Wikipedia:Reference desk/Archives/Science/2016 January 20

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January 20

Strings At Absolute Zero

Hey, I was just wondering if this could be explained to me. I am under the impression that string theory says that all matter is made up of strings, and the way these strings vibrate designates properties to the matter. I am also under the impression that at absolute zero matter has no kinetic energy, and neither does its constituents. If matter lacks any kinetic energy, then the strings would be inert, making them not protons or electrons or neutrons etcetera. If this was the case, and we managed to get (I know this is impossible) say, a hydrogen atom to absolute zero, it would not be a hydrogen atom. In fact, it would not be an atom because it would not be composed of protons, electrons or neutrons. What am I not understanding here? JoshMuirWikipedia (talk) 06:36, 20 January 2016 (UTC)

I know next to nothing about string theory, so I can't make any intelligent comment on that part of the question.

However, I can say that one of your basic premises is wrong. It is not correct that matter at absolute zero has no kinetic energy. There is a minimum zero-point energy. (Of course, as a practical matter you can't get to absolute zero anyway, but that's a separate issue — you can get very close indeed to absolute zero, but there's still a fixed minimum to the kinetic energy below which you can never drop.) --Trovatore (talk) 06:41, 20 January 2016 (UTC)

(edit conflict)That's not quite what absolute zero is. Absolute zero, as our article describes, is the lowest possible energy state of any matter. For an ensemble of atoms, which describes most ordinary matter, this means the atoms are not moving at all, and yes, would have no kinetic energy. But that simple definition does not hold when looking at other types of matter. You don't even have to get as exotic as unproven strings - a gas of electrons never has zero kinetic energy, even at absolute zero (described in our article). So yeah, "absolute zero" does not mean "zero energy", but rather "minimum energy". In fact, even in our "ensemble of atoms" example, the "zero kinetic energy" result only holds if you take a very Newtonian view of matter and energy. In reality, the constituent particles within atoms are always in motion. Someguy1221 (talk) 06:45, 20 January 2016 (UTC)

No, sorry, that's absolutely not true. Ordinary matter does have zero-point energy, and the atoms absolutely are moving at absolute zero. --Trovatore (talk) 07:01, 20 January 2016 (UTC)

Oh, sorry, I didn't read your whole statement. Your last sentence is correct. But it flatly contradicts your third sentence, and your third sentence is blatantly false! Why did you say it, given that you knew it was false? --Trovatore (talk) 07:04, 20 January 2016 (UTC)

It's not false, given as I said, a Newtonian view of matter. A block of wood sitting motionless at absolute zero has no "v" in KE=(1/2)mv2. Of course, that's true of any block of anything with zero measured velocity. It was simply meant to be a train of thought showing that what we think of as "kinetic energy" in high school physics is a very incomplete picture that ignores microscopic properties. Someguy1221 (talk) 00:17, 21 January 2016 (UTC)

It's not false given a Newtonian view of matter? But the Newtonian view of matter is false, so that's meaningless. We should never tell people that matter has no kinetic energy at absolute zero, because it's false. --Trovatore (talk) 00:52, 21 January 2016 (UTC)

And yeah, I did write it in a stupid way, now that I reread it. Someguy1221 (talk) 00:18, 21 January 2016 (UTC)

Also, it's not just "the constituent particles" with in the atoms that are always in motion. It's the atoms themselves. --Trovatore (talk) 07:26, 20 January 2016 (UTC)

You can never get to a zero energy point with matter if only because E=Mc^2. Couple that with other quantum effects and you can't localize an atom which means it can move without consuming energy. --DHeyward (talk) 13:09, 20 January 2016 (UTC)

low boiling point solvents have lower enthalpies of vaporization than water, yet they feel cooler to the touch than water.

