Wikipedia:Reference desk/Archives/Science/2022 November 6

= November 6 =

entropy
How might the universe look if the second law of thermodynamics worked in reverse? Instead of the universe's entropy increasing over time, entropy would generally decrease over time in this alternate universe. Might life have still come into existence, and if so how might it be different? I heard about an MIT study about how the universe's increasing entropy might make the development of life inevitable, which got me interested in this question. How might stars, galaxies, planets etc might look if they even exist at all? 2600:4040:1014:7300:594B:5953:EE8F:274C (talk) 00:49, 6 November 2022 (UTC)


 * Take a film of the universe and play it in reverse. --Lambiam 01:18, 6 November 2022 (UTC)
 * See DMacks (talk) 22:17, 6 November 2022 (UTC)
 * Your question doesn't make sense in physics, because this event didn't happen. In France we say "with if we put Paris in a bottle".
 * But in mathematics you can define this kind of postulate and pose a problem in this new theoretical set. Malypaet (talk) 18:07, 6 November 2022 (UTC)
 * This would make the universe unstable. Ruslik_ Zero 18:43, 6 November 2022 (UTC)
 * Related: arrow of time and entropy as an arrow of time. Shells-shells (talk) 18:16, 7 November 2022 (UTC)
 * This question comes up from time to time.  See Reference desk/Archives/Science/2019 February 9, making the point that time may well be a two-way street. 79.76.42.157 (talk) 13:56, 8 November 2022 (UTC)

first law
Along a similar note, how might the universe look if it was possible to create energy out of nothing or destroy energy? 2600:4040:1014:7300:594B:5953:EE8F:274C (talk) 01:14, 6 November 2022 (UTC)


 * This question is unanswerable. If the law of conservation of energy is broken, all physical laws as we know them are broken, since this implies that the laws of physics are not constant but change with time (see ). --Lambiam 01:29, 6 November 2022 (UTC)
 * It don't get it. I was thinking that the conservation-laws are a inference from symmetry properties. For instance, conservation of Impuls is a conclusion of the possibility to go in every direction in space. 2A02:908:424:9D60:0:0:0:F38B (talk) 11:36, 8 November 2022 (UTC)
 * The constancy of the physical laws is a symmetry property, the shift symmetry of time; see the article section I linked to. --Lambiam 13:21, 10 November 2022 (UTC)

So anything might be possible in that case (and its impossible to predict) because the laws of physics can change over time. — Preceding unsigned comment added by 2600:4040:1014:7300:594B:5953:EE8F:274C (talk) 02:05, 6 November 2022 (UTC)


 * You're waaay over in the region of speculative science fiction or magic rather than anything that can be answere in the science reference desk. NadVolum (talk) 11:54, 6 November 2022 (UTC)

Unit in physics and multiplication by time
In several definitions of physical quantities, we find a multiplication by a second (J.s, A.s) when we refer to a quantity dependent on the time which flows towards the future and therefore expressed by the division of a second. Mathematically, I understand the operation, because it ultimately consists in removing any reference of this magnitude to time. In physics, the division of time on a quantity relates to associating a time interval which flows towards the future with this one (J/s, C/s, m/s), this is regularly experienced. On the other hand in physics, the multiplication of time on a quantity would consist in associating a time interval which flows towards the past, except that has never been experienced (except by reversing the progress of a video!). How can we allow ourselves to use this kind of notion in physics when it only comes under the domain of mathematics or science fiction? Malypaet (talk) 02:49, 6 November 2022 (UTC)


 * I don't know where you get this idea of an association of multiplication or division with time flowing or the past or future. In the case of corn in a field on can have corn per hectare or total corn in the field. Both figures are useful for different purposes but there is no flow involved. In the same way for fuel in a car one can have the total number of litres and kilometres per litre which together give an idea of how far it can go between refills. Speed can be measured in kilometres per hour and together with a distance that can give how long it'll take to arrive. The multiplication or divisions involved imply nothing about the future or the past. NadVolum (talk) 11:46, 6 November 2022 (UTC)
 * You write "Speed can be measured in kilometres per hour" : km/h, this is not a division ? You write "how long it'll take to arrive", this has no thing to do with the futur ? If you don't have the base of language or in physics, don't loose your time here and mine too ! Malypaet (talk) 17:58, 6 November 2022 (UTC)
 * You can also ask how long it took to arrive. Past tense. --Wrongfilter (talk) 19:23, 6 November 2022 (UTC)
 * Yes, but "how long" is between start to end (end is the futur of start), with a speed for a distance divided by (from end minus start ). what you means with "Past tense", is a replay in your imagination, as in a dream (as in mathematics), you're telling a story that happened in the past, it's not the same as reversing time. Reversing time was never experimented in reality (physics), a time action event from end to start. Exept if you can give me an exemple or pointing me on a publication ? Malypaet (talk) 08:29, 7 November 2022 (UTC)
 * Being rude towards those who can (or at least try) help you may turn out counterproductive... --CiaPan (talk) 21:12, 6 November 2022 (UTC)
 * Sorry, you are right Malypaet (talk) 08:12, 7 November 2022 (UTC)
 * Also have you tried figuring out how your idea of flow, the future and the past apply to the corn and land area, and are they really all that relevant? I put them in as things which don't slot into your model and therefore you might feel freer to think about NadVolum (talk) 11:47, 7 November 2022 (UTC)
 * Talking for nothing Malypaet (talk) 13:21, 7 November 2022 (UTC)

