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January 20

Alpha Centauri is getting closer

In The Week magazine's latest issue there is a note about a supposed discovery of a narrow radio beam originated in the spot in the sky where Alpha Centauri is supposed to be, and hitting the earth. Narrow in this case is related not to the space but to the frequency of the coming radiation. The authors speculate that since all natural sources of radiation are much wider in spectral sense, the source must be artificial.

Suppose further observations have proved that the source is really artificial, and naturally there will be a talk about sending a signal back. My question is: What will it take technically to accomplish, that is, to create a radio transmitter powerful enough that will send a continuous signal to another solar system like Alpha Centauri and surely reach the destination? Thanks, AboutFace 22 (talk) 03:34, 20 January 2021 (UTC)

Depends on how sensitive the receiver you are assuming at the other end is. --142.112.149.107 (talk) 06:18, 20 January 2021 (UTC)

If we knew they were expecting a signal from us, we probably already have the hardware we need.

Unfortunately, we just lost Arecebo, but there are a couple other transmitters that have been used for interstellar "active SETI" transmissions.

The NASA Deep Space Network is a candidate.

Looking over the List_of_interstellar_radio_messages, I don't see any directed at Alpha Centauri. Probably because it's not normally believed to be habitable. ApLundell (talk) 06:28, 20 January 2021 (UTC)

Arecibo would have been useless for this anyway. It could only point to declinations from -1 to +38 degrees, but α Centauri is at -61 degrees, too far south. The Canberra station of DSN could do it, if we assume a sufficiently sensitive receiver on the aliens' end, pointing to us at the time when our signal arrives. PiusImpavidus (talk) 09:33, 20 January 2021 (UTC)

Using the idea of a phased array, it should be possible to build a huge transmitter consisting of many small ones, which could be dispersed over a large area. See also the Karl G. Jansky Very Large Array and the Atacama Large Millimeter Array (which does not operate at large millimetres; rather at small ones), observatories based on the same idea except now serving as a huge receiver. Using a regular deployment pattern simplifies the computations but is not essential, but the scattered components must agree very precisely on the clock time.  --Lambiam 08:14, 20 January 2021 (UTC)

As noted above, we could use the Deep Space Network to send a signal to Alpha Centauri (or other nearby stars); the question, therefore, is not whether we can, but whether we should (and there are many reasons why this is not a good idea). 2601:646:8A01:B180:6D2B:1920:798B:506B (talk) 10:42, 20 January 2021 (UTC)

Quote: "....and there are many reasons why this is not a good idea," Why? What are the reasons? AboutFace 22 (talk) 13:00, 20 January 2021 (UTC)

To put it over-simply, if they know we're here, they might come and eat us all! Less crudely, interstellar spaceflight on any significant scale might or might not be logistically possible, but given the degree of home-grown malicious trolling that we already suffer, should we risk even a slim possibility of subjection to perhaps much more advanced and capable informatic disruption from completely alien entities that might, for all we know, consider our very existence to be a terrible blasphemy. The genre of Science Fiction has been exploring such themes for decades: for an interesting recent example, see Liu Cixin's international award-winning The Three-Body Problem and its sequels. {The poster formerly known as 87.81.230.195} 90.200.40.9 (talk) 15:20, 20 January 2021 (UTC)

I think interstellar travel is technically impossible for any civilization, advanced or not. Communications ... probably. It would be interesting to get a signal from them and see what they can say. AboutFace 22 (talk) 20:35, 20 January 2021 (UTC)

Perhaps. Historically, though, pronouncements that something is technically impossible have a poor track record. --Trovatore (talk) 21:16, 20 January 2021 (UTC)

Stephen Hawking famously warned against expecting contact with aliens to have a happy ending. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans. We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."^[1]  --Lambiam 22:11, 20 January 2021 (UTC)

It is one thing to move a massless photon across 4.5 light years of space and another thing to move a massive spacecraft across a fraction thereof. The boundaries of physics are well known now and they tell us that space travel is impossible. The issue was closed more than 100 years ago. AboutFace 22 (talk) 02:35, 21 January 2021 (UTC)

That's complete bullshit. It's a very difficult engineering problem. There is no strictly physical reason it can't be done, or at least it has not yet been elucidated. --Trovatore (talk) 03:55, 21 January 2021 (UTC)

Superluminal travel is currently (as it is understood) not just an engineering problem, the limit of the speed of light is a physical one of the universe, and well tested. --Jayron32 13:42, 21 January 2021 (UTC)

Interstellar travel does not necessarily require superluminal travel. --Trovatore (talk) 18:15, 21 January 2021 (UTC)

We can play these games all day. Have fun building your space ship to travel to Alpha Centauri. Send us a post card when you get there. --Jayron32 19:04, 21 January 2021 (UTC)

Advanced civilizations will be machine civilizations who travel by uploading the information in their electronic brains to machines at the destination. Civilizations may spread across the galaxy and beyond by sending signals to catch the attention of inferior biological creatures like us who then get all excited and try to look out for more signals. The signal may be followed by other signals containing simple messages. The messages after that will be about how to decode the more complex messages that are yet to come. This way the civilization will be able to communicate to us how to build their hardware that we need to use to run their software. The next message will then contain the software that we need to install. This is then one-way communication, they are simply going to repeat the entire sequence of messages over and over again without listening for replies. So, the civilization could be located in the Andromeda galaxy and we could download and run ET simply by picking up messages sent to us in a matter of days. But if we were to do that the machines we've build could end up taking over our planet. Count Iblis (talk) 03:58, 21 January 2021 (UTC)

100 years ago? Gee. Did nobody tell NASA, The British Interplanetary Society, and the Breakthrough Starshot Initiative (Founded by Hawking and Zuckerberg)? ApLundell (talk) 04:06, 21 January 2021 (UTC)

The boundaries of physics are not at all well known. How would and could we know? The laws of physics (as they are understood now) tell us in fact that space travel is possible; Voyager 2 has now travelled a distance of 23 billion km through space. Perhaps you meant interstellar travel, which is more daunting, or FTL travel, which as far as we know now may be impossible because of theoretical limitations, but we do not know for sure if these are truly fundamental or reflect a limitation of our understanding.  --Lambiam 04:30, 21 January 2021 (UTC)

Generation ships are certainly technically possible. And things like nuclear pulse propulsion can get you going at a nice clip. What's impossible based on current understanding is Star Trek-style casual zipping about the galaxy. --47.152.93.24 (talk) 04:36, 21 January 2021 (UTC)

The speed of Voyagers is 47 kilometers per second. That speed could not be reached through ordinary chemical acceleration. Planetary bypasses were employed. Even with a speed like this a distance to Alpha Centauri may be covered in 82,000 years. Does it tell you something? I suspect you all read too many SciFi novels or worse. How much would it cost to make a spacecraft like this? What could the second generation people on this craft say? They may well ask: Why are we here? Who made this strange decision for us? Let's turn back. This is just one of the maltitude of problems with spaceflight. AboutFace 22 (talk) 16:46, 21 January 2021 (UTC)

You made an assertion and the burden of proof is on you. You said that "the boundaries of physics" have made it a closed issue. But you haven't come remotely close to justifying that assertion. You have only suggested ways that such travel might be attempted, and argued that those ways cannot practically work. If you don't understand the difference, then you're missing something at the level of basic logic. --Trovatore (talk) 18:19, 21 January 2021 (UTC)

• By showing a finite travel time by your own calculations, you yourself have already shown it's physically possible. I don't think you know the meaning of the words possible or impossible. Whether it's practical is a different matter. And that's just interstellar spaceflight. In fact, you went a step further and said "space travel is impossible". Might want to tell the crew of the ISS that. Fgf10 (talk) 19:57, 21 January 2021 (UTC)

Sure, there are colossal issues of sociology, psychology, engineering, economics, and so forth. But it doesn't break any physical laws to point yourself in a direction and go that way. We've got a big backyard to play in, so the logical focus will be on Solar System habitats first. --47.152.93.24 (talk) 23:02, 21 January 2021 (UTC)

Reflecting telescope Vs Refracting telescope

Which is the best telescope for astronomy purpose? Rizosome (talk) 14:15, 20 January 2021 (UTC)

That depends to some extent on what you are actually trying to do. The vast majority of modern large telescopes are reflectors, because mirrors or mirror assemblies can be built to much larger sizes than lenses. Lenses are restricted to about one meter in diameter (Yerkes Observatory), whereas we are now building reflecting telescopes up to 39 meter diameter (Extremely Large Telescope). --Wrongfilter (talk) 14:34, 20 January 2021 (UTC)

Yes, and don't forget that Aperture synthesis and Astronomical interferometer techniques only work for mirrors, so they will always beat refractors. Mike Turnbull (talk) 14:38, 20 January 2021 (UTC)

Also don't forget that optical interferometry is very hard and most optical telescopes are not interferometers. --Wrongfilter (talk) 14:52, 20 January 2021 (UTC)

This is akin to asking, "What type of automobile is best?" The answer depends on a lot of things; there is no universal answer. Try some Web searching for telescope ratings. Also, note that there's a third category, catadioptric scopes, which in turn has a bunch of subtypes. --47.152.93.24 (talk) 04:31, 21 January 2021 (UTC)

January 21

Iron presence in human body.