I used dichloromethane to clean up some sticky tape residue the warehouse staff had left on my second-hand hotplate (the tape itself was no longer there -- someone probably just stuck a sign to it using invisible tape but then didn't wipe it off -- and the hotplate otherwise looked clean when I turned it on to test, only for bad smells to evolve and for the black shadow of polymerized sticky tape residue to emerge). I used a combination of soap and water in one hand, and some dichloromethane-wetted towels in the other hand. I immediately noticed that the DCM-wetted-towel felt very cool to the touch, even through gloves, though the towel was getting dry more quickly than the water towel, while the soap-and-water-towel got very warm. After cleaning the burnt sticky tape residue I took off my gloves and threw away the paper towels, I noticed the DCM-wetted paper towels were very cold, despite having way less heat capacity than the soap-and-water, which AFAIK should not have warmed as quickly.

I also notice that hotplate issues aside, that dichloromethane and ethanol both produce a superior cooling effect than water despite their low heat of vaporization. This posed a curious optimization problem: for a given ambient temperature T, assuming that the reservoir being cooled is big enough that it doesn't change temperature significantly as it loses heat to simplify the scope of the problem (our body temperature doesn't drop either). So I note that the power loss for the heat reservoir being cooled by vaporization of the solvent should be the following:

W = amount of moles of solvent in contact with the reservoir (n) * equilibrium percentage of solvent in the vapor phase (K) * rate constant (k) * heat of vaporization (H)

but K = e^(-H/RT+S/R)

so W = n * k * e^(S/R) * H * e^(-H/RT)

e^(S/R) is just a constant dependent on entropy of vaporization (s)

Assuming that k and n are constant and don't vary with H (k and n represent more like the physical engineering issues of surface area and heat flow and whatnot, independent of solvent), and that H doesn't vary significantly with temperature,

• then dW/dH = -k*n*s*H/RT * e^(-H/RT) + n*k*s*e^(-H/RT) = e^(-H/RT) * n*k*s * (1-H/RT)
• dW / dH = 0 when H/RT = 1 or when H = RT (I believe this is a maximum)

Thus, the fastest-cooling solvent for a heat reservoir at room temperature should ideally, be as close to 2.47 kJ/mol as possible, and any lower or higher would result in a lower cooling rate? Yanping Nora Soong (talk) 11:41, 20 January 2016 (UTC)

There is a partial pressure of water vapor in the atmosphere. Solvents have virtually no partial pressure. High humidity and low humidity affect the cooling efficiency of water but not solvents as far as I know. The lower the heat of vaporization, the faster the substance will carry heat away from the source through vaporization.--DHeyward (talk) 13:02, 20 January 2016 (UTC)

Dichloromethane has a partial pressure of ~0.5 atm around 24C. Yanping Nora Soong (talk) 11:39, 21 January 2016 (UTC)

Carbon based Zeolite

Hello,

The International Zeolite Association (IZA) states (http://www.iza-structure.org/databases/ModelBuilding/Introduction.pdf) that zeolites can be built with "tetrahedral TO4 frameworks, where T may be Si, Al, P, Ga, B, Be etc.." - Why not carbon, since carbon leads to tetrahedral structures as well? Is it (theorically) possible to form 3-D ether (?) structures, just like Crown ethers are 2-D ones?

Thanks for any hint. — Preceding unsigned comment added by 192.54.145.66 (talk) 11:43, 20 January 2016 (UTC)

The problem is that these solid polymerized carbon oxides would quickly depolymerize to carbon dioxide. This is due to the very stable nature of the C=O double bond. Whereas Si, P, B, etc. don't form strong double bonds with oxygen, because of poor orbital overlap. Notice that I doubt that the empirical formula of these "3D ethers" would be CO4, as it would require the inclusion of dioxygen (neither is SiO4 accurate for silicon dioxide (silica)). I suppose "3D ether structures" could be possible at very high pressures and very low temperatures. Yanping Nora Soong (talk) 11:53, 20 January 2016 (UTC)

Indeed, the "TO4" is usually written "TO4/2" to reflect both the tetrahedral structure and the fact that oxygen atoms are shared between two "T", thus the global formula is TO2 (like SiO2).192.54.145.66 (talk) 13:18, 20 January 2016 (UTC)

