Wikipedia:Reference desk/Archives/Science/2012 June 28

= June 28 =

skin flora
is mainting your skin flora helpful in preventing skin infections like mainting your vaginal flora is  good?--Wrk678 (talk) 02:19, 28 June 2012 (UTC)


 * Yes. Specifically, killing off the skin bacteria can allow fungi to grow there. StuRat (talk) 02:26, 28 June 2012 (UTC)


 * The desk appreciates contributions from owners of good vaginal flora. DriveByWire (talk) 13:05, 28 June 2012 (UTC)


 * Wife, responding to husband who brought her flowers for no reason: "So now I suppose you expect me to spread my legs ?"


 * Husband: "Why, don't we have a vase ?" StuRat (talk) 07:14, 29 June 2012 (UTC)


 * Oh, the classics... Snow (talk) 00:32, 1 July 2012 (UTC)

trees of interest
is there a tree or a bush that has about 2 inch thorns on it where every leaf is. — Preceding unsigned comment added by 2602:306:CCF5:E7F0:FDBA:5660:E85A:B5BF (talk) 03:19, 28 June 2012 (UTC)


 * According to this site, the red hawthorn has 2-4 inch long thorns:, although I'm not sure how they are spaced relative to the leaves. StuRat (talk) 03:36, 28 June 2012 (UTC) I've taken the liberty of correcting the 'hawthorn' spelling to link it. Richard Avery (talk) 07:42, 28 June 2012 (UTC)


 * Thanks, but unfortunately that doesn't appear to be the same species. Crataegus columbiana, Crataegus chrysocarpa, and Crataegus rotundifolia were the three species listed at my link as "red hawthorn". StuRat (talk) 07:06, 29 June 2012 (UTC)

Acacia tortilis the Umbrella Thorn http://www.google.com/search?q=acacia+tortilis&num=100&hl=en&newwindow=1&safe=off&client=safari&rls=en&prmd=imvns&source=lnms&tbm=isch&sa=X&ei=b-PrT5-lFoyA6QH_k6niBQ&ved=0CPgDEPwFKAE&biw=1027&bih=739 μηδείς (talk) 04:56, 28 June 2012 (UTC)


 * A couple of Berberis species have decent thorns, Berberis julianae and Berberis gagnepainii, the thorns of which are pictured. Richard Avery (talk) 07:39, 28 June 2012 (UTC)


 * In northern Europe, the Blackthorn has the most evil thorns in town, beating the Hawthorn hands down. A friend had one punch through the sole of his work boot and I've heard of people losing eyes to them. If one gets lodged under your skin, it festers horribly. It is considered unlucky to bring blackthorn into the house the UK, apparently because it was believed to have been used for the Crown of Thorns. Alansplodge (talk) 18:52, 29 June 2012 (UTC)


 * As an aside: Ah, Blackthorn and brass splinters ... UK chemist shops used to sell 'Yellow Basilicon Ointment -British Pharmacopoeia'. Put some of that over the afflicted part and slap on a sticky plaster and after a few days out came the festering thorn or splinter. Other than not being kosher, I don't know why it was withdrawn. These days, one's extended family  often ends up having  to take   a trip to the doctor to have it removed and usually  get presented with a  prescription for antibiotics (£7.65) because it was left so long. Is that a step forward?  The ingredients however, are still obtainable from artist suppliers and one's local supermarket (thankfully) but I wont give the receipt  here... as its too off topic.  Call me an old stick-in-the-mud but Blackthorn is a sod, because it's so long and brittle it can brake off under the skin and a 'stitch in time save nine'. --Aspro (talk) 20:02, 29 June 2012 (UTC)

Sorbothane
Can anybody familiar with this material take a look at [http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Language#Homework_help.21_Please_help_me_write_without_close_paraphrasing. this question] on the Language reference desk? Bus stop (talk) 03:22, 28 June 2012 (UTC)

Effect of over speed test on large steam turbine (>600mw) last stage lp blades->900 mm length
As per iso standards over speed test on large(>600mw)turbine lp rotors is carried out to ensure the intigrity of rotor blade assembly and "permanent set" if any.Generally the over speed holding period is between 2 to 10 minutes as per the design requirement.Over speed run normally is carried out at 120% of the rated speed.Such over speed tests are carried out once in the life time of any rotor and not repeated. 1-It has been observed that certain manufacturers of large steam turbines do not carry out the over speed test on l P rotor (with last stage blade height >930 mm) at 120% but do it at 115% holding for two minutes. 2-In other case the above lp rotor is over speeded at 120% momentarily and the speed is brought down. the bladed lp rotor weight is about >90 mt. my questions are a-Is it detrimental to carry out over speed test-run at 120%? b-Can such over speed run for two minutes impair the blade/rotor material strength/properties? c-Such constraint in conducting over speed test and holding for two minutes--can be attributed to dynamic balancing/ over speed set up or design inadequacy? — Preceding unsigned comment added by Rwgkw81 (talk • contribs) 05:01, 28 June 2012 (UTC)
 * Refactored to eliminate shouting. Evanh2008 (talk 05:08, 28 June 2012 (UTC)
 * Wikipedia has an article about Nondestructive testing which is the main intention here, combined with a bedding in process. The given period at given overspeed is chosen by the manufacturer to fulfil the latter process. I expect the turbine is fit to continue indefinitely at the given overspeed, but the manufacturer may be able to give a lifetime estimate based on experience. DriveByWire (talk) 12:55, 28 June 2012 (UTC)

