Wikipedia:Reference desk/Archives/Science/2014 April 16

= April 16 =

all sky surveys
Does anyone know what the best (in terms of resolution and magnitude it goes down to) all sky survey in the visible spectrum that doesn't have any copyright restrictions?©Geni (talk) 04:45, 16 April 2014 (UTC)

Is reality ultimately digital or analog? Can another particle pass through the intervening space while an electron is performing a quantum leap?
When I was young I remember reading or hearing somewhere that you can divide a line up an infinite amount of times. Therefore, when physically moving from point A to point B it's just an illusion. As you're not really moving through the intervening space since it can be divided up an infinite amount of times. I'm pretty sure there's something I'm not understanding there or a flaw in the logic, but ever since then I've still always wondered, is there an ultimate limit to how many times the space between two points can be divided up? Is reality ultimately digital or analog?

I've since learned about the planck length, but recently also learned something else that has now been confusing me: quantum leaps, and that the electron does not move through the intervening space when it leaps from orbital to orbital. My other question then is, is it possible for another particle to pass through the intervening space between electron orbitals? Especially, just as a quantum leap is occurring.. let's say a quark (or whatever) happens to fly through the picture.. is it also subject to 'quantum leaping'? Or can it move through the intervening space while an electron can't? I'm sure I'm missing something or not understanding something about wave/particle duality here, but I'm just a layman. Glad there's some really intelligent folks here that can explain some things or at least point me towards the right articles. Thanks, 70.71.247.169 (talk) 12:28, 16 April 2014 (UTC)
 * Electrons are not little balls that move in circles around another little ball. That's not what an atom is.  Your question presupposes this to be the case; that an electrons moving between energy levels somehow changes "tracks" or moves without crossing the intervening "space".  Electrons don't work like that, and the question shows a fundamental misunderstanding of how electrons work.  One cannot even answer the initial question regarding whether another particle can "pass" through the "space" during a "quantum leap".  Electrons don't change locations.  Per the uncertainty principle electrons are not even localizable enough to say where one is, and if you don't know where it is you don't know where it moves to.  No, electrons change the amount of energy they have, and the change between amounts of energy is confined to certain values.  That's what a "quantum leap" is.  It isn't a physical change of location.  Any electron has a non-zero chance of being at any location in the known universe; though within certain probability limits we can define areas outside of the nucleus of an atom where any particular electron is likely to be (see orbital).  -- Jayron  32  12:48, 16 April 2014 (UTC)


 * I would start with Zeno's Paradoxes, which is almost certainly what confused you as a child. From a mathematical point of view, this is addressed in calculus and infinitesimals: from a philosophical point of view, I'm sure you could find a variety of interesting discussions and arguments that I am ill-equipped to advise on. 86.146.28.229 (talk) 12:56, 16 April 2014 (UTC)


 * As far as not being able to move anywhere because that requires going through an infinite number of intermediate locations, the flaw in that logic is that as you divide the space up you also divide the time it takes to pass through that space. So, you can think of it as taking zero time to pass through each infinitely small space.  But then you might still think zero time multiplied by an infinite number of positions gives us either zero or infinite time, so that seems wrong, too.  However, these are limits we are dealing with, not actual numbers.  For example, if we had the expression x/x, and we wanted to evaluate that as x approaches infinity, we could break it down to just x, which approaches infinity, multiplied by 1/x, which approaches zero.  So, this seems to be the same case of zero times infinity.  However, for any value of x, no matter how small or large, we can obviously see that x/x is just 1. StuRat (talk) 13:08, 16 April 2014 (UTC)
 * For many values of x, at least, that holds true. Mathematicians means something pretty intense when they say "for any value of x."  There exists at least one x for which L'Hôpital's rule can not be applied to the limit of x/x as x goes to zero; that would be an indeterminate form.  The limit might not exist; or, the method to find the limit might be more difficult.  Nimur (talk) 14:59, 16 April 2014 (UTC)


 * "Digital" and "analog" are probably the wrong conceptual models to frame the original question. Those are adjectives we apply to machines we build; if we design a transistor-circuit to usefully operate in a specific way, so that its outputs are thresholded, we call it "digital," and so on.
 * A more useful set of adjectives to describe our formal models of "reality" are the words "continuous" and "discrete". Both of these concepts are mathematical models of the world.  They are ideas constructed by humans as part of our effort to understand the physical world, (using idealized abstraction).  Mathematicians can prove amazing properties about discrete or continuous systems by carefully describing their assumptions and proceeding logically to derive complicated results.  But those mathematical facts apply to an idealized world, as denoted by axioms set up by the mathematician!  When we study the physical world - as observational scientists or engineers - we usually have the luxury of choosing any model that works to suit our needs.  When we zoom to very microscopic scales, we find that some quantities are always discrete, while others vary continuously.  As an example: it is useful to model space as a continuous variable.  It is convenient to study the position of a particle as a discrete value.  In some cases, physical constraints quantize a potentially continuous value so that it is only measured to have a small number of discrete values.  When we discover such a property - like angular momentum - and note that it is always quantized, then we have discovered a simple fact about the nature of our universe.
 * But, even the most enthusiastic physicist would be hard-pressed to say that quantization of certain physical properties demonstrates that "reality is quantized." All we have shown is that certain physical properties behave that way.  "Reality" is too poorly defined for us to use inductive logic on it.  Nimur (talk) 15:12, 16 April 2014 (UTC)


