Wikipedia:Reference desk/Archives/Science/2015 March 24

= March 24 =

Follow on from Fluorescent Lamps question above
The OP states "turning on a fluorescent lamp requires a large energy use". I found this thought odd so I googled "do fluorescent lights use more energy to turn on" (sans quotes) and I got a lot of hits from seemingly reliable sources that this is a myth. So my question is: what gave rise to this misconception? 196.213.35.146 (talk) 13:19, 24 March 2015 (UTC)


 * To my knowledge, it has never been the case that turning on a fluorescent lamp required that much of an energy spike. If you had a massive factory with thousands of bulbs, I am sure you would notice a major spike. In a home with 1 or 2 bulbs, it isn't much of anything. The catch is that old bulbs broke down a little every time they were turned on. So, turning them off and on shortened their life. It was also a bit rough on the ballast. Personally, I replaced ballasts more than bulbs when I worked in a theater in which the bulbs were turned off at the start of every movie and turned on at the end. 209.149.113.207 (talk) 14:08, 24 March 2015 (UTC)
 * It is not so much a misconception as poor understanding of the sources you googled. In order for the gas in the florescent tube to conduct electricity it has to ionize. Mains voltage alone can not achieve this in a florescent lamp. Therefore the luminescent fixture contains a circuit which incorporates a 'ballast' or choke. Upon starting, the ballast windings create a magnetic field. When the ballast is connected to the tube (via the starter) that magnetic field collapses and all that energy within it is dumped  into the tube – so striking the arc. So as watt = joule / second, then yes - they do indeed require more energy to start but it is the volts that do it.--Aspro (talk) 17:50, 26 March 2015 (UTC)

Star life cycles
Hi all! After drawing this graphic, I wondered if I got it right, particularly whether red dwarfs form directly from nebulae or from protostars. Next, where do brown dwarfs fit in — do they start from protostars and end as white dwarfs? Thanks, cm&#610;&#671;ee&#9094;&#964;a&#671;&#954; 13:32, 24 March 2015 (UTC)


 * Brown dwarfs are explained here. They are low mass stellar objects that don't have enough mass to undergo hydrogen fusion. They will not form white dwarfsDja1979 (talk) 15:00, 24 March 2015 (UTC)


 * Who did you create this image for, people who wish to learn/know about stars or people who already know about it...? The whole thing looks like a mess to me, I wish to understand it simply, but beautifully. In other words, work is fine but confusing. Star forming nebula do have colourful stars by the way, can be seen with some UV specs... What colour is it? - Recalling (not sure), pink (active), grey outside, little pink inside (dead)... You have to find out I don't know it well...
 * Btw, what software did you use to create this picture?
 * (SuperGirlsVibrator (talk) 20:28, 28 March 2015 (UTC))

Light Refraction
This question was originally posted to the Help Desk. Robert McClenon (talk) 15:26, 24 March 2015 (UTC)

I am trying to find out what color will you get if you place a red filter over a red object and have searched the internet trying to find out to no avail. My question comes because I was told that if you place a red filter over a red object it will appear white for one red cancels the other. Boxy22 (talk) 09:35, 24 March 2015 (UTC)


 * There's some annoying imprecision in the language about filters, as explained here. Usually a phrase like "red filter" is interpreted to mean a filter that passes red light, and so a red object through a red filter is red.  But sometimes, especially with "UV filter", it can mean one that blocks that color.  In that rare instance, which I'd tend to think of as a misuse of the term, a red object hypothetically could appear entirely black because all its red light is blocked.  However... no filter is absolute, the shoulders are often very wide, and since we're speaking of the intersection of what is filtered out by reflecting off the red object and what passes through the red filter (which includes some red light) I think what you see could be red, orange, yellow, even white ... it's hard to say because you're multiplying two entire absorption curves and seeing your eyes' overall determination of the dominant colors of what is left. Wnt (talk) 16:22, 24 March 2015 (UTC)


 * I would think that white and blue would be the colours you would not expect to see, but I agree with Wnt's comments about imprecise language and filters that can be surprisingly wide. In most cases, you will see red because red light is reflected from the object and is passed by the filter.  Cancelling might happen if the filter is red-blocking ( looks cyan ) and just happens to block the exact extra red light reflected by the object, but even then your eye is unlikely to interpret the result as white, more probably a muddy grey.  Where does refraction come in?  Perhaps you are confusing filters with prisms where you can split up light then recombine it to get white.    D b f i r s   16:53, 24 March 2015 (UTC)

Another question on brown dwarfs
What does [Kulkarni] mean in the lithium burning article? The page cites no sources by Kulkarni or by anyone else. 65.210.65.16 (talk) 15:32, 24 March 2015 (UTC)
 * It is an incorrectlty-formatted citation that had existed for about eight years! Regrettably, I don't recognize the source, so I can't independently verify it.  The name shows up in references for a few related articles, e.g. Brown dwarf, but it's a ton of work to track down a hypothetical ...book/article/webpage ...  from one name alone.  We can more easily find alternate sources.  If I were going to look for an alternate replacement source, I'd pull out my copy of de Pater & Lissauer: Planetary Sciences, which has great content on stellar evolution.  Perhaps later today I can re-source that article.  Nimur (talk) 15:44, 24 March 2015 (UTC)

White dwarf / Chandrasekhar limit questions
All this star talk reminds me that there are still essentials of white dwarf and Chandrasekhar limit that I don't understand. It is clear enough to me that there is electron degeneracy pressure because electrons can't have the same quantum state (whyever that is, the Pauli exclusion principle as observed). The tighter you press plasma together, the faster the electrons have to move. At a certain point, they move so fast that instead the pressure drives them to electron capture. But...

