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There are definitely near perfect pages on Wikipedia, like the German one on Neandertals. But as well dismal nonsense. Academics don't use it as a reference (i recall students being kicked out of seminars), as you say, and so care as much as they do about articles in a local newspaper (not at all). Wikipedia changes each day and we had better things to do.

One can actually lie or simplify pretty well on such a soft subject as this here, because really good publications are rare, and knowledge changes quickly. There are no constants, and the real people have enough other things to do. That leaves space for self appointed experts :-)

Edited by Green Baron
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1 hour ago, Green Baron said:

I am probably talking to a wall since the force is strong with wikipedia. It is too tempting to just click and have a ready made answer. But in principle, what is ready made for a complex situation is almost always made up, willingly or not.

Phew, don't be mad at me :-)

I've been using the phrase "infallible wiki" for a bit under a year.  I never thought anybody would post the slightest suggestion that I was serious.  I've found errors, but never was willing to wade into the politics of wiki to correct it (I have edited a couple local wikis for specific domains).  I find it good enough for "internet discussions", but for ones at least as accurate as here I have to admit when I'm using it as such data can't really be trusted (thus my use of "infallible wiki").

I loved that the Otiz page gave specific years for his birth and death.  Carbon dating must have come a long way.  But in the sciences I've seen at least one case where wiki had the sign wrong in an equation (eventually I figured out the mistake).

Edited by wumpus
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I've had to fix a few mistakes on Wikipedia before, but I think I'm used to the STEM articles on Wikipedia, where mistakes are relatively easy to point out and fix. In most scientific contexts, answers are hard to find, but very easy to verify if you know the basics. So the articles are very rarely wrong on something so fundamental. Hence the habit of falling into a general attitude of, "Wiki is probably right, until proven otherwise."

I'll have to keep in mind to take a different attitude towards historical and archaeological articles.

What @Green Baron says about bronze vs copper use makes perfect sense to me purely from material science / geology standpoint. There are good places to get malachite and related ores in Egypt, so the plank for working with copper is very low. But tin, in any culturally significant quantities, would have to be imported. And while copper has many wonderful properties and uses on its own, it won't replace stone tools and weapons. The metal just isn't hard enough. My confusion has been entirely on chronology of when significant tin trade started to take place.

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@K^2

I'll admit I find observed and theorized things more concrete than things theorized before observation. I am an experimentalist after all! Of course I also value the usefulness and power of inductive reasoning- Theory and Experiment are like the Yang and Yin of science.

You had something cool to bring to the table with the theorized non-relativistic remnant neutrinos, but it doesn't feel like you're trying to have a conversation about it, which could be fun and enlightening, but rather that you're trying to rub my nose in it. I'm not sure if it's on accident or on purpose, but to be honest... no thanks! Peace!

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@Cunjo Carl That is not a constructive attitude, but your call. A statement was made concerning neutrinos that has no basis in fact. You helped reinforce that false notion, and I wanted your help in rectifying it. That was clearly a mistake, because the net result is that you've helped an anti-intellectual remain smugly ignorant, and he will, no doubt, continue spreading his nonsense in other threads.

Personally, to me, knowledge is always more important than pride. As much of the later as I have, I'll gladly swallow it to get the facts straight. And honestly, I don't know why we even have a Q&A thread in the science subforum, if that's not the attitude that people are willing to take. If we aren't prepared to defend a point, because we are afraid of being proven wrong, then we might as well use a magic 8 ball to answer the questions.

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So while removing a part from a CNC machine today, I blow out the inside with the air hose.   The part is a hose coupler, 1.6 in inside diameter (40mm?), and the airhose, is maybe 3/8" (~10mm) outside diameter.    The inside of the part shaped similar to a trombone bell, long and narrow wit a flaring opening on one end.   I am blowing into the flared end. 

I believe it's the venturi effect that is kicking that is accelerating more air through the part than I am actually blowing into it.   It's basically how Dyson fans and hair dryers work.   But, at a certain distance from the hose, the part is drawn towards the hose, like a magnet.  Is it suction or thrust? 

If anybody needs a diagram, I have one drawn, but imgur is not loading... so I'll wait. 

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Still a side-effect of Venturi. Pressure inside the part is lower, due to moving air, so external pressure is squeezing on the outside, pushing it like a wedge towards the hose. There's ram pressure due to incoming air as well, of course, but whether one or another wins out is a matter of geometry and flow speeds. In this particular case, the ram pressure is lower.