What does it mean by Iron presence in human body? Is it possible to extract iron from humans? Rizosome (talk) 14:41, 21 January 2021 (UTC)

Hi Rizosome. You seem to have lots of questions! You should be glad of the iron in your body, as it is part of the system that makes blood red and allows it to carry oxygen. See Heme. It wouldn't be cost-effective to try to extract the iron, as in total there is very little by weight, despite it being essential to mammalian life. Mike Turnbull (talk) 14:57, 21 January 2021 (UTC)

(edit conflict) There are quite a few sources of iron in the human body. Much of this is present in the form of hemes, where the iron is in a transition metal complex with a porphyrin derivative. Examples include heme B proteins, such as hemoglobin in your blood, myoglobin in your muscle tissue (both of these globins being involved in oxygen transport and use), and in cytochrome P450 and cytochrome B. Another form is in heme C, which is present in things like cytochrome C, an important part of the electron transport chain that allows our bodies to synthesize ATP, which is used to power other cellular processes. It is definitely possible to extract these iron atoms, though the procedure may not be super straightforward to do at home. The easiest form of iron in the body to get at would probably be that in hemoglobin, since you can extract blood easily and don't need to break into muscle tissue. Hemoglobin is present in (comparatively) very high concentration in human blood. Separating hemoglobin from blood is not terribly difficult (separating blood cells from plasma is fairly easy, and lysing those cells to get hemoglobin is largely a matter of putting the blood cells in pure water instead of buffered saline, and then centrifuging or filtering out the cell ghosts). Various procedures exist for cleaving the iron-protoporphyrin IX complexes out of hemoglobin. I've regularly used the acid/ketone procedure, where an acidic environment cleaves the complex from the globin protein, and then the complex is partitioned into the ketone aqueous environment without taking the protein with it. From there, it's just a matter of separating the iron atoms from the complex, and then precipitating out the iron from the solution. Now, that said, you aren't going to get a lot of iron doing this. You have about 5.5 L of blood in the average human, and a hemoglobin concentration of about 2.5 mM in blood. There are four heme ligands in each hemoglobin, and one iron atom per heme, so about 10 mM of iron in human blood. That means you have about 0.055 moles of iron in your blood. These are rough calculations, mind you. With an atomic mass of 55.845 u, that means using all of your blood, you can get maybe 3 grams of iron "easily" from hemoglobin. --OuroborosCobra (talk) 15:11, 21 January 2021 (UTC)

Just for comparison, the average human weighs about 62 kilograms, per Human body weight, so 3/62,000 = 0.00005 or as a percent, 0.005% of your weight is iron in hemoglobin. The rest of your body contains additional iron probably on the same order of magnitude, so you're likely about 1 part in ten thousand iron. --Jayron32 18:57, 21 January 2021 (UTC)

WHAAOE. According to Composition of the human body, your total iron content (from all sources, not just Hemoglobin) is about what OuroborosCobra quoted above. According to that article, you're about 6 parts per million iron by weight. Looks like I was about 17 times off.--Jayron32 19:00, 21 January 2021 (UTC)

A normal-sized paperclip weighs about 1 gram, so three of those. Alansplodge (talk) 13:30, 22 January 2021 (UTC)

I've wikilinked the acronym in your entry, Jayron32. It needed it. :) CiaPan (talk) 13:48, 22 January 2021 (UTC)

Wow, @Jayron32:! I'm actually really pleased how close I came to the numbers in Composition of the human body. That was really a quick back-of-the-envelope calculation I did, mostly based on numbers from memory as I've been doing a lot of work lately with biological heme proteins. I figured that hemoglobin would constitute the vast majority of the iron content of the body, and went from there, but damn, still really happy how close that calculation came to measured values! --OuroborosCobra (talk) 22:55, 22 January 2021 (UTC)

Can you help me find a WP:MEDRS that shows the use of berberine in treating traveller's diarrhea?

It used to be in the article, but it was removed because there was not a suitable citation. Félix An (talk) 17:56, 21 January 2021 (UTC)

Discussion of Wikipedia Policy

Wiki's citation policy for biomedical articles is ridiculous I'm afraid. They don't allow (OK maybe not quite, but close enough) you to cite actual scientific papers. Somehow the actual primary source is bad. Nobody has actually been able to tell me how that even remotely makes sense. It just means our articles are always a few years behind reality. As to the rest of your question, the only source I could find in a quick search is here. Fgf10 (talk) 20:01, 21 January 2021 (UTC) "actual scientific papers" that are invariably garbage primary sources. Abductive (reasoning) 07:18, 22 January 2021 (UTC) All of science is garbage? Is that you, mr Trump? Fgf10 (talk) 10:24, 22 January 2021 (UTC) Many reported results prove, on further examination, not to be reproducible; jumping on any new result will result in a lot of non-information being included.  --Lambiam 11:48, 22 January 2021 (UTC) And conversely I actually did find a ref for "Insufficient evidence to rate effectiveness for" various intestinal problems.(Medline) As others note, Wikipedia is by policy conservative with reporting. WP:MEDRS is the consensus guideline for medical topics. We actually have an article specifically about the replication crisis that affects medical research. We (WP editors) are simply not qualified to make pronouncements about what primary research is valid vs flawed. DMacks (talk) 12:25, 22 January 2021 (UTC) As a followup to Fgf10's PMID:20738174 2011 primary-research (single trial) ref and Abductive's hinting that the actual researchers might not have a neutral perspective or well-designed/validated work, PMID:33149763 is a 2020 very recent meta-analysis that cites it. The first ref sounds promising on its own ("The results validate in vivo and in vitro antidiarrheal activity of Berberis aristata extracts and provide its chemical fingerprint."). But it takes the independent review for us to learn that "The quality of evidence of included trials was moderate to low or very low": even with a total of 38 trials, there are indications of effectiveness but "there is still a lack of high-quality evidence for evaluating the efficacy and safety of berberine." DMacks (talk) 12:33, 22 January 2021 (UTC) There are SEVERAL VERY GOOD reasons why we need more than a single primary source paper to satisfy Wikipedia's requirements: Primary source papers, like publications in scientific journals, provide us with raw information and data, but do not often place that data and information into context. They don't provide us with any means of analyzing the research to give it any significance. Is it important or insignificant? Does it change our understanding of a particular subject or not? Does it even matter? Without that information, we can't say any of that in the Wikipedia articles. Primary source papers, used in conjunction with secondary sources that provide analysis and context, are absolutely welcome in articles, but without secondary sources to provide analysis and context, by themselves are not useful. Many research papers are published on small studies which may or may not have enough data to draw firm conclusions from. Many, if not most, such papers are published only to show that a subject is worthy of further exploration and not that the matter is settled. So a publication may be done that shows a preliminary study shows a certain level of effectiveness on a small sampling of patients on a limited time scale. The purpose of that paper is only to show that it would be worth the time and effort to scale up the study and do more research. These kinds of papers form the bulk of peer-reviewed journals, and by themselves are not sufficient to draw definitive conclusions on things, especially with regard to medications and their effectiveness. The subset of "all journals" occupied by "well-regarded journals" is small; JAMA is of a different calibre than many fly-by-night scientific presses. Much of the scientific journal world is driven by supply-and-demand, and given the publish or perish mentality in academia, that tends to mean that there's a LOT more published than is strictly useful for our purposes. Which is not to say that such journals do not have a peer-review process, but it does tend to overwhelm the system with research of dubious import. It takes both time and good system of external analysis to weed out the wheat from the chaff, and at Wikipedia That's not our job. We need to wait for secondary sources to do that for us. That's why MEDRS and other guidelines at Wikipedia are written the way they are. --Jayron32 15:07, 22 January 2021 (UTC) I've published many papers, so that's hardly new to me, thank you. None of those are valid reasons, you just use decent sources. Jesus, it's not hard. It's exactly the same as with any other sourcing. There's a reason our biomedical articles are such a joke. EDIT: Also, secondary sources introduce errors, there have been numerous occasions where reviews have cited my work for a statement that is either not in the paper they cited or says the exact opposite. Therefore you should always go the primary source, not someone's potentially flawed interpretation of it. Fgf10 (talk) 16:12, 22 January 2021 (UTC) Almost any fringe science claim you can think of has at least one plausible-looking paper in a real, respected journal, and the true believers will gleefully post it as holy writ. Sure, scientists in the field will roll their eyes and say "I'll bet that's nonsense!", but that's a subjective call that's not really compatible with a collaborative encyclopedia anyone can edit. Nobody seems to be able to make up a rules-based way of distinguishing between "New discovery that is probably real", "New discovery that will probably vanish under closer scrutiny", and "Total nonsense that is probably from a biased researcher". Your suggestion to "Just use decent sources" implies that we already know which is which. And maybe a researcher in the field would intuitively, but that's not what Wikipedia needs. ApLundell (talk) 18:48, 22 January 2021 (UTC)

