Wikipedia:Reference desk/Archives/Science/2017 May 10

= May 10 =

Several siblings, all male or female
The Boy or Girl paradox article led me off onto a vaguely related train of thought. Imagine that a couple has had six or seven children together, and all of them are girls or all of them are boys. If they have a seventh-or-eighth child, is the baby more likely than not to be of the same sex as its siblings? I'm wondering whether in such cases certain biological factors could come into play, since this isn't merely a coin flip: for example, the father's sperm production is unevenly balanced one way or the other (so it's not a 50-50 chance at conception), or the mother's somehow physically more likely to miscarry one sex than the other (so sex distribution at conception doesn't match the sex distribution at live birth). Or are such families (assuming no intentional abortions, post-birth deaths, etc.) basically just the result of chance, since approximately 1/32 of all six-children families can be expected to have children of just one sex? Nyttend (talk) 04:59, 10 May 2017 (UTC)


 * There are at least three mechanisms at play that may lead to deviation from an exactly 50-50 chance of either sex. First, human sex ratio is not 50-50, but rather the chance of a boy being born is slightly higher than 50%. This may be because the Y-sperm is slightly lighter (and therefore more mobile) than X-sperm, or due to a number of other mechanisms some of which are discussed in the human sex ratio article. Second, families may "want a boy" or "want a girl", in which case they are more likely to stop an the minority sex (n boys 1 girl or n girls 1 boy) rather than at the majority sex (n boys 0 girls or n girls 0 boys). Third, as you say in the question, there are genetic mechanisms that may cause substantial deviations from 50-50 in affected families. I'll see if I can find some more-or-less accurate numbers to quantify these effects. Dr Dima (talk) 07:47, 10 May 2017 (UTC)


 * To the extent that sex at birth is random, Dima's second point is fallacious. Say that every day you start flipping a fair coin and continue until it comes up tails. You will see sequences of T, HT, HHT, HHHT, etc., but it's still true that most probably 1/2 of the total flips will be heads. The longer the run of heads, the less probable it is.  Well, similarly with births, if they are random.  However, this is really irrelevant to the original poster's question, which is asking if the births are random. --76.71.6.254 (talk) 10:17, 10 May 2017 (UTC)
 * I appreciate your comment, but I'd like to point out that you misunderstood my statement. Consider a process where coin is tossed until both heads and tails occur at least one. For such a process, all 6-toss sequences will have either 5 heads and 1 tail or 5 tails and 1 head; indeed, having 6 heads will result in a more than 6-toss sequence, until a tail occurs. I interpreted the OP question "... since approximately 1/32 of all six-children families can be expected to have children of just one sex?" to refer to exactly 6-children families, in which case my statement above is valid, and not fallacious. Dr Dima (talk) 16:03, 10 May 2017 (UTC)
 * Agreed, by that interpretation it's valid. --76.71.6.254 (talk) 19:04, 10 May 2017 (UTC)
 * If we do a Fermi calculation, and ignore the fact that it isn't really 50-50 as noted above, let's assume it is 50-50. The answer to your question " is the baby more likely than not to be of the same sex as its siblings?" the answer is that the 8th baby has a 50/50 chance of being the same sex as its prior siblings.  If there were a 9th baby, it would have a 50/50 chance of being the same sex as its prior siblings.  Odds don't have a memory, so each child has a 50/50 chance of being one gender or the other.  Now, the probability of one mother having, say, nine boys (again, assuming a true 50/50 chance rather than the reality which is slightly different) is 1 in 29 or 1 in 512, which is only a 0.195% chance.  Though not impossible; my grandmother's sister had nine children, all boys.  -- Jayron 32 10:33, 10 May 2017 (UTC)
 * Whilst I applaud the rationality, I have a sneaking suspicion that a given womb-host-system may be slightly more allergic to one sex than the other. So it isn't a pure coin flip. Greglocock (talk) 10:55, 10 May 2017 (UTC)
 * But as the OP says, the question is whether it is 50-50 and by how much it can devy. It is a Bayesian inference problem, but we only have the observation that most children are of either sex, and the prior probability distribution (is "50-50" a good model?) is hidden in a giant heap of biological and environmental data.
 * If you play dice and observe that certain numbers come up more often than others, external clues can give you more confidence that the dice are loaded - the same sequence could be attributed to chance in a high-security Monaco casino, and to loaded dice in a Chicago suburb mafia-run pub. The question is how strong these external clues are in the case of pregnancy - by how much can genetic changes improve odds of either sex and how likely those genetic changes are. Tigraan Click here to contact me 11:07, 10 May 2017 (UTC)
 * I agree that the OP is correct that the are some circumstances when the odds aren't really 50-50 (as with others, let's ignore they are never actually 50-50), but the probabilities of these are likely so low that even with 8 children of one sex these cases will still be lost in the noise i.e. nearly all such cases will just have been due to complete random chance. Nil Einne (talk) 11:29, 10 May 2017 (UTC)


