Wikipedia:Reference desk/Archives/Science/2015 July 30

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July 30

Why does water get loud before it boils?

I notice that when boiling water in a metal or glass kettle or metal pot, a noise slowly builds before the water actually boils. Once the water reaches a rolling boil, the noise lessens. What is causing the noise? The water? The expansion of the glass or metal kettle? --Navstar (talk) 02:05, 30 July 2015 (UTC)

It says here that the explanation is that in the hottest area of the kettle the water is boiling, but the bubbles collapse as they rise into water below the boiling point. The collapsing is what makes the noise, and it stops happening when all the water is at the boiling point. --65.94.50.73 (talk) 03:54, 30 July 2015 (UTC)

Agreed. Note that these bubbles are too small to see, and that the large surface area to volume ratio of such microscopic bubbles allows the water vapor to instantly cool below the boiling temperature and become liquid water again, which takes up much less room, causing the bubbles to collapse. The bubbles get larger as the heating continues, and you might be able to see them collapse briefly, or at least get smaller as they rise. StuRat (talk) 14:17, 30 July 2015 (UTC)

The fancy science term for this is cavitation. Notably, it's also caused by things other than heating, like surfaces passing through a fluid at a high speed. This is an issue for things like propellers and turbines. And when cavitation bubbles form, they are indeed noisy, which is one way you can tell that it's happening to a propeller, pump, etc. --108.38.204.15 (talk) 15:45, 30 July 2015 (UTC)

How many Earth plant species are physically possible?

If we could simulate Earth's entire history octillions of times with different random DNA mutations until we exhaust every possible species how many would there be? Is this even estimatable any time soon? Sagittarian Milky Way (talk) 02:36, 30 July 2015 (UTC)

And also randomize the shapes of the continents and their topography and when and where asteroids hit and the like cause those were random accidents. Sagittarian Milky Way (talk) 02:43, 30 July 2015 (UTC)

It's not possible to do this in a meaningful way. The largest plant genome is 150gb long. There are 4^{150,000,000,000} possible genomes of this size (such a big number I can't find a math program that will even display it in scientific notation). We can think about any particular variation of this genome, but without creating it, we have no way of knowing whether it would be viable in a given environment (or ever), whether it would constitute a species distinct from any other particular variation, or even whether it would classify as a plant. There's also no reason to suspect that 150gb is the upper bound for the size of a plant genome. So while we can imagine all the variations of a genome, we can't know anything useful about most of them - certainly not enough to answer your question. Someguy1221 (talk) 03:04, 30 July 2015 (UTC)

To convert a power of A into a power of B, just multiply the exponent by log A / log B (using logarithms to the same base for both numbers). log 4 / log 10 is just over 0.6, so 4^{150,000,000,000} is about 10^{90,000,000,000}. --65.94.50.73 (talk) 04:01, 30 July 2015 (UTC)

As for the continents...the number of possible outcomes depends on how different two 'shapes' have to be. If a single misplaced atom makes two topographies "different" then the answer is some kind of factorial involving the number of atoms in the earth's crust from somewhere above the height of everest to the bottom of the marianas trench. That's a truly ungodly number. I don't see much value in attempting to estimate it - the mathematical notations for such numbers start to get fairly incomprehensible.

If "different" required a difference of (say) a kilometer in the shape of a continent or the path of a river - then the number is still insanely large - but more manageable. But it's arbitrary - why one limit for "different" rather than another? That's really the problem with these "curiosity" kinds of question. Does it matter how big the number is? I can't imagine why you'd need the answer. Why bother even asking it?

It's really the same deal with the plants - there are an insane number of changes in the DNA of an Oak Tree that would still produce a viable, recognisable Oak Tree - so why count the number of possible DNA strands when it really tells you nothing about how much meaningful variation there might be.

So the best answer here is "Don't Know" - and "Don't Care" comes a close second. SteveBaker (talk) 04:19, 30 July 2015 (UTC)

I meant changes big enough to affect the evolution of species, a kilometer probably wouldn't do it. If the dinosaurs got to evolve for longer or got killed off sooner maybe plants that never existed would happen, though. I'm kind of also wondering how many Earthlike planets would have to gain DNA-based vegetation of the correct amino acid chirality to make a wheat species that could interbreed with the Earth kind. Sagittarian Milky Way (talk) 04:50, 30 July 2015 (UTC)

The problem is that the environment on Earth is 'chaotic' (in the mathematical sense of Chaos theory) - epitomised by the idea that the flapping of a butterfly wing might cause a hurricane halfway around the world a year from now. This effect (which is very real by the way) means that the most insignificant change (one atom displaced by a nanometer or so) is more than sufficient over the very long term to cause extinction or failure to evolve of an entire species. There is no lower limit beneath which you shouldn't care.

