Wikipedia:Reference desk/Archives/Science/2012 June 9

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June 9

"35dB-90dB[μV]" equal to 56 - 31000 mV ?

On the page "How to use the booster." it is said that "35dB-90dB[μV]" is the necessary voltage level for a 75 Ω antenna signal. Is that equalient to 56 - 31000 mV ..? Electron9 (talk) 01:54, 9 June 2012 (UTC)

No, you are 3 orders of magnitude out. To calculate db[μV] take the Log of the voltage in microvolats and multiply by 20, thus 56 mV corresponds to 95 dB[μV] and 31000 mV (31V) corresponds to 150 dB[μV]. 35 dB[μV] corresponds to 56 μV. 90 dB[μV] corresponds to 31.6 mV. However, 35 dB is rather high for the required input at the teminals of a TV set. A modern analogue TV should get a good picture with 20 dB[μV] or even less. 35 db[μV] would be good at the input to the antenna distribution cable system in a high rise building, where there is significant loss in the cable runs and in the splitters. A digital TV should in theory do rather better but in practice you need to allow a good margin to avoid dropouts and friezes. Keit120.145.6.122 (talk) 06:36, 9 June 2012 (UTC)

Like I suspected then, at least the multiplier 20 was correct. Btw, do you have any sources regarding the 20 dB[μV] level? maybe there's even a general difference between analog (CVBS) and digital (DVB) in regard to minimal signal strength? Electron9 (talk) 06:49, 9 June 2012 (UTC)

I answered from memory. However, a quick web search turned up this paper (as well as a lot of useless chat rooms about TV!), which seems to cover things quite well: http://www.eecs.berkeley.edu/~sahai/Presentations/Dyspan_2005_tutorial_part_I.pdf. On page 82 it gives the good picture minimum level for a digital TV as -85 dBm (dBm is an impedance-independent measure referenced to 1 mW). This corresponds to 15.4 μV across 75 Ω, i.e., 23 dBμV. Keit120.145.6.122 (talk) 11:03, 9 June 2012 (UTC)

Why is the oldest person always 114 years old?

For some years now, every time the allegedly oldest person in the world dies, the person's age has been reported to be 114. Just today we saw this item, saying this woman was the oldest person in Europe, and she died today at the age of 114. Why always that same age and never 113 or 115? Michael Hardy (talk) 02:38, 9 June 2012 (UTC)

It's often 115, that number you saw was just for Europe. I would argue it's simply a statistical issue. If you look at the US Social Security Administration's most recent actuarial table, they calculate the probability of making it from 115 to 116 to be only 25%. And making it from 114 to 115 is only a measly four percentage points better. So I would look at that and say that you start with a fixed population maximum of people born in 1897, and have that population experience greater-than-exponential decay from age 10 onward (it is less-than-exponential prior to that). The reason the "oldest person at the moment" is almost always 115 is that the decay function, although a bit noisy at those ages, would predict less than one survivor for all ages past 114. You'll see that creep up in the future as the starting population for each given year is increasing, as is post-adulthood life expectancy. Someguy1221 (talk) 02:53, 9 June 2012 (UTC)

I have seen data on that the maximum life expectancy is 130-150 years for a human, so 114 years is getting close. And thus the deterioration of the body is likely becoming exponential. Electron9 (talk) 03:02, 9 June 2012 (UTC)

Now, modifying the human might allow indefinite lifespan, like genetically rebuilding it and adding repair nanobots. So I'm assuming they're talking about without genetic engineering, cyborgization, or reanimation. (perfect preservation already existing, in the form of liquid nitrogen). So how do they propose living 27.5 years over the record? Calorie restriction? Sleeping through most of it?(/coma/hibernation/near death) Hysterectomy? Sagittarian Milky Way (talk) 18:54, 9 June 2012 (UTC)

You must be a young whipper snapper if you don't remember la chẻre Jeanne Calment. See list of the oldest verified people. μηδείς (talk) 03:15, 9 June 2012 (UTC)

She was a pistol. Her comments about Van Gogh are especially funny. ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:25, 9 June 2012 (UTC)

I guess the disagreement is probably due to low sample size, but this study suggests that mortality per year (i.e. chance of dying in any given year of life if you live that long) is 50% from age 110-115, and they speculate that that number may even increase beyond this age. -RunningOnBrains^(talk) 04:41, 9 June 2012 (UTC)