1. What says how much faster an electron has to move than its neighbors, or at how much of an angle relative to them, in order to get its own quantum state? (Fermi sea is less than informative for me, at least...)

2. When the Chandrasekhar limit is reached, that indicates that electron capture has become a feasible process. Is this determined by kinetics (how close you have to jam electrons and protons before they meet often enough to become neutrons) or thermodynamics (the electrons at this point are moving just fast enough to make neutrons lower energy than proton + relativistic electron)?

3. Above I assume it's protons that interact, based only on the name "neutron star", but presumably white dwarfs should be largely helium. Is it actually helium nuclei that capture the electrons?

4. Our article Chandrasekhar limit references the  Tolman–Oppenheimer–Volkoff limit where neutron degeneracy pressure fails as sort of the next step on, but our article on neutron stars has a cute figure with a number of intermediate levels between white dwarf matter and quark-gluon plasma as one looks deeper and deeper into the neutron star. Oddly, there are no "just pure neutrons" layers listed, which makes me wonder if the TOV limit is just an approximation, something that doesn't purely happen? Are there more precise limits for explaining when each type of nucleus in the white dwarf breaks down by electron capture, or something?

Wnt (talk) 16:14, 24 March 2015 (UTC)


 * A question what says how much faster an electron has to move than its neighbors, or at how much of an angle relative to them? can only be answered indirectly in the framework of quantuum mechanics. Indeed, the way the quantum mechanics is prsently understood, the electrons are (a) indistinguishable and (b) can not be described by an exact value of the velocity vector in any spatiotemporally local sense. We can, however, make an oversimplified mental picture of electrons that (i) do not interact electrostatically with each-other and with the ions, and (ii) are confined to some hypothetical unit volume with impenetrable walls surrounding it. Electrons under such hypothetical conditions can assume single-electron states that look like resonator modes of that unit volume (that is, standing waves), two per mode because they are spin 1/2 fermions. At relatively low temperatures, single-electron modes will be filled from the ground up to the Fermi energy. The full (many-electron) state will be a fully anti-symmetric combination of all the single-electron states. If you do the math right, you will find that the extensive quantities (internal energy, entropy) in this model indeed come out extensive, that is, depend linearly on the volume for a given free electron density.  Pressure, therefore, will depend on the free electron density but not on the (hypothetical) unit volume. Good!  When free electron have a temperature, and the temperature - in energy units - is much lower than the Fermi energy, the most energetic occupied single-electron free electron states will have energy close to the Fermi energy. As you increase the temperature, chances of higher-energy free electron states to be occupied will increase as well. Does this help?   --Dr Dima (talk) 17:30, 24 March 2015 (UTC)


 * (1) I don't know if you can understand fermion degeneracy in the particle picture. If you think of the electrons as waves, then the waves have to be orthogonal in spacetime. Classically, the Nyquist rate is analogous, I think. The number of independent samples you can encode in a signal with a maximum frequency is the same as the number of fermions you can pack into that space with a maximum energy (in the absence of any forces). (Edit: retracting my answers to 2-4 because I'm not knowledgeable about this.) -- BenRG (talk) 20:08, 24 March 2015 (UTC)


 * (1) As others have said, each electron needs to have its own quantum mode. In three dimensions, each mode can be described by three integer wave numbers, $$n_x, n_y, n_z$$ such that the energy of each particle is:
 * $$E_{n_x,n_y,n_z} = E_0 + \frac{\hbar^2 \pi^2}{2m V^{2 / 3}} \left( n_x^2 + n_y^2 + n_z^2\right)$$
 * and where the integer indices are unique for each electron. For a system with a very large number of particles, this implies that some of the particles must have very large indices and be extremely energetic.  In a degenerate Fermi gas, the most energetic particle will have energy approximately equal to the Fermi energy:
 * $$E_F = \frac{\hbar^2}{2m} \left( \frac{3 \pi^2 N}{V} \right)^{2/3}$$
 * Which depends only on the particle number density.


