Wikipedia:Reference desk/Archives/Science/2017 March 17

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March 17

The expanding universe

Wikipedia's article states that the universe is expanding at large scales but that galaxies are not because they are gravitationally bound. This implies there will be an astronomical number of roughly spherical boundaries in space, surrounding every galaxy, on one side of which space is expanding and on the other side of which space is not expanding. The presence of such discontinuous boundaries in space seems extremely unlikely. It seems far more likely there is a smooth function relating the expansion of space to the force of gravity. In which case who is to say that galaxies may not expand with the universe albeit with a much reduced rate. A small residual rate of expansion for galaxies would be extremely hard to observe but could have major consequences. Comments please 165.120.168.239 (talk) 00:50, 17 March 2017 (UTC)

$G_{\mu \nu }+\Lambda g_{\mu \nu }=8\pi T_{\mu \nu }$? Introduction to general relativity is our article on the topic. Nimur (talk) 01:22, 17 March 2017 (UTC)

Would there really be a hard "boundary" or more of a gradual transition, as with the gravitational influence of different objects? ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:03, 17 March 2017 (UTC)

You're asking for a description, in plain English, about the "smoothness" or "gradualness" of the parameters in a very difficult equation. The trouble is, words like "smoothness" or "gradual" have special meaning in mathematical physics. It's not easy to use plain English to describe these complicated topics in a way that is simultaneously simple, complete, and correct.

So if you'll accept a little bit of hand-waving, sure - there's a "smoothness" to the universe, for the most part. Discontinuities in the shape of our universe are not common. But, there also seem to exist at least a few real examples of the gravitational singularity, so those are places in our universe where there is an abrupt discontinuity. Most cosmologists believe that we have enough evidence to say that these theoretical constructions of mathematics do manifest as actual real objects in our universe - and that's a much more important statement than most casual readers realize!

The real take-away lesson, though, is that if you're interested in these complex topics, the best way to understand them is to become extraordinarily adept at using the types of mathematics that physicists and cosmologists use to describe the shape and expansion of our universe. The math isn't just helpful - it's essential to being able to understand it.

If you're interested in the topic but don't want to deal with the equations, here's a great resource: the Public Lectures series archive from SLAC National Accelerator Laboratory. For example, the most recent presentation was Galaxy Clusters and the Life and Death of the Universe (February 2017), in which scientist Eli Rykoff discusses the state of the art of our knowledge about cosmic structure in a format that is accessible to a non-technical audience.

Nimur (talk) 23:26, 17 March 2017 (UTC)

• One qualitative way to look at this: the universe is expanding uniformly, but gravity locally retards the effects of this expansion. But you really need to do the math. Your vision of "roughly spherical" is a simplification that would be valid only if galaxies were point masses of equal mass. A more accurate(?!) image is a universe with a continuous density variation. For some critical local density, gravity is stronger than expansion. At lower density, expansion wins. -Arch dude (talk) 02:19, 17 March 2017 (UTC)

How accurate is this table?

Could someone verify the accuracy of this time dilation table,since I'm no expert in physics? It's the second table. I'm not sure whether the person who wrote the article has any expertise.Uncle dan is home (talk) 02:44, 17 March 2017 (UTC)

Which article? ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:47, 17 March 2017 (UTC)

This one: [1] — Preceding unsigned comment added by Uncle dan is home (talk • contribs) 03:11, 17 March 2017 (UTC)

Have you spot-checked any of the calculations in the table vs. the formula? ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:19, 17 March 2017 (UTC)

The numbers are roughly correct, but are shown with more significant digits than are warranted by the accuracy of the calculation—particularly the right-hand column, which was calculated based on a year being exactly 365 days. --76.71.6.254 (talk) 04:57, 17 March 2017 (UTC)

If you reduce the number of digits too far, the first few entries round to 0, which is not quite accurate. Once the number gets large enough that 365 vs. 365.25 (or whatever) starts to affect the result, rounding to fewer digits would make sense. Also, do you use Tropical year or Sidereal year? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:23, 17 March 2017 (UTC)

