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December 21

What is the pH of liquid magnesium citrate?

I've been advised to take magnesium citrate in liquid (well, powder+water -> liquid) form as a dietary supplement, but am concerned about effects on tooth enamel. The interwebs are returning mixed results, with some saying it's highly acidic and others saying it's alkaline. But none of the sources I've found seem to be reliable.

So, uh, which is it?

And if it's acidic, is there something I can combine it with to neutralize the pH without inducing a chemical reaction that makes the magnesium less bioavailable? -- Avocado (talk) 13:52, 21 December 2025 (UTC)

If you're swallowing it, it should only be in contact with your teeth for a very short period. Ingesting a significant quantity of something else may or may not effect its efficacy, but why take the chance? I suggest you just take it and then wash your mouth out with water. {The poster formerly known as 87.81.230.195} ~2025-31359-08 (talk) 13:58, 21 December 2025 (UTC)

I appreciate the thought. The thing is, people swallow sodas, too -- and yet doing so repeatedly over the course of years can damage tooth enamel.

I'm not asking for a judgement on whether or not I should take it. With enough information, I can make that decision for myself. What I'm asking for is the specific information I still need in order to make an informed decision. -- Avocado (talk) 14:16, 21 December 2025 (UTC)

This data sheet (randomly picked off the web) says pH 3.7 (page 3 section 9), so acidic. -- Verbarson  ^talk_edits 15:33, 21 December 2025 (UTC)

Thank you. That data sheet also makes it very clear that it's not for ingestion, so presumably not the same as is sold as a supplement. Is it possible there are multiple formulations of the stuff? Does the "dibasic hydrate" tacked onto the name have something to do with that? -- Avocado (talk) 16:49, 21 December 2025 (UTC)

Chemicals aren't "different" just by virtue of how they are marketed or where you get them. SDS is a factual assessment of the chemical itself, and is often conservative (or overly cautious) regarding hazards (especially if it's not specifically assayed and certified to have no harmful impurities for a certain use). For example, as a pure active ingredient, it might be too strong to use, which is why you dilute it, or its manufacturing process might leave traces of some byproduct that are generally irrelevant industrially but might specifically be a problem for human medical treatment. The "dibasic" detail simply refers to the ratio of magnesium to citrate—the dibasic salt is a 1:1 ratio of them (see magnesium citrate article for details of that aspect).

That SDS notes pH=3.7 for this substance at a concentration of 50 grams per liter. I have no idea what concentration you are making so that might be wildly off. Check your recipe, and check the ingredients of your powder to see if there are more details about what specific components it contains. DMacks (talk) 10:16, 22 December 2025 (UTC)

(edit conflict) If you dilute an acid aqueous solution by an equal amount of water, the pH increases by about 0.3. But starting from pH = 3.7 you'd have to dilute it by a factor of 2⁶ = 64 or so to reach a pH of 5.5 (3.7 + 6 × 0.3) to neutralize it sufficiently to be safe for tooth enamel.

Magnesium citrate is commercially available, at least in the US, in an aqueous solution.^[1] This seems to be 290 mg of magnesium per fluid ounce, but this is insufficient information to determine the pH. One source offers two versions of what appears to be powders: "Magnesium Citrate, sour" (pH approx. 3–4) and "Magnesium Citrate" (pH 4–5),^[2] so different formulations are possible.  ​‑‑Lambiam 10:21, 22 December 2025 (UTC)

Foodchem.com (which Wikipedia has blacklisted, so I cannot link directly, although I cannot find its entry on any of our blacklists) offers magnesium citrate with a claimed pH of 5.0–9.0; the upper value is rather alkaline. Their "min order quantity" is 500 kg, so this is presumably not immediately helpful.  ​‑‑Lambiam 10:49, 22 December 2025 (UTC)

It's blacklisted via MediaWiki:Spam-blacklist based on this request. See also Wikipedia:WikiProject Spam/Local/foodchem.com. DMacks (talk) 10:58, 22 December 2025 (UTC)

Thank you, @Lambiam -- that probably helps explain the disparity in answers I'm getting from forums and such! Your first reference seems to suggest that some forms have a neutral pH, and others are acidic.

