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May 2

Correspondence section of scientific journals

Some scientific journals appear to have a section of "correspondence", usually addressed "to the editor". They often look more or less like very small articles, complete with graphs etc. What is it all about? Is this what happens when there is not enough data for a full article? —Preceding unsigned comment added by 137.120.53.1 (talk) 01:25, 2 May 2009 (UTC)[reply]

Sometimes a Correspondence section is used for contributions that aren't substantial enough to make a full article, but more frequently they're used for letters that object to or otherwise comment on things that the journal has previously published. It varies a lot, there really aren't any solid rules about what can go there. Looie496 (talk) 01:31, 2 May 2009 (UTC)[reply]

They are also a way to rush a new discovery into print — typically the turn-around on such little things is a lot faster. (Probably because they aren't peer reviewed, just reviewed by the editor? Probably varies by journal). So there are quite a lot of famous discoveries that were first reported in this fashion ("holy smokes, I found something great! Article to follow.") --98.217.14.211 (talk) 12:37, 2 May 2009 (UTC)[reply]

With the lack of solid rules so-called Letters to the Editor can be exploited in various ways. They may be anonymous or signed by a pseudonym, and be concocted to stimulate interest or debate in the journal. A journal may use such a fictive letter to test the interest in an issue or to make editorial statements with subsequent deniability. An unethical example is to print a letter signed by a real person who never wrote it, in order to smear the person or to imply that the person endorses the journal. Such letters may also be used for early announcement of discoveries. Cuddlyable3 (talk) 12:54, 2 May 2009 (UTC)[reply]

Do you know of any legitimate scientific journals that will publish letters anonymously or pseudonymously, or that will use their correspondence section as a pulpit for their editors? Or worse, to post fabricated letters purportedly from another scientist? Really?

Perhaps such things happen in fringe pseudojournals published by kooks, and I suppose such shenanigans might happen in nominally-serious journals published in third-world countries or under repressive regimes (where the press may be government-controlled and propaganda is the order of the day). No serious peer-reviewed journal – again, ruling out the sham 'peer-reviewed' fringe press – would publish an anonymous letter, nor publish an inflammatory letter without taking steps to confirm the sender's identity. No serious peer-reviewed journal – in a country with a functioning legal system – would falsify letters in another scientist's name — that road leads to massive, massive lawsuits. TenOfAllTrades(talk) 14:38, 4 May 2009 (UTC)[reply]

On the positive side, I would argue that letters to decent scientific journals are usually genuine (identity-verified) and can be a forum for scientific discussion that persists (unlike casual forums). They are not peer-reviewed, so generally don't enhance someone's CV (removing one potential source of secondary gain). An exception is Letters to Nature, which are peer-reviewed and prestigious publications. --Scray (talk) 19:25, 2 May 2009 (UTC)[reply]

Physical Review Letters and Astrophysical Journal Letters are also peer-reviewed and prestigious in their fields. Both have a 4-page length limit, and publish results more quickly than the main editions of their journals. Note that I don't think either of these is labeled "Correspondence", as in the OP's question... these "letters" are not usually a direct response to other articles. -- Coneslayer (talk) 14:07, 4 May 2009 (UTC)[reply]

I believe such letters are sometimes (often?) used to stake a claim on a discovery straight away rather than risk taking the time to write up a full paper first during which someone else might publish it (either by discovering it independently or stealing it from you). The credit for a discovery generally goes to the first person to send it in to a respected journal, so a letter is sufficient for that even though a full paper would need to follow before anyone takes it particularly seriously. --Tango (talk) 14:51, 4 May 2009 (UTC)[reply]

Letters to Nature, Phys Rev Letters, etc are not really letters, they are short peer-reviewed papers. A correspondence section is a different thing. Looie496 (talk) 15:53, 4 May 2009 (UTC)[reply]

Term for sounds or smells that trigger past memories

I though I remembered there being an article on this, but I don't know what the term is. When someone hears a song on the radio, or a specific sound, or a certain odor and it instantly triggers some memory. It's psychology-related. Is there a specific term for this? 71.252.216.16 (talk) 02:31, 2 May 2009 (UTC)[reply]

I don't think there is a specific term for it. The famous example is Proust's "madeleine" episode from In Search of Lost Time, but I'm not aware of a special word used to describe it. You might be thinking of deja vu, but that means something a bit different. In psychology it's just called a "cue" as far as I know. Looie496 (talk) 04:02, 2 May 2009 (UTC)[reply]