 * Further complicating these calculations is that estimates range from 25 to 40 percent of pregnancies that result in miscarriage. (Google the subject and you'll see.) So unless the sex of the miscarried embryos is known, you can't really be certain that an apparent trend of all one sex is really a trend. It could be like XmmmXmmmXm for the case of seven successful pregnancies yielding males. If the X's were all female, that might suggest a sex-related problem, but it could also be coincidental. ←Baseball Bugs What's up, Doc? carrots→ 12:00, 10 May 2017 (UTC)


 * Empirical evidence: About 30 years ago I collected extant data (sorry I can't remember where) on sex of a child as a function of sex distribution of a couple's previous children. I found that there is a mild tendency for couples with more children of one sex than of the other to have their next child be of the majority sex. If I recall correctly, having several children all of one sex increased by several percent the probability that the next child will be of that sex. The context was the question of what effect perfect advance selectability of children's sex would have on number of children per couple. Loraof (talk) 14:01, 10 May 2017 (UTC)


 * The article Sex ratio has a good discussion, including the passage sex ratios at birth may be considerably skewed by factors such as the age of mother at birth[3].  Also, the article mentioned by Dr. Dima above, Human sex ratio, has a good section called "Natural factors", which says These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress.  Loraof (talk) 14:12, 10 May 2017 (UTC)


 * Many decades ago I came across (in the New Scientist etc.,) of  couples being able slew the chances of getting a boy or girl  -by diet alone. Hans Eysenck noticed that airline crew had more daughters. Well back then, little girls did not grow up to be airline pilots -like dad, so dad wanted a little boy ( or two tow)  follow in dads footsteps. And hey what?!!! They fathered more little boys when their wives followed the diet. Didn’t think much of it at the time but now as the question has arisen- have googled it. Lots of references that this belief continues. (i.e., https://www.everydayfamily.com/gender-selection-what-is-your-personal-ph/) It makes some biological sense as well. Aspro (talk) 17:33, 10 May 2017 (UTC)


 * There's also the possibility that a couple that produces only daughters carry a mutation for an X-linked dominant disorder that is lethal in males in utero such as incontinentia pigmenti or Rett syndrome. -  Nunh-huh 02:39, 15 May 2017 (UTC)

What does it mean "Negative logarithm" of the concentration of hydrogen ions.
I've read on this site about the definition of pH as follow: "pH stands for potential hydrogen, and is defined as the Negative logarithm of the concentration of hydrogen ions." I understood whole the sentence except of "negative logarithm". what does it mean? 93.126.88.30 (talk) 17:05, 10 May 2017 (UTC)


 * Wikipedia has an article on pH which explains this. The logarithm usually turns out negative. The pH value doesn't include the minus sign, so an alternative definition is "the logarithm of the reciprocal of the concentration of hydrogen ions".  Ask again if you need more help.    D b f i r s   17:30, 10 May 2017 (UTC)
 * Thank you. Iv'e checked this article and honestly I still don't understand. There is a sentence there which says: "For example, a solution with a hydrogen ion activity of 5×10−6 = 1/(2×105)", then I don't understand how existence can be represented by minus (=no existence). 93.126.88.30 (talk) 01:16, 11 May 2017 (UTC)
 * Negative numbers do not mean "nonexistent". If you have a sample of pure water at STP, autoionisation will ensure [H+] = 10–7 mol·dm−3. By the definition of the common logarithm, log [H+] is the exponent 10 must be raised to to give 10−7, which is clearly −7. pH is defined to flip the sign and make it +7, because most common acid solutions have nowhere near a concentration of protons of 1 mole per litre. That's why it's not the logarithm, but rather the negative (additive inverse) of the logarithm. Double sharp (talk) 01:28, 11 May 2017 (UTC)
 * For negative powers of ten, our article Scientific notation might help.   D b f i r s   08:27, 11 May 2017 (UTC)


