Wikipedia:Reference desk/Archives/Science/2010 October 19

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October 19[edit]

Modified Stern-Gerlach device[edit]

In the modified Stern-Gerlach device described by Feynman and Townsend in their respective textbooks where the two beams are allowed to combine after splitting (if you haven't seen what I'm talking about, just ask and I can try to explain), what would constitute a measurement of the particle's spin? If a bunch of electrons were fired at once, and we observed the particles travel, would a measurement have taken place? (talk) 02:06, 19 October 2010 (UTC)

As a preliminary nitpick that doesn't really help answer your question, it doesn't work as intended to send mere electrons through a Stern-Gerlach device. The mass of an electron is so small that the uncertainty of the electron's momentum in the z direction winds up causing there to be only one spot at the end of the device, instead of two. Instead, something more massive like silver atoms are used.
As another qualm about the question, I think it would be better to specify that there are a bunch of particles that go through the device one at a time, rather than all at once. Otherwise, you'd really have to consider interactions between the particles such as potential interactions or exchanges of particles.
With those two caveats out of the way, I'm rather certain that the answer to your question (as modified) is yes. A measurement of the position of a particle in the middle of the modified Stern-Gerlach device amounts to a measurement of the particle's spin. In this situation, if you know what the particle's position is (roughly; you don't want to run into problems with the Heisenberg uncertainty principle totally screwing up the particle's momentum), then you know what its spin is, and vice versa. So a measurement of position and a measurement of spin in this case are one and the same. Red Act (talk) 05:01, 19 October 2010 (UTC)
You're right, I should have said silver atom (which also has the advantage of being neutral). Thanks a lot for your explanation. (talk) 21:15, 19 October 2010 (UTC)

sofa acquiring static electricity from sunlight[edit]

OK so I opened my blinds in my apartment all day to help keep it warm and when I got home the sun was shining directly on the new (microfiber faux suede) sofa so I closed them, thinking that it might get damaged if left under the sun for long periods of time.

Later on that day I sat down on it to feel the hair on my arms standing on end; it was really statically charged, and now, 5 days later! it's still just as strong! can the sunlight coming through a window do this to a microfiber sofa? and how can I neutralize it because it's starting to get annoying! (talk) 03:10, 19 October 2010 (UTC)

I kind of doubt sun could do that, but to neutralize it spray it with some water. I was going to suggest a dryer sheet, but the article says not to use them on microfiber cloth, plus I don't think the chemicals are safe to touch. Ariel. (talk) 05:13, 19 October 2010 (UTC)
If those chemicals aren't safe to touch, then therefore they must not be safe to use on clothes, which touch your skin all day. Which is pretty obviously a silly statement. → ROUX  06:16, 19 October 2010 (UTC)
Dryer sheets contain chemicals that are known carcinogens if ingested. The FDA and similar agencies allow their use in dryer sheets (and also shampoo, laundry detergents, etc.) based on the belief that a negligible amount of these chemicals will pass through the skin. So, if you believe the FDA, dryer sheets would be safe to touch but not to eat. Dragons flight (talk) 06:44, 19 October 2010 (UTC)
There does exist a photoelectric effect, which can result in an object becoming a bit positively charged due to incident light. However, the voltages involved in the photoelectric effect from visible light are much smaller than the voltages involved in static electricity. Plus, I think the energy gap between a free electron and the HOMOs in the microfiber is much too large for the photoelectric effect from visible light to create a non-negligible charge, anyway. There's just no way the problem here is the light.
In fact, I think it's unlikely that it's even the sofa that's charged. I think the sofa is probably actually pretty close to neutral, due to a slow discharge through the humidity in the air. Instead, it's you that's picking up a charge, as you walk across your carpet toward your sofa. I think the solution to the problem should focus on discharging you, rather than discharging your sofa. You can discharge yourself by touching anything that's grounded, such as one of the screws holding on the faceplate over a light switch or outlet near your sofa, if there is one. If you've built up an adequate charge, you will get a brief little shock that way, if you touch the screw directly. If you want to avoid the shock, you can discharge yourself "slowly" (meaning milliseconds instead of under a microsecond) by connecting one end of a resistor to the screw, and then discharging yourself by touching the other end of the resistor. A 10M ohm resistor, which you can pick up very cheaply at Radio Shack, should do the job nicely. Also see Static electricity#Removal and prevention of static electricity. Red Act (talk) 07:12, 19 October 2010 (UTC)
That's nice. I was going to suggest to place a tin foil mat on the floor, ground it, and step on it before stepping on the sofa. --Chemicalinterest (talk) 17:16, 19 October 2010 (UTC)
An 18th or 19th century scientist could have used a proof plane and an electroscope to determine if the sofa was the charged body or if it was grounded. Modern versions of that instrument may be called the Electrometer. A synthetic cloth surface would not seem like much of a ground. A leather surface connected by a wood frame to a wood floor would act as a ground and bleed off charge. My grandmother's ancient auto had a woven seat of some plastic fiber, and scooting across it would build up a strong static charge so I got an audible shock when I touched the metal door handle. The sun might heat your sofa up and dry it out and make it better able to hold charge. The charge might come from friction across the surface of the sofa, or from walking across the floor. You could try walking across the room as to the sofa but touching a grounded object to see if you have built up a static charge from a carpet. Edison (talk) 00:05, 20 October 2010 (UTC)
I wonder if the wavelengths in sunlight would cause a charged body to discharge quicker than one in the dark? If two gold leaf electroscopes were charged so the leaves diverge at the same angle, and one were in sunlight while the other was in the dark, which would discharge faster? I have read that quick loss of charge of an electroscope was an early (pre-Geiger counter) test of ionizing radiation. Edison (talk) 15:37, 20 October 2010 (UTC)