Agreed that Proust usually gets credit (though it is called into question at this blog). I was actually shocked that I couldn't find a "scientific-sounding" name for this phenomenon -- perhaps a term needs to be invented, like "osmnemia" ("osm" from the olfactory sense, as in anosmia, and "mnemia" from the Greek for memory) or parosmnemia (bringing in the idea of paresthesia as the feeling of something that isn't there)??? Anyways most of the papers I Googled use terms like "odor-evoked autobiographical memory" or "long-term odor memory", which are probably much better descriptive terms, but the Wikipedia articles that seem as though they should address this topic (autobiographical memory and episodic memory) don't address the odor-evoked memories at all, and the olfactory memory article focuses mainly on the mother-infant bond. For more information, you may be interested in this review article which discusses the psychology aspects. --- Medical geneticist (talk) 11:36, 2 May 2009 (UTC)[reply]

Involuntary memory seems the most relevant article. Algebraist 12:27, 2 May 2009 (UTC)[reply]

It certainly sounds like a good umbrella term for memories triggered by various stimuli, although that article also doesn't mention olfactory memory. It seems like someone with a psychology background is needed to help expand/unify these various articles... --- Medical geneticist (talk) 13:38, 2 May 2009 (UTC)[reply]

The concept of "involuntary memory" would be considered out-of-the-mainstream in modern cognitive psychology. Most psychologists nowadays think that memories are evoked, not by an effort of will, but by associations. Thus the only thing that distinguishes "voluntary" memory is that it is reached by thinking about related things until something is hit that triggers the memory. In other words, voluntary and involuntary are not different types of memory, they only differ in that one is triggered as a result of a strategy rather than accidentally. Looie496 (talk) 15:12, 2 May 2009 (UTC)[reply]

Hairy

Is it possible to eat the skin of the hairy squash?68.148.149.184 (talk) 07:50, 2 May 2009 (UTC)[reply]

I am not familiar with the variety of squash known as "hairy squash". Generally speaking, squashes come in two variates. If it is a summer squash like, for example, zucchini, the skin is generally thin enough to be fully edible. If it is a winter squash, like for example pumpkin, the skins are generally thick and hard and entirely unpalatable. --Jayron32.talk.contribs 12:18, 2 May 2009 (UTC)[reply]

More commonly known as "moqua" -- I'm surprised we don't have an article about it. I'm not sure whether it has to be peeled, but it wouldn't be very appetizing otherwise. A kiwi doesn't really have to be peeled, but almost everybody does. Looie496 (talk) 15:20, 2 May 2009 (UTC)[reply]

I believe you mean kiwifruit. Eating endangered birds is illegal, peeled or not. 65.121.141.34 (talk) 13:32, 4 May 2009 (UTC)[reply]

In the U.S. people just call it a kiwi, presumably because we don't have the bird. Looie496 (talk) 15:54, 4 May 2009 (UTC)[reply]

Tear duct plugs

After Lasik surgery, a common complication is dry eyes, and a common cure is for an optometrist to put tiny plugs into the channel openings to prevent the tears from draining and keep the eye moist. What are these plugs made off and how do they stay in there so well? Thanks. Mac Davis (talk) 09:03, 2 May 2009 (UTC)[reply]

Punctum or punctal plugs are made from a variety of materials, some temporary and others permanent[1]. Temporary materials include animal collagen, polygloconate (a fibre also used in sutures), gelling polysaccharide, polycaprolactone (PCL, a degradable polyester)[2], while silicone is usual for permanent plugs[3][4]. —Preceding unsigned comment added by 82.41.11.134 (talk) 17:19, 2 May 2009 (UTC)[reply]

Speakers

I've got a pair of speakers for my computer. When I touch the headphone jack feedback occurs, at least I think that is what it is. However, today I had an aluminum MacBook with me. When I touched the MacBook with my right hand and touched the headphone jack with my left, the noise got much louder. In fact, as I rubbed my finger against the metal of the jack I could feel the electricity coursing through. I touched the jack to my face and was painfully shocked.

What is happening here, and is this bad for my speakers? Thank you! Mac Davis (talk) 09:08, 2 May 2009 (UTC)[reply]

See mains hum which is a rough explanation of the effect. Basically, what you are describing as "feedback" is the result of the 60-Hz alternating current from the wall "bleeding through" into the speeker wires and coming out of the speaker as audible interference. The effect becomes more prounced when there is a complete circuit to the earth (which happens when you touch the wire with your hand) and the metal computer improves the conducatance through this circuit as well. --Jayron32.talk.contribs 12:12, 2 May 2009 (UTC)[reply]

