Wikipedia:Reference desk/Archives/Science/2017 November 23

= November 23 =

Science or Fiction ?
In one of his marvels, Crichton, (methinks it was State of Fear), mentions a machine known as "Cavtier", (forgiveness if I misspelled) which we are told is illegal in almost all countries. The great thing about it, which is empowered by very powerful generators required to be carried along in several very big vehicles, can dig a cavity in even the of hardest grounds. Another great feature about it is that you don't even have to bother with the dug-out, which would be heap(s) of mountainous sizes when you will be needing a well of about (say) 20 feet diameter that should be (let's suppose) 60 or 70 feet deep, (dimensions those, as told or implied to the reader, are perfectly within it's capabilities!)  as all that stuff (the dug-out) will get pressed (talk of pressure!) into the "walls" of the brand-new, perfectly dug anti-cylinder of 20' diameter, 60 'or 70' deep. Do such machines actually exist in real life ? Jon Ascton   (talk)  06:26, 23 November 2017 (UTC)
 * Well you can produce a miniature version of your cylindrical hole by driving a fence-post into the ground with a sledge-hammer or a post-driver. Pile drivers are a slightly bigger version, but I've never seen a giant version on the scale that you mention.  I think a conventional excavator is more energy-efficient and environmentally friendly.  On the scale that you mention, I would be worried about triggering earthquakes and shaking the foundations of nearby buildings.  In most areas, soil is not sixty feet deep.    D b f i r s   08:12, 23 November 2017 (UTC)
 * These factual articles describe present tunnel boring equipment and latest methods. Blooteuth (talk) 11:16, 23 November 2017 (UTC)
 * Yes, I was thinking of vertical cylinders, but horizontal cylinders of a much bigger size are common. I've walked through some of them.    D b f i r s   16:21, 23 November 2017 (UTC)


 * The word you are looking for, is cavitation, and we have several related articles on supercavitation and cavitation hammer and soforth.  Crichton is obviously vague on the details. μηδείς (talk) 19:07, 23 November 2017 (UTC)


 * Yeah, I was already doubtful about the spelling, as I've already expressed in the beginning of the question. Anyway thanks for correcting me.  Jon Ascton    (talk)  16:30, 24 November 2017 (UTC)

Impossible asteroidial speed?
How could ʻOumuamua have moved 103 km/s relative to one star, TYC4742-1027-1, quite far (0.25 light year), and only 26 km/s relative to our star? 103 km/s is already about half of the orbital speed of the stars here. --Mortense (talk) 11:06, 23 November 2017 (UTC)


 * Not sure what problem you are imagining? In our local region of space, the stellar velocity distribution has a standard deviation of ~30 km/s.  A relative change of 77 km/s from one star to another would be somewhat unusual, but not so unusual as pose any sort of a fundamental problem.  Dragons flight (talk) 11:33, 23 November 2017 (UTC)


 * [Edit Conflict] :Because that star has a very different Space velocity to the Sun. Whilst on average other stars in our Sun's vicinity are orbiting the Milky Way at roughly similar velocities (i.e. speeds and directions), many are not. {The poster formerly known as 87.81.230.195} 90.208.173.186 (talk) 11:40, 23 November 2017 (UTC)


 * I am imagining that 103 km/s is not the correct figure. That there is an error somewhere. --Mortense (talk) 14:00, 23 November 2017 (UTC)


