Wikipedia:Reference desk/Archives/Science/2014 March 29

= March 29 =

Monitoring CNS activity in fine detail
Is there any way to track individual neurone activity in an animal with a central nervous system? Like what time it depolarises and what effects that has on adjacent neurones with which it synapses etc? Is there any way to fix (as in preserve) a brain so that it could be imaged layer by layer, allowing a 3D map of every neurone and every synapse? "This massive chunk communicates with this massive chunk entirely through a massive release of this single neurotransmitter" isn't very informative. --129.215.47.59 (talk) 00:19, 29 March 2014 (UTC)


 * It's a hot topic. overview fiber optic optical tomography another whole-body fluorescence etc.  And of course the old standbys PETT, fMRI. Wnt (talk) 00:34, 29 March 2014 (UTC)


 * See also brainbow. TenOfAllTrades(talk) 14:08, 29 March 2014 (UTC)


 * If you could combine the Brainbow techniques with those used in the Visible Human Project you would be getting towards what you're looking for in the second part of your question. However, I suspect you would need much thinner slices than those used in the VHP. Richerman    (talk) 00:40, 30 March 2014 (UTC)


 * Not sure you'll have volunteers lined up around the block to have Cre-Lox recombination constructs genetically engineered to express fluorescent proteins in their brains. Really, while Cre/Lox is the better technology, the ability of Golgi's method to stain entire individual neurons while ignoring others is the more remarkable outcome.  I don't know if anyone ever has gotten to the bottom of how it works -- if they do, maybe it will allow a "brainbow" like technique on wild-type samples. Wnt (talk) 02:28, 30 March 2014 (UTC)


 * The question is about "animals with a central nervous system" - not humans. We don't usually require volunteers for animal experiments in my experience. Richerman    (talk) 08:37, 30 March 2014 (UTC)


 * True, I was thinking with a different emphasis there (just saw some interesting news about autism and the six cortical layers) Wnt (talk) 15:12, 30 March 2014 (UTC)


 * As our brain article outlines, a whole-CNS map at the synaptic level was made quite some time ago for the tiny roundworm C. elegans, which has the advantage that every individual animal has the same set of neurons, with the same connections. There is great interest in making such maps for the mouse brain, and eventually the human brain, but that is work in progress.  Regarding activity, large-scale neural recording is one of the main goals of the BRAIN Initiative that President Obama started last year.  I blogged about that recently on my personal web site, see http://weskaggs.net/?p=2214 if you are interested. Looie496 (talk) 19:51, 30 March 2014 (UTC)

Significance of a Term
In eletrical engineering the equation v(t)= L di/dt when integrated gives a constant of integration C i.e., i(t)= 1/Lʃ v(t)dt + C.What is the physical significance of this C.117.202.57.197 (talk) 06:50, 29 March 2014 (UTC)
 * You can have any constant current you like in an inductance L, it won't cause any voltage drop. That's your C. 84.209.89.214 (talk) 15:24, 29 March 2014 (UTC)

If the constant current is 0 then what will be the scenario.i.e. can C be equal to 0.117.194.230.123 (talk) 17:10, 29 March 2014 (UTC)
 * Yes. 84.209.89.214 (talk) 17:24, 29 March 2014 (UTC)

Complex Frequency
What is the physical significance of complex frequency in electrical engineering.What is the physical significance of Laplace Transform.It is not at comprehendible to me from the article.What does it mean by moment in context of Laplace Transform.117.202.57.197 (talk) 06:54, 29 March 2014 (UTC)
 * Searching for "Complex frequency" in Wikipedia links naturally to an article about the S plane. While an isolated sine wave can be adequately defined by its frequency and amplitude, because we don't care about when it has a particular phase, the phase relations between wave components of different frequencies are often critical when they are present together. A communication system may exploit two independant signal channels on a single frequency carrier wave by separately modulating two phases separated by 90 degrees of the carrier, with either continuous (analog) or binary (digital) modulations. These technologies and indeed any broadband analysis of frequency-selective circuits need frequency components to be measured in frequency, amplitude and phase i.e. a complex value for each frequency. The Laplace transform is often interpreted as a transformation from the time-domain, in which inputs and outputs are functions of time, to the frequency-domain, where the same inputs and outputs are functions of complex angular frequency, in radians per unit time. "Moment" has a mathematical meaning; Moment-generating function may be helpful.84.209.89.214 (talk) 14:49, 29 March 2014 (UTC)


 * The complex frequency $$\scriptstyle{S}$$ describes (in general case) exponentially growing or decaying sinusoids. The imaginary part of the $$\scriptstyle{S}$$ is the angular frequency and the real part gives the rate of growth or decay. The general waveform in question is


 * $$v(t) = \mathrm{Re}(V e^{S t})$$


 * where $$\scriptstyle{V}$$ is a complex constant known as a phasor.


