Wikipedia:Reference desk/Archives/Science/2013 October 14

= October 14 =

Do fruit flies spontaneously generate?
I have seen and raised the maggots of Drosophila melanogaster in genetics lab in college. But although I have had briefly self-sustaining colonies of whatever teensy fruit fly is endemic to the NE US come into the house with fresh produce, why have I never seen one of their babies? (And please don't post pictures.) Thanks μηδείς (talk) 02:15, 14 October 2013 (UTC)


 * For the same reason you never see a baby pigeon - you're not looking in the right place! Richerman ''   (talk) 09:19, 14 October 2013 (UTC)
 * Perhaps the best place to look is where fruit or their skins have been allowed to acidify, as might be suggested by the first answer in this link. While they may fly inside to lay their eggs on fresh fruit, they might be developing into maggots and then flies outside in the discarded compost and garbage.  — Quondum 15:52, 14 October 2013 (UTC)
 * History of Animals, although it is not the latest scientific research on the matter, supports the theory of spontaneous generation. Thincat (talk) 09:59, 14 October 2013 (UTC)
 * Aristotle may have been a great philosopher, but as a scientist he was a bust. ←Baseball Bugs What's up, Doc? carrots→ 02:38, 15 October 2013 (UTC)


 * A great white marble bust. Plasmic Physics (talk) 02:42, 15 October 2013 (UTC)


 * As a serious note, Aristotle's science was excellent. He tried to induce principles based on observation.  The problem with his science is that for the next millennium and a half people repeated his conclusions as dogma without using his method or testing his results. μηδείς (talk) 23:06, 15 October 2013 (UTC)

Fruit flies, humans and everything else we have here did in fact spontaneously generate from a cloud of Helium and Hydrogen gas. Count Iblis (talk) 02:57, 15 October 2013 (UTC)


 * 'Everything' requires more than hydrogen and helium, what about the other elements? Plasmic Physics (talk) 03:18, 15 October 2013 (UTC)


 * The other elements can be spontaneously generated from hydrogen. Count Iblis (talk) 14:07, 15 October 2013 (UTC)


 * The same could be said of hydrogen - spontaneously generated from protons and electrons, et cetera ad absurdum. Plasmic Physics (talk) 00:32, 17 October 2013 (UTC)


 * Indeed, but what matters is that you can just start with some gas in a big enough box, sit back for a few billion years and out will come fruit flies, people, cities, cars, Wikipedia etc. etc., all by itself. Count Iblis (talk) 03:18, 17 October 2013 (UTC)

Spectral radiance/Planck's law
The article on spectral radiance, which I was linked to from the Planck's law article, defines spectral radiance as "the quantity of radiation that passes through or is emitted from a surface and falls within a given solid angle in a specified direction".

The Planck's law article states that the spectral radiance of a black-body, $$B(\nu,T)$$, is given by

$$B(\nu,T)=\frac{2h\nu^3}{c^2(e^{\frac{h\nu}{kT}}-1)}.$$

I'm trying to relate this law to the definition of spectral radiance above. I'm guessing the surface referenced to in the definition refers to the surface of the black-body, correct? But what solid angle are we talking about? Solid angles are defined with respect to a point (ie the origin of a coordinate system), but no such point is identified in the Planck's law article.

The reason I'm asking this question is that I'm confused about the derivation of the Stefan-Boltzmann law from Planck's law. I would have thought that, to use the notation from the article,

$$j^{\star}=4\pi \int_0^{\infty}B(\nu,T)d\nu$$,

where the factor of 4π comes from the fact that there are 4π steradians in a sphere. But apparently,

$$j^{\star}=\pi \int_0^{\infty}B(\nu,T)d\nu$$,

and I don't really understand why.

Finally, is the radiation emitted by a black-body at a particular point on its surface travelling perpendicular to the surface at that point? — Preceding unsigned comment added by 74.15.138.165 (talk) 02:36, 14 October 2013 (UTC)


 * A black body radiates in all directions from any part of its surface. A black body also absorbs radiation coming from any direction inpinging on any part of its surface.  The law of reciprocity applies, and you can easily verify that all directions apply by looking at a hot enough black body surface from any angle.  It does not appear dark at any off-normal angle. 121.215.39.252 (talk) 04:35, 14 October 2013 (UTC)


 * You can derive the Stefan-Boltzmann law from Planck's law as follows. Consider a box filled with thermal radiation in thermal equilibrium. This is described by Planck's law it is isotropic. The energy density as a function of frequency is some function u(nu) (so u(nu) dnu is the amount of energy per unit volume in the frequency interval between nu and n + dnu ). Then because the photons move at speed of c and are isotropic, the flux of photons with frequencies between nu and nu + dnu coming from a small solid angle dOmega is c u(nu)dnu dOmega/(4 pi). Now imagine a small black sphere of radius r inside this isotropic photon gas.