And light years are Julian years. (365.25 exactly) Sagittarian Milky Way (talk) 23:18, 17 March 2017 (UTC)

In 1582 His Holiness Pope Gregory XIII blessed a popular calendar but that seems not to have persuaded the IAU that every Light-year should be shorter by 194 300 000 km, about -0.2% -0.002% Thanks for correction.. Blooteuth (talk) 15:32, 18 March 2017 (UTC)

(0.002%) Yes, but it's only 18 760 000 km off from the average of the real year (seasons) and real year (stars) and is the real year (calendar) from AD 1900 to 2100. March 0, 1900 is so long ago and March 0, 2100 is almost a century from now so for all practical civil purposes this is the year. You know what's the most awesome calendar?

365, 365, 365, 366, 365, 365, 365, 366, 365, 365, 365, 366,

365, 365, 365, 366, 365, 365, 365, 366, 365, 365, 365, 366,

365, 365, 365, 366, 365, 365, 365, 366, 365, 365, 365, 365, 366. Repeat forever.

More accurate than the Gregorian calendar and repeats 12 times sooner. Sagittarian Milky Way (talk) 19:22, 18 March 2017 (UTC)

In what way is your 37-year calendar with average Year = $365.{\overline {242}}$ days more accurate and awesome than, say, the Metonic (19 year, 365.2631579.. days), Callippic (76 year, 365.2628205.. days) or Sidereal (365.256363004 days) years? Blooteuth (talk) 14:43, 19 March 2017 (UTC)

My bad, I cloned with copy+paste till I thought I had enough, tacked on the extra 365, wrote the post and forgot to prune. 8 leap days in 33 years instead of 32 is more accurate than the 400-year rule, repeats a dozen times sooner and only has +/- 16/33rds days of non-secular error max instead of 1.09875 like the Gregorian! i.e. spring equinox can be March 22 (i.e. 1903), 21, 20 or 19 (2096) in India's time zone but falls in the same 24 hours in 8/33 calendar and it wouldn't take hundreds of years for the calendar to exceed Julian accuracy. *awesomeness claim limited to precalculated fixed leap-ruleset tropical year calendars (the everyday calendar type of Western Civilization for over 2,000 years). Lunar, lunisolar, sidereal, draconitic, anomalistic, heliacal, Sothic, Besselian, Gaussian, lunar apside-solar, vague, Martian, Cytherean, Platonic, galactic, non-Earth, Coruscantan, Narnian and Middle-Earth calendars not included. Not liable for excessively unforeseeable changes in delta-T. In case of extended suspended animation consult an astronomer. In case of asteroid making a different calendar better consult a priest. In case of the simulation argument making this universe not real prevent strong AI from being invented. Void where prohibited. Sagittarian Milky Way (talk) 03:29, 20 March 2017 (UTC)

You are talking about reducing the number of decimal places, not the number of significant digits. --76.71.6.254 (talk) 07:14, 17 March 2017 (UTC)

How do doctors categorize something as a "disease"?

I know that there is variation within a species. Some individuals have a beneficial mutation; other individuals have a harmful mutation. But the ability of all modifications to pass down to future generations is dependent on the environment. Hereditary hemochromatosis is regarded as a "disease", even though those individuals may be just very efficient at absorbing iron from food. Meat is full of iron, so these individuals would fare poorly in a meat-rich (iron-rich) environment. Huntington's disease is caused by a lethal gene that manifests in adulthood. It is only a "disease" when that person lives long enough to manifest it. If the person reproduces at 15 and dies in childbirth at 20, then the gene may be passed onto the offspring, and the manifestation of the gene may not be shown. People on the autism spectrum seem to live happy, fertile lives. And people with Down's syndrome have children. So, my main question is, what are the characteristics of a disease? How is a disease different from a variation that just happens to be maladaptive in a given condition? 50.4.236.254 (talk) 03:13, 17 March 2017 (UTC)

