Wikipedia:Reference desk/Archives/Science/2015 February 5

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.

February 5

causes of condom failure

According to Wikipedia 1-2 percent of women will test positive for semen residue if no condom slippage or breakage is observed. How is that possible? Could it be that the semen leaked through the pores of the condom even though it was used correctly? — Preceding unsigned comment added by Roger adams49 (talk • contribs) 04:13, 5 February 2015 (UTC)

Could be someone else's semen. InedibleHulk (talk) 04:21, 5 February 2015 (UTC)

Leakage around the edge, maybe. This is why spermicidal foam should be used with it. Although it could be sabotage. I recall a story about three nurses gossiping. One nurse said she found condoms in the doctor's desk. The second nurse said she not only found them but ran a pin through each one. The third nurse fainted. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:32, 5 February 2015 (UTC)

They tested positive after intercourse with a condom — Preceding unsigned comment added by Roger adams49 (talk • contribs) 04:51, 5 February 2015 (UTC)

Use spermicidal foam along with it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:53, 5 February 2015 (UTC)

Well, I remember reading somwhere that condoms lubricated with spermicide are no more effective than condoms without it. Roger adams49 (talk) 05:11, 5 February 2015 (UTC)

You read wrong. ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:21, 5 February 2015 (UTC)

Could be a failure rate in the test. StuRat (talk) 05:14, 5 February 2015 (UTC)

Anyways, is it possible for semen to leak through a condom even if used correctly? I always thought it was impossible, since condoms don't contain pores. Thats what wikipedia seeems to imply.Roger adams49 (talk) 05:36, 5 February 2015 (UTC)

If a condom is used correctly, it's used in a way that prevents anything getting through. Nothing that is successful can fail. A condom with a hole, even unobserved, will fail. Wikipedia has given you four other options to consider, there's no need to dwell on the idea of something permeating the impervious. InedibleHulk (talk) 06:36, 5 February 2015 (UTC)

An item can be used correctly, and yet fail. For example, a measles vaccine, with a later booster shot, are 99% effective, not 100%. StuRat (talk) 12:57, 5 February 2015 (UTC)

Why are people saying random stuff without bothering to checking references? Our own article Condom#Spermicide and to a lesser extent Spermicide#Use with condoms confirm that it unclear if condoms lubricated with spermicide are actually more effective at preventing pregnancy than condoms without. There are plenty of external sites which say the same thing [1] [2] They also have disadvantages namely a shorter shelf life, a possibly increased risk of urinary tract infections in women and a possibly increase risk of HIV transmission. For these reason and perhaps more, they are no longer recommended by many health sources and no longer produced by many manufacturers. Using spermicide packaged seperately together with condoms may however reduce the risk of pregnancy as there is more of it, although still has the earlier disadvantages (well it obviously doesn't effect the shelf life of condoms), so it's generally suggested it should be used with caution (e.g. some only recommend it for a couple in a monogamous relationship where HIV isn't a concern). Nil Einne (talk) 11:48, 5 February 2015 (UTC)

P.S. I appreciate the source used in our article and the ones I could find aren't that great sources. E.g. as this source [3] I saw in our article on condoms mentions, there are a lot of rumours and poorly supported information when it comes to sexual health and preventing pregnancy, even those without a religious reasoning and coming from health advice sources, such as that about "double bagging". I did have a look for more scholarly sources and couldn't actually find any that took a good look at the effectiveness of spermicide lubricated condoms at preventing pregnancy. Heck even this very old source [4] which appears to have been around the time these were first made, only seems to have looked at the effectiveness in a certain group rather than in comparison to condoms without spermicide. I did come across [5] which is perhaps an interest source if you can get it for a somewhat old overview of spermicides in general. I'm not sure why there appears to be so little research on the effectiveness of spermicide lubricated condoms at preventing pregnancy. It may be that research on spermicides in general suggest the amounts used with condoms is probably too small to be of much use. (One source or more sources I read [6] did note there may similarly be a lower risk of many of the complications due to the lower levels. That said, one of the papers on the risk of UTIs was specifically related to spermicide lubricated condoms, although having had a quick look at the paper, they also found a smaller higher risk for people who used condoms in general.) It may also be because while preventing pregnancy is important for many, there are a number of resonably effective methods for pregnancy prevention (with different costs, benefits and risks) but barrier methods particularly condoms, are the only effective method for sexually active people to protect against STIs particularly HIV. And so the recent research focus on condoms with spermicides has been about STIs and similar stuff. (One of the factors may have been there was an interested in condoms lubricated with spermicides to prevent STI particularly HIV transmission due to some in vitro evidence. However further research didn't show this in vivo and combined with the possibility of increasing risk due to the complications from long term meant there was a move away. There is or was still a hope that a perhaps spermicidal agent can be found which would help reduce the risk of HIV transmission and has few complications.) It may also be the concern is when condoms are already lubricated with spermicide, the users don't think much about whether they should be using spermicide so it's better if these are seperate so there's proper consideration of the benefits and risks and times to use them (which is possibly only vaginal sex). Either way, it does seem there has been a move away from them. And it definitely seems misleading to suggest as was done above that it's wrong to state "condoms lubricated with spermicide are no more effective than condoms without it". From the evidence available, I don't think we can conclude this is definitely the case, but we also can't conclude it's wrong. Nil Einne (talk) 12:47, 5 February 2015 (UTC)