The small sample sizes are, indeed, a problem. The mortality tables I use professionally are only based on actual data up to age 95. After that, there just isn't enough data to get robust results, so they arbitrarily extrapolate from age 95 up to age 120, which they set as having a mortality rate of 100%. A detailed explanation of the process can be found here (be warned, it is quite technical). --Tango (talk) 16:31, 9 June 2012 (UTC)

So we will soon lose the last people who have lived in three centuries.Hayttom (talk) 04:25, 12 June 2012 (UTC)

The last ones in the lifetimes of anyone old enough to read this, at any rate. Come 2101, there'll be a whole pile of people born in the 1980s and 1990s who'll still be alive. -- ♬ Jack of Oz ♬ ^{[your turn]} 23:56, 12 June 2012 (UTC)

Baeyer's Reagent

Isn't Baeyer's reagent an alkaline solution of potassium permanganate?? The article on it states it to be neutral. — Preceding unsigned comment added by Roshan220195 (talk • contribs) 10:21, 9 June 2012 (UTC)

I guess that it is a matter of time. A fresh solution of potassium permanganate should be neutral. As time passed the permanganate decomposes slowly, as it does, the solution becomes more and more alkaline. Plasmic Physics (talk) 10:57, 9 June 2012 (UTC)

Plasmic Physics is right. To add the numbers to show why: A pure solution of potassium permanganate should be neutral, because potassium hydroxide is a strong base and permanganic acid is a strong acid, with a pKa of -2.5[1]. The salt of a strong base and a strong acid always forms a neutral solution. However, as the permanganate ion decomposes to the manganate ion, manganic acid has a pKa of about 7.4, making it a weak acid. --Jayron32 12:40, 9 June 2012 (UTC)

What is the decomposition mechanism for permanganate? O=[Mn-](=O)(=O)=O.O=[Mn-](=O)(=O)=O → O=[Mn-](=O)(=O)OO[Mn-](=O)(=O)=O → O=O.O=[Mn-](=O)=O.O=[Mn-](=O)=O ? Plasmic Physics (talk) 13:17, 9 June 2012 (UTC)

It isn't really decomposition, it's oxidation: The permanganate will oxidize just about anything, producing manganate and some sort of oxide, or elemental oxygen. The manganate will spontaneously disproportionate to permanganate and managanese dioxide, so given any trace reductant, there should develop an equilibrium between manganate, permanganate, and manganese dioxide which will account for the rising pH. --Jayron32 15:41, 9 June 2012 (UTC)

In that case, I don't think that it is in an equilibrium - it is not a closed system, oxygen escapes from the solution as singlet oxygen, which then decays into triplet oxygen. According to your process, I think that the reaction would be:

MnO⁻
₄ + 2 H
₂O + 3 e^- ↔ MnO
₂ + 4 HO⁻

4 MnO⁻
₄ + 4 HO⁻
→ 4 MnO²⁻
₄ + O
₂ + 2 H
₂O

3 MnO²⁻
₄ + 2 H
₂O ↔ 2 MnO⁻
₄ + MnO
₂ + 4 HO⁻

Plasmic Physics (talk) 06:47, 10 June 2012 (UTC)

Electronic eavesdropping

How does one find a bug that has been put in a house or car? Kittybrewster ☎ 11:40, 9 June 2012 (UTC)

Make a simple visual search of places where things can be quickly hidden first. Then, search using a radio scanner. Set the scanner sensitivity fairly low so you don't waste time on legit radio transmissions. See http://en.wikipedia.org/wiki/Scanner_(radio). Scanners are very good at picking simple radio transmitter bugs because when they get to the right frequency while auto scanning, you'll hear your own voice(s), and or the scanner will howl. Sometimes bugs placed by professional outfits or govt angencies are placed that display great inguinuity and cannot be found with scanners. Books have been written about this. I'm not trying to imply anything about your goodself, but be aware that there is a common pschological condition, often occuring in people who are otherwise normal, and sometimes brought on by stress, where people believe that they are being spied on, when they are not. Sometimes businessmen think they are just so darn good that the opposition must surely be spying on them. Very very few actually do so with bugs. Most industrial intelligence is obtained quite legally and simply by employing specialist researchers scanning documents in the public domain. I've learnt what I needed to know about competition by sharing a beer in a pub combined with monitoring employment adverts and press releases. Did you check our wiki article http://en.wikipedia.org/wiki/Covert_listening_device ? Also be aware that it is possible to write computer malware that activates the microphone in a laptop and monitor your voice, as well as your keystrokes, without you knowing, over the internet. Always have reputable computer security installed, and make sure your software firewall is set up with optimised port restrictions. If your PC has Vista, that's good - make sure that installing software requires an administrator password. Wickwack124.178.139.104 (talk) 11:55, 9 June 2012 (UTC)