Imagine a single cosmic ray misplacing a single atom in the DNA of the sperm that was to become Richard Nixon. That resulting in it swimming 0.1% more slowly than it otherwise might - and resulting in a different sperm making it to the egg, Richard Nixon never existed but instead we got Sandra Nixon. Despite an unprecedentedly great political career, and a reputation for honesty and a high ethical standard - in 1968, America simply wasn't ready for it's first female president and Hubert Humphrey got the job instead. Being obsessed with solving the Vietnam problem, Humphrey failed in Cold War engagement with the Soviet Union. The resulting nuclear holocaust caused in the extinction of 90% of the species on earth and resulted in the eventual evolution of super-intelligent giant cockroaches who farmed genetically engineered fungi over 80% of the land area of Earth - and that caused the extinction of the wheat plant on earth. One cosmic ray - one nanometer to the left.

So, no - it's not sufficient to assume that one misplaced atom cannot make a difference!

SteveBaker (talk) 17:47, 30 July 2015 (UTC)

That is very funny. Sagittarian Milky Way (talk) 12:57, 31 July 2015 (UTC)

SteveBaker's thought experiment may be funny, but it's perfectly valid, Sagittarian Milky Way. μηδείς (talk) 00:45, 1 August 2015 (UTC)

That was a thought-provoking edit, Steve. The only problem I see with that becoming possible is that cockroaches would be prevented from growing that large by their exoskeletons. Khemehekis (talk) 23:33, 2 August 2015 (UTC)

All of them. (BTW, besides being a flip answer, there are DNA changes that aren't meaningful for species. The human genome is fairly narrow that produces lots of variability without a "species change." I'm not sure how you can specify "species change" with DNA variation. Eye color, skin color, gender, etc, etc, are all DNA differences without species implications and there are genomes that aren't so narrow and allow "inter species" creation (i.e. Ligar) as well large variation within a species such as Dogs.) --DHeyward (talk) 04:28, 30 July 2015 (UTC)

I have lots to say about this, as my research specialty is theoretical ecology of plant communities. Unfortunately I don't have much time work for free today :) The big thing everyone seems to be ignoring is that the number viable plant species depend on the community context, competition, dispersal, life history, predation, disturbance, and many other factors. The number of possible genetic combinations has nothing to do with the number of species you might expect to find in a given situation. For starters, see Chesson (2000 a,b), freely accessible here [1]. The main idea is that number of coexisting species that a system can support is limited by resident-invader differences in the covariances between environmental and competitive effects. The papers go into great detail on this if you can handle the math and follow some basic ecological terminology. If you're still interested next week, drop a line on my talk page and I'll be happy to discuss further. SemanticMantis (talk) 15:22, 30 July 2015 (UTC)

They don't seem to be asking about how many plant species can survive in one particular environment (E), but rather how many could exists (n) in every possible Earth environment (P). It's probably not as simple as n = E×P, either, as E varies widely, P is unknown, and there will be many species that could exist in multiple environments. Also note that in plants (as well as animals, etc.) with asexual reproduction, defining a species is even trickier. StuRat (talk) 15:52, 30 July 2015 (UTC)

Funny, I would have thought that every possible Earth environment would include all specific and particular Earth environments. Please don't try to teach me to suck eggs until you've gotten a relevant PhD and published at least a few peer-reviewed papers about plant ecology ;) SemanticMantis (talk) 16:49, 30 July 2015 (UTC)

Nice argument from authority fallacy. I hardly need a PhD in plant biology to be able to read a question correctly. Specifically, "randomize the shapes of the continents and their topography and when and where asteroids hits and the like" means the OP wants to know about all possible Earth environments, not merely those which currently exist. StuRat (talk) 17:25, 30 July 2015 (UTC)

If you'd taken the time to read and understand the research I linked above, you'd have seen that the framework allows for descriptions of species coexistence in environments that don't exist on Earth, as well as plants that don't exist on Earth. You seem to think I'm interpreting the question incorrectly but don't seem to be understanding what I'm saying. Whether the OP thinks my refs and responses relevant is not for you to decide. Finally, I did not appeal to my authority to support my claims, my refs do that just fine. Rather, I appealed to a well-known saying, and implied that you're trying to give your opinion to an expert in the field, who most likely knows more about this than you do. Whatever, I'm happy to discuss this with OP further as I said above, but I have no more time for you today. SemanticMantis (talk) 17:39, 30 July 2015 (UTC)

