samedi 13 février 2016

The real, the fake, and the true (or false)

A real blind-injection and fake signal of a direct detection of gravitational waves in 2010

A rather strong signal was observed on September 16, 2010, within a minute or so of its apparent arrival at the detectors. The scientists on duty at the detector sites immediately recognized the tell-tale chirp signal expected from the merger of two black holes and/or neutron stars, and sprang into action. They knew that it could be a blind injection, but they also knew to act like it was the real thing. The event was beautifully consistent with the expected signal from such a merger. The figures below show the strength of the signal (redder colors indicate more signal power) in time (horizontal axis) and frequency (vertical axis). The signal sweeps upwards in frequency ("chirp") as the stars spiral into one another, approaching merger. The first plot is what was seen in the LIGO Hanford detector, and the second is what was seen at the same time in the LIGO Livingston detector. Despite apparent differences, the two signals are completely consistent with one another. The dark and light blue regions are typical of fluctuating noise in the detectors. 
The loudness of the signal was consistent with it coming from a galaxy at a distance between 60 and 180 million light-years from ours. The detector network is capable of locating the source in the sky only crudely; it seemed to be coming from the constellation Canis Major (the "Big Dog") in the southern hemisphere (the event was dubbed "the Big Dog" shortly thereafter). They sent alerts to partners operating robotic optical telescopes in the southern hemisphere (ROTSE, TAROT, Skymapper, Zadko) and the Swift X-ray space telescope, all of which took images of the sky on that and/or subsequent days in the hope of capturing an optical or X-ray "afterglow".

Blind Injection" Stress-Tests LIGO and VIRGO's Search for Gravitational Waves 2010

Versus the first true signal or false blind-injection five years later
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10-21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160-180 Mpc corresponding to a redshift z=0.09+0.03-0.04. In the source frame, the initial black hole masses are 36+5-4M and 29±4M, and the final black hole mass is 62±4 M, with 3.0±0.5M⊙ c2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.
                      Signal at LIGO Hanford Observatory            Signal at LIGO Livingston Observatory
B. P. Abbott et al.* 
(LIGO Scientific Collaboration and Virgo Collaboration)
(Received 21 January 2016; published 11 February 2016)

Gravitational waves (GW) ground-based instruments are all-sky monitors with no intrinsic spatial resolution capability for transient signals. A network of instruments is needed to reconstruct the location of a GW in the sky, via time-of-arrival, and amplitude and phase consistency across the network [102]. The observed time-delay of GW150914 between the Livingston and Hanford observatories was 6.9+0.5-0.4 ms. With only the two LIGO instruments in observational mode, GW150914’s source location can only be reconstructed to approximately an annulus set to first approximation by this time-delay [103, 104]. .. the sky map for GW150914 ... corresponds to a projected 2- dimensional credible region of 140 deg2 (50% probability) and 590 deg2 (90% probablity). The associated 3-dimensional comoving volume probability region is ∼10-2 Gpc3; for comparison the comoving density of Milky Way-equivalent galaxies is ∼107 Gpc-3. This area of the sky was targeted by follow-up observations covering radio, optical, near infra-red, X-ray, and gamma-ray wavelengths that are discussed in [105]; searches for coincident neutrinos are discussed in [106].

(Submitted on 11 Feb 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 to the 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.
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

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


samedi 23 janvier 2016

Gravityational waves (rumor of direct detection on Earth and its ) reception spectrum on the blogosphere

From the bright side ...
Masses of both black holes exceed 10 solar masses
... many of us eagerly expect the announcement of a hugely exciting discovery (direct detection) of the gravitational waves...

