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.
I can't find any information on dystonia affecting the lower back. Does it not exist? Is it extremely rare?
Also, are there any muscles in the lower back which, upon contraction or spasm, cause the body to hunch forward? From what I've been able to find out, bending forward is caused by contraction of the abdominal muscles only and the back muscles maintain an upright posture or arch the back.
I have no answer to the first part of your post. Our page for dystonia is not very clear and repetitive in parts (two times "general dystonia" for example). For the second part: It is impossible for muscles in the lower back to cause a forward movement of the body. Muscles are always long when relaxed and "shorten" when they tense. So back muscles work to shorten the back and abdominal muscles contract the abdomen. So both muscle groups work together (called antagonistic muscles) to determine body posture and to perform controlled movements. TheMaster17 (talk) 10:41, 24 March 2009 (UTC)[reply]
Thanks for confirming the info about the back muscles, that's what I thought. I would guess then, that IF dystonia of the lower back does exist, the sufferer would NEVER hunch forward at the time of a spasm? (edit: what is causing me some confusion on this is that I recall, several years ago, pulling a muscle in my neck which caused my head to stay stuck tilted and twisted to one side. I was told that it was the muscle on the opposite side (the elongated one) that was injured, not the one on the side to which my head was sticking. So, while it definitely makes more sense to me that dystonia of the back could not cause a forward bending posture, my experience with my neck reminds me that the body doesn't always seem to work in the most immediately logical way. Or maybe the doc who told me that about my neck was wrong and it was the muscle on the shortened side of my neck that was strained?) bcatt (talk) 16:03, 24 March 2009 (UTC)[reply]
Why is it widely believed that everyone has different fingerprints? It's not beyond the realms of possibility that two people who are distant and unknown to each other could have the same prints. JCI (talk) 02:12, 24 March 2009 (UTC)[reply]
Francis Galton showed back in the 19th-century that statistically the odds of two people having identical prints is pretty much impossible. This is part of what made fingerprints acceptable evidence to courts. I'm fairly sure you are more likely to have 17 matching snippets of DNA (or whatever the standard matching SNP number is) than a full set of identical prints. --98.217.14.211 (talk) 02:47, 24 March 2009 (UTC)[reply]
But, of course, the problem comes in when comparing fingerprints (especially partials) to a database containing millions. With that many to choose from, you're sure to find several fairly close matches. But, when used properly, say by comparing bloody fingerprints at the scene with the prime suspect's, then the chances of getting a false match are extremely low. StuRat (talk) 05:21, 24 March 2009 (UTC)[reply]
What Galton calculated (the calculation was not a proof in the mathematical sense, more an informed estimate like the Drake equation) was the probability of two different prints matching in all their minutiae (Finger Prints, 1892, Ch. VII). There are tens of thousands of minutiae in one print. Fingerprint analysts, however, do not compare all of them. In fact, the number of minutiae legally required for a match is in the low tens, typically (Criminalistics, James Girard, p149). So Galton's huge probabilities, even if they are correct, tell us absolutely nothing about the reliability of modern practice. --Heron (talk) 23:35, 24 March 2009 (UTC)[reply]
It depends on what you mean by "identical" - if you take two objects that are intended to be identical (two pennies for example) - they aren't REALLY identical, each has little nicks and scratches that distinguish it from the other. You can't imagine making any pair of physical objects that were utterly identical in every way. In that regard, I would maintain there are NO pairs of objects of any kind (at the 'macro-scale' at least) that are identical. Sure, you can talk about two Hydrogen atoms being identical - but not anything of any size.
So this oft-repeated claim that no two fingerprints are identical is OBVIOUSLY true at some scale of examination. There are something like 1020 atoms in your finger - it's clearly impossible that all of the atoms in someone else's finger are arranged precisely the same way. But it's not a particularly interesting or useful claim since no two of ANYTHING are utterly the same.
However, there aren't that many patterns of loops and whorls out there - there simply can't be. So at some other scale of description (perhaps a layman who knows nothing of fingerprint analysis using nothing but naked-eye examination) there are clearly pairs of fingerprints that are so similar that we'd need an expert to tell them apart. So I regard this whole thing with a deal of skepticism. Francis Galton's "calculations" cannot possibly be correct - there is simply not a solid point at which you can say "sufficiently identical to count" without very carefully defining you means of measurement and the errors inherent in making those measurements. Remember that your fingertips get cut and damaged - they grow callused if you work with certain tools - they are continually renewed and regrown. They change over time as our fingers grow from little baby fingers into adulthood. They get wrinkled up when you get them wet...so my fingerprint today is not the same as it was yesterday - at some level of examination.
The measure that I think is worth examining is whether your fingerprint at (say) age 5 years is more similar to your fingerprint at (say) age 75 years than to any one else's fingerprint in the world. I think that's a much harder standard to meet and I'd be very surprised if that were true.
It's on, Steve Baker!! You say "However, there aren't that many patterns of loops and whorls out there - there simply can't be". how much informatino entropy does a fingerprint OBSERVATION (for comparison with other fingerprints) contain? answer THAT question. 79.122.44.240 User ID added by Sifaka
What exactly do you mean by "information entropy"? Steve is right in saying that there are relatively common patterns in fingerprints, see Fingerprint#Classifying fingerprints to see what I mean. According to this paper there are more errors when comparing two fingerprints from the same finger but decades apart than two fingerprints from the same finger taken recently using a minutiae based recognition approach. A quote from the aforementioned article:
"From a theoretical point of view it is common sense that aging may not impact the characteristics of fingerprints [l]. However for practical purposes, scaling effects of minutiae based matching algorithms may render older templates useless... In most cases, the ageing process does not change the structure of the fingerprint image. The ridges in the epidermis (dead dry skin) always show the same pattern since the information thereof is stored in the lower layer of the finger (dermis - live skin). If an injury of only the upper skin is sustained, after a certain time the same ridges are formed as before. Even the ageing process cannot change the paths of the ridges. The fingerprint may be a little larger, the ridges may be lower (if they were worn due to working), and the finger may show some wounds. However, the pattern always remains the same. Therefore, it should not be difficult, for the different verification algorithms, to identify fingerprints of the same finger, which only differ in the date of their acquisition, as being identical."Sifakatalk18:28, 24 March 2009 (UTC)[reply]
The reason I addressed Steve Baker is that he is the only one here who knows what information entropy (Shannon entropy) is and can apply it to fingerprint observations!! But hes too squeemish to do so apparently...