See Disease. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:16, 17 March 2017 (UTC)

To answer your second question (from bugs's link) "Disease is often construed as a medical condition associated with specific symptoms and signs". Also "In humans, disease is often used more broadly to refer to any condition that causes pain, dysfunction, distress, social problems, or death to the person afflicted, or similar problems for those in contact with the person". 64.170.21.194 (talk) 03:29, 17 March 2017 (UTC)

In simple terms the word is an concatenation of dis (without), and ease In other words not-at-ease. As medical science has progressed, it now includes such things as high blood pressure that may not cause symptoms of unease at first.--Aspro (talk) 03:38, 17 March 2017 (UTC)

In characterizing conditions such as Autism or Down syndrome, it is ethically and politically imperative to distinguish between a Disease - a medical condition associated with specific abnormal symptoms and signs, and a Disability - an impairment of a person's body that restricts their involvement in life situations. See Down syndrome#Ethics for example of debate where these categories are blurred. Blooteuth (talk) 13:06, 17 March 2017 (UTC)

Nosology may be helpful (linked via the disease article). The differences between disease, disorder, syndrome, and injury are laid out, with a few sub-groups. My impression is that in everyday life, people often think of disease as something contagious or "catchy", with other maladies, such as arthritis or most forms of cancer as "something else". This is possibly caused by an over-application of Koch's postulates. Matt Deres (talk) 14:06, 17 March 2017 (UTC)

The distinction in general can be rather arbitrary - famously, the psychiatric community initially classified homosexuality as a disease, before recognizing it as a natural variation. I think many definitions of mental or even physical conditions now have a term about the disease interfering with a patient's work or being a cause of distress. (But what if the employer, quite legally in some states, discriminates? It seems a rather flimsy kludge...)

Personally, I think in the genetic context the distinction between gene therapy and eugenics is particularly important, and can be made in a more objective way: it is possible, by examining the distribution of alleles in the population, to determine how long ago they arose. If a condition is caused by individuals for whom the first affected ancestor can be identified in recent genealogies, or even can be inferred to have arisen within the past few thousand years, I think the situation can readily be defined as a disease, if patients want it fixed, and could be wiped out of the germline without much apology to history. However, if the alleles are millions of years old, then they represent an important part of the shared human patrimony and removing them from the population means incrementally pushing the human species closer to extinction by depleting its gene pool. (cf. [2]) A sticky point would be something like sickle cell anemia, which clearly has been preserved and favored by natural selection ([3] and especially [4]). Today it would make sense to wipe the allele out --- provided you are willing to swear that malaria will be eradicated, that for tens of thousands or even millions of years the descendants of the people you alter will be free of that scourge. But if you cannot, then curing one person today may mean that thousands of people in the future struggle under the burden of malaria and are ultimately deleted from the population. Of course, not curing them may mean condemning thousands to a slow death from sickle disease. Messing with the essence and fate of humanity is kind of a big deal either way. Wnt (talk) 16:34, 17 March 2017 (UTC)

Mechanism of action?

See this article here What is the possible mechanism for their strengthened immune systems? (If this is true?) 64.170.21.194 (talk) 03:23, 17 March 2017 (UTC)

Read Cold shock response for some cautions. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:27, 17 March 2017 (UTC)

To endorse the claim "children are healthier if they daily run out into the snow in their swimming costumes - and pour a bucket of cold water over their heads" from a source in Russia (average life expectancy: 70.5, USA 79.3, UK 81.2) would amount to giving medical advice that is not the purpose of this Ref. desk. Blooteuth (talk) 12:36, 17 March 2017 (UTC)

A cynic could argue that by immediately killing the less healthy, those who remain are healthier. StuRat (talk) 20:22, 17 March 2017 (UTC)