You don't seem to understand what I said. Say you determine that environment X₁ can support n₁ species, and environment X₂ can support n₂ species. You can not then conclude that the total number of species environments X₁ and X₂ can support is n₁+n₂, because you don't know how many are in common. When you have thousands or millions of possible environments, the overlapping Venn diagrams become absurdly complex. So, how do you propose to calculate the total for all possible Earth environments ? Finding the number for an individual environment is interesting, but simply doesn't lead to an answer to this Q. Then there would be the issue of determining the number of possible environments that could possibly exist on Earth. And, again, no PhD is required to read the Q, and see that it's not what you are answering. StuRat (talk) 17:58, 30 July 2015 (UTC)

I never said we'd sum the numbers of species. The relevant thing to get at overlaps in species distributions and overlapping environmental properties is Beta diversity. There's actually an entire body of research that explicitly addresses your sentences "Say" to "complex". Ok, now I'm done, have a nice day. SemanticMantis (talk) 18:27, 30 July 2015 (UTC)

Thanks for the offer. Sagittarian Milky Way (talk) 13:32, 31 July 2015 (UTC)

vital advances in plant evolution

• How many species are physically possible is a meaningless question, since species can be genetically isolated due to differences in ploidy (chromosome number) but otherwise be indistinguishable. What really matters is the number of niches available and occupied.

Looking at the question ecologically, there are several different breakthroughs in plant evolution.

The first big question is photosynthesis in eukaryotes, assuming we are going to exclude bacteria from our definition of plants. There are plenty of different routes that evolution could have gone down to produce multicellular photosynthetic land organisms. It so happens that this has only really happened with the green plant phylum. But land plants could certainly have evolved from the red algae and green algae had the green algae not beat them to it. The former are still very important in the ocean, with kelp an example of a brown alga. There are several thousands of species of red and brown algae, and other types of algae, the classification is in flux.

The next two major advances on land were the development of vascular tissue, which allowed tall upright forms like ferns, rather than mosses, and of seeds, which allowed evolution independnt of the need of spores to swim through rain water to cause fertilization. These events caused the evolution of forests and the colonization of arid lands. Forests had evolved before amphibians and insects appeared on land, and there were seed plants by the time of the dinosaurs. During this time, however, plant diversity was much lower than it is today due to the vagaries of fertilization. Plant species tended to cover large areas of similar terrane such as todays boreal forests given distribution of pollen and seeds by the wind doesn't tend towards locally specialized species.

It was the development of the flower, and the mutual feedback between pollinators and food plants that allowed the huge boom in plant evolution that occurred with the arrival of guided fertilization. While there are only about 630 species of conifer, a very ancient group, there are some 25,000 species of orchid, a very small but highly specialized branch within the flowering plants. Many plants such as orchids have their own unique species of animal polinator. This means they can become highly specialized to microhabitats that are totally unavailable to plants like conifers. Some orchids are found only on specific mountains. Such evolution doesn't happen with more primitive plants; small ranges in them indicate either relict populations only found on certain islands or species headed toward extinction.

Even then, the total number of flowering plant species, which far outnumbers all other plants combined, is estimated to be only a few hundred thousand species.

In the end it comes down to what ecological niches are available to and accessible by plants. There are parasitic flowering plants, carnivorous flowering plants, flowering plants (e.g., bromeliads) that live in the branches of other flowering plants, flowering plants that are only fertilized by one species of flying organism, and flowering plants that are going extinct because the megafauna that ate their seeds, allowing them to germinate, have been hunted to extinction. μηδείς (talk) 00:53, 31 July 2015 (UTC)

I guess we could've had a red world then, and human connotations of red being the color of bloodshed and war might be merged with peaceful Edenity. That would be more efficient though, sunlight peaks near the color that leaves reflect. And if red algae, which I think can live near the surface, is any good at absorbing blue light then it should be able to photosynthesize deeper than any other algae and be more likely to discover the mutation first cause it can live in more of the ocean. I don't know why we didn't go that route? Sagittarian Milky Way (talk) 13:32, 31 July 2015 (UTC)

Leaves are green because they absorb red and blue light more strongly than they absorb green. See also: purple bacteria. The real point about the "possible" number of plants is that many specialized species exist only because their niches have co-evolved with other organisms. There'd be no mistletoe without birds and oak trees, no orchids without certain insect pollinators. μηδείς (talk) 16:10, 31 July 2015 (UTC)

Wouldn't red or blue leaves be better? Sunlight is strongest near green light and leaves just reflect that away. Maybe purple or even conifer-dark green leaves would overheat but why not have dark purple photosynthetic chemicals for the taiga trees? Or dark red? I guess maybe the mutations were so hard to come up with that only green algae did it. I had no idea there were so few conifer types though, it's interesting that none of the earlier kinds could have anything near the diversity the single invention of flowers can produce. Sagittarian Milky Way (talk) 17:55, 31 July 2015 (UTC)