There are several "traps" that may make you think that the {Laser Interferometer Gravitational Waves Observatory} LIGO  shouldn't work at all (I was tempted to be confused by several such traps) and for decades after the discovery of General Relativity, people felt uncertain whether the gravitational waves could have been physical at all (Mach's principle was the primary misconception that drove those who wanted to say that they were unphysical) but at the end, all of them are wrong. Gravitational waves do exist and LIGO-like detectors may detect them. Note that the lengths of the 4-kilometer arms are measured with the accuracy 100,000 times better than the radius of the atomic nucleus. Because it's so, a LIGO discovery will eliminate all conceivable theories that claim that Nature has an unavoidable error margin in positions that would be longer than 10−2010−20 meters (e.g. the nuclear radius). Nature only prevents you from measuring the positions and momenta simultaneously; but it surely keeps track of the position separately and the position makes sense with an arbitrary accuracy – at most, at the Planck scale, there may be some issues (but not the issues that non-stringy quantum gravity babblers are sometimes imagining). 
... A commenter whose name is known to us has told us that there have been two events (short periods with gravitational waves) detected by the LIGO. A new rumor I got yesterday says that LIGO has "heard" a merger of two black holes into one bigger black hole. (I don't know whether this event is one of the two events from the first sentence of this paragraph.) Because black holes are among the heaviest "stellar mass" objects and allow the shortest orbital radii (and therefore strongest gravitational waves), this situation is obviously an event that creates strong enough gravitational waves, especially when both black holes are said to be heavier than 10 Suns (and most black holes in the Universe probably are: the stellar black holes' masses are believed to be between 3 and 50 Suns or so). 
The frequency of the gravitational wave from the two black holes that the LIGO has supposedly detected is comparable to the aforementioned 100 hertz. It's hard to say how many periods of the radiation they will be able to detect. The truly final moments of the merger only correspond to the number of orbital periods comparable to one. But before the black holes merge, they orbit one another for a very long time. The radiation only gets strong enough during the final stages of the merger and whether the LIGO may "hear" the waves long before the final moment (and how long) depends on its actual sensitivity. LIGO got "advanced" so the sensitivity has been improved by an order of magnitude but I don't want to collect all the engineering data and calculate how good the sensitivity has become. 
...At the end, if the rumor is true, what they should have heard is very similar to this LIGO example of inspiral gravitational waves except that the newly actually observed frequency is "somewhat" lower than the frequency in the example (a deeper sound). Note that the frequency of the vibrations as well as their intensity increases with time up to the final moment when the waves almost abruptly disappear.
Posted on monday, january 11, 2016 by Lubos Motl on his blog The Reference Frame

... to its dark one
On the Theory of Gravitational Wave Rumour Sources 
There has been a great deal of excitement almost nowhere in the astrophysics community since it was announced recently that rumours of the detection of gravitational waves had yet again begun to circulate, so I thought I would add here a brief discussion of the theoretical background to these phenomena. 
The standard theoretical model of such rumours is that they are produced from time to time during the lifetime of a supermassive science project after periods of relative quiescence. It is thought that they are associated with a perceived lack of publicity which might threaten funding and lead to financial collapse of the project. This stimulates a temporary emission of hype produced by vigorous gossip-mongering which acts to inflate the external profile of the project, resisting external pressures and restoring equilibrium. This general phenomenon is not restricted to gravitational wave detection, but also occurs across many other branches of Big Science, especially cosmology and particle physics. 
However, observations of the latest outburst suggest support for a rival theory, in which rumours are produced not by the project itself but by some other body or bodies in orbit around it or even perhaps entirely independent of it. Although there is evidence in favour of this theory, it is relatively new and many questions remain to be answered. In particular it is not known what the effect of rumours produced in this way might be on the long-term evolution of the project or on the source itself.
Posted in AstrohypeThe Universe and StuffUncategorized with tags gossipgravitational wavesRumours on January 12, 2016 by Peter Coles aka telescoper on his blog In the Dark

...with several shades of grey (another drill before the next thrill)

An improbable rumour has started that the observatory has already made a discovery — but even if true, the signal could be a drill.
Davide Castelvecchi 30 September 2015 (NATURE | NEWS: EXPLAINER)

Gravitational-wave rumours in overdrive 
Could a signal be a false alarm?

That is also possible. The LIGO detectors have sophisticated systems for reducing unwanted vibrations, but the team needs to carefully check that any detection is not a false alarm — vibrations produced by a passing lorry, for instance. 
But the rumoured signal could also be the result of a deliberate drill. Three members of the LIGO team have access to systems that can secretly nudge the mirrors and simulate all the hallmarks of an astrophysical phenomenon — a procedure called a ‘blind injection’. Only when researchers are ready to reveal that they have spotted something will the blind-injection team announce whether it has created a deliberate signal. Two such exercises occurred in earlier runs of the LIGO, in 2007 and 2010.

