Black holes are right now thought to be characterized only by mass, angular momentum and charge. Could weak isospin also be a theoretical characterization? Aso I've heard that black holes could pass through us without our noticing. if they had angular momentum,charge, or hefty weak isospin, would they be more noticeable?Rich (talk) 20:55, 21 June 2025 (UTC)[reply]

Black holes vary in size, from tiny micro black holes to supermassive black holes that reside at the centers of galaxies. In theory, if the black hole is small enough, it is possible for a small black hole to pass through the Earth without us noticing. Although black holes have a high density, if the black hole is small enough, the effect of gravity on the tiny black hole will be negligible and we cannot detect it. Stanleykswong (talk) 11:33, 22 June 2025 (UTC)[reply]

It would be funny if it turned out that neutrinos are actually tiny black holes. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:03, 22 June 2025 (UTC)[reply]

It's not impossible. Neutrinos are extremely tiny particles, with a mass far lower than other elementary particles such as electrons. They have almost no size, so the density can be very high. In addition, they have no charge, so a large number of neutrinos can be concentrated in a very small space. Stanleykswong (talk) 16:13, 22 June 2025 (UTC)[reply]

Who told you that? --Wrongfilter (talk) 17:10, 22 June 2025 (UTC)[reply]

Neutrinos have a lepton number (unlike black holes, unless I'm mistaken) and obey the Pauli exclusion principle (so they can't be arbitrarily concentrated). AlmostReadytoFly (talk) 08:26, 23 June 2025 (UTC)[reply]

Note that micro black holes evaporate within a tiny fraction of a second -- and also note that, even for a Planck-mass black hole (10 micrograms), such near-instantaneous evaporation would release an energy of about 900 gigajoules (which, for comparison, is about the explosive power of a 500-lb high-explosive bomb like the ones we drop on Isis and other terrorist groups). So no, such a black hole still can't pass right through you for 2 different reasons -- (1) it will evaporate first, and (2) in doing so it would blow you to bits if it happened to be close enough! 2601:646:8082:BA0:BDCC:8484:2734:F138 (talk) 13:21, 22 June 2025 (UTC)[reply]

However, if such a hole were travelling close to the speed of light, relativistic time dilation would prolong its lifetime. See also Micro black hole#Expected observable effects. {The poster formerly known as 87.81.230.195} 94.6.41.216 (talk) 15:27, 22 June 2025 (UTC)[reply]

Still, assuming a casualty radius of 11 meters for the explosion caused by the evaporation of the black hole (implying blast effects only, no flying debris), the time dilation would have to be at least by a factor of thirty-six orders of magnitude to allow said black hole to pass through you and explode a safe distance away! (And if the explosion would create a significant amount of flying shrapnel, e.g. if it demolishes a parked car nearby, then that would add an extra order of magnitude for the time dilation required!) 2601:646:8082:BA0:F577:E853:35B:8DD9 (talk) 11:52, 23 June 2025 (UTC)[reply]

I've previously heard discussion of black holes passing through other objects. Question, though — how would one be accelerated? I don't suppose anything else (with mass) travels at such high speeds, except for small particles that have been accelerated from a big reaction, like stellar fusion (e.g. solar wind) or an uncontrolled fission reaction (e.g. subatomic particles from a nuclear weapon), and except for the contents of particle accelerators. Nyttend (talk) 07:45, 2 July 2025 (UTC)[reply]

Accounting for DST today, the sun set on Sunday, October 4, 1874, in Oakland, California, at 6:47 PM. This year, in 2025, the sun will set at the same month, day, and location at 6:46 PM, one minute earlier in just 151 years. What factors account for this small difference? Viriditas (talk) 09:51, 23 June 2025 (UTC)[reply]

Milankovitch cycles? Although I suppose there might also be some rounding errors between whole minutes there? I guess we can rule out the unreliability of chronometers in Oakland in 1874? Martinevans123 (talk) 09:59, 23 June 2025 (UTC)[reply]

We have a good calendar that keeps calendar date in sync with the apparent motions of the sun. It ensures that on the same date the sun sets at the same time (with small variations). Or what did you actually expect? --Wrongfilter (talk) 10:08, 23 June 2025 (UTC)[reply]

