lundi 16 octobre 2017

Which neutron star merger with gold-plated nuclear waste mushroom at 130 million light years?

[A new exciting, another boring usual] astrophysical event?

The discovery, announced Monday at a news conference and in scientific reports written by some 3,500 researchers, solves a long-standing mystery about the origin of these heavy elements — which are found in everything from wedding rings to cellphones to nuclear weapons. 
It's also a dramatic demonstration of how astrophysics is being transformed by humanity's newfound ability to detect gravitational waves, ripples in the fabric of space-time that are created when massive objects spin around each other and finally collide. 
"It's so beautiful. It's so beautiful it makes me want to cry. It's the fulfillment of dozens, hundreds, thousands of people's efforts, but it's also the fulfillment of an idea suddenly becoming real," says Peter Saulson of Syracuse University, who has spent more than three decades working on the detection of gravitational waves...
What all the images showed was a brand-new point of light that started out blueish and then faded to red. This didn't completely match what theorists thought colliding neutron stars should look like — but it was all close enough that Daniel Kasen, a theoretical astrophysicist at the University of California, Berkeley, found the whole experience a little weird. 
"Even though this was an event that had never been seen before in human history, what it looked like was deeply familiar because it resembled very closely the predictions we had been making," Kasen says. "Before these observations, what happened when two neutron stars merged was basically just a figment of theorists' imaginations and their computer simulations." 
He spent late nights watching the data come in and says the colliding stars spewed out a big cloud of debris. 
"That debris is strange stuff. It's gold and platinum, but it's mixed in with what you'd call just regular radioactive waste, and there's this big radioactive waste cloud that just starts mushrooming out from the merger site," Kasen says. "It starts out small, about the size of a small city, but it's moving so fast — a few tenths of the speed of light — that after a day it's a cloud the size of the solar system." 
According to his estimates, this neutron star collision produced around 200 Earth masses of pure gold, and maybe 500 Earth masses of platinum. "It's a ridiculously huge amount on human scales," Kasen says...
October 16, 201710:01 AM ET

LIGO, with the world’s first two gravitational observatories, detected the waves from two merging neutron stars, 130 million light years from Earth, on August 17th... VIRGO, with the third detector, allows scientists to triangulate and determine roughly where mergers have occurred. They saw only a very weak signal, but that was extremely important, because it told the scientists that the merger must have occurred in a small region of the sky where VIRGO has a relative blind spot...
The merger was detected for more than a full minute… to be compared with black holes whose mergers can be detected for less than a second. It’s not exactly clear yet what happened at the end, however! Did the merged neutron stars form a black hole or a neutron star? The jury is out.
If there’s anything disappointing about this news, it’s this: almost everything that was observed by all these different experiments was predicted in advance. Sometimes it’s more important and useful when some of your predictions fail completely, because then you realize how much you have to learn. Apparently our understanding of gravity, of neutron stars, and of their mergers, and of all sorts of sources of electromagnetic radiation that are produced in those merges, is even better than we might have thought. But fortunately there are a few new puzzles. The X-rays were late; the gamma rays were dim…

jeudi 31 août 2017

Ondes gravitationnelles et résonances d'orages africains/ Gravitational wave signals and unexpectedly strong Schumann resonance transients correlated noise

Raphaël Enthoven,Jacques Perry-salkow

(The validation of) A great discovery requires a genuinely independent analysis of data

To date, the LIGO collaboration has detected three gravitational wave (GW) events appearing in both its Hanford and Livingston detectors. In this article we reexamine the LIGO data with regard to correlations between the two detectors. With special focus on GW150914, we report correlations in the detector noise which, at the time of the event, happen to be maximized for the same time lag as that found for the event itself. Specifically, we analyze correlations in the calibration lines in the vicinity of 35 Hz as well as the residual noise in the data after subtraction of the best-fit theoretical templates. The residual noise for the other two events, GW151226 and GW170104, exhibits similar behavior. A clear distinction between signal and noise therefore remains to be established in order to determine the contribution of gravitational waves to the detected signals

(Submitted on 13 Jun 2017 (v1), last revised 9 Aug 2017 (this version, v2))

A debate about how to sift the astrophysical wheat from the terrestrial chaff

Recent claims in a preprint by Creswell et al. of puzzling correlations in LIGO data have broadened interest in understanding the publicly available LIGO data around the times of the detected gravitational-wave events. We see that the features presented in Creswell et al. arose from misunderstandings of public data products. The LIGO Scientific Collaboration and Virgo Collaboration (LVC) have full confidence in our published results, and we are preparing a paper in which we will provide more details about LIGO detector noise properties and the data analysis techniques used by the LVC to detect gravitational-wave signals and infer their waveforms.

