Science desk
< April 24	<< Mar \| April \| May >>	April 26 >

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.

April 25

UV resistant germs

Per [1], "UV radiation from the sun is the primary germicide in the environment". Question: that UV radiation has been arriving for billions of years. Why haven't the germs developed resistance by now? Even if a few of them have, why not pretty much all of them? Thanks. 2602:24A:DE47:B270:DDD2:63E0:FE3B:596C (talk) 03:31, 25 April 2020 (UTC)[reply]

The best way to protect against UV is shielding. With a, say, 300μm thick shield, an organism can safely absorb a significant fraction of the UV in layer that can withstand it as the layer is dead already or can be discarded after use. The epidermis works like that. However, a 100nm virus cannot carry a 300μm shield. PiusImpavidus (talk) 11:08, 25 April 2020 (UTC)[reply]

PiusImpavidus, your phrase makes me think of "A five-ounce bird could not carry a one-pound coconut!" :-) Nyttend (talk) 12:27, 25 April 2020 (UTC)[reply]

It's really quite staggering what an accomplishment DNA repair is given how frequently DNA gets damaged. Every day every cell in your body repairs at least 10,000 instances of DNA damage. Now bacteria and especially viruses have much shorter genomes than human cells, and obviously a lot of that isn't caused by UV, but if you're a small bacterium without much protection there will be limits to how effectively you can fix damage. And viruses have particular problems: repair requires a ton of energy, and viruses of course can't do respiration to release energy outside a cell. And RNA is less stable than DNA is. There is actually a case of viruses having a DNA repair enzyme powered by light energy, though. So fundamentally there are constraints on what they can do: it's like asking "why don't bacteria have brains": that's not the mode of living they've evolved into. Viruses have generally evolved to replicate fast and in large numbers rather than replicate good. Blythwood (talk) 11:15, 25 April 2020 (UTC)[reply]

Many RNA viruses don't even have a proofreading mechanism (e.g. influenza viruses). Coronaviruses are an exception; they have a larger-than-average genome that includes genes for proofreading. --Lambiam 15:52, 25 April 2020 (UTC)[reply]

My comment here on the Science Desk is to do with the sloppiness of a scientific source using the word "germicide". Marketers of disinfectants and antiseptics use the word "germs" to emphasise how powerful(?) their products are. "Kills 99.9% of germs" is in a TV ad I see a lot at the moment. Unfortunately, this fails to differentiate between bacteria and viruses. Right now, this matters a lot. Looking at the source, it seems they're actually talking about viruses. I wish they'd made it clearer. I don't blame the IP. The confusion here is coming from academia! HiLo48 (talk) 11:43, 25 April 2020 (UTC)[reply]

The statement by itself is true, whether the microorganisms are bacteria or viruses (see Ultraviolet germicidal irradiation). Both the title of the article and the abstract make abundantly clear that this specific study concerns viruses; the first section of the article also compares bacteria and viruses to clarify the purpose of the study. I cannot agree that in this instance the use of the term "germicide" was sloppy. --Lambiam 15:52, 25 April 2020 (UTC)[reply]

Using the word "germ" at all is sloppy. HiLo48 (talk) 21:01, 25 April 2020 (UTC)[reply]

So the article does not use that term at all. --Lambiam 08:12, 26 April 2020 (UTC)[reply]

As mentioned here: "In a recent study – which looked at whether UVC could be used to disinfect PPE – the authors found that, while it is possible to kill the virus this way, in one experiment it needed the highest exposure out of hundreds of viruses that have been looked at so far. The amount of ultraviolet required varied widely, depending on factors such as the shape and type of material the virus was on....... “UVC is really nasty stuff – you shouldn't be exposed to it,” says Arnold. “It can take hours to get sunburn from UVB, but with UVC it takes seconds. If your eyes are exposed… you know that gritty feeling you get if you look at the sun? It’s like that times 10, just after a few seconds.”" Count Iblis (talk) 19:39, 25 April 2020 (UTC)[reply]