Climate change evolution
I was talking to my grade 9 science (major in biology) teacher about a month ago, and asked if it was possible for humans, as with most other life forms, to naturally evolve to survive in the changing climate (i.e., temperatures). She said that there's not enough time for us to do so, that it would take thousands or millions of years, and the change is happening at a fast rate. However, I was watching a few Richard Dawkins videos about evolution, natural selection, etc. and he mentions that scientists had brought a species of lizards into a new island, and within decades, not millions of years, the species had changed their facial features, as well as their stomach, to adapt to the new environment and foods. So, is my teacher wrong? Could humans, along with other species, evolve in order to cope with the climate change? Thanks, 64.229.5.242 (talk) 07:13, 28 June 2012 (UTC)


 * Even in the most extreme models of climate change, the actual changes taking place are rather benign as far as the integrity of life is concerned. A lot of people might drown due to increased flooding, and others may starve due to crop failures, but we're not talking fireballs coming down from the sky or anything like that. What specific changes did your science teacher think you were talking about? Someguy1221 (talk) 07:26, 28 June 2012 (UTC)
 * If humans do nothing about climate change half the currently inhabited earth could become literally too hot for humans to live in. That's still something I'd want to avoid at nearly all costs. Sagittarian Milky Way (talk) 17:54, 28 June 2012 (UTC)


 * If you think about it, the time between generations in organisms such as lizards is much, much shorter than in humans. Humans have to wait about 20 years to sexually mature and produce offspring, and then that offspring has to wait about 20 years to mature, whereas a lizard may have offspring after 9 months - 1 year, and that offspring can reproduce after 1 year. So 20 generations in a lizard takes 20 years, whereas in a human 20 generations takes 20 times 20 = 400 years. (I'm talking round figures here.) So humans could evolve to take account of climate change, but because their reproduction rate is so much slower than other species, it would take much longer, in which time the changes could wipe the species out. The faster reproducing species are at a greater advantage as far as evolution is concerned when it comes to adapting to rapid changes to the environment. --TammyMoet (talk) 08:38, 28 June 2012 (UTC)


 * It may just be that humans are already naturally evolved to adapt to climate change. Humans have evolved preconditions to produce and modify technology which can act much quicker to address stressful environmental conditions than genetic adaptation can. All that is needed is the proper resource allocation to solve the problems faced with better technologies or societal organisation. Think about it for a minute, a human without technology would die if left exposed to an English winter or the Saharan desert. It will be the other animals without technology that are to be doomed by extreme or rapid climate change. Sky Machine   ( ++ ) 09:04, 28 June 2012 (UTC)


 * Wikipedia has a featured article about Evolution. Evolution works by the selective survival of progeny that adapt best to a changed condition. Human evolution today is driven less by natural selection than by social pressures and economics. A sudden climate change that leads to widespread fatalities might be expected to favour certain inheritable human characteristics that already exist in the population but it is difficult to speculate which ones. Perhaps slim people would survive better than obese people in a sudden global warming. DriveByWire (talk) 12:42, 28 June 2012 (UTC)


 * People are now living quite happily in climates that are much hotter than Europe, say, will ever be even under the most pessimistic scenarios for climate change. It is the rapid change that is dangerous, and that is dangerous for the survival of societies rather than for the survival of the individual. Consequently, climate change does not actually present environmental pressure for humans to evolve biologically. This is in contrast to the lizards which had to adapt to a new environment with different food sources than what they were used to and adapted to before. That's very strong evolutionary pressure, which leads to very rapid evolution - if you can't deal with the new food, you're out. --Wrongfilter (talk) 17:03, 28 June 2012 (UTC)


 * Agreed, to quickly evolve a species must suffer massive fatalities (or infertility) from the condition to which they must adapt to survive. StuRat (talk) 20:01, 28 June 2012 (UTC)


 * The question you have to ask is, how would climate change prevent people from reaching adulthood and/or having children? Lamarckian evolution, where individuals pass on hereditary characteristics acquired during life is a fallacy - evolution is essentially driven by premature death. My guess as to the biggest climate change driven force would be the increased prevalence of tropical diseases such as malaria, with them extending into areas and populations previously unaffected. As said above, humans reproduce (and therefore the species evolves) very slowly however. LukeSurlt c 00:54, 29 June 2012 (UTC)


 * Actually, there seems to be some support for a very minor effect similar to what Lamark proposed. There was a study showing that the descendents of people who had suffered malnutrition (in Norway ?) had altered DNA.  It seems that  periods of starvation cause one's descendents to put on more weight, when the opportunity arises.


