Wikipedia:Reference desk/Archives/Science/2010 October 22

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

October 22

How many generations have there been?

Has anyone produced an estimate of the number of generations that have elapsed between the origins of life and today's human population? (In other words: How many acts of reproduction (asexual and/or sexual) has the average gene in the average [human] body gone through? This phrasing overlooks the fact that most contemporary human genes did not exist at the dawn of life, but that's OK because the focus of my question is on the number of generations that there were.)

This feels like a fun math puzzle ... -user:Agradman, posing as 160.39.222.65 (talk) 06:07, 22 October 2010 (UTC)

I have a feeling that our generations spent single-cellular will completely overwhelm our generations spent as mammals, lizards, fish, etc. It might not be even worth the effort to even count our multicellular generations. APL (talk) 06:37, 22 October 2010 (UTC)

I agree completely with APL that in this context, it's only worth considering the single-celled portion of our evolution.

Our Bacteria article says that bacteria populations can double as frequently as every 9.8 minutes. The article unfortunately doesn't mention what's more of an average time. But if you estimate that our single-celled ancestors took in the very rough ballpark of an hour to divide on average, then there would have been very roughly 2.5 x 10¹³generations in the 2.8 billion years that life existed only as single-celled organisms (see Timeline of evolution). Of course, that's just a very rough order-of-magnitude estimation. Red Act (talk) 08:03, 22 October 2010 (UTC)

Well, let’s start on a more modest note, and with some ball park figures. If we accept that humans began to migrate from Africa around 80 thousand years ago, and we assume that there is a new generation every 30 years (it would probably be closer to 25), then we have had about 2670 generations. If you look back to 0 A.D., then, since we started counting with the Christian calendar, a mere 66 generations have elapsed. That doesn’t sound like very much, does it? It goes to show how incredibly fast the course of human civilization has been. There are only 66 or so hierarchies of ancestors to take you back to Ancient History. 266 generations would take you back 8000 years right to the approx time when agriculture became a dominant force. Being, as they say, an evolutionist, I like to think sometimes that my ancestry was old before the Himalayas were there, before the continents split up. And seeing a new baby today made me think, here is something completely fresh, perfect, and yet she comes from stuff which predates mountains, and her progeny could easily be still existent when the Sun has died of old age, leaving no successor. Myles325a (talk) 03:55, 23 October 2010 (UTC)

Let's ignore the ancient history and consider recent generations of humans. There are plenty of people whose ancestors were related to each other but in different generations; for example, my father's grandparents were first cousins once removed to each other. One of the two was a sibling to the other's mother, so one's parents (let's call them X and Y) were the other's grandparents. On one side, X and Y are my great-great-grandparents, but on the other side, they're my great-great-great-grandparents. How many generations back are they? If this be the case for people living in the last 100 years, how could there possibly be anything close to a single correct answer (whether or not we could discover it) for all of the time since life has existed? Nyttend (talk) 00:46, 27 October 2010 (UTC)

Dark matter: cold gravitationally bound clumps of H and He?

Is there any reason dark matter isn't simply gravitaionally bound clumps of hydrogen and helium in the cold interstellar medium?

Gravitational clumps of cold interstellar hydrogen, helium, and other elements (with abundances in proportion to traditional nucleosynthesis ratios) would have the same spectra as many fewer individual molecules of the same substances because absorbed photons would be stored mechanically as heat and released as blackbody emissions at 50 down to 3 Kelvin and below in addition to much weaker spectral lines than in isolated gas molecules. This would explain why spectroscopy has detected much less cold interstellar media than would be consistent with thermodynamic cooling away from nearby stars as well. This would not require modification of any physical constants or cosmological principles. The spectra would be exactly the same for clumps of many more H and He molecules as for profoundly fewer individual molecules, except for the blackbody emission which is partially absorbed by our atmosphere and certainly overpowered by transmitted starlight through interstellar clouds.

Everyone admits that gravitationally bound clumps of interstellar gas are necessary for star formation, but after it's clumped, it's not really a gas so much as a mechanical fluid, is it?