But Krauss, at Arizona State University in Tempe, tweeted that he has heard rumours that the team detected a signal during tests of the upgraded detectors last summer, before official data-collection began — and before the blind-injection system was in place. (He did not know any more details when asked by Nature.)

The LIGO researchers would still take their time analysing such an early event, says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who heads LIGO's data-analysis team; scientists would not be able to assess such an early signal before gaining a better understanding of how often particular types of false alarm are likely to show up in the data.

The LIGO collaboration declined to comment on whether there was any time when both interferometers were active but no blind injection was possible. 
Are LIGO physicists concerned about the rumours?

González is a little miffed. “I am concerned about creating false expectations in the public and the media,” she says.

But Cadonati says that the buzz around the experiment has been “energizing”. “The fact that leaks started early on meant that they were something we had to learn to live with,” she says. “It means that there is excitement around what we are doing.”
Davide Castelvecchi 12 January 2016 (NATURE | NEWS: EXPLAINER)

Steven T. Corneliussen of Physics Today wrote a somewhat critical text about the wave of LIGO rumors that escalated one week ago:
Cosmologist Lawrence Krauss is often cited as a source, mainly because his rumors are relatively quickly spread through a large number of his Twitter followers. Your humble correspondent is often quoted as the propagator of the most precise rumors. Wait for February 11th to hear the announcement. And you will learn about at least two events. And at least one of them will be the detection of a merger of two black holes each of which weighs at least 10 solar masses. ;-) 
... As far as I can say, there are three basic reasons to be concerned when it comes to similar rumors:

  • Accuracy and balance between hype and underlying evidence
  • Fair distribution of fame and credit
  • Discipline and secrecy for their own sake 
Concerning the accuracy, well, people are told that it's "just rumors". But the track record of similar rumors has been extremely good in recent years. On Friday, March 14th, 2014, rumors spread that BICEP2 was going to claim the discovery of primordial gravitational waves on the following Monday. And it did. 

BICEP2 no longer believes that their 2014 paper was quite good but because the rumor was about the announcement, not the perfection of their actual analysis, the controversy about the BICEP2 results doesn't imply that the rumors were inaccurate.
Posted on wednesday, january 20, 2016 by Lubos Motl on his blog The Reference Frame

Even if all this gives you some feeling of déjà-vu let me wish you a happy new year 2016!

samedi 17 octobre 2015

The theoretical prediction that did not fit a wrong experimental finding and was probably right