PMID 22540597 is just the first of many literature references I found for the carbon analog. Note that it indeed only apparently exists under special/extreme conditions. DMacks (talk) 17:33, 20 January 2016 (UTC)

That's the answer. I should add that "polymeric carbon dioxide" (or CO2) are good search terms. See [1] [2] etc. Wnt (talk) 17:53, 20 January 2016 (UTC)

CD jitter

I am looking for references on how much cd jitter is needed to be audible to the average person.--178.105.166.117 (talk) 17:58, 20 January 2016 (UTC)

This is going to be a complicated problem to define. Do you mean the jitter in the buffering of digital audio frames, or jitter in the analog frequency (or phase) during the digital-to-analog conversion, or jitter along some other axis?

One place to start reading is our article on psychoacoustics, the scientific study of sound perception. This field has a tendency to err towards the "subjective," but it can be conducted in a very quantitative fashion, if you read the right books. I highly recommend the online textbooks of Julius Orion Smith; these are available at zero cost.

For example, from Spectral Audio Signal Processing: Further Reading, links to a dozen research publications on acoustic perception as it pertains to various digital models, including a lot of discussion about sensitivity to noise.

Nimur (talk) 18:38, 20 January 2016 (UTC)

Last time I worked with CD's, the method of decoding audio was through single bit (sometimes multi-bit) sigma-delta modulators. phase noise on the oversampled bitstream to the analog decimation low-pass filter filter I believe is what you are looking to quantify. Jitter associated with reading the disc and buffering is not a factor. Oversampling tends to push noise out of the baseband. --DHeyward (talk) 20:05, 20 January 2016 (UTC)

OP:Final d/a clock jitter- no oversampling. Some sources say you cant hear 2ns pk-pk jitter. Some say it has to be less than 100ps to be inaudible. Who is right?--178.105.166.117 (talk) 17:33, 21 January 2016 (UTC)

Such a specification implies that a listener can hear spectral purity well above the actual spectral purity of the true signal. This seems very unlikely: I suspect neither noise specification is accurate. Nimur (talk) 18:30, 21 January 2016 (UTC)

I don't know enough to comment on the figures, but I do agree with Nimur and think you should be very careful about anything you hear relating to audio (no pun intended). There are a lot of myths about audio that some people who believe themselves audiophiles believe and seem to spread, despite having basically no good evidence for their claims (and often based on flawed understandings of the science or basics of how something works).

I'm not completely sure whether [3] is trustworthy but the writer does at least mention ABX and double blind (albeit only that the test referred to didn't involve that) and Audio Engineering Society who I believe tend to reject such crap. The writer suggests anything less than -100 dBFS although that's for the whole system including recording. As mentioned there and [4] + [5], jitter is one of the areas where there seems to be a lot of myths and misinformation.

Theoretically you'd expect those working as sound engineers for music, movies etc and those who work on mastering to avoid such myths, but I've read enough stuff like [6], to make me think some of them do seem to accept and even spread such myths. It's particularly surprising how often you hear about such people who seem convinced of something yet there's no mention of an ABX or similar test (even when it sounds like it would be easy).

The spread of 96 khz/24 bit or even 192 khz audio for the end human user (as opposed to the processing stage where 24 bit is useful), despite the fact there's very little evidence anyone can hear the difference. Well, other than distortions introduced by equipment which is unlikely to generally be considered beneficial [7], and the possible advantages in processing difference of such audio considering a different target market [8]. And I don't think we can totally blame marketing departments for this either.

Frankly the very high percentage of the overall bitrate of many video formats devoted to audio thanks to these moves and also the move to loseless formats is also IMO another sign. Although at least there's often some evidence someone with super equipement who is looking for the artificats can detect them at common bitrate used for the lossy compression schemes with video. Albeit if you increase the bitrate to significant below whatever the compression of your lossless, you'll probably get transparency for nearly everyone. BTW the Hydrogenaudio forums are always a useful source. [9] while targetted at audio engineers seems a decent basic introduction of the crap that audiophiles can spread.