My third question is: How many additional Daltons (AMUs) per interstellar medium molecule would be necessary to account for the 5.75 times as much mass as has been observed in non-dark matter?

[1] says we need accurate infrared spectrometry and imaging at 28.2, 17.0, 12.3 and 9.7 microns (the Earth's atmosphere is opaque at 9.7 microns) to be able to confirm the possibility. Ginger Conspiracy (talk) 06:50, 22 October 2010 (UTC)

The theoretical models of Big Bang nucleosynthesis predict the primordial abundances of ¹H, ²H, ³He, ⁴He, ⁷Li, etc. These models are only mutually consistent with the observations across all isotopes if you assume that total mass of all normal matter is about 1/6 of the total needed for all matter. As a result, the prediction is that 5/6 of the mass in the universe is in some non-baryonic (i.e. non-atomic) form. Presumably there is some unobserved mass in hydrogen and helium clumps, but the traditional nucleosynthesis arguments lead to the conclusion that most of the dark matter is in some other exotic form. Dragons flight (talk) 07:54, 22 October 2010 (UTC)

Gravitational microlensing is also used to set a limit on the number of dark "clumps" of mater, see Massive compact halo object, max 20% of the dark mater are MACHOs.

I am not sure that this rules out Robust associations of massive baryonic objects.

--Gr8xoz (talk) 09:13, 22 October 2010 (UTC)

Interesting! You could still have protostellar clumps smaller than RAMBOs. What is the limit that gravitational microlensing sets? Is it uniform? Ginger Conspiracy (talk) 19:01, 22 October 2010 (UTC)

MACHO rules out explaning all the dark mater by clumps between 1e-7 and 100 solar mass or between twice the lunar mass and the mass of a realy massive star by microlensing, as I understand it so are RAMBOs groups of MACHOs and the grouping reduces the detection probability since they hide behind each other but that is just a gues. --Gr8xoz (talk) 20:03, 22 October 2010 (UTC)

No, but the BBNS arguments outlined by Dragons flight doput a limit on how much RAMBOs can contribute ... --81.153.109.200 (talk) 19:31, 22 October 2010 (UTC)

Dark matter is also theorized to contain Weakly interacting massive particles, that would pass through ordinary matter similar to neutrinos. ~AH1^(TCU) 18:36, 23 October 2010 (UTC)

Age of plants and animals in Copernican exoplanet expectations

For how long have there been enough oxygen-producing plants on Earth to create an ozone layer in the upper atmosphere? What is that duration as a multiple of the length of time that multicellular animal life has been on Earth? Is there any reason that ratio should not be what we expect as the ratio of habitable exoplanets with plants and animals on them, in accordance with the Copernican principle? Ginger Conspiracy (talk) 06:53, 22 October 2010 (UTC)

The ozone layer formed about 1.9 billion years ago, give or take. See: Great Oxygenation Event. However, that oxygen came from bacteria not plants. The origin of life was at least 3.5 billion years ago, though probably even earlier. Multicellular life at its most basic form predates the ozone layer, though you might set a later horizon depending on what qualities you expect your multicellular organism to have (for example, macroscopic organisms that reproduce sexually from germ cells are probably less than a billion years old). Dragons flight (talk) 08:17, 22 October 2010 (UTC)

Thank you! If you were in charge of the exoplanet colonization budget, how long ago of an Earth would you want to start with? Ginger Conspiracy (talk) 18:12, 22 October 2010 (UTC)

I like 550 million years just before the Cambrian explosion, but I suppose I'd see whether ±0.05 Gyr is likely to work. Ginger Conspiracy (talk) 20:13, 22 October 2010 (UTC)

Once enough oxygen gets into the upper atmosphere, the solar radiation, stratospheric lightning and even small amounts of cosmic rays would have started generating some ozone. ~AH1^(TCU) 18:30, 23 October 2010 (UTC)

Bread made with "100% British Wheat"

Decades ago when I was at school we learned that British wheat was soft wheat, which was suitable for biscuits but not bread making. Bread was made with hard wheat grown in hotter climates, mostly imported from America. There are now loaves from Hovis and Warburtons that use 100% British wheat. What has changed to allow this to happen: the climate, varieties of wheat, bread making technology, or our taste in bread? -- Q Chris (talk) 09:39, 22 October 2010 (UTC)

