Hello, I apologize for the inconvenience, but I'd like to know the list of things that the scientific community criticizes about current cryonics, please. I know that there's the excessive deterioration of neurons, the failure to preserve the excitability threshold of synapses; moreover, there's a hypothesis according to which a certain structure of molecules inside synapses must also be preserved (as we're not sure that this hypothesis is false, we'll have to converse with it as a precaution) ; and else...? 78.240.199.90 (talk) 16:01, 29 June 2025 (UTC)[reply]

IP editor: The section at Cryonics#Obstacles to success has quite a lot of criticisms. Other contributors here may suggest more. Mike Turnbull (talk) 16:58, 29 June 2025 (UTC)[reply]

If the business is not strongly regulated, with good oversight, any bunch of con artists can start a cryonics firm, swearing by all that is holy that they are industriously and meticulously applying best practice using the most advanced science and technology this side of the Milky Way, while not doing much more than keeping up an impressive Potemkin village.  ​‑‑Lambiam 17:41, 29 June 2025 (UTC)[reply]

• There is no inconvenience, OP. We invite questions such as yours. Rest easy. -- Jack of Oz ^{[pleasantries]} 21:40, 29 June 2025 (UTC)[reply]

Imagine that Earth has no clouds and has no topographic relief, so all locations see a clear sky at all hours <edit>and all locations at a given latitude experience the same duration of daytime daily</edit>. Would the poles have the largest amount of time in which any part of the Sun is above the horizon? I'm guessing so, since sunset/sunrise is so extremely slow, and we're counting any moment in which any part of the solar disc is above the horizon. But on the other hand, I wonder if the solar pattern related to the analemma has something to do with this, and because it's nowhere near symmetrical north-south, perhaps it's not as simple as I was guessing. Nyttend (talk) 07:40, 2 July 2025 (UTC)[reply]

It's probably not as simple your current understanding. You might look at Equation of time which presents another simpler view of it, and detailed reasons for why it happens. I investigated the topic when I noticed, when commuting at the same time each day, that the earliest sunset does not happen on, or even close to, the shortest day.

It probably doesn't affect the solution to your problem. Although start and end times vary irregularly day lengths vary the way as you assume. For most of the world the total time the sun is in the sky is virtually the same, but at the poles for days if not weeks the sun will orbit around the pole with part of it above the horizon. If this is counted as day then each pole will have days of 24 hour sun for more than half the year. --2A04:4A43:900F:FA65:B09A:7819:80C0:37A6 (talk) 17:45, 2 July 2025 (UTC)[reply]

On the poles, sunset and sunrise are very slow, but only happen once per year. Still, I think you're right.

On the equator, the elevation of the Sun varies from -90 to +90 degrees; at the poles it only varies from -23.6 to +23.6 degrees. With a smaller variation in elevation, centred on the horizon, I expect it will spend a larger fraction of the time less than a quarter degree from the horizon, giving more daylight hours. But what the distribution of solar elevations looks like exactly isn't so easy, so no mathematical proof here. PiusImpavidus (talk) 10:48, 3 July 2025 (UTC)[reply]

Earth's orbit is not circular, so the angular speed is not constant and the apparent size of the Sun varies through the year, being the least at aphelion around July 4 (today!) and the largest at perihelion around January 3. Earth's considerable axial tilt is normally not aligned with the plane perpendicular to the ecliptic plane that contains the line Earth – Sun; alignment takes place at the solstices, around June 21 and December 21. This is not in phase with the Earth passing through the major axis of its orbit; it is off by about 13°. These three or four facts conspire to make the problem analytically intractable. A precise answer for how much time of sunshine is received per area for different latitudes requires an elaborate numerical computation (possibly one using an existing computational model).  ​‑‑Lambiam 09:35, 4 July 2025 (UTC)[reply]

@Nyttend:Does either of these images help answer your question? cmɢʟee⎆τaʟκ 06:46, 6 July 2025 (UTC)[reply]