News from LIGO Scientific Collaboration
undated (between 7 July and 1 August 2017)
In our view, if we are to conclude reliably that this signal is due to a genuine astrophysical event, apart from chance-correlations, there should be no correlation between the "residual" time records from LIGO's two detectors in Hanford and Livingston. The residual records are defined as the difference between the cleaned records and the best GW template found by LIGO. Residual records should thus be dominated by noise, and they should show no correlations between Hanford and Livingston. Our investigation revealed that these residuals are, in fact, strongly correlated. Moreover, the time delay for these correlations coincides with the 6.9 ms time delay found for the putative GW signal itself...
During a two-week period at the beginning of August, we had a number of "unofficial" seminars and informal discussions with colleagues participating in the LIGO collaboration... Given the media hype surrounding our recent publication, these meetings began with some measure of scepticism on both sides. The atmosphere improved dramatically as our meetings progressed. 
The focus of these meetings was on the detailed presentation and lively critical discussion of the data analysis methods adopted by the two groups. While there was unofficial agreement on a number of important topics - such as the desirability of better public access to LIGO data and codes - we emphasize that no consensus view emerged on fundamental issues related to data analysis and interpretation.
In view of unsubstantiated claims of errors in our calculations, we appreciated the opportunity to go through our respective codes together - line by line when necessary - until agreement was reached. This check did not lead to revisions in the results of calculations reported in versions 1 and 2 of arXiv:1706.04191 or in the version of our paper published in JCAP. It did result in changes to the codes used by our visitors.
There are a number of in-principle issues on which we disagree with LIGO's approach. Given the importance of LIGO's claims, we believe that it is essential to establish the correlation between Hanford and Livingston signals and to determine the shape of these signals without employing templates. Before such comparisons can be made, the quality of data cleaning (which necessarily includes the removal of non-Gaussian and non-stationary instrumental "foreground" effects) must be demonstrated by showing that the residuals consist only of uncorrelated Gaussian noise. We believe that suitable cleaning is a mandatory prerequisite for any meaningful comparisons with specific astrophysical models of GW events. This is why we are concerned, for example, about the pronounced "phase lock" in the LIGO data.
James Creswell, Sebastian von Hausegger, Andrew D. Jackson, Hao Liu, Pavel Naselsky
August 21, 2017

Disentangling the man-made detectors from the Earth-shaped one

As the LIGO detectors are extremely sensitive instruments they are prone to many sources of noise that need to be identified and removed from the data. An impressive amount of efforts were undertaken by the LIGO collaboration to ensure that GW150914 signal was really the first detection of gravitational waves with all transient noise backgrounds being under a good control [4, 5, 6]. 

It was claimed, however, in a recent publication [7] that the residual noise of the GW150914 event in LIGO’s two widely separated detectors exhibit correlations that are maximized for the same 7 ms time lag as that found for the gravitational-wave signal itself. Thus questions on the integrity and reliability of the gravitational waves detection were raised and informally discussed [8, 9]. It seems at present time it is not quite clear whether there is something unexplained in LIGO noise that may be of genuine interest. It was argued that even assuming that the claims of [7] about correlated noise are true, it would not affect the 5-sigma confidence associated with GW0150914 [8]. Nevertheless, in this case it will be interesting to find out the origin of this correlated noise.
Correlated magnetic fields from Schumann resonances constitute a well known potential source of correlated noise in gravitational waves detectors [11, 12, 13]... Schumann resonances are global electromagnetic resonances in the Earthionosphere cavity [14, 15]. The electromagnetic waves in the extremely low frequencies (ELF) range (3Hz to 3 kHz) are mostly confined in this spherical cavity and their propagation is characterized by very low attenuation which in the 5 Hz to 60 Hz frequency range is of the order of 0.5-1 db/Mm. Schumann resonances are eigenfrequencies of the Earth-ionosphere cavity. They are constantly excited by lightning discharges around the globe. While individual lightning signals below 100 Hz are very weak, thanks to the very low attenuation, related ELF electromagnetic waves can be propagated a number of times around the globe, constructively interfere for wavelengths comparable with the Earth’s circumference and create standing waves in the cavity.