 * I'll try this from scratch. Positive logarithms are used for increasing quantities of substances: 10 balls is 1 log unit, 10000 balls is 4 log units.  Negative logarithms are convenient when the quantities are really small: a concentration of 1 in 10 is -1 log units, 1 in 100 is -2 log units, 1 in 10,000,000 is -7 log units.  That's because these concentration numbers would be written as fractions (1/10,000,000 = 0.00000001 in the last example, which is typical for neutral pure water.)  For pH, we also assume specific units - in general, a concentration recipe could be something like 1 teaspoon in 10 cups of porridge, but for pH we're defining it to be mole (unit) per liter, i.e. molarity, more or less.  (There are unimaginable levels of pedantry available about exactly how pH is defined, pH vs. p[H+], activity (chemistry) etc. - not even most scientists have any desire to know these details, but fortunately their effect is relatively small most of the time.  A basic college chem course will use the definition I just gave and stay mum about the rest.) Wnt (talk) 12:49, 11 May 2017 (UTC)


 * Negative logarithm means -1 times the logarithm (i.e. negative refers to reversing the sign of the number.) It isn't a different type of logarithm. RJFJR (talk) 19:55, 11 May 2017 (UTC)


 * Note that the part of the logarithm to the right of the decimal point is always positive.  See common logarithm.  — Preceding unsigned comment added by 86.151.48.31 (talk) 22:36, 11 May 2017 (UTC)
 * I don't think that's a helpful remark in this context.
 * What you're talking about is a convention for calculating with common logarithms, from back when people used to do that. It doesn't apply here.  If you have slightly alkaline water with a pH of 7.3, what that means is that &minus;log10[H] = 7.3, where [H] is the hydrogen-ion activity (roughly the same as concentration) in mols per liter.  So that means log10[H] = &minus;7.3, not 7 &#772;.3, which would equal &minus;6.7. --Trovatore (talk) 22:50, 11 May 2017 (UTC)

As more and more Hydrogen ions exist the more acidic it is?
I've read the information about pH here, then can I conclude that as more and more Hydrogen ions exist the more acidic it is? the more acidic it is, or it is better to say that as more and more the cations exist the more acidic it is while the more and more anions exist the more basic it is? 93.126.88.30 (talk) 17:21, 10 May 2017 (UTC)
 * Read Ocean acidification. --Kharon (talk) 20:50, 10 May 2017 (UTC)
 * You cannot simplify it to all cations and anions, no, since not all cations and anions are involved in acidity/basidity of water. Saturating water with NaCl will have negligible, if any, effect on the pH of the water, despite a huge amount of Na+ cations and Cl- anions now in the water. Additionally, you are going to have an equal amount (charge wise) of cations and anions in water anyways, since the charge needs to balance out. Water with more hydronium ions is more acidic. --OuroborosCobra (talk) 23:01, 11 May 2017 (UTC)