No they don't, because I don't exactly understand them :-\ If I understand them rightly, they provide information for the number of sunlight hours at each latitude on each date, but I'm not sure about that. If I understand them rightly, what I'd like is basically a chart of sums — for each latitude, what is the total above-the-horizon time for the whole year? I don't quite understand why the images show the poles suddenly going from 24-hour daylight to 24-hour night on the same date, since sunset at one pole should happen several days after sunset at the other pole — the date when the first part of the Sun emerges above the horizon at one pole should be close to the date when the first part of the Sun goes below the horizon at the other pole, but there should be days when it's partly above the horizon and partly below the horizon at both poles, right? Nyttend (talk) 21:01, 6 July 2025 (UTC)[reply]

At the equinoxes the Sun should appear as a half disk at either pole.  ​‑‑Lambiam 08:01, 8 July 2025 (UTC)[reply]

Hi.

As a test in a GPT like chatbot I asked it for a hypothetical Illuminant representing sunlight under water (It gave some suggestions).

However, I'd like to check what the chatbot suggesested against actual research. Has anny work on apparent colors underwater been done?

My reasoning is that the apparent color (and any color shifts) would be based on depth, salinity and dissolved suspended contents in the water?.

I'd prefer to rely on cited research to check the chatbot's suggestions of course. ShakespeareFan00 (talk) 11:35, 3 July 2025 (UTC)[reply]

We have an article on ocean color, which cites this introduction to oceanography, which uses a graphic from NOAA, saying "this explains why everything looks blue underwater".

The effect is green at shallow depth in coastal waters due to chlorophyll in algae. Otherwise, the azure blue agrees with what color of water says about pure water. Our ocean color article observes that a diver using a nearby light for illumination underwater will undo the effect, since the light will travel through less water and will be filtered less.  Card Zero  (talk) 16:04, 3 July 2025 (UTC)[reply]

Kind of impressive that a few deep-sea species use red light as an illuminant that can't be seen by their prey. Sean.hoyland (talk) 17:14, 3 July 2025 (UTC)[reply]

The stoplight loosejaw. Inside the gland cells, blue-green light is produced [...] which is then absorbed by a protein that fluoresces in a broad red band [...] it passes through a brown filter, yielding [...] 708 nm (almost infrared).  Card Zero  (talk) 18:45, 3 July 2025 (UTC)[reply]

For salinity, we also have this nice picture of a halocline. This changes the refractive index. (Or something. The picture shows blurring rather than displacement, so perhaps it does something different.)  Card Zero  (talk) 19:02, 3 July 2025 (UTC)[reply]

We also have a graph in the section Electromagnetic absorption by water § Visible region showing that the blue end gets absorbed much less than the red end. If you go deep enough, so much sunlight has been absorbed that it is pitch dark.  ​‑‑Lambiam 20:43, 3 July 2025 (UTC)[reply]

Is there any pan-European system of hardiness zones? Some countries, such as Finland, have their own zones, which in Finland are denoted by Roman numerals from I (warmest) to VIII (coldest). But this Finnish system cannot be extended to any place which has warmer winters than the place in Finland with warmest winters. The USDA system is based on degrees Fahrenheit, so the border values between zones are not round, and the freezing point is not a boundary of any zone, when expressed in Celsius. Do countries such as UK, France, Germany, Poland, Russia, China, Japan and Korea have their own systems? --40bus (talk) 06:49, 6 July 2025 (UTC)[reply]

• The Royal Horticultural Society has its Hardiness Ratings, but these are for plants and there are no maps. The link does give a map of Europe with the USDA colors. Abductive (reasoning) 09:06, 6 July 2025 (UTC)[reply]

I'm not aware of any.

