Wikipedia:Reference desk/Archives/Science/2021 August 27

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August 27

Why perpetual motion considered as free energy if it clearly depending on gravity?

Hydroelectric energy or solar energy never considered as free energy, but why perpetual motion considered as free energy (generator) if it clearly depending on gravity? Rizosome (talk) 04:00, 27 August 2021 (UTC)

Don't you think gravity is free? ;) --CiaPan (talk) 04:35, 27 August 2021 (UTC)

A perpetual motion machine is a machine that does work without an energy source. If it ain't free energy (violating the law of conservation of energy), a "perpetual motion" machine is fake – by definition.  --Lambiam 06:51, 27 August 2021 (UTC)

• A perpetual motion machine works in cycles - meaning if some sort of weight falls down during part of the cycle, that is legit, but it has to go back up at a later point. As gravity is a conservative force, this means that the average energy given during a cycle is zero, so no perpetual motion machine can use gravity.

Solar is a different thing. Are drinking birds perpetual motion machines? Tigraan^{Click here for my talk page ("private" contact)} 08:29, 27 August 2021 (UTC)

Read the second sentence of the article you just linked. --Jayron32 15:42, 27 August 2021 (UTC)

I know. I somehow misread the original post as thinking that the use of solar is kosher for a "perpetual motion machine", so that was a rhetorical question to test their definition. After all, a drinking bird is a machine, and it is in motion, and perpetually if the conditions are kept constant; but of course it does not violate the second law. Tigraan^{Click here for my talk page ("private" contact)} 08:39, 31 August 2021 (UTC)

The Sun generates heat by "burning" hydrogen. The supply of hydrogen will eventually run out, but not for about 5 billion years, so while from a human perspective it is effectively an unlimited source of energy, its not a perpetual motion machine. Solar energy gets its power directly from the sun, and hydro-electricity gets it indirectly (the heat from the sun evaporates water, which condenses, falls as rain, collects in rivers or reservoirs, and flows downhill to turn the turbines in the power station). Because the sun, as stated, is not a perpetual motion machine, things powered by the sun are not perpetual motion machines either. A perpetual motion machine (if it existed, which it doesn't) produces energy by e.g. water flowing downhill, and then uses that energy to move the the water back up hill. But it will always take more energy to move the water back uphill than you can get from it flowing downhill (because you can't create energy from nothing, and some is always wasted due to inefficiencies) - which is why perpetual motion machines are impossible. Iapetus (talk) 08:54, 31 August 2021 (UTC)

How sensitive can microphones be?

Im interested in ways of detecting insects. Like if an ant walks on a linoleum floor, could a micrphone detect it, now or someday with foreseeable technological advances? What if the ant walked on a "clangy" metal table?Rich (talk) 04:13, 27 August 2021 (UTC)

Reputedly the noise floor of sensitive microphones is at the point where they are detecting the brownian motion of the air molecules nearby. Follow this up http://www.noiseaddicts.com/2008/07/sensitive-microphone-heartbeat-of-snail-ants/ Greglocock (talk) 06:34, 27 August 2021 (UTC)

Some spiders living in caves detect their prey by feeling with their legs rock vibrations caused by insects walking. So this level of sensitivity should be possible, I think. --CiaPan (talk) 06:51, 27 August 2021 (UTC)

Hearing worms move through soil is not even cutting-edge (several species of bird can do it, and 2010-vintage hardware was used in the experiments related to it). DMacks (talk) 17:49, 27 August 2021 (UTC)

Snakes

There are many magazine articles that snakes never bite human beings unless someone puts foot on it.

So, all poisonous snakes will never attack a human being without provocation? --Marvel Avenger (talk) 05:05, 27 August 2021 (UTC)

Snakes bite either to subdue prey or to defend themselves. To the best of my knowledge, none of the venomous snakes, extant or recently extinct, prey on humans. Therefore, if a person is bitten, that would be either because the snake perceived a person as a threat ("provocation" as you put it), or as a case of mistaken identity (such as when feeding a captive snake, or having the odor of its food on your hands or clothes). Even when a snake does perceive a person as a threat, it doesn't always inject venom when it bites. It may feint a strike, or it may strike without injecting a venom (a so-called "dry bite") and not waste the venom. A feint or a dry bite is more than a sufficient deterrent in most cases. It will rather likely inject venom if you step on it, though. Dr Dima (talk) 06:15, 27 August 2021 (UTC)

So, all poisonous snakes will never attack a human being without provocation? YES Rizosome (talk) 06:17, 27 August 2021 (UTC)