 * (2-3) These questions are premised on a mistake. The Chandrasekhar limit doesn't exist because of some other process taking over.  It exists because there is a maximum mass sustainable by electron degeneracy period.  Think of it this way, a star's radius can be described as a function of it's mass, $$R(M)$$.  As you add mass, the gravity increases, which increases the central density.  For a normal star, sustained by nuclear fusion, increasing the central density will increase the rate of fusion, make the star hotter, and cause its radius to increase.  Hence, most stars exhibit $$R(M) \propto M^x$$ for some index x, typically between 0.5 and 1.  However, in a white dwarf, the electron degeneracy pressure doesn't increase very quickly as mass increases.  The end result is that radius decreases with increasing mass.  It turns out that the combination of the relativistic electron degeneracy pressure and the Lane–Emden equation (governing pressure-density relationships in gravitational objects) predicts that for a finite mass the radius the white dwarf goes to zero, i.e. $$R(M_{Chandrasekhar}) = 0$$.  If white dwarfs were an ideal Fermi gas, they would collapse into a black hole at the Chandrasekhar limit.  Of course, they aren't ideal Fermi gases, but rather real stars made of atoms.  The result is that as the collapse begins the increased density and temperature reaches helium fusion temperatures and the star detonates as a type Ia supernova.


 * (4) White dwarfs never become neutron stars because the supernova detonation intervenes. Neutron stars are heavily enriched in neutrons compared to ordinary matter (e.g. >10 times as many neutrons as protons), but they aren't pure neutron objects.  A dynamic equilibrium exists between neutrons and protons / electrons depending on pressure and temperature.  Dragons flight (talk) 21:47, 24 March 2015 (UTC)

Thanks for some great answers! I'll start unpacking.

(1) The difference in energy seems recognizable as the classic 1/2 m v2, where v is the momentum divided by the mass of the electron. That gives the 1/2m part; the sum of squares is vector addition for momenta in three dimensions; and the remaining part, the quantization, is Planck's constant h (unreduced) divided by the length - assuming that V is a cube, that is. I assume the cubical star is something of an approximation. :) This is the same as the momentum of a photon with a wavelength equal to the length of the cube.  (But the angular momentum of a photon measured along its axis is h/ 2 pi rather than a length)  Going a bit "out there", the subjective feeling I'm getting from this is that for each quantum state you have to string some distinct number of h's of momentum/angular momentum around some sort of loop to define it.  Pushing the electrons together shortens the loop, and so the amount of momentum at each part of the loop increases, i.e. the pressure and observed momentum increases.

(2) This part is where I go off the rails. I've seen the phrasing that degeneracy pressure 'can't sustain' more than a certain force, yet the math from (1) shows an energy that clearly goes to infinity as V goes to zero. It doesn't make sense to me that you can press something together and create infinite energy; that's why I'm thinking some other process has to interfere (and can find sufficient energy to do so).

(3-4) It makes sense that the helium nuclei simply undergo fusion and set off the supernova; yet in a neutron star itself, the outer layers are something like white dwarf material, so it should be possible to find a continuous gradient... I think. The article is a bit vague about whether the outer layers are all iron nuclei or hydrogen and helium; actually I find myself wondering if there could be a long period of ongoing fusion near the surface of the neutron star. But I suppose it makes sense that at least the bottom layer of nuclei has to be iron, and so the iron nuclei ought to be what (if there is gradually accretion on top of it) would have to give way to capture the electrons? In which case... they aren't iron nuclei anymore, and should undergo more nuclear reactions. Hmmm. I wonder just how complicated this ecosystem really is! Wnt (talk) 02:44, 25 March 2015 (UTC)


 * Thinking about it further, I realize that we're saying the momentum between electrons has to differ by h/ V1/3. We know the position of the electrons is within the star, and so by the Heisenberg uncertainty principle the momentum cannot be measured more accurately than this amount.  So the fundamental limit here is that not only do the electrons have to be in different quantum states - they have to be in measurably different states.  It's funny though... a principle that I tend to think of as something you can't observe under a microscope would seem to be holding up the surface of a star.  The article on electron degeneracy pressure makes this point, but ... it isn't very comprehensible where it does so. Wnt (talk) 11:27, 25 March 2015 (UTC)


 * Yep, the cubical star approximation. ;-)


 * Yes, the particle energy goes to infinity as V -> 0, but the gravitational binding energy also goes to infinity as V -> 0. In fact, black holes have lots of infinities lying around.  Let's look at it a different way.  It takes thermodynamic work to compress a gas.  In the isothermal limit, that work is $$W = P\, dV$$.  The gravitational energy liberated by compressing a star is proportional to $${G M^2 \over R^2}\, dR$$.  A star is unstable towards gravitational collapse if the gravitational energy liberated by shrinking would exceed the work required to compress it.  In that case, the internal energy of the star is increasing, but that increase is driven entirely by the energy liberated by gravitational collapse.  The star's size naturally seeks out a local minimum state of total energy (internal energy - gravitational potential energy), by balancing the tension between the desire to collapse under gravity and the work required to further compress the star.  For certain combinations of mass and temperature (e.g. white dwarfs above the Chandrasekar limit) no local minima in total energy exists except for the one associated with collapsing into a black hole.  During that collapse other processes do intervene, i.e. helium fusion, but the primary effect is that the electron degeneracy pressure grows too slowly ($$P\propto \rho^{4/3}$$) for the work required to compress the star to effectively oppose the energy gained from infalling mass.  Dragons flight (talk) 22:58, 25 March 2015 (UTC)

Star Trek and Science
Dear Ladies and Gentlemen.