The superluminal group velocity difference of neutrinos that was NOT measured by Opera
I take advantage of the Physics Nobel Prize 2015 rewarding the discovery of neutrino oscillations to shine a different light on a famous experiment that made big headlines in september 2011 (and gave me opportunity to start this blog!)
we discuss the possibility that the apparent superluminality is a quantum interference effect, that can be interpreted as a weak measurement [2, 3, 45]. Although the available numbers strongly indicate that this explanation is not correct, we consider the idea worth exploring and reporting – also because it might suggest interesting experiments, for example on electron neutrinos, about which relatively little is known. Similar suggestions, though not interpreted as a weak measurement [6, 7] or not accompanied by numerical estimates [6, 8], have been proposed independently. 
The idea, following analogous theory and experiment [9] involving light in a birefringent optical fibre, is based on the fact that the vacuum is birefringent for neutrinos. We consider the initial choice of neutrino flavour as a preselected polarization state, together with a spatially localized initial wavepacket. Since a given flavour is a superposition of mass eigenstates, which travel at different speeds, the polarization state will change during propagation, evolving into a superposition of flavours. The detection procedure postselects a polarization state, and this distorts the wavepacket and can shift its centre of mass from that expected from the mean of the neutrino velocities corresponding to the different masses. This shift can be large enough to correspond to an apparent superluminal velocity (though not one that violates relativistic causality: it cannot be employed to send signals). Large shifts, corresponding to states arriving at the detector that are nearly orthogonal to the polarization being detected, are precisely of the type considered in weak measurement theory. 
It seems that only muon and tau neutrino flavours are involved in the experiment... The initial beam, with ultrarelativistic central momentum p, is almost pure muon, which can be represented as a superposition, with mixing angle θ, of mass states |+> and |- >, with m> m- ... The two mass states evolve with different phases and group velocities neglecting the spreading and distortion [10] of the individual packets – both negligible in the present case. E± and v± are the energies and group velocities of the two mass states, and we write E±=E±1/2ΔE, v±=v±1/2Δv, x=vt+ξ... in which the new coordinate ξ measures deviation from the centre of the wavepacket expected by assuming it travels with the mean velocity. In the experiment, the detector postselects the muon flavour [1]... thus the shift in the measured final position of the wavepacket [can be interpreted]... as an effective velocity shift, that is  
[where the prefactor, tΔv is the relative shift of the two mass wavepackets, expected from the difference of their group velocities (it is small compared with the width of the packet ... in the neutrino case). The main factor represents the influence of the measurement-that is of the pre- and postselection and the evolution]... The possibility of superluminal velocity measurement arises because the amplification factor in (8) can be arbitrarily large if sin22θ and sin2(tΔE/2ℏ) are close to unity, corresponding to near-orthogonality of |pre> and |post>. 
For neutrinos with momentum p, ... the group velocity [difference Δv is given by -ΔE/p]. Thus Δv<0, so, in order for the apparent velocity to be superluminal, Δveff in (8) must be positive; this can be accommodated by making cos2θ negative. 
Note also that v+ and v-- are less than c if both neutrino masses are nonzero, so the individual mass eigenstate wavepackets move with subluminal group velocities; any superluminal velocity arising from (8) is a consequence of pulse distortion ... associated with the postselection, i.e. considering only arriving muon neutrinos. In the more conventional superluminal wave scenario [10], group velocities faster than light, and the pulse distortions that enable them to occur, are associated with propagation of frequencies near resonance, for which there is absorption, i.e. non-unitary propagation. That is also true in the optical polarization experiments [9] and in the neutrino situation considered here, with the difference that the nonunitarity, which gives rise to the superluminal velocity, is not continuous during propagation but arises from the sudden projection onto the postselected state.
In the [Opera] experiment, the energies of the neutrinos varied over a wide range, with an average of cp = 28.1GeV. For the difference in the squared masses, with electron neutrinos neglected and m+ and m- identified with the standard m2 and m3, a measured value [13] is m+2c4-m-2c42.43×10-3eV2. This gives
Δv/c=-1.5×10-24.                  (16) 
 The apparent velocity measured in the experiment [1] was (1+2.5×10-5)c . Comparison with the quantum velocity shift Δveff in (8) would require knowlege of m+ and m-, not just their squared difference, and the individual masses are not known. But even on the most optimistic assumption, that m-=0, it is immediately clear that it is unrealistic to imagine that the quantum amplification factor in (8) can bridge the gap of 19 orders of magnitude between (16) and the measured superluminal velocity.
(Submitted on 13 Oct 2011 (v1), last revised 14 Nov 2011 (this version, v2))

Remark: for the anecdote the abstract of this article by the distinguished mathematical physicist Michael Berry and his collaborators might be the shortest one ever written since it answered laconically to the question asked in the title : "probably not". And time has proved that it was right...

