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Considering what our sources on ASASSN-15lh say, does it mean that if it was in our galaxy, the light from this hypernova would be seen in the northern sky, even if it exploded in the southern sky? If yes, what would be it's rough intensity compared to the southern sky? Brandmeistertalk 09:19, 18 January 2016 (UTC)[reply]
Well what I saw said brighter than the full moon. The full moon is not apparent on the other side of the world. A fully set sun can't be seen on the fully night side of the Earth. But perhaps you could see some odd lighting on the dark part of the moon, but otherwise if it was below the horizon you should not see anything. Graeme Bartlett (talk) 09:48, 18 January 2016 (UTC)[reply]
"If it was in our galaxy" is somewhat meaningless, given that our galaxy is 100–180 light years across. If it was nearby we wouldn't have much time in which to enjoy the spectacle.--Shantavira|feed me 10:14, 18 January 2016 (UTC)[reply]
Erm, double check your numbers; 100-180 thousand light years. Fgf10 (talk) 10:35, 18 January 2016 (UTC)[reply]
If we see a supernova in our Galaxy it would probably be nearer rather than further because dust blocks anything that's not close or up in the sticks astronomically speaking (page 348). Specifically, that link says most the dust is 100 parsecs wide (326 light years) and Earth is inside it. The part of the Milky Way that's visible is actually less densely populated with stars than the dark rift of dust running down the middle. This is why it's easier to see a much further galaxy through city light pollution than our own Galaxy. Because we are not in Andromeda's central plane but rather see through it at a glancing angle we see through more of the dustless upper or lower suburbs before the line of sight must be stopped by dust. Sagittarian Milky Way (talk) 12:14, 18 January 2016 (UTC)[reply]
Also you can see some southern stars from the Northern hemisphere. At the north pole you cannot see any, but as you approach the equator most of the southern stars will be above the horizon at one part of the day. I suppose you are asking because most of the Milky Way is in the southern sky. Graeme Bartlett (talk) 11:07, 18 January 2016 (UTC)[reply]
The Milky Way is half in each hemisphere, as is required for a great circle. Anywhere but the Arctic you should see more than half reasonably good. The galactic center is in the southern hemisphere (Sagittarius) so more than half the naked eye stars or supernovae (long term average) likely are in the southern hemisphere. But if by southern sky you mean "invisible or hard to see at middle latitudes" that might not be the case, as midnorthern latitudes can see the direction of the galactic center. It would require a majority of stars or visible Milky Way supernovae to be on the southern side of -40° to -50° declination or so and that might not be possible no matter how amazing the sky that far south is. Sagittarian Milky Way (talk) 12:14, 18 January 2016 (UTC)[reply]
The price of many space missions is available, but not how they have reached that amount of money- how much goes to advertising? how much do the materials cost? if I plan my own mission how do I know how much it will cost?
And on the same note- where can I find information about the materials that spacecrafts are used for?