It has come to my attention that many people claim, that the tv shows of the franchise Star Trek are supposed to be scientifically accurate. Even famous scientists like Neil deGrasse Tyson has touted the shows for their "scientific literacy". I have never seen Star Trek until very recently (I am from a different culture, Star Trek isn't as popular here as it is in the United States, in the UK or in Germany) but grew up with the books of various hard science fiction authors like Stanislaw Lem, Arthur C. Clarke and Herbert Werner Franke. Given that I am currently studying physics, it seems to me, that some of the Star Trek shows lack even some very basic knowledge about science. There are episodes, where the crew of Captain Picard encounters planets, that are colder (Theta 116 has a surface temperature of -291 °C. -291 °C is below absolute zero (-273.15 °C)) than the lowest possible temperatures and older than our universe (the same planet's age is estimated to be 7.2*10^10 or 72 billion earth years old; far older than the universe itself). I like Star Trek, but to me, it does not seem more scientifically accurate than Farscape, Stargate or Firefly (like most soft sci-fi products). So why do so many people make rather strange comments about the representation of actual science in these shows? I apologize for not being perfect in English, I started recently with the study of your beautiful language.

Kind Regards.--178.195.98.161 (talk) 17:34, 24 March 2015 (UTC)


 * The difference is the time period. Star Trek is from the late 1960's, and in the time frame "sci-fi" shows were notoriously inaccurate.  So, Star Trek was a step up from those earlier attempts.  Of course, any sci-fi show set in space will have to make compromises, whether to keep from boring the audience with only sublight speed or keeping on budget by having 1 g of gravity in most every scene.


 * I did notice a reduction in the level of scientific realism from the original series to Next Gen, though. For example, in one original episode (The Tholian Web), Kirk is caught in an interdimensional rift, coming in and out of phase with the our universe.  Next Gen did a similar episode, but there instead of being in a space suit with an air supply and floating freely about, the person so affected could still breath the air and walk on the floor, which made no sense whatsoever. StuRat (talk) 17:39, 24 March 2015 (UTC)


 * From the Next Generation series onwards they tried to get things as scientifically accurate as they could within the confines of telling a good story. For instance, having been told that a transporter couldn't work because of Heisenberg's uncertainty principle they introduced a "Heisenberg compensator". Nobody knows what it does but it sounds good and gets round the problem. When Leonard Nimoy died it said on the NASA website that many of their scientists were originally inspired by Star Trek to go into their chosen field.  Richerman    (talk) 18:37, 24 March 2015 (UTC)


 * I strongly disagree! To take *just* the transporter system: The entire point of Heisenberg's uncertainty principle is that it's a fundamental property of nature - you can't conceivably "compensate" for it.  The transporter violates conservation of momentum, conservation of mass/energy, it implies more data bandwidth between source and destination than is reasonable, not to mention the requirement to reassemble large objects without causing the air to move out of the way around them and without having any equipment at the far end.  The range, capability and capricious nature of transporters is poorly maintained from one episode to the next...and the crew conveniently forget to use it in situations where it would be exceedingly useful.  In STTNG, the ship has 'site to site' transport capabilities - and yet everyone still has to go to the "transporter room".  Why do they even need a transporter room?  Transporters frequently function in situations where getting even a simple voice radio link to work fails!  The idea to transport bombs or other weapons on board other ships is rarely used.  The shuttle craft have transporters - but they are never used to get them out of trouble on the surface of planets and such.  We're told that the transporters are also able to 'filter' disease carrying organisms as aliens are beamed aboard - but never to cure people who carry those diseases while on board.  On occasion they transport people aboard while selectively NOT transporting their weapons.  That degree of selectivity would allow an enormous number of industrial processes within the ship to be carried out with them...but no sign of that either.   We're told (in STTNG) that transporters are the safest form of transport devised by man - yet there are dozens of transporter screwups in just one starship over a matter of just a few years.  It's evidently possible to reconstitute a human from copies of their recordings in the transporter "pattern buffer".  Why are the crew not routinely recorded in this way before going on dangerous missions?


 * And that's just the transporters.


 * It's true that many scientists were inspired by StarTrek - but that's true for any number of other sci-fi books, TV and movies.


 * It's safe to say that there is very, very little scientifically accurate stuff in StarTrek. It's rarely even self-consistent from one episode to the next.  HOWEVER, that doesn't prevent the genre from being able to produce some entertaining stories. SteveBaker (talk) 19:10, 24 March 2015 (UTC)


 * Related articles of ours are The Physics of Star Trek and Physics and Star Trek. -- ToE 19:05, 24 March 2015 (UTC)