Thanks! Yep, that's definitely the 60 Hz hum. I do not understand what you mean about "current from the wall bleeding through," and I really don't understand the article very well either. The wall? What wall is that? Mac Davis (talk) 16:46, 2 May 2009 (UTC)[reply]

You probably have a bad headphone jack or plug. Those plugs aren't very robust, and bend or wear out easily. I don't think this can damage the speakers (speakers are pretty robust), but if you keep trying weird things, you could eventually damage your computer. If you can, you should try replacing the plug. (I'm not very confident about this, because I'm not sure I understand what you mean by "touch the headphone jack". It shouldn't be possible for you to touch the jack when something is plugged in to it -- the jack is the hole that the plug goes into.) Looie496 (talk) 15:34, 2 May 2009 (UTC)[reply]

Sorry, I was talking about the 3.5 mm TRS connector. It was not plugged into anything. Also, you said I could damage my computer? This is the normal casing of the computer, not the inside or anything. Why do you think so, just a cautionary guess? Mac Davis (talk) 16:46, 2 May 2009 (UTC)[reply]

You can ignore my response -- I totally misunderstood the setup you were describing. Looie496 (talk) 18:59, 2 May 2009 (UTC)[reply]

::::No, no, no that's not it at all. It isn't that anything is necessarily broken. Look, no shielding is 100% effective, and the alternating current in the wall can cause induction in any other material nearby. So basically there's a 60 Hz radio frequency being broadcast through your house at all times. Normally, its of such low intensity that it doesn't cause a problem. However, if you provide a better means to transmit this frequency, it will improve the means by which it is broadcast. SO basically, instead of being an "over-the-air" broadcast, the signal is using your body like "cable TV" and you are acting like the cable. Your body improves the transmission of this 60 Hz signal from the mains current in your walls (which will bleed a little bit into EVERY device plugged into the walls) into your speakers. The only way to avoid this entirely is to "go wireless" and run everything off of batteries. Its just a symptom of operating in an alternating current world. --Jayron32.talk.contribs 04:24, 3 May 2009 (UTC) Scratch that. I didn't read that bit where you got shocked by the speaker jack. I would say that you have an unstable Ground loop set up which is generating a potential where there shouldn't be one. There is an unavoidable 60-Hz hum that ends up affecting all audio equipment which gets its power from the wall jack; however if the equipment is shocking you then that is a different problem. --Jayron32.talk.contribs 05:17, 3 May 2009 (UTC)[reply]

Why can we envision things from before we were born?

I have an idea, but I wanted to ask here, too.

When I've visited historic sites, I could almost see, for instance, Ty Cobb roaming the outfield in old Tiger Stadium when I was there, though he retired 40 years before I was born. Why is that possible?

My hunch is that it is imagination at work in semantic memory. Episodic memory deals with personal things - what I've seen and heard at baseball games I've been to. But, going to Tiger Stadium, Wrigley Field, etc., causes things I've experienced to merge with my daydreams, and with things I've read about players like Cobb. In other words, our minds don't just insert information to fill in gaps in our experiences later, there can be active filling of information at the time, in certain cases. So that - while a visitor to Independence Hall didn't see the Constitution being drafted, they can come away with a feeling of having been there. Even just from things they read on here.Somebody or his brother (talk) 12:55, 2 May 2009 (UTC)[reply]

We can envision things that never happened at all, why wouldn't we be able to envision things from before we were born? I think you're thinking too hard about this. Looie496 (talk) 15:36, 2 May 2009 (UTC)[reply]

Good point, I do tend to make things too complex for myself sometimes. :-) Somebody or his brother (talk) 20:40, 2 May 2009 (UTC)[reply]

Speed of Electricity Part II

Sorry, I'm just getting around to reading the last responses to this thread [5]. I have a follow-up question but the thread is already in the archives. My understanding is that the electrons themselves flow relatively slowly. Using the garden hose and half-inch ball-bearings analogy, what about the very first time you turn on the water hose? There are no ball-bears in the water hose yet, so shouldn't it take a long time for the first ball-bearing to flow all the way through the hose and out the other side? A Quest For Knowledge (talk) 13:12, 2 May 2009 (UTC)[reply]

Your analogy is faulty: the wire is already full of electrons. As briefly explained at Copper#Electrical_properties, the electrons in good conductors form an "electron sea" in which virtually all of the valence electrons are potentially mobile. As a result, your garden hose is always full of ball bearings, whether or not you've turned on the tap. — Lomn 13:16, 2 May 2009 (UTC)[reply]

A battery is not a source of electrons that dries up when it is out of electrons. It only moves the electrons already present in the circuit. Mac Davis (talk) 16:51, 2 May 2009 (UTC)[reply]