Rhizome, you are trying to elicit a binary (always/never) result from a class of complex and variable real-world situations. The macro-world simply does not work like this – real-world alternatives for nearly all situations are generally more or less probable over a spectrum from "almost always"/"almost never", through "more often"/"less often" to "maybe"/"maybe not", and many if not most situations have three or more different possible outcomes. {The poster formerly known as 87.81.230.195} 2.122.179.94 (talk) 15:43, 27 August 2021 (UTC)

Don't put your confidence in a magazine article. Almost any animal that can hurt you will attack if it feels strongly threatened and there is no safe retreat. It is true that snakes generally prefer to retreat. A retreat is evidently not possible if you step on one, but the animal being startled may already set off the "fight" branch of the fight-or-flight response. Black mambas are feared for their aggression, and many people who were bitten by one did not step on it or willfully provoked it before the attack.  --Lambiam 06:40, 27 August 2021 (UTC)

Snakes may perceive walking through their habitat as "provocation" so we need to be careful with words because that is not what most English speaking human beings understand that word to mean. The same is true of rattlesnakes if they perceive their babies to be at risk. Cullen³²⁸ Let's discuss it 06:45, 27 August 2021 (UTC)

And then, though it's not the happiest topic in the world, there are those (thankfully exceedingly rare) cases of a constrictor preying on a human being [1]: bites typically will happen, incident to such an attack [2], but it's one of the few scenarios in which you've been bitten by a snake (and a massive one at that) and yet that is not your biggest problem at the moment... SnowRise ^{let's rap} 10:35, 27 August 2021 (UTC)

Snakes cannot understand intent. The snake doesn't know if you are or are not trying to harm them. Most snakes will flee as a first line of defense from predators, and if they see you as a threat, they will first try to flee. If they perceive that they cannot flee, because you accidentally step on them, or perhaps come really close to stepping on them, they may reflexively bite you. This is true even if you mean no actual harm. The snake does not know this. Snakes do not seek out humans to bite them, they try to avoid people but may bite if they feel threatened by a human (even if the human doesn't actually intend harm). Some snakes do use threat displays as a means to scare off threats rather than run first, famously Agkistrodon piscivorus, a very common snake in the U.S. known as a "water moccasin" or a "cottonmouth", will often lunge threateningly at people to frighten them, though if the person themselves flees, bites are rare. --Jayron32 12:05, 27 August 2021 (UTC)

The rattlesnake is lucky to evolve a rattle that tells you where the tail is, when the snake is hard to see. Not sure how effective that is when the snake is sleeping. Sagittarian Milky Way (talk) 13:51, 27 August 2021 (UTC)

From what I was taught, in general, snakes (at least my local Aussie ones) do indeed prefer to flee rather than attack a human if (as Jayron rightly notes) the snake perceives that they have that option. That said, there are exceptions. As an Australian, whilst most of our snakes generally avoid attacking humans, the King brown snake (Pseudechis australis) is something of an exception; it has been known to bite even humans who are sleeping. Eliyohub (talk) 07:32, 1 September 2021 (UTC)

Colorless or colored elemental fluids

What causes nitrogen gas (or hydrogen or oxygen) to be colorless whereas fluorine, chlorine and bromine are colored? Sandbh (talk) 13:15, 27 August 2021 (UTC)

If the gas appears colored to you, that is because the molecules of the gas have absorption bands in the visible range; which is to say that when light interacts with those molecules, the light of certain wavelengths is absorbed; as a result the remaining light that is passing through the gas no longer is colorless (white light) but now has a color, created by your eyes receiving some wavelengths of light, but not others. All gases, even colorless ones, absorb light, however if they don't absorb sufficiently in the visible spectrum, and instead primarily absorb in the UV or IR ranges, the gas will appear "colorless" because all of the wavelengths of light you can see are still reaching your eyes. --Jayron32 13:24, 27 August 2021 (UTC)

And along with most other sighted terrestrial organisms, our eyesight has evolved to see in the part of the electromagnetic spectrum in which most commonly encountered gasses, principally nitrogen, are minimally light-absorbent. Life forms that evolved in a mostly chlorine (or chlorine compound) atmosphere (assuming that to be possible) would likely evolve sight in e-m bands in which chlorine (or its prevelant compound) is non-absorbant but rare gasses like nitrogen and oxygen might appear coloured. {The poster formerly known as 87.81.230.195} 2.122.179.94 (talk) 15:49, 27 August 2021 (UTC)

It is quite surprising (to me) how narrow the wavelength range is that we can see: the article Electromagnetic spectrum illustrates this well. Both Infrared spectroscopy and Ultraviolet spectroscopy are of use to chemists who wish to identify materials, since as Jayron32 says, molecules can absorb right across that range. The article on chlorine, for example, has its absorption lines which lie in the visible range shown in its Chembox. Mike Turnbull (talk) 16:35, 27 August 2021 (UTC)