(The questions are for hypothetical mission planning)
77.125.0.41 (talk) 14:41, 18 January 2016 (UTC)[reply]
Don't know what you mean by advertising. This is not generally a commercial product or service. Mars One might be an exception, since they appear to spend most of their funds in marketing themselves. For real programs, go to a project article, for example Space Shuttle program and follow the links. --Denidi (talk) 16:28, 18 January 2016 (UTC)[reply]
Our article on Space advertising covers launch and in-flight/in-space advertising, often a way to bring money into a government space program, not an expense. We don't seem to have any information on private space-launch providers advertising in Earthly sources to attract customers. Rmhermen (talk) 17:43, 18 January 2016 (UTC)[reply]
Indeed, there must be some client-hunting. However flying to space is such a niche industry, with so few players, that everyone must be aware of the existence of all other providers/buyers. Some sort of corporate relationship management must exist though, since there is a series of ancillary providers of products and services. — Preceding unsigned comment added by Denidi (talk • contribs) 18:57, 18 January 2016 (UTC)[reply]
Are we sure the OP referring to advertising by companies or sponsorship of government programmes? It sounds to like the OP is referring to costs, so I'm not so sure. (Of course costs contractors incur from advertising would generally end up being part of the costs they charge, however advertisingwise I think even now the companies probably end up charging less because whatever small amount they spend on advertising, they surely make more back the advertising they get from working on the space programme. That's after all why the government can also get sposorship.) Many space programmes do spend money on what is effectively advertising to promote their programme. A cynic would suggest it's to help bring in government funding but as one of their missions is education, educating people about the mission and getting them interested would also be a core purpose. (And likewise many people involved would be enthuastic about educating and getting people interested as an end goal because they think it's good for these people to learn rather than because it promotes the programme.) However these costs must be tiny, it's not like they do a massive campaign with paid radio, TV, newspaper, online banner ads, Facebook ads, Google ads etc. It's more a case of getting someone to run their website and social media accounts, write press releases, engage with the media etc. Even something like NASA TV or engaging with schools is to some extent partially an advertisement for NASA programmes. Nil Einne (talk) 00:27, 19 January 2016 (UTC)[reply]
At what pressure is the CO2 in a carbonated beverage? Does it make sense at all to talk about the CO2 pressure before opening the can/bottle? I ask because the the CO2 would be dissolved, and not a gas. --Needadvise (talk) 16:18, 18 January 2016 (UTC)[reply]
It varies depending on the brand and the packaging or dispensing method. Slate notes that 12-week old poorly stored plastic bottles can lose 15% of their CO2 compared to a aluminim can packed at the same time and pressure.[1] Fountain sodas can have their pressure manually adjusted. Rmhermen (talk) 17:33, 18 January 2016 (UTC)[reply]
http://hypertextbook.com/facts/2000/SeemaMeraj.shtml gives some clues. The unopened container will have gas in the headspace and this will, of course, have a pressure. The CO2 will, generally, be in equilibrium between the gas and the liquid. Thincat (talk) 19:11, 18 January 2016 (UTC)[reply]
Glass Soda siphons are filled at 60PSIG, plastic bottles at 0PSIG (yes, zero). However things are not so simple - the plastic bottles are filled with the water cold and fully saturated with CO2, the glass with the water at room temperature. CO2 absorbs better in cold water, so when the cold water warms up the pressure in the bottle goes up. They fill cold, at zero PSIG, then put the cap on and let it warm up. The glass bottles are filled using a one-way valve, so all the pressure stays inside.
As the CO2 gets absorbed by the water the pressure drops, until the water holds as much CO2 as it can. So if you fill at a certain pressure, then shake the bottle to mix the CO2, you can noticeably feel the pressure drop. Then you fill again, mix, and the pressure drops less the second time. In my experience it takes 5-10 fills to max out the dissolved CO2. (Yes, I make my own seltzer at home.) Ariel. (talk) 08:33, 19 January 2016 (UTC)[reply]
Asian eyes (epicanthic fold)'s protection from the sun[edit]
Is it true that Asian eyes (the presence of the epicanthic fold and single eyelid) provides greater protection from the sun? I've heard this before, but has there ever been a study that indicates a lower incidence of eye related diseases among East Asians? ScienceApe (talk) 16:27, 18 January 2016 (UTC)[reply]
Epicanthic fold states that the reason is unknown. Some features have no use, so it would be no surprise if no explanation exists. We can speculate that their eyes are more 'squinty' to block out more direct rays of sun. But also we could say that they have more fat to protect against cold weather. These theories could be completely wrong, but they seem to at least have some intuitive merit. --Denidi (talk) 16:33, 18 January 2016 (UTC)[reply]