Forests half blown down[edit]

Resolved: Kerαunoςcopiagalaxies 08:03, 19 October 2010 (UTC)

In the US Rockies, there are forests that have standing trees, but about 60 or 70% of what trees would be the remainder of the forest are flat on the ground. It's very eerie. It almost looks like a logging truck went through and just knocked everything down. The fallen trees are extensive, in the thousands, but the standing trees are numerous enough that satellite imagery shows nothing but "forest". This type of phenomena extends over many miles, not connected, and not just an isolated incident. What would cause this? Microbursts? Alien spaceray wars? I can't find a picture on Google because I have no idea what to look under. – Kerαunoςcopiagalaxies 06:35, 19 October 2010 (UTC)

If you have looked at satellite imagery does that mean you have some coordinates ? They might be helpful. Sean.hoyland - talk 06:54, 19 October 2010 (UTC)
You seem to be referring to windthrow. Although not clearly stated in the article, this is a domino effect.--Shantavira|feed me 07:35, 19 October 2010 (UTC)
Bingo, that's exactly what I'm looking for. Google images of that keyword brings up similar looking areas. Thanks so much! – Kerαunoςcopiagalaxies 08:03, 19 October 2010 (UTC)

How long is my exon?[edit]

Here's the Ensembl record for a gene of interest: What I'm confused by is the different lengths of the exons - why are there two different versions and how do I determine which is correct? ----Seans Potato Business 10:09, 19 October 2010 (UTC)

