Wikipedia:Reference desk/Archives/Science/2014 August 13

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August 13

Gravity lensing of gravity

If gravitons are affected by gravity, couldn't we find some gravity lensing impact from orbiting behind the Sun? Hcobb (talk) 03:56, 13 August 2014 (UTC)

Heh, I personally added the "Solar gravitational lens" section to gravitational lens not more than a few months ago. (Though as I'm not a physicist, maybe somebody else should check it over. :) Wnt (talk) 05:56, 13 August 2014 (UTC)

Tests of general relativity#Deflection of light by the Sun is nowhere near the focal point, but related; it confirmed g-lensing. - ¡Ouch! (^{hurt me} / _{more pain}) 07:17, 13 August 2014 (UTC)

I think our OP is asking about gravitons, or gravity itself, being affected by a another gravitational lens, not light (photons). There is no system to test that on Earth. You'd need a test mass co-orbiting around two large masses; for such binary star system to be stable the planet needs to be far from binary pair's orbit, otherwise even in Newtonian dynamics the planet's orbit is unstable due to changing gravity. CS Miller (talk) 08:47, 13 August 2014 (UTC)

Arthur used a light wave detector able to distinguish the directions of the Sun and nearby stars when he experimentally confirmed Einstein's prediction that star positions appear shifted by the Sun's gravitational field. Doing the same test on gravity propagation would need a similarly sensitive Gravitational wave detector which we don't have. 84.209.89.214 (talk) 12:20, 13 August 2014 (UTC)

There are no real Gravitons known so noone can tell. The origin of Gravitation is still a riddle. --Kharon (talk) 01:19, 14 August 2014 (UTC)

Fields are not explained in terms of particles. The fields are fundamental; the particles are quantized vibrations of the fields. It's hard to see how gravitons (gravitational field quanta) could fail to exist. They're just ridiculously hard to detect because gravity is so weak. -- BenRG (talk) 01:58, 14 August 2014 (UTC)

I don't think so, because then the universe would be self-destructive, being consumed by virtual black holes formed spontaneously by a positive feed-back loop. — Preceding unsigned comment added by Plasmic Physics (talk • contribs)

The equivalence principle implies that everything gravitates, including gravity. We have yet to directly detect gravitational waves, let alone gravitons, but we will presumably find that they are lensed in the same way as light. In other words, they should come from the same apparent place in the sky as light from the same astronomical object. -- BenRG (talk) 01:58, 14 August 2014 (UTC)

I believe you assume gravity waves travel at the speed of light. If a stellar source emits a pulse of light, does that mean that all masses in the Universe simultaneously emit a pulse of gravity? If they do not, can the pulse of light go past another stellar mass before its gravitational field reaches it to cause deviation (lensing)? Or is the idea of gravity waves travelling at the speed of light less tenable than it seemed? 84.209.89.214 (talk) 12:40, 14 August 2014 (UTC)

The field will just be there, it isn't created as the photon goes past, the interaction is purely local (well as far as one can say such a thing without observing it). However if the stellar mass was moving then the field would be different from what one would expect from if it was however far it is away at that moment 'simultaneously' whatever that mean. Dmcq (talk) 13:43, 14 August 2014 (UTC)

I find it easier to think of gravity as a projection of 4D space into our 3D perception of our 2D retinas. Just like a the surface of a balloon is 2D surface curved into a 3D space. A point that moves in a straight direction on that curved plane ends up in the same place. Similarly, lensing is really two paths that arrive at the same point. Both paths are straight 4D paths and lensing is an illusion. Gravity is a measure of curvature that is not much different that the curvature of the surface of the earth only into a 4th physical dimension. Gravity is a measure of curvature and curvature is a function of energy. My 0.02 --DHeyward (talk) 07:48, 16 August 2014 (UTC)

the smell of approaching rain

Yester evening I remarked that the air smelled like rain was likely. Eight hours later it was coming down. (See also confirmation bias.)

What did I smell?

Petrichor from some nearby place already getting rained on? Could ground-level breezes bring that odor so long before upper winds brought the rain itself?