This is most certainly not medical advice "blooteuth".... 64.170.21.194 (talk) 22:21, 17 March 2017 (UTC)

Medical advice is the provision of a formal professional opinion regarding what a specific individual should or should not do to restore or preserve health. Waterboarding is a form of Water torture of adults that cannot be conveniently dismissed as innocuous when freezing water is dumped on children by guardians In loco parentis. Blooteuth (talk) 15:03, 18 March 2017 (UTC)

Regardless of the alleged benefits, the average Russian life expectancy is almost 9 years less. Maybe the core problem is that they're living in Russia. Now, if someone can cite life expectancy in Siberia vs. the rest of Russia, they might have something. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:52, 19 March 2017 (UTC)

Then again, they might not. Confounding factors (like StuRat's "killing the less healthy") in a heads-up life expectancy comparison between any two populations (European Russians vs. Siberian Russians, Russians vs. non-Russians) have to be considered. That said, I agree that we ought to shy far away from comment on the article cited by the OP. If nothing else, it reeks of WP:SENSATION, so as editors, we'd have to discount it as a source, anyway. That leaves either medical advice, or speculation about possible therapeutic effects of exposure to extreme cold (I can't find any such evidence on a cursory Internet search apart from this article). loupgarous (talk) 06:15, 20 March 2017 (UTC)

Another confounding factor would be that the infirm wouldn't take part, so this means those who take part are healthier to begin with. StuRat (talk) 07:20, 21 March 2017 (UTC)

Types of Human genes

I am not a geneticist and don't know much on genetics and I get the feeling that there are two types of genes: Those how go through changes regularly (like genes effecting hair curlyness or color) and genes that seldom go through any change (like genes coding for the very creation of hair or any other body part). Why some genes go through changes so easily and commonly and some others almost never go through changes by means of genetics themselves excluding epigenetic influences? (for example, >99.9% of people with hair that can procreate, could breed children with hair but only <0.1% of these will be totally without hair asuuming there was no epigenetic effect).79.178.144.67 (talk) 08:26, 17 March 2017 (UTC)

Hair colour and curliness don't arise through changes (mutations) in genes per individual. They come from the fact that different people have different variants of the genes responsible for those traits. Both parents of any given child can have different combinations of those variants, and which specific ones the child gets is essentially random. These variants come from mutations many many generations ago. See Mendelian inheritance and Mendelian traits in humans, where many of these things are discusses, and further links can be found. Fgf10 (talk) 11:29, 17 March 2017 (UTC)

I think what OP is getting at is that in a given population (say a large city in EU, USA, AU), there will be more variation in the genes alleles controlling hair color and hair texture, whereas (mostly) all of them have hair, so the bits governing hair production would seem to vary less. The idea that some genes alleles must be tightly conserved for survival, but others are free to take on many forms without any simple strong effect on viability and fitness. Two relevant articles are Fixed_allele and Fixation_(population_genetics), those talk about how core important functional alleles get "fixed" with little or no variation in a population. Neutral_mutation is what created alleles that have "neither beneficial nor detrimental to the ability of an organism to survive and reproduce". (I'd nitpick that nothing in the real world is truly neutral, but it's still a good model for something like eye color). SemanticMantis (talk) 13:48, 17 March 2017 (UTC)

First we should make some distinctions. There is genetic variation which affects how much diversity you see in a population, or even in a family from generation to generation as different alleles randomly turn up in offspring. Genetic variation may result from balancing selection, where certain alleles are more favored under different circumstances so that no one ever loses the genetic contest. Alternatively, there is fixation (population genetics) in which alleles are lost. The success of a new allele arising by mutation depends critically on negative selection or sometimes positive selection -- any disadvantage, however small, can mean that the new allele is lost or becomes universal. So can dumb luck (genetic drift), so population size and population structure matter; for example a founder effect can drastically shape the genetics of an isolated region.