I'm no expert in myostatin, but this is definitely a "teach a man to fish" moment...
<soapbox> My advice would be to enroll in a bioinformatics course at your University as this should be core knowledge for anyone working in biosciences in the post-genome world. Read some of the basic literature on your gene of interest, spend some time browsing the different genome tools to figure out how the evidence for different gene models is accumulated. Compare the sequences of your gene of interest in different species -- how are they similar? how are they different? Ask your faculty advisor for insight, and learn how to do basic sequence analysis -- there's too much to try to explain here in the Ref Desk format. </soapbox>
Now, to answer your questions: One explanation for multiple isoforms of a gene is alternative splicing. Another explanation is a cloning artifact. My best guess is that the chicken transcript of this gene has not been completely sequenced from the 5' cap to the 3' poly-A site. I say this because in the Ensembl record you linked to, the longest transcript (1174 bp) has the initiation methionine at position 1 of the cDNA -- there should be some amount of 5' untranslated sequence upstream of the start codon. There is also very little 3' untranslated sequence in that version. If you look at other complete cDNA transcripts (I found them using the NCBI, which I prefer) you will notice the following:
Total length 5' UTR cds 3' UTR
Mouse 2682 bp 103 bp 1131 bp 1448 bp
Cow 2767 bp 133 bp 1128 bp 1506 bp
Human 2823 bp 133 bp 1128 bp 1562 bp
So, the actual coding region is 1228 bp in human and cow, and mouse has one extra codon somewhere... It looks to me as if the intron-exon structure of the chicken gene has therefore been deduced by comparison to other species and that the reported cDNAs were generated by PCR of the open reading frame. If you notice, the other long transcript in the Ensembl record (1128 bp) is exactly the same length as the open reading frame of the human and cow sequences which suggests that only the chicken ORF was cloned in that version of the transcript. This also seems to be the case for sheep (1228 bp) and pig (1228 bp) transcripts that I found at NCBI. However, the strange thing about the 1228 bp version in Ensembl is that the predicted protein sequence is much shorter and begins at a serine residue at position 301 and it doesn't end with a proper stop codon. There must be something wrong with that annotation. I think I'll let you take it from there... --- Medical geneticist (talk) 13:35, 19 October 2010 (UTC)
Hmmm... I looked at the same thing and reached a different conclusion. First, I don't think it's really a matter of bioinformatics (not like, say, calculating maximum parsimony trees) — just biology, combined with navigating a particular very nice though somewhat confusing web site.
Now as for the two different versions on the original page cited,[1] I assume you mean the difference between the area bolded with caption "ENSGALG00000023961 exons" versus the area highlighted with "Ensembl exons in this region". Note first that the name of the sequence you're looking up (in the URL) is this ENSGALG...23961 — so the bolded exons are probably what you want. Note also that the bolded exon is followed by "GTAAGTAA..." which is always a hopeful sign (though not infallibly so) while the other one is followed with a "GAGTCA..."
Now let's get clicking. Try "Supporting evidence", then on the next screen "align" for each of the five actual cDNAs. Most don't start with M, but the fourth gives a perfect alignment of MQKLAVYVYIYLFMQIAVDPVALDGSSQPTENAE... and the fifth is close - none have any amino acids added or missing. Note by the way all those V,I,A,L,F,Y ... the K before it... the SS after it... this smells like a signal peptide for secretion (compare [2]). And if you're looking at a signal peptide it's pretty likely that's the real start codon, though you might want to hunt around for upstream stops just in case.
But what about that other exon length? Well, under "supporting evidence" click "splice variants". You'll see four different transcripts - the first with the start methionine I mentioned above, the fourth transcribed in the opposite direction which gives the extended sequence that was highlighted as a 5'UTR, and #2-#3 with CDS all the way to the 5' end, it looks like. But click on the source for the second transcript and it gets you to a Uniprot entry that begins MQKLAVYVYI... so the graphic is the only thing that's really different, so far as I can tell.
In other words, you have two genes in the region, each of which start from a promoter, go through a section of 5'UTR that is coding on the opposite strand for a different protein, then some 5'UTR that is shared by both genes in opposite directions, then coding sequence that is 5'UTR for the other protein, then skip over an intron where the opposite gene leaves room for the other gene's promoter. Wnt (talk) 15:34, 19 October 2010 (UTC)
That's a good point. I thought he was referring to the difference in lengths between GDF8_CHICK (ENSGALT00000040372) and Q9I8H5_CHICK (ENSGALT00000003669), so I didn't pay attention to the sequence mark-up below. Boy, do I hate Ensembl. The oppositely transcribed gene that Wnt sees can also be visualized quite nicely by looking at this region in the UCSC genome browser, where it looks like there are two different transcripts on the opposite strand from the myostatin gene that overlap with some of the myostatin exons. By suggesting that Sean learn some bioinformatics I was really meaning just that he needs to learn how to use the tools of the sequence analysis trade, as it were. --- Medical geneticist (talk) 16:55, 19 October 2010 (UTC)
I see the problem you're describing. When I did the alignment of the Q918H5 transcript as described above, it matched the full length sequence with some errors... yet when I page to "exons" from that sequence, it gives the impression that large portions of the first two exons are missing, and gives an odd translation starting in the middle. I didn't even find mention of the Q918H5 on Entrez.
For the moment, I'd ignore it as a bad sequence without thinking much about it. If someone's actually looking for odd splices of myostatin, I'd suggest going back and doing Blast searches with the full genomic sequence against the chicken cDNA database in Entrez, and see what odds and ends match up directly. Wnt (talk) 19:47, 19 October 2010 (UTC)