 * The most pertinent articles I can find on Wikipedia about this topic are Digital physics and Fredkin finite nature hypothesis. But see also Bekenstein bound and Margolus–Levitin theorem. On at least some of the digital physics stuff, it starts getting far enough away from mainstream physics that even calling it "highly controversial" is being kind.  Red Act (talk) 18:06, 16 April 2014 (UTC)


 * Also see Quantum spacetime (or Spacetime for context), the discussion of discrete spacetime in Pregeometry (physics), and Causal sets. Red Act (talk) 20:12, 16 April 2014 (UTC)


 * It’s a question that has intrigued me as well. We call our computers digital because the calculations they make are by discrete numbers, not by physical traits such as weight, mass or temperature. But when we examine the microchip more closely, we see that it too is dependent on analog parameters. There must be natural mechanisms that impel the photons, and the chip will heat up if the channels are too small, and so on. But what about when we really get to the most basic substrate of reality? Is it digital or analog? Popper and other logicians have argued that if fundamental reality was digital, or nomino-deductive, then we could prove empirical observations as being necessary in the way that a triangle necessarily has 3 sides. In the physical world, we cannot prove any empirical observation like that. Snow might be seen as always white, but we cannot prove that it always was and always will be white.


 * In a fundamental way, these kinds of questions are being asked by researchers at the Large Hadron Collider. Can we ever really get a digital picture of reality that provides us with mathematical certainty in the way 1 + 1 = 2 does? What would that kind of reality look like? I don’t think it could even be imagined, and even in sci fi fantasy, there is no fake reality like that which we could look at and say, “Well, it might be a bit like this...”


 * Final reality, if there IS such a thing ( quite a few philosophers and scientists think there is no end to the layers of reality, and thus no bedrock to it), but if there IS such a final reality, it might be something unimaginably different to EITHER digital or analog. It might be something that our brains, rooted in space and time, simply cannot visualise or conceptualise. After all, how bizarre are the findings of Quantum Physics even now? The real truth might be a lot weirder than a particle being in two places at once, and a wave or a particle according to how it is being measured. Myles325a (talk) 09:14, 17 April 2014 (UTC)

Plant identification request.


So that I can label this for commons, any suggestions on what it is?Sfan00 IMG (talk) 12:54, 16 April 2014 (UTC)


 * That's not much to go on. Is it a shrub? Where in the world was the photo taken? Any description of the environment it was in, or how large it was? Extra info would help. SemanticMantis (talk) 15:39, 16 April 2014 (UTC)


 * Posssibly Canada, based on the Uploaders other contributions.. ? Sfan00 IMG (talk) 18:44, 16 April 2014 (UTC)


 * Really hard to say, but my best guess (don't use this for labeling tho) is that this is some sort of maple bud. Maples have these tripartate buds, kinda like a fleur-de-lys, see these pics.  This looks like after the bud starts to put out shoots.  -- Jayron  32  01:20, 17 April 2014 (UTC)
 * I can identify buds and I can identify leaves, but identifying the transitional stage is beyond me I'm afraid. I think maple is a fairly safe bet though. THE MAPLES OF NORTH AMERICA suggests that there are about 13 or so maple species that are native to North America. Alansplodge (talk) 17:15, 17 April 2014 (UTC)

Something's bugging me...
Namely the identification of these insects. Some I haven't even a clue what family they're in, let alone the genus or species... These were all taken in Umbulharjo, Cangkringan, Sleman, Indonesia.

Any ideas? — Crisco 1492 (talk) 15:52, 16 April 2014 (UTC)
 * "The one which looks like a bee and a fly" is probably a hoverfly (family Syrphidae), but as there are 6000 species to choose from that may not be much help... AndrewWTaylor (talk) 17:23, 16 April 2014 (UTC)
 * We don't have an article, but the bluish green grasshopper may be a "Chameleon Grasshopper". Found a few pics in Google Images that may work for that, see this.  -- Jayron  32  17:40, 16 April 2014 (UTC)
 * I don't have anything relevant to add. I never thought I would say this, but that grasshopper is gorgeous! Justin15w (talk) 22:02, 16 April 2014 (UTC)
 * Kosciuscola tristis or similar? Yes, it is quite a gorgeous creature. Aularches miliaris is also a beaut. — Crisco 1492 (talk) 00:53, 17 April 2014 (UTC)
 * Kosciuscola tristis or another Kosciuscola, perhaps (strangely the genus isn't even mentioned on Wikipedia yet)? There are five species, and I think I saw a source (didn't keep the link) that said all of them have the ability to change color. — Crisco 1492 (talk) 00:53, 17 April 2014 (UTC)


 * The "one the the very long antennae" is a cricket, possibly a tree cricket. Mikenorton (talk) 06:49, 17 April 2014 (UTC)
 * That looks to be a reasonable guess (although this one was out at 11 a.m., which isn't quite "nocturnal"... however, we may have disturbed it by tromping through the woods) — Crisco 1492 (talk) 08:16, 17 April 2014 (UTC)