Sure, it could be due to a lot of things, or maybe nothing (e.g. genetic drift). Maybe sexual selection played a role. Especially in human biology and evolution, we should all be wary of a just-so story. SemanticMantis (talk) 17:35, 18 January 2016 (UTC)[reply]
Protection from what regarding the sun? Eastern Asia, which includes China and Japan, is on the same latitude as the Mediterranean Sea where the native population have no natural epicanthic fold. Similarly the United States is on the same latitude and in either place I am unaware of any higher incidence of eye pathology caused by solar radiation. Richard Avery (talk) 07:47, 19 January 2016 (UTC)[reply]
A feature does not have to be an adaptation to the environment where it is common. You can also find white people nowadays in places like the Mediterranean region or the Southern US. The epicanthic fold could have arisen in a different region. But even then, given the feeble evidence, I stand by the position that the reason why the epicanthic fold exists is unknown, and only speculate about a possible evolutionary advantage. --Denidi (talk) 12:52, 19 January 2016 (UTC)[reply]
It is indeed unknown. But the sun sounds pretty absurd to me. If I wanted to *try* to find a guess for this, I'd need to compare the poorest of the poor - people starving, flies crawling over their bodies laying eggs, pus coming out of their eyes. It might be that one morphology or the other provides some sort of protection against some region-specific pathogen. Wnt (talk) 13:33, 19 January 2016 (UTC)[reply]
With respect, you're substituting one absurd, speculative, almost-certain-to-be-untrue hypothesis for another. SemanticMantis has put forth the only explanation that is to any extent consistent with understanding of the evolution of this variety of superficial feature: Sexual selection. Indeed, this is basically exactly the same archaic/urban myth that continues to persist for why certain people have darker skin tone than others; that too was attributed by folk evolutionary theory / racial segregationists to the notion of natural selection. And it remained popular amongst those who know just enough about the origins of human diversity to accept natural selection but don't know enough to really understand how it works. Today, like most other questions of morphology/phenotypic variations between the features of different races, it is understood to be the result of sexual selection, not natural selection. I know of no serious research to suggest the epicanthic fold is any different in this regard than variations in the morphology of the bridge of the nose, or skin tone, or any other feature which tends to vary between the races--be the source sunlight, pathogen, or glowing mutagen. Please avoid searching, speculative answers of the "it might be that" variety, unless you can provide at least some reliable sourcing that lends WP:WEIGHT to such a notion.
For ScienceApe, you might be interested in some light reading on this topic courtesy of Jared Diamond in the The Third Chimpanzee, where he debunks many of this class of popular myth about how the races are the result of adaptation to localized environments. He wasn't the first to do so, of course--Darwin himself understood that sexual selection was the more sensible explanation of this divergence than his notions on adaptive natural selection, and he was very cognizant of the fact that those inclined towards racist ideology would be tempted to leverage his work on natural selection to imply that humanity was actually composed of subspecies; that's part of the reason The Descent of Man is twice the size of On the Origin of Species. But, in any event, Diamond sums up the contemporary research rather nicely. If you would like more specific and detailed references to particular features (or just more detail on how sexual selection leads to racial divergence) please feel free to ping me here for them or inquire on my talk page. Snowlet's rap 03:24, 22 January 2016 (UTC)[reply]
@Snow Rise: If you're claiming that skin color is the result of sexual selection, that does not agree with the Human skin color article. Any very white kid who has played for a few hours in the Florida sun knows how poorly adapted this phenotype is to lower latitudes! But the article raises points of skin permeability and folate depletion that I didn't even think of ... reminding me that Just So Stories can lead you wrong even when they are right. I don't deny my random suggestion was just one of many conceivable guesses ... nonetheless, I hesitate to attribute everything to sexual selection. I mean, why are epicanthic folds sexually selected for in one place and sexually selected against everywhere else, and how can that be stable for thousands of years? I mean, my own perception of the attractiveness of the trait changed greatly over the course of a decade (much less than that if sexes are considered individually) ... so it scarcely seems like something set in stone that can drive evolution! Wnt (talk) 21:51, 22 January 2016 (UTC)[reply]