One reason why minor differences in hair structure are more neutral is that they don't matter as much as losing it entirely. (Being bald on the steppes of Central Asia, where many Europeans and Asians trace back to, would probably not be fun... having your whole scalp being bitten by flies in Africa might be worse?) Another is that changing the hair protein is perhaps (presumably) less likely to have pleiotropic effects. That said there are rare conditions affecting curliness and color like amelogenesis imperfecta and POMC deficiency that do have pleiotropic effects, and I'm not very sure this is really true. I wonder if there might also be some balancing sexual selection (different people like different things) but I have not done any background research on this.

Another thing to bear in mind is that some genes are simply more prone to mutation, such as CAG repeat expansion. The sequence of the gene itself simply makes it mutate more. (See also transition (genetics) vs. transversion). On average the genes you notice more variation in are presumably somewhat more likely to mutate on this basis, but I doubt it's by any large degree, and I haven't looked up to see if anyone came up with a way to put a number on it. Wnt (talk) 13:48, 17 March 2017 (UTC)

What were the differences in physical appearance between Pleiosaurs and Mososaurs?

Liopleurodon ferox, a primitive relative of turtles and lizards

a recent Mosoosaur snake relative

Despite the fact that they were not closely related, Mososaurs and Pliosaurs had similar physical characteristics and the same ecological niche as apex marine predators. How do you tell them apart by just looking at them side by side? 70.95.44.93 (talk) 09:54, 17 March 2017 (UTC)

Tails. Mosasaurs swam with powerful tails (and thus musculature and skeleton), pliosaurs paddled with their legs. The tails of pliosaurs and pleisiosaurs have small or no flukes. Andy Dingley (talk) 11:29, 17 March 2017 (UTC)

Mosasaurs had a body shape similar to that of modern-day monitor lizards (varanids), but were more elongated and streamlined for swimming. They had double-hinged jaws and flexible skulls (much like those of snakes), which enabled them to gulp down their prey almost whole. Their limb bones were reduced in length and their paddles were formed by webbing between their long finger and toe bones. Their tails were broad, and supplied their locomotive power.::Pliosaurs were distant cousins of modern turtles and distinguished by a short neck, massive toothed jaws and an elongated head. Their bodies were flat and their tails were short. Blooteuth (talk) 12:12, 17 March 2017 (UTC)

• You do realize that Pliosaurs are basal sauropsids, while Mososaurs are rather derived varanid lizards? It's like comparing turtles and armadillos. μηδείς (talk) 02:07, 18 March 2017 (UTC)

File:Aepyornis_skull.JPG vs File:Skeksis-duel.jpg PaleoNeonate (talk) 04:39, 18 March 2017 (UTC)

Isn't that rather what the OP's first question stated? They're asking about synecology, not autecology. Andy Dingley (talk) 21:29, 18 March 2017 (UTC)

Well, then look at Alfred Romer and Euryapsid. μηδείς (talk) 23:13, 18 March 2017 (UTC)

Gravity on a denser planet

Take an object weighing 1 pound (0.45 kg) on Earth and put it on a denser planet, one with the diameter of the Moon and the mass of Earth. It will weigh more, since it's closer to the center of gravity on an object with the same mass, but how much more? I've never understood the maths in this area well at all. Nyttend (talk) 12:39, 17 March 2017 (UTC)

• First of all pounds and kilograms are units of mass, not weight, so weighing 1 pound (0.45 kg) should read with mass 1 pound (0.45 kg). (I will assume you are aware of the distinction though.)