In response to the specific question about semen residue, I looked at the sources from Condom#Causes of failure where this claim is present. There are two papers.

One a metholodogy verification paper with a relatively small sample size [7]. It seems semen residue primarily means prostate-specific antigen, although there was also a check for sperm in some cases. (The check for sperm found it in a few cases without PSA and even when there was claimed abstinence for 24 hour or more but always at low levels. In the PSA cases, sperm was always found, but sometimes at low levels. PSA was always found in samples taken immediately after unprotected sex, but never after 24 hours. BTW, to avoid confusion, I don't think sperm at the low levels was counted as a 'semen residue', actually the primary intention of one of the papers seems to have been to demonstrate that PSA seems to be a better indicator of recent "seminal exposure" than sperm.)

Anyway in this paper, they didn't include any cases where PSA was found in the pre-coital samples. And whether due to this, the small sample size or whatever, there was no PSA found in those cases that were considered where there were no reported problems with the condom.

Note also that samples collected after 72 hour abstinence didn't contain any PSA. BTW, all female participants had tubal ligation (and part of the study included unprotected sex as well participant punctured condoms). One final interesting thing is that even in cases where breakage or slippage did occur, PSA levels did vary wildly.

The second paper is where PSA was detected in some cases with reported failure is interesting [8]. Although in the earlier methodology paper by largely the same authors it was suggested (as was done in that case) that pre-coital samples should be taken to rule out PSA being present from a previous sex act, it doesn't sounds like this was done here. May be they had found this was too odorousonerous and so lead to low compliance. It's also possible they felt it wasn't that important since PSA doesn't generally seem to be detectable for long and the couples were supposed to record coital acts etc in a diary (and so if they didn't comply there, they may not have properly complied with the pre-coital swab anyway).

They do mention it's possible the PSA came from unreported previous unprotected sex. (This being a different sexual partner as suggested above could be one reason it was unreported, but there could also be other reasons ranging I'm guessing from the partners bringing drunk, to one of the partners not knowing.) They don't mention sabotage. They do mention undetected failure is unlikely, but I'm a bit confused whether they're saying they don't believe the cases were undetected failure, or they think if it does occur, it's low as evidenced by only find the 1.2% of cases with PSA without reported breakage/failure.

Interesting of the 3 cases where they did find PSA, 2 of them had it at a low level, significantly below that expected from unprotected sex (so whether the sex had been a while, or it wasn't unprotected, who knows). One of these cases (I presume the one with the higher level of PSA, but this isn't stated) did have a clinically significant level of sperm.

Anyway beyond the suggestion it could be due to unreported unprotected sex, they don't actually comment much on the 1.2% of cases with PSA without reported failure, I think the general point is the rate was fairly low and it's ultimately impossible to know from their results why PSA was detected despite their being no recorded failure. Note that they did have 2 pregnancies in couples where no failures were recorded and the instructions are supposed to have been followed every time. (They also had 8.1% of their participants withdrawing of the research due to breaking up.)

Nil Einne (talk) 14:30, 5 February 2015 (UTC)

Corrected "odorous" to "onerous". Thanks to User:Thinking of England for letting me know. Nil Einne (talk) 11:55, 11 February 2015 (UTC)

Also when Wikipedia mentions slippage and breakage rate does it mean slippage and breakage after one year of use? — Preceding unsigned comment added by Roger adams49 (talk • contribs) 19:05, 6 February 2015 (UTC)