From the Foodchem site:

Magnesium citrate (1:1) (1 magnesium atom per citrate molecule), called below by the common but ambiguous name magnesium citrate (which can also mean magnesium citrate (3:2))... It contains 11.3% magnesium by weight. Compared to magnesium citrate (3:2), it is much more water soluble, less alkaline, and contains 29.9% less magnesium by weight.

Their pH of 5-9 also puts that in the neutral range, I suppose, with variation from mildly acidic to mildly alkaline.

Sadly, supplements don't indicate which chemical formula they use. The one I have lists "magnesium (as magnesium citrate)" as the only ingredient. It does not taste sour, which if that table is correct suggests it's one of the less acidic forms. But who knows.

Maybe the safest thing to do is just rinse my mouth with not just water but a weak solution of baking soda after drinking the magnesium. -- Avocado (talk) 14:51, 22 December 2025 (UTC)

You've moved beyond what we can discuss here. Please talk to your health-care professionals to help clarify the nature of what products they are recommending, whether any other products or activities would be adviseable, and what side-effects or interactions with other products might be relevant to your specific condition and the reason for the current treatment plan. DMacks (talk) 19:54, 22 December 2025 (UTC)

Of course, not looking for actual medical advice! Just information, so that I can make informed decisions. Sorry, I guess that last sentence (musing on probable decisions) sounded like looking for advice. That was not the intent. -- Avocado (talk) 22:00, 22 December 2025 (UTC)

If I remember correctly from my school chemistry, citrate is a conjugate base of citric acid, which has no fewer than 3 carboxylic groups, each capable of donating one proton when dissolved in water. Colloquially, magnesium citrate could be used to refer to the 1:2 salt (in which citric acid has donated one proton), the 1:1 salt (in which citric acid has donated two protons) or the 3:2 salt (in which citric acid has donated 3 protons), giving a progressively higher pH when dissolved. In practice, you always have a mixture.

I suppose adding some base can increase the pH. Once it enters the stomach, enough hydrochloric acid is added that the pH goes down to about 1–2 anyway. PiusImpavidus (talk) 13:45, 22 December 2025 (UTC)

By definition, pH depends on concentration of the solution, not just what is being dissolved to make it. A larger volume of a weaker-strength recipe is a less-extreme pH (closer to neutral). Our Magnesium citrate article has several refs for the bioavailability of magnesium from it, which might have further information about pH effects. DMacks (talk) 10:03, 22 December 2025 (UTC)

December 22

Rocket trail

Rocket Dec 18, 2023

Two years ago I took this time exposure of a rocket launch (Falcoln 9, I think).The bright spot where it is coming up is where it stages. Then it gets too dim for me to see with the naked eye, but I kept exposing. The photo shows a broader, dim trail a little later, which was too dim for me to see. I'm wondering if this can be ionized gas in the ionosphere. Bubba73 ^{You talkin' to me?} 05:49, 22 December 2025 (UTC)

There were two Falcon launches on 28 December 2023, both from Cape Canaveral:

• one, with lift-off at 20:07, being a launch of the Boeing X-37 using a Falcon Heavy as launch vehicle, into high Earth orbit,^[3]
• the other of a Falcon 9, launching 23 Starlink satellites into low Earth orbit orbit at 23:01.^[4]

Was this one of these launches? If it is the first one, I think it may be possible that the second visible part of the trail is lit by direct sunlight. What does the timestamp of the photo say?  ​‑‑Lambiam 09:46, 22 December 2025 (UTC)

I did one on that day too, but this one's EXIF says December 18, 2023 at 11:38PM. It can be an hour off because I don't reset for whether or not it is daylight savings time, so it can't be direct sunlight. Bubba73 ^{You talkin' to me?} 23:20, 22 December 2025 (UTC)

Actually that was the time I was editing it. It was taken about 11:04PM, EST. I just uploaded a version with higher resolution. Bubba73 ^{You talkin' to me?} 23:30, 22 December 2025 (UTC)

The sharp trails are time exposures of the very hot glowing gases immediately as they leave the rocket exhaust nozzles; as the exhaust gases cool, they cease to glow, but can still be lit up by the glow at the exhaust nozzles, and by sunlight if the rocket has risen into it.

In the denser lower atmosphere, the expelled gases remain in a fairly tight 'tube' because they are confined by atmospheric pressure, but as the rocket ascends, atmospheric pressure drops and the exhaust can expand more widely, which I think is what you see in the second-stage trail.

