Wikipedia:Reference desk/Archives/Science/2015 February 12

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February 12

What are stem cells?

I know that stem cells are the basic building blocks of human development, are sometimes called “magic seeds.” But I want to submit a detailed paper on this topic. I am searching for the credible sources of information to gather detailed information. I collected some study materials from available academic reference. But I want more. Please provide detailed authentic information. — Preceding unsigned comment added by Mauriciogordon847 (talk • contribs) 06:02, 12 February 2015 (UTC)

Although Wikipedia itself should not be considered a reliable source, the 'References' and 'External links' sections of the Stem cell article might be useful for finding sources, and our article does "provide detailed authentic information".  —71.20.250.51 (talk) 06:24, 12 February 2015 (UTC)

If you eat fresh veggies, you eat stem cells all the time, e.g. apical meristem. Tasty! SemanticMantis (talk) 14:01, 12 February 2015 (UTC)

A stem cell is just a cell that can go through Cellular_differentiation to make other forms of tissue. SemanticMantis (talk) 14:03, 12 February 2015 (UTC)

Faraday cage and holes in one dimension

In a Faraday cage if the holes are smaller than the wavelength it will block it. What if the holes are smaller only in one dimension? e.g. long wires near each other, but no cross wires so the gaps are longer than the wavelength, but narrower than it. Will that block just as well as holes in two dimensions? Or somewhat worse? Ariel. (talk) 09:29, 12 February 2015 (UTC)

So you're asking if an arrangement like this: ========= will block the same wavelengths as this ######, assuming all gaps are equal sized? I don't know, but it's an interesting question and I just wanted to clarify :) SemanticMantis (talk) 14:13, 12 February 2015 (UTC)

The blocking is never perfect. That arrangement won't block the signals quite as well as small round holes, but it will still do a very good job of blocking if the gaps are thin enough. Looie496 (talk) 15:27, 12 February 2015 (UTC)

I'm not sure you'll get a Faraday cage effect if a closed circuit is not made. For the effect to work, you need to induce current loops in the cage, and the current needs a closed path to flow.

There are other means by which a metal grate can interfere with electromagnetic radiation, so Looie496 is correct that some signal will be blocked.

We discussed the details of Faraday cages a few weeks ago, and last time I ended up referring to Griffiths Introduction to Electromagnetism... so again, referring to theory from Chapter 2.5, to satisfy a condition that ensures ${\vec {E}}=0$, you need to ensure a closed path exists. If there is no closed path, there is no Faraday cage effect. Any other signal attenuation you observe is caused by something else - most probably, dielectric heating, which is not very effective signal attenuation mechanism in a metal conductor!

So, if the size of the gaps is sufficiently large that the impedance of the conductor attenuates any induced current at the frequency of the incoming wave, then the holes are "too big." The conceptual idea is that a path can be electrically short or electrically open for a specific frequency, and this response is governed by the material properties and the geometry. This is a really complicated geometry, and I don't think the signal behavior can be usefully solved analytically. At best, we could assume an "infinite sheet" of this material with a repeating geometry and try to solve for an incoming plane-wave. In practice, we would need an experimental setup or a rigorous computer simulation.

I think the only way to be sure is to test the theory by experiment! Nimur (talk) 16:46, 12 February 2015 (UTC)

I thought we were talking about something akin to a stack of looped wires, separated by small gaps (my crude diagram would just be one side then, and ignores whatever we do for the floor and ceiling). Each wire ring is a closed loop, but there's no cross hatching like you see in traditional mesh cages. So many separate loops are formed, if not one continuous circuit, and that should be enough to stop most/some of the voltage drop, down to some wavelength, right? SemanticMantis (talk) 21:23, 12 February 2015 (UTC)

Yes. Or even if the individual wires are connected together in one or two places, but not in the direction the EMF is coming from. Ariel. (talk) 23:57, 12 February 2015 (UTC)

I don't think closed loops are necessary. In order to block EM radiation, a Faraday cage or polarizer has to react within a time comparable to the wavelength divided by c, which means that electrical connectivity on scales significantly larger than the wavelength can't matter. Polarizers don't have closed wire loops at a scale comparable to the wavelength, yet they work. A Faraday cage with no electrical connection between the horizontal and vertical wires is equivalent to two polarizers offset by 90°, so it will work too. -- BenRG (talk) 00:23, 13 February 2015 (UTC)

An electromagnetic wave exhibiting linear polarization which has a direction matching the direction of the wires might be able to slip through. StuRat (talk) 16:01, 12 February 2015 (UTC)

Can't polarization always be separated into two vectors? So if you are correct wouldn't this mean that with random polarization half the energy would be able to slip in? Ariel. (talk) 00:00, 13 February 2015 (UTC)

Your cage of parallel wires is a polarizer. I think it's as simple as that. Re StuRat's answer, note that (quoting the article) "the notion that waves 'slip through' the gaps between the wires is wrong." -- BenRG (talk) 00:08, 13 February 2015 (UTC)

