Wikipedia:Reference desk/Archives/Science/2014 January 25

= January 25 =

Gravity wells


How is this a two-dimensional slice? Unless "two-dimensional" refers to the image itself (it's a 2D picture, not a 3D model) in the style of The Treachery of Images, I can't understand; it looks very much 3D to me. It's not simply a mistake in the image caption, since the gravity well article says A plot of this function in two dimensions is shown in the figure. If it's in relation to the 3D stuff mentioned in the article's section on relativity, could you explain (basically) how that works? I don't understand the section at all. Nyttend (talk) 00:42, 25 January 2014 (UTC)


 * [Sorry, this is wrong – see below] It's a two-dimensional slice of the (intrinsically) curved four-dimensional geometry, embedded in flat three-dimensional space to show that (intrinsic) curvature. The 3D embedding space is not part of the slice. Likewise, to mathematicians, a sphere is a two-dimensional object with positive intrinsic curvature. -- BenRG (talk) 01:21, 25 January 2014 (UTC)
 * Yes, well said. I'll just include links to embedding, topological dimension, and cross section, though the articles are a bit obtuse . In short, it's a 2d surface embedded in a 3d euclidean geometry. The information displayed in the 3rd dimension could also be encoded in a pure 2d image, e.g. with color, as in a heat map, or with isoclines, as in a topographic map. SemanticMantis (talk) 02:23, 25 January 2014 (UTC)


 * Schwarzschild_interior.jpg, now that I look at it, this is not an embedding at all. It's simply a graph of the gravitational potential on a flat 2D slice of Newtonian space, with the spatial coordinates on the horizontal axes and the gravitational potential on the vertical axis. It's two dimensional because there are two spatial coordinates. The "height" of the potential could have been shown in other ways in a 2D image, as SemanticMantis said.
 * The second image on the right shows what I was actually talking about. The gravity well (first image) is often mistakenly presented as an embedding (second image) because they look so similar, and now I'm adding to the confusion. They are really quite different not only in interpretation but also in shape: the gravity well is a paraboloid surrounded by a hyperboloid, while the embedding diagram is a portion of a sphere surrounded by a paraboloid (but, as I said, is often not drawn that way). -- BenRG (talk) 05:00, 25 January 2014 (UTC)


 * Our retina's are 2D surface and our brains are very good at turning 2D images into a perceived third dimension. Whence illustrating a plane warped into a third dimension is easy to perceive even on a 2D surface of a computer screen. On an infinite 2D plane, moving in single direction will never bring you back the origin.  Curve that plane into a sphere, and you end up at the starting point.  We imagine that very well as our brains are suited for it.  Brains are not well suited for warping 3D space into a 4th physical dimension.  There is no picture to show it.  I believe one of the outcomes is that the earth is actually traversing a straight path while we perceive it as an elliptical orbit.  The distinction between the gravity well and general relativity seems more like a modelling issue than a distinction.  Sort of like Newtonian laws and relativity.  They are different but not unrelated.   --DHeyward (talk) 17:47, 25 January 2014 (UTC)

TrES-2b: Gas giant without clouds?
Curious. I was reading the article on TrES-2b and I noted that it is classed as a gas giant, but it is thought to not have clouds... what other forms would this gas be taking? — Crisco 1492 (talk) 13:17, 25 January 2014 (UTC)


 * The article says it's without reflective clouds, which is why it's so dark. There may be non-reflective clouds.  Either way, it's perfectly possible to have an atmosphere without clouds.  Clouds (at least as we know them) are masses of liquid or solid particles suspended within the gas.  Rojomoke (talk) 13:36, 25 January 2014 (UTC)


 * Do we know of any gas giants without clouds? — Crisco 1492 (talk) 13:47, 25 January 2014 (UTC)


 * While it's possible to have an atmosphere without clouds, as on Mars (at least part of the time), I'm not sure it's possible to have a gas giant without clouds. That's because there is so much atmosphere, even a very small percentage of particles in suspension will be visible as clouds.


 * For comparison, consider liquid water. A thin layer of water may or may not be clear, depending on the amount of particles in suspension.  However, water that's miles thick is never completely clear.  (Yes, water has a faint color all it's own, but even without that, I don't think you'd be able to see through water that's miles thick, due to the particles in suspension, unless that water was distilled in a lab.) StuRat (talk) 16:56, 25 January 2014 (UTC)

Converting motor to generator
Hello-

I am trying to convert a mechanical fan motor to a generator. Where should I go from here?

Thank you, Seattle (talk) 15:27, 25 January 2014 (UTC)
 * Figured it out, but advice here would be helpful anyway. Seattle (talk) 18:27, 25 January 2014 (UTC)
 * A good place to go is the article Electric motor where you can identify the principle of your motor. It does not appear to be a PM DC motor which is the only type that can be simply driven as a dynamo. If your purpose is to charge a battery or to power low-voltage lighting, the article on car alternators may suggest a better choice. 84.209.89.214 (talk) 19:16, 25 January 2014 (UTC)

BCAAs positive or negative affect on health
The article on BCAAs is confusing to me. It reads to me like BCAAs negatively impact health. My specific question would be in regards to the ALS paragraph that states in the article a link between excessive BCAAs and ALS, while a quick amount of referencing I've done from other website indicates BCAAs may be used to TREAT ALS. I'm no doctor or expert, but I feel like this article is leading and does not contain full accurate information on this suppliment. — Preceding unsigned comment added by 174.71.15.179 (talk) 18:15, 25 January 2014 (UTC)
 * This is about branched-chain amino acids. That is a poorly written article, like many on Wikipedia.  Unfortunately it would take somebody who knows more than I do to improve it. Looie496 (talk) 18:44, 25 January 2014 (UTC)

Carat
What is the highest carat for gold/gemstones?--78.156.109.166 (talk) 19:55, 25 January 2014 (UTC)
 * They are referring to two different things. When talking about gold, it refers to Carat (purity), and in this context 24 carat is the highest possible, i.e. pure gold (12 carat gold is 50% gold and 18 carat is 75% etc.) When talking about diamonds and other precious stones, the carat is a unit of mass equal to 0.2 grams. That means that there is no theoretical limit. One can have a gemstone of 100 or even 1000 carats, but they would obviously be very expensive. - Lindert (talk) 20:05, 25 January 2014 (UTC)
 * And this diamond is a lot more than 1000 carats. Richerman    (talk) 20:52, 25 January 2014 (UTC)
 * Woah, never thought a planet could be a diamond--78.156.109.166 (talk) 20:44, 27 January 2014 (UTC)