Nil Einne (talk) 17:29, 23 January 2016 (UTC)

This is also related to the mythos surrounding perfect pitch, the reputed ability of certain "musical prodigies" to produce and/or identify frequency-accurate tones. All you need to refute them is a double-blind test with a microphone and a modern digital oscilloscope. Audio signal frequency accuracy, in the real world, is probably accurate to less than 1Hz. This works out great, because the bandwidth of a "perfect sinusoidal tone," even from one of the more perfectly-spectral instruments like a flute, is still quite wide. If you look at the timbre of a resonant instrument like a grand piano, it becomes clear that we are hearing very broadband, noisy tones - and it is this quality that we find aesthetically pleasing.

Again, I point to the texts I linked earlier, in which the formal mathematics are worked out and compared against psychological studies of sound perception. We humans really like phase noise to be added in to our music. Most electronic instruments have to add phase noise so that they sound more natural. Trying to "perfectly" construct a signal waveform is overkill; humans can't hear accurately enough for bit-perfect, sample-for-sample recreation to actually matter very much below, oh, maybe 10 microseconds (this is, without coincidence, pretty close to the time-scale that corresponds to the upper frequency range of our hearing).

What we do hear, and find annoying, are spectral noise spurs, shot noise (unwanted impulses and jump discontinuities), and very high amplitude white noise. Technically, all of these noise can be mathematically transformed to represent as phase noise, (equivalenlty, "jitter" in the time domain); but if you want to play mathematical games, ... well:

Here's a fantastic white-paper from Maxim Semiconductor on the theory and practice of random noise contributing to timing jitter. Pay close attention to the frequency axis on their Bode plots! Maxim mostly makes high-frequency RF and mixed signal devices: so when we build a 200MHz amplifier or a 20 GHz amplifier, then ... yes, we worry about a few nanoseconds of timing jitter. In audio devices, at this magnitude, phase noise sources are literally and actually quantitatively thousands of times too small for you to notice. They are way below your noise floor. It is a near-certainty that your stereo picks up more noise from the next-door neighbor's refrigerator compressor motor. Any "real" audiophile who says they can hear audio jitter at one gigahertz probably needs to put themselves and their Hi-Fi stereo in an RF-clean remote location in the desert, and while they're at it, they should probably ask the Air Force to shut down the GPS satellites for a few hours so remote radio signals from hundreds of miles away won't interfere with their amplifier noise floor.

Those high voltage power lines a few miles out of China Lake sure do put out a lot of radio-noise in the audible frequency spectrum!

I doubt even a trained musician - even one who claims to have perfect pitch - could detect as huge an error as 10 microseconds of peak-to-peak jitter, provided there no samples are dropped and no gaps in playback (because a gap would be zero-fill, in other words mathematically equivalent to shot noise, not phase error).

Nimur (talk) 19:17, 24 January 2016 (UTC)

Well I guess, when using 44.1 kHz, only 22.05 kHz exactly can be transmitted a square or rectangle. It might be the same when displaying lower and not exact half resolution on a TFT monitor. TFTs have fixed number of pixels. The picture can be fitted to the screen, but it is not that sharp as in the TFTs physical resolution. Those losts will occur. --Hans Haase (有问题吗) 17:54, 21 January 2016 (UTC)

Natural habitat of birds in the city

If birds (i.e. hawks) live in the busy city, is the city their "natural habitat"? Would it still be the "wilderness" if the raccoon digs into the trash bin or dumpster behind a restaurant? Although the "wilderness" seems to evoke imagery of trees and rocks, does the wilderness have to be trees and rocks and things that Mother Nature made directly? Can the "wilderness" be man-made structures that are abandoned and overtaken by rodents that may dig into trash? 140.254.77.208 (talk) 19:15, 20 January 2016 (UTC)

• "Wilderness" is the wrong word here, since it implies lack of human settlement. You want "environment" which can include an urban environment. μηδείς (talk) 20:35, 20 January 2016 (UTC)