That's the very distinction of sexual selection when compared against natural selection; it doesn't require adaptive properties in terms of the same kind of survival mechanisms; it's instead about mate selection and broadcasting (or faking) fitness in order to mate (rather than survive long enough to mate). In terms of research into why these traits get preserved over generations, such that certain populations exhibit increasingly particular traits, it is (in humans and many other social species) governed by the fact that individuals tend to imprint for their notions of attractiveness at a young age. Many studies have shown that if you measure very particular feature's of an individual's spouse (size and shape of earlobe, relative size of brow, eye and nose shape, ect.) they tend to show a close correlation to those of that same individual's siblings and other close relations, relative to the average for those features in the the population to which the individual immediately belongs. Similar studies of other highly social mammals have dyed the fur of parents and siblings a colour which does not occur in nature (hot pink for example) and the offspring from those litters are vastly more likely to select a similarly-coloured mate once entering sexual maturity. Now, for humans, this situation is likely changing and will continue to do so as we become a more global community with more people of mixed ethnicities in our peer groups and immediate families. Nevertheless, this helps explain one of the major mechanisms which answer your inquiry as to how these changes have persisted historically (and often grown more pronounced over time). There are of course other factors involved in Sexual selection--it can include direct ques as to reproductive or general health, for example--and I recommend that article and the books I linked above for the OP if you're interested in the science involved (I'm also happy to collect together and provide links to more specific research if you'd rather be reading the niche experts directly rather than secondary overview; just let me know).
In any event, while it is your own prerogative to "hesitate to attribute" causality with regard to your own impressionistic views that you choose to adopt on a given matter or concept, for the purposes of the ref desks, please refrain from automatically passing this speculation along unless you know you have sourcing for those assumptions, be they affirmative theories of your own or speculation as to how much weight should be given to the consensus notions of a field of research. As to human skin color, I hesitate to comment as I have not yet reviewed its content, but I will certainly do so to make sure it is consistent with the contemporary scientific understanding of this topic--it is, afterall, an immensely important topic that we should have well-vetted coverage on! I will say this much: the notion of skin tone being the result of natural selection amongst humans is not just a just-so story, it's really the just-so story for human morphology, the very one that (arguably more so than any other) helped lead to the widespread understanding of the pitfalls involved in this kind of thinking. If you look at the data for the distribution of the traditional ranges of light or dark skinned peoples and then compare it against the amount an intensity of sunlight that people receive in those ranges, this traditional/folklorish theory falls apart instantly. Snowlet's rap 23:35, 22 January 2016 (UTC)[reply]
I don't think I was sounding all that dogmatic about my idea, and you're blowin' pretty hard here. Comparing skin color to latitude seems like a strawman to me - it could depend on how much vitamin D in the diet (more = darker) , how much folate in the diet (more = lighter), apparently humidity (more = lighter), not to mention tree cover, clothing customs (and the weather that influences them), even the prevalence of unknown factors that increase or decrease cancer risk. But even today, a black family in Scandinavia or Britain (see rickets) will be warned to be sure to get enough vitamin D in the diet, and a white family in the tropics or the outback will need few reminders to keep the sunscreen and the floppy hats close at hand. To deny non-sexual selection applies in this case at all seems absurd - it's one thing to write a just so story, another to live under the selective pressure. Wnt (talk) 02:08, 23 January 2016 (UTC)[reply]
I don't recall ever saying you were being dogmatic or anything remotely in that vein. I said you were speculating, without sources to back up that wild guesswork, on the reference desk, which, at best muddies the waters for the OP's understanding of the issue he came seeking insight on here, and, at worse convinces him to believe your pet theory (Though I rather suspect ScienceApe probably knows better). You defended that decision and, for good measure, decided to further speculate in minimizing a mechanism that has been thoroughly investigated for its role in creating racial variation, a mechanism which I did reference, however briefly (Jared Diamond, with more sources to follow bellow).