what do you do with a hypothesis? "decide" it?[edit]

Do you "decide" a hypothesis? (It sounds very mathy like that to me) or what?

"resolve" doesn't sound right to me either, and I can't think of other words like that at hte moment. —Preceding unsigned comment added by (talk) 11:12, 19 October 2010 (UTC)

Do you mean like "support" or "disprove"? That's what you might do if you already have a hypothesis; your question isn't very clear. You could also write a hypothesis, sing a hypothesis, ignore a hypothesis etc etc... ----Seans Potato Business 11:17, 19 October 2010 (UTC)
I would usually suggest a hypothesis, then test it. After I've thoroughly tested it, I'd hope to have either confirmed it or disproved it, but I'd usually settle for having found evidence to support or oppose it. (talk) 11:33, 19 October 2010 (UTC)

sorry I wasn't clear guys. This is not my hypothesis. It's really a conjecture. (it's called hypothesis for historical reasons). For example, take "Riemann Hypothesis". What do you do with the riemann hypothesis when you prove or disprove it: do you decide it? Do you resolve it? What do you do with it, meaning you have dispatched with it as an open problem. (I'm looking for the general word, not the more specific word which would be prove or disprove). (talk) 11:59, 19 October 2010 (UTC)

I don't expect to resolve the Riemann Hypothesis any time soon. But should that happen, it will no longer be a hypothesis. It may then be called an accepted theory or an abandoned proposition but either way, to resolve a hypothesis deletes it as a hypothesis. One solves a problem, resolves a hypothesis or settles an argument. Cuddlyable3 (talk) 13:21, 19 October 2010 (UTC)
Yes, one tests a hypothesis, in order to then either accept or reject it according to the outcome of the test.WikiDao(talk) 16:34, 19 October 2010 (UTC)
One proposes or introduces a hypothesis due to perceived patterns or intuitions of how something works. To see if a hypothesis is valid, one proves or disproves it. -- Sjschen (talk) 17:06, 19 October 2010 (UTC)

In experimental science where hypotheses are tested statistically, you either reject a hypothesis, or fail to reject it. Those are the only two possible outcomes. Looie496 (talk) 22:28, 19 October 2010 (UTC)

Lithium hydroxide reaction with carbon dioxide[edit]

I made lithium hydroxide by tossing lithium in water. I let it sit out for a couple days (skimming the slimy petroleum jelly from it) and a hard crystalline solid formed on the top. Is that lithium carbonate? It formed a layer on top of the lithium hydroxide. --Chemicalinterest (talk) 17:13, 19 October 2010 (UTC)

Test it with acid - you'll get CO2 forming again if it is indeed Li2CO3:
Li2CO3 + 2HX → 2LiX + H2O + CO2
Ben (talk) 17:23, 19 October 2010 (UTC)
I did get an amount of CO2. And I wondered why my potassium hydroxide stored for months in an open egg carton fizzed so much when acids reacted with it! --Chemicalinterest (talk) 19:00, 19 October 2010 (UTC)
Don't underestimate the Lewis acidity of carbon dioxide. The effect is even more potent if the ensuing carbonate salt is less soluble in water (mole for mole) than CO2 -- it precipitates out of solution, allowing more CO2 to dissolve and driving the reaction forward. John Riemann Soong (talk) 03:20, 21 October 2010 (UTC)
I do wonder what happens if you add alcohol to the solution -- will you form carbonic acid esters? John Riemann Soong (talk) 03:25, 21 October 2010 (UTC)