 * 103 km/s matches the cited source . If you think the source is wrong, there is an email address on their paper, so you could ask the author.  Otherwise, I'm not sure what we can tell you.  I am assuming that no one here is likely to rerun their simulation to try and check it.  Dragons flight (talk) 14:16, 23 November 2017 (UTC)
 * There are definitely stars (belonging to the halo or thick disk) that have high velocities relative to the Sun. One example is Barnard's star. So, a velocity of 103 km/s is not that unusual. Ruslik_ Zero 18:51, 23 November 2017 (UTC)
 * Which is well known in astronomical circles for being likely the fastest star in the universe (in degrees per millennium, it moves 1 Full Moon width per 2 centuries or so relative to most stars (the ones too far, slow and/or non-tangentially moving to have much proper motion). By star standards a degree/360 years is runaway. Sagittarian Milky Way (talk) 20:44, 23 November 2017 (UTC)
 * The fastest proper motion in degrees per millennium depends on the relative velocity being high and the star being relatively close. It's irrelevant to the question of relative velocity alone. --69.159.60.147 (talk) 06:34, 24 November 2017 (UTC)


 * May I direct attention once again to the link I provided earlier: namely Stellar kinematics. These are stars "moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighbourhood" [my italics]. Stars moving up to 100km/s relative to the local average (see Local standard of rest) are not even considered high-velocity stars. Those relative velocities are not all in the same direction (and the Sun itself is moving about 20km/s relative to that average motion – see Solar apex), so obviously a particular body can have differing velocities relative to two different stars of this order of magnitude. {The poster formerly known as 87.81.230.195} 90.208.173.186 (talk) 09:38, 24 November 2017 (UTC)

Phone camera science
Phone cameras have come a long way but why is it that the one area they consistently fail against DSLRs continue to be low light conditions? DSLRs also generally seem to be superior on colour contrast, depth and detail. 82.132.232.28 (talk) 13:03, 23 November 2017 (UTC)
 * Camera lenses, amongst other things, have to capture light. The bigger the front element (the hole at the front), the more light they can potentially capture. So phones lose out.
 * OTOH, the smaller the aperture (which is related) the greater the depth of focus (see Ansel Adams for what you can achieve with that technique). So phones also gain from this, and the problem of focussing is easier. Andy Dingley (talk) 13:41, 23 November 2017 (UTC)
 * Aperture? Isn't it the f-number? The focal length can't be very long in a phone, thus a small f-number, thus lower depth of focus. Or is there more to this? --Mortense (talk) 14:15, 23 November 2017 (UTC)
 * Yes, f-number (the ratio between aperture and focal length) would be a more general measure here. I was trying to keep it simpler. Andy Dingley (talk) 15:41, 23 November 2017 (UTC)
 * According to which metric do you believe phone cameras "fail" relative to a DSLR? If you can specify, I would be happy to provide a well-referenced explanation and lots of further reading material.  The quality of a complete camera product is a composite of its parts (including the sensor, other electronics, and lens); its software and control; and the skill and subjective design choices of its designers; and it is constrained by its energy-, mass-, volume-, and monetary cost- budgets.
 * A fun place to start reading is the product line-up website of Sony Semiconductors, the commercial conglomerate that manufactures many of the image sensors in both large- and small- consumer cameras. They publish short-form "datasheet" flyers for "mobile" product markets and for "camera" product markets.  A well-informed reader can easily compare and contrast the sensor technical specifications and draw some conclusions.
 * Nimur (talk) 21:18, 23 November 2017 (UTC)

How are 'bits' of information placed on magnetic tape
In a traditional magnetic tape (like cassette) how are the 'bits' of magnetic impulses placed along it? Are they just a single row along the plastic film? The tape head seems to have a single metal reader, so I wonder whether it could read in parallel at all. But if they are just a row, it seems a waste of space. --B8-tome (talk) 14:44, 23 November 2017 (UTC)


 * See history of multitrack recording. Gandalf61 (talk) 15:00, 23 November 2017 (UTC)


 * On the compact cassette there are either two mono tracks (side A and side B) or four tracks for stereo, indeed just running parallel to the tape. It is somewhat inefficient, but technologies like helical scan come with complexity and expense, and due to greater accuracy they may be poorly suited to a portable recorder (jogging with your Walkperson).  And the C cassette is analog, so no bits. 91.155.192.188 (talk) 15:36, 23 November 2017 (UTC)