Per Newton's law of universal gravitation, the weight is proportional to the product of the masses divided by the square of the distance. For an object that is much smaller than the Moon, we can assume that the distance is equal to the radius of the celestial body under consideration. The Moon's diameter (hence radius) is about a quarter that of the Earth, so the gravitational force will be about (1/4)^(-2) = 16 times higher (closer to 13.4, using the value from the NASA link). Tigraan^{Click here to contact me} 12:46, 17 March 2017 (UTC)

One wonders what sort of matter this planet could be made up of to have such a high density. How could an object the size of the moon compress matter to the enormous pressure required? Dbfirs 13:09, 17 March 2017 (UTC)

A pound is certainly a unit of force as well as being a unit of mass. Edison (talk) 15:30, 17 March 2017 (UTC)

The Earth has a mean Density of 5515 kg/m³. The Moon's diameter is 0.273 of Earth's so the hypothetical planet in the question would have a density of 271055 kg/m³ which is more than any known element, and is only approached at the core of the Sun. This does not affect the math that may be clarified by noting that the distance in Newton's law is measured between the Center of mass (gravity) of each body. Blooteuth (talk) 13:34, 17 March 2017 (UTC)

Also, if we want to be more precise, we assumed the Earth/Moon to be spheres with homogeneous mass distributions, which allowed to consider them as point particles because of Gauss's law for gravity. I doubt that is the level of detail Nyttend wanted though. Tigraan^{Click here to contact me} 14:14, 17 March 2017 (UTC)

Nitpick- Gauss' law also applies if the sphere consists of concentric shells of constant density, the whole sphere doesn't have to be uniform. Greglocock (talk) 19:55, 17 March 2017 (UTC)

It seems vaguely entertaining to look at this in relation to tidal force. The rule with tidal force is:

${\vec {a}}_{t}$(axial) $~\approx ~\pm ~{\hat {r}}~2\Delta r~G~{\frac {M}{R^{3}}}$

i.e. a unit vector multiplied by twice your distance from the center of the object you're on times the gravitational constant and the (idealized spherical center of gravity) mass of the remote object. If you suppose that has a radius x then the tidal force depends on the density of the object multiplied by its size in the sky. (My attempt to derive that was less than compelling, but [5] agrees) For example, the Moon is the same size as the Sun, so it exerts a bit over twice the tidal force because it is a bit over twice as dense as the Sun.

That said, the rule for gravitational acceleration is different:

${\vec {a}}_{g}=-{\hat {r}}~G~{\frac {M}{(R\pm \Delta r)^{2}}}$

In other words, the gravitational force is greater for an object of a given density and the same apparent size in the sky in proportion to how far away it is. That's because the object must be bigger (hence heavier) by a factor of R^3 the further away it is, but the gravity goes down as R^2.

In the case of your example, of course, this is hard to apply because the object fills half the sky. It isn't the angular size that matters and it's not the horizon either - I think it's the apparent diameter of the mid-point of the spherical body, if you could see it, in terms of a straight ruler measurement without reference to angles. Still, it's at least vaguely amusing - if you ever happen to see a rogue planet growing big in your planet's sky you can make a fair guess as to the tide it's raising, but to guess how it's messing with the orbit you need a distance. Wnt (talk) 16:11, 17 March 2017 (UTC)

An even weirder thing is that orbital period's determined only by density (as long as the orbitee's a sphere way larger and massiver than the orbiter, there are no third party gravitational perturbations and it's the smallest possible orbit).Sagittarian Milky Way (talk) 16:36, 17 March 2017 (UTC)

So my £1 object would weigh £16 if I intentionally use the wrong symbol for "pounds". Impressive; I had no idea it would be that much of a change. And yes, weight can be measured in pounds, just as it can be measured in newtons; I originally was going to use kg until I remembered that they're only mass, and I didn't want to use newtons because I don't have a good sense of how much 1N weighs. The density was just made up, a convenient figure without regard for realism. Curious: what would be the Moon's mass were it 100% bismuth, the heaviest element without significant radioactivity? And finally, I don't understand the bit about tidal force; how is it relevant? A person weighs the same on a household bathroom scale regardless of the time of day, and I wasn't wanting anything more precise than that. Nyttend (talk) 00:00, 18 March 2017 (UTC)