 * Also tangentally related are Clarke's three laws, especially the third law. Clarke's Third Law the sine qua non for science fiction writing, and is what separates good, readable science fiction from the sort of "Moby Dick in Space" in the sense of being a massive technical work with some minor story going on in the background.  Really good science fiction doesn't try to explain or even "get right" its violations of the laws of physics as we know them.  The basic application of Clarke's Third Law for good writing is: get right what your audience would notice if it were wrong, and the rest isn't important to get right".  That is, any technology which would allow faster-than-light travel would have to be "magic" under the current understanding of physics, so: let it be magic.  Maybe you throw a MacGuffin into your work like "dilithium crystals" or "warp drive" or something like that, but as soon as you try to make the magic "real", you lose your audience (read any of the criticisms of midichlorians in the Star Wars universe to see what happens when you treat the Third Law with disrespect).  Some authors do deal creatively with the "faster than light" problem in more realistic ways than Star Trek.  For example the Enderverse by Orson Scott Card deals with the problems of relativistic aging due to the twin paradox and time dilation in FTL travel.  But he still has to play the "it's magic" game when it comes to other aspects; for example in the Enderverse, instantaneous communication between distant points in space is still possible, and FTL travel still occurs, both of which require the author to just ignore some fundamental rule of physics.  The balance must be made between "writing an engaging story set in a futuristic world" and "not getting the physics so bad that it distracts from the writing".  A good way to avoid the latter is to ignore discussions of physics in general.  The audience will forget that your story is impossible so long as you don't actually try to actively prove to them it is... -- Jayron 32 19:23, 24 March 2015 (UTC)


 * Star Trek (original series), like lots of "science fiction" wasn't originally about science so much as about exploring the human condition, in a setting unfettered by present society, politics, and technology. So Star Trek could talk about things like socialism, communism, inter-racial and multicultural relationships (etc. ), that would have been much harder to do on TV at the time with a contemporary setting. Lem does a bit of exploring the human condition too, but Lem of course also knew a lot of math and science. Think of it another way: has anyone ever criticized Kurt Vonnegut for lack of scientific realism in his works? Not really, because although they have elements of Sci-fi, Vonnegut's novels aren't really even about science. SemanticMantis (talk) 19:28, 24 March 2015 (UTC)
 * Well, to flip it back on you, name any well regarded science fiction work which isn't fundamentally about "exploring the human condition". It's true that the level of realism with regards to the laws of physics is often what separates hard science fiction from soft science fiction, but ultimately, even the "hardest" science fiction in the world is still fiction, and still deals with issues of character development and plot arcs.  You mention Lem as someone knowing a lot of "math and science", but even he had to leave as unexplained such issues as FTL travel and instantaneous communication, and has a lot of unexplained "Magical" phenomena in his works.  You can't say Solaris isn't a work about the human condition; it's a novel which uses a science fiction setting to explore an aspect of the human condition in unique and novel ways, but it still does that.  Without that, it wouldn't be fiction.  -- Jayron 32 19:46, 24 March 2015 (UTC)


 * Well -- and don't take this to as critical as it's going to sound, but... -- I don't think you're telling SM anything he (or anyone here) doesn't already know; we're all familiar with the meaning of fiction, and of course we can all agree that any work of fiction bears on the human condition.


 * What SM was referencing was the specific bent of the show and the intentions behind it, which are the subject of a factual record that is more substantial than that which exists for probably any other work in the history of science fiction. As others have noted here, Star Trek has often been noted as a work which illustrates the difference between two broad approaches in science fiction: one can explore how scientific developments or concepts could change the nature of the human experience, or they can use such concepts as allegory (often in a manner that requires liberal reinterpretation of the actual science involved) to make observations about the world as it currently is/has been, typically focusing on social issues rather than empirical matters.  Alternatively, you can use the setting as an excuse to have some Buck Rogers adventure or to shoe-horn in some exploding robots for the purpose of eye-candy or to tell stories in which the sci-fi elements are largely window-dressing, but we'll leave those confounding factors be here.


 * Gene Roddenberry and the Star Trek showrunners that followed him were incredibly open (indeed, in Rodenberry's case, incredibility vocal) about which of the above ends they were pursuing. Many episodes of the original series are (or contain) specific allusions to the major social issues which were facing American and global culture in the sixties, be it racism (and eugenics), gender roles, or war (including particularly military interventionism, and, in the midst of the nuclear threat of the Cold War, fear of annihilation as the result of runaway technical capability for destruction), to say nothing of the general themes of multiculturalism and social unity that the show consistently advocated upon.


 * So yes, as you say, there is always a balancing act between realism in the science and the other story elements that one might which to emphasize, but the creators of Star Trek decided very early on which of those priorities to focus on and they've mostly stuck with that decision ever since -- though to what extent this represents devotion to the original artistic/social vision and how much it is the result of other factors (not wanting to confuse an audience of highly varying degrees of science literacy and who may drawn to non-factual elements anyway; convenience for writers, who very often do not have a formal background in science themselves; the franchise lore taking on a life of its own), I wouldn't venture to guess. But I think SM's point was to the overall intention associated with the show, not a suggestion that good story-telling can't incorporate strong realism and hard science, that bad stories can't use weak or fantastical science and still be bad on other levels altogether (that clearly happens a lot), or that even the greats of "hard" sci-fi wouldn't find it necessary to "cheat" (as they often have, as you pointed out).