Thermodynamics of a permanent magnet
Does a permanent magnet's field weaken every time it is used to pick up a paperclip from my desk? It is after all doing work in lifting a mass, thus there must be waste heat. Roger (Dodger67) (talk) 18:50, 10 May 2017 (UTC)
 * No. Why is it, you think, they called it "permanent"? --Kharon (talk) 20:49, 10 May 2017 (UTC)
 * Proton decay. Sagittarian Milky Way (talk) 21:36, 10 May 2017 (UTC)
 * This is a red herring. ApLundell (talk)
 * Nothing with such a high percent of protons in it may be permanent. Of course that is irrelevant to whether the work wears out the magnet or just come from the person lifting the magnet and proton decay takes longer than human timescales to possibly happen. Sagittarian Milky Way (talk) 22:33, 10 May 2017 (UTC)
 * No. This question is a based on a very common misunderstanding of how magnets work. They do not expend energy that must be replenished.
 * They are simply converting potential energy to kinetic energy and back. Similar to moving a weight up a heavy hill and letting it roll down. The Earth does not wear out its gravity field when things move up and down.
 * This common misunderstanding often leads people to believe they have an idea for a perpetual motion machine. (They don't.)
 * As a practical matter, some magnets do weaken over time, but that's unrelated to the 'energy' they seem to expend. (But don't really.)
 * ApLundell (talk) 21:58, 10 May 2017 (UTC)
 * Was the comment way up above about proton decay intended to answer the very reasonable question, or was it an attempt at humor? And the claim that the magnets are "permanent" is also untrue, since they weaken over time. Horseshoe magnets in science demonstration sets came with an iron "keeper" with instructions to leave the keeper across the poles to reduce the loss of magnet strength over time. If I use a permanent magnet to an iron weight some distance, as in when the object flies up from a desk to the magnet, travelling a small distance through the air, work has certainly been done by the magnet, and the energy has to come from somewhere. Edison (talk) 00:17, 11 May 2017 (UTC)
 * Proton decay is mostly humor. Maybe insufficient coercivity or other things like orbiting the galaxy till hitting a black hole would demagnetize anything before the protons decay so if that's proven would only be a weak disproof of permanence by contradiction. Sagittarian Milky Way (talk) 01:43, 11 May 2017 (UTC)
 * Please do not make jokes or other pointless statements that seem like they might be answers to the question, but definitely are not.
 * It is very confusing to people asking questions. ApLundell (talk) 18:57, 11 May 2017 (UTC)
 * I like deadpan-type humor but very well. Point taken. I could never become a physicist and I know it. Not trying to impress. Who hasn't heard of proton decay? Sagittarian Milky Way (talk) 19:33, 11 May 2017 (UTC)


 * The question here covers a lot of different things which are peripherally related.
 * A magnet does work in picking up a paperclip, but the amount of work done is quite small, and the energy must be put back into the system from an external source if you expect to get your paperclip back.
 * In theory, a magnet might not need to produce much waste heat. For example, you might set up a little plastic track near a magnet and roll a ball along it as if you were rolling it in a dish under the effect of gravity.  In such a situation the ball might go back and forth quite a few times, dissipating relatively little energy in rolling friction on the track, before losing all its kinetic energy and coming to the lowest potential energy state at the closest point on the track.
 * That said, a conductor such as a steel ball moving through a magnetic field is going to experience some kind of eddy current, which will produce a sort of "friction" not seen with a ball in a dish. The current will tend to oppose the motion and slow the ball sooner than it would have otherwise, I think.
 * There is energy in the magnet that can be lost if it loses its magnetization. To illustrate this, consider what happens if you have a magnet you're screwing around with and you break it.  I suppose if you break it perpendicular to the N-S line then the north pole of one piece will stick to the south pole of the other, but definitely if you break it the other way you'll have two N-S magnets next to each other that want to end up flipped relative to each other.  Well, magnets are made of a whole lot of little paramagnetic atoms (or in theory molecules, I think) which in ferromagnetism form up into large domains where they are all lined up the same way for various reasons; nonetheless, beyond a certain scale the force of the opposing field will flip them if you simply heat up the magnet and cool it again, leaving you with an ordinary piece of metal or ceramic.  You can also ruin a magnet by bashing on it hard enough, I think.
 * Which brings me to the interesting center of your question: I'm not entirely sure if the small magnetic field induced in an object approaching the magnet due to eddy currents will have any incremental effect on the alignment of domains in the magnet.  After all, these are very small regions which, by random chance, will tend to lose their alignment anyway over time.  I suspect the effect of that and even the vibration of the paperclip hitting the magnet might truly be insignificant relative to other effects, but I don't know that. Wnt (talk) 13:06, 11 May 2017 (UTC)
 * Wnt's language may make someone believe that you can break the "north" and "south" poles of a magnet. You can't. Tigraan Click here to contact me 16:45, 11 May 2017 (UTC)

Why do fat people have a lower pain threshold?
We can read here: "An extra layer of fat won't provide a cushion against pain -- in fact, obese people are more sensitive to pressure pain than those who are not overweight, and they are equally susceptible to extremes of hot and cold."