There are red leaves on plants. Poinsettias are one of many examples. Interestingly, the leaves are green in summer and only turn red in winter, based on longer nights. This suggests that the red color helps with photosynthesis when less light is available, although I don't know if that is actually the case. (Of course, many plants also have red flowers, but that's all about attracting pollinators.) StuRat (talk) 21:13, 31 July 2015 (UTC)

The answer to your question is somewhat counterintuitive, SMW, and commonly confuses people. All other things being equal, the color of an opaque object depends on the relative amounts of which frequencies it absorbs. So leaves look green, because the light at the red and blue end of the spectrum is highly absorbed, while green is much less so. Leaves arent green in the sense of giving off (shining in) green light like a green neon sign. Instead, they are reflecting some green and absorbing the rest. Indeed, if you excite chlorophyll by shining a light through it the chlorophyll glows in a rich red color. (Blue is absorbed by auxiliary pigments in a leaf which transfer electron to chlorophyll. Because this is indirect, the chlorophyll itself only glows in red.)

One of the reasons for red in shoots and dying leaves is that again, there are auxiliary pigments in leaves beside chlorophyll. These pigments are less expensive to the plant metabolically, and hence are produced more quickly during early growth; and while a deciduous plant will actively reabsorb the components of chlorophyll in the fall, they don't waste energy pumping the other pigments out of their leaves, so these pigments get left behind causing the pretty colors of autumn.

Red leaves in Poinsettias are effective "painted" red with a pigment that reflects very strongly in the red area of the spectrum. Poinsettias are "actively" reflecting red, while green leaves are passively not absorbing as much green as they are other colors.

To forestall objections, the above is very simplified, there are lots of other side issues, but I have addressed the essence of the matter. μηδείς (talk) 00:42, 1 August 2015 (UTC)

Aren't you just saying that leaves reflect green light the best as a byproduct of absorbing everything they can? Um I've watched the Magic School Bus as a kid I know that opaque color is reflected light. I Googled chlorophyll absorption spectra and absorption in green seems to be pretty small. And reflection in green is almost twice red twice blue. If leaves fluoresce red then it can't be a big contributer to their appearance because reflectance in green isn't even twice that of red. Leaves should look banana yellow by the time those two colors equalize. But if you have to reflect some color then why not reflect a color that's not yellow or green? I'm guessing that it wouldn't help many leaves as leaves sometimes get too hot and dry out as it is, absorbing even more sunlight would cause more waste heat. There are bare forest floors (too dark), and the tree line where tree needles have trouble staying warm enough. A pigment that reflected a less major color in the sunlight spectrum would be more efficient. But if evolution didn't take alternative photosynthetic substances beyond the algae stage then it'll have to be chlorophyll. I do wonder though why no alpine or Arctic plants invented some extra non-photosyntetic pigment(s) just to become completely black (to the naked eye) for maximum warmth. Sagittarian Milky Way (talk) 02:13, 1 August 2015 (UTC)

I am not "just saying" anything, and, "um", I am not inclined to answer patronizingly worded retorts about children's shows. There does happen to be an answer why leaves absorb blue but chlorophyll fluoresces only red and why leaves don't tend to be black, but I'll leave it to others or the staff of Magic school bus to correct your misunderstanding of what has been said so far. μηδείς (talk) 02:53, 1 August 2015 (UTC)

Um, that episode where the colored light bolt is absorbed by a different color? Never mind, it was on in the 90s. Maybe you could've realized I meant the solar blackbody peak shouldn't be near the reflectance peak for max efficiency instead of thinking I thought leaves are green cause they glow or absorbed green the best. Sagittarian Milky Way (talk) 16:46, 1 August 2015 (UTC)

It's worth clarifying that creating species (according to the biological species concept) doesn't necessarily mean creating ecological niches or new morphological forms. A very simple process in speciation is the formation of chromosomal inversions. You have two populations, one of which has the genes in a chromosome A B C D E and the other A B E D C or something. When meiosis tries to do recombination between the strands at the C-D region, you end up with some number of A B C D E B A and C D E chromosomes, which tend not to be viable. So simply collecting a bunch of these inversions (which are relative between the two species; there's no 'right' order) can make two species essentially unable to interbreed. Just having one already creates aspects of speciation, because the relative lack of recombination in the region of the inversion means that you accumulate more and more differences there that can't be reshuffled into both populations. See http://www.ncbi.nlm.nih.gov/pubmed/?term=drosophila+inversion+speciation for some of this. For our hypothetical purposes though, this means that you can have a *ridiculous* number of different shuffles of the same genes in different orders on parallel Earths, *none* of which can interbreed effectively even though they have the same genes and in fact have many genes in the same order on local segments of chromosome. (I don't think this is really what the OP meant, but trying to nail down what he really is looking to ask as actually calling for some philosophy that isn't really part of modern biology, i.e. substantial forms and intelligible forms, Theory of Forms... ideas which I think are very meaningful but which are hard to draw up into a pipette. Wnt (talk) 23:49, 3 August 2015 (UTC)