No, I'm pretty sure the breakage and slippage rates are per use. See for example this source [9] (11/1001 latex condoms broke during intercourse), which is one of the sources used in our article albeit indirectly. Similarly the source I mentioned above [10] (not available in full text for free unfortunately) had 16 condoms breaking and 39 slipping off during intercourse or withdrawal, out of 3677 condoms used for intercourse for a total failure rate of 1.5%. There was also one condom which broke when being removed from the penis after intercourse (which should carry minimum risk of pregnancy or STIs if done carefully). BTW, these rates are only breakage or slippage during use. Breakage when opening are normally counted seperately I presume because while they may influence the cost, they ideally shouldn't influence the "failure rate". In practice, they may influene the typical use failure rate, as a couple who find themselves without a condom because the only one that had was broken may just have sex without one (or use the broken one). In fact, one thing the sources mention is that the biggest contributor to typical use failure rates of condoms tends to be inconsistent use. (In a related fashion, our article suggests education and I guess greater concern could reduce breakage and slippage. Notable for slippage, it seems to me there's a big difference between slippage during intercourse and slippage after, probably why some sources count them seperately (although some also count breakage seperately). Slippage during intercourse, particularly if it happens a while before orgasm can potentially be detected and correct. Slippage during withdrawal after intercourse will probably be reduced by good practice, i.e. holding the the base during withdrawal.) BTW, you may want to read the source our article directly uses for the breakage data [11], it appears to try and estimate how much semen may exposed as a result of breakage etc. I would also suggest you take a more careful look and read of sources as plenty of health related journal articles now have full text versions available for free. And while it may be difficult to understand some details of the articles, you should be able to answer questions like what is mean by breakage. Nil Einne (talk) 12:33, 11 February 2015 (UTC)

cold dark matter and nuclei

Is cold dark matter, such as the hypothetical axions, expected by theorists to be able to pass through or even exist inside, without interacting, atomic nuclei?Rich (talk) 05:30, 5 February 2015 (UTC)

• If they are particles then yes, otherwise the particles would be somewhat like neutrons, and be absorbed in the Earth or particle detectors. It would be like super dense low speed neutrinos, or perhaps even golf ball size pieces of rock. Although the concentration of deuterium formed in primordial nucleosynthsis puts limits on how many protons and neutrons there should be around to form normal matter. Graeme Bartlett (talk) 07:54, 5 February 2015 (UTC)

Space Elevator

Inspired by a question above, wouldn't the space elevator be quite dangerous? After all, at such a height (way above the ISS, it would be more likely to be hit by space junk (more space, more junk). I know that most space junk is only small, and hundreds of miles apart, but by building something so big, the likelihood of it being hit by something would be higher, logically. Also, there would need to be some permanent staffing for maintenance purposes and periodic checks on the structure, probably on a constant basis (22,000 miles is a long way). KägeTorä - (^影_虎) (Chin Wag) 11:07, 5 February 2015 (UTC)

Yes. See Space elevator safety. I suspect the hazards outweigh the advantages, and we will never see terrestrial one in practice. Nevertheless the concept could lead to advances in other areas.--Shantavira|^{feed me} 11:51, 5 February 2015 (UTC)

Bear in mind that the space elevator probably wouldn't be a vehicle for getting people into orbit. At 180mph the elevator would likely take five days to complete the trip to the top, that's a pretty high speed for something that's climbing a cable...which is worth an absolute fortune and can't be damaged. This would result in you spending a long time in the Van Allen radiation belt - which is an exceedingly dangerous place to be. It's not a huge problem for rocket flights because you spend so little time within that danger zone. That means that the elevator car would need an enormous amount of shielding and life-support for (let's say) at least a week to allow for potential delays and mechanical issues. The resulting elevator car would probably wind up being heavier than a reasonable cable could support.

As a cargo transport mechanism, we may not care about an occasional debris hit on the elevator car itself. The risk is mainly of damage to the cable. Consider the cable more as a broad ribbon than a traditional circular cross-section cable...the width of the ribbon varies substantially along it's length in order to balance weight against strength. But it'll be really quite thin. There is no reason (in principle) why the cable can't be aligned with the orbital direction of the most common equatorial-orbit debris - which reduces the probability of a hit considerably - and provides more of a safety factor in the unlikely even that it happens. The cable is also going to have to be made from the toughest materials known to mankind...that helps too.

The problems with the space elevator currently seem insurmountable. Instabilities in the cable itself due to coriolis forces as cabs ascend (and perhaps, descent) are significant - and it might require active correction in the form of rocket motors mounted at intervals up the cable. The mathematics of these issues only really came to light a considerable time after large research efforts were underway and the concept had lodged into the popular psyche - so problems like this, which may well scupper the entire concept, tend to get swept under the carpet in popular accounts of the idea.