In the right circumstances, when the launch is just after sunset or just before sunrise (at ground level) the Sun can illuminate the high altitude expanding gases into a beautiful "jellyfish" effect. {The poster formerly known as 87.81.230.195} ~2025-31359-08 (talk) 19:58, 22 December 2025 (UTC)

One thing, though - there is a part of the second-stage trail that is just barely visible in the time exposure. Bubba73 ^{You talkin' to me?} 00:05, 23 December 2025 (UTC)

The Falcon 9 stages at 70-80 km. The ionosphere starts about 60km. So is it shining because it is hot or because it is being ionized? Bubba73 ^{You talkin' to me?} 02:22, 23 December 2025 (UTC)

I don't think ionisation is involved. It's 'shining' because, close to the rocket, it is reflecting the glow of the second stage's exhaust nozzle, I suggest. {The poster formerly known as 87.81.230.195} ~2025-31359-08 (talk) 16:29, 23 December 2025 (UTC)

Ok, maybe, but the second-stage exhaust doesn't seem to be visible. Bubba73 ^{You talkin' to me?} 23:04, 23 December 2025 (UTC)

When you watch the video from the camera mounted on the second stage of a Falcon rocket, you don't see a glowing exhaust either. The bell of the engine glows red-hot and you see some black soot coming out, but no flames. This is expected. Like many rocket engines, it runs slightly fuel-rich. The exhaust still contains unburned or partially combusted fuel. Early in the first stage burn, this excess fuel can react with atmospheric oxygen, causing a flame, but not at high altitude. What you see from the ground is the inside of the nozzle glowing. Changing the viewing angle can change the apparent brightness.

Water vapour from the exhaust may freeze onto the soot particles, making a kind of high altitude clouds. They can be lit by anything: sunlight, moonlight, starlight, artificial light from a nearby city. I don't expect airglow to be responsible. PiusImpavidus (talk) 16:47, 24 December 2025 (UTC)

December 24

Contesting diagnosis

Hello. I recently received a diagnosis of a mental disease from a psychiatrist, which I don't believe in. I don't plan to "contest" it officially by getting a second opinion, nor do I believe I have something else than the thing I was diagnosed with. I have a fundamental issue "getting" psychiatry, and for some time now. I could go on a rambling on how it's mostly bullshit, but, to be brief, I think it's wrong to "classify" so-called "dysfunctional personalities" (which aren't comparable to real, physical impairments). As long as you're not hallucinating or committing suicide, everything's fine in my eyes. I'm searching for good counter-arguments to my view, none have been able to convince me (sure, addictions are bad, but only to those who admit they have one. The majority of so-called 'cannabis addicts' or 'alcoholics' have happy lives and might as well not care about what some doctor, themselves struggling in private life, thinks. As to the real "evidence", like brain scans etc.: we don't know how the brain functions, and that's really it. The fact that some people diagnosed with the very likely inexistant ADHD [at least in the form it is diagnosed] might indeed have any brain condition at all which is observable doesn't prove the legitimacy of this illness being forced onto kids who don't greet their teacher…). I'm mentioning this precisely because I want to be convinced, and right now I can't bring myself to believe in the "truth" of it (it feels more like useful lies). ~2025-42877-78 (talk) 14:16, 24 December 2025 (UTC)

Apparently one user thought this was trolling… Whatever, but it's not. I thought that was clear by my way of writing. ): To clarify again, before it derails, I am searching for arguments in favour of the idea of psycho-therapy and all of the useful labels they have (for more information, see my above message). I'm not some conspiracy militant or anything, just an average person somewhat frustrated by my own disbelief, in search of arguments against my current belief (which can be summed up as "it's all BS anyway"). ~2025-42877-78 (talk) 14:31, 24 December 2025 (UTC)

A number of your assertions are false. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:39, 24 December 2025 (UTC)

Considering their vague nature, that's not really helpful… (and I don't think I gave any "assertions". The closest thing to one might be that diagnoses made on the basis of supposedly dysfunctional personality traits [and given on the basis of said traits] aren't really "true", as in: a brain scan might prove that certain groups of people diagnosed with something might indeed have 'something' which alters ways of being, but that most likely doesn't correspond to all or even most people diagnosed, hence it isn't really linked to the original diagnosis, which might as well be BS). Anyway, it doesn't really help me if you just call this "incorrect" (and it seems a little too vague and argumentative to call it that). I want arguments, explanations, whatever, which will make me understand how all of this functions. Regards, ~2025-42389-73 (talk) 16:04, 24 December 2025 (UTC)