As already noted, the hose is ALWAYS full of ball bearings. What you are adding to the system is not more ball bearings, but the energy necessary to move the ball bearings. You don't add material to the system, you add motion to it. In a battery, you have two materials, one attached to each terminal. One material "wants" electrons more than the other does, so if you can provide a mechanism to get the electrons from one to the other, they would do just that. The wire provides that mechanism. Basically, if we go back to the ball-bearings-in-a-hose analogy, the "+" terminal of the battery takes an electron out of the end of the hose, and the electrons all shift in that direction to take up the space, and the "-" terminal dumps another electron into its end of the "hose" to take up the space. Now, there needs to be a way to counterbalance the electrons building up at the "+" terminal, so inside the battery there is a little piece of porous filter paper that acts as a Salt bridge, which allowed little positive ions to pass through and balance out the build up of negative electrons at that terminal. The net result is that the battery contains the same number of electrons both before and after it has been used up, as does the wire, its just that the electrons have been "shuffled around" a bit. Those moving electrons, like all objects in motion, are able to do work for us, which is what we want them to do in the first place. --Jayron32.talk.contribs 04:16, 3 May 2009 (UTC)[reply]

So...after reading through some our articles, the electrons that flow through a wire (such as copper) are part of an atom but are so far away from the nucleus that they easily break free from the atom? Is my understanding correct? A Quest For Knowledge (talk) 13:42, 4 May 2009 (UTC)[reply]

You didn't say which articles you read, but the one you should be reading is metallic bond which does not seem to have been mentioned above on a quick scan through. Saying the electron has broken free is not quite accurate, it is more akin to the electron sharing of covalent bonds, but in this case the electrons are shared by the entire crystal structure and is not localised to any particular bond. For a substance to be a metal, you must not only have this electron sharing but also there must be a surplus of energy states for the electrons to exist in. Such an energy band structure is called a conduction band and in metals it is usually without an intervening energy gap or a very small one. SpinningSpark 14:05, 4 May 2009 (UTC)[reply]

B vitamins and stress?

I've read on countless pages, most if not all unsources, that "stress depletes the body of B vitamins". Does anyone have a reliable source on this? It sounds plausible, but I haven't found any credible evidence to back it up, and after some (admittedly quick) looking through Wikipedia, I found nothing either confirming nor contradicting this, with the same results on google and pubmed. -- Aeluwas (talk) 15:08, 2 May 2009 (UTC)[reply]

The term "stress" covers a huge amount of ground. I think there is some evidence that a variety of stressful things such as intense exercise or illness increase the body's usage of Vitamin B6 -- beyond that I shall not venture. I will, however, venture to note that this is the sort of thing that tends to get hyped way beyond reality by "alternative nutrition" sources. Looie496 (talk) 20:49, 2 May 2009 (UTC)[reply]

Another thing to keep in mind is that the "B vitamins" are a catch-all category for water-soluble vitamins that for whatever reason weren't able to talk researchers into giving them their own letter. I don't see any particular reason to expect them to behave the same way under "stress" or any other circumstance. --Trovatore (talk) 21:18, 2 May 2009 (UTC)[reply]

B-vitamins are important co-enzymes in a variety of various metabolic processes. If you are under "stress" it means that your heart may be working harder, your brain is working harder, etc. etc. and since your body is essentially working at a faster rate, it is going to consume things at a faster rate, and this includes especially those B-vitamins. As noted, there are lots of ways to define "stress" but regardless of how you define it specifically, the general idea that your body is in a heightened state of activity is a pretty universal part of any definition of stress. So, B-vitamins are likely to be consumed faster during stressful times. Being water soluble vitamins, however, your body has no mechanism to build up a storage of B-vitamins, so you have less residual B-vitamins in your body at any given time than other nutrients, like say Vitamin A. So while, during stress, ALL nutrients are being depleted at an accelerated rate, there are less excess B-vitamins lying around your body, so these may tend to be depleted faster. --Jayron32.talk.contribs 04:08, 3 May 2009 (UTC)[reply]

Thanks everyone!

"I will, however, venture to note that this is the sort of thing that tends to get hyped way beyond reality by "alternative nutrition" sources." -- indeed, this is the biggest reason I asked. I figured there might be some truth in it anyways, and Jayron's answer makes a lot of sense (almost "duh, of course"-much now that I've actually read it ;).