You can't apply that fundamental information-theoretical approach when the terms involved are so vague. Let's read what our article says:
In the Henry system of classification, there are three basic fingerprint patterns: Arch, Loop and Whorl.[8] There are also more complex classification systems that further break down patterns to plain arches or tented arches.[7] Loops may be radial or ulnar, depending on the side of the hand the tail points towards. Whorls also have sub-group classifications including plain whorls, accidental whorls, double loop whorls, peacock's eye, accidental, composite, and central pocket loop whorls
So if I take that at first sight: There are three kinds (Arch,Loop,Whorl) - but there are really two kinds of arch, two kinds of loop, seven kinds of whorl...so there are eleven kinds of fingerprint. The probability of two fingerprints being identical is one in eleven and certainly there are billions of people with "identical" prints. Well - no. The print can be bent and twisted or closer to the fingertip or off to one side. Suppose we use the distance between the skin fold at the joint to the center of the arch/loop/whorl - well, if we measure that distance accurate to the nearest millimeter - then perhaps there is between 1 and 20 millimeters between 'fold' and 'feature' - so now there are 20 different kinds of eleven different features - so we have 220 different fingerprints in the world...but suppose we measure accurately to half a millimeter - now there are 440 different prints - but if we only measure accurately to 2mm - then there are only 110 different prints. If you alter the precision and complexity of what you measure - you can make the answer come out to anything you want. If I want it to come out that all 7 billion people in the world have "different" prints - I can just measure enough subtle parameters to enough precision and claim that statistically, it must be so. But it's meaningless. If I measure to less precision - there are fewer "unique" prints...if I just look at the gross 'shape' then there only 11 unique prints. You just can't attach a number to that. All we can say is that fingerprints are more unique than (say) pennies - but less unique than (maybe) snowflakes. But that's just a gut-feel thing - it's not science. In scientific terms - the print you have now - is different from the one you had when you started reading this sentence because a few skin cells have fallen off in the meantime. This is truly a bullshit thing...it's politics on behalf of crime-fighters and generally the stuff of urban legends. They say nobody ever got mis-identified because of their prints - but how would we know? If there were then they were misidentified for chrissakes! SteveBaker (talk) 00:49, 26 March 2009 (UTC)[reply]
tsk tsk Steve Baker, you're not thinking hard enough. You can give a range of entropies, depending on how good of an expert you pick, but the fact is, there are going to be an n number of fingerprints for which a given expert will say they are that of a different person, and the 2-based logarithm of n will give you the number of bits of entropy in the coding of that observer. You follow? If you want you can say the number of bits of entropy "range from" and then go 6-whatever, because the worst "expert" actually makes observations according to criteria by which fingerprints fall evenly into one of 64 possible groups (where he would say that two fingerprints in that group are "of the same person", according to his observation [=his observational criteria]!). The best expert might make observations by criteria according to which fingerprints fall evenly into about a million possible groups. In this case it would have 20 bits of entropy. So you could say "fingerprint observations for comparison purposes have 6-20 bits of entropy depending on the expert making the observations" -- you would make this statement if in your estimation the worst experts WOULD, for a given print, answer "yes it is the same person" for the prints of every 64th discrete person whose prints you could ask them to compare to the one in front of them, and the BEST experts would answer "yes it is the same person" for every every millionth discrete person whose fingerprints you could ask them to compare it with. For the first, the expert compares the prints by 6 bits of observational entropy, for the second, by 20 bits. However I am just making these bit numbers up!!! I am calling you out, Steve Baker, to propose an informational theoretical number of bits of entropy in actual, real, honest-to-goodness, expert fingerprint observational criteria for comparison purposes!! And the reason I'm calling you out is because you're the only one here who can possibly comprehend what I'm even talking about.Ball's in your court, Steve Baker.79.122.75.197 (talk) 17:41, 26 March 2009 (UTC)[reply]
Your conduct is bordering on the disruptive. You are being argumentative. We don't call out individual editors here. Cut it out. - EronTalk17:49, 26 March 2009 (UTC)[reply]
For people other than Steve Baker: although it may look like I am being argumentatitve with Steve Baker, in fact he (unlike anyone else here) knows exactly what I'm talking about and will answer with a better attempt soon enough. I'm only "calling him out" in that 1) he is one to respond to a good-natured challenge, and 2) he's the only one here who can possibly answer the question addressed to him. However I will give others a chance to see the record, which will be Steve Baker's correct result given shortly. That's why I'm doing it here, so everyone can see the answer.79.122.75.197 (talk) 18:03, 26 March 2009 (UTC)[reply]
This isn't your private chatroom with SteveBaker, or any other editor for that matter. And I'd suggest that most editors here don't much like being told that they can't possibly comprehend you. - EronTalk18:31, 26 March 2009 (UTC)[reply]
While obviously, not being Steve Baker, my poor little mind cannot comprehend what you say, I will note that your capitalised 'would' seems rather odd, dismissing the whole probabilistic/expected value thing. After all, if people are giving the wrong answer at regular intervals, that's easy to correct for :P Or is the weird detached thing below written by you and supposed to fit into the above paragraph? 79.66.127.79 (talk) 20:23, 26 March 2009 (UTC)[reply]