 * Cassette is analog, that's way I said 'bits' and not bits. How would you refer to the unit of information on them?B8-tome (talk) 16:47, 23 November 2017 (UTC)
 * An analog signal is a continuous stream, which in the case of audio has instantaneous amplitude proportional to air pressure variation. The pattern of magnetism put on a compact cassette has superimposed cycles of an ultrasonic frequency. This is an inaudible "bias" signal that overcomes distortion caused by the magnetic hysteresis of the ferric oxide tape coating. Blooteuth (talk) 22:46, 23 November 2017 (UTC)


 * If the tape were narrower, with narrower tracks, wouldn't the noise to signal ratio be even higher than on the usual cassette tape? Wnt (talk) 16:02, 23 November 2017 (UTC)
 * Also see Digital Audio Tape and Digital Data Storage. Basically they worked like hard drives tho with different and much less delicate mechanics and thus way cheaper and more sturdy. The earliest ones only managed to record a few megabytes of data on one tape but it was a revolution since everyone could suddenly copy and record music, and later even video, with little cost. --Kharon (talk) 16:37, 23 November 2017 (UTC)
 * In case there's some confusion, tapes for digital storage are still in regular use. LTO-8 (Linear Tape-Open) is the final stages of development with 12TB cartridges (raw capacity) [//www.theregister.co.uk/2017/07/06/spectras_lto8_prepurchase_programme/] . IBM's proprietary TS1155 has a 15 TB raw capacity [//www.redsharknews.com/technology/item/4600-ibm-releases-mammoth-15tb-tape-drive]. Many small scale operations are abandoning their use of tape for backups/archiving but the cloud storage providers they're turning to may themselves use tape at some level [//www.theregister.co.uk/2017/06/14/spectralogic_foresees_ibm_becoming_the_sole_tape_drive_supplier/] Nil Einne (talk) 19:26, 23 November 2017 (UTC)


 * See Magnetic tape data storage. There have been many different ways to do this. Usually, but not always, there are one or more parallel tracks along the tape. When there are parallel tracks, they have been either treated as independent tracks, or read and written in parallel. One alternative to parallel tracks was the helical scan, which was implemented for video signals (VHS) and then adapted for digital signals. The most recent standard is LTO, which used independent parallel tracks. -Arch dude (talk) 04:24, 24 November 2017 (UTC)

Nerve flossing
I encountered some sites claiming that "nerve flossing" is a thing to deal with nerve pain. E.g.   The idea is at least superficially plausible; the nerve is to be slid this way and that in the hope of somehow working it into a less annoyed position. The specifics, though, in some cases include extending and flexing the head in order to help shift the position of the sciatic nerve, which sends my battered bullshitometer some distance into the red. (It missed the peg on account of the figure at right - I don't actually know the sciatic sensory tract can't move relative to the others, though such a thing would imply a level of lubrication to rival a synovial joint, I think! Also it is possible just that part is bullshit but the lower part isn't.)  Searching it on PubMed pulled up one paper about "neurodynamic sliders" that used "neural flossing" as a keyword; there are also a handful of mentions of "neural sliders". The sources tend to be self-help sites and chiropractors, but at the same time, I note the free dissemination of information and the lack of an obvious profit angle from recommending the exercise. So at the moment I'm on the fence between whether this is a science-free delusion or an interesting idea for which I missed the evidence. Can anyone weigh in on its reality? Wnt (talk) 15:50, 23 November 2017 (UTC)
 * The flexing of the head is useful in order to alleviate the pressure and muscular tension from the upper back and shoulders area toward below. I do not think this part is intended for having a direct effect on the sciatica, but that it will be useful for the purpose of the exercise however. Calling the thing "flossing" is perhaps not the uppermost delicate idea indeed ( as it tends to become complicated of getting kinesiatrics everytime it may seem needed, it's not surprising if self-help sites tend to proliferate, possibly starting polluting subjects and their vocabularies as well ). --Askedonty (talk) 12:23, 24 November 2017 (UTC)