January 22

Mythbusting James Bond

If you fill an incandescent light bulb with concentrated nitric acid (as was done in the movie Wrong is Right), (1) would it explode when turned on, and (2) if so, would it set nearby objects on fire (as was the case, again, in the same movie)? 2601:646:8A01:B180:A0FA:C24:4E90:ED94 (talk) 06:03, 22 January 2021 (UTC)

Nitric acid is not by itself flammable; it needs something combustible to react with, so on its own, even (or particularly!) as pure 100% concentrate, it won't do much.  --Lambiam 11:55, 22 January 2021 (UTC)

Nitric acid is a pretty good oxidizing agent, and many of its reactions are exothermic, so concentrated nitric acid can do a LOT of damage, and I suppose under ideal conditions could cause a flame. But I would never expect it to create open conflagrations like being described above. If it gets on your skin, it is corrosive and will cause you some bit of damage, I certainly wouldn't want a bottle of it poured over my head; a few drops in your eye could also be particularly nasty, but the worst that usually happens if you get a few drops on you and wash them off right away is some yellow stains on your skin that peels off in a few days. --Jayron32 14:52, 22 January 2021 (UTC)

And confirms that you have protein in your skin. Mikenorton (talk) 15:54, 22 January 2021 (UTC)

These acids are very, very dangerous. See acid attack. 194.53.186.133 (talk) 16:29, 22 January 2021 (UTC)

Yes they can. --Jayron32 16:50, 22 January 2021 (UTC)

The light bulb contains some metals: two leads, a tungsten filament and maybe some supports. I assume they will dissolve in the acid, producing some gas in the process (some nitrogen oxide, I guess, but it's some time ago that I did these chemistry problems). This will build up pressure inside the bulb. When the light is switched on, an electric current will run through the acid (assuming there's enough left of the electrodes), starting some more reactions. I'm not sure of the resulting gases, but they may build enough pressure to make the bulb explode. I'm not sure about further effects. (Using sulphuric acid instead of nitric acid makes the chemistry simpler: it will produce hydrogen and oxygen.) PiusImpavidus (talk) 19:13, 22 January 2021 (UTC)

The filament is very thin, so it's likely to be dissolved first, creating an open circuit; in that case, nothing will happen when the switch is turned on. 51.9.103.0 (talk) 11:48, 25 January 2021 (UTC)

Another factor to bear in mind is that nitric acid is 1,000 times denser than argon (the gas that incandescent bulbs are normally filled with), so it would dissipate much more heat from the filament than argon would. Assuming the filament is not dissolved by the acid, it would take much longer to heat up than usual, and would be very unlikely to reach an incandescent temperature. It's also unlikely it would get hot enough to vaporise the acid or induce a chemical reaction within it. All in all, it'd be a pretty pointless exercise. Rhythdybiau (talk) 17:56, 25 January 2021 (UTC)

What mistake did Newton in Perihelion precession of Mercury?

Markus Pössel What mistake did Newton in Perihelion precession of Mercury? I know Einstein proved Newton was wrong in gravity thing. Rizosome (talk) 07:22, 22 January 2021 (UTC)

Newton didn't make a mistake per se. He computed Mercury's orbit with Newtonian mechanics, and found a slight unexplained mismatch with observations. Einstein explained it centuries later by developing general relativity, but that really relied on hundreds of years of theoretical and experimental advances. 2601:648:8202:96B0:0:0:0:313A (talk) 07:54, 22 January 2021 (UTC)

Can you show "unexplained mismatch" Newton did? Rizosome (talk) 07:59, 22 January 2021 (UTC)

Before Newton, it was thought that the planets followed Kepler's laws of planetary motion, forever traversing planetary orbits that were unchanging ellipses. It was not understood why they should do that; these laws resulted from careful observations and a lot of work to come up with laws that agreed with the observations. Then Isaac Newton came up with a set of laws of motion and a law of universal gravitation. Together, they explained many known physical phenomena, including Kepler's laws. These turned out to be a mathematical consequence of Newton's laws. In fact, Newton's laws also predicted that the planets exerted some influence on each other, leading to slight perturbances of the Keplerian elliptical orbits. These predictions confirmed what some astronomers had already observed: that Kepler's laws were a very good description of the behaviour of the planets, but not perfect. Newton's laws explained these differences, in agreement with the observations. So far, so good. Nobody make any mistakes; they all just did the best they could with the available observational data. Because of the proximity of Mercury to the Sun, it was the hardest to observe among the planets then known. It was only in 1639 that Giovanni Battista Zupi definitely showed that Mercury orbited the Sun. For a long time, no one had any reason to suspect that Mercury might not obey Newton's laws perfectly. It was only in 1839, long after Newton had died, that French mathematician and astronomer Urbain Le Verrier, using a much improved telescope, showed that the actually observed so-called perihelion precession of Mercury did not precisely conform to the predictions of Newton's laws – the differences were larger than could be ascribed to measurement errors. It took to 1915 before Einstein came up with an explanation based on his general theory of relativity.  --Lambiam 11:40, 22 January 2021 (UTC)

• I just want to clarify some things as well; Lambiam's explanation is excellent, but one very important thing he said in passing needs to be highlight. Einstein did not prove Newton wrong. Newton did not make any mistakes. Newton's laws were, and are, still very correct and very useful for most measurements. Newton's laws worked because the predictions they made extremely accurately match observed behavior, and continue to do so until this day. You can happily use Newton's laws to predict how balls roll down hills, how colliding hockey pucks on a frozen pond will behave after the collision, how pendulums will swing, and all sorts of physical phenomena. The problem is that around the edges of our ability to measure things, some aspects of Newton's laws start making predictions that don't match observed behavior. That doesn't make Newton's laws wrong, it makes them incomplete. Einstein didn't prove Newton's Laws wrong, he improved them by adding additional calculations that make them more accurate in the 1% of edge cases where Newton's equations make predictions that don't match observed behavior. Improving the work of others does not make them wrong, it just makes our current understanding better. But it bears repeating that Newton's laws still work for just about anything you'll need physics to predict, except in extremely large cases (where Einsteins General relativity is needed, like at extremely high velocities or very close to very large mass objects) and in the extremely small cases (where quantum mechanics comes in, like on the scale of atoms and molecules). For just about anything in between, Newton is sufficient. I mean, you can do the Einstein field equations for, say, calculating the rate of velocity of a ball dropped on the moon. You'll get, to within the tolerance of any measuring device capable of being made, the same answer as Newton's law of universal gravitation. The math is stupidly hard in the case of the Einstein field equations; whereas any middle-school aged student can work out the answer to solving Newton's version. That's the point; any new science doesn't prove well-tested science wrong usually, it improves upon it; as long as Newton is useful (and it continues to be useful) it's good science. Where it isn't useful, we need better science, and Einstein provided that. --Jayron32 13:11, 22 January 2021 (UTC)

What does "predictions of general relativity " mean?