attn steve baker: don't read the rest of this paragraph! -- No, the "weird detached thing" is for people like you, not to bug me about it. Steve Baker can understand information (shannon) entropy, which is about uncertainty and hence automatically includes...uncertainty. But why am I wasting your breath, you are re not Steve Baker -- so look, there's the weird detached thing written below, just for you! 79.122.75.197 (talk) 20:35, 26 March 2009 (UTC)[reply]
Was merely pointing out that you seem to think you are discussing this in a very deep, arcane sense, but appear to be addressing the problem in a very simple manner with the addition of a few well-known mathematical concepts. And while doing so, you appear to be skipping some basic and important ideas. But I'll leave this for Steve, if he feels like addressing it in your terms. 79.66.127.79 (talk) 20:46, 26 March 2009 (UTC)[reply]
Sure, I certainly I know about information theory - and so do many other here (and we have this big, impressive encyclopedia for the smart people here to go look it up in). But that doesn't matter because what I do' know is that it just isn't applicable here because we have no data to work with. There are bold assertions that no two humans have the same print so there are at least 7 billion different fingerprints - and yeah - we can take log2 of that and come up with a figure of about 32 or 33 bits of information content. But that's MEANINGLESS. If I observe the position of every molecule making up the print (let's say there are 1020 of them maybe) - we could measure the position of each one within a 1cm cube accurate to (say) a nanometer in three dimensions - measure the rotation of each molecule in nanoradians in all three axes and come up with some number which is something ungodly like a trillion bits of information. Or I, personally, might be only capable of recognising the two kinds of arch, two kinds of loop and seven kinds of whorl and come up with between 3 and 4 bits of information. Using information theory doesn't help in the slightest here - the answer is still "between maybe 3 and 1,000,000,000,000 bits of information depending on the quality of the observer" - that's such an astronomically vague answer as to be absolutely freaking useless. Hence you can keep chucking out stupid challenges and upsetting everyone here until hell freezes over - but applying information theory really doesn't help very much without carefully defining the limits of your observation. Hence the clearest answer for the OP is "Yes, all fingerprints are unique at a sufficient degree of observational precision - but it is far from clear what the observational precision of forensic labs is - so in that sense, it's still perfectly possible for there to be "non-unique" fingerprints at that level of observation.". Shannon's theorem has nothing whatever to do with that. SteveBaker (talk) 02:36, 27 March 2009 (UTC)[reply]
Suppose we use the distance between the skin fold at the joint to the center of the arch/loop/whorl — huh? Why not suppose we use the branching topology of the grooves/ridges, which (I gather) is what's used in fact? —Tamfang (talk) 22:25, 28 March 2009 (UTC)[reply]
Fingerprint evidence has been used in countless court cases. But there never seems to has been two persons found to have identical prints - not even for one finger, let alone all 10 fingers. If matching prints had ever been found, the occurrence would have received a lot of publicity. Also, from then on every defense lawyer would have fingerprint evidence thrown out. Even identical twins do not have matching prints, although their DNA is identical. Of course, matching prints of different people would have to be stumbled on by chance - fingerprint classification and searching is far from perfect. But with all the criminal cases that have used prints, two persons with the same fingerprint would surely have been found by chance. The problem with prints is that there is no known way to digitize the pattern. If there was, a computer could easily find identical prints. (Threshold scoring with a computer is useful when comparing two known fingerprints, but that is not a random search.) Illustrating this, a friend of mine had his place burgled and the burglar cut himself on the glass of the window he broke to get in, and he left a bloody fingerprint. The police said they could not try to match the print unless my friend could name 10 persons whose prints (if on file) could be checked for a match. That shows there is no way to make a full search of all fingerprints. (The FBI has a database of over 51 million prints.) Even when a match for the same person is found by other means, it is occasionally wrong due to police sloppiness (wrong name on the fingerprint card, etc.). See "Criticism" and "Errors in identification or processing" in the Wikepedia article Fingerprint.– GlowWorm.
The FBI has recently made false positive identification of an innocent person, Brandon Mayfield, as a terrorist bomber. Duplicate fingerprints or malfeasance? The proof of validity of fingerprint identification is generally lacking and is based mainly on hand-waving and unverified statistical assertions. Any forensic technique's validity should hold regardless of whether the government is out to get the individual. Edison (talk) 04:45, 25 March 2009 (UTC)[reply]
Yeah - exactly. It's observer bias: "Nobody ever gets misconvicted because of fingerprint evidence"...except perhaps for the people who were indeed misconvicted! If you could say "Everybody who was ever convicted of a crime on the basis of fingerprint evidence subsequently made a full confession" - then maybe. But I'm 100% sure that plenty of people who were convicted on this basis have screamed and kicked and protested their innocence all the way to jail. How do you KNOW you didn't misconvict one of two of them because their prints happened to be identical to the real criminal? SteveBaker (talk) 00:49, 26 March 2009 (UTC)[reply]
The chances for two people having the same fingerprints is slim enough, but iris patterns are about 6 times more unique, and each eye has a different pattern (they are actually using this technique for identification in some countries).