I don't have a good demonstration with that exact geometry, but I believe this demo shows identical effect with different geometry.

 

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On 9/12/2018 at 4:13 PM, Cunjo Carl said:

[snip]

Ah man, the whole thing gave me the willies. With quantum computing on the horizon, I wonder how many more decades we have before HAL is a reality? On the flipside, the possible benefits are just too tempting to ignore, and I think there's a good possibility that AI won't be any better/worse than people... 'May you live in interesting times', I guess!

The problem with HAL was that the military gave HAL a very high priority order to keep the actual destination/purpose of the mission a secret from the crew that was awake, not realizing that HAL, as a computer will obey the highest priority order absolutely, even to the point of trying to kill the crew if they were about to learn the secret that was supposed to be kept.  (part of killing the cryo crew members was because they knew the secret and if they were woken up they might tell the awake crew members)  The entire 'homicidal HAL' was just a simple case of 'be careful what you ask for' where the person who asked for something was not the one who suffered from that order.

On 9/12/2018 at 5:47 PM, Green Baron said:

I have tried again lately, but until now i have not yet made it through the hole film 2001 without fast forwarding. Without an explanation from the "special features" i didn't even dig what it is about; that aliens save the world by letting all the nuclear weapons vanish ...

---------------

Will quantum computing really be faster than the classic electric stuff ? Or must we search for problems to the solution in order for it to be so ?

The enthusiasm and google style hype seems to slow down or am i mistaken ?

'2001: A Space Odyssey' is a book that was made into a movie.

premise/plot:  

Spoiler

 

0) In pre-history some strange object(s) appeared and helped homo-sapiens learn to use tools/become intelligent (The cave-man/monolith scene, where the monolith is actually taking control of their bodies to demonstrate how to do those things)

1) An odd magnetic field was found on the moon during an orbital survey

2) when they dug it up, they found a clearly artificial object with the dimensions 1x4x9 (ie 1^2 x 2^2 x 3^2) to their best ability to measure it.  They theorize that it may continue this progression into higher dimensions.

3) When sunlight hit the previously buried object, it sent a signal to something in orbit of Jupiter(and stopped having the magnetic field that let it be found)

4) Scientists figured that this was a message left for us to find once we were advanced enough to do so, with the signal pointing us towards the place we were supposed to go

5) A mission is sent to Jupiter supposedly to explore one of the moons, but in reality to investigate where the signal was sent.  HAL and the cryo-sleep crew knew the real purpose, but the awake crew did not, and HAL was told to keep the real purpose a secret by someone who did not know much about computers, but had lots of authority.

6) The conscious crew is in danger of discovering the real destination, so HAL starts killing them to keep the secret (this is where most of the drama comes in)

7) HAL is disabled and the lone surviving crew member does a space-walk to investigate the monolith(1x4x9 object) at the ship's destination.

8) The surviving crew member is taken by the monolith to a dimension where he is maintained for the rest of his natural life and is then re-born as the 'star child'   During this time he learns that the monoliths are an ancient self-replicating tool with the purpose of cultivating intelligent life, but it is also specifically prevented from making it's own decisions in some cases

9) the Star Child learns that Jupiter has native species of gas-type creatures, but as it is impossible for them use tools or ever leave their world on their own, it wants to turn Jupiter into a star so that the fledgling life on Eurpoa(that comes to exist with and then dies out with thermal vents) will have a chance to evolve into intelligent life.  But as a tool, it is not permitted to make decisions of genocide such as this, and requires the help of an organic being(the star child) to make such decisions(this was the purpose of the monolith on the moon and the signal out to jupiter: to bring an being here that can make that decision.)

10) the star-child agrees that destroying the life on Jupiter which has no chance of developing intelligence to give the life on Europa the opportunity to do so is a reasonable choice

11) the monoliths convert more and more of the atmosphere of Jupiter into Monoliths until the density of the planet is such that it ignites into a star.

12) Shortly after Jupiter ignites, a message is received by earth, which just saw Jupiter ignited, saying 'All these worlds are yours, except Eurpoa, attempt no landings there'

Note: 8, 9, and 10 happen between 2001 and 2010

So no, not much to do with nuclear weapons.

 

Quantum computing allows you to work with values that are a super-position of both a 1 and a 0, letting you thereby do math with a large set of numbers in parallel using one one set of (Q)bits.

The more Q-bits you have, the larger the set of parallel numbers can be(3 q-bits can be 0-7, 4 can be 0-15, etc).

