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December 20

What is the melting point of diamond?

In the article it just says "pressure dependent". Well, at least what is diamond's melting point in standard pressure?--Inspector (talk) 01:21, 20 December 2012 (UTC)[reply]

I don't think it has one. If I recall correctly, at one atmosphere, carbon sublimes rather than melting. --Trovatore (talk) 01:36, 20 December 2012 (UTC)[reply]

It doesn't just say "pressure dependent", the entry in the infobox is a link to Carbon#Characteristics. There it says At atmospheric pressure it has no melting point as its triple point is at 10.8 ± 0.2 MPa and 4,600 ± 300 K (~4,330 °C or 7,820 °F), so it sublimates at about 3,900 K. Tonywalton ^Talk 01:40, 20 December 2012 (UTC)[reply]

The scientist who wanted to find this out made a 6 foot (~2 meters) magnifying glass filled with alcohol (why? to reduce the weight? to reduce the cost?) to focus sunlight. He put a diamond in front of it (bought with personal money) and it disappeared. For some reason I remember it turning into carbon dioxide under the glass, though.?? And what substance managed to hold it long enough for the diamond to be destroyed? Platinum? How did he not go blind, at least temporarily, or did he look away? Maybe the diamond was quite small. And it must feel like an oven next to that thing. Sagittarian Milky Way (talk) 02:52, 20 December 2012 (UTC)[reply]

I've seen an experiment where someone heated a diamond until it was white hot (or close to it) using a blow torch and then dropped it into a container of liquid oxygen, after a while of bubbling there was no diamond left. I'm sure if you googled it you could find a video on youtube. Expensive way to prove diamonds are not indistrucable. Vespine (talk) 03:00, 20 December 2012 (UTC)[reply]

See the diagram (above, right) for the most eloquent answer. μηδείς (talk) 03:21, 20 December 2012 (UTC)[reply]
- The diagram is good, and goes to show that room temperature/room pressure diamond is essentially metastable, as it isn't the ground state phase at those conditions. --Jayron32 04:42, 20 December 2012 (UTC)[reply]

Graph appears to show full graphite at 10 atmospheres and absolute zero. It does say it's not pixel perfect and based on theory, though. Sagittarian Milky Way (talk) 06:01, 20 December 2012 (UTC)[reply]

So it says it changes to graphite before it melts/sublimes?--Inspector (talk) 07:16, 20 December 2012 (UTC)[reply]

Sorry, I just mistaken the Gpa in that diagram as MPa.--Inspector (talk) 08:34, 20 December 2012 (UTC)[reply]

Graphs like that are always a bit inaccurate and fuzzy. As SGM notes above, it isn't "pixel perfect". There's probably a time-dependency on certain phase transitions: a slow, gradual heating at low pressure may provide the ability for diamond to rearrange into the more stable graphite form before melting/subliming while a sudden burst of extremely high heat may cause it to essentially instantly vaporize without waiting to rearrange to graphite, for example. --Jayron32 14:01, 20 December 2012 (UTC)[reply]

The problem is all in the kinetics of the reaction - the change just happens so slowly at even fairly high temperatures that it's difficult to get it to happen. These researchers tried their hardest using both high temperatures (~1000°C) and bombarding with Argon ions, but only managed to observe what they interpret to be the first stages of such a change. Mikenorton (talk) 19:02, 20 December 2012 (UTC)[reply]

Myans accuracy

If the Myans were so accurate in astronomy then did they know about relativity, the speed of light, gravity's relationship to curved spacetime?165.212.189.187 (talk) 14:06, 20 December 2012 (UTC)[reply]

No. --Jayron32 14:12, 20 December 2012 (UTC)[reply]

Definitely not. 217.158.236.14 (talk) 14:43, 20 December 2012 (UTC)[reply]