So I guess that means it's astronomically easier to find Cannabis indica phenotypes on another planet than to breed it with any you brought from Earth. You can still eat it though. Sagittarian Milky Way (talk) 02:52, 4 August 2015 (UTC)

It is still exceedingly unlikely even that will happen, but yes. We can't readily say how unlikely because it isn't always clear how much evolution determines what a species becomes. We know that different continents after a split will develop species that not only fail to interbreed but often look different (allopatric speciation) but it is harder to say when two separated populations (like recently reported for the golden wolf) are similar because speciation amounts to mostly neutral mutations, versus when they have both undergone convergent evolution. We can tell from the number of "trees" that have evolved on islands that if you go to another planet, you'll probably find something we'd call forests. From the comparison of birds and bats we might speculate they have winged creatures of a certain size in their skies. The similarity of molluscs and brachiopods makes me expect "clams" on their beaches. Yet all these things are rich with details that the taxonomist can use to say that this is not one of that, and which presumably reveal the amount of random variation that can arise on the same planet. Yet... it's not utterly impossible that an herb evolves with photosynthesis and a distinctive leaf and a certain kind of pharmacologically active sunscreen, somewhere in the universe. Wnt (talk) 11:21, 5 August 2015 (UTC)

Galaxy merger - Andromeda and Milky Way

What will happen to the two supermassive black holes at the centers of the two galaxies when these two galaxies will merge? Will these two black holes merge to from a single black hole? --IEditEncyclopedia (talk) 05:28, 30 July 2015 (UTC)

yes Void burn (talk) 05:30, 30 July 2015 (UTC)

But supermassive black holes are much more powerful. Will not they engulf all the matter if they are disturbed? --IEditEncyclopedia (talk) 05:33, 30 July 2015 (UTC)

Why would they engulf all matter? Black holes obey the same laws of gravity as anything else. Putting two of them together will release a great deal of energy in the merger, but the gravity won't be any stronger than the sum of the two black holes. Someguy1221 (talk) 05:51, 30 July 2015 (UTC)

Why guess what could happen, when astronomers have already been studying it happen: let my type super massive black hole merger into google for you. 209.149.113.45 (talk) 13:25, 30 July 2015 (UTC)

Incidentally, this week's Cosmic Video from Keck Observatory was Black Holes and the Fate of the Universe, presented by Dr. Günther Hasinger, directory of the Institute for Astronomy at University of Hawaii. This is part of the Keck Cosmic Summer School, and the videos are available at no cost. These videos are a great way to hear real scientists talking about cutting-edge research: it will help you see how they actually frame their questions.

Attention physics students: note that in his only slide with equations, the Director has conflated orbital velocity with escape velocity. Astronomers are the only scientists who can get away with such errors - a minor factor of 2x or 10x or 100x is just a "practical detail" in the field of astrophysics. Directors can get away with this type of thing, but when you calculate orbit velocity for your rocket, don't use the equation for escape velocity!.

The presentation discusses "mergers" of massive black holes, and black holes eating galaxy-sized masses, around 30 to 40 minutes into the video.

Nimur (talk) 13:38, 30 July 2015 (UTC)

Might want to be a little careful. At the event horizon, the orbital velocity is the speed of light. It's also the escape velocity as it creates Hawking radiation. Ta daaa! --DHeyward (talk) 02:51, 31 July 2015 (UTC)

Watch the video carefully. His statement, which I quote here verbatim, adding wikipedia article links is:

"When we are trying to send a rocket to the moon or even to the International Space Station, we have to give the rocket a certain speed in order for it to leave the earth... If I could throw a stone, the stone would go up and it would come down again. But if I could throw the stone with a velocity faster than the escape velocity, then it would leave the earth and go into orbit. Now, this velocity on the Earth is 11 kilometers per second."

Now do you see why I say he conflates escape- and orbit- velocity? He's talking about low earth orbit and he even specifically calls out the International Space Station, whose orbital speed is closer to 7 kilometers per second. In fact, it is possible to throw a stone with a speed slower than escape velocity, and the stone can enter a stable orbit - it may never fall back down again, provided that it is thrown in the right direction (your added velocity must impart the correct angular momentum to achieve a stable orbit)!