I would add an unlisted option: what about a change in advertising regulations that define "100%"? Nimur (talk) 13:30, 22 October 2010 (UTC)

Botham's of Whitby says it's due to new varieties and added gluten here. Syngenta sells a number of such varieties here and extols Gallant's breadmaking prowess here. -- Finlay McWalter ☻ Talk 10:10, 22 October 2010 (UTC)

Thanks, that's interesting. -- Q Chris (talk) 10:30, 22 October 2010 (UTC)

I wonder from where the "added gluten" comes, given that gluten is typically produced from wheat and related grass crops. If that still comes from Canadian wheat, then you'd have to parse "100% British wheat" rather economically. -- Finlay McWalter ☻ Talk 10:43, 22 October 2010 (UTC)

I suppose they could process the British wheat to extract what gluten it does have. Perhaps we can do a bit better than that. There was a time before the English could import Canadian wheat, and then we used locally grown wheat (and rye, perhaps a bit of barley) to make their bread. We had to use local wheat again during the Second World War. Long-standing techniques to get the best out of English wheat include starting with a batter left to rise over night, followed by multiple risings. These methods not only help the gluten to do its work but bring out the full flavour of the grain. Itsmejudith (talk) 11:25, 22 October 2010 (UTC)

The Irish manage with soft wheat for their traditional soda breads. They do taste a bit different to the English loaf though. -- Q Chris (talk) 11:35, 22 October 2010 (UTC)

Oddly enough, our article on gluten talks about industrial extraction and the addition of gluten to flour to increase the hardness. It gets used for all kinds of purposes, so adding it to wheat flour would be trivial. Also, if I understand the Good Eats episode correctly, raw wheat seeds and flour do not contain gluten per se, but instead have gliadin and glutenin, which combine to make gluten during the kneading portion of bread making. That might explain why our article says that flour that has been protein hardened with added gluten "[M]ust be worked vigorously if they are to rise to their full capacity, so a bread machine or food processor may be required for their kneading." Matt Deres (talk) 13:38, 22 October 2010 (UTC)

If they're using the Chorleywood Process, they won't have to worry about whether it's kneaded enough! And the Chorleywood Process was created precisely in order to make bread with low-gluten British wheat. We could do with a redirect on Chorleywood Process and Chorleywood process. 86.163.212.182 (talk) 15:31, 22 October 2010 (UTC)

I understand that the amount of selenium in British diets has plummeted to below guideline amounts since we stopped importing North American wheat. See for example http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WJH-45S940V-3&_user=10&_coverDate=12%2F31%2F1995&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a2e206ac51e8d88f6736c19e1e497504&searchtype=a 92.15.28.203 (talk) 19:57, 22 October 2010 (UTC)

Feeling of blood circulation

One has asked in Arabic Wikipedia: Why can't we feel blood moving (circulating) inside the body, though we can feel the pains inside?--Email4mobile (talk) 12:14, 22 October 2010 (UTC)

Sometimes after exercise I can feel my pulse, but not the flow of blood. -- Q Chris (talk) 12:18, 22 October 2010 (UTC)

Did you ever feel the difference when your blood wasn't flowing? You also don't feel the heavy atmospheric pressure until it is removed. --Chemicalinterest (talk) 14:21, 22 October 2010 (UTC)

The brain and nerves have a filtering system - if they feel a constant sensation for a while, they filter it out and you don't notice it anymore. Have you ever not noticed a sound till it stopped? Or for someone who always wears a ring, necklace or hat; you don't feel it till you remove it. Ariel. (talk) 18:08, 22 October 2010 (UTC)

Some people indeed feel the blood flow in parts of the body, or hear the blood flow in the head. More of a swishing than a pulse thumping. Edison (talk) 20:00, 22 October 2010 (UTC)

See venous hum. the wub "?!" 21:26, 22 October 2010 (UTC)

Wow, that's odd. I always feel (arterial) blood as it circulates throughout most parts of the body; it's hard for me to picture that someone wouldn't. Wnt (talk) 02:17, 23 October 2010 (UTC)

"HMS Astute: world's most advanced nuclear submarine runs aground" -- world's most advanced? why?