Renal Medullary Carcinoma (we don't have an article on this yet -- Done!) is a rare type of cancer that affects the kidney. It tends to be aggressive, difficult to treat, and is often metastatic at the time of diagnosis. It is not the same thing as sickle cell disease but the references in PubMed suggest that most individuals with this type of cancer have sickle cell trait or sometimes sickle cell disease -- meaning that the sickle cell trait may be a risk factor for this type of cancer (although it is still incredibly rare in people who carry the trait). See this for one of the first reports. There are also reviews available (here and here). I hope this helps. Clearly, anyone who has potential concerns about this disorder should see their physician as soon as possible. --- Medical geneticist (talk) 04:48, 24 March 2009 (UTC)[reply]
If humans manage, through war, climate change or any other mechanism, to cause our own extinction, then is it likely that some other sentient creature (not necessarily of a currently existing species, not necessarily originating on Earth, and potentially including the Creator if one exists who isn't already omniscient) will study our mistakes and learn from them? If so, does our capacity to serve as an example in this way increase with our peak population? NeonMerlin10:17, 24 March 2009 (UTC)[reply]
Given the openness of the scenario - yes clearly if some creature capable of study came about after the end of humanity then provided they can uncover a history of our demise they could use it as an aid to preventing their own demise. Of course knowing what causes something and avoiding that occuring are two different things. I don't see how a larger population makes it more obvious - apart from perhaps an increased chance of their being 'evidence' of our existence perhaps...but then i'd say a small population of technologically advanced citizens are more likely to be 'findable' than a mass-population without technological advancement. 194.221.133.226 (talk) 10:25, 24 March 2009 (UTC)[reply]
Well, given the difficulty of interplanetary travel - it's unlikely that aliens would come here. Doubly so if there were no intelligent lifeforms left here. So I think that's really highly unlikely. Another possibility might be that without humans, some other species that could survive whatever we screw up might evolve to eventually reach our levels of intelligence, curiosity and creativity. They might well be able to use archeological approaches to discover who we were and how we screwed up. Sadly, the most enduring things we'll have left behind are things like non-biodegradable plastic waste in landfills and nuclear waste bunkers - which won't speak well of our good sides. What's sad is that it's unlikely that our crowning achievements will survive - art, music, Wikipedia, architecture - all of that will have crumbled to nothing within half a million years...and it would probably take at least that for a race of intelligent cockroach-descendents to take over. I would hope that whatever fate befalls us - we'd have time to consider our legacy and build something so enduring that future species would be able to understand us. SteveBaker (talk) 11:00, 24 March 2009 (UTC)[reply]
NB: My reason for asking these questions is that if the answer is yes, then they add a positive component (which I've heretofore neglected) to, respectively, the total utilitarian value of humanity and the marginal utilitarian value of each new human born. Possibly even enough to shift the sign of these quantities from negative to positive. NeonMerlin11:06, 24 March 2009 (UTC)[reply]
That entirely depends on what you are measuring. This "utilitarian value" thing is nebulous at best and downright nonsense at worst! In order to define a "value" you have to know what you are measuring and why it's important? The universe doesn't give a damn what happens to us or what we do. This is merely a matter of philosophy - and this is the Science desk - not the standing-around-making-specious-arguments-while-being-a-waste-of-quarks desk. SteveBaker (talk) 11:32, 24 March 2009 (UTC)[reply]
In numerous science fiction works, humans have found ancient alien civilizations somewhere in the universe which had flaws leading to their own destruction. Us learning from their mistakes was a plot element, but was never a very convincing one. See also the poem Ozymandias. It would seem to be within our present technology to leave behind a better archive than the crumbling statue of Ozymandias, or some nonbiodegradable plastic bottles in a landfill and some nuclear waste casks. Take some durable substrate and archive a copy of Wikipedia, various great books, copies of world-class art and music, and park it on a space probe at the L3 point or on the moon, where it will be out of harms way for a few million years. Something like that was done on the Voyager space probe, by means of the Voyager Golden Record. Edison (talk) 13:49, 24 March 2009 (UTC)[reply]
We might ourselves be able to go pick up Voyager in a few years when faster space travel is possible. However, a new species which would evolve millions (if from chimps) or billions of years later would have a lot farther to go, since it's headed out into space, and wouldn't know where to look. If there are plastics that last forever, why can't we use them to record Wikipedia, etc. ? If no inks last that long, we could burn letters into plastic pages. Perhaps we could also make something like a DVD out of it, although that would be inherently more difficult for a future species to read, requiring that they create a DVD player. StuRat (talk) 14:22, 24 March 2009 (UTC)[reply]
If we kill ourselves through climate change and leave proof of it, than any new species that comes along will have proof that anthropic climate change can kill you, so they won't cause it. If we kill ourselves through nuclear war, it will a) give proof that nuclear war will kill you, which we know and thus must not prevent us from causing it, and b) tell them how to make nuclear bombs. By the way, this reminds me of a demotivator. — DanielLC15:07, 24 March 2009 (UTC)[reply]
(EC) Your question is very broad and has 2 very different main components. Aliens visiting our planet by definition have made it off their own dust ball. If our demise was due to the fact that we failed to establish a sustainable population elsewhere, they'd not be likely to make that mistake. (Said aliens presumable visiting our now empty world for that very purpose.) Local species would need time to evolve. Animals that are closest to us in the "use of tools" and "problem solving" department (e.g. chimpanzee, parrots) have only small populations left and it is doubtful that they will be able to multiply fast enough to become a dominant species. Animals with large populations (e.g. cockroaches, rats) will need time to evolve. By the time they get to a point of evaluating past events, there is likely to be very little left. They might dig up our "religious temple to energy" and start a traveling exhibit with our nuclear waste. Warning signs tend to be ignored, because risk taking is part of the process of developing s.th. new. (For example: Warnings on the walls of the pyramids were not paid much attention to.) Plastic will get crumbly after about 50 years, give or take. [1] It may make it for a couple of centuries under ideal conditions. Archiving Wikipedia in a durable manner would either require a consecutive population to produce backups on emerging technology or some advancement in data storage. Just consider what you'd do if you found a modern computer and a stack of punch cards, some magnetic tapes or a stack of floppy disks? ...and that would have been from within just a couple of decades. The magnetic storage will probably be unreadable anyway. CDs will merely last for about 5 years. Inscribing data in a crystal is still only a neat trick in the lab. History tells us that a) even things designed for the ages aren't necessarilyintelligible to others and b) very few blunders get avoided the second time around. (aka. History is repeating itself.) Taking what is left behind for a following civilization as the only measure for an individual's worth is a pretty measly standard. Usually his/her contribution to the species while it's still around and to it's future existence and well being might be better gauge. ("If you are going downhill, at least enjoy the ride." :-) 76.97.245.5 (talk) 16:20, 24 March 2009 (UTC)[reply]