 * How much radiation will this sphere absorb per unit time? We don't need to calculate any integrals to find out, if we look at photons coming from any particular direction, then the intercepted flux by the sphere is the same as what a disk perpendicular to that direction of radius r would absorb. So, the sphere absorbs an energy per unit time of pi r^2 c u(nu)dnu dOmega/(4 pi) from the incident photons coming from photons from any solid angle range dOmega. Sice both the radiation and the sphere is isotropic, this doesn't depend on Omega, so we can integrate over all solid angles by multiplying by 4 pi. The total flux through the sphere from the photons from any direction is thus pi r^2 c u(nu)dnu =  A/4 c u(nu)dnu where A is the surface area of the sphere. Since a black object at temperature T will emit as much radiation per unit time and frequency as it would absorb when placed inside a photon gas of temperature T, this means that a sphere will emit thermal radiation per unit frequency and unit area of c u(nu)dnu/4. This is then also valid for a black object of any arbitrary shape, because each emitted photon comes from some point on the surface and how much radiation each surface element emits per unit time, doesn't depend on the orientation of the other surface elements, as the emission process is a local process. Count Iblis (talk) 14:58, 14 October 2013 (UTC)

Another way to think about it: A piece of hot surface emits only into a half-space, so maybe the factor should be 2 &pi; instead of 4 &pi;. But if it really would be 2 &pi then it would have to emit the same energy into every direction of that half-space, and so if you start with a hot glowing sheet of some material being perpendicular to your line of sight, and then you tilt it, you should still receive the same amount of light from the surface, despite the fact that the surface's image on your retina is now smaller. I.e. the glowing sheet should actually look brighter if you tilt it. This obviously isn't true; the correction factor is equal to the cosine of the angle. Thus, we need an integral not only over the frequencies, but also over the angles:

$$j^{\star}= \int_{\Omega}cos(\theta) \int_0^{\infty}B(\nu,T)d\nu d\Omega$$

Written out in full:

$$j^{\star}= \int_0^{\frac{\pi}{2}} \int_0^{2 \pi} cos(\theta) \int_0^{\infty}B(\nu,T)d\nu sin(\theta) d\phi d\theta$$


 * Thanks, I understand now. 74.15.138.165 (talk) 17:26, 15 October 2013 (UTC)

Icek (talk) 09:17, 15 October 2013 (UTC)

Are neutron star collision energies as large as supernova energies?
I've read that the nature of the energy emitted is mainly in gamma rays, and that unusual magnetic fields are created, so it's somewhat different than a supernova, but is the "pow" comparable in energy?76.218.104.120 (talk) 04:18, 14 October 2013 (UTC)


 * Well, a supernova certainly gives off more energy at once, when it explodes, but it's quite possible that the total energy given off from a neutron star, or even a normal star, over it's life, might be more. StuRat (talk) 12:41, 14 October 2013 (UTC)


 * I imagine that the OP was referring to the energy emitted at the time of a collision between two neutron stars, as might happen when they spiral into each other. This is one of the speculative sources of gamma-ray bursts. — Quondum 18:00, 14 October 2013 (UTC)
 * Yes, I certainly was. Stu misread my question.76.218.104.120 (talk) 08:15, 17 October 2013 (UTC)
 * Well, assuming that the proportion of gamma ray bursts observed attributable to neutron start collisions is not vanishingly small, that the number of collisions is not markedly larger than of supernovas (per unit time), that you are referring to the energy of each in the gamma ray spectrum, and that the spread over time of the emission is comparable for each, the logical conclusion is that the amount of energy is "comparable". This is essentially a ballpark conclusion; it would make sense to check the premises here a little more closely.  There are other forms of radiation that can carry away enormous amounts of energy rapidly, especially neutrinos.  — Quondum 18:36, 17 October 2013 (UTC)