 * As for malaria, yes, that is a major killer, and humans have evolved a response, in the sickle cell gene, which might become more prevalent as malaria spreads. StuRat (talk) 06:59, 29 June 2012 (UTC)


 * I'd say your teacher has oversimplified matters a bit. In fact, there are occasions in which organisms with complexity equatable to that of a human have, in the context of a drastic environmental change, develop highly efficient adaptations within a handful of generations,  and some of these are quite transformative to the species and it's interaction with it's surrounding ecology.  As others have noted above, there are limiting factors -- the relative length of the human life cycle (less overall individuals and thus less occurrences of random mutations to generate traits that will be selected for as favourable) and our tendency towards developing technical solutions to problems -- though that second one could be viewed as an adaptation to the changing environment as well, since, if we solve the problem through technical means, it is (speculatively) possible that technical inclination might have been selected for.   But I gather when you say "adapt" you mean specifically overt physiological changes, like different profiles and body sizes, increased water retention, adaptations of the eyes to deal with glare or the ability to mentally enslave giant sandworms.  While these types of changes (bar maybe one or two of the examples) can occur (and indeed, as has been suggested above, some of these traits might exist in a sub-portion of the general population as genes just waiting for a big break like this), the long and the short of it is this - we just don't know.  Short of actually going there, we can't be absolutely positive about what traits might be selected for, nor what new changes to genomes might occur.


 * If the underlying question for your inquiry boils down to "Can we count on just adapting on time?" then the answer is no; even at an absolute most "optimistic" appraisal of how much we could physiologically adapt in the next few centuries (barring directed genetic manipulation, which is certainly not just the stuff of sci-fi as it used to be, but is a whole other can of worms), we could not change enough to avoid the significant fallout of a genuine global ecological collapse and colossal numbers of people would suffer and die.  You must also take into account that evolution is a balancing act: organisms make trade-offs, in the theoretically sense, for new adaptations.   They may necessitate a higher consumption rate of resources, influence life cycles,  or otherwise inchworm us away from other adaptations that may have proven more beneficial in the long run.  Lastly, you must consider the context -- it's not just the individual organism's (or even the individual species') ability to cope with a different temperature or do with less of increasingly scarce resources that matter.  In the event of severe food shortages, societal cohesion would suffer and our ability to affect novel new technical solutions -- or even to sustain the stop-gap measures and normal application of technology that we already posses -- would be greatly diminished.  Likewise, in the event of widespread environmental collapse, both natural selection and technical ingenuity can only go so far in countering the effects of mass extinctions.  Even if a majority of species find ways to adapt, the inability of a few keystone species to do so could none-the-less cause the collapse of entire food-chains.  So finding technical solutions to avoid such rapid changes is definitely a more secure approach to the problem than hoping for slightly more advantageous perspiration rates or what-have-you.   Although it would be fun to ride a giant sandworm... Long live the fighters! Snow (talk) 08:55, 30 June 2012 (UTC)


 * The question is actually pretty irrelevant:


 * We know humans can survive much higher temperatures than many regions ever experience.


 * Humans have actually "de-adapted" to the weather, with many being practically dependent on air conditioning even in mild climates like we have in the northern U.S.


 * It's not the heat that will kill you, it's having your farmland turn into desert.


 * That said, I would agree that in a comparatively short term (10-20 generations) you can see many effective adaptations in experimental populations. A 200-400 year timeline is compatible with global warming; by the end of that time, I suppose, there won't be any fossil fuel left to burn and there will have been some very remarkable changes.  If those changes can be survived, they will be survived, by people with some degree of genetic advantage.  Note that populations in Africa, which have the largest reservoir of genetic variation, will likely do better at this if the most extreme case is tested than Caucasion, Asian, and Native American populations which were adapted for thousands of years to high cold terrain and now have fewer alleles to work with.


 * In addition to this, epigenetic changes may or may not help humans adapt to the change faster. My guess is that they will do so very effectively, because this isn't the Earth's first rodeo where global climate change is concerned.  The epigenetic changes may help both for children to inherit and build on key regulatory responses by adults, and potentially, might even make the genes involved more prone to mutation (e.g. transitions at methylcytosine; I also still have my suspicions about repeat element insertions).