@Nyttend: Note bismuth has a density of under 10 g/cc, less than half that of platinum. The densest known element is osmium at 22.59 g/cc. I would assume a denser alloy is possible but all I quickly found was speculation of the same thing. Anyway, to calculate the weight of a denser Moon just take whatever density you can arrange over 3.344 g/cc (its density - basically rock like) and multiply by the current weight. Wnt (talk) 12:46, 21 March 2017 (UTC)

Time crystals

I wonder if anybody could explain it in plain English? What are time crystals? [6]. There is also an article in Wikipedia but it is a difficult stuff to follow. [7] --AboutFace 22 (talk) 17:33, 17 March 2017 (UTC)

PBS Space Time on YouTube just put up a video about the topic that I think is fairly accessible. --47.138.163.230 (talk) 17:57, 17 March 2017 (UTC)

Its just a badly madeup term to put some fake magic into some extreemly boring, jet irritating details of quantum physics. Likely just some silly physicists hoping for more/new funding for their research. --Kharon (talk) 21:10, 17 March 2017 (UTC)

Space-time crystal might be the most "sufficiently advanced or Star Trek BS?"-sounding invention ever that already exists. Sagittarian Milky Way (talk) 23:33, 17 March 2017 (UTC)

Maybe someone will pay me to tool around on my room temperature macroscopic time crystal array simulator.--Wikimedes (talk) 20:33, 18 March 2017 (UTC)

@Kharon, boring? yes. I listened to the youtube lecture and fell asleep. When I woke up the man was still talking. I shut him off. Still I want to understand the issue eventually. --AboutFace 22 (talk) 21:32, 18 March 2017 (UTC)

User:AboutFace 22, Do you understand the bit about having structures that are periodic in time, as opposed to periodic in space, as conventional crystal lattices are? A checkerboard is periodic in space, but four on the floor is periodic in time. This is making headlines because there is both a) a theoretical basis and conjecture and b) a few labs claim to have made stuff that does this - it oscillates freely in a periodic manner. Importantly, the claim is that the materials exhibit periodic fluctuation in time without any added energy, and without any dissipation due to friction. This is seen by the community (and by popular science promoters) as counterintuitive, interesting, potentially useful, and rather unprecedented. That is my attempt to explain in plain English, as a professional (non-physics) scientist and educator. If you have a more specific questions, please let us know. SemanticMantis (talk) 22:01, 18 March 2017 (UTC)

Yes, I also read the time crystal article in Wikipedia, especially the first paragraphs and it seems I understood it better. It sounds like a perpetuum mobile of sorts, a thermodynamic violator or whatnot. I need to go over this a few more times to comprehend. But it is becoming more clear. The fascinating part is that it was first predicted theoretically and then implemented. --AboutFace 22 (talk) 00:06, 19 March 2017 (UTC)

What I find annoying about the coverage of the concept is that something is obviously moving and changing, yet it isn't described in the normal way that moving/changing objects are described. For example, if you have a loop of current in a superconductor I would think that's a "time crystal" in the sense that, at a small scale where their spacing can be analyzed, the electrons ought to line up with the atoms sometimes and not at others. But we just think of it as motion. Here they made some atoms change spin 100 times in a trap and they call that a "time crystal", but it just seems like a fancy version of the thing with the five swinging metal balls that geeks with fancy offices inevitably end up keeping to look sophisticated. I'm not impressed by the perpetual motion aspect unless you can show me it is truly frictionless. Wnt (talk) 12:56, 21 March 2017 (UTC)

Time dilation and warp drive

If a spaceship were to travel using warp drive,would there still be time dilation?Uncle dan is home (talk) 18:28, 17 March 2017 (UTC)

Science fiction assumes not (see Warp drive). For a scientific possibility, see Alcubierre drive. Dbfirs 18:43, 17 March 2017 (UTC)

"If I were driving at the speed of light, and I turned on the headlights, would anything happen?" -- Stephen Wright

←Baseball Bugs ^{What's up, Doc?} carrots→ 19:32, 17 March 2017 (UTC)

