Wikipedia:Reference desk/Archives/Science/2017 November 22

= November 22 =

Why aren't odd carbon number fatty acids more common in biology?
At least for plants and animals. Are odd fatty acids more common in other kingdoms? Why aren't there more short fats like triformin and triacetin in bulk fat/oil? Why are only a few omegas common like omega-3, -6, -7, and -9? Sagittarian Milky Way (talk) 04:11, 22 November 2017 (UTC)
 * Fatty acid synthesis would be a good starting point for helping you research your own answers to this question.-- Jayron 32 04:17, 22 November 2017 (UTC)
 * Fatty acid is more to-the-point. tl;dr: in most organisms the synthesis pathway creates fatty acids by adding two carbon atoms at a time. --47.157.122.192 (talk) 09:14, 22 November 2017 (UTC)
 * As for Why are only a few omegas common like omega-3, -6, -7, and -9?, hmm, Omega-3 fatty acid might be informative, particularly that huge image caption (which really should be moved into the article body). It might have something to do with their properties vis-a-vis cell membranes. The omega nomenclature is a means of classifying different types of polyunsaturated fatty acids; the relative amount of saturated-to-unsaturated fatty acids in cell membranes affects their properties. In particular, DHA is a very important component of brain neuron cell membranes. But I'm not sure of an exact answer to your question; maybe someone who knows more about biochemistry can help. --47.157.122.192 (talk) 21:18, 22 November 2017 (UTC)
 * It's clear that there is some biological specificity here that is dominating, especially in organisms like humans that scavenge what PUFAs they can from external sources, but chemistry may also be a factor. To put the double bond at omega-2, for example, would make the end an allylic carbon, which would be more susceptible to oxidation (true, from this it is apparent that other positions are also vulnerable, but perhaps those are protected to some degree by the membrane structure?).  Exposing the alkene directly would also invite troublesome reactions.  Within the lipid, adding adjacent conjugated double bonds would give it a color, causing it to absorb light, which might lead to unwanted reactions or otherwise prove undesirable.  (If you only had two conjugated it might not be visible but it could still absorb more UV). Wnt (talk) 11:53, 23 November 2017 (UTC)

General Relativity questions
I recently watched a youtube playlist on General Relativity titled "Curved SpaceTime and General Relativity" and I have a couple questions.

The playlist is a very quick overview of some of the major muscle movements regarding General Relativity. It builds up to the conclusion that 4-dimensional spacetime is not flat but curved, which brings me to my first question: The curvature of a 4-dimensional object implies that there is a fifth dimension, correct? If so, is there any scientific understanding of what this fifth dimension is?

Second, one of the videos in the playlist, titled Is Gravity An Illusion?, speaks to the difference between Newton and Einstein's description of gravity. The main point being that Newton viewed gravity as an actual force and Einstein viewed it as an implied force. The video seems to claim that Newton would view an apple falling to earth as an apple falling down toward a earth & Einstein viewed an apple falling to earth as the apple being more stationary and the earth moving up toward the apple. I suspect this depiction is less intent on being scientifically accurate than it is to get people to stop relying on their "common sense" notions of science - as a necessary step for grasping General Relativity.

So, can anyone confirm which is actually the case regarding the apple? The playlist as a whole seems to be saying that falling objects are under the influence of spacetime curvature not the Newton concept of gravity. That said, what of a falling apple on earth? Does curved spacetime bring the apple towards earth or earth towards the apple? If it is the earth towards the apple, can anyone point me to good explanations of how that is so because I'm still not clear on that point.

128.229.4.2 (talk) 14:35, 22 November 2017 (UTC)
 * Of course, any curved or flat space-time can be immersed in a space-time of a higher dimension. However such an immersion is not a necessary condition for the existence of space-time curvature. As to an apple and the Earth, you should note that any motion in General Relativity is relative. Therefore, which object moves and which stays at rest, depends entirely on your choice of reference frame.
 * In General Relativity all free floating objects move along geodesics. In curved space-time they are such that two object, which are initially at rest in some reference frame, will move towards each other. Ruslik_ Zero 16:46, 22 November 2017 (UTC)
 * Extra link for the win: geodesics in general relativity. --47.157.122.192 (talk) 21:04, 22 November 2017 (UTC)
 * To answer your first part, an N-dimensional curved space can always be mathematically embedded in an (N+M)-dimensional flat space. However, M is not necessarily one.  The Whitney embedding theorem guarantees that M is no more than N, but it isn't obvious if 4D spacetime is embeddable in a 5D flat space or how many extra dimensions would be required.  (This rather depends on how complicated the topology of spacetime actually gets.)  That said, whether such an embedding has any physical meaning is an unanswerable question at present.  As far as we know, there is no method of interacting with anything outside of 4-dimensional spacetime, and by extension there is no way of probing the properties of anything outside it.  As long as that remains true, the question of whether higher dimensions have any physical meaning is largely unanswerable.  (String theory, M-theory, and some of the other hypothetical extensions of general relativity posit additional accessible dimensions, but so far such effects are untested.)  Going further, though a flat embedding space does exist (at least as a mathematical construct) it isn't necessary to refer to such a space in order to study the geometry and effects a curved spacetime.  The broad field of differential geometry has developed methods of studying curved spaces purely from the properties as they exist in the space itself (e.g. how objects in the space move relative to each other), without the need to reference any higher dimension.  The field of general relativity generally applies these methods to do calculations within 4d spacetime without the need to talk about a fifth dimension.  Dragons flight (talk) 17:17, 22 November 2017 (UTC)


 * Thank you Ruslik_ Zero & Dragons flight (talk). I find this stuff fascinating & I appreciate your help in progressing my understanding!
 * 128.229.4.2 (talk) 20:05, 22 November 2017 (UTC)

Civil engineering and programme management
In civil engineering and programme management, which type of organisation is seen as the most prestigious to work for? Client, contractor, design consultant or programme management consultant? 193.240.153.130 (talk) 16:22, 22 November 2017 (UTC)
 * Define "prestigious". ←Baseball Bugs What's up, Doc? carrots→ 17:52, 22 November 2017 (UTC)
 * Which is the one most people want to work for? 82.17.228.129 (talk) 18:05, 22 November 2017 (UTC)
 * The one that pays the best? ←Baseball Bugs What's up, Doc? carrots→ 18:17, 22 November 2017 (UTC)
 * This question has been asked and answered here some weeks ago. Please visite the archive. --Kharon (talk)
 * Yes, by this same user, several times this year. It might help if he would explain why he's asking these questions, and maybe he could get more satisfactory answers. ←Baseball Bugs What's up, Doc? carrots→ 06:34, 23 November 2017 (UTC)