 * Genetic engineering might also be a factor. We know that some species maintain a higher body temperature than humans, and adapting proteins to survive higher temperatures is pretty standard biotechnology by now.  It is conceivable that in the future, for example, a sperm sample could be run through a machine, coherent terahertz radiation could be used to identify the sperm carrying only ten or fifteen of the preferred thermostable alleles, and only those used for breeding.  Or, a more active approach of altering the genes directly could be perfected.  All this and much more will probably occur long before things start getting hot, until you have people running around with body temperatures of 45 Celsius.


 * All that said, if the heating causes methane clathrates at the bottom of the ocean to rapidly bubble up, and the methane further enhances global warming, and hydrogen sulfide starts getting released from the heat, and we start seeing a Permian extinction going on, then the Earth could be a very hostile and alien planet by the time it's over. And there's even a theoretical point, I think, where the tropical oceans could start boiling, the steam absorbs more infrared, and the Earth goes the way of Venus.  Over billions of years the Sun gets hotter and hotter - it's only a matter of time until it does anyway.  I haven't looked into the current status of such speculations, which I assume are difficult to assess because there is no past data we can extrapolate from. Wnt (talk) 14:11, 30 June 2012 (UTC)
 * Apparently the current status of those speculations is "virtually no chance". 112.215.36.172 (talk) 02:57, 1 July 2012 (UTC)
 * Thanks for the link to that section... but that's not what I'm getting from reading it! Specifically . Wnt (talk) 21:02, 2 July 2012 (UTC)

Resistivity of plastic cover to heat in presence of water
WHEN A PLASTIC COVER FILLED WITH WATER, HELD TIGHTLY AT THE OPENING OF THE COVER[AT THE TOP] AND IF THAT COVER IS EXPOSED TO HEAT WHETHER THE HEAT SOURCE MAY BE ANYTHING THE PLASTIC COVER MAY NOT DAMAGE NOTHING HAPPENS TO THE COVER..........,,,WHY ? WHY ? WHY ???????? — Preceding unsigned comment added by Madhu Sagar (talk • contribs) 10:48, 28 June 2012 (UTC)


 * Please note that typing all in capitals is like shouting - it is bad manners. In any case, what you have written is poor english and I cannot work out what it is you are asking.  You may like to clarify.  Wickwack120.145.74.189 (talk) 11:35, 28 June 2012 (UTC)


 * The water is absorbing the heat and pulling it away from the container fast enough that it can't get hot enough to burn. A common example is using a candle to boil water in a paper cup. 209.131.76.183 (talk) 11:58, 28 June 2012 (UTC)


 * It's the same with paper. Just Google "paper kettle instructions", or see here for a paper frying pan.--Shantavira|feed me 11:19, 29 June 2012 (UTC)

Where was the first ever plastic bag ban implemented?
After asking all these questions about plastic bags bans, I was wondering, once and for all, where was the first plastic bag ban in the world implemented? I know that San Francisco was the first American city to ban such bags, and Bangladesh was the first country to do so, but where was the first ever such ban implemented? Was it Bangladesh, a small village in Australia, or somewhere else? Narutolovehinata5 tccsdnew 11:24, 28 June 2012 (UTC)
 * "Ireland began charging 19 cents for each single-use plastic bag taken at the checkout in 2002, reducing consumption by 95% and raising millions of dollars for environmental programs. In 2001, Taiwan instituted a 3-cent fee and reduced single-use bag consumption by 69%. In 2002, floods caused by plastic bags choking drainage systems led Bangladesh to ban plastic bags entirely." from http://plasticbaglaws.org/wordpress/wp-content/uploads/2010/02/The-Evolution-of-SFs-Plastic-Bag-Ban.pdf --Canoe1967 (talk) 19:17, 28 June 2012 (UTC)


 * As noted above by Canoe1967, it seems that the first total ban was Bangladesh in 2002. See Top 5 places with plastic bag bans. Coles Bay, Tasmania gets a mention here. National Geographic article here, Plastic-Bag Bans Gaining Momentum Around the World. See also the only 3 weeks old WP page,  Phase-out of lightweight plastic bags - 220  of  Borg 05:53, 30 June 2012 (UTC)

12th Century people in 21st Century times
I remember watching a scene in the film Just Visiting where the two 12th century characters were vomiting in a car, saying that it was going too fast. I was just wondering if the 12th century people were time traveled to today, what shocks to the system whould they experience medically, psychologically, etc.? Reticuli88 (talk) 18:33, 28 June 2012 (UTC)


 * They would have no immunity to many current diseases and would likely be carrying a few, as well as lice and worms, themselves. μηδείς (talk) 19:51, 28 June 2012 (UTC)
 * what kind of worms? Reticuli88 (talk) 19:57, 28 June 2012 (UTC)
 * Cestoda - tape worms. --Tagishsimon (talk) 19:59, 28 June 2012 (UTC)


