Wikipedia:Reference desk/Archives/Science/2014 April 14

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April 14

Some questions re: Malaysia 370:

1. How can a black box give out a signal that can be mistaken for other signals?

Black box should send a signal that unambiguously identifies it as such, and also incorporate signal that identifies the aircraft it came from. Is that too hard to do?

2. Black box should surely be tethered to outside of craft, and fitted with a mechanism that releases it if and when the plane crashes. Is that too hard to do?

3. How can a plane turn off its transponder from the cockpit, and fly unnoticed half way round the world?

Surely, all major aircraft should be beaming signal continuously back to base or to satellites in such a way that no one can turn it off. Myles325a (talk) 07:35, 14 April 2014 (UTC)

The article to read is Flight recorder. Answering your questions:

1. The prority of an Underwater locator beacon is to provide a signal that can be detected at maximum range through water. A pulsed burst of ultrasound has been specified because a narrow-band receiver can detect it unambiguously even at very poor Signal-to-noise ratio. Specifying a more complex signal would demand a wider bandwidth receiver which would reduce the detection range. That could only be compensated for by increasing the transmitter power which would shorten the battery life. This and the increased complexity involved make identifying the aircraft in the beacon signal an undesirable added complexity. (There is a different approach that has not been implemented, namely that flight recorders could be equipped with transceivers so that their stored data could be downloaded even without physical recovery of the unit. For this to work, quite complex circuits and an antenna would have to survive on the beacon and the communication range would be short.)
2. An externally releasable flight recorder is not "surely" going to survive better than one placed in a part of the plane least likely to be burned or crushed by impact.
3. Planes don't operate switches, pilots do. Pilots must have control of all electrical systems on their aircraft. Do you have an example of anyone flying unnoticed half way round the world?

In a war theater aircraft operate IFF radio to allow the kind of continual identification that you suggest, as a matter of survival from potential Friendly fire. Civil aviation, including commercial carriers like Malaysia Airlines, can only operate in peacetime where flight risks are minimised by extensive international regulation, not by permanent radio monitoring of every moment in every flight. Malaysian Airlines has suffered one probable fatal hijack event in 1977, one non-fatal crash probably due to pilot error and the cause of the loss of MH 370 is not yet resolved (and may never be resolved). Key evidence of what happened lies in what was said in the cockpit before MH370 diverted from its scheduled course, but that speech has probably been overwritten in the voice recorder (if that is ever found) and could only have been recovered if ground-based receivers recorded a continuous transmission of cockpit sound from every airplane in the sky, which is an impractical idea. 84.209.89.214 (talk) 14:41, 14 April 2014 (UTC)

Actually, deployable FDRs that eject from the plane during a crash are already used in several military aircraft, including some based on civilian models like the E-3 Sentry (based on the 707) and Air Force One. There have been several attempts in the US Congress to require them on commercial aircraft, but it died in committee every time.

I'm not sure why continuous transmission of sound from the cockpit is impractical. It would be expensive, but not obscenely so. It's not as if sound transmission requires a lot of bandwidth. I imagine most commercial aircraft are already equipped with satellite communication capabilities. The reason it wouldn't be done is political, not technical. The reason the CVR only records 2 hours and why there are no video recorders is because the pilots' unions have opposed it, saying that it would be an invasion of privacy. Mr.Z-man 15:19, 14 April 2014 (UTC)

Pilots are given the option to turn off the transponder for two reasons: 1) turning it off while on the ground to avoid interference, etc; and (2) turning it off in case of malfunction or fire. Justin15w (talk) 21:23, 14 April 2014 (UTC)

An aeronautical transponder requires manual control by the pilot who sets the code as instructed from the ground; the pilot can also use the transponder to signal the ground in an emergency when voice communication fails. The OP may believe aircraft transponders can communicate with satellites but they are designed to respond to local control radars, ensuring aircraft are identified on surveillance radar displays see article. 84.209.89.214 (talk) 22:34, 14 April 2014 (UTC)

Op back live. Can't figure out why transponder should be turned off in case of fire, or in case of malfunction for that matter.