Increasing population takes almost no time at all, when compared to the millions or billions of years it takes for new species to evolve. If a population can double every generation, which isn't that difficult in ideal conditions, that would mean it would increase 1000-fold in 10 generations, a million-fold in 20, and a billion-fold in 30. Given a 20 year time frame for each generation, that would only take 600 years. StuRat (talk) 17:33, 24 March 2009 (UTC)[reply]
600 years would indeed be under ideal conditions. It took humans considerably longer to make it from caveman to computers. Tool use and problem solving abilities help to reduce some population pressures, but by no means all. Natural selection is a lot more complex and messy than the simplified "survival of the fittest." Sometimes the fittest get their heads bashed in by the second fittest or they just can't get a prom date :-). One lowly pathogen can wipe out populations in entire areas, so simple mathematical progression doesn't quite apply. 76.97.245.5 (talk) 22:31, 24 March 2009 (UTC)[reply]
Modern humans (Homo sapiens sapiens) are only 200,000 years old, so our population increased from almost nothing to several billion in that time. While much longer than 600 years, that's still nothing compared to how long it would take another species to evolve on Earth to the same level of intelligence as we currently have. Even starting from chimps, which are 98% of the way there, it would still take millions of years. StuRat (talk) 23:57, 24 March 2009 (UTC)[reply]
So given the timescale for another species to evolve to replace us - and assuming it's not chimps or dolphins because they'll probably die right along with us when whatever befalls us comes - we'd probably have to find a way to preserve our Wikipedia backup for a billion years. Sadly, it's not just a matter of finding a good material to write it on. We couldn't count on avoiding vulcanism, earthquakes, continental subduction, rising sea levels, rock deposition, being ground to dust by a kilometer of ice in an ice age...one of those could bury our best efforts beyond the ability of any advanced race to find it. Putting it somewhere more quiet like the moon or one of the Lagrange points would make sense - but remember: that time-capsule that the super-intelligent race of dinosaurs left for us it still sitting in a crater somewhere and the backup copies at the two lagrange points are still there. So even at our level of development - there is no certainty that we'd find these information sources. If you are a pessimist - you might successfully argue that the time it takes a new species to find a carefully preserved archive could easily exceed the typical lifetime of a civilisation. SteveBaker (talk) 01:27, 25 March 2009 (UTC)[reply]
There's another good reason to leave that stuff at the Lagrange points, because when we get there we may find them cluttered with time capsules from all the previous civilizations. :-) StuRat (talk) 05:19, 25 March 2009 (UTC)[reply]
In my attic are boxes of punchcards, reels of magnetic tape,and floppy discs. There is also punched paper tape with a Fortran 2 program for a PDP-8. I have no doubt that all would be readable. Edison (talk) 04:42, 25 March 2009 (UTC)[reply]
How long do CDs last? I have some that are nearly 15 years old that work well, so I know its more then 5 years, but would they last centuries in a place like a bank vault? 65.121.141.34 (talk) 20:34, 24 March 2009 (UTC)[reply]
Plastics degrade rapidly when exposed to UV light, but I imagine they'd last far longer if buried. You'd probably also want to keep ground water away from them and bury them below the frost/freeze line. Hermetically sealed, under ideal conditions, I'd guess centuries, at least. StuRat (talk) 20:41, 24 March 2009 (UTC)[reply]
It depends very much on the specifics. In general, commercially stamped CDs last longer than writable media. But some early batches used an unsuitable glue, with CDs deteriorating after a few years only. --Stephan Schulz (talk) 20:47, 24 March 2009 (UTC)[reply]
I bought three CD's from the Philips Research Labs staff shop - about 2 months before the first CD players went on sale to the general public. All three still play just fine (In case you care - they are: Dire Straits: Brothers in Arms, Some Bach Fugues and a recording of Glenn Miller taken from the original pre-magnetic tape wire-recordings and remastered especially for CD). Those must be close to being the oldest mass-produced CD's in existence. There have been a few snafu's with disk manufacturers over the years - so some disks have behaved badly - but on the whole, they do pretty good. There is certainly no obvious fixed lifespan that you could point to. They don't all die after X number of years. (Mind you - I bet they are all three on about their tenth replacement jewel cases!) SteveBaker (talk) 01:08, 25 March 2009 (UTC)[reply]
There was a court case somewhere (EU?). The industry representatives were unwilling to guarantee their CDs for more than 5 years. So that's how long they think they'll last. That is for ones subjected to ordinary use, though. Under some carefully managed storage conditions they might last as long as the plastic stays intact. 76.97.245.5 (talk) 02:42, 25 March 2009 (UTC)[reply]
I have CD's from 1982, reel-to reel tapes from the 1960's, LPs from 1950, 78's from 1909, and cylinders from the 1890's which still play fine. It is a matter of preservation. Edison (talk) 04:37, 25 March 2009 (UTC)[reply]
There was one period where the formulation of the aluminium that's evaporated onto the disk to make the mirror-surface wasn't quite right - and CD's from many manufacturers over a period of several years had a tendency to develop tiny pin-holes in the aluminium layer - and (weirdly) an effect similar to surface tension in liquids was making the holes grow slowly over time - eventually ruining the disks. But once the problem was known, it was fairly quickly rectified - a lot of the bad press that CD's have had over the years can be attributed to that incident. SteveBaker (talk) 05:26, 25 March 2009 (UTC)[reply]
Fungus can also be a problem for CDs in some tropical countries, I've experience it personally and there are widrespread reports on the internet. It seems to eat the aluminium layer. See [2] as an example image. Nil Einne (talk) 11:17, 25 March 2009 (UTC)[reply]
As noted above, the commercially stamped media is generally far more durable than user writable CDs. The same chemistry the makes most writable CDs able to be encoded by laser heating also makes them degrade over time. I remember seeing a report that most writable CDs developed significant errors in under 5 years. This was even true of the CDs that had never been burned, they also became unusable after only a few years of shelf-life. That said, there are also some companies now that will sell you "archival quality" writable CDs. I have no idea if they really do the job, but I have heard of one company that even offers a 100 year guarantee which at least says they are serious. Of course, their product is also like $5 / disk as opposed to $0.25 for the cheap stuff that dies after a few years. Dragons flight (talk) 06:13, 25 March 2009 (UTC)[reply]
Certainly writable CD's have a short life - and indeed, their shelf-life is problematic even before they've been written to. I worked on the project that produced the first CD-ROM ever - we had to press the disks in a CD factory because the projected life of the early experimental writable CD technology was so short that it would be tough to get them out of the factory and into our hands before they stopped working! The 100 year guarantee is only for the value of the blank media - it's really no comfort at all when 20 years from now you find you've lost all of your data and all they give you is a couple of bucks to buy a new blank disk. SteveBaker (talk) 22:18, 25 March 2009 (UTC)[reply]
The only place that fingernail growth occurs is at the base. The entire hard part of the 'nail' is non-living; the nail extends as new material is deposited by the nail matrix: living soft tissue that sits under the nail. The visible portion of the nail matrix is the lunula, that whitish semicircular bit at the base of the nail. (The lunula may not be visible on all your fingers and toes, and it is often most conspicuous at the base of the thumbnail.) TenOfAllTrades(talk) 15:58, 24 March 2009 (UTC)[reply]
What's the hottest a human can stand for more than a few seconds?