• The solstice - the exact moment that the Earth's axis lines up with the sun - drifts around a little bit due to the fact that the year isn't a perfect 365 days. According to timeanddate.com, summer solstice 1874 was 21 June 15:07, and summer solstice 2025 was 21 June 02:42 UTC. That means that 4 October this year starts - in an astronomical sense, compared to the Earth's rotation around the sun - 12 hours earlier this year compared to 1874. Sunset times changes quite quickly in late September/early October (it looks like in Oakland, it's a minute or two per day), because it's near the equinox, and it's quite possible that that 12 hour difference is enough to push the sunset from one minute to another. In another year, when we have an unusually late solstice, October 4 will have a slightly later sunset, but most of us are unlikely to live to see it. The solstice date naturally gets slightly earlier every year by about 6 hours and 13 minutes, and that's why every 4 years we have a leap year - but the leap year correction still leaves about 13 minutes of drift per year. We try to cancel that out by skipping a leap year once a century (not counting years divisible by 400), so normally you get late solstices following a round century year (1800, 1900 etc) because the leap years are skipped in these years and that resets the calendar, but 2000 was a leap year so the calendar didn't get the usual reset. There won't be another late solstice until 2103, when it will occur at 22:47 UTC on 21 June - and that year Oakland will gain a minute and the sun will set at 6:48 PM (according to Wolfram Alpha). Smurrayinchester 10:35, 23 June 2025 (UTC)[reply]

  Thank you! Viriditas (talk) 10:59, 23 June 2025 (UTC)[reply]

  Many thanks, Smurrayinchester. Just for my info, are sunrise and sunset times always given in whole minutes? Martinevans123 (talk) 11:02, 23 June 2025 (UTC)[reply]

I think so. You can work it out more precisely, but the terminator (line of sunrise/sunset) moves at 463 metres per second at the equator, and slower the further towards the poles you go - you only need to walk a short distance to change the sunrise by a second. The sun rises and sets approximately 10 seconds earlier at the Tower of London compared to at Big Ben, for example, not taking into account all the other factors like the precise shape of the Earth and atmospheric effects that refract the sun's position. It's just not worth publishing sunrise times in seconds - it would be an example of false precision. Smurrayinchester 12:35, 23 June 2025 (UTC)[reply]

Many thanks for that very clear detail. Is that walking North or South, or East or West? Perhaps both. Obviously, in many places on land, the horizon will be obscured by terrain and/or structures. Is there an agreed official location for the sunset and sunrise in London? I would have guessed Royal Observatory, Greenwich. Sorry to keep asking! Martinevans123 (talk) 12:48, 23 June 2025 (UTC)[reply]

East-west. And I don't know if there's an official single point for astronomical calculations in London, but Greenwich certainly produce their own sunrise and sunset tables for astronomy. Smurrayinchester 15:01, 23 June 2025 (UTC)[reply]

Chart by Tomruen of the variation in December solstice date over 500 years. —Tamfang (talk) 22:51, 2 July 2025 (UTC)[reply]

The terminator moves across the Earth's surface with a speed of 360° of longitude in one day of 24 × 3600 s – not uniformly in October's Oakland, but the average is good enough here. The longitudinal width of Oakland is about 0.2°, so for the terminator to glide across Oakland takes about 48 seconds. Can we be certain that the reference point for 1874 is at the same longitude as that for 2025?  ​‑‑Lambiam 18:22, 23 June 2025 (UTC)[reply]

Yes, it's quite long, isn't it. Martinevans123 (talk) 18:30, 23 June 2025 (UTC)[reply]

There was no standard time in the United States in 1874 (History_of_time_in_the_United_States#Railway_time). So was 6:47 pm recorded in municipal time? Local solar time? Or something else? --Amble (talk) 22:05, 23 June 2025 (UTC)[reply]

(Just to be clear, I think the OP is talking about calculations of the kind you get from modern astronomical software, not measured local clock time. Even for precisely the same coordinates, the calculated sunset times vary for the reasons I listed above). Smurrayinchester 08:48, 24 June 2025 (UTC)[reply]

Your reply was fantastic, those effects can easily explain the variation. You are probably correct about the OP's data source as well. I initially thought it might relate to the long record of timekeeping data from Chabot Space and Science Center, which "served as the official timekeeping station for the entire Bay Area, measuring time with its transit telescope", but Chabot was only founded in 1883. --Amble (talk) 14:54, 24 June 2025 (UTC)[reply]

Smurrayinchester got this the wrong way round. It's easier to visualise using the autumnal equinox, which in 1874 fell on 23 September at 5:23 AM GMT and this year will fall at 6:19 PM GMT on 22 September. So this year 4 October falls 11h 4m further from the equinox than it did in 1874 and thus 11h 4m nearer to winter, meaning the sun sets earlier. The equation of time, which is partly affected by the time the apparent sun crosses the meridian, has little effect. In 1853, for example, this variable (which is subtracted from apparent (sundial) time to obtain mean (clock) time on 3, 4 and 5 October was respectively 11m 0.17s, 11m 18.33s and 11m 36.14s. You can check you are applying the equation correctly by considering the clock time at which the sundial shows noon: in early October the sundial is ahead of a clock set to local mean time (in practice standard (zone) time is used). With the cessation of direct astronomical observation from Greenwich parameters were recalculated using a zero meridian line a few yards from the one marked on the ground. 2A00:23C8:9626:8F01:F150:6B5A:4C04:3F82 (talk) 12:22, 25 June 2025 (UTC)[reply]