Another idea: Voice recorder should be like other recorders elsewhere. When there is silence, it does not record. When someone speaks, it does. Black box was Aussie invention, and I'm proud of it, but it just seems like they could be doing a lot lot more with it, and with airplane safety in general. (This would also have the effect that if a pilot said something that might possibly incriminate him, he would have to talk complete rubbish, continuously, for two hours and a few minutes so that it would be erased from tape.) Myles325a (talk) 03:12, 15 April 2014 (UTC)

An aircraft cockpit always has a significant noise level, and some of this noise may also have significance in incident analysis. Safety systems in general are designed to be as simple as possible - just imagine they retrieve the recorder from the ground of the Indian Ocean, only to find that the voice detection circuit had failed, and it did not record anything. Pilots need to switch off transponders on the ground because they would all reply to radar requests from the local airport SSR. With several aircraft typically being parked very closely to each other on the ground (as opposed to the minimal safety distances in the air), that would generate overlapping transponder replies and essentially drown out any relevant replies (this is called "garbling", and it's a problem even for aircraft in the air). A transponder can fail in different ways - if it simply goes dead, there is no reason to turn it off. But it can also fail actively, i.e. emit uncontrolled signals or noise on the 1090 MHz frequency. In that case, it potentially disrupts secondary radar coverage for all aircraft in the vicinity, which is a large problem. ATC can easily guide a single aircraft safely through IFR airspace using just radio, but they get in real trouble if reliable position information for all aircraft vanishes from the screen in an area where there is significant traffic and radar separation is used. In general, a lot of though has gone into defining the world wide civil aviation system. It's not perfect, of course, but overall the rate of accident due to problems in the procedures is exceedingly low. --Stephan Schulz (talk) 05:55, 15 April 2014 (UTC)

OP myles325a back live. Ok, Stephan, you obviously know something about that of which you speak, but the impression I get is that there is absolutely nothing that can be improved in ANY of these procedures. Hey, expert, don't tell me what's wrong with my ideas, tell me how they can be improved and MADE to work. I get the idea that folk like you would have found 29 excellent reasons for why a black box recorder was not a good idea in the first place. (In fact, there was a lot of opposition to it from the aviation industry in various parts of the world, especially in Australia, where it was, ironically, invented. And the pilots didn't like it either). If today we can have billions of mobile phones all transmitting at once, then don't tell me contemporary technology could not come up with a system that tracked a plane, everything on the plane, and sent it in real time to some location. It's easy peasy. Over to you for some CONSTRUCTIVE thinking. Leave the "it can't be done" stuff to the Luddites. Myles325a (talk) 07:06, 15 April 2014 (UTC)

@Myles325a: A proposal for deployable recorders in civil aircraft has been circulating in the US Congress for several years. This proposal has recently been re-introduced as a result of the search for MH370. See the information supplied by User:BBoniface about the SAFE Act - diff. Dolphin (t) 07:37, 15 April 2014 (UTC)

The original Luddites were saboteurs of technology such as labour-saving textile machinery, acting on their premise that "It can be done but we'll hinder it to keep our inefficient jobs". Penalties from 1721 included Penal transportation to Australia where disgruntled sentiment occasionally still surfaces. Sitting in an armchair demanding that precious radio frequency spectrum shall be expropriated to build a vast network to fulfil an Orwellian dystopian fantasy of omnipresent bugging of every airplane cockpit is easy but short sighted.

Op myles325a back here. I’ve had 20 years working as a management consultant, and am a big fan of armchair theoreticians, and those who come up with a lot of ideas. I’m a big picture man, it’s up to others to work out the details. It is the lab boffins and admin fat cats who will be the first to tell you that “It can’t be done”. Original inventor of “black box” was an “armchair” theoretician, and among those who screamed that they would have nothing to do with it were the Luddite pilots, who have, like teachers and doctors, fought any and every attempt to curtail any aspect of their privilege. (That is why the flight recorder was originally derided as a “black box”.) Your objection to any of my notions could be sent back in time, and it would be EXACTLY the same kind of rubbish that was spouted against the introduction of any new safety device. Mine operators used similar against new innovations like lifts which are made safe from crashing down the pit right up until the 1930s. It was Orwellian, it was a waste of money, it interfered with their God-given right to run their business exactly the way they wanted, a few thousand people could die, c’est la vie...twas ever thus, there’s a lot more where they came from. Airline pilots are not taxi drivers, soon they will be flying a couple of thousand souls and I’d like to know they aren’t banging cocktail waitresses and slugging booze up there. They don’t like being observed. Stuff ‘em. Myles325a (talk) 08:36, 17 April 2014 (UTC)

Stephan made a valid point that all the cockpit sounds, not just speech, can be important: these include engine vibrations, sounds of circuit breakers, buzzers and switches. A consumer mobile telephone employs a Vocoder, typically subjecting speech to Linear predictive coding in GSM networks, which is a lossy data compression process that ruins the original signal.