A superluminal group velocity of photons that was effectively measured
While the theoretical prediction from the last paragraph has not been tested by the Opera experiment and will stay quite hard to test empirically given the smallness of the effect, the physics behind it is pretty sound and falsifiable in other contexts. I think the article below is a nice illustration:
The physics of light propagation is a very timely topic because of its relevance for both classical [1] and quantum [2] communication. Two kind of velocities are usually introduced to describe the propagation of a wave in a medium with dispersion ω( k): the phase velocity vph=ωk and the group velocity vg=∂ω/∂k . Both of these velocities can exceed the speed of light in vacuum c in suitable cases [3]; hence, neither can describe the speed at which the information carried by a pulse propagates in the medium. Indeed, since the seminal work of Sommerfeld, extended and completed by Brillouin [4], it is known that information travels at the signal velocity, defined as the speed of the front of a square pulse. This velocity cannot exceed c [5]. The fact that no modification of the group velocity can increase the speed at which information is transmitted has been directly demonstrated in a recent experiment [6]. Superluminal (or even negative) and, on the other extreme, exceedingly small group velocities, have been observed in several media [7]. In this letter we report observation of both superluminal and delayed pulse propagation in a tabletop experiment that involves only a highly birefringent optical fiber and other standard telecom devices. 
Before describing our setup, it is useful to understand in some more detail the mechanism through which anomalous group velocities can be obtained. For a light pulse sharply peaked in frequency, the speed of the center-of-mass is the group velocity vg of the medium for the central frequency [3]. In the absence of anomalous light propagation, the local refractive index of the medium is nf , supposed independent on frequency for the region of interest. The free propagation simply yields vg=L/tf where L is the length of the medium and tf =nL/c is the free propagation time. One way to allow fast- and slow-light amounts to modify the properties of the medium in such a way that it becomes opaque for all but the fastest (slowest) frequency components. The center-of-mass of the outgoing pulse appears then at a time t = tf+<t>, with <t> the mean time of arrival once the free propagation has been subtracted; obviously <t><0 for fast-light, <t>>0 for slow-light. If the deformation of the pulse is weak, the group velocity is still the speed of the center-of-mass, now given by  
                          vg=Ltf+<t>.                                                                         (1) 
This can become either very large and even negative (<t>→−∞) or very small (<t>→∞) — although in these limiting situations the pulse is usually strongly distorted, so that our reasoning breaks down.


(Submitted on 20 Jul 2004 (v1), last revised 10 Jan 2005 (this version, v2))

mercredi 7 octobre 2015

Neutrino oscillations : experiment validated and awarded the 2015 Physics Nobel Prize ...

... but theory is still under [discuss]{construct}ion

Neutrino physics is one of the most interesting and vividly discussed topics in high-energy physics today. Especially the question whether the neutrinos can oscillate or not (i.e. different neutrinos can change into each other) gave rise to a huge number of experiments to actually observe these oscillations. At least since the results from the Super Kamiokande ... and the SNO experiment ... are published, it is widely believed that neutrino oscillations are an experimentally verified fact. However, the first hint has already been found in 1964 when the Homestake experiment ... discovered the solar neutrino problem. That is, the number of measured electron neutrinos from the sun is by a factor of 2-3 less than the number of neutrinos predicted by the standard solar model (SSM). 
Since within the standard model (SM) of particle physics the neutrinos are massless, and consequently cannot oscillate, their measurement shows that new physics beyond the SM exists. And indeed nowadays the experiments on neutrino oscillations are important to measure the unknown parameters of the SM and its minimal extensions. In particular, these unknown parameters are the neutrino masses and the entries in the neutrino mixing matrix. 
From all the measurements made to discover neutrino oscillations one should think that the theory behind [them] is well established and understood. But surprisingly this is not the case. The first who mentioned the idea of neutrino oscillations, though he assumed neutrino-antineutrino oscillations, was Pontecorvo in 1957 [Pon57, Pon58]. A few years later Maki, Nakagawa and Saka were the first to consider oscillations between the electron and the muon neutrino [MNS62]. Then it took around 20 years before Kayser in 1981 showed that the up to that point used plane-wave approximation cannot hold for oscillating neutrinos and he proposed a wave packet treatment [Kay81], which then has again not been discussed for around 10 years. In the early 90s the discussion on the theoretical description of neutrino oscillations finally started with several seminal papers. First, Giunti, Kim and Lee explicitly calculated the oscillation probability for the neutrinos in a wave packet model [GKL91] and then showed that the state vectors used for the quantum mechanical description are, in general, ill-defined [GKL92]. In 1993 they published together with Lee a calculation of the probability in a quantum field theoretical framework without using state vectors for the neutrinos [GKLL93]. And finally, in 1995 Blasone and Vitiello showed that the description of mixed particles in quantum field theory (QFT) yields unexpected problems for the interpretation of neutrinos as particles. By only using exact—without perturbation—QFT methods they calculated an oscillation probability which differs significantly from the other results [BV95]. All these different approaches are even today still under discussion, but however under the assumption of relativistic neutrinos which have tiny mass squared differences, all approaches give the same result. Thus, the theoretical discussion on the right description of the neutrinos does not spoil the experimental results, because today we are only able to measure ultra-relativistic neutrinos whose energy is at least a few orders of magnitude higher than their mass. 
Diploma Thesis On Theories of Neutrino Oscillations (Summary and Characterisation of the Problematic Aspects) Daniel Kruppke September 2007