This is one of those cases where we need to understand conventional physics in order to describe what it is about black hole physics that makes them actually weird! The escape velocity of a black hole, measured relative to a point inside the Schwarzschild radius, is greater than the speed of light. At a different radius, the orbital speed is equal to the speed of light: this is exactly 1.5 times the Schwarzschild radius and it is called the photon sphere. Photon escape by Hawking radiation - which is a real phenomenon - defies the mathematics of conventional orbital mechanics, and is usually treated in a statistical fashion! Photons emitted by Hawking radiation are neither exceeding the escape velocity nor are they even exceeding the speed of light: they are photons, and they always travel at c - yet they are emitted! There is the novel physics of a black hole, and that is the reason that Hawking radiation is so groundbreaking. It is a different process that we cannot describe using only the equations of general relativity.

Nimur (talk) 14:42, 31 July 2015 (UTC)

The theoretical escape velocity is largely irrelevant to actual spacecraft launches because it applies to ballistic objects, not rockets. (And also because it neglects air friction.)

You can't throw an object into an orbit with a perigee higher than your hand unless there is some sort of friction or n-body dynamics. Absent that, it will follow a conic-section orbit, and a spiral (a launch from the ground converging to a closed orbit) isn't a conic section.

The idea that black holes are objects whose escape velocity is c makes little sense for a number of reasons. For starters, the calculation is Newtonian and black holes aren't. The answer is determined up to a constant factor by dimensional analysis, and I think the agreement of the constant factors is a coincidence (especially since they only agree if you use a certain definition of radius in GR). See e.g. [2]. Also, escaping from a black hole is not much like exceeding an escape velocity. Objects thrown upwards at exactly the escape velocity do escape to infinity, and self-propelled objects can leave at arbitrarily low speeds, but nothing at the event horizon can get any higher even if self-propelled or moving at c.

The slide with equations in that video shows up at 6:21. Most of what he says about it is really inaccurate. Aside from not seeming to understand escape velocity, he says that stars are "pulled inward by the gravity of the sun", which is backwards (if he means that they appear closer to the sun due to gravitational lensing, and I don't see what else he could have meant). He also seems to say starting at 13:30 that the fluctuations in the cosmic microwave background are pre-inflationary fluctuations that were magnified by inflation, which is wrong (they're from the end of inflation).

Hawking radiation can escape the black hole because it comes from outside the event horizon. Hawking radiation is still not well understood, but I think everyone agrees on that part. -- BenRG (talk) 02:08, 1 August 2015 (UTC)

To elaborate a bit, you seem to be exhibiting a common misconception about black holes: that they're all-consuming monsters that want to devour everything. Black holes are just objects that obey the same laws of physics as everything else. The only point of difference is we aren't currently quite sure what happens inside their event horizons. For that, we need a full-fledged theory of quantum gravity. But outside the event horizon, the gravitational force of a black hole works the same as that of anything else, including you. Large (meaning "having a high mass") black holes are just very very massive, so they have a correspondingly strong gravitational pull, but things on the right trajectories will still orbit a black hole just like they orbit stars and planets—indeed, we are orbiting the black hole at the center of the galaxy right now. If our Sun were replaced by a black hole of equivalent mass, the whole Solar System would continue orbiting it just the same. Of course, most life on Earth would die since it's ultimately powered by the Sun, but nothing would change Earth's orbit. --108.38.204.15 (talk) 15:32, 30 July 2015 (UTC)

It's probably a little early to say it's quantum gravity that is missing. All physics is a mathematical representation of observation. At the end of the 19th century, theoretical physics was "being wrapped up" as Newton and Maxwell had described evreything and there were just some small details that didn't fit. Those small details turned into quantum mechanics and relativity. We were wrong at both ends of the spectrum. Black holes appear as a singularity and I suspect the mathematics needed to describe will be as revolutionary as QM and GR. Planck time and all the things that currently defy the rules (or rather the things that make our observable approximations follow our models) --DHeyward (talk) 02:51, 31 July 2015 (UTC)

What will happen will depend on how much angular momentum they have, and the relative orientation of the spin. The binary black hole that comes about will orbit faster and faster. When a merger takes place an apparent non conservation of momentum can happen with a "kick' where the result can be pushed out at 1000km/second, gravitational waves carry the complementary momentum. Maybe the black hole will be ejected from the galaxy. Perhaps 5% of the mass will be lost as gravitational waves. Material taken for a ride may be ejected at high velocity, so stars may be sprayed in all directions from the merging core. Graeme Bartlett (talk) 01:19, 31 July 2015 (UTC)

Does eating oatmeal increase or decrease the iron in the human body?