the question on actually experiencing boiling water weeks back brough to my mind a story - don't recall where, but my money's on it being James Bond - where the bad guy locks the protagonist in a sauna or somesuch and knobs it up to about 165 degrees or so.
My question is, how much can the human body stand for more than a few seconds? I imagine the writer did some research on how high to have that turned up (why would one have the option to have it dangeruosly high, anyway?), but ISTR someone saying a really hot sauna can get up to 212 F, which is boiling? Is that really doable?
It makes me wonder about people near a blast furnace, too, where I imagine as you get real close it can be what, several hundred degrees? Or just what firefighters face, ignoring the problem of smoke.172.130.27.46 (talk) 15:44, 24 March 2009 (UTC)[reply]
This depends heavily on what the hot thing in question is. Hot metal, hot water and hot air of the same temperature will have different effects, due to differing thermal conductivity and heat capacity. A human can easily stand hot air at well above boiling temperature (try sticking your hand in an oven sometime), but boiling water is another matter. Algebraist15:50, 24 March 2009 (UTC)[reply]
Real Finland style saunas routinely go up to 100℃, and sometimes reach 110℃, significantly above the boiling point of water. People can stand this with low humidity for several to many minutes - the body manages to regulate its temperature fairly well for short periods of time. On the other hand, long-term exposure to 45℃ is very unhealthy, as is, of course, exposure to hot water quite a bit below boiling point. --Stephan Schulz (talk) 15:54, 24 March 2009 (UTC)[reply]
The key factor is humidity. In low humidity (such as the saunas Stephan mentions), the human body can cool itself very effectively through sweat even at temperatures significantly above the boiling point of water. In high humidity, sweat doesn't work, and you get into trouble at temperatures well below the boiling point of water - 50℃ will have you suffering severe heat stroke pretty quickly in tropical humidities if you aren't careful (and probably even if you are careful - without access to some form of refrigeration, at least an cool box full of ice, I can't see what you could do to survive more than a couple of hours, if that). --Tango (talk) 18:49, 24 March 2009 (UTC)[reply]
I remember reading in the Guinness Book of Records in the 1980s that humans had experienced temperatures of over 500 deg C in US Army trials and survived! Unfortunately the GBWR website is not at all search-friendly so I can't instantly confirm this. --TammyMoet (talk) 19:04, 24 March 2009 (UTC)[reply]
The melting point of Aluminium is 660.32°C, so not far off. I agree that 500°F is far more likely (that's 260°C - still rather toasty!). --Tango (talk) 20:45, 24 March 2009 (UTC)[reply]
Thanks; yeah, I thought of looking at Guinness, but yeah, their site isn't too easy to move around in? Wow, 500 degrees F is still amazing. the stuff on how much of a different humidity makes is really helpful, too. I'd have thought something that high would do something really bad to the blood or skin or somethingg even after a couple seconds; just like I've read putting one's hand in liquid nitrogen (or is that oxygen) freezes it instantly. (Then again, that's a liquid, but at that low a temperature, I think you'd get frostbite anyway.).209.244.187.155 (talk) 21:42, 24 March 2009 (UTC)[reply]
So my recollection is a little different. The way I remember it, they were NASA experiments rather than Army. The numbers I recall were that you could tolerate 400 F unclothed or 500 F if bundled up (at those temperatures a heavy jacket keeps you cool, relatively speaking). I don't know what the time frame was; I can't believe it was a really long time but presumably it was long enough to accomplish some task, or maybe make it through re-entry and have the ship pick up a living person rather than some barbecue. --Trovatore (talk) 21:59, 24 March 2009 (UTC)[reply]
There is a big difference between liquid nitrogen and air - liquid nitrogen conducts heat far better than air. If your skin ever got to 100 degrees, you would be in serious trouble within a fraction of a second, but heat doesn't go from air to skin very quickly so the body's cooling methods can prevent the skin ever getting hot enough to burn even when in direct contact with 200 degree air. --Tango (talk) 22:17, 24 March 2009 (UTC)[reply]
Putting your hand in liquid nitrogen doesn't freeze it instantly, anyway. It's perfectly possible to dip your hand in for a brief period without taking any harm (be very careful if doing this at home!). Algebraist00:38, 25 March 2009 (UTC)[reply]
Having had my face briefly immersed in a large propane flame (probably around 1000°F), walking through a turbocharged diesel exhaust plume from a military vehicle (about 400°F), and having inadvertantly swallowed drops of splashing liquid nitrogen, I can attest personally that you neither burn nor freeze instantly. My eybrows and eyelashes got singed, but otherwise I was unharmed. With the nitrogen, I was also unharmed. Frostbite can happen after a few seconds of liquid nitrogen exposure but for the most part it evaporates so fast it never directly contacts the skin (swallowing a drop results in belching out a white cloud of vapor a few seconds later). I highly recommend not trying any of this yourself. In my case, they were accidents. ~Amatulić (talk) 01:07, 25 March 2009 (UTC)[reply]
I've heard stories from friends of "liquid nitrogen fights" (similar to water fights). They splash it on each other, apparently with no ill-effects. While the thermal conductivity is pretty high, the thermal capacity of a small drop can't be much - there probably just isn't enough "cold" there to harm you. --Tango (talk) 20:25, 25 March 2009 (UTC)[reply]
"... Chantrey was engaged at that period in casting a large bronze statue. An oven of considerable size had been built for the purpose of drying the moulds. I made several inquiries about it, and Chantrey kindly offered to let me pay it a visit, and thus ascertain by my own feelings the effects of high temperature upon the human body. ...