The explanation offered in the article didn't sound all that credible to me:

" "It could be the case that a person who is more sensitive to pain is less likely to do physical activity and therefore more likely to gain weight and become obese," says Dr Tashani."

You would then expect the result to apply to just a fraction of the obese people, if their results then affect the average of the entire group of obese people, you would end up with a significantly larger standard deviation in that group. Count Iblis (talk) 22:14, 10 May 2017 (UTC)


 * I think the important comment is: "The team plan to carry out further research into the factors that make people more susceptible to pain. This includes examining the chemicals secreted by fatty tissues in the body which could affect the response of pain receptors." The team's results show a possible correlation but not a cause.    D b f i r s   23:47, 10 May 2017 (UTC)


 * You have given us the source. You ask us "why" and then argue "You would then expect..."  It seems like you want us to give our opinions, or validate your conclusion.  Is there something else you are looking for? If you want anecdote, I can tell you that having lost some 100lbs, I can now walk barefoot without pebbles and twigs being excruciatingly painful to step on, yet I find I still act as if I am at my heaviest weight.  Weighing now what I did in the early nineties I am still very careful about motions that before might have caused injury, but which now are presumably much safer.  (For example, I used to take two steps at a time going up stairs, now I put each foot on a step before feeling fror the next one, expecting a fall.)  I notice no difference to my reaction to burns or splinters, though. μηδείς (talk) 00:16, 11 May 2017 (UTC)


 * If you want a hypothesis off the top of my head, well, POMC mutation causes obesity, and involves the loss of an endogenous opioid chopped up out of that protein. Perhaps that contributes to "runner's high" and also reduces pain sensation in other settings.  The mutation I describe also causes red hair, which is not mentioned in your source article and might be useful to look at if the statistic is available.  However, the effect of the opioid could also be inhibited if its receptor is less active, or something its receptor sets off is less active, and none of those mutations would have this particular effect.  That said -- this is just a random hypothesis and the odds that this is what the researchers come up with is quite low.  There are a lot of ways to invoke biochemistry to relate a particular cause with a particular effect - so many things interact with so many other things, it's like looking at a complicated roadmap and trying to say which route a car must have taken. Wnt (talk) 13:14, 11 May 2017 (UTC)
 * Okay, but obesity is multifactorial. My dad was 260lbs (6'2) when my parents got married, and both sides of the family had historic experience with famine.  While I and my older sister have always been overweight, and she has a BMI over 45, my youngest sister was very thin.  She died of a heart attack at 20; one of those "athlete drops dead" stories.  I suspect my middle sister and I inherited genes from our parents which, in combination, led to obesity.  But there's also the possibility that the two eldest of us got adenovirus 36 which is linked to obesity as well.  In any case, the only thing I have noticed with pain sensitivity is that as I have aged I ca disassociate from it.  I don't think the intensity is any less, but the attention I pay it is.  I know it is normal, and will pass.  Little kids live in the moment.  Adults live in the lifetime. μηδείς (talk) 22:20, 12 May 2017 (UTC)


 * Thanks everyone. Count Iblis (talk) 19:52, 13 May 2017 (UTC)

Universe expansion
I've tried to use Hubble's law to estimate an approximate value of the size of the observable universe and found some difficulties to reach the same value (93 billion light years). If I assume the furthest point where light can still reach us regardless expansion, I realize it must be less than 13.8 Bly, specifically 13.8/e and so, the universe should then expand by a factor of e till light reach us. If I assume an initial size of 13.8 Bly, I realize light cannot reach us due to the fact that expansion is almost the same rate as light speed at that point. Is there something I am missing?Almuhammedi (talk) —Preceding undated comment added 22:35, 10 May 2017 (UTC)
 * The visible universe is about 93 billion light years across. In any direction we look we see the cosmic background radiation which is now a little over 46 billion light years away, this is because not only has it been moving away from us over a little more than 13 billion years, the metric expansion of space has also been increasing the overall distance. μηδείς (talk) 00:08, 11 May 2017 (UTC)
 * But how will you apply Hubble's law in this case? — Preceding unsigned comment added by Almuhammedi (talk • contribs) 09:23, 11 May 2017 (UTC)
 * See Particle_horizon. Dragons flight (talk) 13:13, 11 May 2017 (UTC)