 * As to the OP's original question (why someone would say the series is scientifically sound) that's obviously an open-ended and highly subjective question which we can't answer in concrete fashion here with references, and is thus more appropriate to a fan forum than this reference desk, and the initial response to that question probably should have said as much, instead of allowing this becoming a jumping-off point for discussion of the scientific merits of the show itself in a very forum-like fashion. I would say the only speculative answer I think I could suggest to answer that question is that actual scientists probably by and large have very little time for science fiction and for comparing the qualities of works within the genre, since Star Trek, by modern standards, is about as "soft" on science as they come, but if you're thinking of other major past landmarks in the genre that everyone is aware of (Star Wars, for example) it probably looks pretty rooted in science by comparison.   S n o w  let's rap 12:10, 25 March 2015 (UTC)


 * Can you point to a source where Tyson (or anyone) is quoted as saying that Star Trek is scientifically accurate? It seems unlikely he'd say that, since, well, it obviously isn't. People write books called "The Physics of Star Trek" because most people are not willing to buy books about physics unless they have "Star Trek" or "Einstein" in the title. They make tenuous connections between real science and Star Trek because they don't know how else to reach the lay audience that they want to educate. -- BenRG (talk) 20:36, 24 March 2015 (UTC)
 * (To BenRG's first question, can you point to a source ... saying that Star Trek is scientifically accurate?): Quite the opposite! A famous piece of Star Trek memorabilia is "The Writer's Guide", (widely believed to be) written by Gene Roddenberry.  Unofficial copies are available online, and the most official copy is presently kept in the Houghton Library at Harvard University.  In this very famous document, Gene Roddenberry (Creator of Star Trek) specifically warns his writers not to fixate on scientific accuracy, but to focus on good story-telling and character development.
 * (If you'd like to engage in hearty debate about whether the writers adhered to Roddenberry's advice - or whether Roddenberry himself backtracked on his own admonitions when he produced the later series and movies... there's a vibrant community of internet folks who would love to argue minutia with you. Consider perusing the official list of Star Trek fan sites to locate a forum).
 * Nimur (talk) 02:44, 25 March 2015 (UTC)


 * See here for the view of one of the NASA scientists and a discussion about the science. To sum up he says "The real science is an effort to be faithful to humanity's greatest achievements, and the fanciful science is the playing field for a game that expands the mind as it entertains. The Star Trek series are the only science fiction series crafted with such respect for real science and intelligent writing. That's why it's the only science fiction series that many scientists watch regularly... like me." Richerman    (talk) 09:14, 25 March 2015 (UTC)


 * In general I think the Stargate SG-1 TV series (but more the first one than its knockoffs) had a somewhat more science-oriented approach than Star Trek. (Maybe some of the later Star Treks were roughly equivalent, but watching those stuffed shirts run their corporate empire is just too much to take.  I mean, the only thing that could make Star Trek Voyager interesting would be a really bad transporter accident that left lumps of Janeway embedded in bulkheads and crew members throughout the ship) Wnt (talk) 11:41, 25 March 2015 (UTC)


 * Not meaning to give offense, Wnt, but let's try to keep WP:NOTAFORUM in our sights here; we could spend years comparing the relative merits of any of a countless number of sci-fi shows with regard to realism, but pointing out that another show was better than Star Trek in this regard is not going to get us any closer to a definitive or sourceable answer to the OP's question about why such shows get credited as scientifically accurate when they often aren't. And this is not otherwise the place for such comparisons. Besides which, while Voyager did take Star Trek to new heights of silliness, SG-1, despite initially being rooted to some extent in more realistic physics, ultimately "jumped the shark" early in its run and ended up being one of the biggest examples of goofy sci-fantasy out there by the end, despite retaining some good writing elements as regards...errr...nevermind... ;)  S n o w  let's rap 12:52, 25 March 2015 (UTC)


 * The OP may be referring to this . I didn't actually listen to it, but if is right, Tyson said “They’ve made admirable attempts to do the correct things with physics.” Nil Einne (talk) 12:38, 25 March 2015 (UTC)


 * I'll just bring up Larry Niven as one author who makes a huge effort to stick to hard science in many of his stories, such as Lucifer's Hammer and his Belter (asteroid belt) series. Nevertheless he does introduce teleportation devices, FTL travel, and scrith.  You pretty much have to have FTL travel or wormholes (in The Mote in God's Eye they are called jump points) to allow any inter-alien interaction on the scale of an interesting story. μηδείς (talk) 17:23, 25 March 2015 (UTC)  Russell's The Sparrow is rather hard scifi, except that she places the aliens at Alpha Centauri for convenience's sake.  (BTW, I most strongly recommend both those books.) μηδείς (talk) 17:23, 25 March 2015 (UTC)


 * I've heard that Larry Niven stopped writing Known Space stories because the setting had accumulated too much ‘magic’. Who said good sf gets to use one gross violation of physics? —Tamfang (talk) 08:44, 26 March 2015 (UTC)
 * The Sparrow is pretty good but unfortunately the numbers for time dilation are way off, iirc, not to mention the fuel requirement. —Tamfang (talk) 08:44, 26 March 2015 (UTC)
 * Not being a physicist, and only interested in the plot I did not check The Sparrow for accuracy, but at least she paid lip service to the principles. Yes, Niven did say at some point that once everything had become possible in his Known Space universe he found further writing in it fruitless.  I wonder where I read that. It's probably in the forward to one of his later books. μηδείς (talk) 18:18, 26 March 2015 (UTC)