Radio scanners will miss spread spectrum digital burst transmissions. Use a digital spectrum analyzer to find bugs. As for computers, disable recording devices like microphone and webcams physically and audit software. If you use any Microsoft software your computer is f-cked by design. Electron9 (talk) 18:53, 9 June 2012 (UTC)

You are correct, however a spectrum analyser requires a trained operator or electronic technician to operate it and interpret the display. Anybody with at least average intelligence can operate a radio scanner, and if there is an ordinary FM bug, the scanner operated in the same room as the bug will make the presence of the bug obvious. Wickwack124.178.60.220 (talk) 03:00, 10 June 2012 (UTC)

Note that radio scanners only work for devices which transmit radio signals. Other approaches are a hardwired bug, with the wires going outside the house to where somebody taps into them, a device that records and is retrieved later, or one that uses existing communication channels, like the phones lines, cable, wireless internet, cell phones, etc., to transmit signals. StuRat (talk) 18:52, 9 June 2012 (UTC)

Measuring vibrating glass is not that uncommon technique. Electron9 (talk) 18:55, 9 June 2012 (UTC)

Yes, that's another one, where you bounce a laser off the window at a shallow angle, and the refracted laser beam vibrates as the window vibrates, due to sounds in the room, allowing someone at the receiver to listen in. StuRat (talk) 23:45, 9 June 2012 (UTC)

I think that might be an urban myth, Stu. Being a bit of a nerd, I once made a laser communication system. I tried it out bouncing the beam off a window. I did detect sound from within the room to degree, and noise from inpinging wind to a greater degree, but as for understanding converstions, it was a dead loss. That doesn't mean someone else could not have succeeded, but it seems unlikely. The laser I used was a visible wavelength. A covert device would have to be infrared, which window glass doesn't reflect as well. Wickwack124.178.60.220 (talk) 03:10, 10 June 2012 (UTC)

Indeed, this technique is easy and cheap, as it can be done using a laser pointer, photodiode, and any old device with an audio-in jack. My physics advisor proposed that we make one as our senior project, even had all the materials ready, but I (stupidly) chose a different one. -RunningOnBrains^(talk) 03:14, 10 June 2012 (UTC)

Was your physics advisor talking about a voice link (ie from laser pointer to photodiode, nothing in between except air) or a bounce-off-glass eavsdropper system? I just tried my ear against the window (with my other ear plugged) while my lady was inside talking on her phone. All I could hear was the wind. Wickwack124.178.60.220 (talk) 03:25, 10 June 2012 (UTC)

Correct, you need a window. We were going to have a laser pointer, and a photodiode hooked up to the audio-in jack of a computer set up on one end of the hallway, with the laser pointer pointing at the glass on the other end of the hallway, reflecting back to the photodiode. Theoretically you could use other surfaces too, but windows are best because they are relatively rigid, often single-layer, and not weight-bearing so they are free to vibrate. In retrospect it probably would have been a bad idea for privacy reasons, since that glass was to my advisor's office :D -RunningOnBrains^(talk) 05:52, 10 June 2012 (UTC)

You would have been lucky to get that to work. The window vibration will not impart amplitude modulation, it will only impart phase modulation (http://en.wikipedia.org/wiki/Phase_modulation) via doppler effect. A photodiode connected to the audio input of something will not inherently recover the audio, unless you position it just right so that the beam is centred not quite on the diode. That would be extremely critical. In the system I tried, I used circuitry to pulse the laser diode on/off at 40 MHz. That allowed me to use a 40 + a bit MHz oscillator at the recive end to mix with the incomming beam signal and recover the doppler modulation imposed by the vibrating glass. Wickwack120.145.7.109 (talk) 06:32, 10 June 2012 (UTC)

Such eavesdropping would certainly be easier on a day with no wind. However, even on a windy day, it might be possible, if the wind causes different frequency vibrations than the sounds inside. The choice of window also might be critical, as some are more free to vibrate than others. I suspect a large, single pane would vibrate more, and a wooden or vinyl frame would tend to absorb vibrations more than a metal frame. StuRat (talk) 14:46, 11 June 2012 (UTC)