Although it has been claimed (by e.g. Paul Davies) that an intelligent caveman could have theoretically derived the equations of General Reletivity and electromagnetism by demanding covariance under general coordinate transforms and local gauge transforms. Count Iblis (talk) 15:01, 20 December 2012 (UTC)[reply]

But to make that very straightforward demand, one would need a well-developed wave theory of electromagnetism as a prerequisite, which would require the preceding discoveries about electrostatics. Otherwise, we must ask: "what" remains constant under the general transform? Not everything; not length; not mass. The answer, of course, is that the most fundamental constants remain constant: the electric and magnetic permissivity/permittivity of free space, which in tandem define the speed of light (rather, the constant of propagation in the wave theory of electromagnetism). I have no doubt that an (arbitrarily-intelligent) caveman could deduce such facts, but reasoning alone would not substitute for the necessary intermediate experimental discoveries. Particularly, the experimental evidence that ε₀ and μ₀ are in fact perfectly constant regardless of frame speed requires some very well-calibrated machinery that is easier to build after the cavemen invent machine-tool industrial society. After these substantial developments to validate this not-so-obvious fact, the next caveman could state with confidence what every earlier caveman might have already speculated about the very strange nature of our universe. Nimur (talk) 21:02, 20 December 2012 (UTC)[reply]

Strange, compared to what? -- Jack of Oz ^[Talk] 07:39, 21 December 2012 (UTC)[reply]

Relativity, the speed of light and gravity's effect on spacetime are much more to do with physics and astrophysics rather than astronomy, which deals with the actual objects in the universe. So probably not. douts (talk) 15:47, 20 December 2012 (UTC)[reply]

Count Iblis (or anyone else), could you elaborate on Paul Davies' claim or provide any references? I can't see how that statement means anything other than the rather self-evident claim that we have derived those things and we are just latter-era cavemen. 87.112.53.136 (talk) 17:23, 20 December 2012 (UTC)[reply]

Greg Egan's novel Incandescence is about the development of General Relativity theory by primitives ... in some very peculiar circumstances. —Tamfang (talk) 16:40, 27 June 2013 (UTC)[reply]

Would their projections require knowledge of these in order to be truely accurate?165.212.189.187 (talk) 15:57, 20 December 2012 (UTC)[reply]

What, exactly, do you think they predicted? The answer is probably 'no', but people seem to be talking a lot about things that Mayans did not, in fact, predict at all, so it's best to be sure. AlexTiefling (talk) 16:06, 20 December 2012 (UTC)[reply]

I'm not really concerned or know too much about it but I thought that they claimed that there would be no more earth after tomorrow? Or is that just when they got tired of chiseling stone for the time being?165.212.189.187 (talk) 16:15, 20 December 2012 (UTC)[reply]

The Mayans didn't predict the end of the world on that date. The current fad is mostly fabricated by sensation hungry journalists, with a bit of support from some Russians and New Agers. --Saddhiyama (talk) 16:39, 20 December 2012 (UTC)[reply]

They could accurately predict the movement of stars and planets in the night sky as seen from earth, based on how they had moved in the past. Knowledge of gravity and relatively are not required for that. Greek, Persian, Egyptian, and Chinese astronomers, among others, did the same thing back in the day. thx1138 (talk) 16:49, 20 December 2012 (UTC)[reply]

Why would someone need to know all this stuff? And where do you get the idea that the Mayas were accurate? They were unique among all people who didn't have access to a telescope, but did they know something that we didn't know? OsmanRF34 (talk) 18:34, 20 December 2012 (UTC)[reply]

You don't need a telescope to calculate the motions of the planets and stars. Someguy1221 (talk) 08:24, 21 December 2012 (UTC)[reply]

Right, but knowing something about the Orion Nebula without one is amazing. OsmanRF34 (talk) 13:30, 22 December 2012 (UTC)[reply]

They didn't do a very good job of predicting the end of their world due to discovery by Spain. Gzuckier (talk) 07:27, 21 December 2012 (UTC)[reply]