Note that there exists some day-night variation of the resonance frequencies, and some catastrophic events, like a nuclear explosion, simultaneously lower all the resonance frequencies by about 0.5 Hz due to lowering of the effective ionosphere height [16]. Interestingly, frequency decrease of comparable magnitude of the first Schumann resonance, caused by the extremely intense cosmic gamma-ray flare, was reported in [17]. Usually eight distinct Schumann resonances are reliably detected in the frequency range from 7 Hz to 52 Hz. However five more were detected thanks to particularly intense lightning discharges, thus extending the frequency range up to 90 Hz [18].

...  For short duration gravitationalwave transients, like the three gravitational-waves signals observed by LIGO, Schumann resonances are not considered as significant noise sources because the magnetic field amplitudes induced by even strong remote lightning strikes usually are of the order of a picotesla, too small to produce strong signals in the LIGO gravitational-wave channel [4].

Interestingly enough, the Schumann resonances make the Earth a natural gravitational-wave detector, albeit not very sensitive [20]. As the Earth is positively charged with respect to ionosphere, a static electric field, the so-called fair weather field is present in the earth-ionosphere cavity. In the presence of this background electric field, the infalling gravitational wave of suitable frequency resonantly excites the Schumann eigenmodes, most effectively the second Schumann resonance [20]. Unfortunately, it is not practical to turn Earth into a gravitational-wave detector. Because of the weakness of the fair weather field (about 100 V/m) and low value of the quality factor (from 2 to 6) of the Earth-ionosphere resonant cavity, the sensitivity of such detector will be many orders of magnitude smaller than the sensitivity of the modern gravitational-wave detectors

However, a recent study of short duration magnetic field transients that were coincident in low-noise magnetometers in Poland and Colorado revealed that there was about 2.3 coincident events per day where the amplitudes of the pulses exceeded 200 pT, strong enough to induce a gravitational-wave like signal in the LIGO gravitational-wave channel of the same amplitude as in the GW150914 event [21]...

The main source of the Schumann ELF waves are negative cloud-toground lightning discharges with the typical charge moment change of about 6 Ckm. On Earth, storm cells, mostly in the tropics, generate about 50 such discharges per second.

The so-called Q-bursts are more strong positive cloud-to-ground atmospheric discharges with charge moment changes of order of 1000 Ckm. ELF pulses excited by Q-bursts propagate around the world. At very far distances only the low frequency components of the ELF pulse will be clearly visible, because the higher frequency components experience more attenuation than the lower frequency components...

In [22] Earth’s lightning hotspots are revealed in detail using 16 years of space-based Lightning Imaging Sensor observations. Information about locations of these lightning hotspots allows us to calculate time lags between arrivals of the ELF transients from these locations to the LIGO-Livingston (latitude 30.563◦ , longitude −90.774◦ ) and LIGO-Hanford (latitude 46.455◦ , longitude −119.408◦ ) gravitational-wave detectors...

We have taken Earth’s lightning hotspots from [22] with lightning flash rate densities more than about 100 fl km−2 yr−1 and calculated the expected time lags between ELF transients arrivals from these locations to the LIGO detectors... Note that the observed group velocity for short ELF field transients depends on the upper frequency limit of the receiver [21]. For the magnetometers used in [21] this frequency limit was 300 Hz corresponding to the quoted group velocity of about 0.88c. For the LIGO detectors the coupling of magnetic field to differential arm motion decreases by an order of magnitude for 30 Hz compared to 10 Hz [4]. Thus for the LIGO detectors, as the ELF transients receivers, the more appropriate upper frequency limit is about 30 Hz, not 300 Hz. According to (2), low frequencies propagate with smaller velocities 0.75c-0.8c. Therefore the inferred time lags in the Table1 might be underestimated by about 15%...