How did you end up with more than one temporary ID? ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:35, 24 December 2025 (UTC)

I'm on my phone and remove tabs after using them, I hope it doesn't cause any issues… ~2025-42776-23 (talk) 16:47, 24 December 2025 (UTC)

This entire screed is the biggest argument in favor of getting pschiatric care that I have seen in quite some time.--User:Khajidha (talk) (contributions) 16:52, 24 December 2025 (UTC)

That's far from relevant nor is it an answer to my question… ~2025-42828-40 (talk) 20:36, 24 December 2025 (UTC)

Your posting is a sequence of assertions about what you believe is wrong with diagnoses of mental disorders and contains no question. Maybe there is an implied question, like, "Where is the scientific evidence that falsifies my beliefs?". But they are insufficiently clear and specific to answer this question – if it is your question – in a meaningful way. The section Diagnostic and Statistical Manual of Mental Disorders § Criticisms may be informative (or not, depending on the nature of the implicit question).  ​‑‑Lambiam 23:39, 24 December 2025 (UTC)

Thanks very much for providing a somewhat helpful answer. To clarify: I am searching for explanations of why psychology (outside of somehow helping those who hallucinate or commit suicide, which are true issues) is in fact not 'just BS', as I currently am convinced that it is. I thought there might be some professionals to answer the question, or otherwise informed individuals, but I guess I'll have to wait a little. My 'assertions' are just vague 'facts' (the way most everyday diagnoses of psychic illness are made; the fact that these labels are born from behaviour studies; etc.) mixed with criticisms of the current state of affairs. I'm searching for an explanation of why psychology isn't just the way I described it above (or maybe counter-arguments, but that just seems like being extremely bothered, without really giving me any answers). It's a science-related reference desk, and 'medicine' was under the description too, but if the question is too vague I guess that's on me. Regards, ~2025-42898-99 (talk) 08:33, 25 December 2025 (UTC)

The Rosenhan experiment might be of interest. Notice that the article also contains accusations of fraud. In any case, you only need to have some acting talent to get a "desired" common psychiatric diagnosis. Icek~enwiki (talk) 12:54, 25 December 2025 (UTC)

Psychology is a broad scientific field commonly placed within the branch of social science, but the study of mental disorders is firmly in the province of psychiatry, a medical specialty within the branch of health science. Naturalists, observing an array of organisms, could not help but noticing similarities and dissimilarities, leading them to assigning categories such as "mammal", "reptile", "amphibian", and so on, thereby creating a taxonomic system that made further study easier. Likewise, doctors, observing an array of complaints of a not obviously somatic origin, with sometimes striking similarities and dissimilarities, have assigned categories such as "paranoia", "schizophrenia", "manic depression", and so on, trying to systematize this to enable further study. The success of their efforts has been limited. It is a reasonable assumption, and, I think, also the scientific consensus, that mental disorders (becoming manifest through a patients behaviour or through their complaints) arise from abnormalities in the cognitive functioning of the brain. Getting to understand the relation between the brain as an anatomically complex organ and its "normal" cognitive functioning is a hot field of scientific pursuit that has, thus far, yielded very few solid results. Major inroads may be needed before scientists can hope to begin to understand the relation between abnormalities in the biological functioning of the brain and abnormalities in a patient's cognitive functioning that can be labeled as mental disorders. Once this gets off the ground, I think we may expect to see a revolution in the systematizing of symptoms and diagnoses, hopefully also leading to better treatments. I fear it is not yet possible, though, to predict when or even whether this will come to be.  ​‑‑Lambiam 14:16, 25 December 2025 (UTC)

Thanks for your very informative response. What have been these small advancements made so far, concretely? Are there any labels created exclusively on the basis of this model (I know trisomy is based on biological data, but it has been identified for a long time already)? Regards, ~2025-43047-68 (talk) 16:16, 25 December 2025 (UTC)

The "limited results" I referred to are advances in the understanding of the normal cognitive functioning of the brain, such as described here. The study described there does not validate the theory known as "Spatial Computing theory", but contributes a minute piece of evidence. Another theory for which there is some, but scant, evidence is that of "Neural Manifolds"", described here. And these are not even competing theories; they operate at different levels. I am not aware of any studies showing concretely how abnormalities in the functioning of the brain as an anatomical organ lead to abnormalities in its cognitive functioning. Studies exploring this, like here for the degenerative process in the progression of Alzheimer's disease, remain at the level of speculation, as they are based on still tentative theories of the brain's normal cognitive functioning.  ​‑‑Lambiam 09:38, 26 December 2025 (UTC)

December 26

How can one tell whether a set is infinite or finite?