Wind noise is essentially white noise, covering all audible frequencies, and cannot be filtered out. Wickwack121.215.63.236 (talk) 00:50, 12 June 2012 (UTC)

Stalin's little thing worked. DriveByWire (talk) 00:05, 13 June 2012 (UTC)

The Nonlinear junction detector (NLJD) is a device that detects semiconductor devices (diodes/transistors), as used in the majority of listening devices. However, the heavy use of electronics in cars and homes nowdays might render the NLJD much less useful in 'bug' detection than before the widespread use of electronics in almost every electrical appliance. It might not be able to sort the bug from the many other electronic devices in most houses/cars. It would also, most likely, be unable to detect the Thing (listening device) mentioned by DriveByWire as the 'Thing' uses no semiconductors. See Also Technical surveillance counter-measures, which has a link to "Listening In: Electronic Eavesdropping in the Cold War Era" (PDF). (4.1 Mb). "Bugging" from Popular Science, Aug. 1987 may also bbe of interest, though well out of date. - 220 of ^Borg 16:48, 14 June 2012 (UTC)

Crepuscular rays

I saw some interesting atmospheric phenomena the other day (gallery is here; I've included all the shots I took, but the first, fourth, and fifth are the best). So, I'm assuming these are some kind of crepuscular rays, but the shape of them is what I'm curious about. I've seen crepuscular rays on countless occasions, of course, but the shortness of these is something new to me. The sky looked like someone had gotten crazy with a clone brush. What exactly is at work here? Are the rays only showing up in places where there's a certain amount of humidity/water vapour and then disappearing in the drier air below? These pictures were taken around 8am, facing (roughly) east; the sun is off-frame to the left. As you can see, there was a variety of clouds out that morning. I'm afraid all I had on me was my iPhone, so the quality is less than ideal. The images have not be manipulated in any way (other than the standard jpeg compression). Matt Deres (talk) 13:26, 9 June 2012 (UTC)

The weird thing is, sun rays (crepuscular rays) are lighter than the background, while yours appear darker. I might say they were smoke in the upper atmosphere blown into lines, but that doesn't explain why they would appear to radiate from the Sun. StuRat (talk) 18:47, 9 June 2012 (UTC)

What you are seeing is the shadow of the cloud. I suspect that you are right that there is a layer of smoke that is causing the short length of the rays. The normal cloud free condition of the sky would be for it to be as bright as the crepuscular rays, and the cloud shadow makes it darker. It would be easier to see a bright ray against a dark background than a slightly darkened sky against the bright sky, but the difference in this situation would be the smoke making higher scattering of light in that part of the sky, bringing up the contrast. Graeme Bartlett (talk) 23:15, 9 June 2012 (UTC)

I'm not sure what smoke has to do with it. I didn't see any smoke, nor smell any (not that I necessarily would, if it was that high) and we'd just had a few days of gentle rain, so there's no particular reason to think there'd be a forest fire or something. Also, none of the pictures appear smokey. I'm wondering if the cloud on the right is involved; its left edge seems to indicate that it was gradually thinning out towards the phenomenon. Matt Deres (talk) 15:42, 10 June 2012 (UTC)

Yes they are, although faint. As a (formerly obsessive) sky watcher and optics enthusiast, I have no problem seeing them. 129.2.171.55 (talk) 22:11, 10 June 2012 (UTC)Nightvid

These are indeed a type crepuscular rays, although you are seeing them from an atypical angle, which is part of the reason for their strange appearance. Were you near a body of water perchance? My theory is that the rays seem to end mid-sky because the sun is reflecting up off a body of water nearby, interrupting the cloud's shadow nearer the ground. Pretty much speculation on my part though, tough to tell just from a few photos. -RunningOnBrains^(talk) 00:30, 11 June 2012 (UTC)

Nope, no sizable water nearby. Matt Deres (talk) 01:02, 11 June 2012 (UTC)

How many times did sex evolve independently?