By the way, by "stress" I (well, the alternative medicinists, I guess) mean things like worrying about a sick relative, too much to do it little time, general anxiety etc., rather than the kind of stress you subject the body to when running 10 miles. -- Aeluwas (talk) —Preceding undated comment added 09:01, 3 May 2009 (UTC).[reply]

When physiologists talk about stress, nowadays they mostly mean things that activate the Hypothalamic-pituitary-adrenal axis, which includes things like hurry and worry, but also a variety of purely physical "stressors". Looie496 (talk) 15:31, 3 May 2009 (UTC)[reply]

Indeed, many things that go on in your body make no distinction between physical and psychological stress. The deciding factor is the elevated levels of certain hormones like adrenaline and cortisone which basically act to speed up your metabolism. If cortisone levels are high because a) you ran ten miles or b) your boss is breathing down your neck to get that report in by Friday or c) you're being chased up a tree by a lion it doesn't really matter. They all elevate your metabolism levels in the same manner. In short bursts, such stress is not an issue. However, imagine that you needed to run away from lions every few hours. Your body begins to have permanantly elevated levels of cortisone, which can mean that your metabolism gets more and more out of whack. Now, replace the lion with your boss who is a prick and always riding you and the effect is the same. --Jayron32.talk.contribs 17:45, 3 May 2009 (UTC)[reply]

Are all sexually reproducing animals genetically unique?

I'm not too sure on this one - I was wondering if anyone could assess my thought process. I thought clones aren't necessarily genetically unique because if you take the nucleus of a cell and put it into an egg cell (which had had its nucleus removed) taken from another organism, the resulting organism is not a clone and therefore not genetically unique because of the mitochondrial DNA in the egg cell which does not belong to the cell from which the nucleus was derived. (I think that was the process by which dolly the sheep was cloned.) But then I thought what if you took an egg cell from an organism, removed the haploid nucleus and then inserted a diploid nucleus from the same organism - would you then have a proper clone which is not genetically unique? If this is possible then is it incorrect to say that all sexually reproducing animals are genetically unique? Anyone else got any ideas? Thanks in advance for suggestions.

What about identical twins? they also share identical mitochodrial DNA. Dauto (talk) 16:07, 2 May 2009 (UTC)[reply]

I believe Dolly the sheep was cloned in exactly that way. As Dauto says, though, identical twins also have identical DNA and are far more common. --Tango (talk) 16:49, 2 May 2009 (UTC)[reply]

The human genome contains 3 billion bases[6], each with 4 possible values, so (by my own calculation) that's 4^(3x10^9) possibilities, a number of the order of 1 followed by 2000000000 zeroes. Not all combinations are equally probable (due both to viability and availability), but you can see it's quite unlikely to occur by chance. Even earthworms, a simple sexual animal, have about 1 billion bases[7]. --82.41.11.134 (talk) 17:36, 2 May 2009 (UTC)[reply]

Yes, there are 3 billion nucleotides, but most of them are the same in all humans, otherwise they wouldn't be human. According to Human genetic variation, two randomly chosen humans would be expected to vary in only 1 in 1000 of them. And that is with random selection, if you chose people from the same family it is going to be even rarer for nucleotides to be different. --Tango (talk) 18:07, 2 May 2009 (UTC)[reply]

Thanks for the point about identical twins - I somehow forgot about that! Is my above idea about taking an egg cell from an organism, removing the haploid nucleus and then inserting a diploid nucleus from the same organism a feasable idea that would also be an exception? —Preceding unsigned comment added by 92.17.247.142 (talk) 17:49, 2 May 2009 (UTC)[reply]

Yes, as I said, that's how Dolly was made. --Tango (talk) 18:07, 2 May 2009 (UTC)[reply]

There are lots of animal species that are capable of reproducing both clonally and sexually. Looie496 (talk) 18:56, 2 May 2009 (UTC)[reply]

I think the point might need some qualification even when discussing identical twins. They might be genetically identical when they first divide one from the other, early in the gestation process. Do they stay that way, even long enough to be born? I doubt it. --Trovatore (talk) 21:39, 2 May 2009 (UTC)[reply]

Genetically they do remain identical overall, other than occasional mis-repaired damage to DNA in a few individual cells caused by radiation, replication mistakes etc, which is usually insignificant unless it leads to cancerous growth. However, development is also influenced by factors in the physical environment, such as exposure to the hormone levels of their mother and each other, blood supplies via their placenta(s), and relative orientations in the womb, which are all likely to differ slightly. Post partum, physical and social experiences will begin to differ much more even when (as is usual) the twins are raised in the same family, so slight physical and greater mental/personality differences will accumulate. 87.81.230.195 (talk) 22:17, 2 May 2009 (UTC)[reply]

Well, that's not really "identical" then, is it?

No, identical twins are not actually identical. But the point of the question is about being genetically identical which they are.