Or is rain sometimes preceded by a distinctive change in ionization that I can (directly or indirectly) notice? —Tamfang (talk) 09:18, 13 August 2014 (UTC)

It's True, You Can Smell Oncoming Summer Rain. Article in Scientific American mentions Ozone, Petrichor and Geosmin. 84.209.89.214 (talk) 10:20, 13 August 2014 (UTC)

Thanks. I just thought it was the smell of wet dust though I wondered why it was stronger after a dry spell, it never occurred to me that it was organic. Dmcq (talk) 13:50, 14 August 2014 (UTC)

Nd:YAG laser question

Hello, I'm wondering how well a YAG laser couples to brass. I found this graph which suggests Cu has quite a low absorption of ND:YAG light but I can't seem to find the absorption coefficient for brass. Anyone know of a good reference which will provide this? Best wishes Polyamorph (talk) 11:10, 13 August 2014 (UTC)

NIST publishes a list of accredited optical laboratories: Optical Radiation Calibration Laboratories. If you care about the correctness of the answer, you should track down a publication from one of those groups, or a publication that can trace their data to equipment thus calibrated. Anything else you find on the internet is just something you found on the internet, and while it could be correct, it's probably not a "good reference"!

For a small fee - about $2000 - you can ship your specimen directly to NIST and they will measure it for you. This eliminates the guesswork about exact alloy composition, surface finish, and so on. After all, where's the value in an "accurate" chart of material properties if they're properties for a material that is different from your sample? If you only care about accuracy to a few percent, we can tell you right now: brass absorbs less than 80% of the incident light, plus or minus twenty percent. Building imprecision into your engineered design can be a great thing, because you don't need a graph or chart for it!

Now, all this is probably overkill - whatever hobby project you're working on probably doesn't need calibrated measurement. You can get reasonable accuracy by measuring this kind of thing yourself - home-brew up a photometer and a digital multimeter and test it yourself! - why do you need better accuracy than you can measure? But, if you were manufacturing large volumes of optical material for "special needs," as many of us often do, you'd really want some guarantee of accuracy and precision. You'd want to exactly define your material, your surface finish, light polarization, laser beam profile, and bunch of other standard parameters. If you don't know where to start, and you don't have time to take a four-year degree in optical physics, NIST even has a five day short-course on optical metrology for materials. Nimur (talk) 15:27, 13 August 2014 (UTC)

Add a sort of Emotion to main Article

Halo! I'm interested in, why in Article "Emotions" an Emotion "Schadenfreude" isn't included. I would put this Question to the originators. I have an Article above this in Internet located (Title: "Schadenfreude as a Consumption-Related Emotion"): http://business.utsa.edu/marketing/files/phdpapers/Sundie1-et-al_JCPS59.pdf Thanks! Kartoffeldip (talk) 15:38, 13 August 2014 (UTC)

If you wish to propose changes to the article, the best place to start is Talk:Emotion. You should however note that per Wikipedia:Identifying reliable sources your paper is unlikely to be usable as a source, and in any case, Schadenfreude would appear to be beyond the scope of the article as it is currently written - though we do have an article on the subject. AndyTheGrump (talk) 16:03, 13 August 2014 (UTC)

I just now added Schadenfreude to the template that lists a broad set of emotions, which appears on the Emotion article among other places. But schadenfreude is probably too narrow of a topic to discuss in the Emotion article body. Red Act (talk) 16:18, 13 August 2014 (UTC)

This was my thought, to add this term to the template; thanks Red Act!

Can you help me (re-)define in the main article Schadenfreude as Emotion?

Kartoffeldip (talk) 17:53, 13 August 2014 (UTC)

Psychology is outside of my main areas of interest, but Talk:Schadenfreude, Talk:Emotion and WP:WikiProject Psychology would be good places to look for collaborators on changes to either of those articles. Also, the WP:Welcome page has links that you may find helpful for editing the article(s) yourself, or for getting help from somebody pertaining to editing articles. Red Act (talk) 18:27, 13 August 2014 (UTC)

Thanks! Kartoffeldip (talk) 05:38, 14 August 2014 (UTC)