Further, you seem to have lost the thread on what is being discussed. Of course human skin (and form generally) has adaptations to make the best of the environment, and these traits can vary between populations to some limited degree. The issue in question (after you put the notion forth) is whether the visually-recognizable racial characteristics are the result of natural selection. (i.e. does skin pigmentation vary in populations as a direct result of the level of exposure to sunlight, are epicanthic folds defenses against some speculative parasite, ect.) Correlation≠causation. Yes darker skin protects well against UV light, but you have to remember that this darker skin was the original phenotype of human populations. You go further to imply that human skin tone lightened due to adaptive pressures to allow more UV light into the skin for Vitamin D synthesis. This source helps explain how that isn't so--by the way, you might want to broaden your academic searches if you don't find what you are looking for in PubMed, which is hardly the be-all and end-all of biomedical/genetic reference material--and will go some way to explaining where your inductive reasoning is failing you here, as will this.
If you can put sources which bring this well-established line of research into question, by all means, please do so; I'm always happy to eat my words in exchange for new knowledge. Otherwise, I ask that you remember that this isn't reddit, it's Wikipedia and claims need to be sourced, and every bit as much on the reference desks as elsewhere on the project. And regardless of whether I have managed to convince you of how the mechanics of human skin variation are thought to work, your original epicanthic fold suggestion that got us on to this topic is clearly just a notion that occurred to you, not something supported by research in the slightest.Snowlet's rap 07:46, 23 January 2016 (UTC)[reply]
I should add I just checked at PubMed and I'm not seeing the evidence you talk about. There's scarcely anything on skin color and sexual selection, and even that recognizes the role of UV. I found this recent paper that says that even beyond visible skin color, northern Europeans have other mutations to allow more UV in to increase vitamin D production. I feel like you're pushing a fringe view. Wnt (talk) 02:19, 23 January 2016 (UTC)[reply]
Please see above. The question your comments raised isn't whether human skin has adapted to protect against UV, it's whether human variation in skin tone is a result of this factor. Snowlet's rap 07:46, 23 January 2016 (UTC)[reply]
Specific gravity of Urine is typically reported in either g/cm3 or kg/m3. Can anyone tell me which unit of measure is correct for the reference ranges provided in the article? — Preceding unsigned comment added by JohnSnyderDTRRD (talk • contribs) 18:46, 18 January 2016 (UTC)[reply]
Technically, specific gravity doesn't have units. Instead, it's the ratio of the density of a substance to that of a reference substance, usually water. That said, the density of pure water is normally taken as 1 g/cm3, so the specific gravity and the density in g/cm3 are usually numerically identical. (The density of water is 1000 kg/m3, so orders of magnitude considerations should probably have lead you to rule that out.) - P.S. If you're attempting to use these numbers for anything important, I wouldn't trust the values given on Wikipedia. Get them from a more reliable source, one which you're confident that you can interpret correctly. -- 19:02, 18 January 2016 (UTC) — Preceding unsigned comment added by 160.129.138.186 (talk)
Do we have anything existing supporting electric transferring from a battery to a any motor whatsoever without a wire(s)? -- Mr. Zoot Cig Bunner (talk) 19:20, 18 January 2016 (UTC)[reply]
There is an article about it here: Wireless power. It has many examples of different ways to do this. Ariel. (talk) 19:36, 18 January 2016 (UTC)[reply]
This is not sufficient, you have to keep it touched...
