Wikipedia:Reference desk/Archives/Science/2015 December 29

= December 29 =

Fire simulator
I wanted to know how fire works, or how it's "visible physics" work, I've been looking through fire in slow motion videos, simulators, animations and drawings, but I can't predict how fire will move, wave or react, so is there a book in the topic? Is it possible to simulate fire? 186.146.10.154 (talk) 01:41, 29 December 2015 (UTC)


 * In a mass way, yes, by averaging out all the unpredictable effects, although predicting where an specific tendril of flame will rise may be impossible, due to quantum uncertainty, or perhaps it's just too complex for us to model yet. StuRat (talk) 15:35, 29 December 2015 (UTC)
 * Here's a nice introduction fire modeling:. There is a package for the Rothermal Fire model in R on CRAN. Some models focus on spread, while others focus on combustion and flame properties. No one model covers all scales and processes, but some of them are Very advanced and detailed. I'm on mobile and can't link easily but drop me a line on my talk page if you'd like more along these lines. SemanticMantis (talk) 15:40, 29 December 2015 (UTC)

Bacterial growth number of phases are 4 or 6?
Hi in this page Bacterial_growth there are six phases but in the picture at the right we cannot see the E and F phase. there are only four phases universally admitted right? thanks in advance — Preceding unsigned comment added by 176.92.28.124 (talk) 08:00, 29 December 2015 (UTC)
 * Thanks for picking this up. I've changed the article back to the previous version (which only lists the standard four phases) and added a reference.  This paper adds a fifth phase ("long-term stability") after the death phase, so that might be a possible addition - however, the only other reference I've found to the six-phase description is here, which I suspect may have been copied from our (incorrect) article. Tevildo (talk) 11:00, 29 December 2015 (UTC)

The Universe is Flat?
Somewhere in here the esteemed Prof Krauss talks about the universe being flat. Wasn't it not too long ago that most scientists were talking about the curvature of the universe? One of his main arguments is that if the universe were curved, then eventually light would bend around the entire structure and you would be able to see the back of your head. Isn't that rather weak, especially since the universe might not be spherical and we cannot see the entire width of it yet? I am a fan of Prof Krauss but I just don't understand the assertion. It smacks of an uncertain time in cosmology, with multiverses and other absurdities being thrown around. Dare I say that it's similar to the period when the "earth was flat" and people were ridiculed for thinking it was a ball? Sandman1142 (talk) 12:55, 29 December 2015 (UTC)
 * There are a lot of folks who have looked at the data - and the general consensus right now is that the universe is shaped rather like a pancake. Collect (talk) 13:26, 29 December 2015 (UTC)
 * That'll be a treat for the Langoliers Sandman1142 (talk) 13:36, 29 December 2015 (UTC)
 * See Shape of the universe. The experimental data, so far, enable us to put an upper limit on the curvature of the universe: $$\Omega = 1.00 \pm 0.02$$.  More accurate observations might detect a slight positive or negative curvature, but it's flat as far as we can tell at the moment. Tevildo (talk) 14:14, 29 December 2015 (UTC)
 * Thanks for that article. It says that a flat universe implies a Zero-energy universe. However the only way that is possible is because of Negative energy. However that article states 'Current observations support the scenario in which the universe is expanding at an increasing rate and is therefore dominated by negative energy'. Isn't that a contradiction then, based on observation? Sandman1142 (talk) 14:52, 29 December 2015 (UTC)
 * You're right, the article negative energy is incorrect to equate dark energy with negative energy. The "therefore" is someone's misunderstanding, which should be fixed. --Amble (talk) 02:35, 30 December 2015 (UTC)
 * Thanks but this is not the contradiction I was referring to. If indeed there is observational evidence that the universe is expanding and is therefore dominated by negative energy, this implies that it cannot be a zero-energy universe, and therefore it cannot be flat. This bothers me. Sandman1142 (talk) 08:28, 31 December 2015 (UTC)
 * A spatially flat universe is not necessarily a zero-energy universe. (Note the article says "can be", not "must be").  You're also confusing an expanding universe with accelerating expansion, and dark energy with negative energy.  This isn't your fault... they are confused in the negative energy article.  That's what needs to be fixed.
 * Maybe we are "able to see the backs of our heads" except we haven't figured it out yet. ←Baseball Bugs What's up, Doc? carrots→ 14:23, 29 December 2015 (UTC)
 * Wouldn't the current crowd look silly if we ever do... Sandman1142 (talk) 14:52, 29 December 2015 (UTC)
 * Perhaps the X: The Man with the X-ray Eyes was seeing his own eyes? ;-) Dmcq (talk) 15:22, 29 December 2015 (UTC)
 * The total energy of the Universe is not a well defined quantity and the concept of "zero-energy universe" is not standard. "Flatness" refers to the geometry of the space part of space-time (space-time is always curved); this has to be distinguished from the topology of the universe ("being able to see the back of one's head"). Flatness has been robustly measured, notably from measurements of the cosmic microwave background. These measurements span the largest part of the observable universe; they are fairly simple measurements to do, essentially geometric in nature and leave little room for doubt as to their interpretation. People have also looked for signatures of a compact shape of the universe (which is also possible for a flat universe!), but have not found anything. You ought to distinguish between the robust results of observational cosmology (including flatness) and the speculative ideas of the more outlandish parts of theoretical cosmology ("multiverses and other absurdities"), which (unfortunately?) hold a very prominent place in the public imagination. --Wrongfilter (talk) 16:51, 29 December 2015 (UTC)


