Science desk
< September 12	<< Aug \| September \| Oct >>	September 14 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 13

physical dimension of Hertz, wavelength and Planck's law

In Wikipedia, Hertz has the physical dimension "T⁻¹", the speed "LT⁻¹", so with the wavelength which is the ratio of speed to frequency in Hz, you get the physical dimension "L" .

In Planck's law for frequency you have Si units " W·sr⁻¹·m⁻²·Hz⁻¹ ", as "W" is "J.s⁻¹" why not apply the same simplification as wavelength and get " J·sr⁻¹·m⁻² " ?

Or why the wavelength does not have Si units " m·s⁻¹·Hz⁻¹" ? Malypaet (talk) 02:47, 13 September 2023 (UTC)[reply]

The choice of units has nothing to do with Planck's law per se, but with the traditional treatment of spectral radiance. It comes in two kinds, spectral radiance in frequency and spectral radiance in wavelength. Generalizing for the kind and writing X as a placeholder for the unit of the physical quantity per which, we have for the SI units:

W·sr⁻¹·m⁻²·X⁻¹.

Replacing X by Hz gives the SI units for spectral radiance in frequency. Replacing X by m (or nm) gives the SI units for spectral radiance in wavelength. While your proposed simplification is a valid one, it hides the conceptual correspondence. In a formula said to be for something per frequency, one should expect to see Hz⁻¹, even though it is equivalent with s. Replacing W·Hz⁻¹ by J obscures the relationship with frequency even further. --Lambiam 07:18, 13 September 2023 (UTC)[reply]

It does not obscure the relationship with frequency, it simply misses the cycle in the physical dimensions of a frequency. Because if we remove the unit of time which is shared between power and frequency, at the same time, what remains is the quantity of energy and the number of cycles that there was in this unit of time, as with kilowatt-hours for electricity billing.

Above all, what I show here with Planck's law for frequency is that there is an anomaly, where we measure a power, the equation gives energy. In both cases, the value obtained will be the same, power or energy. Doesn’t that shock anyone? Malypaet (talk) 08:10, 13 September 2023 (UTC)[reply]

Why stop at J·sr⁻¹·m⁻²? That can be simplified further to kg·sr⁻¹·s⁻², and now you have mass. Shocking! --Wrongfilter (talk) 09:34, 13 September 2023 (UTC)[reply]

Except that my simplification is recognized, I took the precaution of doing the // with the kw-h, because here my Joule is also W-s (https://en.m.wikipedia.org/wiki/ Joule#Watt-second) in the accounting sense over a time intervalle (1s), which I was told here was equivalent.

And what is the meaning of your simplification, if there is one ? Malypaet (talk) 12:19, 13 September 2023 (UTC)[reply]

Your statement shocks me because it does not make sense. I suspect the anomaly lies solely in your grasp of physics. --Lambiam 12:27, 13 September 2023 (UTC)[reply]

You pay well for your electricity, right? You are paying for a quantity of energy over a period of time, 1 kw-h= 3,600,000 Joules. To do this, we multiply the average power over this interval by the value of this time intervalle, in this case I hope you find this normal and yet we find the same reasoning there. Average power is a flow of energy. There's not only physics in it, but also accounting and logic. The physical dimensions are there to verify consistency and you have recognized that my simplification was correct, so... Malypaet (talk) 16:29, 13 September 2023 (UTC)[reply]

Plank's law gives a relation between a whole bunch of physical quantities. The statement "where we measure a power, the equation gives energy" is devoid of meaning. --Lambiam 07:52, 14 September 2023 (UTC)[reply]

The fact that you cannot find the meaning does not mean that there is none. I found it indirectly, during our discussions last June. Do you know about “brain storming” in problem solving and the textbook case of Swatch watches? No censorship and let your imagination run wild to solve a seemingly insoluble problem. I spent a lifetime solving problems in electronics and then in an application/data center, a Sherlock Holmes profile. Now I'm testing the reactions here, to better respond to them in my next publication, voilà. Malypaet (talk) 11:45, 14 September 2023 (UTC)[reply]