I feel the OP would benefit from a course in calculus explaining the operator ρ/ρt that means the differential with respect to time of anything that varies with time, evaluated at one given instant in time. Becoming able to understand Maxwell's equations that employ differential operators to reveal the interdependence of electric and magnetic fields, and their importance in the technologies of Electromagnetism (light, radio, TV, radar) should remove any doubt that physicists "can allow themselves" to use these calculations. Of course they are aware that any practical measurement of a ρ/ρt value will require collecting values of the varying quantity during a finite time span which may be as short as measurement accuracy allows. For example: two photographs of a car showed it was at time(hh:mm:ss)/distance: 6:00:00 / 5 miles and 6:00:01 / 5-1/60 miles. The photographs are enough to convict the driver of exceeding by 10 mph the local speed limit of 50 mph but nothing is, nor can, be implied about his behavior before or after the measurement. After calculating velocity = ρ (position)/ρt for a car one may further calculate its acceleration = ρ (velocity)/ρt = ρ2 (position)/ρt2, also applicable to a given instant with no implications for the past or future. Philvoids (talk) 16:08, 7 November 2022 (UTC)


 * "with no implication for the past"? It is obvious, the past is over, we cannot act on it. But, a position, a speed, a direction, and an acceleration allows you to know where you will be in 10 minutes, so a precise implication on the future, remove some of this information, there will always be an implication on the future, even vague. Too focused on equations, makes you forget the meaning of things. Malypaet (talk) 21:36, 7 November 2022 (UTC)
 * Your advice is noted. The example of instantaneous measurements taken of a car was wasted if you think they predict whatever the driver may do later. I think a person seeking late in life to reform the physics that they neglected or forget will not be helped by more rambling debate and that this one may now be closed, citing the rule here: "We don't answer requests for opinions, predictions or debate." Philvoids (talk) 15:19, 8 November 2022 (UTC)


 * From their user page: "Currently I am interested in the origins of Einstein's theory of relativity and want to participate in discussions on this vast subject, to perhaps correct it." I dare say we can safely ignore this guy.--User:Khajidha (talk) (contributions) 21:48, 7 November 2022 (UTC)
 * Here can we speak freely of science ? "Einstein's theory of relativity": It's questionable science or dogma ? Malypaet (talk) 22:05, 7 November 2022 (UTC)
 * Maxwell was a professor of natural philosophy, at the time when nature and physics were linked. I reread his original article "A Dynamical Theory of the Electromagnetic Field". His text is as powerful as his equations. You should read it. Malypaet (talk) 22:00, 7 November 2022 (UTC)

Cats' eyes
I have two cats; each cat has eyes that reflect in a different color. One reflects red, the other green. I had assumed eye reflection color would be species dependent, but evidently it is not. What determines the color of eye reflection? 136.56.52.157 (talk) 03:09, 6 November 2022 (UTC)


 * Humans can have red/pink, green, blue, black etc eyes. Even purple I think.
 * . Sagittarian Milky Way (talk) 03:14, 6 November 2022 (UTC)
 * Humans have only red, because we lack the tapetum lucidum. Foxes eyes reflect green; supposedly a murder case hinged on this fact, as the shooter claimed he was aiming at a fox at night and accidentally shot a person. Your cat may be explained further down in the tapetum lucidum article, or s/he might have a damaged or missing tapetum lucidum in one eye. Abductive  (reasoning) 03:34, 6 November 2022 (UTC)
 * Interesting article, thanks for the link. Accordingly, green reflection  ("eyeshine") is normal for cats, and blue-eyed cats are an exception, glowing red.  In my case, the cat with red eyeshine does indeed have blue eyes.  The article doesn't explain why a blue iris would affect eyeshine color; presumably it is coincidentally caused by genetics. 136.56.52.157 (talk) 04:31, 6 November 2022 (UTC) ... Edit: I should clarify that each cat has same color in both eyes; but one cat reflects green in both, the other (blue-eyed cat) reflects red in both -- sorry for the confusion.
 * I wonder if blue-eyed cats don't see as well in the dark (?). 136.56.52.157 (talk) 04:51, 6 November 2022 (UTC)
 * If the cat's eyes really don't reflect green from any angle then I would say, yes, their night vision is impaired. Supposedly cats can detect a single photon under the right circumstances, while our detection level is something like 4 photons. Cat vision says their night vision is about seven times more sensitive than ours. Abductive  (reasoning) 08:56, 6 November 2022 (UTC)

Dubious animal with only one layer of cells
I remember reading about an animal that may or may not actually exist. It was microscopic and had only a single layer of cells in its body. It was like a sausage in shape, with a pore and extra cillia on each endd Quick Trundleteacher (Talk) (Inputs) 18:52, 6 November 2022 (UTC)
 * Are you maybe thinking of rotifers?--User:Khajidha (talk) (contributions) 11:10, 7 November 2022 (UTC)
 * Perhaps Placozoa (or Myxozoa, some of which are unicellular)? --Amble (talk) 19:02, 7 November 2022 (UTC)
 * Choanoflagellate can be a ball shape of a single layer of cells, not classed as an animal. Embryos may be a single cell layer, eg the sponge embryo. Graeme Bartlett (talk) 21:06, 7 November 2022 (UTC)

Jiufotang Dinosaurs
Why is there such a lack of variety of non avian dinosaurs in the Jiufotang formation compared to the Yixian formation which has iguanodonts, compsognathids, therizinosaurs, sauropods, ornithomimosaurs and troodontids, while the Jiufotang only has microraptorines, ceratopsians, ankylosaurs, and proceratosaurs which the Yixian also has? CuddleKing1993 (talk) 19:44, 6 November 2022 (UTC)
 * I can't answer myself, but Jehol Biota may contain some clues. {The poster formerly known as 87.81.230.195} 5.64.163.219 (talk) 08:12, 7 November 2022 (UTC)