Special PCR machine for touchdown PCR?[edit]

I was just wondering whether there are special PCR machines for touchdown PCR that just let you specify a few parameters and then it does the rest. I don't know for sure, but I imagine some of the older ones you'd have to specify each increment individually, making the programming of the machine time-consuming and irritating. ----Seans Potato Business 17:31, 19 October 2010 (UTC)

I think most if not all "normal" PCR machines on sale today allow you to program touchdowns easily. At least with the Bio-rad and Applied biosystems ones I've used you need only to specify how many degrees you want to lower (or raise) the Ta per cycle and for how many cycles. Otherwise the programming does not differ from a regular PCR program (which can of course pe painful, especially with some of the Bio-rads) BTW: what do you consider old? Oldest of the machines I've used like this was about 10 yrs old and on this field of science, that's a lifetime:-) --Albval (talk) 13:20, 20 October 2010 (UTC)

NIR Technology[edit]

what is NIR Technology on clothes. is it a weave or a chemical treatment —Preceding unsigned comment added by Kj650 (talkcontribs) 17:33, 19 October 2010 (UTC)

It appears that NIR refers to near infra red. Googling 'NIR fabric' reveals several papers about using NIR optics to analyze fabrics for quality control, but 'NIR compliant' seems to be used to refer to military applications, wherein fabric is designed to mask the presence of soldiers to those using infrared ('night vision') surveillance devices. See [3]. In short, NIR is not a technology used to make or treat fabric directly. Many technologies (such as chemicals/weaves that you mention) may be used to reduce the NIR signature of fabric, but I don't have any info on that. --SemanticMantis (talk) 18:38, 19 October 2010 (UTC)

how is NIR fabric made then —Preceding unsigned comment added by Kj650 (talkcontribs) 18:44, 19 October 2010 (UTC)

"NIR" is near infrared. You can think of "near infrared" as a sort of "color" or "shade" of infrared light. It is called "near" because this "color" of light is "near" to regular visible red - it has a very similar wavelength - but is just barely invisible to the human eye. It is just outside the range we are sensitive to. NIR is not a type of fabric, nor is it a process applied to fabric. It so happens that much night-vision technology is sensitive to this "color" - so "NIR technology" might be used to analyze how a particular fabric looks when viewed with NIR equipment. This would be compared to, say, "far infrared" - a different "color" of infrared that is "farther" from the regular colors we see with our eyes. Just like any piece of cloth might be a mix of regular colors (it might contain red, green, and blue), it may also be brightly colored in the infrared spectrum. If you wanted to hide from night-vision goggles, you would prefer a cloth that was not bright when viewed in the infrared spectrum. Nimur (talk) 21:36, 19 October 2010 (UTC)
I did a little looking around on the Web, but it's not my field. From forums and searches it's apparent that a large fraction of NIR-compliant fabric is 50% nylon, 50% cotton ("50/50 NYCO"), but this is chosen to resist ripping of uniforms. There are many near infrared dyes, but I've had trouble finding which is good for dyeing fabric, which is a very special function. "Vat-dyed" fabric seems to be desired to resist exposure by wear and wrinkling. Also there are people advising to avoid non-"free" detergent mixes, which could include brighteners that could be visible, and never to starch clothing.
The most practically interesting thing I found was that carbon black is black to all frequencies of infrared,[4] which is not unexpected considering the recent Nobel Prize for the scientist who discovered that single layers of graphite absorb light dependent only on the fundamental alpha constant of physics, not wavelength. So in an emergency situation or novel where one finds himself being hunted by commandoes in a helicopter, charred wood or copier toner should offer a basic way to smudge up and conceal oneself in infrared. Though I'd like to have a picture before I say that with confidence - maybe we should beg someone with an IR camera to ruin some old clothes for the 'pedia? Wnt (talk) 05:51, 20 October 2010 (UTC)

Multiple partners[edit]