Hertz and radians are SI derived units. One has to be careful using them for dimensional analysis. ~~For example: 1 Hz = 2π rad/s. Using physical dimensions, this leads to 1 T⁻¹ = 2π T⁻¹, which is incorrect.~~ Giacomo Prando says "the current state of affairs leads inevitably to ghostly appearances and disappearances of the radian in the dimensional analysis of physical equations" (from Radian#Dimensional analysis) Alien878 (talk) 11:47, 13 September 2023 (UTC)[reply]

Can you provide a reference for your definition if 1 Hz = 2π rad/s? The fourth paragraph in Hertz#Definition contradicts explicitly. Angular (pseudo-)units can cause confusion, but I don't think that's the case here. --Wrongfilter (talk) 11:58, 13 September 2023 (UTC)[reply]

The magnitude of physical quantities plays no role in dimensional analysis. Although they differ by 12 orders of magnitude. the speed of light has the same dimension as the furlong per fortnight. --Lambiam 12:22, 13 September 2023 (UTC)[reply]

Okay, that was a bad example. One cycle is not necessarily the same as one rotation. However, assuming cycles are unit-less has similar pitfalls as assuming angles are unit-less. Alien878 (talk) 12:41, 13 September 2023 (UTC)[reply]

Complete sidetrack, but what property would be best measured in furlongs per fortnight? Spread of slugs in a field, maybe? {The poster formerly known as 87.81.230.195} 51.194.81.165 (talk) 15:05, 14 September 2023 (UTC)[reply]

An earlier version of the article informed us that "a garden snail has a top speed of about 78 furlongs per fortnight". Sadly, this eminently useful but uncited fact was mercilessly deleted as being "tr[ivia]". --Lambiam 11:38, 17 September 2023 (UTC)[reply]

Your answer begs the problem.

Speed is the number of meters per second. The meter is defined as a unit in Si by a method of measurement.

Frequency is the number of cycles per second. The cycle is not defined as a unit in the SI, yet we know how to measure it.

What good and do the slugs here? Malypaet (talk) 11:51, 15 September 2023 (UTC)[reply]

And the wavelength is the length of a cycle, right? Malypaet (talk) 12:00, 15 September 2023 (UTC)[reply]

What I meant was: units are often chosen so that the quantity being measured comes out as a small (and therefore humanly graspable) number; one commonly measures one's height in metres and/or centimetres, or feet and inches, not millimetres or chains, for example.

I was speculating that slugs might spread (in a farmer's fields, say) at the speed of a few furlongs per fortnight. It was a quip about Lambian's mention of the term, which is why I put it in small face (a thing we do here when we are trying to lighten the tone). My indentation and positioninig may have misled you: I meant to put it immediately below Lambiam's post, not Alien878's: apologies for the confusion. {The poster formerly known as 87.81.230.195} 51.194.81.165 (talk) 12:41, 15 September 2023 (UTC)[reply]