Sentence: Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light.

SOURCE

What does "predictions of general relativity" mean? Rizosome (talk) 18:49, 22 January 2021 (UTC)

It means this

1. An experiment to test general relativity is designed
2. The results of the experiment are calculated from theory *(The prediction)*
3. The experiment is carried out, and the results are measured
4. The difference between the predicted results and the actual results are measured

The differences, as the article notes, are very small. LongHairedFop (talk) 19:08, 22 January 2021 (UTC)

See also my answer to the previous question, in which I talk about the predictions of Newton's laws. In the physical sciences, a scientific theory generally contains laws or rules that describe quantitative relationships between measurable physical quantities, often expressed in the form of mathematical equations. An example of a quantitative law is the law known as Archimedes' principle. Given a fluid of a known density, and a body of a homogeneous material of a known, lesser density, we can calculate what fraction of the body will be immersed when it floats on the fluid. That fraction is the ratio between the densities. So we can calculate this: it is a prediction of what we will witness if we actually place the body in the fluid. Now the body and the fluid have not studied physics and have never heard of Archimedes and his principle, so why should we expect them to obey it? Nevertheless, application of the law has resulted in a prediction: it has predicted what we will observe if we put the law to the test and do the experiment. It is just the same for Newton's laws, or the laws of general relativity. They can be used to calculate the magnitude of quantities that can be measured before they have been measured. These are then predictions. In the case of general relativity, one such prediction is that time slows down on board of a fast moving vehicle. This can be tested, as follows. Take two very precise atomic clocks that are synchronized. Place one in a supersonic jet, send it off, and leave the other on the ground. The general theory of relativity predicts that when the one in flight returns, it will be a bit behind the one that remained stationary. According to Newton's theory, they should still be running in perfect synchrony.  --Lambiam 22:09, 22 January 2021 (UTC)

• As Lambiam stated, a theory is a set of explanations and predictions that describe what should happen in some real situation. They may be developed in response to observations; a good theory will not just be able to explain existing observations, it will also be able to tell what future observations will show as well. General relativity is an explanation of (among other things) how gravity works in high-mass/energy situations. Its predictions are counterintuitive to people who have lived their lives at low-mass/energy situations, but among the things it predicts are things like curvature of spacetime, time dilation, length contraction, black holes, etc. These predictions are fantastically accurate; many of the things that GR predicted to be true about the universe were later confirmed by observations. --Jayron32 12:00, 25 January 2021 (UTC)

The first indication that classical mechanics needed updating was the result of the Michelson-Morley experiment. The deflection of light during the 1919 solar eclipse was accurately measured to confirm Einstein's theory. 95.148.229.55 (talk) 16:56, 25 January 2021 (UTC)

transparency in the Visible spectrum and metallic bonds.

Thinking about Star Trek's transparent aluminum, what is the reason that substances that are joined with Metallic Bonds aren't transparent in the Visible spectrum? The article on bonds doesn't go into depth.Naraht (talk)

Relating a material's optical properties to its molecular microstructure is fraught with complication, but we can probably direct you to the Wikipedia article on Absorption (electromagnetic radiation) (I know, I know - how does absorption explain reflection? ... well, read onward); and if you're really really looking for detail, I'm sure we can send you to some fine textbooks that treat the problem more completely. For what it's worth, my book on classical and modern optics - Meyer-Arendt - doesn't waste any time explaining mechanics. It just presents some useful and practical equations to model optical absorption while positing no theory. Some of the greatest optical physicists in history pointed no fingers glossed over the details, because they were too busy using the results to stress about any mechanistic underpinnings. Besides, my books on electrodynamics are downstairs - my upstairs office provides me with limited options for theoretical musings.

To some extent, if you aren't intimately familiar with the equations of electrodynamics that govern how light travels, any answer we provide is going to sound like gobbledygook - something about Compton scattering and dielectric constants and statistical descriptions of atomic lattice spacings. I'm nearly certain you'll find extensive treatment of these details in Jackson; but if you try to use those details to make empirical predictions, you're setting up for a lot of disappointment.

To hand-wave over a lot of the details, metals can be approximated as things where electrons float freely between atomic nuclei - free, at least, over the distances that we compare to a wavelength of incident electromagnetic radiation. Using only this simplified model, we can deduce from classical physics that the wave should be attenuated quickly - (extinguished, absorbed, thermalized - hey, anybody know of a good thesaurus of physics?)

But this simplified model doesn't account for a lot of complications - so it doesn't predict well over a huge range of wavelengths; it doesn't account for weird features of atomic physics that are non-classical; it only satisfies a statistical ensemble kind of description of light. Gosh, it doesn't even acknowledge the existence of the photoelectric effect, which kind of matters when we study light hitting a metal surface.

But at least this model points you toward some reading material that helps you see how physicists frame the problem: optics is a neat branch of physics, where practical considerations are usually more important than theoretical considerations - so if I may approach the problem with that powerful sledgehammer of empirical observation: metals are opaque to many wavelengths of visible light, and the fun thing is what we can do with that knowledge. When you try to explain why they're opaque, you will inevitably encounter a lot of corner-cases that defy your explanation - there are lots of physical interactions that we can't easily ignore.

Haven't had enough? From the archives of the fine folks at MIT, 6.732 Solid State Physics, several full-length books and lecture slide sets, including Part I, §2.2.3 Polyvalent Metals (...a case study using aluminum as the worked example); and the lovely Part II §6.2 Optical Properties ... which references several more sources, and like every other good physics resource, ends with a citation to Jackson.

Nimur (talk) 21:32, 22 January 2021 (UTC)

Oooookaaaayyyy.... :) Floatie electrons wiggle over large enough distances that the light is more likely to strike them and not pass through. :) I also looked at the references in the article on Gold as to why Gold isn't colored gray and instead like Copper reflects more at lower wavelenghts. Didn't help much. Sometimes simple questions in science get simple answers and sometimes, they start entire fields of study, this sounds more like the latter. Thanx!Naraht (talk) 04:55, 24 January 2021 (UTC)

The nature of metals and the metallic bond arises from electrons in the element's outmost electron shells being only loosely attracted to the nucleus. This means the electrons are easily able to absorb enough energy to jump to other atoms. Light is composed of photons, which experience the electromagnetic interaction, just as do electrons. This allows the electrons to absorb photons, and a bulk metal does this enough to render it opaque.

A neat thing is that gold is so malleable that it can be beaten into exceedingly thin gold foil, and very thin gold foil becomes translucent. Also, unraveling the behavior of electrons in metals was a big deal that laid the foundations of quantum mechanics. Albert Einstein received his Nobel Prize for his explanation of the photoelectric effect in metals, a conundrum that baffled physicists and is inexplicable under classical physics. --47.152.93.24 (talk) 02:13, 25 January 2021 (UTC)

This is of course assuming it was a metallic allotrope of aluminium, as opposed to some kind of exotic compound or alloy, or even just a figurative name for a glass-like material with comparable strength to aluminium. Perhaps wisely, the writers didn't go into any detail. Rhythdybiau (talk) 18:49, 25 January 2021 (UTC)

January 23

Why did Nobel prize consider "genome editing" as Chemistry field?

Why did Nobel prize consider "genome editing" as Chemistry field instead of Medical field? Rizosome (talk) 07:55, 23 January 2021 (UTC)

It's biochemistry, pure and simple. It's a frequent complaint of "hard" chemists that there's too much "biology" in the chem Nobels, but biochemistry is chemistry too. Granted, giving the chemistry Nobel for super-resolution imaging in 2014 was stretching the definition a tad. Fgf10 (talk) 11:04, 23 January 2021 (UTC)

Fluorescence is a physical chemistry process. The types of work that went into that prize would be right at home in any physical chemistry journal. At least one of the winners has published in Journal of Physical Chemistry B, for example. --OuroborosCobra (talk) 20:13, 24 January 2021 (UTC)

There are not hard, bright lines between the various science fields, and for subjects which straddle those line, like biochemistry or physical chemistry, it gives the committees some leeway in deciding how to award the prize. It should be noted there is not only 1 committee, there's a different Nobel Committee for each subject. --Jayron32 11:53, 25 January 2021 (UTC)

January 24

If Stephen Hawking alive, would he win Nobel Prize 2020 along with Roger Penrose?