Some finite sets are so large that it seems that there would be no practical way to distinguish them from an infinite set. For example, (see Talk:Von Neumann universe#Could someone elaborate. ) consider the set V₈ minus any of its elements which can be defined by a first-order formula of set theory containing fewer than googolplex symbols. How could that be distinguished from an infinite amorphous set in say Kripke-Platek set theory? JRSpriggs (talk) 00:22, 26 December 2025 (UTC)

"Imagine the largest number you can think of. That number is no closer to infinity than is the number 1." - Carl Sagan. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:24, 26 December 2025 (UTC)

There are a number of ways to prove that a set is finite or infinite. A set S is finite if and only if there is some integer N such that there exists a bijection between S and the set of integers less than N. If you can construct such a bijection, then you have proven that S is finite (even if N is very large). If you can prove that such a bijection is impossible, then you have proven that S is infinite.

Another approach is to consider whether there is a bijection between S and a proper subset of S. If you can construct such a bijection, then you have proven that S is infinite. If you can show that such a bijection is impossible, then you have proven that S is finite. CodeTalker (talk) 03:28, 26 December 2025 (UTC)

Is there a reason to pose this question in the science section of the reference desk, instead of the mathematics section?

How should we interpret "can be distinguished"? For example, consider the question whether algebraic numbers can be distinguished from transcendental numbers. A mathematician may respond, Yes, algebraic numbers satisfy a univariate polynomial equation with integer coefficients, and transcendental numbers don't. But if they can be distinguished, how come mathematicians don't know whether Apery's constant, Catalan's constant or Euler's constant is transcendental?  ​‑‑Lambiam 08:53, 26 December 2025 (UTC)

I first came here for another question which was a science (biology) question. By the time I thought of this question, I had forgotten that there was a mathematics section separate from the general science section, sorry.

This is an issue for me because I was trying to write Σ₁ formulas to define addition, multiplication, exponentiation, tetration, etc. of ordinal numbers in the context of Kripke-Platek set theory. I was able to do multiplication using a Cartesian product of the inputs as an intermediate step. But to do exponentiation, I figure that I need to get the set of all finite subsets of the Cartesian product. Since powerset is not available in KP, I cannot apply separation to it. So I needed to find another way.

A model of KP with the axiom of infinity would include all of V_ω and then some. But defining an injection from the set I defined above to an initial segment of ω (i.e. a natural number) without either the axiom of choice or the axiom of power set seems impossible to me at this time. JRSpriggs (talk) 13:27, 26 December 2025 (UTC)

Such an injection would exist in V_ω, but one would need to pick it out. That is, identify it as the solution to the problem. JRSpriggs (talk) 15:33, 26 December 2025 (UTC)

Or, you could do a Ramanujan and just say "It is so because I know it is so". So much simpler than pesky prooves. -- Jack of Oz ^{[pleasantries]} 18:54, 26 December 2025 (UTC)

I get the impression that the situation with "infinite" is similar to the one with "countable", i.e. it is model dependent. Thus Skolem's paradox doesn't mean model M of set theory, which contains all of M's reals can really be put in bijection with the "true" integers. It just means that M itself doesn't contain a bijection between M's reals and M's own integers. Similarly, any nonstandard model of Peano arithmetic will contain a nonstandard element x which is "finite" in that model, but transfinite in the "true" model. ~2025-43214-75 (talk) 01:23, 28 December 2025 (UTC)

I doubt that I can be helpful, but just in case, where does the program get stuck? Is it impossible to define $\omega$ in KP + axiom of infinity, or is this possible, but KP does not provide the means to define injectivity?  ​‑‑Lambiam 11:41, 28 December 2025 (UTC)

I thought of another method to calculate the exponential function which avoids having to identify the finite subsets of a set. So I am working on that now.