If the strong lightnings and Q-bursts indeed contribute to the LIGO detectors correlated noise then the distribution of lightning hotspots around the globe can lead to some regularities in this correlated noise. Namely, extremely low frequency transients due to lightnings in Africa will be characterized by 5-7 ms time lags between the LIGO-Hanford and LIGO-Livingston detectors. Asian lightnings lead to time lags which have about the same magnitude but the opposite sign. Lightnings in North and South Americas should lead positive time lags of about 11-13 ms, greater than the light propagation time between the LIGO-Hanford and LIGO-Livingston detectors. 

(Submitted on 27 Jul 2017)

mercredi 22 février 2017

{Bohmian mechanics, is} [subtle, malicious] (?)

Here is my post consisting as usual in quotes from some scientific articles fully available online, underlining (or emphasizing with a bold font) selected parts in order to sketch a draft response to the question in its title. This time, I was mostly inspired by reading this post at another blog named Elliptic Composability.

Inconclusive Bohmian positions in the macroscopic way ...
Bohmian mechanics differs deeply from standard quantum mechanics. In particular, in Bohmian mechanics particles, here called Bohmian particles, follow continuous trajectories; hence in Bohmian mechanics there is a natural concept of time-correlation for particles’ positions. This led M. Correggi and G. Morchio [1] and more recently Kiukas and Werner [2] to conclude that Bohmian mechanics “can’t violate any Bell inequality”, hence is disproved by experiments. However, the Bohmian community maintains its claim that Bohmian mechanics makes the same predictions as standard quantum mechanics (at least as long as only position measurements are considered, arguing that, at the end of the day, all measurements result in position measurement, e.g. pointer’s positions).  
Here we clarify this debate. First, we recall why two-time position correlation is at a tension with Bell inequality violation. Next, we show that this is actually not at odd with standard quantum mechanics because of some subtleties. For this purpose we do not go for full generality, but illustrate our point on an explicit and rather simple example based on a two-particle interferometers, partly already experimentally demonstrated and certainly entirely experimentally feasible (with photons, but also feasible at the cost of additional technical complications with massive particles). The subtleties are illustrates by explicitly coupling the particles to macroscopic systems, called pointers, that measure the particles’ positions. Finally, we raise questions about Bohmian positions, about macroscopic systems and about the large difference in appreciation of Bohmian mechanics by the philosophers and physicists communities... 
Part of the attraction of Bohmian mechanics lies then in the assumption that • Assumption H : Position measurements merely reveal in which (spatially separated and non-overlapping) mode the Bohmian particle actually is.   
A Bohmian particle and its pilot wave arrive on a Beam-Splitter (BS) from the left in mode “in”. The pilot wave emerges both in modes 1 and 2, as the quantum state in standard quantum theory. However, the Bohmian particle emerges either in mode 1 or in mode 2, depending on its precise initial position. As Bohmian trajectories can’t cross each other, if the initial position is in the lower half of mode “in”, then the Bohmian particle exists the BS in mode 1, else in mode 2.

Two Bohmian particles spread over 4 modes. The quantum state is entangled... hence the two particle are either in modes 1 and 4, or in modes 2 and 3. Alice applies a phase x on mode 1 and Bob a phase y on mode 4. Accordingly, after the two beam-splitters the correlations between the detectors allow Alice and Bob to violate Bell inequality... Alice’s first “measurement”, with phase x, can be undone because in Bohmian mechanics there is no collapse of the wavefunction. Hence, after having applied the phase −x after her second beam-splitter, Alice can perform a second “measurement” with phase x′ .