SteveBaker (talk) 14:58, 5 February 2015 (UTC)

Aslo: Fine in theory but what about in practice? A sky-hook, such as this, will send off lightning bolts right-left-and-centre as it approaches terra-ferma (due to dielectric quality of air). Thus, the carbon-fiber tether must be able to with stand this current (and thus need be thicker and heavier than pure physics dictates). Mix that into the equation and sky-hooks wont happen soon. But I do like the idea.--Aspro (talk) 21:42, 5 February 2015 (UTC)

How about on a lesser body, like the Moon ? There I'm thinking tides caused by the Earth might be a problem, as the Moon moves closer and farther from the Earth. StuRat (talk) 15:29, 5 February 2015 (UTC)

For the Moon, the cable would have to go through L1 or L2. I think stability is not a problem, though variation in tension could be. —Tamfang (talk) 06:47, 7 February 2015 (UTC)

Especially in a cable limited to cargo, it seems to make sense to have many small "cars". I am thinking that provided you keep a stream of such carriers going up and down the cable, they ought to be able to balance the Coriolis forces from one another (a sort of "countercurrent exchange" for orbital momentum). Of course, unless your ore miners are already hard at work, one expects a net outward flow of mass, so this balance shouldn't be perfect; nonetheless, there ought to be some stable state where the cable droops behind its attachment point on the ground, supported by the propulsion from the cargo. Is that a workaround you've seen proposed?

A wackier idea I doubt has been proposed: what if you have two counter-revolving skyhooks that don't go all the way to the ground, which rotate in a plane to either side of the space elevator? You could have your cars spew out little gluey tentacles designed to touch and break on impact after collecting a certain amount of momentum from whichever skyhook is moving the right way. The counterrevolving skyhooks could keep rotating full pace with a simple motor to push one center of gravity against the other, with no need for rockets. Yeah, it's a little absurd, but my point is to illustrate that once you build structures on this scale, I think you can use them for a wide range of forces. Wnt (talk) 18:34, 5 February 2015 (UTC)

What keeps these skyhooks in the neighborhood? —Tamfang (talk) 06:47, 7 February 2015 (UTC)

Forgetting the space junk problem - much closer to the ground, I'd be concerned about an airplane hitting it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:43, 5 February 2015 (UTC)

Don't want to spoil too much, but it you read Red Mars, then it talks about the dangers of space elevators and how bad things could hypothetically get. I'd recommend reading it just because it's a good book as well though. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 16 Shevat 5775 20:24, 5 February 2015 (UTC)

Our article debunks many of the Red Mars claims. It strained my credulity even as fiction, actually. The whole premise of a space elevator cable is that the weight of (most of) the cable can hang from a single tiny section of cable. So how much damage can such a weight really do to a planet by hitting it? Wnt (talk) 15:06, 6 February 2015 (UTC)

I'm not an expert in the subject, but the article talks about it in the context of Earth, not Mars which is a planet with different overall conditions from Earth. I'm not saying Robinson's portrayal is accurate, but any opinion on what would happen on Mars is lacking from that article—as it does mention Red Mars even though the section lead is only discussing Earth and does not say specifically that Red Mars deals with a Martian space elevator rather than Terran (which you wouldn't know unless you'd read the book). Also missing is any citation for what the article says would happen on Earth. (Tl;dr the article needs some fixing and is unsatisfactory for answering the question). Anyway, I don't remember the mass of the cable from Red Mars, but I think the idea there was that it does the people of the planet damage more than the planet yourself (unless you mean the planet in the context of its inhabitants). Again, not saying Robinson was accurate or anything in his fictional portrayal, but what's presented in that article is not enough to formulate any sort of opinion on the matter with regard to Earth or Mars. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 17 Shevat 5775 16:48, 6 February 2015 (UTC)

I've never seen anyone suggest it, but it seems to me that the only reasonable guard against catastrophic failure is to have multiple interconnected elevator cables. I am imagining something like three or four cables spaced an appreciable distance apart (much larger than an aircraft wingspan, for example) but horizontally interconnected every few kilometers. The cables would need enough spare capacity that the remaining cables could catch and hold the weight of any single cable failure. In that case, a single cable failure wouldn't bring the whole system down but rather leave dangling ends that could potentially be repaired. Of course, building three space elevator cable would substantially increase the cost of a project that would already be ridiculously expensive, but that redundancy might still be cheaper than the effects of a catastrophic failure. Dragons flight (talk) 04:54, 6 February 2015 (UTC)