The problem with the subset of V₈ is that by removing all the practically definable elements, we are left with a set which has no place to start counting. Which element should be counted as the 0-th element? Or the 1-st? Or the 2-nd? In principle, you could use the ordering given by: W_n = {W_k : k<n ∧ (n mod 2^k+1) ≥ 2^k} for n<ω. W lists every hereditarily finite set exactly once and is based on the binary numeral for n which has a 1 in each place corresponding to a k with W_k ∈ W_n. But there is no practical way to pick out the subset and the values of n involved would have more binary digits than there are atoms in the observable universe. So using the axiom of separation on it is out of the question. JRSpriggs (talk) 16:33, 28 December 2025 (UTC)

What is the problem in using $\exists \,n\in \omega :n\sim S,$ in which $\sim$ means that a bijection between the arguments exists?  ​‑‑Lambiam 17:43, 28 December 2025 (UTC)

That says that n is finite in the ambient model of ω, but in the case of a nonstandard model sitting inside some set universe U, the same n could have infinite cardinality in U. So "infinite", like "countable", not absolute. That's all I'm getting at here. ~2025-43214-75 (talk) 21:02, 28 December 2025 (UTC)

To Lambiam: According to Lévy hierarchy#Σ1-formulas and concepts "x is countable." is Σ₁.

$\exists \,n\in \omega$ is OK. But the bijection would be a problem. And specifying $S$ would be an even bigger problem. JRSpriggs (talk) 21:20, 28 December 2025 (UTC)

In $n\sim S,$ the variable $S$ is meant to stand for the set whose finity is being questioned. For the bijection, can't we define $S\sim T$ to mean, $\exists R\subseteq S\times T:(R\circ R^{\circ }\!=I_{S}\,\land \,R^{\circ }\!\circ R=I_{T})$?  ​‑‑Lambiam 08:50, 30 December 2025 (UTC)

December 28

Quantum entanglement detection

I have an electron in a box sitting in my lab, with a (for now) unknown spin state. I suspect my electron might be entangled with another electron somewhere, maybe in the same building, but maybe even in another galaxy. I prefer to not measure the spin without knowing what else might be connected, but it's ok if I do have to take a measurement. Either way, is there a way to tell? Is there a way to locate the entangled counterpart? Thanks. ~2025-43214-75 (talk) 01:26, 28 December 2025 (UTC)

Have you read our article Quantum entanglement? MinorProphet (talk) 14:42, 28 December 2025 (UTC)

This would be a very useful invention for locating one's keys: keep one entangled particle of a pair in a box on the key chain, and the other one on a chain around your neck. If an entangled particle can be located by consulting its partner in crime, one might also use this to store an almost arbitrary amount of information in one particle by placing it in a specific location uniquely associated with the information, and retrieve the information from the other particle. But the "No-communication theorem" states that it is impossible to use entanglement to communicate information.  ​‑‑Lambiam 17:39, 28 December 2025 (UTC)

MinorProphet, yes I did look at that article a while back, and just read it again. Lambiam's point is good that being able to locate the entangled particle would give a way to communicate, hmm. Or for that matter, the entanglement itself. If someone at the other end measures the spin of the other electron, then it's no longer entangled, so being able to detect that would communicate 1 bit. Hmm. This is still awfully suspicious. What if the other electron is inside a black hole? If that's even possible. Double hmm. ~2025-43214-75 (talk) 05:35, 29 December 2025 (UTC)

Anything is possible, given an infinite amount of money, patience and the control of time itself. Triple hmm, which is not like the triple jump: hop, skip, and damn it's another black hole. MinorProphet (talk) 13:49, 31 December 2025 (UTC)

Why does salt lower the freezing point of water, and why is there a limit to how cold it can make ice get?