Because there's no significant selection pressure to drive the development of such a trait. Any trait has a cost, taking resources that could be invested in something else. Let's look at the history of life on Earth. There was little life on land for about half of Earth's history, because there was no oxygen in the atmosphere and thus no ozone layer to block high-energy UV from the Sun. Essentially all life was in the ocean. (All life was microscopic as well.) The Great Oxidation Event produced an oxygen atmosphere, which was essential to the development of large organisms. Some organisms have some kind of UV resistance, such as in endospores that some bacteria produce to survive harsh environments. But that gives a good illustration of why such traits are not everywhere. Endospores are "vegetative". They don't move around or "do" anything; the bacteria basically erects a fortress and goes into "hibernation" to try to survive until the environment is more hospitable. Or another good example: human skin color. Humans developed dark skin when our ancestors lost fur, to shield against UV. Then some human populations migrated to higher latitudes and went back to having light skin, since this was not as necessary in environments with less sunlight. In this case there was also selection pressure driving the switch back to the light skin phenotype, as UV is needed for vitamin D production. --47.146.63.87 (talk) 22:17, 25 April 2020 (UTC)[reply]

I disagree with this–as I said above, clearly it's beneficial to at least some viruses to have DNA repair enzymes, as they package enzymes with them inside the virus particle. And bacteria have tons of DNA damage pathways. Blythwood (talk) 01:29, 26 April 2020 (UTC)[reply]

Uh, I don't see how we're in disagreement. Some level of DNA repair is necessary in basically all organisms because otherwise their DNA would turn to mush from just random unwanted chemical reactions, if nothing else. I was just describing why more microbes don't have the ability to withstand prolonged exposure to high amounts of UV. Some do, like radiotrophic fungi. But it's just not that beneficial in most microbial niches. --47.146.63.87 (talk) 01:57, 26 April 2020 (UTC)[reply]

some bacteria have resistance, for example Deinococcus radiodurans about 1000 times as resistant to radiatin than humans. Graeme Bartlett (talk) 10:32, 26 April 2020 (UTC)[reply]

To actually answer the question, deductions from the article tells you. The most effective UV to inactivate viruses is 260 nm. But the UV at Earth's surface is above 290 nm. Also, I do know from past research, that the vast majority of viruses in the world, are in ocean's. 67.175.224.138 (talk) 04:17, 28 April 2020 (UTC).[reply]

The Great pyramid of Giza: a question to Egyptologists

still pseudoscientific nonsense

The following discussion has been closed. Please do not modify it.

THE GREAT PYRAMID AND THE DIMENSIONS OF THE EARTH Much has been written on the mysteries of the Great pyramid and writers like Graham Hancock have used them to support the idea that a technologically advanced civilization was present on Earth more than 10.000 years ago and, before being wiped out by a cometary impact, left traces of its advanced knowledge, like the Precession of the equinoxes, as a message to successive civilizations, from Egypt to South-America (an alternative is that visiting aliens provided such information). The speculations by Hancock and company did not stand critical scrutiny and have all been shown to be without any foundation. All but one: a relationship between the dimensions of the Great pyramid of Giza and the dimensions of the Earth. This is discussed in one of Hancock’s books (Magicians of the Gods, 2016), where the fact that multiplying perimeter and height of the pyramid by the number 43.200 gives-with an excellent approximation- the dimensions of the Earth’s equatorial circumference and polar radius respectively, is taken as one example of ancient knowledge left for future generations. The Egyptians had no knowledge of the Earth’s dimensions and Precession of the equinoxes (the “magic” number 43.200 is, according to Hancock, related to the phenomenon of Precession of the Earth’s axis), therefore only a civilization with such knowledge could have encoded these data in the pyramid, hence there must have been a very advanced civilization long before the Egyptians. Very advanced indeed: understanding the Precession of the Earth’s axis requires modern astronomy. The pyramid/Earth relation, originally proposed by John Taylor, an engineer with Napoleon’s expedition in Egypt and later by the Astronomer Royal of Scotland Charles Piazzi-Smith in 1864, was based on a suggestion by the astronomer John Greaves in 1706 about the “pyramid inch”, a unit of measure used by the builders of the Great Pyramid, a fact later dismissed by the Egyptologist Flinders Petrie in 1883. We can therefore forget the pyramid inch and the Taylor-Smith theory, and the number 43.200 has of course nothing to do with Precession, all the numbers supposedly related to an ancient knowledge of Precession have been shown to be pure fantasies (see e.g. “Pick a Card, Any Card - How Hancock Finds Precessional Numbers” by John Wall 2005, The Hall of Ma’at on the Internet).