... There is no doubt that according to Bohmian mechanics there is a well-defined joint probability distribution for Alice’s particle at two times and Bob’s particle: P(rA, r′A, rB|x, x′ , y), where rA denotes Alice’s particle after the first beam-splitter and r′A after the third beamsplitter of {the last figure above}... But here comes the puzzle. According to Assumption H, if rA∈′′1′′, then any position measurement performed by Alice in-between the first and second beam-splitter would necessarily result in a=1. Similarly rA ∈′′2′′ implies a=2. And so on, Alice’s position measurement after the third beam-splitter is determined by r ′ A and Bob’s measurement determined by rB. Hence, it seems that one obtains a joint probability distribution for both of Alice’s measurements results and for Bob’s: P(a, a′ , b|x, x′ , y). 
But such a joint probability distribution implies that Alice doesn’t have to make any choice (she merely makes both choices, one after the other), and in such a situation there can’t be any Bell inequality violation.
... Let’s have a closer look at the probability distribution that lies at the bottom of our puzzle: P(rA, r′ A, rB|x, x′ , y)... now comes the catch... as the Bohmian particles’s positions are assumed to be “hidden”... they have to be hidden in order to avoid signalling in Bohmian mechanics. ... it implies that Bohmian particles are postulated to exist “only” to immediately add that they are ultimately not fully accessible... Consequently, defining a joint probability for the measurement outcomes a, a ′ and b in the natural way: 
P (a, a′ , b|x, x′ , y) ≡ P (rA ∈ “a“, rA ∈ “a ′ “, rB ∈ “b“|x, x′ , y) (10) 
can be done mathematically, but can’t have a physical meaning, as P(a, a′, b|x, x′ , y) would be signaling.
In summary, it is the identification (10) that confused the authors of [1, 2] and led them to wrongly conclude that Bohmian mechanics can’t predict violations of Bell inequalities in experiments involving only position measurements. Note that the identification (10) follows from the assumption H, hence assumption H is wrong. Every introduction to Bohmian mechanics should emphasize this. Indeed, assumption H is very natural and appealing, but wrong and confusing.

To elaborate on this let’s add an explicit position measurement after the first beam-splitter on Alice side. The fact is that both according to standard quantum theory and according to Bohmian mechanics, this position measurement perturbs the quantum state (hence the pilot wave) in such a way that the second measurement, labelled x ′ on Fig. 4, no longer shares the correlation (9) with the first measurement, see [4, 5]...

From all we have seen so far, one should, first of all, recognize that Bohmian mechanics is deeply consistent and provides a nice and explicit existence proof of a deterministic nonlocal hidden variables model. Moreover, the ontology of Bohmian mechanics is pretty straightforward: the set of Bohmian positions is the real stuff. This is especially attractive to philosopher. Understandably so. But what about physicists mostly interested in research? What new physics did Bohmian mechanics teach us in the last 60 years? Here, I believe fair to answer: not enough! Understandably disappointing... 
This is unfortunate because it could inspire courageous ideas to test quantum physics. 

Probably surrealistic Bohm Trajectories in the microscopic world?

... we maintain that Bohmian Mechanics is not needed to have the Schrödinger equation "embedded into a physical theory". Standard quantum theory has already clarified the significance of Schrödinger's wave function as a tool used by theoreticians to arrive at probabilistic predictions. It is quite unnecessary, and indeed dangerous, to attribute any additional "real" meaning to the psi-function. The semantic difference between "inconsistent" and "surrealistic" is not the issue. It is the purpose of our paper to show clearly that the interpretation of the Bohm trajectory - as the real retrodicted history of the atom observed on the screen - is implausible, because this trajectory can be macroscopically at variance with the detected, actual way through the interferometer. And yes, we do have a framework to talk about path detection; it is based upon the local interaction of the atom with the photons inside a resonator, described by standard quantum theory with its short range interactions only. Perhaps it is true that it is "generally conceded that.. . [a measurement]... requires a ... device which is more or less macroscopic," but our paper disproves this notion, because it clearly shows that one degree of freedom per detector is quite sufficient. That is the progress represented by the quantum-optical whichway detectors. And certainly, it is irrelevant for all practical purposes whether "somebody looks" or not; what matters only is that the which-way information is stored somewhere so that the path through the interferometer can be known, in principle.

Nowhere did we claim that BM makes predictions that differ from those of standard quantum mechanics. The whole point of the experimentum crucis is to demonstrate that one cannot attribute reality to the Böhm trajectories, where reality is meant in the phenomenological sense. One must not forget that physics is an experimental science dealing with phenomena. If the trajectories of BM have no relation to the phenomena, in particular to the detected path of the particle, then their reality remains metaphysical, just like the reality of the ether of Maxwellian electrodynamics. Of course, the "very existence" of the Böhm trajectory is a mathematical statement to which nobody objects. We do not deny the possibility that some imaginary parameters possess a "hidden reality" endowed with the assumed power of exerting "gespenstische Fernwirkungen" (Einstein). But a physical theory should carefully avoid such concepts of no phenomenological consequence.  
B.-G. Englert, M. O. Scully, G. Süssmann, and H. Walther
received October 12, 1993