Why do some men find it so appealing to have sex with lots of partners? Is it bragging rights - in my experience sex does not feel dramatically different from one woman to the next (still fun though!) and it's always better when you're in a communicative relationship. TheFutureAwaits (talk) 20:15, 19 October 2010 (UTC)

"Variety is the spice of life." ? :-) It's personal preference, with a dash of primitive biological urge to impregnate as many different females as possible. The Masked Booby (talk) 21:43, 19 October 2010 (UTC)
Darwin's theory of natural selection would suggest this behaviour has an advantage in preserving and broadcasting the relevant gene(s). Males with the relevant gene are likely to be more successful at producing larger numbers of offspring than males without this gene, and as a result the gene(s) become increasingly prevalent among male populations.
The reason this behaviour has not become universal, and has not expanded to extreme proportions, is probably because this behaviour imposes a cost, and an optimum condition exists. Perhaps males with the propensity for extreme promiscuousness are less successful at producing offspring than those with the propensity for being monogamous. For example, in humans perhaps females find this promiscuousness unattractive, preferring instead a male with good prospects of being monogamous and sticking around to help raise the brood. Dolphin (t) 21:46, 19 October 2010 (UTC)
And it's not just men. thx1138 (talk) 21:52, 19 October 2010 (UTC)
Actually, Dolphin, there seems to be a lot of evidence that women will settle down with the nice stable monogamous guy... but have it off with the promiscuous handsome man. Best of both worlds; get the good genes for one's child, and the fathering capabilities of the decent man. I believe I read this in The Meme Machine. → ROUX  21:58, 19 October 2010 (UTC)
That is until they realise that the kids aren't there's theirs,(silly me) leaving the mother a single parent. To gain some insight from other animals, check out the links in mating system. Smartse (talk) 22:18, 19 October 2010 (UTC)
In my case, I just find it hard to say no, and hurt the feelings of all those girls. Myles325a (talk) 04:30, 23 October 2010 (UTC)

Tin(II) oxide hydrate[edit]

Black tin(II) oxide

What color is this chemical? White? --Chemicalinterest (talk) 20:53, 19 October 2010 (UTC)

Yep! Default choice, I'm afraid, nothing interesting in its colour. Physchim62 (talk) 21:06, 19 October 2010 (UTC)
Not sure it's white or that it's not interesting--seems like it depends on preparation, particle-size, level of hydration--per this lead ref: doi:10.1021/j150342a015. DMacks (talk) 21:17, 19 October 2010 (UTC)
How can it be converted to the interesting red or blue-black tin(II) oxide? Would gently heating it in air suffice? --Chemicalinterest (talk) 21:33, 19 October 2010 (UTC)
Tin(II) oxide, I would imagine that the references have more specifics. --shoy (reactions) 13:39, 20 October 2010 (UTC)
I can try heating it gently, although it probably all oxidized to tin(IV) oxide. --Chemicalinterest (talk) 19:15, 20 October 2010 (UTC)
It was blue-black tin(II) oxide after I heated it. --Chemicalinterest (talk) 11:57, 21 October 2010 (UTC)
Until you read given refs or look up the exact conditions you are using or specifically test, you cannot know which oxidation state you have. Whole papers get retracted when they guess at chemicals rather than actually analyzing them--especially when there is evidence that the same chemical formula can exist in different forms. DMacks (talk) 16:52, 22 October 2010 (UTC)

Wave function for large objects[edit]

Newton's laws are only equipped to handle point particles, but something like a baseball can still be analyzed by considering the motion of its center of mass as a particle. To what extent does this work in quantum mechanics?