If I'm understanding you right, you're asking whether it's possible to point a battery at a motor from across the room and make it turn, without close-contact or near-field-transmission. What you're looking for is described at Wireless power#Far-field or radiative techniques; basically, unless you're fitting both transmitter and receiver with perfectly-aligned microwave dishes, any transmission is going to be staggeringly inefficient and become exponentially less efficient with distance owing to the cube-square relationship. (Tesla claimed that the World Wireless System could theoretically transmit power over any distance but general consensus is that even if one could get it to work, a system transmitting enough power to power an electric car at a distance of more than a few meters would (1) get you the kind of electricity bill more normally associated with industrial smelters, (2) kill every living thing in the vicinity, and (3) overheat spectacularly and catastrophically.) ‑ Iridescent 19:45, 19 January 2016 (UTC)[reply]
(adding) There have been some proof-of-concept experiments using high-intensity lasers aimed precisely at photovoltaic cells as a method of transmitting energy over long distances (the particular envisaged application is recharging spacecraft), but this is not something you'd want to try at home, since a laser powerful enough to transmit enough power to run a motor is also a laser powerful enough to blind you instantly. ‑ Iridescent 20:04, 19 January 2016 (UTC)[reply]
Woah...no! Check out a Crookes radiometer (for example) - this is a "motor" that's powered by sunlight - so if you had a laser that produced the same energy per meter-squared as sunlight, it could power the radiometer - and walking through the beam would be no different than crossing a beam of sunlight. Staring right into it might be a bad idea - but no worse than staring at the sun. So long as it was a visible-light laser, I don't think it would be a hazard. Our article on this gizmo says that it can be powered from the heat from your hand...so clearly a very low energy source is sufficient. Also, note that the size and power of the motor is not being asked about here - so we could imagine a tiny, super-lightweight motor that could be powered by light alone from a very dim source. This is far from impossible. SteveBaker (talk) 20:19, 19 January 2016 (UTC)[reply]
Oh, sure, one could power a radiometer with a low-power laser, in the same way one could power a micromotor with a Tesla coil, but I'm assuming that's not what the OP means; given that he says "power any motor", I'm assuming he's talking about something with practical applications, in which case to run a decent-sized motor constantly (as opposed to five minutes of operation followed by five hours charging) would involve lasers or microwaves at ray-gun intensities. ‑ Iridescent 20:27, 19 January 2016 (UTC)[reply]
Yeah "any motor whatsoever" is not reasonable. There are some VERY large motors out there! There are some 100,000 horsepower motors out there (being used by Boeing and NASA in their wind tunnels)...that's 74 Mwatts. There are petawatt lasers out there - but they only fire for the briefest time. There are, however, continuous power megawatt-range lasers (Boeing YAL-1, for example) - so it's not too implausible that we could deliver the power from A to B wirelessly. Obviously you'd need some fancy equipment to turn the resulting light beam into electricity...probably you'd need to boil water and have a steam-powered generator or something. It might be possible...but definitely not easy!
I've read thoroughly, both of your statements made it easy to understand.
One thing I wish to say i.e., this is not mandatory, otherwise humans would've found a way to contain it. Thanks guys. -- Mr. Zoot Cig Bunner (talk) 20:20, 20 January 2016 (UTC)[reply]
Pshh, who needs electricity? Attach blades to a shaft, then laser ablate the blades to turn the shaft. Bob's your uncle. --Link(t•c•m) 21:50, 20 January 2016 (UTC)[reply]
I recall a prototype suit that feeds your body food. Does it exist? If so, why is it not out yet? -- Mr. Zoot Cig Bunner (talk) 19:20, 18 January 2016 (UTC)[reply]
This is an article about it, and this is the appropriate page on the artists' website. Note that it's designed as performance art rather than a practical method of nutrition. Tevildo (talk) 19:57, 18 January 2016 (UTC)[reply]
Looks disgusting; I was expecting something awesome Tev.
Just to clarify, you still have to put something in the stomach right?
Yeah - it's an art concept. I don't see anywhere where they crunched the numbers to see whether adequate calories could be delivered with such a thing (seems highly unlikely) - or whether all of the required nutrients would be available. Meh.
If you consider Spirulina (dietary supplement), 100g of dried algae provides 290 calories - so for a 2000 calorie daily diet, you'd need to consume about 700g of the stuff - and that's dry weight. If you can't dry it out (which would take a lot more equipment and some source of energy beyond what the body could provide), then you'd need to consume around 7 to 14 kg of the filtered wet stuff per day.
This NASA report suggests that with optimal conditions, the amount of Spirulina in an optimum experimental setup with enough CO2, water and sunlight, doubles about twice per day. But wrapped around a human body and getting only whatever sunlight and CO2 is available is FAR from optimal - and the doubling time could easily be more like one doubling every 10 days. So, for example if the temperature in the algae tank/tube is only 25 degC instead of the optimal 38 degC - you'd need to be hauling around 70kg (wet weight) of Spirulina per day to keep yourself in food...add the tubes/tanks and the other equipment to support that vast amount of algae and you're probably hauling around maybe twice your body weight in gear.