There are a large set of problems that have been proven to be mathematically equivalent to factoring large integers(such as breaking most modern cryptography, which literally requires factoring a large number that is known to be the product of two large numbers), these are the problems that quantum computing is known to be capable of helping perform in greatly reduced time-scales, but will also require large numbers of q-bits(cracking a 2000 bit HTTPS session for example probably requires 1000 q-bits to crack in a single operation(although 100 q-bits may be able to crack it in more or less real-time))

Edited by Terwin
Forgot that the Jupiter Ignition does not happen until 2010 book/movie
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I have that from the special features on the DVD and interviews with the actors. Film and book were made side by side, they said. It wasn't my idea, the nuclear weapon thing ... *shrug*

-------------------------------

[snipped]

I took out what i linked and wrote about "scientists" reasoning about lost civilisations and their fingerprint because game forum ... :-)

Edited by Green Baron
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19 minutes ago, ARS said:

...the pointy-shaped tip creates less drag and more streamlined than rounded tip

This statement is not correct, although it depends on what you mean by "pointy" and "rounded".

Not all areas of fluid dynamics are equal, hence a 747 airliner has a rounded nose and the Concorde was super-pointy, both were designed to be optimal in different regimes.

Hydrodynamics has its own rules so optimal shapes are different again.

In the case of torpedos there are other consideration as well, such as optimising for transmission of sonar signals. Modern sonar use a phased array of transducers which prefer a flat plane:

TorpedoF21propagandagrande.jpg

 

There is also a rumour that a flat nose profile can facilitate a cavitation effect which would reduce skin drag along the fuselage of the weapon. Not a dramatic supercavitation like with the "Shkval" but similar.

And finally, different types of torpedos have different missions, some are faster and some slower, some for shallow/surface/antiship and some for deep-diving/antisub, long/short range, ship/sub/air launched/dropped, and some are multi-purpose, so there are a lot of design trade-offs.

 

Here's some bedtime reading (note this is from 1972, it may not entirely cover modern torpedos):

http://www.dtic.mil/dtic/tr/fulltext/u2/744314.pdf

 

(Torpedos are borderline strategic weapons, so i expect there is a fair amount of stuff that we wont see due to it being classified, eg: the drag coefficients of modern weapons etc.)

 

 

Edited by p1t1o
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You are right for high speeds, but not at the low speeds of a torpedo.

A drop-shape would be the least resistant one, round head, pointy end.

Also ships have a round bulb below the waterline to reduce drag.

 

Edit: ninja'd with a better explanation by @p1t1o. Though i find "safer" in conjunction with a weapon to ... depend on the view point :-/

Edited by Green Baron
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Rule of thumb, rounded for sub-sonic, pointy for supersonic, and speed of sound in water is much, much higher than in the air. But as people said, it depends on what you are optimizing for.

The only torpedo in operation that I'm aware of that benefits from pointy nose is VA-111 Shkval, and even that because it generates a gas envelope to travel through, which has some characteristics similar to supersonic shock. That does allow it to be the fastest underwater vehicle (unless one of the other supercavitating prototypes has beaten it recently), but one may argue that moving underwater not through water is almost cheating.

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6 hours ago, K^2 said:

Rule of thumb, rounded for sub-sonic, pointy for supersonic, and speed of sound in water is much, much higher than in the air. But as people said, it depends on what you are optimizing for.

The only torpedo in operation that I'm aware of that benefits from pointy nose is VA-111 Shkval, and even that because it generates a gas envelope to travel through, which has some characteristics similar to supersonic shock. That does allow it to be the fastest underwater vehicle (unless one of the other supercavitating prototypes has beaten it recently), but one may argue that moving underwater not through water is almost cheating.

Worth noting that at the extreme nose, the Shkval has a flat plate and some other features to generate the supercavitation bubble, the fluid dynamics here are a whole other kettle of fish.

20170121_STP002_0.jpg

Edited by p1t1o
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5 hours ago, p1t1o said:

 Worth noting that at the extreme nose, the Shkval has a flat plate and some other features to generate the supercavitation bubble, the fluid dynamics here are a whole other kettle of fish.

 20170121_STP002_0.jpg

Looks like the direction control, this is wire guided all the way to the target as it can not do acoustic tracking like standard torpedoes.
As I understand an hollow point bullet has less water resistance than  standard pointed bullets because supercavitation  effects, note this would not work with an regular hollow point as it would just expand so you would need an massive copper front with an depression on the tip. 