Pride is one thing, claiming world's best status is quite another. In describing the recent grounding, the Telegraph.co.uk headlined the article as "HMS Astute: world's most advanced nuclear submarine runs aground" -- I checked the Wikipedia article and saw nothing that made me feel the title of "world's most advanced" was warranted. Is this really the best sub in the world? If so, why? The Masked Booby (talk) 13:05, 22 October 2010 (UTC)

HMS Astute (link to Telegraph story) is the lead ship of one of the newest submarine classes in the world. We note that she is the most advanced sub in the Royal Navy, and the Royal Navy's sub force compares well qualitatively. So is there some national pride in claiming the most advanced sub? Probably. But "most advanced" is subjective anyway (what standard defines "advanced"), and the hard numbers in question are going to be classified, both for Astute and for her competitors. So sure, it's a reasonable label. — Lomn 14:42, 22 October 2010 (UTC)

The Royal Navy's website [2] says "Astute is the largest, most advanced and most formidable vessel of its kind ever operated by the Royal Navy. She incorporates the latest stealth technology combined with a world beating sonar system and equipped with Spearfish torpedoes and state of the art Tomahawk land attack missiles to make her a supremely effective naval asset." I suspect that they're not going to tell us the exact details so that we can make an independant analysis. Alansplodge (talk) 15:32, 22 October 2010 (UTC)

The puffery makes it all the funnier when it is stuck in the mud, like a remake of The Russians Are Coming, the Russians Are Coming wherein a fictional Soviet sub gets stuck in the mud off the US coast while trying to get a look at America. How deep was the water, and what is the draft of the sub? Were the charts incorrect? Did they make a wrong turn? Did an unmarked sand bar suddenly appear? Edison (talk) 19:36, 22 October 2010 (UTC)

The BBC says that there is a well marked (by buoys) channel, and that the locals were very surprised that such a large vessel would be out of that channel; I rather think someone mistimed the tides. She was reportedly performing a transfer of men with a smaller boat. I wouldn't be a bit surprised if it was the boat's cook, set to bicycle into Kyle of Lochalsh to buy some vegetables from the Co-op. -- Finlay McWalter ☻ Talk 19:46, 22 October 2010 (UTC)

It seems probable that the submarine was undergoing field-tests to determine exactly how close to shore it can get before getting stuck. Perhaps we should call the test a success - "can operate in waters no shallower than ___ " is now an easy-to-answer question. Nimur (talk) 19:50, 22 October 2010 (UTC)

This is the second sub grounding near Isle of Skye since 2002. Some subs in the past had wheels on the bottom. Might help. Edison (talk) 19:56, 22 October 2010 (UTC)

There's a seat on the naughty step waiting for someone if this[3] precedent (or this[4]) is anything to go by. Alansplodge (talk) 22:24, 22 October 2010 (UTC)

nerve regeneration and pain?

i think i read it somewhere but i'm not really sure... Is nerve pain or discomfort a sign that the nerve is regenerating? thanks

Usually no, the nerve is working well and something on the end of the nerve is in trouble, or the nerve itself is damaged. See Wikipedia articles Nerve and Afferent nerve fiber. Please sign your posts. Cuddlyable3 (talk) 13:39, 22 October 2010 (UTC)

Nerve regeneration feels more like tingling and weird sensations, not really pain. Ariel. (talk) 18:08, 22 October 2010 (UTC)

In some cases of injury to a nerve, a neuroma can develop. These traumatic neuromas can be painful. If you have an injury or are in pain and you are not sure why, please see a health professional. Mattopaedia ^{Say G'Day!} 05:31, 23 October 2010 (UTC)

Tin(II) oxide hydrate oxidation

Does tin(II) oxide hydrate oxidize in air when dry? --Chemicalinterest (talk) 14:20, 22 October 2010 (UTC)

The same ref I mentioned last time you asked about properties of this chemical is still applicable. DMacks (talk) 16:50, 22 October 2010 (UTC)

This may be a dumb question, but how do I get to that DOI? --Chemicalinterest (talk) 23:11, 22 October 2010 (UTC)

Type the doi in google! John Riemann Soong (talk) 07:35, 23 October 2010 (UTC)

good bacteria

Is it true that they are good bactera in the human body?