When salt is spread on icy roads, it lowers the freezing point of water and causes ice to melt. At a molecular level, how does dissolved salt interfere with the formation of an ice crystal lattice? Why does this process stop being effective below a certain temperature (around −21 °C for sodium chloride)? Additionally, how do other de-icing substances like calcium chloride differ in their effectiveness, and why are they able to work at lower temperatures? This! ~2025-43471-36 (talk) 09:51, 28 December 2025 (UTC)

See Eutectic system. Dolphin (t) 09:57, 28 December 2025 (UTC)

Also see freezing-point depression and road salt. PiusImpavidus (talk) 10:26, 28 December 2025 (UTC)

Salt disrupts ice formation by dissolving into ions (Na⁺, Cl⁻) that get between water molecules, preventing them from forming the rigid lattice needed for ice, a phenomenon called freezing point depression. This process becomes less effective below about -9°C (15°F) for sodium chloride (NaCl) because there isn't enough liquid water to dissolve the salt, or the water-salt solution becomes too concentrated to freeze. (as a temporary account|he/him) ~2025-43569-93 (talk) 17:19, 30 December 2025 (UTC)

December 29

How do black holes get bigger?

By "get bigger", I mean the radius of the event horizon increases from the perspective of someone on the outside. From outside the BH, it's impossible to literally throw stuff in, right? You just see the stuff get more and more redshifted as it approaches the EH, but you never actually see it cross. Does the EH grow just from acquiring more mass on the -outside-? Do you see the stuff you threw disappear as the EH crosses it, or what? By "throw stuff in" I mean large objects like stars, I guess. Thanks. ~2025-43214-75 (talk) 18:51, 29 December 2025 (UTC)

You are right and an external observer will never see any downfalling object crossing the event horizon. Ruslik_Zero 19:45, 29 December 2025 (UTC)

Wouldn't a downfalling object disappear once it crosses the event horizon? ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:28, 29 December 2025 (UTC)

Not to an outside observer. Because of the relativistic effects of gravity, although from its own point of view the object does cross the event horizon quite quickly, from that of the outside observer the object progressively slows (or rather, time slows for it) the closer the object gets to the event horizon, such that the 'moment of crossing' never happens. Both points of view are equally "real", because Relativity.

This, incidentally, was a crucial plot point in Frederik Pohl's classic sf novel Gateway. The main protagonist and three others, including his lover, are in two linked spaceships about to fall into a black hole. They plan to escape by crowding into one ship and ejecting the other into the black hole, thus propelling theirs away from it, but the plan goes wrong and only the protagonist is in the ship blasted free. Thereafter the seemingly doomed, infalling ship remains permanently visible, crippling him with guilt which informs his actions in the sequels. {The poster formerly known as 87.81.230.195} ~2025-31359-08 (talk) 21:13, 29 December 2025 (UTC)

Yay, let's hear it for Frederik Pohl, one of the best: The Coming of the Quantum Cats deals partially with similar concepts. If you like that sort of thing, obvs. MinorProphet (talk) 13:13, 31 December 2025 (UTC)

Let's say some glowing object is heading for the event horizon. It continues to emit photons. If I'm reading you right, to the observer it never goes past the event horizon. But to itself, it does. Is it in two places at once? ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:38, 30 December 2025 (UTC)

"At once" doesn't apply, because in a relativistic setting gravitational time dilation means that there is no longer any "moment" that is definitively the same in both inertial frames (those of the object and the observer), or to put it another way, simultaneity breaks down. You can't think of relativistic events in terms of the classical physics our minds are used to; you just have to trust the mathematics. Yes, the object (which may contain an observer) and the distant observer experience two contradictory things, but neither is "more correct" than the other.

In the extreme gravitational field and orbital velocities close to/at a black hole's event horizon, these relativistic effects become extreme: what to the object very close to the event horizon takes a millisecond, to the more distant observer may take years. This is why Pohl's protagonist (who because of his mission's success and his survival became very rich) came to realise that he actually had many years to try to discover a means of rescuing the others. Of course, alien science and technology (such as the ships) more advanced that our own is involved in all this. {The poster formerly known as 87.81.230.195} ~2025-31359-08 (talk) 01:16, 30 December 2025 (UTC)

Ruslik_Zero, yes, that's what I mean. If outside observers never see downfalling objects cross the EH, how do they see the BH itself get bigger? Like the SMBH's in the centers of galaxies are big because they spent billions of years eating stars. All that mass is now inside their EH's. So from an outside perspective, the stars got to the inside without actually crossing. What happened? Tx. ~2025-43214-75 (talk) 22:00, 29 December 2025 (UTC)

You do not actually see the black hole; you see its effects on things around it, especially the bending of light from stars behind it. I think that the red-shifting of the in-falling matter happens relatively quickly as seen from outside. So the last photons to escape will probably all do so in short order. Any that are long-delayed may be indistinguishable from the Hawking radiation. JRSpriggs (talk) 01:18, 30 December 2025 (UTC)