And yet such relation exists, as anyone can find out with simple math using the metric system. The dimensions of the original pyramid (according to Egyptologists) are: perimeter=921.44m, height=146.60m. Multiply by 43.200 and find 39806.208Km and 6333.12Km respectively. Modern measurements give the Earth’s equatorial circumference as 40035.41Km and the polar radius as 6356.988Km. An incredible “coincidence”! The probability that given a man-made structure, any structure, a number can be found such that, if multiplied by the dimensions of the structure, it will reproduce the dimensions of any celestial object, is exceedingly small and the case of the Great pyramid can therefore hardly be attributed to a coincidence. And certainly it cannot be attributed, following Hancock, to the existence of an ancient advanced civilization, the evidence against such hypothesis is overwhelming. A visit by aliens providing mankind with information about Precession and Earth dimensions is a possibility but it has an exceedingly small probability of being true. As I argued in my book “Il mondo dell’improbabile Homo sapiens”(The world of the improbable Homo Sapiens), the probability that there is life elsewhere in the Universe is so close to 1 that we can take it as a certainty, but the probability that a life-form developed somewhere beyond the stage of single-celled organisms is so small that we can take it as null. And single-celled organisms do not study Precession and do not travel to other planets. Sherlock Holmes said: ”When you rule out the impossible, what is left, however improbable, must be the truth”. But what is left? Do Egyptologists have any suggestions for this incredible case? 151.73.116.99 (talk) 16:37, 25 April 2020 (UTC)[reply]

Your numbers are written in a way that is confusing. Is 43.200 in fact 43,200? There was no advanced civilization before Egyptians, there was no aliens. The earth radius was measured by Eratosthenes[2] in 3rd century before CE. No computer was needed AboutFace 22 (talk) 17:43, 25 April 2020 (UTC)[reply]

Note: many countries write one thousand as 1.000 and not 1,000 --OuroborosCobra (talk) 18:19, 25 April 2020 (UTC)[reply]

Yes, but then he should have written 39806.208Km as 39806,208Km AboutFace 22 (talk) 19:22, 25 April 2020 (UTC)[reply]

Not really. While the OP's comments have been nonsense, I don't think it makes sense to make a big deal over a mistake. I think most people with sufficient level of English to be able write a coherent (even if nonsense) paragraph are also aware of the norms for delimiters, and decimal separators for most English speakers and therefore generally use the decimal point as the decimal separator when communicating with generic English speakers to avoid confusion. However if what you're use to the the comma, it's likely fairly easy to make a mistake on occasion. Nil Einne (talk) 18:16, 26 April 2020 (UTC)[reply]

I should clarify that I'm not saying there's anything wrong with seeking clarification or providing guidance if there is uncertainty. I simply mean we should treat it as some major mystery or something that makes no sense, when even before the OP's comment below, it seemed likely based on familiarity with the differing conventions that it was simply a mistake most likely arising from trying to write in a convention you're not used to. Nil Einne (talk) 02:14, 28 April 2020 (UTC)[reply]

Reply to comments: first comment:True I should have used a coma rather than a dot,but-as stated in the second comment- many countries,including mine,use the dot.Anyway the meaning of numbers should be obvious,everybody knows that the Earth radius is of order 6000Km,coma or dot! Of course there was no advanced civilization and no aliens, this is exactly what is said in my talk!We all know that Eratosthenes measured the Earth's radius but it is irrelevant to my argument, the pyramid was built thousands of years before! The comments are totally irrelevant to my question and so is the general heading "pseudoscientific nonsense".My talk has nothing to do with pyramidology or pseudoscience,all the nonsense by Hancock,Taylor or Piazzi-Smith is clearly criticized and set aside leaving only a "fact":the relation between the dimensions of the pyramid and the Earth's,this can be checked by anyone, it is a mathematical fact with no relation to any pseudoscientific nonsense. I only asked if Egyptologists have any answer for this "fact": where is the pseudoscientific nonsense?151.73.116.99 (talk) 14:41, 26 April 2020 (UTC)[reply]