A quantum field theory for flavor states ...
The study of mixing of fields of different masses in the context of Quantum Field Theory (QFT) has produced recently very interesting and in some sense unexpected results ... The story begins in 1995 when in Ref.[1], it was proved the unitary inequivalence of the Hilbert spaces for (fermion) fields with definite flavor on one side and those (free fields) with definite mass, on the other. The proof was then generalized to any number of fermion generations [7] and to bosonic fields [2, 5]. This result strikes with the common sense of Quantum Mechanics (QM), where one has only one Hilbert space at hand: the inconsistencies that arise there have generated much controversy and it was also claimed that it is impossible to construct an Hilbert space for flavor states [16] (see however Ref.[6] for a criticism of that argument). In fact, not only the flavor Hilbert space can be consistently defined [1], but it also provide a tool for the calculation of flavor oscillation formulas in QFT ..., which exhibit corrections with respect to the usual QM ones [20, 21]. From a more general point of view, the above results show that mixing is an “example of non-perturbative physics which can be exactly solved”, as stated in Ref.[13]. Indeed, the flavor Hilbert space is a space for particles which are not on-shell and this situation is analogous to that one encounters when quantizing fields at finite temperature [22] or in a curved background [23]. In the derivation of the oscillation formulas by use of the flavor Hilbert space, both for bosons and for fermions, a central role is played by the flavor charges [9] and indeed it was found that these operators satisfy very specific physical requirements [6, 8]. 
(Submitted on 23 May 2003 (v1), last revised 10 Jun 2003 (this version, v2))

... with an unfinished taste
Blasone and Vitiello (BV) have attempted to construct a Fock space for neutrino flavor states [4]... Giunti conclude that “the Fock spaces of flavor neutrinos are ingenuous mathematical constructs without physical relevance” [3]. 
... there is another issue that plagues the scheme in [5]. The problem is that the neutrino flavor vacuum defined in [5] is time-dependent and hence Lorentz invariance is manifestly broken. Recently, BV and collaborators attempted to tackle this issue by proposing neutrino mixing as a consequence of neutrino interactions with an external non-abelian gauge field [7]. Under this framework, the Lorentz violation of the neutrino flavor vacuum can be attributed to the presence of a fixed external field which specifies a preferred direction in spacetime. However, at the moment, there is not a single sign of such a non-abelian gauge field in neutrino experiments. They proposed that this scheme can be tested in the tritium decay, but again the indefinite mass mνα becomes an observable quantity. Also, given the current stringent bounds on Lorentz violations [8], it is unclear whether this scheme will survive... 
In this article, we first gave a detailed review on the current status of the understanding about the neutrino flavor states. At the end of the review, we were led to conclude that it is currently unclear how to construct a consistent and physically relevant Fock space of neutrino flavor states. We proceeded to prove that if one insists on second-quantizing the neutrino flavor fields and thereby constructing the flavor states, then they are approximately well-defined only when neutrinos are ultra-relativistic or the mass differences are negligible compared to energy...   
However, we showed that one can consistently describe weak interactions by only neutrino mass eigenstates. At the same time, we argued that the second quantization of neutrino flavor fields generally lacks physical relevance because their masses are indefinite. Thus, neutrino flavor states lose their physical significance and they should simply be interpreted as definitions to denote specific linear combinations of mass eigenstates involved in weak interactions. Under this interpretation, there is no physical motivation to construct the Fock space of neutrino flavor states from the first principles of quantum field theory. 
(Submitted on 16 Sep 2012 (v1), last revised 26 Nov 2012 (this version, v2))

mardi 25 août 2015

My quantum ostinato : the standard model can not stay out of the revolution of spacetime

This short post aims at two things:
  • to thank Jackson Clarke author of the very informative blog Syymmetries to put Transcyberphysix  in his recent list of "recommended (active) high energy physics news and blog links".
  • to suggest to any internaut arrived here through the former blog links to visit Quantum Ostinato another blog of mine which is currently more active than this one and might bring piece of information less covered by other blogs but relevant for people interested in high energy physics and astrophysics. 