Actually, what I've just written may be misleading: the spectrum shown for chlorine is its emission spectrum, not its absorption spectrum and nitrogen has many emission bands in the visible range, so based on that it might be thought it should be colored also. The relationship between absorption and emission depends on the Einstein coefficients. Mike Turnbull (talk) 16:49, 27 August 2021 (UTC)

Not exactly; the Einstein coefficients determine the intensity of the absorption and emission (stricto sensu, the determine the likely hood that an absorption or emission event will occur). The actual color of the lines exactly matches up, so if a black line appears at a specific color in an absorption spectrum, that color line will appear in the same element's emission spectrum. --Jayron32 17:04, 27 August 2021 (UTC)

Chlorine's emission spectrum

Nitrogen's emission spectrum

Thanks for the clarification, Jayron32. So now I'm becoming puzzled as to why nitrogen is colorless, given all the lines in comparison to chlorine! ;-) Mike Turnbull (talk) 17:50, 27 August 2021 (UTC)

Because the Einstein coefficients are so low. Basically, there are lots of absorbed colors of light, but the likelihood that an actual absorption event takes place is very very low, making it essentially colorless. In high-energy environments (like in stars or flame or things like that), there's usually enough emission/absorption so you can easily see the spectrum. In room temperature, atmospheric pressure, some substances have such low likelyhood of absorbing the light that you don't see any meaningful colors. --Jayron32 17:56, 27 August 2021 (UTC)

The colour can be helped along by condensing the gas (which, of course, greatly increases the density and hence the likelihood of getting an absorption event). That's why liquid oxygen shows a clear blue colour whereas gaseous oxygen doesn't (and here it's quite bad because the absorption event involves pairs of O₂ molecules which are harder to get together as a gas). Double sharp (talk) 03:17, 28 August 2021 (UTC)

Thank you!

I did some more research further to Jayron32's answer; the subsequent discussion; and clues provided therein. An explanation somewhat related to that of Jayron32 is provided by this site.

Noting H, N, O are relatively light, strongly bonded molecules, the site mentions that:

"The inviting blue of a mountain lake or a sea is unique in nature, in that it is caused by vibrational transitions involving hydrogen bonding…

…Most molecules have vibrational energies that are lower in frequency (longer in wavelength)…falling in the range of far infrared or thermal vibrations rather than in the visible light range. The hydrogen atoms in water are very light, and the bonds between hydrogen and oxygen very strong, which shifts them to higher frequencies (with shorter wavelengths), with overtones that lie in the range of visible light. Just as the pitch of a vibrating string is raised if the mass of the string is reduced and the tension applied to the string is increased, so too the highest-frequency vibrations occur with the lightest atoms (hydrogen) when most strongly bonded (to oxygen in water)."

For heavy water, this is heavier enough than ordinary water to apparently shift its absorption spectrum to higher wavelengths outside of the visible spectrum of light (never mind H bonding). Heavy water is thus colorless. I guess this is why H₂O₂ hydrogen peroxide is also colorless.

Things get confusing at this point.

In this JChemEd article[3] on why liquid oxygen is blue, the author writes:

"A substance appears colored to the human eye when it absorbs a portion of the visible spectrum (4000 to 7000 A). Since the energy of a quantum of visible light is very much greater than that required to excite vibrations and rotations in molecules, absorption of visible light can normally be traced exclusively to the excitation of an electron from one energy level (state) to another."

It seems then that the lightness of H, N, O or the strength of their bonding (436 to 945 kJ/mol) compared to the fluid halogens (159 to 242) doesn't have anything to do with their appearance(?), if this is instead related to "excitation of an electron from one energy level (state) to another."

For H, this forum[4] says:

"Molecular hydrogen does not begin to absorb until you get into the vacuum ultraviolet. The hydrogen atom in its ground electronic state (n=1) also does not absorb any light until you get into the vacuum ultraviolet. For the atom, the first absorption is Lyman alpha (121.5 nm). For molecular hydrogen, the first absorption is at even shorter wavelengths (ca. 110 nm)."

For N, this journal article[5] notes, "Strong absorption bands in the range 80–100 nm shield the Earth's surface from the extreme ultraviolet (XUV) part of the solar radiation."

