Café des sciences

[Freezbee]

Je crée ce fil destiné à recevoir tout ce qui concerne les sciences sans pour autant rentrer dans un des fils existants : articles à partager, anecdotes, curiosités, expériences, images, animations, conseils de lecture... Si un sujet venait à s'enrichir, il sera toujours possible de le déplacer ou de créér un nouveau fil.

 

Le titre de ce fil fait référence à la taverne du forum (pour l'aspect fourre-tout) et au site éponyme, qui agrège des contenus divers : blogs, podcasts, etc.

[Freezbee]

J'ouvre le bal avec ce thread de John Baez :

 

Citation

An "optical vortex" is a beam of light that turns like a corkscrew as it moves. It's dark at the center. There's one type of optical vortex for each integer m.

You can use an optical vortex to trap atoms! They move along the dark tube at the center of the vortex.

(1/n)

Photons have spin angular momentum, and in circularly polarized light this equals +1 or -1. An optical vortex is different: it exploits the fact that photons can also have orbital angular momentum!

So, some hotheads call an optical vortex a "photonic quantum vortex".

(2/n)

But you can study optical vortices without quantum mechanics, using the classical Maxwell equations!

The electromagnetic field is described using a complex function that in cylindrical coordinates is exp(imθ) times some function that vanishes at r = 0: the dark center.

(3/n)

The phase of the electromagnetic field, exp(imθ), is undefined at the center of the optical vortex. It turns around m times as you go around the vortex. So this number m has to be an integer. It's a simple example of a "topological charge".

(4/n)

People make optical vortices using many different technologies, including spiral-shaped pieces of plastic, "computer-generated holograms", and computer-controlled liquid crystal gadgets called "spatial light modulators".

(5/n)

For more, try this:

en.wikipedia.org/wiki/Optical_v…

The scary formula came from a nice article on Gaussian beams of light:

en.wikipedia.org/wiki/Gaussian_…

All pictures came from Wikicommons!

(6/n, n = 6)

 

[Tipiak]

Les salamandres des grottes peuvent vivre 100 ans, vivre 10 ans sans manger et vivre 7 ans sans bouger. Vu sur lematin.ch, avec ce titre tout en sobriété :

Cette feignasse n'a pas bougé depuis sept ans

[Rübezahl]

L'histoire de l'électricité (1/3)L'étincelle

 https://www.arte.tv/fr/videos/050724-001-A/l-histoire-de-l-electricite-1-3/  vraiment très très bien foutu.

De l'histoire des sciences à fondre.

[Freezbee]

 

[Freezbee]

 

Le pdf en question : https://arxiv.org/pdf/1910.06389.pdf

[Freezbee]

 

[Boz]

Ca fait moins flipper que Jurassic Park, en tous cas.

[Freezbee]

 

[Freezbee]

 

[Freezbee]

 

[Freezbee]

 

[Freezbee]

 

[Rincevent]

Fouloscopie, now.

[Freezbee]

Citation

Researchers create new state of light

Scientists have known for decades that light rotates around a longitudinal axis parallel to the direction in which it travels. However, some specialist researchers are currently trying to establish whether there are other forms and states, and to what extent it would be possible to control this. Recently, researchers from the University of Dayton managed to create a new “state of light”, by making it “rotate” around a transverse axis perpendicular to the displacement.

 

After two years dedicated solely to their study, Andy Chong and Qiwen Zhan, researchers from the University of Dayton in the United States, have for the first time managed to create a new “state of light”. As part of their experiment, they show that a beam of light can also rotate around a transverse axis perpendicular to the direction in which it moves, like a vortex. The results of the study were published on February 24 in the specialized journal Nature Photonics.

"The sabbatical allowed us the time to fully concentrate on this research and was very instrumental in putting us in a position to make this discovery," Chong said.

Zhan and Chong didn't go into their research with preconceived notions on what to look for or what they would find.

"It was more of a curiosity. Can we do this or make light do that?," said Zhan, a professor of electro-optics and photonics and managing director of the UD-Fraunhofer Joint Research Center. "Once we discovered we're able to do this, we then asked 'what's next?'"

"What's next?" may be a while off for the researchers and others who will examine the pair's basic research findings for applications, but they surmise this new state of light could be used to improve the transmission of large amounts of data with greater security, among many other potential applications.

 

a) Experimental device for generating and measuring spatiotemporal vortices (ST) of light; BS: beam splitter. b) Diagram showing the method of measuring the phase of light. The figures in italics represent the relative phases for the vortexes. The numbers in italics represent phases relating to various places. Note that the phase increases clockwise. Credits: Andy Chong, Chenhao Wan / University of Dayton

The researchers demonstrate in particular that a three-dimensional wave packet that is a spatiotemporal (ST) optical vortex with a controllable purely transverse OAM. Contrary to the transverse SAM, the magnitude of the transverse OAM carried by the ST vortex is scalable to a larger value by simple adjustments.

Since the ST vortex carries a controllable OAM uniquely in the transverse dimension, it has strong potential for novel applications that may not be possible otherwise. The scheme reported here can be readily adapted for other spectral regimes and different wave fields, opening opportunities for the study and applications of ST vortices in a wide range of areas.