See gut flora. Vimescarrot (talk) 18:00, 22 October 2010 (UTC)

And the endosymbiont hypothesis. –Henning Makholm (talk) 18:34, 22 October 2010 (UTC)

Yes. Your body is estimated to contain 10x as many microbial cells than human cells. See human flora for a more general discussion. --- Medical geneticist (talk) 22:51, 22 October 2010 (UTC)

How does the immune system tell the difference between good and bad bacteria?

The good bacteria stay in the gut or on the skin. Ariel. (talk) 18:11, 22 October 2010 (UTC)

Also, the immune system is able to acquire tolerance to antigens that come into contact with the mucosal surfaces. --- Medical geneticist (talk) 22:54, 22 October 2010 (UTC)

If there are good bacteria are there good viruses?

Probably not, but maybe. Bacteria are "good" because they digest things, and release useful chemicals and vitamins. Viruses don't do anything except reproduce. But see bacteriophages which are virus that infect (and kill) bacteria, which can be useful if there is an infection of those types of bacteria. This is a developing field of research, but I don't think there are any current treatments using bacteriophages. Ariel. (talk) 18:14, 22 October 2010 (UTC)

There are viral vectors used in genetic engineering. Does that count for your purposes, OP? Or are you talking about viruses out in the wild that are beneficial to the body? --81.153.109.200 (talk) 19:27, 22 October 2010 (UTC)

Acording to Retrovirus "Most insertions have no known function and are often referred to as "junk DNA". However, many endogenous retroviruses play important roles in host biology, such as control of gene transcription, cell fusion during placental development in the course of the germination of an embryo, and resistance to exogenous retroviral infection." This is about Endogenous retrovirus, viruses that has inserted its genes in the germline and no longer are functioning as a virus. --Gr8xoz (talk) 19:43, 22 October 2010 (UTC)

Bacteriophages have long been used in phage therapy, which has been used effectively in Russia for half a century. They're bureaucratically unacceptable in the U.S. There have been at least two questions about these here in the past 2-3 months. I'm not sure they really count for purposes of "good viruses" because they're actually being used as biowarfare against another species. However, they can be beneficial to infected bacteria; while they're insert into the genome their point of view is that of an ally rather than an enemy. For example they can introduce antibiotic resistance into new strains in the wild.[5] I believe that certain integrated retroviruses could have a beneficial effect in the human germline in the more limited sense of providing immunity to infection with others of their kind. (There's a somewhat analogous situation with P elements in fruit flies, which are jumping genes but don't form viral particles around themselves. You might say they have a tendency to facilitate evolution, for example by putting themselves into introns and creating a spot where recombination is possible to make new genes; but they do so at the cost of many badly mutated flies. In such cases the definition of good versus bad becomes rather philosophical...) Wnt (talk) 01:22, 23 October 2010 (UTC)

It appears that humans share a certain set of multiple genes with a virus (not sure which one specifically), that are not present in chimpanzees and may have resulted from a sideways insertion of genes. Does anyone know which virus this is? I read it somewhere on Wikipedia, but cannot currently find it. ~AH1^(TCU) 18:11, 23 October 2010 (UTC)

See HERV#Human endogenous retroviruses which was already referred to. HERV-FRD looks interesting, but tells us nothing. Graeme Bartlett (talk) 21:20, 23 October 2010 (UTC)

What are the advantages and uses of the good bacteria in the body

See Probiotics. Ariel. (talk) 18:12, 22 October 2010 (UTC)