Cytomegalovirus and HIV
This letter to Nature, dated 1990, suggests a relationship between cytomegalovirus (CMV) and Human Immunodeficiency Virus (HIV) infection where CMV increases susceptibility to HIV infection. I can't, for lack of google-fu, find any more recent discussion of this relationship between these two viruses. I don't really expect an answer to the question, but I'm looking for instruction that my google-fu might become stronger and am expecting that the masters at the reference desk will know how to find such things. Specifically, it'd be nice to have an actual paper that discusses this link, rather than just a letter. 71.231.186.92 (talk) 04:39, 14 October 2013 (UTC)
 * Have you searched Google Scholar, http://scholar.google.com/schhp?hl=en, rather than plain old Google? μηδείς (talk) 04:56, 14 October 2013 (UTC)
 * Yeah, a couple of ways. 71.231.186.92 (talk) 05:24, 14 October 2013 (UTC)
 * I have read the same, that there is a strong relation. Electron9 (talk) 07:30, 14 October 2013 (UTC)


 * Three things - firstly, you might not be aware that a "letter" in Nature is an actual, proper, peer-reviewed academic paper in it's own right, for presumably historical reasons Nature calls it's shorter papers "letters" and it's longer papers "articles". The difference is explained in more detail here . Secondly you might want to have a look through these results, which are (most of) the papers which cite your paper (I got to this be searching for the title of your paper in google scholar and clicking the "Cited by" link below it). Google scholar doesn't have quite as good coverage as the proprietary databases that also do this, but those are really expensive. I find this search approach is normally the best for looking for more recent discussion on the topic of a paper. Finally you should keep in mind that the paper claims to have found a link between CMV and HIV infection in human fibroblasts, grown in vitro. This may or may not be in any way reflective of the situation in vivo. You should also bear in mind that fibroblasts are not the primary site of infection in HIV, although they can become infected. As far as I can tell from my quick read of the abstract this paper suggests one of the mechanisms by which this can occur, and would probably have no bearing on the primary site of infection which is the CD4 T cell (it would not affect these cells in the same way as the virus utilises the CD4 receptor to gain entry to these cells, rather than an Fc receptor, which is what is induced by the CMV in the McKeating et al. system). Equisetum (talk &#124; contributions) 12:31, 15 October 2013 (UTC)

Is light from light-emitting diode emitted with same phase?
Does light from light-emitting diodes have the same phase ..? Electron9 (talk) 07:29, 14 October 2013 (UTC)


 * No, a normal LED is an incoherent source.  Spinning Spark  07:34, 14 October 2013 (UTC)


 * ...but then again, so am I. :-) StuRat (talk) 20:02, 16 October 2013 (UTC)


 * Not with a normal LED, but a laser diode is a closely related device which does produce coherent light. Red Act (talk) 15:24, 14 October 2013 (UTC)

What is the opposite of Wikipedia's List of mental disorders?
Is there a list of things that are unique to a healthy, able, sane mind? What is the study of ideal brains called? — Preceding unsigned comment added by 174.65.23.49 (talk) 11:42, 14 October 2013 (UTC)


 * Mental health and positive psychology study that sort of thing. Unique though, hmm, what is unique to a healthy animal? it can't be six legs like an insect or a trunk like an elephant. Dmcq (talk) 13:29, 14 October 2013 (UTC)


 * Yes, I would have said "things that are common to a ...", instead of "unique". One example might be having all parts of the brain be operational, such as can be shown on a blood-glucose utilization scan.  However, some people have been amazingly functional with even large portions of their brains disabled or absent. StuRat (talk) 13:42, 14 October 2013 (UTC)

Thank you. — Preceding unsigned comment added by 174.65.23.49 (talk) 16:13, 14 October 2013 (UTC)
 * My brother-in-law wrote a book about that and titled it Optimal Human Being. It seems to get the idea across. Looie496 (talk) 20:23, 14 October 2013 (UTC)
 * Is he this guy, Kennon Sheldon? Must make for interesting dinner conversations. 220  of  Borg 18:14, 18 October 2013 (UTC)

Can axon tracts travel *around* the internal capsule?
Can axon tracts travel *around* the internal capsule, or must all the axon tracts travel *through* the internal capsule thereby making the internal capsule a gateway canal-like thing between the cortices and rest of the body? I am having trouble visualizing the bigness of the internal capsule in 3D in my head, based on 2D myelin-stain representations. (Hey, it isn't easy, particularly if this brain sliced in coronally at one or a few locations!) 164.107.102.151 (talk) 17:59, 14 October 2013 (UTC)
 * If you are asking whether there are any direct pathways between the cerebral cortex and spinal cord that don't travel through the internal capsule, the answer as far as I know is no. But there are a number of indirect pathways that go by completely different routes.  Our internal capsule article ought to be helpful to you for visualizing it. Looie496 (talk) 20:17, 14 October 2013 (UTC)