Where in the world will you find an analogue calendar that goes on forever? Answer: nowhere. The rules for the calculation of such a calendar are available, but when people write down the actual calendric details for the next X years, X is always a finite number. Why would the Mayans have been any different? They had to stop somewhere, and here we are. When you look in your desk diary, and see the calendars for the next 3 years but no further, does anyone ever think this means the world is going to end in 3 years? Obviously, no. -- Jack of Oz ^[Talk] 07:46, 21 December 2012 (UTC)[reply]

I use a calendar called Microsoft Outlook, so I am absolutely certain that the world will never end! --Lgriot (talk) 10:11, 21 December 2012 (UTC) [reply]

Champ

How many recorded sightings of Champ have there been in Port Henry, NY? — Preceding unsigned comment added by 216.239.242.7 (talk) 19:32, 20 December 2012 (UTC)[reply]

Over 300, according to the dead link cited by Champ (cryptozoology). Red Act (talk) 22:18, 20 December 2012 (UTC)[reply]

I've updated that link to the new URL on that site. However, that's the total number of sightings, not just those from Port Henry. Rojomoke (talk) 13:34, 21 December 2012 (UTC)[reply]

Possible footprint in Curiosity photo.

I know that this is a government picture, and not just some hobo trying to prank people, but still, why in the name of our savioris a footprint doing directly to the left of the closest rover wheel? Tread marks? Buggie111 (talk) 23:10, 20 December 2012 (UTC)[reply]

There is something that looks vaguely like a shoe print immediately to the left of the wheel in the low resolution version, but downloading the highest resolution version makes it clear that it is merely an artifact of a wheel being steered. Floda 60.230.197.188 (talk) 00:24, 21 December 2012 (UTC)[reply]

This reminds me that if you take any image of Mars or Moon from wikipedia and zoom in, at the end you see rectangular structures like blocks of a street map. There are people beliving that this is the prove for intelligent life in both places.(I know that this is called a pixel)--Stone (talk) 07:11, 21 December 2012 (UTC)[reply]

SteveBaker made an excellent post about this, showing you could find what looks like an alien city zooming in far enough on some boring image. ~~I forget where in the archives he posted this.~~ Someguy1221 (talk) 08:25, 21 December 2012 (UTC) Correction, that was actually really easy to find: Wikipedia:Reference_desk/Archives/Science/2009_October_23#Apollo_photo_number.3F Someguy1221 (talk) 08:28, 21 December 2012 (UTC)[reply]

Wow! I'd forgotten all about that post. I was definitely on a rant that day! The "footprint" in the OP's photo is more a case of Pareidolia than of inappropriate image enhancement...but User:Stone's point is very true. You can produce all sorts of interesting images that look like alien cities or whatever just by the process of repeatedly loading and saving the image in a lossy image format like JPEG or GIF and/or by using image "enhancement" tools. In my post that User:Someguy1221 linked to, I deliberately mis-used these techniques to "find" a convincing photograph of an alien city from a screen-shot of the OP's post here on Wikipedia! The alien city map was "hidden" in a period at the end of his post. The resulting conspiracy theory is pretty much on a par with what most web sites who deal with these kinds of claim would have said. SteveBaker (talk) 14:33, 21 December 2012 (UTC)[reply]

Are you talking about the scoop marks from sampling the sand ripple ? Sean.hoyland - talk 07:22, 21 December 2012 (UTC)[reply]

It's interesting that this 'arm' which carries the camera in order for it to take a picture of itself, is actually not visible. KägeTorä - (影虎) (TALK) 20:33, 22 December 2012 (UTC)[reply]

Generally for these photos they take a series of shots (a sweeping panorama) and stitch them together. They try as far as possible to "erase" the self-image of the arm by stitching those parts of each photo that have the arm excluded. Zunaid 20:51, 23 December 2012 (UTC)[reply]