All fundamental particles have wavefunctions which obey the Schrodinger equation, right? So would something like a proton, which is composed of elementary particles, also have a wavefunction satisfying the Schrodinger equation? What about a baseball? The reason I ask is that my teacher talked about the de Broglie wavelength of a baseball, and I was wondering how we could assume the baseball is still a wave obeying the de Broglie equation. (talk) 21:25, 19 October 2010 (UTC)

A baseball is composed of many individual atoms, each composed of individual sub-atomic particles, and each one of those has a wavefunction. The baseball as an aggregate of molecules behaves as an ensemble - and a non homogeneous one, at that. So, if you want to properly model the quantum behavior of a baseball, you must treat every constituent quantum particle and all their quantum interactions. This is impractical (it is computationally infeasible, because the number of interacting quantum particles would be in the billions-of-billions-of-billions). But, you can apply quantum statistical mechanics to some extent, though in the case of the nonhomogeneous baseball, correct application of this approach is still probably computationally impossible. Or, you can "handwave" and pretend that a baseball is a single particle defined only by its mass and momentum (and other quantum numbers, as if it were an incredibly massive "electron" or something) - but while this might be a fun thought-experiment, you will get meaningless, useless results if you make such assumptions. Nimur (talk) 21:28, 19 October 2010 (UTC)
The baseball example is a common one, and is just meant to illustrate that the wave function, while it does technically apply to macroscopic objects, is essentially invisible at such scales. E.g. the de Broglie wavelength of an electron is something like ~10-9 cm, and a baseball is more like ~10-30 cm. The wave-like nature of the baseball is effectively invisible at the macroscopic scale and thus doesn't really come into play at all. An electron's de Broglie wavelength is large enough that it actually interacts meaningfully on an electron's scale. Such read some notes I took of a QM lecture a long time ago... Anyway, all of this is to say that while the results like the above are certainly hand-wavy, they do illustrate the basic principle pretty well for beginners, no? That would rank them above "useless" to me. --Mr.98 (talk) 21:54, 19 October 2010 (UTC)
Sure - it's a thought experiment; it has pedagogical use. But if you tried to calculate the baseball's collisional cross-section based on its de Broglie wavelength (10-30 cm, you say?) the errors due to a single piece of microscopic dust on the surface (that you forgot to account for) would be billions of times more relevant than the de Broglie wavelength. So, the quantum treatment is useless for practical predictive purposes. Nimur (talk) 23:03, 19 October 2010 (UTC)

You can factor out the center of mass motion of an extended object like a baseball, just like you can do in classical mechanics. Unlike in case of classical mechanics, this only works when the object is freely moving, i.e. not acted on by external forces. The proof is trivial, all you have to do is observe that the total momentum operator is the generator of translations and that the Hamiltonian is invariant under translations. So, the center of mass motion is that of a free particle for which momentum is conserved.

In a more realistic situation, there will be environmental influences acting on the system. This means that a single wavefunction description for the center of mass motion is not possible, as this will get entangled with environmental degrees of freedom; this happens extremely rapidly. This entanglement (also called decoherence) leads to the cente of mass of macroscopic objects being localized. Otherwise, you could imagine large objects being in free momentum states that are completely de-localized.

Now, what you still can say is that there exists a many particle wavefunction that describes the object and the environment. If you look a how this behaves as a function of the center of mass of the object, you see that for very small variations of the position of the order of the thermal deBroglie wavelength, h/sqrt(m k T), the environmental part of this wavefunction does not change appreciably. Over this small range, the environment "does not know" where the center of mass of the object is, and the single particle wavefunction for the center of mass will still be valid over that range. The wavefunction is of a Gaussian form with this width of order h/sqrt(m T), but it doesn't evolve and spread like that of a free particle evolving under the free Schrödinger equation, because the interactions with the environment effecively leads it to collapse to within this range.

What is interesting about this is that the localization of the center of mass to within a range of h/sqrt(m T) due to decoherence implies that the momentum is uncertain to within a range of order sqrt(m k T). For macroscopic objects this uncertainty purely due to quantum mechanics, is actually quite large. This makes it possible to actually do quantum experiments, like the two slit experiment where one can observe an interference pattern. If you imagine some interferometer containing mirrors where the photon can take different paths, you can ask why the momentum imparted by the photon reflecting off a mirror doesn't give away the "which path information" that should destroy the inteference pattern.