"We don't know yet? But the sky's the limit," Zhan said. The duo is most interested in how the light interacts with materials. "We want to better understand how this state of light interacts with materials in space and time," said Chong, associate professor of physics and electro-optics and photonics.

 

--> Generation of spatiotemporal optical vortices with controllable transverse orbital angular momentum

[Rübezahl]

Si on regarde attentivement un troupeau de moutons en marche, ça fonctionne bel et bien comme un tore, avec frottement max sur les bords.

 

Et quand on essaye de bloquer un troupeau de moutons tout seul, l'analogie avec un fluide saute aux yeux !

[Freezbee]

 

[Freezbee]

 

[Freezbee]

 

[Freezbee]

 

[Freezbee]

Le Krakatoa est rentré en éruption cette nuit :

 

 

Citation

The Indonesian volcano Krakatoa, also known as Krakatau, has reportedly erupted.

 

Reports say it sent a plume of ash several kilometres into the air, with a boom that could be heard from far away.

 

"Please stop making that booming noise and go to bed, Anak Krakatau. It's late and we've already got plenty of other things to worry about," one person wrote on Twitter. 

 

"Krakatau is erupting nonstop for 2 hours," wrote another. "I live in bogor and I can hear the noises as clear as everybody else hear. This feels like a nightmare."

 

Volcano tracking site Volcano Discovery called it a "large magmatic eruption", at least the biggest since 2018, which caused a 150m-high tsunami wave that hit an uninhabited island. That boom was so big, the Anak Krakatau lost two-thirds of its height.

 

"Both the webcam on Anak Krakatau Island and from the coast (in 40 km distance) show strong lava fountaining from the volcano," Volcano Discovery said, saying the plume was 14km high.

 

A local expert told CNN Indonesia the lack of traffic at the moment, thanks to COVID-19 lockdown measures, made it easier to hear Krakatoa's rumbles.

 

Magma Indonesia said the "main crater smoke is gray and black with moderate to thick intensity about 200-500 meters from the summit".

 

When Krakatoa erupted in 1883, it created a tsunami that was still a whopping 42m high when it hit land, killing 36,000 people.

 

[Mégille]

L'année 2020 continue en beauté, on dirait.

[Adrian]

Next : le big One sur la faille de San Andreas

[Rübezahl]

Chez moi, il y a des sauterelles qui ont passé l'hiver.

La pyrale du buis, ça va chez elle, merci.

Les chenilles processionnaires aussi.

Egypte.

[RaHaN]

Je ne sais pas si ça répond à la demande de ce topic, mais la semaine prochaine a lieu la 19ème édition du Forum des sciences cognitives, avec à cette occasion différentes conférences sur le sujet.
 

 

http://cognivence.scicog.fr/forum-des-sciences-cognitives-2020/

[Rincevent]

il y a une heure, Adrian a dit :

Next : le big One sur la faille de San Andreas

Arrête de me faire rêver, il y aura aussi des innocents frappés. 

[Calembredaine]

Le 11/04/2020 à 12:01, Freezbee a dit :

Le Krakatoa est rentré en éruption cette nuit :

 

 

 

Krakatoa :
Liste des éruptions: 1530, 1680-81, 1684, 1883, 1927-30, 1931-32, 1932-34, 1935, 1936, 1937, 1938-40, 1941, 1942, 1943, 1944, 1945, 1946, 1946-47, 1949, 1950, 1952, 1953, 1955, 1958-59, 1959-63, 1965(?), 1969(?), 1972-73, 1975, 1978, 1979, 1980, 1981, 1988, 1992-93, 1994-95, 1996, 1997, 1999, 2000, 2001, 2007-8,  2009, début 2010, Oct 2010 - Mars 2011, Juillet et octobre 2011, Janvier, mai et septembre2012, 2015, 2017, 2018, 2020

[Sekonda]

Est-ce que quelqu'un a regardé de plus près les annonces de Wolfram ? 

 

https://writings.stephenwolfram.com/2020/04/finally-we-may-have-a-path-to-the-fundamental-theory-of-physics-and-its-beautiful/

Son nouveau projet : https://www.wolframphysics.org/

 

[POE]

Un article en parle ici https://www.wired.com/story/stephen-wolfram-invites-you-to-solve-physics/

J'ai survolé, il semble qu'il ouvre encore davantage l'accès à ses recherches à tous de manière à profiter de l'occasion pour impliquer un maximum de personnes. C'est vrai que la crise du coronavirus nous a montré qu'il était possible de collaborer mondialement sur des projets scientifiques, est ce que cela produire des fruits, je n'en sais rien. Pour ma part, j'ai peur que mon niveau en physique ne permette pas d'aider grand monde.

[Mégille]

La croyance aux bienfaits des huiles essentielles est corrélée à l'adhésion à du bullshit généré automatiquement.

 

L'article vulgarisé : https://www.cerveauetpsycho.fr/sd/psychologie-sociale/huiles-essentielles-la-trace-du-bullshit-19376.php

La publication : https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0229779#pone.0229779.ref045

Et les générateurs de bullshit (ils sont magiques) :

http://wisdomofchopra.com/

http://sebpearce.com/bullshit/