All of the replies above are quite excellent, and I would like to add to them that the unit of choice is often one of convenience for the application in question. As has been shown above, through dimensional analysis, many of these choices in units are interchangeable or interconvertible with each other, and say the same thing. Let's take that basic one of energy, which happens to be quite applicable to this conversation regarding wavelength (at least as applied to thing like electromagnetic radiation). The unit we are all often taught first in "Physics 101" is the Joule, often explained to us as a N·m. Both mean the same thing. A Joule also is expressed, in SI base, as a kg·m·s^-2, but in your classic "Physics 101" thought experiment of pushing a ball or something, that's not a very unit expression. Now, what if we are looking at mid-infrared light, such as that absorbed during molecular vibrations (FTIR spectroscopy, or the energy difference of scatter in Raman spectroscopy)? Let's take the bending vibration mode of a single molecule of water. If we express that in Joules, we get 3.28×10^-20 J, which isn't easy to comprehend, to but on an x-axis, or to distinguish between different vibrational energies. We could express it in nm, and we would get 6061 nm, which isn't so bad, until you consider the entire range these vibrations usually take place (and the fact that it can be handy to use a different set of units for vibrations than you do for visible light, just so people immediately know what you are talking about on a plot). Now we get anything from 200000 nm to 2500 nm, which is a somewhat annoying range to work with, and when you're primarily caring about higher or lower energies, having to do the mental gymnastics of "the smaller wavelength is the higher energy" is just an annoying and unnecessary step for this application. So, in the field of vibrational spectroscopy, we use the "wavenumber" unit, which is cm^-1. 1 cm^-1 = 1/(nm·(cm/10⁷nm)), which might seem annoying math at first, but if you just work in wavenumbers to begin with, that single molecule water bending vibration becomes 1650 cm^-1, and the range you are likely to work with in any molecule is 50 cm^-1 to 4000 cm^-1, which is a lot easier to work with mentally or plot on a graph. The units of choice are about the application often more than anything. In x-ray spectroscopy, nm and cm^-1 and J would all be terrible units, but keV (kilo-electron volts) works pretty well, since the range there is usually no more than 0.1 - 2000 keV (usually a smaller range, based on the type of x-ray spectroscopy being conducted, whether you are studying ejection of core electrons or just exciting them between orbital energy levels, etc.) --OuroborosCobra (talk) 14:12, 15 September 2023 (UTC)[reply]

Yes, but here the fundamental question was not about a choice of equivalent units, because between power and energy there is a subtlety. For example, kinetic energy is independent of time if there is no disturbing event. Whereas in a flow of energy, therefore a power, there is an addition of the energy elements composing the flow over time, at least in the field of accounting. In these two cases the notion of time does not have the same effect. Malypaet (talk) 18:05, 15 September 2023 (UTC)[reply]

Taxonomy/phylogeny question

If you have two species in the same family, and a third species in a sister family, am I correct in thinking that that third species is equally closely related to the first two? (I was sure this was the case, but got into an argument on Reddit with someone who claimed otherwise, and now I'm doubting myself). Iapetus (talk) 09:12, 13 September 2023 (UTC)[reply]

It depends on how you define interspecies distance. For example, one might use the edit distance between their genomes. Then it is extremely unlikely that your claim will stand. Since the split of the two confamiliar species from their MRCA, the first species may have undergone far more extensive mutations than the second, giving it a larger distance to the extrafamiliar third species. In traditional biology families are not necessarily clades and it is even conceivable that the third species is genetically closer related to one of the two confamiliar species than they are to each other. --Lambiam 12:43, 13 September 2023 (UTC)[reply]

There is no agreed upon measurement of relation between biological species. A usually obvious difference between any two species is that they cannot interbreed. Conventional cladistics declares species related by assigning them to the same genus. Thus in binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus. The article Genus details the varying criteria used by taxonomists. Biological family is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy under which genera (plural of genus are sorted.

Example: Panthera leo (lion) and Panthera onca (jaguar) are two species within the genus Panthera. Panthera is a genus within the family Felidae. Philvoids (talk) 13:14, 13 September 2023 (UTC)[reply]

Our article at cladistics may be of assistance here. Cladistsics is an approach to phylogeny that attempts to pare down relationships to simply the last common ancestor. (I'm summarizing; our article seems pretty decent). The upshot for your question is that, you can often see which is the odd one out since the clades will show that A diverged from B/C first and then later B and C split. So, you'll know that A is less related to B and C than B and C are to each other. You'll also know that B and C are equally distant from A - neither is closer to A. Where it gets complicated is that traditional taxonomists did not have access to the tools that are available today; families and orders and so on were built up based on things like anatomical similarities. A huge amount of work has been done over the last few decades to rectify life's "family tree" with the knowledge gained by cladistics (which ultimately doesn't really care about stuff like families and orders and so on) and other tools. Matt Deres (talk) 16:38, 13 September 2023 (UTC)[reply]

So in this viewpoint the distance between two species is effectively the time elapsed since their splitting off from their MRCA. Using that as the definition, the claim that the third species is equally distant to the first two is correct. --Lambiam 07:45, 14 September 2023 (UTC)[reply]