 * Psychologically, culture shock is the relevant concept. 12th century to 21st century will be far more extreme than our article suggests, quite possibly to the point of being functionally debilitating. &mdash; Lomn 20:12, 28 June 2012 (UTC)


 * They would be psychologically disturbed by the fact that everyone else would say they smelt bad. HiLo48 (talk) 20:22, 28 June 2012 (UTC)


 * It's highly unlikely they would understand what people are telling them. Is there any current language that is mutually intelligible with it's 12th century "ancestor"? Roger (talk) 20:39, 28 June 2012 (UTC)


 * Church Latin is a good bet. --Carnildo (talk) 01:00, 29 June 2012 (UTC)
 * How about danish? Plasmic Physics (talk) 03:34, 29 June 2012 (UTC)
 * Icelandic, maybe. --  ♬  Jack of Oz  ♬  [your turn]  04:50, 29 June 2012 (UTC)
 * Hebrew. 112.215.36.174 (talk) 07:39, 30 June 2012 (UTC)

Well, in the movie, a car moving at 20 miles/hr was too much for them and kept telling the driver "too fast" while barfing, providing comedy relief for the movie. I wondered would that happen to someone in reality if they were never in a car before? Also, the culture shock thing was referenced in the movie, when the two characters stepped out into a busy city street. They were overcome with the noise and the movements because they could not comprehend what was happening and ran to the nearest shelter to hide. Another scene was when they approached a modern toilet. One of the characters thought of it as a wash bowl and promplty washed his face with toilet water.

I wonder what would happen if someone transplanted us to 700 years from now and how we would be 'shocked' or what behaviors or mannerism would be considered backward or old fashioned, etc. Reticuli88 (talk) 21:23, 28 June 2012 (UTC)
 * A horse at full gallop can run well over 20 mph, and they had horses then, right? ←Baseball Bugs What's up, Doc? carrots→ 02:47, 29 June 2012 (UTC)


 * There would be some rough real-world parallels - Uncontacted peoples are generally expected to be living a stone age-like existence. There will be records of what happens when first contact with between the outside world and these isolated tribes occurs. That said, such peoples don't get transported to "very modern" urban environments such as Chicago. LukeSurlt c 00:41, 29 June 2012 (UTC)
 * A hint of what it might be like is sometimes made public when someone awakens from a coma after several years. ←Baseball Bugs What's up, Doc? carrots→ 02:47, 29 June 2012 (UTC)
 * The behaviour of the visitor from the past would resemble that of this rat but with (hopefully) some ability to empathise and adjust. DriveByWire (talk) 15:23, 29 June 2012 (UTC)


 * May I recommend the French film Les Visiteurs which has exactly this scenario. Tres drole. Alansplodge (talk) 18:26, 29 June 2012 (UTC)
 * D'oh! I've just seen that the film you linked is a Hollywood remake of the French original. I still recommend it though - US remakes of French films are always a bit poor in comparison. Alansplodge (talk) 18:40, 29 June 2012 (UTC)

recent xkcd cartoon. SCNR Уга-уга12 (talk) 19:34, 29 June 2012 (UTC)


 * There's a New Zealand (iirc) film where a child is transported from the 14th century to 1980s Auckland (iirc) in order to get some iron work done. The child's religious world view allows them to adapt and assimilate the variety of wonders, and eventually find a 24 hour forging metal shop.  Fifelfoo (talk) 03:09, 1 July 2012 (UTC)
 * The Navigator: A Medieval Odyssey Fifelfoo (talk) 03:10, 1 July 2012 (UTC)

Evolution
''Non-question removed. Please don't use the reference desk as a forum to post links for the purpose of general discussion.''112.215.36.172 (talk) 04:48, 29 June 2012 (UTC)
 * Link to the deletion: . Buddy431 (talk) 00:16, 30 June 2012 (UTC)

Cost of interstellar colonization
I'm trying to understand the construction cost assumptions in Ceyssens et al (2011) relative to the much more rigorous Moir and Barr (2005) which doesn't discuss construction costs, only acceleration costs. Assuming a sleeper ship is more economical than a generation world ship in this case, are Ceyssens' estimates unreasonably high? 75.166.192.187 (talk) 22:22, 28 June 2012 (UTC)


 * I'm not familiar with the papers you cited (I will look into them), but isn't estimating the cost of a sleeper ship kind of difficult given that they don't actually exist? A generation world ship is, roughly speaking, plausible with our current level of technology. Sleeper ships are a whole nother deal, and to my knowledge actually getting one up and running would be contingent on some unforeseeable advancement in cryonics, the cost of which is surely incalculable (in the true sense of that word, not the "really, really big" sense). Evanh2008 (talk 03:24, 29 June 2012 (UTC)