For O, this JChemEd author[6] mentions the six lowest energy states of oxygen lie in the IR, near visible, or UV, and that, "There are no other states of O₂ which can give rise to absorption bands in the visible region." For liquid O₂ they mention the possibility of one photon simultaneously elevating two electrons on two different molecules to excited states…twice the energy required to excite a molecule…is possessed by a photon at 6340 Å (= red). I gather such a possibility is more likely in liquid oxygen, where the molecules are closer together, perhaps in the form of nano-meter scale clusters of solid oxygen.

Ozone O₃ is pale blue, and heavier than oxygen, and has a lower average bond strength of ca. 300.[7]. I'd've thought that being heavier and having a weaker bond presumably shifts its absorption spectrum to lower (visible) wavelengths. Instead it appears to be due to electron excitations, perhaps in the same manner as what happens to liquid oxygen?

The halogen fluids remain puzzling, including the violet color of iodine vapor. They're colored and get progressively darker going down the group. If this isn't related to their weak bonds and increasing atomic weights, but instead to electron excitations, why is it they're colored whereas H, N, O aren't, and why does the color get progressively darker?

Our article on Absorption spectroscopy says:

"The frequencies where absorption lines occur, as well as their relative intensities, primarily depend on the electronic and molecular structure of the sample. The frequencies will also depend on the interactions between molecules in the sample, the crystal structure in solids, and on several environmental factors (e.g., temperature, pressure, electromagnetic field). The lines will also have a width and shape that are primarily determined by the spectral density or the density of states of the system."

Unfortunately, this doesn't help.

I know that in going down a group such as the halogens, atomic radius increases, the pull on the valence electrons from the nuclear charge becomes less strong; and that (presumably) their valence electrons become easier to excite.

But what is that influences the energy levels of the excited states, which determine the colors, or lack of colour, involved? Is it related to the atomic radius, and to Z, both of which figure in the Rydberg formula and the Rydberg constant?

I'm asking these questions in the context of bringing nonmetal up to FAC standard. It's currently undergoing peer review. Sandbh (talk) 07:22, 28 August 2021 (UTC)

The problem is that the Rydberg formula ONLY works for two-particle systems, essentially hydrogen or any ion that contains only a single electron, like Li²⁺ or something like that. Once you deal with real atoms and molecules, there is no meaningful mathematical solution; it becomes a quantum n-body problem, and there's no way you can tie the observed spectrum to any mathematical formulation which would predict that pattern of lines. Heck, even the Schrödinger equation has only been quantitatively solved for hydrogen atoms. For any larger atoms or molecules it can only be approximated. --Jayron32 20:44, 29 August 2021 (UTC)

List of main IR spectroscopy bands

In case that sounds too pessimistic, it is worth pointing out that chemists regularly look at IR spectra, note the presence of an absorption band and say "this shows that the molecule contains such-and-such functional group". Carbonyl groups are especially useful in this context. On the other hand, as Jayron32 implies, the details of the so-called "fingerprint region" are usually a mystery. Mike Turnbull (talk) 10:36, 30 August 2021 (UTC)

Yes, but the IR bands are determined empirically. Like "We know this is the carbonyl band because we've checked a bunch of C=O containing compounds and found this band and worked out it comes out here". There's no way to work out the electron energy levels ahead of time and mathematically figure out that there should be a C=O stretch band at that location, it's purely a result of experimental results. --Jayron32 11:54, 30 August 2021 (UTC)

Cutting steel temperatures.

You know the metal spinning wheel used to cut metal bars? I believe it is easier to cut metal when the bars are hot. Someone said "not all metals." So some metals are actually easier to cut at room temperature? I find this hard to believe. Both the wheel and the bars get hot. 67.165.185.178 (talk) 17:29, 27 August 2021 (UTC).

You seem to be talking about Grinding (abrasive cutting). That article may be a good place for you to do your research. --Jayron32 18:02, 27 August 2021 (UTC)

What was the earliest time in the history of the universe for which there is direct evidence?

In the context of the Big Bang theory, what is the earliest time (or temperature scale, or the smallest scale) for which there is direct evidence? The CMB shows that the observable universe definitely existed at temperature of about 3,000 K, with the observable universe being something like 40 million light years in radius [8]. I would say that the observed isotopic ratios in the present universe constitute direct evidence that something like Big Bang nucleosynthesis occurred, which shows that the entire observable universe was hot enough for nucleosynthesis to occur. This apparently constitutes indirect evidence that the universe was at such-and-such scale factor at that time, but maybe it's direct evidence.

But is the Big Bang nucleosynthesis epoch the earliest stage for which we have direct evidence? In popular science presentations, it's typical to point to particle colliders like the LHC to show that we have direct evidence of how the universe behaves at energy scales and densities which would correspond to the Electroweak epoch. We surely know that portions of our observable universe can reach those energy scales with no issue. However, do we actually know that the entire observable universe was once at that electroweak temperature (and scale, and time corresponding to the Big Bang theory)?