OP myles325a back live. If “original signal” is “ruined”, how can I use Skype, and how can I watch High Definition colour TV in stereo? There are prob only a couple of hundred big planes flying every day. Can it really overwhelm the entire system to have them sending signals back to some base, while the rest of the world is sending literally billions of signals for nothing more than idle chat. Of course, there might be probs with having voice activated recordings. Well, that doesn’t mean, one simply slams the book on that idea. Let’s find a way around it. Myles325a (talk) 08:36, 17 April 2014 (UTC)

Thus the most constructive response that engineering can offer to facile plans encouraged by Mr.Z-man and Myles325a for spending public money is a major disruption of radio communication resources to accumulate thousands of hours a day of virtually useless recordings. Voice-activated recording is a bad idea here. It destroys the exact time line recreation that is essential to Accident analysis. A presumption that an air accident is caused by a negligent pilot whose words should be used to incriminate him displays an unprofessional approach. 84.209.89.214 (talk) 13:21, 15 April 2014 (UTC)

OP myles325a back live. How much “public money” has been spent in sending dozens of big planes, helicopters and ships, and hundreds of engineers and searchers in the Malaysia 370 case? And what about the billions lost in compensation, and replacement aircraft. You factored in this in petty cash, did you? And since when is flight recorder money “public money”? Who pays for the current installation and operation of Flight Recorders? Social Security? I don’t think so. What are you blathering on, about?

See, this is typical of reactionaries I’ve faced on any site where progress must be made. I’m no techie, and I don’t wanna be, but I DO know that voice-operated recordings COULD easily mark the time whenever a voice activated them. And there could be OTHER recorders for non-voice sounds. I mean, don’t just say “It can’t be done”. Like, as if 50 years from now we are going to have the same system we have now. If it was up to you, we would still be taking canaries in cages down mines. As for “a presumption” being made that pilots are unprofessional, that is no more true than that security measures in banks are “presumptions” that customers are all crooks. Myles325a (talk) 08:36, 17 April 2014 (UTC)

I didn't say constant recording and transmitting to the ground was necessarily a good idea, merely that it wasn't that infeasible. According to Honeywell's specifications for one of their CVRs, the audio is not stored uncompressed, but uses lossy G.723 24 kbps encoding. So apparently engineers and crash investigators have already decided that a perfect recording is unnecessary. Mr.Z-man 14:08, 15 April 2014 (UTC)

(ec) The amount of effort considered here seems a bit mystifying to me. A pilot can get a notion to detour into a skyscraper on my way to the destination any time he wants. If he is simply incompetent I could easily die in a crash at the airport. Do I really care whether one time in a billion someone might not figure out which species of shark ate my meat? I'd rather have my fifty cents, please. I should add that I doubt the recorder does a forensic secure erase on previous rounds of cockpit recording... we'll see how much is really recoverable with a million dollars and top notch facilities. Wnt (talk) 13:23, 15 April 2014 (UTC)

G.723 Adaptive differential pulse-code modulation is an audio compression method that is appropriate to digital recordings where an unstable signal-to-quantising-noise is tolerable because the goal is voice intelligibility. "Perfect" sound recording is not a meaningful goal; what matters is a recording's fitness for a purpose. The Honeywell SSCVR (Solid-State Cockpit Voice Recorder) product just needs to meet the existing FAA specification for the analog recorders that it replaces; previous generations used steel wire, later Magnetic tape. (Its solid-state memory may introduce an unspecified EMP vulnerability.) Two examples of crash enquiries where the CVC recording is critical are:

• the crash of TWA Flight 800 where controversy persists, focussed on the last few tenths of a second of sound recovered on the CVR and questions about its analysis that have nothing to do with speech quality
• American Airlines Flight 587 where it eventually emerged that an NTSB sound spectrum analysis of the CVR revealed dramatic energy level increases during the time of the pilot's aggressive rudder control inputs that contributed to breakup in midair, also nothing to do with speech quality. 84.209.89.214 (talk) 18:32, 15 April 2014 (UTC)

Prosthetic skin?