 * I don't think so. The capacity of vitrification freezers increases at a fairly constant rate. We can make assumptions such as that suspending and reviving someone would generally require an anesthesiologist and a couple clinicians on hand, except for the first people revived, which would have to be done by robot. I think the biggest expense is for the landing craft. 75.166.192.187 (talk) 04:55, 29 June 2012 (UTC)


 * And another option is the have the vast majority of the crew in stasis, while a small crew, perhaps multi-generational, maintains the ship. Or, instead of one group of people kept awake permanently, you could wake each person in stasis, for maybe a 1 year maintenance shift, then put them back under. StuRat (talk) 06:28, 29 June 2012 (UTC)


 * For voyages on the order of 50,000 years, I think everyone is going to want to sleep through them in stasis. 75.166.192.187 (talk) 06:39, 29 June 2012 (UTC)


 * Another reason to wake occasionally is that the stasis won't be quite perfect, and waking them would allow their body to repair accumulated damage from gamma rays, etc. StuRat (talk) 06:48, 29 June 2012 (UTC)


 * Also, having the entire crew in stasis, perhaps even frozen in liquid helium where the human body becomes far more hardy, you could accelerate the ship at rates that would kill an unfrozen human. As for coming out of stasis to repair damage, even with future technology the act of freezing/unfreezing may still cause far more damage than the inbetween. Someguy1221 (talk) 06:51, 29 June 2012 (UTC)


 * I agree with waking occasionally, on systems alarm or timer if all clear, to measure health and radiation exposure (which you would want to minimize with asteroid rubble shielding) because nobody will have ever been in stasis for that long. Primary power would probably be aneutronic fusion but a sleeper ship could probably work on hydrocarbon fuels. The difference would be how fast it could accelerate. However, I'm not sure how much of a difference that would actually be, because propellant is limited in both cases.  I am not sure about the radiation/acceleration trade-off.  Slow acceleration over lengthy periods of time can indeed achieve impressive speeds, but you need almost the same amount of propellant to slow down at the other endpoint. The optimization problem is probably about the same with and without the relativistic component. 75.166.192.187 (talk) 18:05, 29 June 2012 (UTC)


 * I don't think the acceleration rate would be much of a problem. The ship could accelerate at 1 g for around a year to get close to the speed of light, then cruise weightless, and again decelerate at about 1g for another year at the end.   You could accelerate faster, but that would require bigger engines, and more fuel, etc., and only save a few months off your 50,000 year trip, and make any humans who are awoken early during an emergency unable to move, so it seems like a bad idea. StuRat (talk) 04:18, 30 June 2012 (UTC)


 * You're forgetting the relativistic effects. It only shaves a few months off the journey from the point of view of someone staying behind on Earth. From the point of view of someone on the ship, a 50,000 light year journey done by accelerating at 1g for the first 25,000 light years and then decelerating at 1g for the second 25,000 light years would only take only 21 years (it takes 50,037 years from the point of view of someone on Earth). Increase the acceleration to 2g and it only takes 11 years. See Space travel using constant acceleration and the calculator on the linked page (it's not a very good article, but the facts are right). If you can sustain accelerations of 1g or so indefinately, and can create a ship that can survive travelling through the interstellar medium at relativistic speeds (from the ship's point of view, it is getting bombarded by high energy particles constantly), then for the traveller journeys can be pretty short (a million light years at 1g only takes 27 years). Unfortunately, creating a ship that can do all that would be very difficult, so you're likely to have to travel at non-relativistic speeds. --Tango (talk) 14:01, 30 June 2012 (UTC)


 * But there's little diff between 11 years and 21 years, if you are in stasis at the time. And doubling the size of the engines to get 2g might not be practical. StuRat (talk) 20:16, 30 June 2012 (UTC)


 * Closer to 0.01 g for something on the order of months, I would think. I'll raise the question again when I get some decent mass estimates for what the generation ship people say they would need to end up with. Designing the beaching yachts is difficult. Being able to refuel and move on to a secondary destination undetected is a must. 75.166.192.187 (talk) 18:18, 30 June 2012 (UTC)


 * Undetected by whom? If there are alien civilisations capable of detecting you, then I would think making contact would be a primary goal of the mission. And where are you planning to refuel? I think you would be better off going directly to your destination. There isn't likely to be another system en route without going a long way off course. --Tango (talk) 18:39, 30 June 2012 (UTC)


 * There are some civilizations you would want to wait before establishing contact with. Unstable civilizations, well, you might want to buzz them and turn off their nukes, but not much else. 75.166.192.187 (talk) 19:07, 30 June 2012 (UTC)
 * What civilisations are you wanting to wait before contacting? If you mean primitive ones (ie. a Star Trek style prime directive), then they won't be able to detect you anyway. You wouldn't go to an inhabited planet to refuel, there are plenty of better places in a solar system (without the big gravity well of an Earth-sized planet). If you want to stay undetected, you only have to worry about space-faring civilisations (or, perhaps, ones with big ground-based radio telescopes). --Tango (talk) 23:05, 30 June 2012 (UTC)
 * Potentially dangerous civilizations. 75.166.192.187 (talk) 01:24, 1 July 2012 (UTC)