Put another way, let's imagine we had a particle collider which could probe the Grand unification scale. That would show that parts of the universe can get that hot, but it alone doesn't demonstrate that the observable universe actually was that hot at some point in the past. So, what's the earliest time for which we can definitely say that the universe existed? BirdValiant (talk) 18:13, 27 August 2021 (UTC)

It depends on what your standard for "direct evidence" is. For example, the fact that nuclei exist could be seen as direct evidence that nucleosynthesis happened at some point. If you mean "what is the earliest we can look out into the sky and see stuff from", that's the CMB, which represents a sort of visual horizon earlier than which we can't see. That's because the CMB is the oldest light we can see, we literally can't detect anything else from any earlier point. --Jayron32 18:21, 27 August 2021 (UTC)

Detection of the cosmic neutrino background would bring us to 10 billion Kelvin or one second after the Big Bang. The PTOLEMY experiment is being developed to do that. --Wrongfilter (talk)

@Wrongfilter: So at the current state of the art, would it be correct to say that the earliest time in the chronology of the universe for which we have direct evidence is 370,000 years after the theoretical beginning of the Big Bang? And also that we have direct evidence that something like Big Bang nucleosynthesis occurred, which then is indirect evidence that the universe was at the scale and temperature regime of a few minutes after the theoretical beginning? BirdValiant (talk) 18:54, 27 August 2021 (UTC)

Disregarding all philosophical subtleties as to what constitutes "direct" evidence, I'd say yes. Being pedantic, the direct evidence is that there is a sea of photons now at a temperature of 2.75 K. That these photons were last scattered at a temperature of 3000 K is inferred, and I leave it to you whether you want to call that direct or indirect. --Wrongfilter (talk) 19:09, 27 August 2021 (UTC)

@Wrongfilter: Beyond the cosmic neutrino background (for which there is already indirect evidence in the CMB), are there any avenues of research on the horizon which could push the timeline back further from the 1 second point? I'm reminded of the B-mode polarization from 2014 which pretty much has been discounted due to cosmic dust, but maybe primordial gravitational waves could be found somehow. I guess that if we had a really good model for baryogenesis and tested it in other ways, and then used it to predict the predominance of matter over antimatter, that would be somewhat similar to the way our pre-existing understanding of nuclear fusion combines with primordial isotope data to provide evidence of BBN. Or maybe if cosmic strings or magnetic monopoles or something were found, that'd be something too. But maybe there are other avenues of which I am not aware. BirdValiant (talk) 00:09, 28 August 2021 (UTC)

There is no evidence, direct or otherwise, that is accepted by Young Earth creationists. They believe that the universe was created no more than 10,000 years ago, and that the Creator stuffed the young Earth with newly created fossils to make it appear much older. The cosmic background radiation from which scientists extrapolate early events was created at the same time as well. In short, the Creator created a new universe that was indistinguishable from one that already had a history of more than 13 billion years; any "evidence" that makes scientist adhere to the Big Bang theory was actually created relatively recently.  --Lambiam 20:49, 27 August 2021 (UTC)

This as a science reference desk, not a theology reference desk or forum. What young Earth creationists believe is of no relevance here. AndyTheGrump (talk) 00:34, 28 August 2021 (UTC)

Is there direct evidence that the universe was not created 2.5 seconds ago (or at 3pm last wednesday), along with all of its inhabitants complete with their memories? I think there is a name for this puzzle but I don't know what it is. Also, idk much physics, but I had the impression that at very high densities like in the big bang era, the concepts of space and time didn't really apply like we normally think of them. Something similar happens near black holes. 2601:648:8202:350:0:0:0:2B99 (talk) 03:34, 28 August 2021 (UTC)

The name you want is the omphalos hypothesis. It is of no scientific value because it cannot be used to make any verifiable predictions, so I suggest there's no reason to discuss it here. --184.144.99.72 (talk) 04:46, 28 August 2021 (UTC)

The theory that the universe has existed for more than two days does not produce much in the way of verifiable predictions either.  --Lambiam 07:58, 30 August 2021 (UTC)

Thanks, five-minute hypothesis (linking to part of that article) was the version I had in mind. I dunno that verifiable predictions are that important for a theory, if it has good explanatory power (e.g. GR's predictions about black hole interiors are unverifiable because there is no way to make observations there and get the data out, but GR is still a good theory). The 5-minute hypothesis of course doesn't explain much either ;). 2601:648:8202:350:0:0:0:2B99 (talk) 09:07, 28 August 2021 (UTC)