If you divide something by something you get a number. Here some of the numbers are 43502 (perimeter_p/perimeter_Earth) and 43362 (side_p/r_polar), which are I suppose somewhat close to 43200 if that number meant something. The precession of the Earth number would be the axial tilt, which is quasiperiodic with the period being somewhere around 41,000 years, but motion is irregular and not quite periodic. Judging from WP knowledge, the average period lately has been ~41,040 years. You'd think that if the mystic pyramid builders could measure Earth size and axial tilt period so closely, they would know better than to get it wrong by 6%. And I say that all while shutting an eye to the sheer quantity of ratios, lengths, distances etc. out there, not to mention random pretty numbers like 43200 = 3³40². You can get it a lot closer than 6% by choosing the right measurement (and probably a different pyramid inch). Think of the Ley lines, uncanny arrangements between prehistoric sites in Britain. You'd never expect to be able to fit so many of relatively few objects on a straight line (give or take a few miles to the side) by pure chance, right? Wrong. 93.136.55.42 (talk) 15:41, 26 April 2020 (UTC)[reply]

Your formulation is just another way leading to my question.Is it not strange (very strange) that the ratios of the exact dimensions give very similar numbers? Identical within some approximation and both close to 43200,a number with no particular meaning, certainly not related to the precession of equinoxes. If you repeat your calculation with any other celestial object get very different numbers (for the Moon,for instance, the ratios are 23703 and 11890, quite different!).Only for the Earth these numbers are "almost" identical. I find it very strange and, although I believe that,improbable as it may be, it must be a coincidence with no hidden meaning, I wandered if Egyptologists have any explanation for this. Again:where is the pseudoscientific nonsense? 151.73.116.99 (talk) 20:16, 26 April 2020 (UTC)[reply]

Since the circumference of our planet divided by its radius is close to 2π (within a fraction of 1%), the whole thing amounts to the observation that the ratio height : base-length of the Great Pyramid of Giza, which equals 146.7m/230.34m = 0.6369 (using the data from our article), is approximately equal to 2/π = 0.6366. No need to get any astronomy involved. My money is on this being a coincidence, but should the architect, for whatever reason, have chosen to use the ratio circle-diameter : half-circle-circumference for the pyramid, they surely could have done that to this precision with the means available to them. --Lambiam 04:59, 27 April 2020 (UTC)[reply]

At last a clear answer to my question. Thank you.Elementary!(and of course my calculation for the moon was wrong, used wrong data) 151.73.116.99 (talk) 16:09, 27 April 2020 (UTC)[reply]

The angle of the Great Pyramid is based on centuries of experimentation by Egyptian architects in determining the angle of repose for such a structure. Calculations involving angles (i.e. trigonometry) frequently have π pop into them somewhere, so it is unremarkable that a well-constructed pyramid at it's ideal angle of repose would end up with π coming out of the ratios somewhere; it doesn't actually require knowledge of π to produce it in a calculation. It pops out of lots of things like this all the time. It just comes from the geometry, and will arise naturally in the ratios of such constructions. I can't be bothered to work out the math, but I've seen enough similar calculations to be entirely unsurprised that the ratio of the base of a well-constructed pyramid to its height would have a factor of π somewhere. --Jayron 32 13:12, 27 April 2020 (UTC)[reply]

The Angle of repose of a granular material depends on its (NOT "it's") density, the surface area and shapes of the particles, and the coefficient of friction of the material. Unless he has access to a reliable formula for calculating the repose angle that Wikipedia lacks, it should surprise an intelligent person if a correct value of π (see link) were found here. DroneB (talk) 15:39, 27 April 2020 (UTC)[reply]

The angle of repose can (in principle) be anything between zero and a right angle; most angles do not lead to π in any simple way. —Tamfang (talk) 02:37, 2 May 2020 (UTC)[reply]

I once saw a conjecture that the side of the pyramid was measured with a roller of known diameter, and the height was a multiple of that diameter. —Tamfang (talk) 02:38, 2 May 2020 (UTC)[reply]

First you boxed-in my reply and now you're all engaging in pseudoscientific nonsense!! Really... If you understand French read this or use Deepl or something and stop hemming and hawing about surprises and coincidences. 93.136.55.42 — Preceding unsigned comment added by 89.172.65.59 (talk) 04:49, 2 May 2020 (UTC)[reply]