The point I was making is that, as I understand it, even a single individual is not genetically uniform from cell to cell. There is genetic drift going on all the time. It may not have any noticeable macroscopic consequences, but the word "identical" is too strong. --Trovatore (talk) 22:22, 2 May 2009 (UTC)[reply]

By that criteria nobody would be even genetically identical to theselves. Dauto (talk) 22:46, 2 May 2009 (UTC)[reply]

Do you mean by those criteria, or by that criterion? --Trovatore (talk) 22:49, 2 May 2009 (UTC)[reply]

Identical twins are identical genetically, but they usually differ epigenetically. Looie496 (talk) 01:58, 3 May 2009 (UTC)[reply]

I do not think that Dolly was made by the process described above - I don't think the article on dolly the sheep specifies either. I believe the nucleus from which dolly grew was taken from a finn dorset sheep and the egg cell which had its nucleus removed was taken from a Scottish Blackface. Therefore Dolly was not a clone of the finn dorset as she had the Scottish blackface's ribosomal DNA. —Preceding unsigned comment added by 92.17.56.94 (talk) 15:11, 4 May 2009 (UTC)[reply]

Science Channel Documentary and Quantum Uncertainty

I don't really know how to best phrase this question since I don't really understand quantum mechanics and some the terminology I use might be wrong. But I'll try my best. I was just watching a documentary on the atom on the Science Channel. At the end when explaining Shroedinger's Cat thought experiment, the narrator stated that atoms have no location until measured and that an atom is spread out. I believe he was referring to Heisenberg's uncertainty principle. I thought this uncertainty only applies to subatomic particles, specifically electrons, not to atoms as a whole. Can someone clear this up for me? Does this apply to atoms as a whole or just to subatomic particles? A Quest For Knowledge (talk) 16:53, 2 May 2009 (UTC)[reply]

The Heisenberg uncertainty principle applies to everything (it's just more relevant the smaller an object is), but that's not what the narrator was talking about. The uncertainty principle is a fundamental limit to how precisely you can measure somethings position and momentum (the more accurately you measure one, the less accurately you measure the other). Schroedinger's cat is to do with the idea that things in quantum mechanics exist in terms of probability. It is meaningless to say where something it, you can just say how likely it is to be in different places. When you make an observation the "probability waveform collapses" (whatever that means - nobody is quite sure!) and you get a specific result (to with in a certain margin of error, the uncertainly principle giving you the minimum that that margin can be). This also applies to everything, regardless of size (but is, again, more relevant for smaller objects). --Tango (talk) 18:13, 2 May 2009 (UTC)[reply]

I took the liberty of fixing your link. Dauto (talk) 19:42, 2 May 2009 (UTC)[reply]

Oops, forgot to go back and cross my t's! Thanks. --Tango (talk) 19:46, 2 May 2009 (UTC)[reply]

I lost a trivia question once on "what two things can't be measured simultaneously according to the Heisenberg uncertainty principle?" The answer on the sheet was "position and velocity". Writing out the equation of delta-p X delta-x >= h/2 didn't help me either. Damn those triviamasters who don't have PhD's! (We won the match though :) Franamax (talk) 10:24, 3 May 2009 (UTC)[reply]

IT is not incorrect to say that position and velocity cannot be measured simultaneously. But it is at least misleading to say that those are the two things that cannot be measured simultaneously. There are other things besides position velocity that are not compatible and cannot be measured simultaneously. The answer might as well have been the z component of angular momentum and the x component of the angular momentum of the same body. Dauto (talk) 15:24, 3 May 2009 (UTC)[reply]

I thought that it was more than simply not being able to precisely measure position and momentum simultaneously, but that position and momentum have not been determined until measured. IOW, an electron doesn't make up its mind (so to speak) as to where it is and how fast it's travelling until someone decides to measure it. A Quest For Knowledge (talk) 13:47, 4 May 2009 (UTC)[reply]

Infinite speed communication

A method of communication across vast distances (lightyears) was proposed once and I don't recall how it was refuted, though I have no doubt it was. The method is thus: Take a long stick (say, one lightyear long) and push it. The person at the other end (one lightyear away) will see the stick move, and thus be able to interpret the movement as communcation. Why wouldn't this work? Vimescarrot (talk) 17:46, 2 May 2009 (UTC)[reply]