Is realistic permanent prosthetic skin available to injured people instead of grafts? — Preceding unsigned comment added by 174.65.135.44 (talk) 08:17, 14 April 2014 (UTC)

See Artificial skin. --Jayron32 11:05, 14 April 2014 (UTC)

I was thinking of a special effects prosthetic skin but realistic and porous and permanent, or semi-permanent, that can be laid over the patient's missing or damaged skin after their wounds are effectively healed. Is that an option? Since it's much more simple than making skin out of spider silk or shark cartilage I would think this was a better surgical solution. — Preceding unsigned comment added by 174.65.135.44 (talk) 20:19, 14 April 2014 (UTC)

Read the article I linked. --Jayron32 21:57, 14 April 2014 (UTC)

You can't wait until the wounds are healed to put on artificial skin. Some type of "skin" is needed during the healing process to do what skin does (prevent dehydration, block bacteria and viruses from entering, etc.). There are some temporary skin options, and those could later be replaced with something more permanent, if that's what you mean. StuRat (talk) 22:27, 14 April 2014 (UTC)'

Hiv antibody and antigen tests

If it takes 4 weeks- 3 months for antigen and antibodies to develop for HIV tests, why do sexual health clinics recommend being tested 2 weeks after exposure? Wouldn't it save them money and false results, if they just test everyone at 4 weeks? 2.221.71.95 (talk) 08:59, 14 April 2014 (UTC)

Are they recommending antibody and antigen tests? The typical nucleic acid test can pick up a 4-day-old infection, and it can be modified to detect it even sooner if there is special reason to believe someone has been exposed. Someguy1221 (talk) 09:03, 14 April 2014 (UTC)

they call it a 4th generation test, which I believe picks up both. 2.221.71.95 (talk) 09:30, 14 April 2014 (UTC)

According to the FDA, the 4th generation test can take up to 20 days to detect an infection [1]. I can't tell you though, why some clinics are providing the guidance they do. Someguy1221 (talk) 08:51, 15 April 2014 (UTC)

All Matter made of photons?

I think all matter is made of photons- the energy or light particle. Because electrons can accept or emit photons. They are still electrons disregarding they are exited or in ground state. And another reason is gamma radiation. In this case photons are emmitted from nucleus. So it seems that proton, neutron, and electrons are all big clumps of photons. So a photon should also have some mass. (Black hole attracting light supports this). Am I correct or not?--G.Kiruthikan (talk) 10:52, 14 April 2014 (UTC)

No. You can read the articles titled photon and standard model for more details, but no, you are not correct. Photons have no rest mass. All of their mass comes from their energy, so they only have relativistic mass. Photons are also not the "energy particle". They are a gauge boson, which is a type of particle that carries one of the four fundamental forces. Photons carry the electromagnetic force. Electrons are leptons, and as far as we can tell, are fundamental particles themselves (they have no smaller internal structures). Protons and neutrons are composed of quarks, which are themselves fundamental particles, so like electrons also have no internal structure. --Jayron32 11:03, 14 April 2014 (UTC)

What you're saying is not proper physics, but it's not really that far from proper physics. It is widely believed that early in the universe, during baryogenesis, that there was a very free equilibrium between high energy photons and particles via pair production. We know that an electron and a positron, a proton and an antiproton can disappear and leave nothing but photons behind; it's just that by some mysterious process the antimatter has all but entirely gone away. Nonetheless, whatever sense in which these particles "are" photons is fairly intangible - aside, possibly, from dumping matter into a black hole and waiting for the Hawking radiation to come out, there's no really good way to extract all the hypothetical "photon" out of what is now matter without antimatter. An equivalence you can't actually do something with isn't worth much in physics. Wnt (talk) 16:00, 14 April 2014 (UTC)

No, it is very very far from being anything remotely resembling proper physics. The fact that you can combine a proton and anti-proton to make photons doesn't mean that a proton is made of photons, any more than the fact that you can make cake from milk means that milk is made of cake. --140.180.255.188 (talk) 20:43, 14 April 2014 (UTC)

Remember that by some means, which we don't understand, antimatter was converted to matter. In concept, photons can be converted by pair production to antiparticle + particle, the antiparticles somehow converted, and therefore whatever type of matter is desired can be made from them. It is also likely that some reverse process is at least conceivable by which matter becomes antimatter, allowing these particles to be matched up and cashed in for photons again. We don't know how to do it but it ought to be conceptually possible. Wnt (talk) 21:25, 14 April 2014 (UTC)

that doesn't sound correct, WNT. Rmhermen (talk) 01:54, 15 April 2014 (UTC)

In fact, Wnt's style of thinking was debunked by Richard Feynman in this interview, part of the series on The Pleasure of Finding Things Out. The electron doesn't "contain" photons, any more than your mouth "contains" words. You don't "run out of words" after your mouth emits them. You can't squish words back together and get a mouth.