 * If it's not clear to you why this represents a sinusoid, then see Euler's formula. Writing $$\scriptstyle{V}$$ as an amplitude and phase $$\scriptstyle{V = |V| e^{i \arg(V)}}$$ and $$\scriptstyle{S}$$ as real and imaginary parts $$\scriptstyle{S = \sigma + i \omega}$$ and applying Euler's formula gives


 * $$\begin{align}v(t) & = \mathrm{Re}(V e^{S t}) \\

& = |V| \mathrm{Re}(e^{i (\arg(V) + \omega t)} e^{\sigma t}) \\ & = |V| \cos(\arg(V) + \omega t) e^{\sigma t} \end{align}$$


 * That is (as promised), an exponentially growing sinusoid of angular frequency $$\scriptstyle{\omega = \mathrm{Im}(S)}$$ and rate of growth $$\scriptstyle{\sigma = \mathrm{Re}(S)}$$. The amplitude is determined by $$\scriptstyle{|V|}$$ and the phase by $$\scriptstyle{\arg(V)}$$. Each point on the complex S plane is one complex frequency. The imaginary axis of the S-plane represent sinusoids that are neither decaying or growing, the left half of the plane decaying sinusoids and the right half growing sinusoids. The real axis represents exponential decay or growth with no oscillation and the origin is d.c.. --catslash (talk) 22:47, 29 March 2014 (UTC)

Component of a force
When a force is resolved into two components why is it that we always only consider a horizontal component and a vertical component that are mutually perpendicular. The forces can be balanced only in these two direction. However a force can be resolved into two components in infinte number of ways as long as it obeys parallelogram law of forces. Did Sir Isaac Newton consider forces as vectors and and apply parallelogram law?117.202.57.197 (talk) 07:00, 29 March 2014 (UTC)
 * You are correct about the parallelogram of forces. When resolving a force into two components, it is usually to serve a purpose that necessitates the two forces be mutually perpendicular. Those two forces don't have to be parallel to the local horizontal and vertical, but those two orientations are often the only useful ones. Dolphin  ( t ) 11:03, 29 March 2014 (UTC)


 * In problems of statics or dynamics involving an inclined plane it is common to resolve forces along directions parallel to and perpendicular to the plane instead of in horizontal and vertical directions. Newton certainly knew about the parallelogram law - he states and proves it in Corollary I of Book I of the Principia Mathematica, straight after he states the three laws of motion. Gandalf61 (talk) 11:32, 29 March 2014 (UTC)


 * Gandalf61's link goes to our article on Principia Mathematica (1910) by Bertrand Russell. He probably would have meant Principia, (1687), by Newton.  Nimur (talk) 14:26, 29 March 2014 (UTC)
 * Oops ... yes. Thank you. Gandalf61 (talk) 18:36, 29 March 2014 (UTC)
 * Similarly to resolving forces in those two directions, in ballistics it's also useful to resolve velocity into both horizontal and vertical components. This is because the horizontal velocity will remain unchanged (ignoring air resistance), while the vertical velocity will be affected by gravity. StuRat (talk) 17:48, 29 March 2014 (UTC)

In a Warren truss one would resolve the axial tension of the horizontal member at the end at the top into two parts, each at 60 degrees to the horizontal. Generally one resolves forces in the directions of interest, not necessarily oriented with respect to the gravity vector. If you research triangle of forces you'll probably find some good examples. Greglocock (talk) 22:44, 30 March 2014 (UTC)

Dispersion
Why is it that only prisms are used for optical dispersion. For any optical medium of any arbitrary shape does dispersion takes place or does not take place.Is there restriction on the shape of the object for dispersion to take place.Is it possible to subject this type of dispersion if it occurs to mathematical analysis.117.202.57.197 (talk) 07:05, 29 March 2014 (UTC)
 * Glass prisms have been used to demonstrate dispersion ever since Isaac Newton in 1666 but they are not the only objects that display optical dispersion i.e. spreading of a white light beam into its constituent spectral colours. Dispersion occurs in raindrops as every natural Rainbow proves, as undesirable Chromatic aberration in simple lenses and in Diffraction gratings. Only the latter seem to make good practical use of dispersion, when they are incorporated in a Spectrometer that can be used to analyse the dispersive properties of different optical glasses. 84.209.89.214 (talk) 14:12, 29 March 2014 (UTC)


 * Good flat surfaces are much easier to make than high quality lenses, too.
 * With a fixed effort (or at a fixed price), a prism will me much more precise than a lens, and so the spectrum will be superior, too. ooze — Preceding undated comment added 05:10, 1 April 2014 (UTC)

Inorganic chemical synthesis
When the chemists especially the early chemists wanted or wants to produce a particular compound or preparation on what basis does he chose the reagents.Is theresystematin theory or logic or method to arrive at the solution.At the initial stage how does the chemist know about the chemical nature of the product(s) that are produced in a reaction or side reaction.Is it so that in experimental chemistry while finding the chemical nature of a chemical do the chemists react with all other chemical compounds  in all possible combinations.Is it possible to predict if two or more chemical substances with known composition react under given conditions.How do the chemists devise or dicover the specific tests for radicals.How and when were the dry and wet tests for acdic and basic radicals discovered or devised.My questions can be extended to the organic chemistry also.I could not find answers to these intiguing questions in any chemistry text at any level.117.202.57.197 (talk) 07:24, 29 March 2014 (UTC)
 * See Outline of chemistry and Chemistry. Chemical reactions are governed by certain laws which have become fundamental concepts in chemistry. Wikiversity has started a school of chemistry. 84.209.89.214 (talk) 13:48, 29 March 2014 (UTC)
 * Mostly, chemists already have. So if a chemist wants to know how something behaves chemically, he looks it up.  In past generations, reference works like the Merck Index and the CRC Handbook of Chemistry and Physics.  Of course, the existence of the Web makes such print references superfluous.  Further more, even if trying to predict the behavior of novel compounds, they are able to use the results of hundreds of years of chemistry to do Computational chemistry to predict the results of even novel chemical reactions.  Of course, they still do the new reactions to confirm the predicted results.  -- Jayron  32  20:41, 29 March 2014 (UTC)