Within the Netherlands, going from one kilometre inland to a hundred kilometres inland, the coldest night in winter gets about 10°C colder, but that doesn't appear very relevant for what plants grow where. The number of days with afternoon temperature exceeding 10°C appears more relevant, or rainfall, but soil types are most important of all. Of course, we can calculate the number in the American hardiness zone system. Using my own data, collected in the east of the Netherlands (100 km from the sea), the lowest temperature of the year is ${\displaystyle T=0.17\cdot (year-2010)-8\pm 2.5^{\circ }\mathrm {C} }$, so that used to be zone 8, but now it's zone 9. Yes, it increased two standard deviations or one full hardiness zone over the past 30 years; that's climate change. PiusImpavidus (talk) 09:43, 6 July 2025 (UTC)[reply]

I have thought of devising my own system, where zones are marked by letters A (warmest) to N (coldest) optionally with number 1 (upper half) and 2 (lower half) and each zone spans 6 °C and each half 3 °C (corresponding to 10 °F and 5 °F increments) and runs from 24 °C to -60 °C. Also, summer temperatures could also be taken account, what USDA zones don't do. --40bus (talk) 14:02, 6 July 2025 (UTC)[reply]

Why not A to Z? ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:47, 7 July 2025 (UTC)[reply]

As I understand it, the grandfather paradox and the killing of baby Hitler omit the issue of what would happen to the present time and everyone living in it when time traveller goes to the past and alters it. For the sake of argument, dropping the impossibility of time travel, seemingly, one of two options would be true: either the present instantly dissappears with everyone in it, or nothing would happen after any alteration of the past because the present already happened once (and continues to exist after time traveller's mess in the past). ChatGPT admitted it's a shortcoming, but what do other sources say about present time? Brandmeister^talk 08:44, 6 July 2025 (UTC)[reply]

One of the wilder hypotheses around is that changing the past would result in parallel universes, thus allowing both of your "two options" to be true. This kind of wacky theory illustrates what an old math professor of mine once said: "When you start with incorrect assumptions, you're liable to get interesting results." The incorrect assumption here being that backwards time travel is possible. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:59, 6 July 2025 (UTC)[reply]

The fact that there is no one plausibly likely answer is what leads to there being so many Science Fiction stories that entertainingly posit different ones. Some suggest that on returning to their 'home present', the time traveller would find it altered subtly or radically (see for example the story 'A Sound of Thunder' by Ray Bradbury), others that two opposing factions instigate "Time wars" of changes and counter-changes that may envelop increasing swathes of past and future history (or histories) (e.g. Fritz Leiber's novel The Big Time and its sequel stories). {The poster formerly known as 87.81.230.195} 90.210.159.137 (talk) 12:32, 6 July 2025 (UTC)[reply]

If I understand the last comment correctly, another story that matches the "Time wars" scenario is Harry Potter and the Cursed Child. The whole thing is rather confusing, since functionally you have the "same time" happening at different times (Albus and Scorpius write on the blanket kind-of simultaneously with their parents reading it, despite the reading happening decades later), and when the parents figure out the situation, they quickly jump back to the previous time, hoping they're not too late. (I recently read a book analysing the whole thing as a fantasy-within-the-storyline, i.e. it's all imagined by the characters and not real even within the fictional canon.) Several parts of the story don't make sense, but at the same time the alternatives don't make sense. Nyttend (talk) 21:23, 6 July 2025 (UTC)[reply]

You may periodically verify that the article on A Hitler still exists. If not, ie A Hitler, you can conclude to live in a parallel universe. If this PA is any better, please post a brief review and how to get there... --Cookatoo.ergo.ZooM (talk) 16:34, 7 July 2025 (UTC)[reply]

First make sure, though, that consulting Wikipedia is not a crime, punishable by hanging. Also consider the possibility that the page exists but starts with, "A Hitler may refer to: A ruler of the Hitlerian Empire; ...".  ​‑‑Lambiam 07:51, 8 July 2025 (UTC)[reply]

We have nothing to Führer but Führer himself. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:07, 8 July 2025 (UTC)[reply]

Somewhere it is recorded that the weekly rations for marines and male convicts in New Holland was: "7 pounds of bread or in lieu thereof 7 pounds of flour, 7 pounds of beef or in lieu thereof pork, 3 pins of pease, 6 ounces of butter ... ". The only definition of pin as a unit of weight or volume I can find is 0.5 firkins or 4.5 imperial gallons (20 L; 5.4 US gal), but 3 litres of (presumably pease porridge) per day (2 on Sunday) seems rather a lot. Doug butler (talk) 23:47, 6 July 2025 (UTC)[reply]