Pancreatin Reaction

I want to do an experiment in which i will put 10 mL of potato starch into a test tube and add 5 mL of pancreatin. This will be done at various temperatures (21 C, 37 C, 100 C, 0 C). I will time the reaction for 1 minute and then record the change in volume. I think that the volume will decrease (it was 15 and after the reaction and it will be less than 15 after since pancreatin is a digestive enzyme and will break down the potato starch). This information will be used to calculate the rate of reaction at different temperatures. Can anyone tell me if this experiment is going to work? Or is one of my variables wrong? (this is a one time thing so i can not afford mistakes) —Preceding unsigned comment added by 24.86.167.133 (talk) 19:16, 22 October 2010 (UTC)

I think the whole point of doing experiments is to see if your hypothesis is correct (formed by your background research), not just prove what someone else tells you will happen (based on his own experiments or backround research). Lots of times experiments do not support a given hypothesis, and the goal is to figure out why not, not just say "yay, I was right" or "I was wrong, oh well". Those "didn't work" cases are really the only place you actually learn something, because you have some other result to explain (have to form a new explanation and also an explanation of where you went wrong in your hypothesis) just know that you read the background material properly. "Change of volume after a fixed time at different temperatures" is a great example of a dependent and independent variable and you have made a testable hypothesis (with rational support from background information). Be careful: are you really using X mL of "starch" and of "pancreatin", or of solutions of them? Does your answer to that question affect your thoughts about volume change? DMacks (talk) 19:33, 22 October 2010 (UTC)

Yes but i am getting marked on a lab i have to create myself. If nothing works and none of the tests produce any results, what am i going to show on my data processing? —Preceding unsigned comment added by 24.86.167.133 (talk) 19:37, 22 October 2010 (UTC)

I think that DMacks' point is that instead of "do an experiment" you ought to say "measure the reaction rate" and instead of "is this experiment going to work" you ought to say "will this measurement give reliable results?"

That being said, do you even know anything about how the breakdown products behave in solution that would allow you to infer the amount of reaction from a volume change? The enzyme, if it works, is not going to make the starch disappear into nothing; it will cut it up into other chemicals that also take up space. How much space is that? –Henning Makholm (talk) 19:45, 22 October 2010 (UTC)

I tried to be careful to separate the ideas of "data" from "conclusions" from "supported-hypothesis". Common mistake is to confuse them (question is unclear what "works" means--no data (that's the only real meaning of "works", but is obviously not what he meant) vs uninterprettable/random data (uncontrolled variables, which is something he seems to be asking about, and that borders on {{dyoh}}) vs results match expectations (mis-understanding the background material or making a key systematic mistake in experimental design, which I'm definitely not going to answer, except to (as Henning also does) suggest key ideas to consider).

You will get results. I guarantee (assuming you know how to use your measuring equipment) you will be able to measure a temperature, and time "1 minute", and measure the volume(-change)s. You will be able to make a table of numbers. You will be able to graph those points. You will be able to make a conclusion stating whether there is an effect of temperature on volume-change, and if so whether it's linear or has a maximum-peak, or whatever. There--right there are your results. Doesn't seem like "I get no data" is a reasonable possibility, and therefore no chance of having "no data-processing" or "no results" of your experiment. The results might surprise you, but you will still get "some results". Your conclusion can be 100% correct for your data, even if it completely contradicts your hypothesis--part of an experimental conclusion is recognizing that situation and proposing why (figuring out the pattern to the contradiction is at least as good a conclusion as merely concluding that your background research was correct). DMacks (talk) 19:53, 22 October 2010 (UTC)

positive on-off switch

Resolved

The Washington State weatherization specs demand that "Thermostats shall be digital, have a built in anti-short-cycle feature and include a positive on-off switch". What is a positive on-off switch? — Sebastian 23:29, 22 October 2010 (UTC)

I think they mean a "manual on", "manual off" switch, in addition to a thermostat that will turn on/off based on current temperature. Nimur (talk) 00:02, 23 October 2010 (UTC)

It's a widely used term that it's remarkably difficult to find a definition for, but I finally found a source that explains that a "positive" switch is one that makes a definite click when it is moved to the on position. Looie496 (talk) 00:15, 23 October 2010 (UTC)

Ah, cool! Thanks a lot! — Sebastian 00:17, 23 October 2010 (UTC)