Testosterone and the voice

I heard a transgender man on the radio this morning, speaking about the impact of taking testosterone. The interviewee had quite a deep voice. It made me wonder. If a woman started taking testosterone, their voice deepened, and then they stopped taking the hormone, would their voice return to its previous pitch? --Dweller (talk) 09:56, 12 February 2015 (UTC)

Our Castration article says in the "Medical consequences" section: "Castrations after the onset of puberty... The voice does not change." Working on the principle that a female ceasing to take testosterone supplements has the same effects as castration in a normal man. Alansplodge (talk) 11:35, 12 February 2015 (UTC)

Thanks. I had a look around the vocal chords article and some others and it led me to an understanding that the voice change is prompted by growth of the chords, which I suppose wouldn't reverse itself just because the hormone that prompted it had stopped. --Dweller (talk) 11:38, 12 February 2015 (UTC)

That's right, here is a source that directly addresses testosterone taken by women: "If a woman wants to become a man, the use of male hormones will permanently deepen the pitch of the voice, because the vocal folds become thicker" (Voice Work: Art and Science in Changing Voices by Christina Shewell, John Wiley & Sons, 2013, p. 556 [1]). - Lindert (talk) 12:03, 12 February 2015 (UTC)

The Keen senses of the Gray Wolf, Jaguar/Panther and Puma

On a scale from 1 to 10, how keen are these three animal's senses relative to the rest of the animal Kingdom?

I'm well aware that these animals would score very high, but how high exactly? Personally, I would think that some of their senses are almost unmatched, such as the wolf's sense of smell.

1) Sense of Sight 
2) Sense of Sight in the dark 
3) Sense of Hearing 
4) Sense of Smell 

If you have first-hand knowledge or good sources to back up your opinions, then all the better. ;) 

2A02:FE0:C711:5C41:54B6:6EEA:430A:1573 (talk) 13:07, 12 February 2015 (UTC)

I doubt a wolf can beat moths on sense of smell. Some moths can detect a single molecule of sex pheromone: Manduca sexta's abilities are described here [2]. SemanticMantis (talk) 13:57, 12 February 2015 (UTC)

However, I suspect that moth is very much tuned to smelling that particular molecule, at the cost of not smelling other molecules nearly as well. I'd go with size of the nose as being a good indicator of sense of smell. The longer the nose, the more room for olfactory receptors. Now some noses might not be 100% packed with receptors, or may not have the brain processing to use that info, but still it's hard to see how a moth, with so much smaller of a sensing area (the antennae, I believe, in their case) could compete on sense of smell with a wolf overall. StuRat (talk) 15:24, 12 February 2015 (UTC)

More generally, ranking things on a scale of 1-10 isn't a very scientific approach. For instance, suppose a bat can hear much higher frequencies than a wolf. But suppose a wolf can detect sounds at a much lower decibel level. Which one gets a higher score? it's all very subjective at that point. Similar arguments go for sight. No wolf will ever see what a mantis shrimp can see (broad spectrum, polarization, more cones, etc.), but a wolf probably has better visual acuity at a 10 m range. So the more scientific thing is to look for quantifications of these animals' sensory abilities. Along that line, here's an article that discusses the retina of dogs and wolves, and draws some conclusions about vision [3].SemanticMantis (talk) 15:07, 12 February 2015 (UTC)

I guess I can see your point, even though I was thinking within their domain/habitat. You mention shrimp, and of course in a watery world an aquatic creature will have senses better suited to an aquatic life than the wolf or a feline cat, and vice versa.

It is also easier to comprehend how most other mammals and many vertebrates (such as the animals I mentioned) sense the world around them than how many invertebrates and non-mammals do (like the ones you mentioned; moths and shrimps), since they are so vastly different. Saying that probably only strengthens your argument that it is often hard to compare. I accept that.

Thanks for links. I'll take a look at them. 2A02:FE0:C711:5C41:54B6:6EEA:430A:1573 (talk) 15:29, 12 February 2015 (UTC)

I'd suspect that exclusively nocturnal animals with big eyes, like the aye-aye, would have better night vision. Of course, if you count echolocation, then bats can "see" in absolute darkness (aye-ayes seem to use echolocation, too). I also suspect that in animals that use echolocation, their brain combines inputs from their eyes and ears to form a single image, so hearing and vision are really one in the same for them. StuRat (talk) 15:35, 12 February 2015 (UTC)

Sure, it will be easier to compare if we restrict to terrestrial vertebrates. This paper says that the eagle Aquila audax has "acuity is between 132 and 143 c/deg and with decreasing luminance acuity declines sharply." [4]. That's the highest number I can find at a glance, will be interested to see if anything else does better in terms of c/deg (the cycles per degree measure of spatial acuity is described with good illustrations here [5]). This [6] (rather odd slide show) gives humans at 60 c/deg, and a cat at 5 c/deg. I haven't yet found a reference for wolf vision in c/deg. SemanticMantis (talk) 16:02, 12 February 2015 (UTC)