 * My impression is this is much more trivial than people think. The universe is "flat" in the sense that Euclidean geometry approximates what we see - indeed, no one has found a measurement inconsistent with it.  But is it "flat" in the sense of absolutely, positively, 1.00000000000000... 00000 .... 00000?  Well, at any given point, with a known range of values to choose from, the odds seem heavily against it. :)  (Alright, I'm being a bit facetious since more data tends to fall in the 'open' range, but it's not like you get a money back guarantee) Wnt (talk) 18:45, 29 December 2015 (UTC)


 * We know from general relativity that the universe is not globally Euclidean. It's true that where gravity is relatively weak, the local geometry is approximately Euclidean. --71.119.131.184 (talk) 20:02, 29 December 2015 (UTC)


 * Spatially, the universe is very close to flat at large scales, but there are deviations from flatness at small scales (demonstrated by the anomalous precession of Mercury's orbit, for example). If the spatial flatness is forced by something like inflation, then the large-scale spatial curvature might actually be 0.00000000000000... 00000 .... 00000 with a large number of zeros. Spacetime curvature is not zero at large scales (because the density of matter is not zero), but it can be very close to zero locally. -- BenRG (talk) 18:59, 30 December 2015 (UTC)


 * I think comparing discussion of multiverses to belief in a flat Earth is a bit unfair. Multiverse hypotheses have been proposed to explain various observations about the universe, and most scientists proposing such things are quite clear that these are speculative hypotheses that need testing before they are taken more seriously. In contrast, belief in a flat Earth is an assumption that is contradicted by observations you can make with nothing more than your own two eyes: ships disappear over the horizon, the Earth's shadow on the Moon during a lunar eclipse is round, and so on. This is why many people figured out thousands of years ago that the Earth is a sphere. --71.119.131.184 (talk) 20:02, 29 December 2015 (UTC)
 * "people figured out thousands of years ago that the Earth is a sphere": More exactly, the concept has been around at least since the 6th century BC, according to Spherical Earth, the counterpoint to flat Earth. --Denidi (talk) 21:45, 29 December 2015 (UTC)
 * Correct. One has to distinguish between the term "multiverse" as it is used by A) a cosmologist to describe theoretical models that would involve and extension of the universe into a kind of composite, wherein the other "universes" are conceptual abstractions that don't even necessarily obey the same basic physical principles of "our" universe and B) as it is used in various science fiction tropes as a synonym for "alternate history reality"/the bizzaro dimension.  When cosmologists reference the possibility of a multiverse they are discussing these concepts, not this nonsense.  S n o w  let's rap 02:33, 31 December 2015 (UTC)