No object is perfectly rigid. When you pushed the stick you would send a compression wave down it, that wave would travel slower than the speed of light. In fact, this thought experiment can be taken as proof that perfect rigidity is impossible. Another interesting thought experiment is giant scissors - very short on one side of the hinge but light years long on the other. You close the scissors and the tips of the blades should move faster than light - this also doesn't happen since the scissors just bend, however rigid you make them they will always bend (or break, I guess). There is discussion on it here. --Tango (talk) 18:00, 2 May 2009 (UTC)[reply]

Thanks. I understand the scirssors one (sort of...I think...once you start approaching light speed crazy things happen) but since lightspeed was never achieved itself in this case...but yes, you answered me fine. Thanks Vimescarrot (talk) 18:07, 2 May 2009 (UTC)[reply]

If you pushed the stick or moved the scissors, the information in the form of vibration would be transferred at the speed of sound in that substance. The speed of sound is considerably less than the speed of light. In your mind it only seems to work because of the order of magnitude we are at. If we move something at our size and energy level, the entire object seems to instantly move. In reality, it is more like wiggling jello. Mac Davis (talk) 18:32, 2 May 2009 (UTC)[reply]

"The speed of sound is considerably less than the speed of light" - that is why some people appear bright until you hear them talk... —Preceding unsigned comment added by 79.122.41.137 (talk) 20:43, 2 May 2009 (UTC)[reply]

You can often find answers in our archives (box at the top-center of this page). In this case, I searched "faster than light communication", and here's the first link. --Scray (talk) 18:48, 2 May 2009 (UTC)[reply]

When I've heard the gedankenexperiments about the scissors, it involves the point at which the scissors cross each other in fact being able to go faster than light, in the same way that if you flicked a powerful spotlight across the moon the point of light would be able to exceed c, even though no information has done so. --76.182.94.172 (talk) 02:01, 3 May 2009 (UTC)[reply]

The stick is held together by the electromagnetic force. As such, any force on the stick cannot travel faster than it. The messenger particle for the electromagnetic force is photons. Hence, the waves will move slower than light. — DanielLC 04:50, 3 May 2009 (UTC)[reply]

Have there beeen any experiments documenting this? Wikivanda199 (talk) 16:10, 4 May 2009 (UTC)[reply]

Liquid to solid phase change due to shock

We just bought a chemical baby bottle warmer that needs no power supply, and it's re-usable. You "recharge" it by heating in boiling water. A non-illuminating description of the product is here.

I'm trying to learn how this thing works!

It's basically a clear plastic bag filled with a liquid. You wrap the bag around the baby bottle. The plastic bag contains a coin-sized disc of metal that is slightly concave. One must "snap" the disc by squeezing it to invert the concavity. This "snap" apparently sends a shockwave into the liquid. The liquid then crystallizes starting at the disc and spreading outward. The bag has completely solidified in about 30 seconds. Heat is released during this phase change.

Once the bag is solidified, it stays warm for an hour or so. Heating the bag for 15 minutes in near-boiling water will change the phase back to a liquid, and after it cools down, it's ready for re-use.

I've had fun playing with this thing today, and our baby isn't even here yet. The shock needed to initiate the reaction apparently needs to be fairly strong. Throwing the bag against the floor, or giving it a firm snap with my finger, won't activate it. Maybe it requires a bit of cavitation rather than a shock.

Is there a term for the process I described, where an acoustic shock results in a phase change from liquid to solid? =Axlq 22:51, 2 May 2009 (UTC)[reply]

I don't know a special term, but I found an explanation of the process at this web page. Pretty nifty! Looie496 (talk) 23:05, 2 May 2009 (UTC)[reply]

Thanks! I guess the nearest term would be supercooling. I didn't know that's what was happening here. Apparently the bag contains sodium acetate, and sodium acetate remains stable in a supercooled state. There's a description at Sodium acetate#Heating pad. Thanks again. =Axlq 23:28, 2 May 2009 (UTC)[reply]

The bag contains sodium acetate in a liquid meta-stable phase. Once you seed a crystal with the snapping piece of metal this crystal grows releasing heat. The crystal is the real stable phase. You can see some interesting video here http://www.youtube.com/watch?v=fhG18zF1HGw. Dauto (talk) 23:36, 2 May 2009 (UTC)[reply]