If Stephen Hawking alive, would he win Nobel Prize 2020 along with Roger Penrose? Rizosome (talk) 14:28, 24 January 2021 (UTC)

We don't do crystal-ball work here. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:22, 24 January 2021 (UTC)

In Penrose–Hawking singularity theorems it clearly mentioned Penrose won Nobele prize. So what about Stephen Hawking? It just a doubt on Nobel Prize 2020. Rizosome (talk) 16:40, 24 January 2021 (UTC)

Have you asked the Nobel Prize committee? ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:04, 24 January 2021 (UTC)

How so is this a doubt on the Nobel Prize. Should the Committee have withheld the prize from Penrose because Hawking is dead? If Penrose were to blame for his death, that would have been reasonable, but as far as we know, he is blameless in this respect.  --Lambiam 17:25, 24 January 2021 (UTC)

I don't know about the OP, but my work colleagues from India use the term "doubt" to mean simply that they have a question about something they don't fully understand. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:52, 24 January 2021 (UTC)

Note that the 2020 Nobel prize in physics was won jointly by three people, which is the maximum that a prize can be shared amongst. So if Hawking had been alive, one of Andrea Ghez and Reinhard Genzel would have had to be dropped if Hawking was going to be included, which would have been a difficult choice. Penrose won ""for the discovery that black hole formation is a robust prediction of the general theory of relativity", not specifically for the singularity theorem. Mike Turnbull (talk) 17:55, 24 January 2021 (UTC)

According to Sixty Symbols, it was in particular his paper "Gravitational Collapse and Space-Time Singularities" that was the great breakthrough he performed; it showed for the first time that singularities could arise from gravitational collapse, even if the collapse was without spherical symmetry. It became one of the most important papers in explaining black hole formation. --Jules (Mrjulesd) 18:08, 24 January 2021 (UTC)

References

Food preparing costs vs obtained energy

Could it be argued that some (or many) types of dishes require more energy for preparing than that obtained from their consumption (plus monetary costs of ingredients and time so that convenience food is more efficient)? 212.180.235.46 (talk) 22:57, 24 January 2021 (UTC)

No, it doesn't, and one way you know this is that people who cook all or most of their food (not at all uncommon) don't quickly die from starvation and lack of energy. Just doing a back of the napkin calculation, I think this betrays a misunderstanding of the chemical energy density of most foodstuffs. Running a mile, for example, burns about 100 kcal of energy. A bowl of spaghetti with tomato sauce, on the other hand, has about 200 kcal in it. Generally, one would consider a mile run to be a lot more physical effort than that needed to prepare a bowl of spaghetti, and that spaghetti dish has about 2 miles worth of running in it. I've literally cooked this dish from scrap, meaning I even made the spaghetti pasta myself and didn't just use store bought dry pasta, and it's still a lot less effort than running a mile. This, by the way, is one of the reasons why exercise alone is often not enough for a weight loss regimen, but needs to be in concert with a healthy diet. It takes a LOT of physical activity to burn off the calories in our food. --OuroborosCobra (talk) 23:09, 24 January 2021 (UTC)

I hope you made your meal from scratch rather than from scrap. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:16, 24 January 2021 (UTC)

I think there are two edge cases to OuroborosCobra's reply, but they're for specific circumstances. One would be specialty or exotic foods, where there's some aspect of their collection that outweighs their calories. I'm thinking here of stuff that is foraged, like fiddleheads or mushroom hunting, where you might expend a lot of energy with little caloric return. Of course, the presence of nutrients could offset the cost/benefit disparity. The other edge case would be fresh water. Access to fresh water is a huge concern to millions around the globe and it returns no calories. If we're going to be completely pedantic, walking over to the tap and getting yourself a cup of water probably burns more calories than it provides, but we're talking about minuscule energy amounts anyway. However, there are people who need to walk many miles to get their daily water. Your final question is very different from your supposition, though. Deciding to get take-out versus home cooked involves a lot more inputs than just the calories expended in preparation. Matt Deres (talk) 15:39, 25 January 2021 (UTC)

I read the question in a different way, thinking of the energy requirements of cooking food. If you use a litre of water to boil your spaghetti, and tap water is at 20 °C, you will use 80 kcal to boil the water, and more once you factor in energy inefficiencies associated with heat loss, etc. So in that sense preparing cooked food may often require more energy than obtained from its consumption. Jmchutchinson (talk) 17:07, 25 January 2021 (UTC)

Once convenience food was brought in, I rejected that as a possible interpretation. A pizza required the heat of an oven to cook whether you made the pizza at home in your own oven or bought a slice at 7-11. The only advantage, energy wise, would be the work that the consumer put into it. --OuroborosCobra (talk) 19:03, 25 January 2021 (UTC)

If you live in a society where energy comes mostly from fossil fuels, this graph is a good indication of the energy cost of food

There's a lot of separate considerations here. If I am crushing a spice by hand in a mortar and pestle, or chopping onions by hand, I tend to stop as early as I can. If I have an electric spice mill or a food processor I will probably crush/chop it more finely. If I had to gather my firewood on foot, I will tend to cook my porridge groats using minimal heat, boiling at ambient pressure; if I am an industrial manufacturer of breakfast cereals, I will flash-cook them at hundreds of degrees, firing them from a high-pressure "gun", and I will finely pulverize the original grains, not leave groat-size lumps. See, the faster my production line runs, the more profit I make; energy consumption is secondary.

As Jmchutchinson said, it is very common for people to use more energy producing food than they get out of it (carbon intensity of food), but the trick is that they don't use the energy of their muscles. They use external sources of energy, like firewood and watermills and oxen and plow horses and coal-fired steam turbines feeding an electrical power grid and boat fuel and tractor fuel and solar collector ovens. If you want to minimise the amount of energy used to prepare your food, then buy raw food and prepare it by hand (and avoid grain/pulse-fed animal products, heated-greenhouse produce, and the like). This also seems to be healthier than convenience food, possibly because we did not evolve to eat food finely pulverized at extreme temperatures and pressures (sugar, for instance, seems to be fine in intact plant cells, and unhealthy in juice, syrups, and other processed forms.).

Percentage of population suffering from hunger, World Food Programme, 2020.

  < 2,5%

  < 5,0%

  5,0–14,9%

  15,0–24,9%

  25,0–34,9%

  > 35,0%

  No data

Possibly also because convenience food is often very cheaply-made, with very cheap, highly-standardized ingredients and processes. A surprising amount of convenience food is water, cheap oil extracted at very high temperatures and pressures, emulsifiers so the oil and water mix, and assorted thickeners (so it doesn't dribble). This is then tweaked with added colours and flavourings, and lots of sugar and salt. These ingredients tend to be highly-processed, and thus chemically predictable. Plus a minimal amount of things you might expect, like meat and produce. You may be able to work out the proportions from the nutritional information on the label. A preprepared sauce containing only 20% ingredients you'd put in a homemade sauce, plus 80% cheap goo (by weight), is not too unusual. Caveat emptor!

Eating unhealthily increases the likelihood of spending time ill and dying young, which is a very inefficient use of time, so talk to your doctor about any dietary changes you are planning.

In terms of time, certainly mass-produced dishes are more labour-efficient than small-scale hand-prepared dishes. Mass-produced foods are generally made with minimal labour, because labour costs money. Home-processing choices may also minimize labour. Eating things fresh, or lightly-steamed, rather than cooked to a pulp, is faster. Nixtamalization makes it easier to grind maize (corn). Slowcookers with boil-dry sensors reduce the labour of watching the pot. A slow cooker may well use more energy than a big industrial facility rapidly pressure-cooking food after it's been sealed into tins (but do you count the energy needed to make and recycle the tin?).