"The iron folding-doors of the small room or oven were opened. Captain Kater and myself entered, and they were then closed upon us. The further corner of the room, which was paved with squared stones, was visibly of a dull-red heat. The thermometer marked, if I recollect rightly, 265° [130°C]. The pulse was quickened, and I ought to have counted but did not count the number of inspirations per minute. Perspiration commenced immediately and was very copious. We remained, I believe, about five or six minutes without very great discomfort, and I experienced no subsequent inconvenience from the result of the experiment."
Kind of continued from this thread. Do we know whether space is expanding all over the universe (i.e. all space around us is stretching) or whether it's just the outermost sections of our universe expanding? I mean is the space around us now, Earth, the Moon, expanding with the rest of space around the universe? I'm not even sure if this is testable or not, because if all space around us was expanding at the same rate as everything else, then it wouldn't be detectable. —Cyclonenim (talk · contribs · email) 18:41, 24 March 2009 (UTC)[reply]
The expansion of space on a local scale would be detectable (at least theoretically). We measure the distance to the moon, for example, by bouncing laser beams off mirrors left by Apollo astronauts and timing how long it takes for them to get back. The speed of light isn't changed by metric expansion, so we would notice the time taken increasing (actually, the moon is moving away from the Earth due to tidal forces, but that's irrelevant!). Gravitationally bound systems (anything on the scale of galaxy clusters or smaller) aren't expanding. That doesn't mean it is just "outermost" sections of the universe expanding (whatever that means - there is no centre to be far away from) - expansion happens on large scales, but not small scales. Any two objects a large enough distance apart will be moving away from each other, regardless of where they are in the universe. --Tango (talk) 18:57, 24 March 2009 (UTC)[reply]
It sure seems like there's an influx of "expansion of space" questions lately. Anyway, this is answered in the lede of the metric expansion of space article linked in the first answer of the above question: no, small-scale gravitationally-bound systems (including the Earth-Moon system and the solar system, but also including the Milky Way as a whole and yet larger systems) do not expand within themselves. As for detectability, the "measuring distance in a metric space" subsection addresses this.
However, along a related line, it's possible that all space (and other related constants) expand at a particular rate, shared equally, and that it's therefore completely undetectable. Of course, it's also completely irrelevant -- if everything continues to function in such fashion as if there were no inexplicable immeasurable force, then we might as well assume that there is not. — Lomn18:59, 24 March 2009 (UTC)[reply]
How would you define such an expansion? Our definition of "distance" depends on various constants, and the units we express those constants in depends on our definition of distance. I can't see how you could define things in a way that makes the kind of expansion you describe make any sense. --Tango (talk) 20:35, 24 March 2009 (UTC)[reply]
I don't know how it would work, either -- I'm just noting that while such a philosophical concept could be bantered about, it's not a meaningful discussion. I've seen a few thought experiments before along the lines of "what if everything is expanding, all means of reference included?" and thought it worth addressing why they're not really interesting. — Lomn20:54, 24 March 2009 (UTC)[reply]
People have already kind of said this, but anyway: your questions would make sense if space were more like a substance (a liquid or a gas or a loaf of bread), but it isn't. Space does have some properties of its own (like curvature), but it doesn't satisfy a continuity equation—it isn't "conserved". If you have a liquid-filled region that's getting larger, it makes sense to ask whether it's because the existing liquid is expanding or because more liquid is being added at the edges. You can distinguish those two cases because you can trace the motion of the liquid over time—you can draw worldlines in spacetime showing what happens to individual bits of liquid. If the liquid region is getting larger then either the lines must be diverging from each other or someone must be adding new lines at the edge. When you're talking about expanding space you still have the spacetime but not the lines in it, so that distinction disappears. Galactic superclusters have worldlines and they're generally diverging, so the gas of superclusters is expanding (and it really is very much like a gas, strange as that may sound). Smaller objects within the superclusters have their own worldlines and those are not generally diverging (within a single supercluster), so individual superclusters are not expanding. -- BenRG (talk) 19:11, 25 March 2009 (UTC)[reply]
Hmm... as I understand it, a simple answer to your question doesn't exist: there is no universal "rate of expansion". We suspect that much of the universe is, due to expansion, now beyond our light horizon. As such, it is unknowable to us (save that it has receded at a rate greater than c). Within the observable universe, the rate of recession varies object to object; however, scientists currently believe the general trend (the deceleration parameter) is that those rates are increasing. — Lomn19:21, 24 March 2009 (UTC)[reply]
Locally, things can't travel faster than the speed of light, but on cosmological scales it doesn't quite work like that - you can think of it as the galaxies staying still and space being created inbetween them (BenRG will tell us this isn't the case, but I'm still not convinced!). --Tango (talk) 00:30, 25 March 2009 (UTC)[reply]
Well, it isn't the case but it isn't not the case either. It's just meaningless to ask whether space is being created as far as the theory is concerned. What worries me is saying that space is being created in some cases (the supercluster motion) but not other cases (other relative motion), since that distinction doesn't exist in the theory. -- BenRG (talk) 19:22, 25 March 2009 (UTC)[reply]
I realize, but space is, at least locally, homogenous, so shouldn't the observed values be the same everywhere in our observable universe? I know that most scientists believe that the rates are increasing; I am looking for the exparimentally measured redshifts that back this up. *Max* (talk) 20:12, 24 March 2009 (UTC)[reply]
There is a universal rate of expansion (or large enough scales), it's just proportional to separation. See Hubble's law. It is theorised that Hubble's Constant varies over time (increasing due to dark energy, decreasing due to gravity - observations suggest more of the former than the latter, so a net increase). I can't find any estimates of its value at other times than now, though, sorry. --Tango (talk) 20:30, 24 March 2009 (UTC)[reply]