You need to work within the framework of a theory that violates special relativity, such a theory must be consistent with all experimental data, which imposes some constraints. In this article the details of what is allowed are worked out. E.g. it may happen that lighter particles will decay to heavier particles, so the protons in your spaceship may decay to neutrons. Count Iblis (talk) 20:18, 17 March 2017 (UTC)

... but the point of warp drive is that you are not travelling through space, so the restrictions of relativity in space-time do not apply. I agree that this is all fictional, and may never be possible. Dbfirs 20:41, 17 March 2017 (UTC)

The only point of "warp drive" is to fool the amateur physic knownledge reader with jet another maybe possible way to tell a faszinating story. Faszinating! --Kharon (talk) 21:02, 17 March 2017 (UTC)

The idea of a wormhole seems fairly well regarded; and if one could exist and, more incredibly, be traversible, the question comes up of how its endpoints are defined, and whether they can move, and if they do move, then consisting of bent spacetime rather than matter, can they move at a rate faster than spacetime. The Alcubierre drive is a weird looking thing, not quite a wormhole, but it also involves highly bent space. There is a whole field of looking at transient distortions of spacetime, starting with gravity waves, gravitomagnetic forces and so on - it clearly is a long way from detecting a distant black hole merger to making a Tipler cylinder, and this would be somewhere in the middle. We don't know it's not doable, and for a decent sci-fi author that is more than enough license. Wnt (talk) 01:54, 18 March 2017 (UTC)

@Wnt:Gravitational waves is what you probably meant. Gravity wave seems to be a term mainly in Hydrodynamics. These "ripples in spacetime" are a nice example how preposterous many physical concepts in Sci-fi are. Imagine, if these tiny ripples need 2 black holes colliding to "fuel" a tiny, microsecond "ripple", how much force would be needed to "bend" space into making a wormhole. Very likely all the force of the complete universe would not be enough to make a wormhole over a distance of 1 metre. --Kharon (talk) 03:19, 18 March 2017 (UTC)

@Kharon: You're right about the link. But -- it takes two black holes colliding over a billion light years away to make a ripple we can detect. If you were in the room with the black hole I dare say you would see space bent a bit further. Think Benjamin Franklin flying a kite... there was some work to be done after that before you could use the principle to microwave beans or to execute ceremonial sacrificial rites in state-run chambers... yet it didn't really take that long to get there. Wnt (talk) 22:49, 18 March 2017 (UTC)

Brass magnetic?

I know pure gold or silver is not supposed to stick to an average magnet, but I'm wondering if brass does. I have a magnet, but I don't have anything I'm sure is pure brass to test this. White Arabian Filly ^Neigh 21:18, 17 March 2017 (UTC)

Brass is not magnetic in the sense you're using (see ferromagnetism). It's composed of copper and zinc, both of which are weakly diamagnetic, so it can be influenced by magnetic fields, but not attracted to them. Matt Deres (talk) 21:27, 17 March 2017 (UTC)

Magnetism is ordinarily limited by the Stoner criterion, but this can be finessed by adding other materials - like buckyballs - if only in a thin film of copper separated by other materials. See [8]. I'd have to know a lot more to have any sense of how much you could get away with if you had the right structure, but it should be obvious that if you define "brass" in a very standard way, such as to be the exact substance ordinarily found to be non-[ferro]magnetic, it won't be magnetic... and if not... Wnt (talk) 01:43, 18 March 2017 (UTC)

The Stoner Criterium ususally requires at least an ounce to show its effects, no? μηδείς (talk) 02:01, 18 March 2017 (UTC)

A criterium seems to be a sort of bike race. I doubt doing it stoned is the best way to win, but who knows. --Trovatore (talk) 18:23, 18 March 2017 (UTC)

It's the next Robert Ludlum film, set in Seattle, and starring Matt Damon as Rachel McConaughey. 23:06, 18 March 2017 (UTC)