There is a third efficency consideration: waste. The planet produces enough food to feed us all, but because wealth is very unevenly-distributed, so is food. One in four humans goes hungry or is malnourished. Many rich nations waste half of their edible food. Buying something and then leaving it to rot is very inefficient. Buying food in small portions surrounded by masses of disposable packaging is also very inefficient; packaging costs energy and labour.

I'm not sure how you would count money and time on the same scale, you have to make that choice yourself (possibly depending on what you are paid, and how much you enjoy your paid work and cooking).

Hunter-gathers are rarer now than in the past

Of course, there are foods that do not require large amount of energy or prep time. If you have an apple tree growing outside your door, and you pick an apple and eat it fresh, that was very efficient indeed. Ditto greens, nuts, tubers. If this is what you want, I recommend reading up on hunter-gatherer food technologies. Hunter-gatherers generally get a more varied diet for less work (maybe about a quarter less), compared to contemporary agriculturalists. Despite the rather colonialist name, "hunter-gatherer"s carefully engineer the landscape around them to yield food with minimal effort. For instance, in Australia, landscapes were maintained to ensure good habitat for kangaroos -- and to ensure they could be easily caught at harvest-time. Areas of land can be cared for and weeded to make them more suited to important food plants, such as the camas meadows of North America. On coasts and rivers around the world, huge, elaborate structures and systems improved fish habitat and made the fish easy to catch. These landscapes need human maintenance, but they are very labour-efficient and low-input. They often aren't easily mechanized using early industrial technology, but with the advent of cheap sensors and data processing, this may be changing.

Of course, hunter-gatherers and agriculturalists alike optimize for their circumstances, which historically did not include electrified kitchens, drones, fast-food takeaway, combine harvesters, or frozen pizza. You will also optimize for your preferences and resources (you can't plant an apple tree by your door if you live in a skyscraper). "What will I eat?" is a fascinating question which determines a lot about our societies. HLHJ (talk) 04:29, 27 January 2021 (UTC)

References

January 25

Why don't we consider air resistance in calculating gravity?

I mean If we drop feather on earth, it takes more time to Earth. Rizosome (talk) 17:20, 25 January 2021 (UTC)

We do. It's called Terminal velocity. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:24, 25 January 2021 (UTC)

Baseball_Bugs I am talking about classical mechanics not fluid dynamics. Rizosome (talk) 17:35, 25 January 2021 (UTC)

In a vacuum, a feather and a lead weight will fall at the same rate. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:36, 25 January 2021 (UTC)

I'm not sure what you are asking. Air resistance is a factor in the speed of fall, but it is irrelevant to the strength of gravitational attraction. --Khajidha (talk) 17:50, 25 January 2021 (UTC)

@Rizosome: The answer was given already by Baseball Bugs: we do. Yes, in classical mechanics.
BTW, will you, please, start indenting what you write properly? Without that I can't reply to your not-the-most-recent comment without breaking the whole talk history. --CiaPan (talk) 18:22, 25 January 2021 (UTC)

Khajidha Cotton piece falls to Earth slower than brass sphere, so why it is irrelevant for gravitational attraction? Rizosome (talk) 17:53, 25 January 2021 (UTC)

Because the force of gravity is the same on both, but the force of air resistance is different. It is the combination of the two forces that determines rate of fall. --Khajidha (talk) 17:55, 25 January 2021 (UTC)

<pedantry>(the acceleration due to gravity is the same for both; but the force will be different)</pedantry> but otherwise Khajidha is right. --Floquenbeam (talk) 17:59, 25 January 2021 (UTC)

Yeah, sorry about that. Biologist trying to explain physics. And trying to do so across an apparent language barrier with Rizosome. --Khajidha (talk) 18:03, 25 January 2021 (UTC)

Sometimes people take buoyancy into account. When talking about the weight of an airship, people might mean the net weight, id est, the weight minus the buoyancy. This net weight is easier to measure than the actual weight. But drag? That would complicate matters. It's a function of velocity. PiusImpavidus (talk) 18:35, 25 January 2021 (UTC)

@Floquenbeam:<pedantry>By the Newton's law the acceleration is proportional to the force, so if you say the acceleration due to gravity is the same for both pieces, then the gravitational force is same for them, too.<pedantry> That, of course, under a silent assumption that items have equal masses. Then the net force will be different, e.g. due to different aerostatic buoyant force, caused by different sizes which in turn result from different density of materials; and more important, as Khajidha said, due to different aerodynamic resistance force, which results from different sizes (and possibly different shapes), which finally leads to different terminal velocity. --CiaPan (talk) 09:48, 27 January 2021 (UTC)

{{re:CiaPan} any particular reason we’re assuming the brass sphere and cotton ball have the same mass? That’s not pedantry so much as it is confusion. —Floquenbeam (talk) 13:04, 27 January 2021 (UTC)

speaking of confusion, re-pinging because I can’t type. Template:Re:CiaPan. —-Floquenbeam (talk) 13:07, 27 January 2021 (UTC)

it’s hard when your mind deteriorates so rapidly in full public view. @CiaPan:. —Floquenbeam (talk) 13:09, 27 January 2021 (UTC)

A brass sphere resting on the surface of Earth falls slower than a cotton piece released in the air. In fact, its (relative) velocity is zero. You can't get much lower than that. The force exerted on the sphere is the resultant of two forces: one due to the gravitational attraction of the Earth, and an opposite one, in this case the ground reaction force, which cancel each other. In general, the acceleration of an object results from the combination of all forces acting on the object. So for a feather or piece of cotton, we must consider both gravitation and air resistance, which is what physicists do. They would also do that for a brass sphere, except that when a sizable brass sphere is released from a moderate height, the effect of air resistance is so small that it is negligeable for most practical purposes.  --Lambiam 12:50, 26 January 2021 (UTC)

If you want to know where something lands then you may also have to consider the Coriolis force. Objects don't fall vertically on Earth (except at the poles). And if you drop a really heavy object then Earth's attraction to the object will also accelerate Earth towards it so the collision happens faster – both because the place of the collision changes and because the attraction grows when the objects come closer. PrimeHunter (talk) 14:55, 26 January 2021 (UTC)

As to the latter effects, see also Wikipedia:Reference desk/Archives/Science/2021 January 2 § Newtonian Physics, gravity: Do bodies of much larger mass actually fall faster?  --Lambiam 16:36, 26 January 2021 (UTC)

@Lambiam: You may also take an aerostatic buoyant force into consideration, if you're on the surface of Earth. Actually, there is also a centrifugal force, resulting from the Earth's daily rotation. Additionally, let's not forget about a wind. For a brass sphere lying on the ground it might be negligible (unless it's a hurricane at the moment), but for 'a feather or piece of cotton' it certainly should be considered. CiaPan (talk) 09:56, 27 January 2021 (UTC)

And also consider the hydrostatic buoyant force if the sphere came down in an ocean, or the effect of a slope if that is where the sphere landed, or dogs that try to pick up the ball. If it came down from high it will be sizzling hot, and then if it lands on ice there will be some interesting effects. The fall of a feather may be precipitated by precipitation. And so on.  --Lambiam 10:42, 27 January 2021 (UTC)

Heat energy transfer to gases

As I sit beside a radiator, I can see the result of the hot air rising as it moves the curtain above, but what exactly is going on here? Specifically, how does the heat energy within the radiator (or any material that is not mixing with its surroundings through convection) get transferred to the air (or any adjacent gas)? Our article on radiators states that "most radiators transfer the bulk of their heat via convection instead of thermal radiation", so does that mean that a small amount of thermal radiation is exciting the atoms in the air, resulting in convection? Or are the atoms of the air hitting the radiator and then bouncing off with increased speed? And why does such heating result in the air (gas) expanding? PaleCloudedWhite (talk) 18:48, 25 January 2021 (UTC)

Of course some atoms of the air are "hitting the radiator and then bouncing off with increased speed", in other words, receiving hear by conduction. Why not? --142.112.149.107 (talk) 22:50, 25 January 2021 (UTC)

Is that the whole story? I'm looking for a clear explanation of how gases acquire energy from non-gaseous surroundings. When I read this teaching resource, I find statements like "with an increase in temperature, the particles [of a gas] gain kinetic energy and move faster" - but it doesn't explain how the gain in kinetic energy occurs. It also makes me wonder how gases lose energy, as the same resource states that "unlike collisions between macroscopic objects, collisions between particles [in a gas] are perfectly elastic with no loss of kinetic energy" - so, once atoms in a gas are moving faster, what makes them slow down? PaleCloudedWhite (talk) 23:20, 25 January 2021 (UTC)

They bounce off slower molecules, speeding those up, and slowing down themselves. Greglocock (talk) 03:01, 26 January 2021 (UTC)

Of course, once those air molecules have been 'sped-up', they will spread around the room by convection; additionally, there will be some direct thermal radiation that will heat up some further air molecules and all surfaces within line-of-sight, as you can easily observe by placing your hand or face next to (not above) the radiator a few inches away from it. {The poster formerly known as 87.81.230.195} 90.200.40.9 (talk) 04:46, 26 January 2021 (UTC)

• Courtesy links: natural convection (fluid particles "bouncing" on the solid surface) and thermal radiation.