Could that be a typo for pints? Presumably imperial ones, ~600 ml.-Gadfium (talk) 00:47, 7 July 2025 (UTC)[reply]

That seems likely to me. HiLo48 (talk) 01:13, 7 July 2025 (UTC)[reply]

And supported by this article, but of dried peas not my presumption of the cooked dish. Doug butler (talk) 01:36, 7 July 2025 (UTC)[reply]

diff (a large unreferenced addition). I will try contacting the editor. Doug butler (talk) 12:15, 7 July 2025 (UTC)[reply]

Or "tins" perhaps? cmɢʟee⎆τaʟκ 12:19, 11 July 2025 (UTC)[reply]

The Violet Town rail accident involved two trains colliding head-on, and its article notes that their closing speed was later determined to be 172 km/hr. Closing speed redirects to Collision, which says that "closing speed" is "the magnitude of the velocity difference just before impact". Is this correct? (No sources to check, since the whole section is unsourced.) If so, two questions:

• If it's a matter of velocity, why is it called "closing speed" rather than "closing velocity"?
• If it's a matter of velocity, how should the rail accident article report this? Since the article has already established that it was a head-on collision, it seems a bit redundant to say that their closing speed was 172 km/hr in opposing directions, and I'm not sure how this fact should be written even in an isolated sentence without prior context.

Thank you. Nyttend (talk) 20:03, 8 July 2025 (UTC)[reply]

• Tangentially related, many of the stupider members of the driving public think things like "it's better to speed up before a head-on because the faster car wins". Abductive (reasoning) 20:22, 8 July 2025 (UTC)[reply]

As Google confirms, the closing speed of two objects colliding head on is the sum of the individual speeds. I don't see how this is equivalent to "the magnitude of the velocity difference just before impact". -- Jack of Oz ^{[pleasantries]} 20:30, 8 July 2025 (UTC)[reply]

Velocity is a vector quantity; it has a direction in space. If one object has a velocity ${\displaystyle \mathbf {v} ,}$ an object going equally fast in the opposite direction has a velocity ${\displaystyle -\mathbf {v} .}$ They have the same speed ${\displaystyle v=\|\mathbf {v} \|=\|{-}\mathbf {v} \|,}$ which is the magnitude of the velocity. The magnitude of a vector is not a vector.

The closing speed of two objects colliding because they are going in the same direction but the one behind is faster is the absolute value of the difference in speeds. But, just as in the case of a head-on collision, it is the magnitude of the difference in their velocities.  ​‑‑Lambiam 20:49, 8 July 2025 (UTC)[reply]

Ah, that makes sense. Thank you, Lambiam. Trouble is, in general parlance "velocity" is just a synonym for speed, without regard to direction. People reading our closing speed article who are not scientifically educated to know the special scientific meaning of "velocity" will find it just as confusing as I did. Similarly, in general parlance mass and weight are synonyms, and any text that uses weight in its scientific sense as distinct from mass, without explaining the difference to readers, will confound them. -- Jack of Oz ^{[pleasantries]} 21:00, 8 July 2025 (UTC)[reply]

The word "velocity" comes from Latin for "speed".[1] ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:26, 9 July 2025 (UTC)[reply]

(Why not refer to our sister project, Wiktionary?)

And speed comes from Middle English spede ("prosperity"). To make this sound more scientific, we should call it rapidity (from Latin rapiditas).  ​‑‑Lambiam 09:36, 9 July 2025 (UTC)[reply]

Just to add to the confusion in Australia, Virgin Australia is currently running an ad campaign telling everyone that Velocity means Fast. Velocity is the name of Virgin's frequent flyer program. HiLo48 (talk) 05:49, 11 July 2025 (UTC)[2][reply]

The term rapidity is indeed in use in physics. PiusImpavidus (talk) 17:38, 9 July 2025 (UTC)[reply]

In a one-dimensional problem one doesn't really care about the difference between vectors and scalars. Technically, a speed can only be positive, a one-dimensional vector can be positive or negative, for a multiple-dimensional vector positive and negative don't apply. In practice, in the one-dimensional case one uses a scalar that can go negative and the words velocity and speed are used interchangeably. PiusImpavidus (talk) 17:54, 9 July 2025 (UTC)[reply]