On the plus side, spirulina contains almost all of the nutrients your body needs (there is some debate about vitamin B12). However, as NASA helpfully point out: "The most difficult problem in using algae as food is the conversion of algal biomass into products that a space crew could actually eat over a long period of time."
So this "body suit" is either art or fiction. There is no chance of it being real.
The suit seems like a low-tech, very low efficiency way of trying to copy Elysia chlorotica. Looking around I found this cool paper (though I haven't checked the nitty-gritty yet). I don't know if chloroplasts in animals have a chance of providing meaningful amounts of energy, but at least it is attemptable. Wnt (talk) 21:22, 19 January 2016 (UTC)[reply]
I recalled guys, the one's I've read of in the newspaper is called Synthetic Biological Suit/System. Its a armless jacket; like a waistcoat or a cold wastcoat lookalike jacket - the description said something like it... -- Mr. Zoot Cig Bunner (talk) 20:21, 20 January 2016 (UTC)[reply]
To put it simply, water is a liquid that boils at a low temperature, while sodium chloride (salt) is a solid that boils at a really high temperature. So whenever salt water evaporates, the salt stays behind. The water in the air then cools and condenses somewhere - the top of a still, or a rain cloud - and that is fresh water produced by distillation. The process of reverse osmosis is used in industry but rarely occurs in nature, mangroves being the exception. Wnt (talk) 21:30, 19 January 2016 (UTC)[reply]
Speed is relative correct? There is no "absolute" speed as newton pointed out. If you are moving in a car going at 40mph, you are still at rest with respect to the car. So if you have a rocket ship moving at 99%c that ship is moving at 99%c relative to the Earth. And time slows down for the rocket with respect to the Earth. However since speed is relative, you could say that the rocket is at rest and the Earth is moving at 99%c with respect to the rocket. So why does time speed up on Earth with respect to the rocket? As far as the universe is concerned, neither is moving in the absolute sense, so what determines what gets slowed down and what gets sped up? ScienceApe (talk) 20:21, 18 January 2016 (UTC)[reply]
The common belief that "time slows down when you are moving fast" is false. What is true is that each observer "sees" time appear to move more slowly on an object moving at a relative speed. This observation is entirely symmetrical, so the rocket observes time to move more slowly on earth. In fact time is progressing quite normally for both observers in their own reference frame. The Twins paradox is still true because of the change in frames as the rocket accelerates. Dbfirs 20:30, 18 January 2016 (UTC)[reply]
I believe there is another misconception in your question. It is in the bit 'no "absolute" speed as newton pointed out'. According to special relativity, there is one absolute speed: the speed of light.--Denidi (talk) 21:27, 18 January 2016 (UTC)[reply]
Yes, in the sense that your speed relative to light coming from any direction will always be c in free space, and the same fraction of c in any given medium (air, water, etc). Dbfirs 21:35, 18 January 2016 (UTC)[reply]
I believe precisely this question is addressed by the Twin paradox. Vespine (talk) 21:46, 18 January 2016 (UTC)[reply]
Yes, I accidentally put an "s" on the link I gave above, but it still goes there via a redirect. Dbfirs 21:49, 18 January 2016 (UTC)[reply]
I'm still having my 1st coffee of the day, completely missed you already linked it:)Vespine (talk) 21:53, 18 January 2016 (UTC)[reply]
No problem, it's nearly bedtime here. Better to link twice than not at all. Enjoy your day. Dbfirs 22:00, 18 January 2016 (UTC)[reply]
These types of questions about spacetime have analogues in Euclidean geometry. The Euclidean analogue of your question is the question of which of two non-parallel lines is "denser" than the other. If you draw perpendiculars to one line at regular intervals and extend them until they intersect the second line, the second line will be longer between each pair of consecutive perpendiculars (by where m is the slope of one line relative to the other), so it's "denser". But if you switch the role of the two lines, the first line is the one that's "denser". How can that be? Well, it just is. There would be nothing surprising about it if I hadn't introduced the confusing concept of "density" of lines, which is nearly identical to "time dilation" in spacetime.