The correct term is actually supersaturation and I'll explain how it works, but first lets take a little side trip. There are two factors that determine if a chemical process, like Precipitation in this example, will actually occur or not. They are calledthermodynamics and kinetics. They both deal with energy, but from different perspectives. Thermodynamics deals with the start and end state energies, but ignores what is happening "along the way". Kinetics deals with the sorts of energy changes that happen during the reaction. Another way to look at it, is if you look at a chemical reaction as a trip from Point A to Point B; thermodynamics looks at the difference in energy between A and B, while kinetics looks at the trip from A to B.
So lets look at this situation. From a thermodynamic perspective, the precipitation of sodium acetate (which is what is happening here) is highly exothermic, which means that the process gives off heat. Normally, this is a Good Thing from a chemical perspective, because it means that the end result (solid outside of liquid) is more stable than the starting stuff (solid dissolved in liquid). So, when you have this cool, supersaturated solution of sodium acetate, it really wants to get out. You know that it does, because it gives off a LOT of heat when it does get out. So why doesn't it just do it spontaneously? Thats because of the other part of our process, which is kinetics. According to the kinetics of this reaction, in order for the material to precipitate, the little sodium bits in the water have to collide with the little acetate bits in the water with enough force to produce the sodium acetate. This is refered to as activation energy, and regardless of how much the material would become stabilized by changing its form, if there is not enough residual energy around to get overcome this activation energy (i.e. not enough energy to make the sodium bump into the acetate hard enough) than no reaction will occur. Look at it this way. Picture you have a giant stone you want to throw off of a cliff. Its pretty easy to make a stone fall down the cliff. Well, the bottom of the cliff is out end products, and the top of the cliff is our starting materials, and the loud "crash" the stone makes is the energy given off in our reaction. If, however, between you and the cliff is a huge hill, you first have to get over the hill before you can make the stone fall down the cliff. The hill is what we call "activation energy" and it is what is keeping our stuff from happening here.
So lets go back to our reaction in our little "heating pad". Inside the heating pad is a solution of sodium and acetate which is supersaturated. That is, theres so much sodium and acetate stuffed in the water the two REALLY want to get out. If they can get out, they become more stable and release a whole lot of pent up energy in the form of heat, which is what we are after. However, the sodiums and acetates aren't moving fast enough to collide hard enough to actually get together and settle out of the solution, so regardless of how much they want to get out, they cannot. Enter that little snappy coin-sized disc. When you crack the disc, it creates a little shockwave that pushes a tiny amount of sodium and acetates together hard enough to precipitate. When this little bit precipitates, it gives off heat which speeds up some more sodiums and acetates, which are now moving fast enough to collide hard enough, so THEY precipitate, and give off yet more heat, and you have a chain reaction which feeds itself. The whole point of the snappy disc is to provide enough initial energy to "get you over the hill". Once that happens, the reaction generates enough energy to support itself, all the sodium acetate precipitates out, and there gobs of excess heat, which you are using to heat your baby bottle. --Jayron32.talk.contribs 03:52, 3 May 2009 (UTC)[reply]

Jayron, that's an absolutely spectacular explanation. Leaves me with one question, though, for my own understanding. Would it be more accurate, less accurate, or the same, to describe the "hill" of activation energy as something actually fairly small? Activation Energy isn't always a large obstacle, is it? If it's a 300-pound boulder, and all I have to do is get it over a scrap piece of 2x4 that somebody dropped on the ground -- I can throw my shoulder into that as hard as I can, and can't move it.

(Really drifting off topic... there's an urban legend about a locomotive being unable to start if one wedges a nickel or 20p piece between the track and the front of one wheel. Same concept?)

--DaHorsesMouth (talk) 20:35, 3 May 2009 (UTC)[reply]

It depends entirely on the reaction involved. Some reactions have small activation energies, such that the residual heat at room temperature is enough to overcome them. Some reactions have HUGE activation energies, such that they require a large push. If you can envision a scenario (exothermic, endothermic, large activation energy, small activation energy, etc. etc.), there exists hundreds of reactions which meet that profile. There are LOTS of reactions that go on in the human body which are part of regular metabolic pathways which have outrageously large activation energies; the equivalent of rolling the stone up Mt. Everest in order to get it to go. The reason they happen at all is because of catalysis which is a means of altering the kinetics of a reaction to provide an alternate pathway between the start and end to allow the reaction to occur with lower activation energy. Back to our analogy, a catalyst is basically digging a tunnel through the hill rather than going over it. However, this precipitation reaction has a relatively small activation energy. Think of it as getting a roller coaster over the hill. All we have to do is get the first car over and the rest of the train gets dragged along behind it.

Understand too that my explanation above was a bit of a simplification; it ignores the solvation process which keeps the sodium and acetate ions in solution, for example, and I also don't discuss the process of nucleation; however the basics are still all there. I was trying to get it across without reteaching all of freshman year chemistry to everyone again... --Jayron32.talk.contribs 01:20, 4 May 2009 (UTC)[reply]

Thanks for the detailed answer. I understand what's going on now. =Axlq 20:51, 3 May 2009 (UTC)[reply]