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The direct influence of water vapor is debated / questionable / poorly understood. But the argument is sometimes used by climate change deniers to say "See, all not that bad !". I know you are none of those !

Anyway, water vapor does by far not have the same green house effect as Co2 or Methane. It is just extremely abundant and so plays the main role in keeping our planet's temp 15K above equilibrium. Which is quite convenient, if i may say so. Also, water vapor is part of cycles and feedback loops, like much of chemical erosion and physical transport of sediments, etc. Thus accelerating some processes or slowing them down, depending on its abundance and form of appearance. The weathering of a mountain range can have a global cooling effect on a geological timescale by binding atmospheric Co2, extreme weathering can even play a role in initiating an ice house. Now, if the weathering processes based on precipitation change, what ? Or will more water in the atmosphere even rise the albedo if temp stays low enough for clouds to form, thus having a much more direct impact on climate ? Idk ...

Otoh changing Co2 or Methane concentrations in the atmosphere by only tiny amounts rises water vapor levels as well indirectly because warmer air holds more water, besides and additionally to their direct adverse effects on temperature and affecting climate on a much shorter timescale than for example weathering, like regional weather as well as climate.

Since a switch to global use of hydrogen cars is not coming soon, the question isn't that important yet and i know of no serious study on that.

One can determine the relative fractions of atmospheric or sea water by measuring of fractionation of stable isotopes of O (18 and 16). The lighter ones tend to evaporate faster, thus leading to a higher fraction of lighter isotopes in the atmosphere than in the water. But it is yet not possible to give a definitive absolute value, since too much influences the measurement, like other reservoirs / wells / sinks, temperature, transport in sea or atmosphere ....

Here is an example of a recent study that uses this technique:

https://www.nature.com/articles/s41598-018-25522-5

----------

Hope my stuff isn't too chaotic. Specific questions are always easier to answer than general ones :-)

 

Edited by Green Baron
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49 minutes ago, Aperture Science said:

Apparently water vapor has a greater greenhouse effect than carbon dioxide, so if we start using hydrogen fueled cars, isn't global warming going to get worse than by using conventional fuels?

Short answer - Whilst water vapour does have a greenhouse effect, it also tends to condense and fall out of the sky. So rather than production of water vapour being a problem, it is the average temperature of the globe that affects the held water vapour content of the atmosphere, so in that regard, it is relevant to global climate change discussions, but more as a symptom rather than a cause.

Increase water vapour production (with hydrogen cars) and all you do is increase rainfall.

But if other persistent greenhouse gases cause a global temperature increase, the capacity of the atmosphere to hold water vapour increases, adding its greenhouse effect. Potential for a feedback loop here is the basis for a "runaway greenhouse effect". But that is worst-case scenario territory.

 

Its also worth noting that hydrocarbon exhaust contains significant amounts of water vapour already but CO2 is still the main gas of concern.

Edited by p1t1o
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That is the point i couldn't express well enough: The amount recent rise of water vapor in the atmosphere is an outcome of temperature rise, controlled by CO2 emissions, not the cause of warming. Edit: the overall amount is one of the reasons for our planet being so life friendly.

Edit: I found a link on the subject that appears to be relatively neutral, though ten years old, i'd say still valid. It leaves the tectonic stuff aside.

https://www.yaleclimateconnections.org/2008/02/common-climate-misconceptions-the-water-vapor-feedback-2/

 

Edited by Green Baron
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31 minutes ago, Green Baron said:

That is the point i couldn't express well enough: The amount recent rise of water vapor in the atmosphere is an outcome of temperature rise, controlled by CO2 emissions, not the cause of warming. Edit: the overall amount is one of the reasons for our planet being so life friendly.

Edit: I found a link on the subject that appears to be relatively neutral, though ten years old, i'd say still valid. It leaves the tectonic stuff aside.

https://www.yaleclimateconnections.org/2008/02/common-climate-misconceptions-the-water-vapor-feedback-2/

 

Water vapor generate greenhouse effect in the atmosphere, co2 in the stratosphere. 
And its an significant effect, its why deserts are so cold during night compared to other places. Was about to say that clear winter days are colder than cloudy but that might be the other way around as in no clouds then its very cold. 
As water vapor is local it would contribute to urban heat islands, on the other hand an humid air help wash out pollution and its hard to reduce as its mostly because of weather. 
Finally fossil fuel also generate lots of water vapor then burning, 

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