 * There is a science-fiction short story by A. E. van Vogt called "Far Centaurus" that explores precisely this idea. There are four crew-members who each get woken up for a few hours every 50 years or so to monitor the systems. It also points out the big problem with such long journeys (in the story, I think it was 300 years, but the problem would be much greater for a 50,000 year journey). It's a really good story, so if you don't want to know the plot twist, stop reading now! They get overtaken by faster craft that were invented during their journey and arrive to discover an entire colony has been established. --Tango (talk) 22:46, 29 June 2012 (UTC)


 * Why is that a problem? Is that not the optimal outcome for the colonists, and would yield them professorships of history at the colony's universities? 75.166.192.187 (talk) 18:13, 30 June 2012 (UTC)


 * Their goal was to be the first to get there and they failed (they weren't colonists, they were just a small team of explorers). It meant their mission was pointless. If they just wanted to visit the future, they could have gone into stasis on Earth. --Tango (talk) 18:39, 30 June 2012 (UTC)


 * From the colony backers' point of view, a soft landing is a sub-optimal success with flying colors. There is no reason that the faster ship shouldn't be able to send a skiff to pick up the passengers, or simply bring the slower ship in tow. 75.166.192.187 (talk) 19:12, 30 June 2012 (UTC)
 * But what is the benefit of sending the slower craft if it is likely to get overtaken? You are better off just waiting until you've invented a faster craft. --Tango (talk) 23:05, 30 June 2012 (UTC)
 * And that's why I haven't bought a new computer since 1975. 112.215.36.172 (talk) 00:44, 1 July 2012 (UTC)

Clearly, humans are not suitable to colonize the galaxy, this will be done by machines. Self replicating machines will travel from one solar system to the next, making copies of themselves using solar energy and the materials they find. If the time to travel from one solar system to the next and to get ready to move away from there, is 1000 years, then the entire galaxy will be colonized in about 1000 Log(10^100)/Log(3) years = about 210,000 years (the galaxy is approximately a flat disk, so you can model the colonization process as an exponential growth where in each step you can move to 4 neighboring solar systems, one of which is where you came form, so 3 new places to move to). The entrire energy output of all the stars in the solar systems in the galaxy will be available to this huge machine civilization. Count Iblis (talk) 23:14, 29 June 2012 (UTC)


 * Then why haven't machines from elsewhere gotten here? Bubba73 You talkin' to me? 06:52, 30 June 2012 (UTC)


 * See Fermi paradox. (WP:WHAAOE) --Tango (talk) 07:55, 30 June 2012 (UTC)


 * I strongly disagree. Humans are the original von Neumann machines on this planet, and therefore the most suited to explore. Robot probes will seem quaint with another decade of telescopy. 75.166.192.187 (talk) 18:13, 30 June 2012 (UTC)


 * So either technological civilizations are very rare or that type of colonization of the galaxy isn't feasible. I'd put my money on the latter. Bubba73 You talkin' to me? 18:16, 30 June 2012 (UTC)


 * Best estimates are that technological civilizations are separated by about 5,000 light years on average, not counting colonies. However, the most discriminate factor in expanded Drake equations at present is the proportion of planets with the right amount of water to support surface life, so we should be looking for exoplanet ozone. 75.166.192.187 (talk) 18:37, 30 June 2012 (UTC)


 * How many would that be in our galaxy? Bubba73 You talkin' to me? 19:34, 30 June 2012 (UTC)
 * About fifteen. 75.166.192.187 (talk) 21:41, 30 June 2012 (UTC)


 * Those "best estimates" are worthless, based on fabricated numbers. We have no real data to support any estimate of that sort. "Humans are the original von Neumann machines on this planet, and therefore the most suited to explore" is a bizarre statement. Humans are not the original von Neumann machines on this planet, and are obviously incredibly ill suited to extraterrestrial travel. -- BenRG (talk) 22:24, 30 June 2012 (UTC)


 * The Technical Director of Scientific Space Exploration of the International Academy of Astronautics does not share those opinions. For what reasons do you say that humans aren't the original von Neumann machines? 75.166.192.187 (talk) 01:24, 1 July 2012 (UTC)
 * What does that even mean? Wouldn't the original von Neumann machines be single-celled? And if somehow this were true, how would it make humans suited for exploration? It's a non sequitur, and the consequent is false in any case.
 * I'm not going to watch an hour-long Youtube video, and I'm not sure why you think that the International Academy of Astronautics has any credibility, or why it would matter if it did. Real science is based on empirical evidence and the careful identification and minimization of all sources of error. The Drake equation is a good example of what Feynman called cargo cult science. It's a formula into which people substitute made-up numbers and believe that the result is more scientific than it would have been if they'd made it up directly, because it came out of a formula. -- BenRG (talk) 06:52, 1 July 2012 (UTC)
 * Von Neumann machines are aware of their own construction and can treat their instructions as data. Perhaps you think some national academy of astronautics is more credible? Maccone's statistics have been reviewed by both frequentists and Bayesians. 75.166.192.187 (talk) 22:44, 1 July 2012 (UTC)