Photons interact with matter. Sometimes, the interactions take the form of scattering; sometimes the interactions take the form of annihilation. Neither of those interactions should be interpreted to mean that the particles are made of photons. Fundamental particles aren't made of anything. They're localizations of physical quantities that are able to interact with other localizations of other physical quantities. Nimur (talk) 02:03, 15 April 2014 (UTC)

Well, if a loud word could create a mouth and an antimouth, things would be different. I don't feel like enabling the dozens and dozens of ad tracking sites and/or XSS it takes to try to play that minute-long video clip from Feynman - either cite a transcript, or as far as I'm concerned it can be abandoned and lost from the sum of human knowledge for all eternity; let its owners clutch it in their hands like Gollum on his way into the fiery pit. Wnt (talk) 04:49, 15 April 2014 (UTC)

Wnt, I did cite a transcript. It is called The Pleasure of Finding Things Out, and it is available in published form. You can buy it online or find it at a library. The one-minute excerpt, provided by the BBC at no cost, is a tiny fraction of the totality of the explanation. If you're interested in the nature of matter and its interaction with photons, and you're willing to invest more than a few moments to inform yourself, I'll gladly provide several additional reading recommendations. Nimur (talk) 14:24, 15 April 2014 (UTC)

Wnt, you're quite allowed to be wrong, but the problem comes when your writing things here has the chance to convince people that your wrongness might be correct. It would be much better for you to keep your wrongness to yourself. --Jayron32 11:20, 15 April 2014 (UTC)

I viewed the clip of Feyman on utube, who said his father was unsatisfied by his explanation that the reason for photons being found in nature is that they are created. Thus he didn't actually debunk his father's disbelief in the theory... heaven forbid that we follow each other blindly. That said, this is a reference desk, so we should be avoiding speculative questions and answers. --Modocc (talk) 12:31, 15 April 2014 (UTC)

The majority of the "rest mass" of your body is kinetic energy. Of course the total mass and energy of the universe is zero, so any mass you might see around you is just a rounding error. Hcobb (talk) 02:28, 15 April 2014 (UTC)

Op's argument that photons must have mass because black holes attract them ignores Einstein's general relativity theory. Mass creates space time curvature, and it is this curvature that the photons follow, and not its mass which is attracted. This I believe. Myles325a (talk) 03:18, 15 April 2014 (UTC)

If you shoot photons into a black hole, that increases its mass, no? Wnt (talk) 04:49, 15 April 2014 (UTC)

Because of mass-energy equivalence yes. It's important to recognize, however, that the equivalence is not the same is identity, and the difference between rest mass (which an object has because of the Mass generation mechanism such as the Higgs mechanism) and an objects relativistic mass (which is the mass an object has because of it's kinetic energy]]. Photons have no rest mass. --Jayron32 11:20, 15 April 2014 (UTC)

Photons have no rest mass, but two photons do, provided they are not going in exactly the same direction. (To be picky, a single photon has a rest mass; it's just zero.) --Trovatore (talk) 20:00, 15 April 2014 (UTC)

So gauge bosons are the energy particles? If I was incorrect why does Einstein's equation say E=mc²? Doesn't that mean that all matter can be converted into energy and vise versa? And another equation say that wavelength= Plank's constant/ (velocity*mass). It is applicable to electrons and all matter. If we apply it to EM radiation, we get a mass to photon. And if we combine this equation with E=ch/wavelength, then we get E=muc. Doesn't all these say that photons have mass and they may be the only elementary particle? --G.Kiruthikan (talk) 06:56, 15 April 2014 (UTC)

No, gauge bosons are not the energy particles. There are no energy particles. There are particles that have energy (well, all particles have energy), but that's not the same thing. --Jayron32 11:21, 15 April 2014 (UTC)

I don't pretend to understand the Higgs mechanism. I don't understand what particles are supposed to fill space, or how they're supposed to agree on what position in the Mexican hat potential the whole universe is supposed to have. I should however note that our article's explanation says that "the Higgs mechanism is a type of superconductivity which occurs in the vacuum. It occurs when all of space is filled with a sea of particles which are charged..." and goes on to explain that superconductivity permits the existence of permanent edge currents. (Edge of what? where?) If this is accurate then it sounds to me like mass is generated from the energy implicit in these currents, i.e. that the Higgs mass is just as "relativistic" as any other. I would welcome further explanation here. Wnt (talk) 13:11, 15 April 2014 (UTC)