Reason for Worthing exponent of metal electrical resistivity
Reference manuals for engineering often give constants ρT1 and α for use in an algebraic modification of Mitchell's formula for resistivity:
 * ρT2 = ρT1[1 + α(T2 - T1)]

where ρT2 is the resistivity at temperature T2 and ρT1 is the resistivity at temperture T1. However this formula is a rough approximation usually good enough where T1 is around 300 K and T2 is around 300 to 400K. For all metals above the scattering point (typically << 300 K) and below the melting temperature or Curie temperature for ferromagnetic metals, Worthing's formula is accurate within a few percent and often much better:-
 * ρT = ρT1K TP

where ρT is the resistivity at any temperature T, ρT1K is the notional resistivity at 1 Kelvin (not to be confused with the actual intrinsic resistivity at 1 K) and P is an exponent characteristic of the particular metal. For many metals, P is around 1.2, which means that the slope dρ/dT increases slightly with increasing temperature. For ferromagnetic metals, P is around 1.7, so the curvature of dρ/dT is very pronounced. For some metals, e.g., palladium, Tantalum, P is around 0.8, which means that dρ/dT decreases with temperature. What is the reason for P being in three groups, P ~ 1.2, P ~ 1.7, and especially P ~ 0.8? 124.178.128.203 (talk) 16:33, 16 April 2014 (UTC)


 * I can't answer directly, but it seems similar to why most substances expand when heated, but there are exceptions, like with water, which expands when it freezes. There I believe the reason is that the structure of ice crystals contains more empty space. StuRat (talk) 16:40, 16 April 2014 (UTC)


 * No. Some metals are face centered cubic (eg Nickel, P = 1.747), some body centered cubic (eg tungsten, P = 1.205, or Iron, P = 1.768), and some are hexagonal (eg Cobalt, P = 1.687).  Palladium is face centerred cubic and has P = 0.803 yet Tantalum is body centred and has P = 0.820.  No correlation.  124.178.128.203 (talk) 16:50, 16 April 2014 (UTC)


 * Do any phase changes take places over the temperature ranges in question ? StuRat (talk) 17:09, 16 April 2014 (UTC)


 * In the case of iron (Fe), a phase change occurs at 1183 K (from body centred to face centred), but the Curie point is below this at 1043 K. The resistivity curve follows Worthing's formula accurately (better than +,- 1%) with P = 1.768 up to the Curie point then continues with P close to unity up to and past 1183 K.  There may be an inflection point at 1183 K but it is difficult to detect in measured data.  ( A paper by V E Zinovev & others in the Soviet Journal of Physics in 1972 claimed a slight step at about 1200 K but to my knowlege this hasn't been replicated.  It may have been due to unidentified impurities, or more likely an instrument connection problem - only one test on one sample was done), In fact, in engineering, we usually take the resistivity curve above the Curie point as a straight line.  If phase changes have any significant effect, surely one would expect a point of inflection at the phase change temperature, not a smooth curve.  Palladium resistivity is very curved with P = 0.803 and is fcc right up to melting.  124.178.128.203 (talk) 17:24, 16 April 2014 (UTC)


 * You're assuming the phase change takes place instantly throughout the sample. I'd expect some overlap of the two phases around the phase transition temperature, with the resulting resistivity being somewhere between those of each phase.  This would be predicted to produce a smoother curve. StuRat (talk) 12:45, 17 April 2014 (UTC)
 * Again, no. Definately not.  Resistivity is by definition a steady state property.  One does not measure it while changing the temperature - one measures it at in a series of constant temperatures, waiting as long as is necessary for the sample to come to thermal equilibrium at each step.  On either side of the transition temperature, one of the two phases is unstable and will not sustain.  Under these conditions, the phase must be the same thoughout the sample - we are talking about sensibly pure metals and supercooling, quenching, and the like phenomena do not apply.  121.221.225.6 (talk) 13:25, 17 April 2014 (UTC)

Longevity documents
What other documents, aside from birth certificate and passport, I may need to submit in case of longevity (assuming that the Gerontology Research Group and the Guinness World Records would still exist)?--93.174.25.12 (talk) 18:11, 16 April 2014 (UTC)


 * Unless you are very old such documents are likely archived. Historians also use birth announcements in newspapers and religious (baptismal, Bris) records.  Good luck. μηδείς (talk) 21:54, 16 April 2014 (UTC)


 * Since your passport was most likely obtained using your birth certificate, the birth certificate is the only document needed. Unlike your passport, your birth certificate is a vital record.--Shantavira|feed me 08:53, 17 April 2014 (UTC)


 * I suspect you would need to produce evidence that it is you all your life. Let me explain what I mean here - I mean that the person claiming the longevity is not your son or daughter masquerading as you! --TammyMoet (talk) 09:28, 17 April 2014 (UTC)
 * List of the verified oldest people states that a supercentenarian would need at least three documents, so I wonder which is the third. 93.174.25.12 (talk) 17:14, 17 April 2014 (UTC)