What's the purpose of this ring on the hard drive motor?
I found a hard drive motor as shown with a smooth metal ring that goes around the shaft on the left. What, does the central shaft expand when the thing heats up and friction gets the ring to catch on and spin? To what purpose does the ring spin? Or is it for bearing purposes? Please someone tell me about this ring's purpose and the details of what goes on with it during operation. Peter Michner (talk) 15:28, 29 March 2014 (UTC)


 * Just a guess, bit if they had a vibration problem due to the hard drive hitting it's natural resonance frequency, then adding a metal ring like that to change it's mass, and thus it's harmonic frequency, might be the quick and dirty way to solve the problem. StuRat (talk) 17:57, 29 March 2014 (UTC)

Lowest temperature during last ice age
Does science have a ballpark of the lowest average low temperature normally occuring on Earth during the last ice age? I know it gets around -60°C quite a bit in Antarctica and some parts of Siberia on a more or less yearly basis — Preceding unsigned comment added by 78.1.186.238 (talk) 17:18, 29 March 2014 (UTC)


 * It wouldn't have been anywhere near that low, at least in the tropics. Those temperatures come from getting no light for 6 months.  The tropics would have gotten plenty of light in the ice age.  Also, if the whole world was that cold, the oceans would have frozen over.  This may have happened during the Snowball Earth period, but not during any recent ice age. StuRat (talk) 18:02, 29 March 2014 (UTC)


 * Two common errors there, StuRat. Polar night does not last for six months throughout Antarctica. In fact some of Antarctica is north of the Antarctic Circle and so won't experience polar night or the midnight sun. Also take a look at Antarctica for an explanation of why the place is cold. By the way check Form B and use 0 degrees east or west longitude and 90 degrees south latitude (and then 89 south latitude) to see the sunrise/sunset times. CambridgeBayWeather (talk) 05:13, 30 March 2014 (UTC)


 * Yes, I meant at the South Pole, not all of Antarctica. StuRat (talk) 14:58, 30 March 2014 (UTC)


 * Surely it was lower than today, right? I'm not talking about the tropics, I'm talking about the mean low winter temperature in the then coldest places on Earth. 78.1.186.238 (talk) 18:05, 29 March 2014 (UTC)


 * Yes, lower than today, possibly with a few exception areas due to changing currents, jet streams, etc. StuRat (talk) 18:13, 29 March 2014 (UTC)


 * "Scientists estimate that global average surface temperature was about 5 °C lower than at present, with larger changes at the poles."The Science Museum - Past climates: Ice ages. Alansplodge (talk) 18:16, 29 March 2014 (UTC)


 * And in case it isn't obvious, one major reason is the white ice reflecting sunlight back into space, although typically there is some other cause of the ice age initially. StuRat (talk) 18:20, 29 March 2014 (UTC)

Bioinformatics
Hi, I am starting to take my first steps in this field. I am trying to understand global and local alignments, using dynamic programming. I'm OK with the bio, but not really with the computer stuff. I am doing this alignment with a pen and paper, and want to make sure I'm doing it right. Is there a somewhere online I could put in tow DNA sequences & decide the scores, and as an output get the matrix itself {similar to what appears on this page: http://www.biosym.uzh.ch/modules/models/DNA_Alignment/align.xhtml }? — Preceding unsigned comment added by 164.138.124.238 (talk) 19:50, 29 March 2014 (UTC)

Cantilever bridges
2 questions about cantilever bridges 1- do cantilever bridges have a free end when're complete or is it only a method of construction. Where is the free end if it does? 2- Are bascule bridges such as Tower Bridge, cantilevers if we assume the movable part is a fixed end? Clover345 (talk) 22:07, 29 March 2014 (UTC)


 * See cantilever bridge. Typically they are attached at both ends, so act ads a dual cantilever (one from each end).  However, I can imagine a case where they might not be attached at one end, say to span a fault line.  This would allow it to survive a quake, although there might then be a large gap to span to make it safe for traffic again.

Single cantilever bridge over fault line before quake: --- -- -     \/

Single cantilever bridge over fault line after quake: --- ---   \         \/


 * StuRat (talk) 23:06, 29 March 2014 (UTC)


 * Regarding the Grand Canyon Skywalk, I certainly hope it has an anti-slip surface, as otherwise it might become slick when predictably covered with urine. :-) StuRat (talk) 01:31, 30 March 2014 (UTC)