According to the ΛCDM model, the value of the Hubble parameter at different times is given roughly by , where t is the "time since the big bang" (see Age of the universe#Explanation for what that means) and k ≈ 1 / (11 billion years). The image on the right is a graph of the coth function which I found on Commons. The x < 0 part isn't physically meaningful. The present day (14 billion years after the big bang) is in the vicinity of x = 1.2. The horizontal asymptote (y = 1) corresponds to a Hubble parameter of around 60 km/sec/megaparsec. Three caveats: (1) I didn't find this in a textbook, I derived it from the Friedmann equations and I may have made a mistake; (2) it's only valid for t > a few thousand years, before that other physics comes into play; (3) it's only valid in the future if the ΛCDM model is correct, and there's not enough data yet to be sure of that. -- BenRG (talk) 13:43, 25 March 2009 (UTC)[reply]
I used the first Friedmann equation in the form given at the bottom of Friedmann equations#The density parameter. I took (consistent with the evidence) and (close enough for t > a few thousand years) and plugged the rest into a CAS, which found the solution . (I've been basing Ref Desk answers on that formula for ages, so I hope it's right.) Then H = a'/a. The factor of 2/3 comes from the exponent. It's canceled by the 3/2 from inside the sinh, but I absorbed that factor into my k. -- BenRG (talk) 19:23, 26 March 2009 (UTC)[reply]
Looking at earth from near another star, how bright would it seem? Brighter than the sun? One of the brightest things in the sky? I would like to ignore the time delay caused by the speed of light and assume that the radio brightness in 2009 is what is observed. 78.146.178.204 (talk) 23:01, 24 March 2009 (UTC)[reply]
The only terrestrial signals likely to be detectable from another star are those intentionally sent into space - either specific attempts at signalling to aliens, or from studies of planets and asteroids using radar. Those could only be detected if they were pointed directly at the star in question. So, from virtually all stars, the Earth would not be visible in the radio spectrum. If one of these radar signals happened to go to the star, I'm not sure how bright it would be - it would, obviously, depend on how far away it was, but I'm not even sure how it would compare to the Sun... --Tango (talk) 23:32, 24 March 2009 (UTC)[reply]
Using our current technology, detecting an Earth-sized planet with a level of technology similar to ours (i.e. looking at "earth from another star") would be impossible on any level, and at any frequency, visible or radio or any other. Earth is just too small. Nearly every extrasolar planet we have found has been basically a Jupiter-sized planet orbiting at rediculously close distances; such planets cause their parent stars to "wobble" and also "dim" as they pass in front of it. From a distance of a few light-years or more (remember the closest star to us is about 4 light years away) Earth would be entirely undetectable. Looking for the Earth would be like trying to resolve a specific grain of sand on a beach if viewing the beach from the moon. We are REALLY small, and we just don't give off enough general radiation to be detectable, and we don't reflect enough light, block enough of the sun's light, or gravitationally effect the sun enough to be seen. MAYBE an outside-of-the-solar-system viewer could detect Jupiter, but not earth. Possibly, if we were beaming a tight, high energy radio signal directly at another star, it could be detected, but otherwise we would be invisible. --Jayron32.talk.contribs01:41, 25 March 2009 (UTC)[reply]
I seem to recall that there have been at least two signals intentionally sent into space from radio telescopes which would have made the Earth far brighter than the sun. I strongly question the wisdom of such efforts to call attention to us. Edison (talk) 04:28, 25 March 2009 (UTC)[reply]
I'm picturing a tentacled, drooling alien creature noticing the radio emissions from Earth, then heading our way as he straps on a bib... :-)
Well - if he's that hungry, he may be in a lot of trouble because getting here is (in all likelyhood) going to take a couple of centuries. SteveBaker (talk) 05:20, 25 March 2009 (UTC)[reply]
Yes - that's true - the actual amount of power in those transmissions isn't really that great - but being sent from a radio telescope means that the beam is highly directional...I forget which stars they were aimed at - but only beings orbiting around those stars would have any chance of seeing the signal. For the rest of the universe, the earth would still have been pretty much invisible. There are other people claiming to do this though - there used to be (and maybe still is) a company on the web someplace that'll beam any short ASCII message you care to give them out in some direction or other for just a few bucks. Aleksandr Leonidovich Zaitsev[3] has been sending all sorts of things out on a really powerful narrow-beam transmitter. So this does happen...but the odds of anyone out there picking it up are really slim. SteveBaker (talk) 05:20, 25 March 2009 (UTC)[reply]
If you add up the power of all the radio, tv, and other radio-wave type emmissions (including those from power lines or electrical wiring, even thunderstorms) it must be a lot. Either it gets absorbed, or it leaks into space. I'm wondering how this would compare with the power of similar wavelengths from the sun - if these emmissions from modern technology would be a beacon in some wavelengths. 89.243.177.130 (talk) 11:39, 25 March 2009 (UTC)[reply]
You have to remember that numbers that may be enormous by human view-point, can be miniscule by astronomical standards. Some order-of magnitude comparisons:
So even if you assume that all energy generated/consumed by man is radiated out into space, it would be much smaller than the reflected solar radiation, and about 0.000000000001×total solar radiation. This should give you some idea of the difference in magnitudes we are talking about!
Of course these calculations will change, if the radiation from Earth is very narrowly directed in terms of its frequency or direction, but barring that detecting Geo-radiation from another star is a long shot, if not completely hopeless. Abecedare (talk) 19:31, 25 March 2009 (UTC)[reply]
Frequency is the key consideration - the Sun emits enormous amounts in the visible part of the spectrum, but orders of magnitude less in the radio frequencies. A unidirectional radio transmission from the best technology we have would be detectable by the best technology we have from other stars (I'm not sure how distant those stars could be, though - the other side of the galaxy might be a challenge, but the time required for a signal to get there makes it irrelevant). If we sent a visible light laser beam to another star, I doubt they would notice. --Tango (talk) 20:30, 25 March 2009 (UTC)[reply]