lundi 29 juin 2015

Two scalars to rule the m(ass for almost) all (particles)? / Deux scalaires pour gouverner (presque) toutes les masses

The advanced art of massware in electroweak and QCD quantum vacua/ L'art subtil de la génération de masse dans les vides quantiques électrofaibles et chromodynamiques
This post is a follow-up to this one. / Ce billet fait écho à celui-ci vieux de plus d'un an.
In the standard model the masses of elementary particles arise from the Higgs field acting on the originally massless particles. When applied to the visible matter of the universe this explanation remains unsatisfactory as long as we consider the vacuum as an empty space. The QCD vacuum contains a condensate of up and down quarks. Condensate means that the q pairs are correlated via inter-quark forces mediated by gluon exchanges. As part of the vacuum structure the q pairs have to be in a scalar-isoscalar configuration. This suggests that the vacuum condensate may be described in terms of a scalar-isoscalar particle, |σ>=(|uu̅>+|dd̅>)/√2, providing the σ field. These two descriptions in terms of a vacuum condensate or a σ field are essentially equivalent and are the bases of the Nambu–Jona-Lasinio (NJL) model [28] and the linear σ model (LσM), [9] respectively. Furthermore, it is possible write down a bosonized version of the NJL model where the vacuum condensate is replaced by the vacuum expectation value of the σ field. 
In the QCD vacuum the largest part of the mass M of an originally massless quark, up (u) or down (d), is generated independent of the presence of the Higgs field and amounts to M = 326 MeV [1]. The Higgs field only adds a small additional part to the total constituent-quark mass leading to m u = 331 MeV and m d = 335 MeV for the up and down quark, respectively [1]. These constituent quarks are the building blocks of the nucleon in a similar way as the nucleons are in case of nuclei. Quantitatively, we obtain the experimental masses of the nucleons after including a binding energy of 19.6 MeV and 20.5 MeV per constituent quark for the proton and neutron, respectively, again in analogy to the nuclear case where the binding energies are 2.83 MeV per nucleon for 31 H and 2.57 MeV per nucleon for 3He. 
In the present work we extend our previous [1] investigation by exploring in more detail the rules according to which the effects of electroweak (EW) and strong-interaction symmetry breaking combine in order to generate the masses of hadrons. As a test of the concept, the mass of the π meson is precisely predicted on an absolute scale. In the strange-quark sector the Higgs boson is responsible for about 1/3 of the constituent quark mass, so that effects of the interplay of the two components of mass generation become essential. Progress is made by taking into account the predicted second σ meson, σ′(1344) = |ss̅> [7]. It is found that the coupling constant of the s-quark coupling to the σ′ meson is larger than the corresponding quantity of the u and d quarks coupling to the σ meson by a factor of √2. This leads to a considerable increase of the constituent quark masses in the strange-quark sector in comparison with the ones in the non-strange sector already in the chiral limit, i.e. without the effects of the Higgs boson. There is an additional sizable increase of the mass generation mediated by the Higgs boson due to a∼24 times stronger coupling of the s quark to the Higgs boson in comparison to the u and d quarks. In addition to the progress made in [1] as described above this paper contains a History of the subject from Schwinger’s seminal work of 1957 [10] to the discovery of the Brout-Englert-Higgs (BEH) mechanism, with emphasis on the Nobel prize awarded to Nambu in 2008. This is the reason why paper [1] has been published as a supplement of the Nobel lectures of Englert [11] and Higgs [12]...

The masses of constituent quarks are composed of the masses Mq predicted for the chiral limit and the mass of the respective current quark m0q provided by the Higgs boson (EW interaction) alone. For scalar mesons the sum of Mq and m0q leads to a zero-order approximation for the constituent-quark mass mq, but there are dynamical effects described by the NJL model which modify the simple relation mq=Mq+m0q  , except for the non-strange sector where this relation is a good approximation. Similar results are obtained for the octet baryons. A difference between the scalar mesons and the octet baryons is that that for scalar mesons binding energies do not play a rôle whereas they are of importance in case of octet baryons... 

(Submitted on 1 Jun 2015)