Curious, why don't positive and negative apply in a multiple-dimension vector? Is that because in such a situation we always specify the direction where it's going ? Nyttend (talk) 20:55, 10 July 2025 (UTC)[reply]

They do apply. If vector ${\displaystyle \mathbf {v} }$ is equal to ${\displaystyle (1,-2,3),}$ its additive inverse ${\displaystyle {-}\mathbf {v} }$ equals ${\displaystyle (-1,2,-3).}$ There is no simple formula for the magnitude of the difference between two vectors in terms of the magnitudes of the components, unless they are aligned. When they are orthogonal, the Pythagorean theorem applies. In the general case, the cosine formula is needed; see Law of cosines § Using vectors.  ​‑‑Lambiam 05:13, 11 July 2025 (UTC)[reply]

The components of a vector, in a numerical representation using a particular base, can be negative, but the vector as a whole can't. PiusImpavidus (talk) 08:13, 11 July 2025 (UTC)[reply]

The rail line at Violet Town is straight and flat, at least as far as you can tell when standing at the station or looking at Google Maps, so it's functionally a one-dimension problem. But what if there were a head-on crash on a sharp curve? Would we talk about it being a two-dimension problem? Obviously the impact locations would be different — one corner of the locomotives would take the initial impact, and the other corner would be affected only after some energy had been absorbed — and the trains might be at a greater risk of falling over, even if the collision happened at a slow speed. Instead, I'm curious about the physics themselves, which could apply even if you were rolling two spheres against each other. Nyttend (talk) 21:02, 10 July 2025 (UTC)[reply]

Let us model a loc as a rectangular block with a length of 21 m and a width of 3 m. The front of a loc on the curve makes an angle with an orthogonal cross section of the track in the curve. Assuming a curve radius of 150 m (below which the trains would need to go slow in order not to derail), this angle is very close to 1⁄2 × 21/150 = 0.07 rad. At the moment of collision, the angle between the two loc fronts would be twice that. On a loc width of 3 m, that amounts to a gap of 0.42 m between the corners on the outside of the curve. Not much energy will be absorbed over that small a distance. The trains will derail; the momentum will carry the cars outwards.  ​‑‑Lambiam 04:58, 11 July 2025 (UTC)[reply]

150 m is really sharp; I'd only expect that on tramways, mountain railways and yards, and the latter two of those probably narrow gauge. The speed limit would be no more than about 30 km/h. The problem isn't toppling over, but the flanges on the wheels will contact the rail, giving noise, friction, wear and at higher speeds the flange will climb over the rail, leading to derailing. This can be avoided by using wheels, which are conical, with a coarser top angle, but that worsens hunting oscillations. Trams sometimes use independently spinning wheels, but that requires additional tricks to follow the track. For mainline speeds, don't expect curves with a radius less than a kilometre or so. PiusImpavidus (talk) 08:56, 11 July 2025 (UTC)[reply]

By "tramway", do you mean Tramway (industrial) or just normal tram tracks? Here in Melbourne the standard-gauge tram network has some very sharp curves at intersections, with Balaclava Junction being the most prominent — a left turn involves going around the kerb, and a right turn involves crossing just two lanes of traffic — but because they're at intersections with tram stops and traffic lights, trams often have to slow down for reasons unrelated to the rail geometry. Nyttend (talk) 21:21, 11 July 2025 (UTC)[reply]

While on the subject, can anyone shed light on this (manually operated) railway point, which appears to have no purpose other than to send the vehicle careering into the bush. Can't remember the location but probably somewhere in mid-New South Wales. The large structure is a wheat silo. Doug butler (talk) 05:34, 11 July 2025 (UTC)[reply]

The article is catch point. It's a safety measure to prevent a train on the siding from running onto the main track. --Wrongfilter (talk) 06:01, 11 July 2025 (UTC)[reply]

Great link. Thanks.

Resolved

Doug butler (talk) 06:23, 11 July 2025 (UTC)[reply]