Dicke at Princeton. Linking the two groups was someone who never published anything on the subject under his own name, but whose activity is mentioned in both papers — Philip K. Chapman, who had earned a doctorate in Instrumentation at MIT’s Department of Aeronautics and Astronautics before joining NASA as a scientist-astronaut.
Peter R. Saulson 1998
//Update February 20 2016

(I thank the JETP editorial office in general and Natalia Tserevitinova in particular to have made this article recently available online).

Marco Drago?
...on September 14, 2015, at just before eleven in the morning, Central European Time, the waves reached Earth. Marco Drago, a thirty-two-year-old Italian postdoctoral student and a member of the LIGO Scientific Collaboration, was the first person to notice them. He was sitting in front of his computer at the Albert Einstein Institute, in Hannover, Germany, viewing the LIGO data remotely. The waves appeared on his screen as a compressed squiggle, but the most exquisite ears in the universe, attuned to vibrations of less than a trillionth of an inch, would have heard what astronomers call a chirp—a faint whooping from low to high.

The LIGO team includes a small group of people whose job is to create blind injections—bogus evidence of a gravitational wave—as a way of keeping the scientists on their toes. Although everyone knew who the four people in that group were, “we didn’t know what, when, or whether,” Gabriela González, the collaboration’s spokeswoman, said. During Initial LIGO’s final run, in 2010, the detectors picked up what appeared to be a strong signal. The scientists analyzed it intensively for six months, concluding that it was a gravitational wave from somewhere in the constellation of Canis Major. Just before they submitted their results for publication, however, they learned that the signal was a fake.

This time through, the blind-injection group swore that they had nothing to do with the signal. Marco Drago thought that their denials might also be part of the test


Aucun commentaire:

Enregistrer un commentaire

Cher-ère lecteur-trice, le blogueur espère que ce billet vous a sinon interessé-e du moins interpellé-e donc, si le coeur vous en dit, osez partager avec les autres internautes comme moi vos commentaires éclairés !
Dear reader, the blogger hopes you have been interested by his post or have noticed something (ir)relevant, then if you are in the mood, do not hesitate to share with other internauts like me your enlightened opinion !