 * There are at least 3 kinds scientific blunders:
 * The Story Blunders. Some, like "All Aliens speak English" are extensions of well known narration conventions, which save time and keep the story interesting. Same for the near-omnipresent "Instant Translator" handwave. Without some (like FTL drives and aliens), there wouldn't even be a story to tell.
 * I.e. these are, from a story-telling perspective, virtues rather than blunders.
 * The Economy Blunders. The "Most Aliens look Human" cliché belongs here. This is not about telling the story, or telling an enjoyable story, but telling an affordable story. The teleporter belongs here, too; the first season didn't have a budget for small craft.
 * The "Nobody Gave a Fuck" Blunders. Laser beams moving like bullets, visible laser beams in vacuum, etc. Somebody did it that way, and almost everybody followed. The really stupid mistakes, like Theta 116's temperature and age, and the near-omnipresent "Engines quit, Ship stops" (on TV, only Babylon 5 got that one right) belong here, too.
 * The "Nobody ever teleports a torpedo" rule has actually been violated twice; they seem to "forget" about the teleporters when it's convenient for the plot, though. Sometimes, it's about an otherwise good story, but sometimes, it's an ugly case of NGaF. - ¡Ouch! (hurt me / more pain) 13:05, 27 March 2015 (UTC)

From electricity to electromagnetic wave and back
If I heat the filament of a light bulb with light, would electricity start flowing through the attached cables? That is, it would be the reverse of powering the light bulb to emit light. Instead of transforming electricity into light, we would be transforming heat into electricity.Fend 83 (talk) 18:34, 24 March 2015 (UTC)
 * Sometimes yes, and sometimes no. In your specific example, no, you would not.  Heating the filament would produced stochastic motion of the atoms in the filament, and that motion and energy would not then generate an electric potential; no potential, no electromotive force, no EMF, no electricity.  However, some electrical processes are reversible.  Consider two examples, which are basically the same device used in reverse.  Consider first a speaker and a microphone.  What is the difference between them?  Actually, basically nothing.  A speaker is a device which vibrates in response to electric current fluctuations.  If you send an AC current to a speaker, you get a sound from the speaker.  If you do the reverse: if you take a speaker and shout into it, you generate an AC current in the wires attached to the speaker.  That's all a microphone is: a speaker you shout in.  Secondly, consider a motor and a generator.  A motor is a device which spins in response to an electric current.  If, however, you take a motor and drive it by hand, you've just made a generator: you can detect a current in the wires attached to the motor if you physically spin the motor.  Why do these systems work that way, where the lightbulb doesn't?  It has to do with the specific way they are constructed.  Both a motor and a speaker work by means of a moving magnet.  In the case of a speaker, fluctuations in the electric current in the wire causes an electromagnet (technically a solenoid) to vibrate back and forth; this vibration is amplified by the speaker cone, generating a sound.  In the case of the motor, the motor spins because a fluctuating magnetic field (generated in this case by a commutator) caused by the electric current causes the magnet attached to the axle of the motor to spin.  In each of these cases, the process is reversible because moving electrons generates a magnetic field, AND moving magnetic fields cause electrons to move.  That's where electromagnetism comes in as a concept: The two processes are two sides of the same coin; you don't get electricity (moving electric charge) without generating a magnetic field in response.  And if you move a magnetic field, you generate an electric current in response.  So, reversible electric devices would be ones where the transducer is one that relies on magnetism to transfer its work.  Processes that rely on heat to transfer their work are fundamentally non-reversible because heat is always lost as entropy.  And entropy cannot be recovered as usable energy.  The second law of thermodynamics is a mean bitch that way.  However, processes that don't depend on heat are usually in some way reversible.  -- Jayron 32 18:53, 24 March 2015 (UTC)
 * I should also note that it is possible, however, to generate electricity by shining light on things. Once we take heat out of the situation, there are some materials that generate electricity in response to shining light on them, which is the reverse process of using electricity to generate light (though not the exact reverse process that happens in an incandescent light bulb).  See photoelectric effect, the explanation of which was one of Einstein's Annus Mirabilis papers of 1905.  However, the photoelectric effect does not depend on heating anything.  -- Jayron 32 19:01, 24 March 2015 (UTC)