Oatmeal is rich in iron, but it seems that it hinders the absorption of iron by the body (see [3]].

Would consuming regularly some commercial product as Dr. Oetker's Oatmeal increase or decrease the iron level in the blood? Do producers enrich the product with iron to avoid a decrease of iron in the body?--Yppieyei (talk) 10:10, 30 July 2015 (UTC)

Does Dr. Oetker's actually sell plain oatmeal ? A Google search yielded products containing oatmeal, but not the oatmeal itself. Here's their oatmeal muffin mix nutrition: [4]. At only 2% RDA iron, they apparently haven't added any.

And note that when iron is added, it's just to make their numbers look better, they don't actually care if it can be absorbed or not. StuRat (talk) 14:06, 30 July 2015 (UTC)

Note that you're just being cynical and not doing any research or citing any references. Most forms of iron fortification do get absorbed by the body Human_iron_metabolism#Dietary_iron_uptake explains quite clearly that the common form of added iron is absorbed by the body at a rate of 10%-20% of intake. Animal sources of iron are absorbed at 15%-35% intake, but that is easily compensated by ingesting a bit more iron salts (Iron(II)_sulfate#Nutritional_supplement). Other forms of iron used for food fortification include Ferrous gluconate. Here's a nice report from the WHO that covers the use of food fortified with iron [5]. OP might like to take a look at that last link, it does list several foods and compounds that interfere with iron absorption. Finally, you don't even need fancy iron compounds if you get enough total iron, the Lucky Iron Fish [6] has already made huge improvements in anaemia levels, and that's just a simple lump of iron that you boil with your soup. Anemia#Oral_iron also has some good info on some of the common supplements and food additives. SemanticMantis (talk) 15:01, 30 July 2015 (UTC)

I included a reference listing the iron content of one of their products. StuRat (talk) 15:24, 30 July 2015 (UTC)

Duly noted, I should have said "any references that support your final claim" ;) SemanticMantis (talk) 15:27, 30 July 2015 (UTC)

Also, in many countries, including the U.S., the government requires that refined grains be fortified with nutrients, including iron, that are found in the bran and cereal germ, which are removed during processing. Whole grains retain these, so they aren't required to be fortified. --108.38.204.15 (talk) 15:19, 30 July 2015 (UTC)

Train deceleration

Why do trains in Asian countries such as Japan and South Korea accelerate and decelerate so fast? — Preceding unsigned comment added by 176.251.146.3 (talk) 17:41, 30 July 2015 (UTC)

Because they can? Fast acceleration gets the train up to its very high maximum velocity quicker, and fast deceleration permits it to continue at maximum speed longer before stopping. Robert McClenon (talk) 17:44, 30 July 2015 (UTC)

Hmm, let's look at some factors:

PROS:

1) Reduces total trip time.

CONS:

A) Uses up more energy, particularly in the rapid decel (as opposed to allowing the train to slowly decelerate due to friction). Some type of regenerative braking could reduce that.

B) In the case of traditional brakes, rapid decel causes wear on brake pads. However, in a maglev train, probably not, although heating might cause wear.

C) Can be uncomfortable for the passengers to undergo either rapid accel or decel, although depending on if the seat faces forwards or backwards, one of those should be easier on them than the other. Getting caught walking with a cup of coffee would be bad, too, so some warning would be appreciated. StuRat (talk) 18:09, 30 July 2015 (UTC)

So, they must feel the pros outweigh the cons. The main factor that might vary in Asian nations is more maglev trains, reducing concern over B. StuRat (talk) 18:09, 30 July 2015 (UTC)

It doesn't actually use more energy for the acceleration or deceleration itself. It uses more power, but uses that for a shorter time. It also uses more energy due to the higher air resistance at the higher speeds achieved earlier, of course. Regenerative braking should be fairly standard for electric trains in most advanced countries today. --Stephan Schulz (talk) 18:19, 30 July 2015 (UTC)

Of course, that's assuming that the energy-efficiency for the train's power plant is uniform at all power levels. Is this accurate for, say, the Shinkansen bullet train? Nimur (talk) 18:46, 30 July 2015 (UTC)

I believe, at least in the case of rapid braking, that it does use more energy, since more of the braking is done by the train, and less by air resistance and rolling resistance (in the case of trains with wheels). StuRat (talk) 14:15, 31 July 2015 (UTC)

In reply to StuRat's C), the comfort of passengers is determined mainly by the rate of change of acceleration (see Jerk (physics)). This can be high even on slow British trains, and is regularly felt on slow buses especially if badly driven. Dbfirs 20:08, 30 July 2015 (UTC)