This implies that the October 4, 1874, sunset time for locations at Oakland's latitude should be equal, within a few seconds, to the mean of their sunset times for October 4, 2025 and October 5, 2025.  ​‑‑Lambiam 20:19, 25 June 2025 (UTC)[reply]

I'm currently doing detailed planning for my trip to New England as mentioned 14 days ago (and will go on July 1), and I have some more specific questions to ask: I've found 2 other potentially promising locations for butterfly watching, one in Yarmouth, Maine and another in Waterville, Maine -- however, the question I will need answered before the end of this month is, in these 2 towns which species of tiger swallowtail is predominant, Papilio canadensis or P. glaucus (note no link to the article about the latter species)? 2601:646:8082:BA0:4C47:A4C8:EC7A:C5C6 (talk) 03:33, 26 June 2025 (UTC)[reply]

A quick Google finds:

...both eastern and Canadian tiger swallowtails are common, with Canadian tiger swallowtails more common in Maine. [1]

Alansplodge (talk) 21:58, 27 June 2025 (UTC)[reply]

And distribution maps within Maine are here and here. Alansplodge (talk) 22:04, 27 June 2025 (UTC)[reply]

The scary butterflies are all confined to Hancock County! I hope somebody's told them that. The county is home to Acadia National Park, the only national park in New England - a possible reason for the distribution.  Card Zero  (talk) 22:12, 27 June 2025 (UTC)[reply]

Thanks! And I've actually been told by a local expert from Coastal Maine Botanical Gardens that P. glaucus is also predominant in Lincoln County, just to the west, and that the reason for this is the much milder winter due to the river nearby (and also that in recent years this species has been aggressively expanding its range northward, as is also the case with Papilio cresphontes) -- but he also told me that this far north, P. glaucus has a non-scary size (in fact, no bigger than P. canadensis), which makes it OK! (In fact, it's the P. cresphontes which is more likely to be a problem for me due to its size -- the expert told me that even in the Boothbay area, they can reach a size of 5-6 inches, although he might have meant (I hope) the ones in the butterfly house at the botanical garden, not the ones in the wild -- I guess I'll just have to see!) 2601:646:8082:BA0:D5E1:EC31:5434:1B47 (talk) 04:21, 28 June 2025 (UTC)[reply]

Carry a pair of strong nearsighted glasses; they make everything look smaller.  ​‑‑Lambiam 06:23, 28 June 2025 (UTC)[reply]

User:Lambiam, your comment brought to mind a memory from 30+ years ago, when I was a small child just getting his first glasses. I clearly remember my sense of "wow everything looks small", even though I don't remember any impressions of my newfound ability to see distant things properly. Nyttend (talk) 21:14, 2 July 2025 (UTC)[reply]

Hahaha, I actually don't think I'll need to bother -- this source says the P. cresphontes in Canada have a size range of 3 1/4 to 4 1/2 inches (the two pinned specimens at the Royal Ontario Museum in Toronto had a wingspan of only about 3 inches or so, I measured with my finger against the glass), and since Maine is not any farther from the northern edge of their range than Toronto (or even Ottawa), they should be no bigger than that up there either -- which, for a non-tiger-striped species like this one, is perfectly fine by me! (That guy must have been talking about the ones in the butterfly house, not those in the wild!) And if I do see a giant swallowtail which is too giant for me, I expect I'll be able to see it from afar and make myself scarce before it gets too close -- but I don't expect this will happen! 2601:646:8082:BA0:B4C5:E96C:7826:47EE (talk) 13:45, 28 June 2025 (UTC)[reply]

We can't wait to hear your trip report once you made it (hopefully safely) back.  ​‑‑Lambiam 05:22, 29 June 2025 (UTC)[reply]

Not to worry, will do! And seriously -- barring some freak accident as a result of a panic attack (a vanishingly small possibility, as even having a panic attack in the first place would require some improbable turn of events, for the reasons I've stated above, and even if it happens it's unlikely to lead to anything worse), the worst that can happen is that instead of being desensitized I'd end up traumatized worse than before -- but even that is highly unlikely, given how gradually I'm doing this (hence my choice of location, selected specifically so that I have a good chance to see P. canadensis and not run into its bigger cousins like P. glaucus) and how much progress I've already made in the past 7 years! Of course, with an animal phobia, going from pictures and figurines to real live animals is always a big step (because a real live animal has a mind of its own, so you don't have any control over what it does) -- but I've made every effort to make this step go as smoothly as it possibly could! 2601:646:8082:BA0:E08A:96DE:28D8:2E6F (talk) 02:22, 30 June 2025 (UTC)[reply]

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]

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]