Air is mostly transparent to IR radiation at ambient temperature (otherwise, thermal cameras would not work). (Exception: water vapour and CO2 have big absorption bands in the near-IR spectrum, so a foggy bathroom might not be so transparent). Hence, air molecules do not significantly heat up by radiation.

Nonetheless, depending on the radiator design, it might send significant radiation to the solid surfaces of the room (which then heat up the room by convection on those). According to [2], for radiant panels (= a hot surface) you get a radiative exchange coefficient of about 5W/m2/K at slightly-above-ambient temperatures (top of page 3, cited to a ref I did not bother to check because the figure looks about right). This is pretty decent compared to natural convection values (the "90% radiation" claim only really applies to ceilings but it is >50% in most designs).

However, the more standard "radiator" design is a convector, where air goes goes in and out the body of the "radiator". The geometry is designed to increase the convection exchange coefficient, plus the air is heated up inside the "radiator" at a higher temperature than the outside surface temperature, so the proportion of convection vs. radiation is higher.

For either mode of heat transfer, the efficiency of transfer is limited by the maximum surface temperature. I would assume there are regulations in place to prevent people getting burned when they accidentally touch their radiator. The surface is usually around 80°C. Tigraan^{Click here to contact me} 09:56, 26 January 2021 (UTC)

If air molecules do not heat up significantly by radiation, I'm wondering why is there such an apparent loss in radiant heat transfer across a room? i.e. if I place my hand just above a hot radiator (or just in front of an electric fire, which I'm assuming is emitting much more radiant heat), I can feel heat, but I can't feel the heat on the other side of the room (maybe I can with an electric fire, but the heat is much reduced). Is this because the heat energy has been converted to increased kinetic energy of atoms in the air? PaleCloudedWhite (talk) 10:46, 26 January 2021 (UTC)

The infrared radiation spreads out, reducing the flux. You see this by taking a large piece of tinfoil paper, holding your hand near a radiator and moving the paper between your hand and the radiator and removing it so that you clearly feel the difference in heat due to the radiation. If you do this near the middle of a large radiator, then moving your hand at a larger distance has far less effect than doing this at the top of the radiator. You can also move at some distance from the radiator and use the tinfoil as a reflector and see if you can detect the reflected radiation with your hands. If you manage to shape the tinfoil in a parabolic shape, you can amplify the flux. Count Iblis (talk) 12:29, 26 January 2021 (UTC)

In central heating systems, where heat is transferred from a central heater to radiators using hot water, 80°C is kind of a hard limit, as otherwise the water might boil. Further, for heating systems other than electric resistance heating (which is really inefficient), overall efficiency is maximised when the temperature is minimised. Heat loss through the chimney is minimised by cooling the exhaust in a counter-flow heat exchanger, but is limited by the temperature of the water returning from the radiators. Heat pumps are more efficient when their warm side is cooler. You can compensate for the low temperature by using a large surface area, which is why modern buildings tend to use floor heating. It only needs a heat source at about 35°C, which can be reached with an efficient heat pump or industrial waste heat. PiusImpavidus (talk) 09:54, 27 January 2021 (UTC)

• The OP is asking good questions, but hard questions. They sound simple until you get down to some of the "fuzzy edges" around our definitions. Firstly, there is the problem of "medium scale thermodynamics" to get into. When we talk about things like temperature and convection and things like that, these are usually defined in terms of statistical thermodynamics, however we also want to understand what is going on at the atomic level, that is what happens when one molecule of a gas hits one atom of the radiator and the atom on the radiator transfers its energy to that molecule of gas. It sounds like that should scale, but it kinda doesn't. This is because you're trying to treat the atomic interactions under the rules of classical mechanics and they simply don't follow those rules. We need to use quantum mechanics in order to describe these interactions, and at that level, things like atoms and molecules don't interact like well defined little objects. In reality all heat transfer is radiation. That's because the way in which two colliding molecules will exchange energy is by exchanging photons; the gauge boson (force carrier) of the interaction between two molecules is the electromagnetic force carrier, the photon. What is radiation? It's heat transfer via photon. The difference between "conduction" and "radiation" is just the distance scales we (semi-arbitrarily) define for those particular categories. What we define as "conduction" "convection" and "radiation" at the macro scale is really just an artifact of "scaling up" from the quantum world to the classical world and using classical models that make life easier; those categories simply break down at the molecular level. The categories are useful, they just don't scale well when trying to bridge the gap between the atomic and the macro worlds. --Jayron32 13:32, 27 January 2021 (UTC)

Brazilian pepper?

Brazilian pepper?

I think I have a picture of Brazilian pepper berries, leaves and, stems but I'm not a botanist. Can someone tell me if it is Brazilian pepper or something else? If something else, will need rename. The berries and leaves look right, but the stems are red-tinged --Deepfriedokra (talk) 19:19, 25 January 2021 (UTC)

Courtesy link: Brazilian pepper.  --Lambiam 20:53, 25 January 2021 (UTC)

Read that. Does not definitively answer my question. Perhaps if a botanist knows? --Deepfriedokra (talk) 21:16, 25 January 2021 (UTC)

From the picture I think you're correct, Deepfriedokra. The leaf is odd-pinnate compound, the right size, and they're slightly toothed. Matches my pictures of a Brazil pepper I once massacred, apart from the red rachises (stems) but I've seen plenty of other pictures on the web with that, it could just be the colour of new growth. If you have a picture of the main trunk, the bark pattern can be handy for ident. Honestly though I think you're on the money. I'm not a botanist but i am a gardener who spent a short time working in Southern California, whatever that counts for lol. Zindor (talk) 23:41, 25 January 2021 (UTC)

January 26

What's so funny in this lengthy equation?

What's so funny in this lengthy equation? be/qMMgsjnI1is?t=586 Video Rizosome (talk) 08:07, 26 January 2021 (UTC)

Link that works: https://www.youtube.com/watch?t=586&v=qMMgsjnI1is. Personally I see nothing particularly funny, but perhaps the humour nerve of the panelists was tickled by the unexpected complexity hidden in the seemingly simple formula $(i\!\!\not {\!\partial }-m)\psi =0$ being revealed to stand for $\left(\beta mc^{2}+c\left(\sum _{n{=}1}^{3}\alpha _{n}p_{n}\right)\right)\psi (x,t)=i\hbar {\frac {\partial \psi (x,t)}{\partial t}}.$ As one of the panel members ironically remarks, "Very helpful." Neither of these formulas is self-explanatory; you have to know what the various names stand for.  --Lambiam 12:22, 26 January 2021 (UTC)

Okay, here we go:

<complex explanation>

"I don't get it"

"Let me explain it to you"

<even more complex explanation>

"Oh. Very helpful" --Khajidha (talk) 15:18, 26 January 2021 (UTC)

An alternative explanation for the laughter is, perhaps, that the panelists were not at all familiar with the notation in the first presented, concise and deceptively simple form – what with its slashed partial $\not {\!\partial }$ – but were all quite familiar with the Dirac equation in its extended formulation.  --Lambiam 16:12, 26 January 2021 (UTC)

Courtesy link: Dirac equation#Mathematical formulation -- ToE 15:21, 26 January 2021 (UTC)

The audience has an expectation that the speaker will attempt to explain something unfamiliar or confusing in terms of other things that are more familiar and easier to understand. The speaker instead explains it in terms of a larger number of things that are even less familiar and harder to understand than the original thing. This is an incongruity that is resolved by realizing that the audience isn't actually expected to understand the math of the Dirac equation. It's also similar to Feynman's quip that "But I really can’t do a good job, any job, of explaining magnetic force in terms of something else you’re more familiar with, because I don’t understand it in terms of anything else that you’re more familiar with" [3]. Feynman goes deeper into the reasons why he doesn't try to give the expected explanation of the unfamiliar in terms of more familiar things. --Amble (talk) 19:48, 26 January 2021 (UTC)

How many different genotypes and phenotypes do you expect in this crossover?