To take a slightly different epistemological angle, countries like Japan and South Korea have invested heavily in high-speed rail. This means the rail infrastructure tends to be designed for higher speeds. High-speed rail also generally is designed to allow trains to accelerate and decelerate more rapidly, since the whole point is to get people to their destinations quickly. It's kind of a waste if your train has a high top speed but it takes forever to get to that speed! Designing for high top speed and rapid velocity changes generally goes hand-in-hand anyway. Tracks need to be sturdy, level, and as straight as possible, trains need to be able to withstand high stresses and have powerful engines, etc. --108.38.204.15 (talk) 18:28, 30 July 2015 (UTC)

They can accelerate and decelerate because each axle is a traction motor (see Electrical multiple unit). By distributing the traction engine, the train isn't limited by a central traction system. Every car is driven and the distributed mass and drive improves both acceleration and deceleration. It's all electrically driven. By contrast, diesel electric trains have electric motors only on the locomotive. Synchronizing all the axles is not trivial and is why it's not done everywhere and is one of the reasons high speed trains in Europe don't accelerate or decelerate as fast (see AVE Class 102 for example of high speed train in Spain that has only 2 drive units). Most braking is done using with either regenerative brakes or more commonly, Eddy current brakes so pad wear isn't an issue. --DHeyward (talk) 21:06, 30 July 2015 (UTC)

No, this is quite wrong. Provided that there is sufficient weight on the powered axles for the necessary traction to be achieved, the maximum acceleration of a train is determined by its power-to-weight ratio. It doesn't matter if there are a large number of small motors or a small number of large ones. The two "power cars" (locomotives) of that AVE class 102 have a total of 8 motors developing 1,000 kW each, and the train weighs 322 t (metric tons), so that's a ratio of 8,000/322 = about 25 kW/t. The N700 Series Shinkansen, built about the same time, has 56 small motors developing 305 kW each, and weighs 716 t, giving almost the same ratio at 17,080/716 = 24 kW/t. The acceleration capacity of the two trains should be about the same, provided that the AVE locomotives are heavy enough for the force from the motors to be practically used. Multiple-unit trains do have two big advantages (against a downside that is mostly in cost): they're easier on the track (no heavy locomotives) and they can be made to divide into shorter trains by simply uncoupling. But they don't have an advantage in performance. Also, there is no need to "synchronize the axles". --65.94.50.73 (talk) 04:50, 31 July 2015 (UTC)

The original question implies that trains in Asian countries DO accelerate and decelerate at a greater rate than similar trains outside Asian countries. I'm skeptical. What is the evidence before us that there IS a significant difference in rates of acceleration and deceleration? Dolphin (t) 06:44, 31 July 2015 (UTC)

Go ride them :). Japan most certainly accelerates faster. Spain acknowledges it in their design docs but it's not an issue on a long run, only when there are lots of stops. --DHeyward (talk) 07:23, 31 July 2015 (UTC)

Your caveat is why it's not equal (and also why a car with wide driving tires accelerates quicker than narrow tires for the same power to weight ratio). In practical high-speed trains power to the motor is regulated and measures slippage and creep. Power has to be reduced when wheels starts slipping too much but slippage is required for maximum acceleration. By distributing the weight and a driving motor each with it's own power control, the available torque is much higher. There's much more driving wheel surface in contact with the rail, all the weight of the train is on driving wheels (it's all the normal force of the full train weight, not just normal force of the mover weight), and each motor is driving with the optimum slippage. Top end speeds are the same but acceleration is much greater in the distributed case because the motors are running with maximum tractive force throughout the acceleration cycle. Japan has many stations and stops so they use distributed motors to maximize acceleration but it doesn't help top speed. When acceleration is a significant factor in trip time, distributed is better. For that reason, N700 Series Shinkansen accelerates to top speed much quicker than the AVE class 102 even though top speed is about the same and your power to weight is about the same. Distributed power delivery and control must be coordinated (synchronized was a bad word considering its meaning wrt motors) and it is more complicated with EMU's and is not simple and it's more expensive. --DHeyward (talk) 07:23, 31 July 2015 (UTC)

As a peripheral observation, this was also why the Southern Railway was in the forefront of electrification in the UK: it had the largest proportion of urban, suburban and rural commuter traffic (mostly to/from London) the associated high number of stations and stops, the passenger-driven motivation to minimize journey times, and the ever-present desire to minimize energy costs. Electrification enabled both better acceleration and improved efficiency (over coal/diesel). {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 13:33, 31 July 2015 (UTC)