Yes, mass and energy are equivalent. However, energy isn't the only quantity that's conserved in a system. Other examples of conserved quantities are charge and lepton number. An electron has a charge of -1 (in appropriate units), and a lepton number of 1. Photons have a charge of 0, and a lepton number of 0. So no matter how many photons with just the right energy you get converging at just the right spot to engage in some complicated system of particle interactions (pair productions and annihilations), the resulting set of particles is going to have a total charge of 0, and a total lepton number of 0. I.e., you can't get a bunch of photons to form just an electron. It's possible for just photons to create a set of particles which include an electron, with other output particles that "cancel out" the electron's charge and lepton number. But if you just ignore those other particles, that's kind of like saying that "oxygen is made out of water", because you can apply an electric field to a water molecule such that it separates into an oxygen atom and a couple hydrogen atoms that you just ignore. Red Act (talk) 13:00, 15 April 2014 (UTC)

We agree on this much, and yet... well, if you dump matter into a black hole it has only mass, spin, and charge remaining. Photons carry (relativistic) mass and spin away from the hole later on. And at a high enough temperature, photons become unified, somehow, with weak force mediators that can be charged. Wnt (talk) 13:16, 15 April 2014 (UTC)

At high enough temperature, photons simply don't exist. The electroweak interaction is governed by other particles, and photons cease to be created. Someguy1221 (talk) 09:10, 17 April 2014 (UTC)

Earthworks mass haul diagram

On an earthworks mass haul diagram, is the average haul distance simply the length of the line between the balance line and the free haul line? Clover345 (talk) 15:26, 14 April 2014 (UTC)

Moles of plasmid being transfected

I'm reading a research article. It says "we observed a notable additional seven-fold increase in specificity upon further decreasing transfected DNA from 9.0 × 10^11 to 1.8 × 10^11 nmol/cell (50 ng to 10 ng plasmid; Fig. 4c)."

Since a mole is 6.02214129(27)×10^23, does that mean there are about 10^11 plasmids? That's a lot of plasmids! --129.215.47.59 (talk) 16:41, 14 April 2014 (UTC)

nmol is nano mole. Nano is the prefix that means 10^-9. 10¹¹ nmol is therefore 10² moles. Each mole is about 6 x 10²³ things, so 2 x 10¹¹ nmol would be about (2 x 10²) x (6 x 10²³) = 12 x 10²⁵ things (in this case, plasmids in each cell). Is that clear enough? 86.146.28.229 (talk) 17:56, 14 April 2014 (UTC)

Yeah, that was really dozy of me! Thanks! 129.215.47.59 (talk) 21:28, 14 April 2014 (UTC)

(ec) Searching with your phrase gets me [2] which I read as the vastly more plausible "Whereas specificity increased gradually by nearly fourfold as we decreased the transfected DNA amount from 7.1 × 10−10 to 9.0 × 10−11 nmol/cell (400 ng to 50 ng plasmid)," So that's roughly 10^-10 nmol = 10^-19 mol * 6.02 x 10^23 = 60000 plasmids per cell. Wnt (talk) 21:31, 14 April 2014 (UTC)

I know but the point was I had ignored the n in nmol. 86.146.28.229 doesn't need to know how many plasmids so I didn't point out that my exponents were missing minus signs. — Preceding unsigned comment added by 129.215.47.59 (talk) 12:07, 15 April 2014 (UTC)

It is kind of unusual to see a prefix like nano- (10^-9) being used with a number that small, and therefore it may invite misreading. I'd've expected them either to use whole moles (and numbers like 7.1 × 10^-19) or to go all the way down to attomoles (10^-18) or zeptomoles (10^-21). But it's not incorrect to use nanomoles, of course, and I suppose it might be conventional in the field or something. --50.100.193.30 (talk) 10:11, 16 April 2014 (UTC)

I suspect it is because molecular biologists are used to expressing things in micro- and nano- prefixed amounts; micrograms and nanograms of solute per microliter (giving nanomolar concentrations) are typical concentrations and volumes at the bench. Typical transfections involve the use of thousands or millions of cells; while the per-cell amounts are very small, the amounts of material actually used to do the experiment (50 ng or 400 ng of plasmid in this example) are quite reasonably expressed in the units chosen. Making multiple interconversions between different prefixes then becomes an invitation to error as the decimal place gets bounced back and forth. TenOfAllTrades(talk) 16:23, 16 April 2014 (UTC)