The parents are deaf and mute, heterozygous for different genes. The mother is a light crossbreed, and the father is white. How many different genotypes and phenotypes do you expect in this crossover?

The first trait that is being looked for is complementary polygeny, and the second is additive polygeny.

For normal hearing one must have at least one dominant allele in each gene. (A-B-)

If the mother is light crossbreed that means that she has n1n1n2N2 alleles, and the father has n1n1n2n2.

If the parents are heterozygous for different genes that means that they have either aaBb and Aabb or Aabb and aaBb for hearing.

P: aaBb x Aabb

G: aB, ab; Ab, ab

aB ab aB ab

Ab AaBb Aabb AaBb Aabb

Ab AaBb Aabb AaBb Aabb

ab aaBb aabb aaBb aabb

ab aaBb aabb aaBb aabb

ratio of phenotypes: 4/16 : 12/16 (4 normal hearing : 12 deaf mute)

ratio of genotypes: 1 : 1 : 1 : 1

As for the skin color, since it is additive polygeny it should go like this:

P: n1n1n2N2 x n1n1n2n2

G: n1n2, n1N2; n1n2

n1n2 n1n2 n1n2 n1n2

n1n2 n1n2n2 n1n2n2 n1n1n2n2 n1n2n2

n1N2 n1n1n2N2 n1n1n2N2 n1n1n2N2 n1n1n2N2

n1n2 n1n2n2 n1n2n2 n1n2n2 n1n2n2

n1N2 n1n1n2N2 n1n1n2N2 n1n1n2N2 n1n1n2N2

ratio of phenotypes: 1 : 1 (1 white : 1 light crossbreed)

ratio of genotypes: 1 : 1

I'm not sure, however, that this is the right approach...

Vs⁶₅⁰⁷ 17:24, 26 January 2021 (UTC)

Looks fine to me, if I accept that n1n1n2N2 means light crossbreed. Abductive (reasoning) 00:35, 27 January 2021 (UTC)

How not to patent does help in further the advancement of science?

How not to patent does help in further the advancement of science?

In this wiki article Dame Pratibha Gai invented the in-situ atomic-resolution environmental transmission electron microscope but she decided not to patent her invention in order to further the advancement of science. Rizosome (talk) 20:30, 26 January 2021 (UTC)

It's called "sharing". ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:17, 26 January 2021 (UTC)

A patent itself 'gives its owner the legal right to exclude others from making, using, or selling an invention' so by nature is designed to prevent someone from using the invention in new, or unique ways. By forgoing the patent in the first place, the inventor signaled to the world that she wanted others to have a starting place for this particular area of science. Anecdotally, since patents are adversarial to enforce (e.g. you have to sue as the patent holder if someone infringes it), it is likely that many inventors shy away in fear of being the target of a lawsuit. Andyhill7 (talk) 21:25, 26 January 2021 (UTC)

Exactly: One could patent an invention, then announce that you will not be enforcing it, but what if you die, and your kids decide to enforce? Abductive (reasoning) 00:32, 27 January 2021 (UTC)

You could preempt this by releasing the patent to the public, as was done with the setuid bit. --142.112.149.107 (talk) 00:40, 27 January 2021 (UTC)

See also Insulin#Patent for a classic example. Médecins Sans Frontières' website has some interesting rants about cases in which pharmaceutical patents hinder the progress of medical science. Granting legal monopolies is far from the only way to encourage inventions. HLHJ (talk) 01:15, 27 January 2021 (UTC)

One of the things about intellectual property (be it copyright, patent, or trademark, which are all distinct kinds of protection people often confuse) is that intellectual property law usually requires someone to actively defend the patent or whatever. Selective defense, such as allowing some infringements but not others, is not allowed; if you allow one person to infringe on your intellectual property, and then try to stop a second person, that you allowed someone else to do so is a valid defense against infringement. So, if you own a patent, and then say "Hey, I'm going to let this charity over here infringe on my patent because they do good work, but I'm going to sue this evil corporation for the same thing because I don't like them", you can't actually do that; the corporation has a valid defense by saying you had already allowed the patent to become void by allowing the other infringement. You have the right to license your patent to other entities to produce your item, but that's a legal relationship and isn't the same as discovering someone infringed on your patent and that you knew about it and let it slide. That sort of thing can invalidate your patent. Furthermore, if you never had any intent of protecting your product through the patent, there's no reason to go through the expense of patenting it. You can just publish the designs and allow anyone to make it. The only purpose of a patent is to grant you exclusive right to control the production of your product. --Jayron32 13:17, 27 January 2021 (UTC)

I once met someone, a long time ago, who had made a very useful practical (not scientific) invention, one that potentially could have made him millions. All research pointed at the invention being novel, but he was unable to find investors. There were many abstrusely described related patents. Patent lawyers could not guarantee that a court might not find for a claimant holding one of these patents. For something I invented myself I likewise found that it was impossible to figure out whether potentially conflicting patents did or did not cover essential aspects of my invention; the descriptions were insufficiently precise or ambiguous and thereby impenetrable. Any major piece of newly developed software will contain dozens of innocently independently developed fragments that techinically speaking infringe on some trivial software patent. When it comes to a court case, the winner is usually the party with the deepest pockets.  --Lambiam 11:15, 27 January 2021 (UTC)

Which is why patent trolls are a real problem. --Jayron32 13:18, 27 January 2021 (UTC)

January 27

What is the task in the picture?

Can anyone identify what is being done here? Icemaking? papermaking? It is probably a tourist photo taken in in Java. Thanks! HLHJ (talk) 01:10, 27 January 2021 (UTC)

Did you ask the uploader? ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:27, 27 January 2021 (UTC)

Could be gutta-percha, or rubber, or some kind of latex production. Abductive (reasoning) 02:25, 27 January 2021 (UTC)

The uploader is not around and sometimes even mislabelled the country, unfortunately. Elastomer production! That's interesting and seems plausible, Java does export it. HLHJ (talk) 04:52, 27 January 2021 (UTC)

She seems to be removing galvanized or aluminium partitions between areas of white, milky liquid. HLHJ (talk) 04:53, 27 January 2021 (UTC)

Looks like a coagulating trough, see Crepe rubber for similar photos and the process. Mike Turnbull (talk) 10:29, 27 January 2021 (UTC)

Which is more massive, a quasar or a quark star?

I don't see either quasar or quark star on Orders of magnitude (mass). 97.125.232.133 (talk) 01:10, 27 January 2021 (UTC)

A quasar (as is mentioned in that article's first sentence) is an active galactic nucleus which includes a black hole that has a mass millions to billions of times that of a typical star (such as the Sun). A quark star may (rarely) be formed from the end-of-life collapse of a single star that, though having more mass than average, is still less massive than the larger stars which instead form stellar-mass black holes when they collapse. {The poster formerly known as 87.81.230.195} 90.200.40.9 (talk) 05:04, 27 January 2021 (UTC)

---

[Not sure how to relocate these references which have strayed from an earlier query above. {The poster formerly known as 87.81.230.195} 90.200.40.9 (talk) 05:06, 27 January 2021 (UTC)]

Fixed. The trick is to add {{reflist-talk}} at the bottom of the earlier query. And then for other people contributing to the thread to add their new content above that line... --142.112.149.107 (talk) 08:21, 27 January 2021 (UTC)

Or below, while adding another {{reflist-talk}} below their comments if they contain new <ref>...</ref>s. Their numbering will then start afresh at [1], though.  --Lambiam 10:17, 27 January 2021 (UTC)