 * Note that the Drake equation was not meant as a predictive tool, but as a curiosity to guide discussion at a Green Bank conference. To be sure, people misuse it all the time, but to claim that it's cargo cult science is like saying evolution is cargo cult science because some people try to use it for eugenic purposes.  --140.180.5.169 (talk) 15:46, 1 July 2012 (UTC)
 * There are hundreds of peer reviewed papers expanding on the Drake equation, some by Drake himself. 75.166.192.187 (talk) 22:44, 1 July 2012 (UTC)


 * Indeed, we don't really have any estimates worth calling "best". We do, at least, now know that planets are commonplace and habitable planets are very likely to be commonplace (which is a start - we didn't even know that 10 years ago). We still only have one data point for determining how likely it is for life to arise and, if life does arise, how likely it is for a technological civilisation to arise. One data point is enough to conclude precisely nothing (although taking the Earth at different points in its history as separate data points can give us something - the speed with which life arose on Earth after the oceans formed is suggestive of a very high probability of it happening, the very long period before multicellular life arose, though, suggests that is less likely). We don't even have one data point for what I consider to be the most significant factor, which is how long technological civilisations exist for (and, therefore, how likely there are to overlap). We know it is possible for a civilisation capable of radio communication to exist for a hundred years, but that's all we know. It could be that such civilisations usually wipe themselves out within a few decades (and we've just been lucky) or it could be that such civilisations tend to exist indefinitely once they arise. We have no way to know, and that makes a really big difference to any estimate of how many civilisations we may be able to find out there. --Tango (talk) 23:05, 30 June 2012 (UTC)


 * And you can't do statistics with one data point. And we should exclude ourselves at any rate.  Bubba73 You talkin' to me? 23:10, 30 June 2012 (UTC)


 * It could be that machine civilizations degenerate over time, losing the capability to coordinate things on a large scale. If we become robots in the future, then we can travel by uploading our brains from one machine to another machine. So, we don't need spacecraft to travel, we simply travel at the speed of light by radio communication. Some individuals will want to make copies of themselves, and if this is not limited in some way, the person who is inclined to make the most copies will dominate the civilization. The question is then how one should police against people making copies of themselves so that the integrity of the machine civilization is maintained on the long run... Count Iblis (talk) 00:03, 1 July 2012 (UTC)
 * Write a novel about it. Don't treat it as "science", please. The state of science education is bad enough as it is. -- BenRG (talk) 06:52, 1 July 2012 (UTC)
 * The state of science education is bad because we only teach the same stupid boring sums over and over again in school. Count Iblis (talk) 15:31, 1 July 2012 (UTC)


 * Hypothetically, if machines are going to colonize the galaxy, then I wonder which planets / moons they would prefer? In terms of building durable, long-lasting technology there would seem to be some real advantages to avoiding both weather and biological life.  Robots, at least the simple ones that humans have been able to make, tend to operate best is simple and predictable environments.  A robot colonist first arriving in the solar system might well think that the Moon or Mars is a more inviting first stop than a place like Earth.  Engineers motivated by "moral" concerns might even tell the robots to always avoid planets (or even whole systems) with biological life.  Dragons flight (talk) 23:57, 30 June 2012 (UTC)


 * Position preferences are things that life has, but I'm not sure if machines would have strong position preferences. 75.166.192.187 (talk) 01:31, 1 July 2012 (UTC)


 * Large robots might prefer asteroid belts or planetary rings, as those provide lots of materials, sunlight (for solar power), and less gravity/atmospheric resistance to fight against (especially if they want to leave orbit). Smaller bots would have insufficient shielding to protect their electronics, control systems, etc., from radiation damage and micro-meteors. StuRat (talk) 02:26, 2 July 2012 (UTC)
 * That is why Asteroid 2010 SO16 should be explored. 75.166.192.187 (talk) 19:06, 2 July 2012 (UTC)


 * The cost will inevitably depend on the technology. We're speaking of a centuries time scale here.  Could Charles Babbage give you a cost estimate for a good gaming PC?  It makes a difference whether your rocket fuel is bulk hydrogen and oxygen lightly mixed versus nuclear power or antimatter or something goofy like Zero-point_energy.  The cost estimate in the end, of course, will be "a lot, but not more than we can afford", because until then the project will not be started. Wnt (talk)`