• (ec)"Wilderness" implies uninhabited by humans, or nearly so.[10] Some would half-joke that NYC is a "wilderness", but there is no question that it has plenty of human habitation. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:37, 20 January 2016 (UTC)

• It does? <g> Coyotes and other "wild animals" have been found in Manhattan - the fact is "wilderness" may apply to any area not under direct use by Man within any reasonable length of time. Cities have been around for a vey long time now (likely well over 10,000 years from current reports) meaning that yes - some birds can and do treat urban areas as "natural habitats". Collect (talk) 20:42, 20 January 2016 (UTC)

• Well, it's alleged to be, at least. :) Many types of creatures dwell in cities, and it is their "normal" habitat, for sure. It's not "wilderness", though. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:45, 20 January 2016 (UTC)

• The term "urban natural habitat" is a bit of an oxymoron, but "anthropogenic biome" avoids that. Also see Urban ecology. BiologicalMe (talk) 20:50, 20 January 2016 (UTC)

• It also avoids being pronounceable. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:52, 20 January 2016 (UTC)

I wouldn't call a city their "natural habitat" unless they have actually evolved to adapt to it, and can no longer survive outside the city. StuRat (talk) 04:13, 21 January 2016 (UTC)

What if they can survive outside the city but survive better in the city? Isn't the city then their natural habitat? Examples include urban rats and urban pigeons. Robert McClenon (talk) 16:17, 21 January 2016 (UTC)

From the same window in Manhattan, I've seen a peregrine falcon fly by with a pigeon in its talons on two separate occasions. That falcon has adapted to city living as the cliff-dwelling pigeons did. There is no reason to require that populations to evolve specialized traits if they are well suited to the urban environment. The fact that they have developed the ability to find sustainable food supplies and nesting sites to produce new generations indicates that they have colonized the city successfully. BiologicalMe (talk) 17:01, 21 January 2016 (UTC)

Commensalism and synanthrope are relevant here. E.g. Norway rats and the house mouse are only found at very low densities outside of human habitations. There are many critters whose natural environments are primarily human enclaves. We also have an article on urban wildlife.

How much time can a fish survive outside water?

I asked in IRC but they sent me here. I don't want information about big fishes like sharks, I want information about small fishes which are about the size of human palm. Thank you 188.42.233.35 (talk) 21:33, 20 January 2016 (UTC)

7 hours, according to the Daily Mail. And there are also lungfishes, which live even longer out of water. --Scicurious (talk) 21:57, 20 January 2016 (UTC)

Amphibious fish is worth a read if that's what you're interested in, mudskippers can survive for a very long time out of water in the right environment 95.148.212.178 (talk) 22:16, 20 January 2016 (UTC)

For a fish without lungs that doesn't get lucky, maybe a few minutes. StuRat (talk) 04:10, 21 January 2016 (UTC)

The problem is not that of lungs, because fishes have initially no problem breathing in air, where they can find far more oxygen than needed. Their problem is that outside water, the gills eventually dry out and cannot exchange oxygen any more - this is when they sllowly suffocate. See Fish gill#Breathing without gills 192.54.145.66 (talk) 14:19, 21 January 2016 (UTC)

Decades ago when I was (more) interested in fish and angling, I read that (British) hobbyists who raised prize carp, would routinely take them on train trips to Fish Shows in the UK or the nearer Continent (France, Belgium etc.) by wrapping them in wet newspaper and carrying them under their arms. Different species and families of fish differ in their ability to absorb oxygen through their skins, per 192.54's link above, so a general answer may not apply to a particular species. {The poster formerly known as 87.81.230.195} 185.74.232.130 (talk) 14:44, 21 January 2016 (UTC)

But didn't the owners experience nerve pain when they passed through a tunnel?98.213.49.221 (talk) 20:03, 21 January 2016 (UTC)

I had always thought that rather than the gills drying out, when fish are taken out of water, the gills are no longer supported by the water so the gill lamellae collapse on each other causing a dramatic (fatal) decrease in the ability for oxygen exchange.DrChrissy ^(talk) 17:12, 21 January 2016 (UTC)