The diagram to the right shows the slightly more complicated case of the Euclidean twin paradox. You can solve this with the perpendicular lines, and get the right answer (as shown). It's not wrong, just stupidly overcomplicated. The simple answer is that the two sides add up to more than the one side because of the triangle inequality, or the fact that a straight line is the shortest distance between two points. It's the same thing in spacetime, except that because of the opposite sign of the spacetime version of the Pythagorean formula, a straight line (inertial motion) is the longest distance (elapsed time) between two points (events), so the stay-at-home twin is older.
Is there any device that can measure the depth underground from the perspective of a man below, such in caves, subways or how deep a man is in catacombs below the ground level? Seemingly sonar-based devices wouldn't work in that case. [[--93.174.25.12 (talk) 21:41, 18 January 2016 (UTC)[reply]
A basic Pressure altimeter should still work as long as the chamber is not sealed air thight. Vespine (talk) 21:50, 18 January 2016 (UTC)[reply]
(ec) Interesting question. I suppose an pressure altimeter could work, although with a few caveats:
1) The cave would have to be open to the air, so air pressure in the cave would not be disconnected from the atmosphere. So, an air pocket in an undersea cave wouldn't work (or at least it would need to be adjusted to reflect the water pressure from the water above it).
2) It would tell you how far above or below sea level you were, and you would need to know how high above or below sea level the surface is, to determine how far below the surface you were.
3) It wouldn't be all that accurate, as air pressure also changes with the weather, etc. Taking an accurate air pressure reading at the entrance of the cave would help to account for that. Air pressure changes in caves also tend to lag the changes outside, so you would need to account for this lag, or better yet, take your readings when the air pressure has been stable for many hours. So, there would be a fair amount of calculation to be done, but this could all easily be automated. If you took one pressure altimeter with you and left one at the entrance, then you could use data from both (after you leave), to calculate the depth at various times in your expedition. StuRat (talk) 21:52, 18 January 2016 (UTC)[reply]
Not a realistic answer, but apparently muons penetrate deep underground, but may be stopped by tens of meters of rock, so in theory....... hmmm, I dunno, but this came up when I searched. Wnt (talk) 13:24, 19 January 2016 (UTC)[reply]
For more precision than you can get with a pressure altimeter using air, you can use a tube filled with water that leads (possibly by a circuitous route) to the surface and measure the pressure at the bottom of that. The difference in pressure at the ends of the tube (knowing the density of the water) gives the elevation - and the large values of pressure should make it easier to measure it accurately. The incompressiblity of water should eliminate the 'lag' effect you get when there are pressure changes at the surface. However, variations in temperature may make the density estimage inaccurate - but that's true when you do it in air also. This approach is called "Hydrolevelling" (Hmmm...no article on that?!). SteveBaker (talk) 18:44, 19 January 2016 (UTC)[reply]
I thought of suggesting that, but the difficulty in snaking a water line deep into a cave dissuaded me. StuRat (talk) 22:11, 19 January 2016 (UTC)[reply]
And of course, you can just drop a rope with marks on it down a vertical shaft, or use sonar to measure depth there. Any horizontal chambers or tunnels will have the same depth. For angled tunnels, you can then use the same methods listed previously, but measuring from where the tunnel meets the vertical shaft, instead of from the surface. Temperature variations will hopefully be less there. StuRat (talk) 03:57, 20 January 2016 (UTC)[reply]
This is not sufficient, you have to keep it touched...
-- Mr. Zoot Cig Bunner (talk) 20:20, 20 January 2016 (UTC)
--Link (t•c•m) 21:50, 20 January 2016 (UTC)
-- Mr. Zoot Cig Bunner (talk) 20:21, 20 January 2016 (UTC)