Hypothetical heart control
The medical sources I've looked into say a person fades out after cardiac arrest in some 8–12 seconds (and "rather longer if the patient is recumbent"). In theory would it mean that if a particular person were able to control his heart, he would be able to stand or seat seemingly well for those 8-12 seconds (or at least, say, 5 seconds) with stopped heart before restarting it? Brandmeistertalk  19:31, 24 March 2015 (UTC)
 * The heartbeat is controlled by the Autonomic nervous system which, by definition, is not under conscious control, except tangentially (for example, being able to increase one's heart rate by imagining a stressful situation, for example). I am not aware of anyone who is actually able to control their own heart to the point where they can stop it and start it on command, and given the number of people who have lived in the history of history, that's a pretty good experimental sample for saying it isn't likely to be possible.  -- Jayron 32 19:39, 24 March 2015 (UTC)
 * I know, that's why I wrote "in theory", despite several claimants like Guy Bavli or Cristian Gog. For the sake of argument, let's assume some ultra-rare mutation similar to dysautonomia or Ondine's curse, which enables a person to control the heartbeat until feeling of impending fadeout. Brandmeistertalk  20:05, 24 March 2015 (UTC)


 * "For the sake of argument"? Please don't post random suppositions without backup and ask us to cooment. μηδείς (talk) 21:16, 24 March 2015 (UTC)
 * Per Medeis, if you're going to invent a fictional magical power, you get to decide how it manifests itself. We have nothing to say on the matter in this forum, where our role is to provide you with references to further research factual questions you may have.  Questions where we speculate about the effects of magical superpowers are not what we do here.  -- Jayron 32 00:27, 25 March 2015 (UTC)
 * Indeed this is starting to sound like how much wood would a woodchuck chuck... it's not truly magical of course, since there's nothing impossible about such a "power", but certainly there's no reason to evolve it, and so we can't really predict the form it would take with any accuracy.  As a rule, though, cardiac myocytes want to beat; they'll beat in a tissue culture dish; so they should keep contracting on their own if whatever mechanism is interrupted by unconsciousness.  Fibrillation is the main worry.  But of course fibrillation isn't really an intended mechanism and it's no great stretch to suppose the person has a good cardiac pacemaker that prevents death by this means, at least until it advances the plot to start dealing with it.  So basically you need to mumble mumble inhibitory synapses mumble neurotransmitters mumble Eastern mystics mumble and you're good to go. :) Wnt (talk) 02:52, 25 March 2015 (UTC)


 * Superman supposedly had the ability to control his own heartbeat. However, Superman is most likely a fictional character. ←Baseball Bugs What's up, Doc? carrots→ 03:08, 25 March 2015 (UTC)


 * Brandmeister, you write that a person "fades out" in 8-12 seconds after cardiac arrest, and rather longer if the patient is recumbent, according to "medical sources" you have looked into. I would be interested to know what those medical sources are. The time seems way to long to me, if "fades out in 8-12 seconds" is interpreted to mean "maintains conciousness for 8-12 seconds". --NorwegianBluetalk 12:58, 25 March 2015 (UTC)
 * See ("After some 8–12 seconds of cardiac arrest, rather longer if the patient is recumbent, there is loss of consciousness"),  ("Transient disruption of cerebral blood flow for 8 to 10 seconds results in loss of consciousness"). Brandmeistertalk   09:09, 26 March 2015 (UTC)
 * Thanks! The source seems reliable, but contrasts with other reliable sources linked to in Physiology of death by decapitation, which states that "[Consciousness is] probably lost within 2-3 seconds, due to a rapid fall of intracranial perfusion of blood.", although anecdotes where the decapitated head appeared to have been concious for longer are mentioned. In orthostatic syncope and micturition syncope, the time between the cause and the syncope may be several seconds, but then there is not a total loss of blood pressure. --NorwegianBluetalk 16:43, 26 March 2015 (UTC)


 * The heartbeat can't be stopped or started by the nervous system, only speeded up or slowed down. The muscle cells in the heart are intrinsically oscillatory -- in other words, they contract rhythmically all by themselves.  Even during a heart attack the heart doesn't actually stop contracting:  what happens is that the parts of the heart lose their synchrony, resulting in a chaotic pattern of contraction waves called fibrillation. Looie496 (talk) 13:50, 25 March 2015 (UTC)

Section on electron mobility in the article about Indium-Gallium-Arsenide semiconductors (specifically related to Gain-Bandwidth product)
I posted three questions on the article's talk page a few days ago, but I haven't gotten any response. Talk:Indium_gallium_arsenide Could someone knowledgeable about semiconductor physics or electron mobility please look it over?

Also, I'm wondering if maybe that particular section should be included in the specific article about Gain-Bandwidth products, since it deals with math that is generally applicable to more than just InGaAs semiconductors? 97.84.96.60 (talk) 21:45, 24 March 2015 (UTC)


 * If I don't get a response to either this comment or my comment on the talk page sometime tonight, I'm going to at least correct the formatting on the relevant equations, as I laid out on the talk page. 97.84.96.60 (talk) 04:31, 25 March 2015 (UTC)


 * The equation doesn't look correct to me... at the very least, that equation is being quoted out of context, and in my opinion, it's just flat wrong. A source is provided: Photonics Essentials.  Anyone have that book to check what the original authors meant?  Nimur (talk) 04:54, 25 March 2015 (UTC)


 * I'm not sure what the SOP is here, so I removed the material in question and added a note about it on the talk page. It can be added back in after this gets sorted out. 97.84.96.60 (talk) 09:54, 25 March 2015 (UTC)