Thanks. That's what I thought. For the record, the argument was about the relationship between the megalodon, mako, and great white. The other guy was claiming that megalodon was more closely related to the mako than to the great white, citing a Smithsonian article that stated that but didn't show any cladograms or phylogenies. I contended that the mako and greate white were in the same family, while megalodon is now considered to be in a different family and so was equally close to both, which he rejected. Iapetus (talk) 09:39, 14 September 2023 (UTC).[reply]

Our article Megalodon has this: "It was formerly thought to be a member of the family Lamnidae and a close relative of the great white shark (Carcharodon carcharias), but has been reclassified into the extinct family Otodontidae, which diverged from the great white shark during the Early Cretaceous." This statement is unsourced. If correct, the reclassification may have taken place after February 2019, when the Smithsonian article was written. --Lambiam 15:34, 15 September 2023 (UTC)[reply]

Burning of the Temple of Jupiter

The Temple of Jupiter Optimus Maximus article states that the second and third building both burned downed to the point that a new building would be built on the site. However, assuming that both the second and third building were most likely made mostly from stone, marble or some fire-resistant material (unlike the first building from wood), how can such non-wooden structure burn down entirely? 212.180.235.46 (talk) 15:14, 13 September 2023 (UTC)[reply]

As the recent fire at Notre Dame Cathedral demonstrates, large ostensibly stone buildings have some underpinnings made of wood. These types of mega-structures tend to push the limits of physical laws and can collapse when wooden support is weakened by fire. What you can end up with is a pile of rubble. --136.54.106.120 (talk) 18:53, 13 September 2023 (UTC)[reply]

Also read Lime kiln. This is a kind of kiln where limestone is "burned" into lime, which is a powdery substance. Even if a building doesn't rely on wooden underpinnings, it may have many inflammable contents, and if these burn inside a limestone building (which have been common for millennia), they may produce enough lime that the remnants of the blocks collapse. Nyttend (talk) 19:41, 13 September 2023 (UTC)[reply]

Just to add support to that, marble suffers from the same problem. It isn't so much that limestone and marble "burn," but rather that carbonate containing minerals, such as limestone (primarily calcium carbonate) and marble (primarily calcium carbonate and calcium magnesium carbonate) will decompose (technically, calcinate) at high temperatures, releasing the carbonate as carbon dioxide gas and leaving behind calcium oxide. When that happens, you've entirely disrupted the structure of these materials, and they crumble to powder. Quite often, stones in these areas are also high in carbonates; that's why they can mine for limestone and marble there. So, even stone itself can suffer from this problem. --OuroborosCobra (talk) 20:49, 13 September 2023 (UTC)[reply]

Thanks. Presumably there was a lot of flammable material inside the temple to sustain such a destructive fire. 212.180.235.46 (talk) 09:03, 14 September 2023 (UTC)[reply]

[Biology] What are the differences between the Cheek pouch, Gular skin, and Crop (anatomy)?

There seems to be a lot of overlap in their functions, especially regarding food. 202.190.69.201 (talk) 21:48, 13 September 2023 (UTC)[reply]

The articles indicate they are in different locations. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:52, 13 September 2023 (UTC)[reply]

Cheek pouches are normally found in some primates and rodents, and are used to store unhusked nuts, grains, etc so that the owner can grab a big load of food and carry it away to a more secure location to eat it. In a captive primate situation I have seen them used to store "stolen" keys, stones, even small scraps of a broken mirror. Crops are found in birds and are used to store grain after the bird has husked it. It can then be pushed down into the gizzard or regurgitated to feed chicks or a mate who is sitting on eggs or babies. Gular skin is not used in food storage or transport, it's purpose is more for sexual signaling or cooling. 49.177.90.146 (talk) 02:10, 15 September 2023 (UTC)[reply]

The gular skin of pelicans forms a large pouch used to store fish. --Lambiam 13:14, 18 September 2023 (UTC)[reply]