Not sure why an aircraft wingspan should matter much. A couple of these, these or even these with TN warheads should keep those little pests away. No way anybody would build a space elevator without an accompanying no-fly zone (or no-pruning-shears zone for that matter), ever. - ¡Ouch! (^{hurt me} / _{more pain}) 08:33, 9 February 2015 (UTC)

This makes for some interesting reading on why there will never be a "space elevator" http://www.rense.com/general54/wef.htm Not sure of all the facts, but there's a lot of reasons here why it can never work. 217.158.236.14 (talk) 16:11, 6 February 2015 (UTC)

I'm unconvinced. The ionosphere thing is cool -- that's a great source of electric power to tap into, assuming you use a modest amount of insulation to control how much current flows. But if the ionosphere is so ready to discharge, why don't space launches and asteroid strikes discharge it? It's just a long way up, many places where a little resistance can add up. The ice buildup is serious, but ultrahydrophobic materials are being invented as we speak. The temperature stresses, micrometeorite strikes... all of those sort of arguments assume that the space elevator is a static structure. But static structures, buildings that are designed simply to stay in one conformation and slowly wear away, are already beginning to become obsolete as architects become more sophisticated with counterweights and absorption mechanisms. Give the Chinese 50 years to colonize the Pacific Ocean and get service on their high speed trains to North America, and the sort of mechanisms that will dampen waves and storms and tsunamis will be mature for use on a space elevator that can react to changing stress and unexpected damage. Wnt (talk) 21:34, 6 February 2015 (UTC)

That article may make a whole lot of valid points, but part of it is twaddle: The cable must move through space at the same speed as the surface of the earth it is anchored to. It will be moving at approximately 22,500 MPH near the surface of the earth. (Er, what?) But it will be at a MUCH higher velocity the further from earth as you travel length of it. Imagine in your mind the end of a yardstick attached to the side of a baseball. Rotate the baseball slowly, and the end of the yardstick moves many, times faster. Now you have an idea of what will happen to the cable, the further from earth it is. So what? The main problem there is that the lower parts are moving more slowly than things in orbit. Relative to things not in orbit around Earth, like micrometeoroids, one speed is little better or worse than another; time of day may be the most important variable in sand damage. —Tamfang (talk) 07:01, 7 February 2015 (UTC)

Why should it be dangerous? The space elevator isn't fixed at one point on the earth and in the space. The earthern base is in the middle of an ocean on the equator and floats. So it can dodge thunderstorms and in the space it would be connected to a satellit for dodging space fragments to evade collisions.--Calviin 19 (talk) 10:18, 7 February 2015 (UTC)

Strength of bronze

How come copper/tin bronze is significantly stronger than copper, let alone tin? Zarnivop (talk) 20:48, 5 February 2015 (UTC)

Alloy#Theory have some information, but not terrible much. WegianWarrior (talk) 21:06, 5 February 2015 (UTC)

In general, pure metals tend to be fairly soft and pliable. The bonding resulting from a proper mix of two or more metals yields a much more durable substance. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:44, 5 February 2015 (UTC)

I thnk copper-arsenic bronze is generally stronger than copper-tin bronze though it generally meant shorter lifespans for the smelters. Sir William Matthew Flinders Petrie | ^{Say Shalom!} 16 Shevat 5775 21:56, 5 February 2015 (UTC)

However, the ancients did not intentionally add arsenic (it was a natural inclusion to some malachite ores) unlike tin, so methinks this is off topic to the OP's question.--Aspro (talk) 22:34, 5 February 2015 (UTC)

At low tin content (less than ~10%, depending on cooling rate) solid solution strengthening is the main mechanism. At higher tin contents, like bell metal it forms 2 phases, Sn and Cu in a solid solution, and δ_CuSn, which is a hard, brittle intermetallic compound that dominates the mechanical properties. Mr.Z-man 05:30, 6 February 2015 (UTC)

Thanks! Zarnivop (talk) 13:49, 6 February 2015 (UTC)

• An intuitive way to think of this is to imagine a face-high pile of marbles. What would happen? Now, imagine a face high pile of marbles and sand. What would happen? Obviously the marbles alone will have far less cohesion than the marble and sand mixture. μηδείς (talk) 23:59, 6 February 2015 (UTC)

Metallurgy goes far deeper than my comprehension of it. Just now I read someone has invented a better form of steel, claiming it is superior to titanium alloys. [12] Amazing to think there is still this much room for improvement in what you'd think would be the most mature technology. Wnt (talk) 15:36, 7 February 2015 (UTC)