The reason is that the center of mass momentum of the mirror is not sharply enough defined. So, when the photon imparts its momemtum, you can picture that as a big fat wavefunction in momentum space shifting by an amount that is far less than its own width. The overlap between the two possibilities of the photon having imparted a momentum or not, is then very large. So, the state the mirror is in gives away almost no information about the photon's path. Count Iblis (talk) 03:38, 20 October 2010 (UTC)

Okay, thanks a lot. (talk) 00:07, 21 October 2010 (UTC)

Oyster name[edit]

Does anyone know the english name for this shellfish? External Forum Link. It's a rare delicacy i often see (and enjoy!) in greece, but i only know that the greek name is φουσκα (bubble). Many thanks for any help. (not sure if science or misc is the best board...?) Capuchin (talk) 23:20, 19 October 2010 (UTC)

A bit of googling tells me that it is a grooved sea squirt, Microcosmus sulcatus, also known as a tunicate. It is also mentioned in our own article Ascidiacea#Culinary. It is not a shellfish, by the way -- it's actually more closely related to a human than to an oyster, in evolutionary terms. Looie496 (talk) 23:52, 19 October 2010 (UTC)
Well, sort of. It's probably among the most primitive member of the chordata phyllum, which also contains humans. Let's just say its about half way between oysters and humans. --Jayron32 02:48, 20 October 2010 (UTC)
Jayron, if it is a cordate than it is closer to a human than to an oyster since humans also are cordates and oysters are not. (talk) 02:55, 20 October 2010 (UTC)
Yes, but depending on how far back the Most recent common ancestor is to the oyster and the tunicate. It's entirely possible for a primitive chordate to be a closer cousin to a molusc than to another, more descended chordate. --Jayron32 02:59, 20 October 2010 (UTC)
No, that's not possible, assuming we are talking about extant species. (talk) 03:27, 20 October 2010 (UTC)
(Further taxonomic de-rail) Interesting point 174. So, for Jaryon's hypothetical case, the primitive chordate that is more closely related to oysters than humans must perforce be extinct. SemanticMantis (talk) 14:14, 20 October 2010 (UTC)
No, the oyster must be perforce extinct. That's a side point though. What I'm argueing with Jayron about is, off course, semantics. What do we mean by closeness? I say that the best measure of closeness is indded the most recent common ancestor (MRCA), and that the further back in time the MRCA happens to be, the further apart the species are. Well, The MRCA of any coordate with any non-cordate will be more ancient then the MRCA of any two cordates. That is why it makes sense to put all the cordates into a single bin (labeled cordates obviously). All the species in that bin are more closely related to each other than they are related to any species outside of the bin. If that was not the case than the definition of cordates would be an arbitrary pointless useless definition. BTW, I'm (talk) 15:22, 20 October 2010 (UTC)
It's an interesting question, though, and one that pretty much turned biology on its head for a bit as cladistics redefined how we think of things being grouped together (as opposed to the familiar Linnean hierarchy). I'm out of the loop a bit now, but bird taxonomy was one of the ones that I recall got some major rewrites over the last couple of decades. Talking about MRCA can be a bit unsettling when we're including squishy stuff (to use the technical term) in with humans, but it really is no more complicated than asking: "Am I more closely related to my father than my cousin is?" If you figure that the answer is yes, then you pretty much have to agree that all chordates are more closely related to themselves than to anything outside of the chordate group. Matt Deres (talk) 16:51, 20 October 2010 (UTC)
Ok, an attempt to reconcile 174 and Matt's comments. Extant cordates A and B must have a more recent MRCA than A and X, for any extant non-cordate X. I think I had it wrong in my comment above, but my understanding in light of 174's correction is that cordate A could be closer (in sense of MRCA) to an EXTINCT non-cordate Y than it is to extant cordate B. Anyone care to confirm or reject this scenario? --SemanticMantis (talk) 16:20, 21 October 2010 (UTC)
Yes, that's correct. The thing to watch out for when talking about historic relations like that is conceit of talking about species. Consider the last million generations of your "family"; it would reach back to the days before humans existed, but there'd be no place to draw a line and call one side species A and the other side species Y. Matt Deres (talk) 18:44, 21 October 2010 (UTC)