Hawking radiation is negligible for large BH. What I'm wondering is how very big BH got that way. Like maybe today it has a billion solar masses. How many did it have a billion years ago, if it's been growing over that time? Growing = mass inside event horizon is increasing. But how can that happen if infalling matter can never be observed to cross over? ~2025-43679-43 (talk) 08:24, 31 December 2025 (UTC)

One way is by two smaller black holes merging: we have already detected mergings of stellar-mass black holes via observations of gravitational waves. Many galaxies contain supermassive black holes at their core, and galaxies also merge, as would the black holes within them. The origins of some if not all SMBHs may lie in the Big Bang itself, and Dark matter likely has a role: theorising and observations are actively striving to clarify these aspects of the history of the Universe, but it may take some years/decades or longer before we gain a fuller understanding. {The poster formerly known as 87.81.230.195} ~2025-31359-08 (talk) 22:32, 31 December 2025 (UTC)

December 30

Weather questions

1. Why doesn't every country in the world have historical weather data available through national meteorological organization's website?
2. Why doesn't every country in the world have climate normals published by national meteorological organization?
3. Is there any European country that measures snowfall similarly to what US, Canada and Japan do, separate from snow depth and precipitation?

--40bus (talk) 20:59, 30 December 2025 (UTC)

Not all countries have the means to fund a full-scale meteorological service. And there may be more pressing task for what little funding is available than to publish data, especially historical data (which may be sparse and/or pre-digital, if available at all). Also, the idea of open science/open data is fairly new. Many countries may be encumbered by contracts with respect to some data, both historical and present, especially if they contract with private weather services for some of the data.

I don't understand your last question. From what I know, e.g. Switzerland has excellent records, at least for modern times, but it's not clear to me what kind of data you expect. --Stephan Schulz (talk) 21:17, 30 December 2025 (UTC)

I ask whether daily, monthly and seasonal snowfall is recorded at weather stations in any European country like it is done in US, Canada and Japan. And I think that US and Canada, as they are large and developed countries, have historical weather data dating back to 19th century in uniform format throughout the countries (such as by NOAA in US and Environment Canada in Canada) and I think that there should also be a similar application in Europe, a joint website between countries with historical weather data. --40bus (talk) 23:12, 30 December 2025 (UTC)

And are search engines unavailable to you somehow? Anyone wanting to answer this will search for "european historical weather data", just like you could, and find lots of useful pointers to useful data. --jpgordon^{𝄢𝄆𝄐𝄇} 23:44, 30 December 2025 (UTC)

Why do you keep asking "Why..." questions to things that don't have a "Why" to them? 1. Why should they? 2. Why should they? 3. huh? --jpgordon^{𝄢𝄆𝄐𝄇} 22:04, 30 December 2025 (UTC)

It seems fitting that "Why" comes from out of left field. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:46, 31 December 2025 (UTC)

Fitting? Why, if I may ask?  ​‑‑Lambiam 09:44, 31 December 2025 (UTC)

See Out of left field and Who's on First? ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:36, 31 December 2025 (UTC)

I don't give a damn. --User:Khajidha (talk) (contributions) 18:57, 31 December 2025 (UTC)

January 2

Old paper of Edward Witten

Several years ago, I have found a freely accessible scientific paper of Edward Witten, clearly written in a pre-LaTeX text system, so probably still from the 1970s or early 1980s. I do not remember what it was about, and I do not find it anymore, but it included some interesting mathematical notation I would like to recover. Does anybody know any Witten paper which was written before the invention of LaTeX and which is (maybe still) online? --KnightMove (talk) 17:30, 2 January 2026 (UTC)

Bit of a long shot. How about this one from 1979? Otherwise you'll have to sift through his publication list. More typewritten stuff in this list. --Wrongfilter (talk) 17:39, 2 January 2026 (UTC)

That has helped already, thank you very much! --KnightMove (talk) 15:55, 3 January 2026 (UTC)

January 3

chemical treatment of cancer

chemical treatment of cancer ~2026-59813 (talk) 22:34, 3 January 2026 (UTC)

Read Chemotherapy. AndyTheGrump (talk) 23:26, 3 January 2026 (UTC)