Beyond moths, bears [7] [8] and African elephants [9] [10] are said to have great sense of smell in some fashion (these aren't great sources particularly the bear ones, I'm presenting them only to make further research in to the claims easier). Nil Einne (talk) 17:37, 12 February 2015 (UTC)

Sports science - runners' flexibility test

A few years ago I watched Eddie Izzard's program Eddie Iz Running, in which he ran a succession of marathon distances almost every day. Before this undertaking, he was subjected to some fitness tests by sports scientists (I think from Loughborough University). One of the tests was for his flexibility and balance, I think. If I recall it correctly, it worked like this: the subject stood on one leg, and touched the big toe of his other foot to the floor at various points of a notional clockface around him (without overbalancing). It seems like the "fitter" person could achieve a greater radius, and presumably it highlighted angles at which the subject was weaker or less flexible. I assume this is a standard test that's well documented in the sports science literature. What is the name of this test, and where can I read more about it? Thanks. 81.174.196.2 (talk) 16:45, 12 February 2015 (UTC)

I found Five Key Biomechanical Tests, but it doesn't include the one that you describe. The search continues.... Alansplodge (talk) 18:05, 12 February 2015 (UTC)

Metric Expansion of Space Article Removed by IP

Hi Folks,

I wanted to refer to an article called Metric Expansion of Space which I had read before, but I found that it was basically removed by an unidentified editer last month (Metric expansion of space).

I felt that since the content of this article is the valid subject of a hot debate, it shouldn't simply be excised at the will of an anonymous person. Perhaps the science / physics / cosmology expert volunteers out there could make pages that clarify the ideas and the nature of the debate about the ideas for the rest of us?

Thanks,

E.S. — Preceding unsigned comment added by 146.23.3.250 (talk) 19:32, 12 February 2015 (UTC)

What goes in the article can be discussed on the article's talk page, but the question of the metric expansion of space being science or nonsense is appropriate here. I thought that the metric expansion of space was an integral part of modern cosmology, but the IP editor removed most of the content calling it "pseudoscientific garbage:"[11]. Perhaps someone who has studied cosmology recently could take a look and see if the previous content of the article was supported by reliable sources or if the version of expansion there was pseudoscience as the IP redactor stated. Edison (talk) 20:03, 12 February 2015 (UTC)

The trouble with "metric expansion of space" is that it has caught on like a catch-phrase. "Metric" is an adjective and it has a specific meaning in mathematics and cosmological physics. "Expansion" of the universe refers to Hubble expansion. If you study cosmology in great depth, you can spend tons of time modeling that expansion and adapting the model to fit with our knowledge of gravity, relativity, conservation of energy, and so on - or to fit specific observational data to match a particular theoretical model. It has been my experience, though, that if you fixate on this buzz-word-esque phrase, you'll end up in pop-sci territory, rather than real science. That's not quite the same as "pseudoscience" - but if you want real science, here's a much better place to start: the NASA/IPAC Extragalactic Database's reading list, maintained by scientists at Jet Propulsion Laboratory (California Institute of Technology).

Those books are hard to read. I've only got a couple of them; and they are hard to read. But they are real science about cosmology, written for a scientific audience.

Nimur (talk) 20:21, 12 February 2015 (UTC)

The article should have some other name, such as "expansion of the universe" or "cosmological expansion" or "Hubble expansion". "Expansion of space" doesn't make sense in general relativity; there's no continuity equation for space that would let you say that there's new space appearing in some spacetime location. But renaming the article would probably involve endless arguments on the talk page, and I don't have the stomach for that. -- BenRG (talk) 21:10, 12 February 2015 (UTC)

The versions by the IP editor are nonsense. The versions they're trying to delete are fine, though not great. -- BenRG (talk) 21:10, 12 February 2015 (UTC)

It's probably the same editor that has been active at Ultimate fate of the universe with similar edits. Sjö (talk) 20:38, 12 February 2015 (UTC)

You can cite a particular timestamped version of a Wikipedia article to avoid problems like this. Click the history tab then click on any article version to get a permanent link to it, or click "permanent link" under "tools" in the sidebar to get the most recent version. -- BenRG (talk) 21:10, 12 February 2015 (UTC)

The deletion of the large amounts of material from the article by the IP is an example of vandalism. Each of the deletions is reverted. My advice to the original poster would be to create a registered account and use the watchlist feature to look for changes to the article, and revert the vandalism. One of the advantages of creating an account is the ability to create a watchlist, which has various uses including detecting vandalism to particular articles. Robert McClenon (talk) 23:15, 13 February 2015 (UTC)

Another way to deal with repeated vandalism of an article by IP addresses is semi-protection. A registered editor can request semi-protection of an article, which prevents edits by IP addresses. (That is yet another reason to create an account, to avoid being locked out by semi-protection.) Robert McClenon (talk) 23:17, 13 February 2015 (UTC)