How many times did sex evolve independently? 82.31.133.165 (talk) 17:17, 9 June 2012 (UTC)

Are you sure that it did ? (As opposed to only evolving once and being passed down to all species which evolved from that one.) Our evolution of sexual reproduction article says, in the 2nd sentence, that "All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species.", and provides source(s) to back up that claim. StuRat (talk) 17:40, 9 June 2012 (UTC)

That sentence by itself wouldn't mean much more than that all sexually reproducing species are eukaryotes. More strongly, it seems very unlikely that meiosis -- the special type of cell division involved in sexual reproduction -- evolved more than once, since it requires a large number of special mechanisms in order to happen, and as far as I know those mechanisms are always implemented in essentially the same way. Looie496 (talk) 19:06, 9 June 2012 (UTC)

It appears that isogamy was the first stage of sexual reproduction that involves gametes that look alike and so cannot be classified as "male" or "female." In several lineages (plants, animals), this form of reproduction independently evolved to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is very much larger than the male and has no ability to move. There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction, see Isogamy#Evolution. DriveByWire (talk) 19:21, 9 June 2012 (UTC)

If you mean meiotic reproduction, just once, per above. But the transfer of genes between bacteria and the recombination of genes in viruses are separate phenomena. μηδείς (talk) 18:04, 10 June 2012 (UTC)

What is: "colonic sorting" in biology?

thanks.

As I understand it, it's a mechanism by which the colon of certain animals such as rabbits separates small particles and fluids from larger, less digestible particle. Looie496 (talk) 18:56, 9 June 2012 (UTC)

a simple table which sums up all types of reproduction?

1.monoecious: types, dioecious: male-female, male or female with intersex. thanks.

This list of modes of reproduction would be very long indeed. Here are a few that you might not be familiar with: Apomixis#Types_of_apomixis_in_flowering_plants, and Fungus#Reproduction. Combined, these links present a few dozen different modes, and only cover a small fraction of what plants and fungi actually do. Some fungi have thousands of mating types. Animals have slightly less variety in reproductive modes, but there is still parthenogenesis, which occurs in things like aphids and even some lizards and fish. So- good question, but I've never seen a comprehensive list that covers all forms of life :) SemanticMantis (talk) 13:15, 10 June 2012 (UTC)

I don't understand how to mount the camera on a barn door tracker

Doesn't the angle between the camera body and the tracker matter? What are the guidelines for tilting and aligning the camera, once the polar finder has been aligned with the poles? 76.104.28.221 (talk) 19:10, 9 June 2012 (UTC)

I'm not sure I understand your confusion. Where else would the camera be pointed, if not towards the object you're trying to photograph? See this photo: http://www.astropix.com/BGDA/SAMPLE2/207A.JPG

The barn door tracker acts like a simple equatorial mount with only a right ascension adjustment knob. If you imagine moving the angled board around a full circle, the direction in which the camera points will also move in a full circle. The size of the circle depends on how far the camera lens' direction is from the axis. If it's pointing along the axis, the circle has 0 size, and the camera only rotates. If it's pointing 90 degrees away, the camera traces a great circle around the sky. This is exactly how a star behaves in relation to the celestial pole. --140.180.5.169 (talk) 20:14, 9 June 2012 (UTC)

i.e. does it matter how the camera is oriented, in order for the barn door tracker to work? Can the camera point towards any object in the celestial sphere and still track the object? 207.114.92.194 (talk) 17:45, 11 June 2012 (UTC)

Yes, if your polar alignment is accurate, you can point the camera at any part of the sky. The earth only rotates around one axis, if the "mount" counter-rotates around the same axis, it doesn't matter where the camera is pointing; in relation to the "sky", it will remain stationary. Maybe have a look at Equatorial mount. Vespine (talk) 22:54, 11 June 2012 (UTC)

Jamming the GPS system on a global scale

Apparently, it's quite easy to jam GPS signals, GPS jammers are even available commercially. But this raises the question of how reliable this system is. Could e.g. China shut the GPS system down globally using satellites in case of war with Taiwan? Count Iblis (talk) 23:31, 9 June 2012 (UTC)

Yes, along with GLONASS and GALILEO. But most major militaries have frequency hopping positioning satellites ready to be switched on in an emergency. 71.212.248.104 (talk) 04:47, 10 June 2012 (UTC)

Do you have a source on the frequency hopping GPS satellites? Even if this technology actually exists it's not available in any of the portable GPS receivers that the army uses. Anonymous.translator (talk) 05:30, 10 June 2012 (UTC)

I'd also like to see a source on China being able to jam the GPS system. Seems like it's possible on a local scale, but on a global scale? You'd need a hell of a transmitter. -RunningOnBrains^(talk) 05:46, 10 June 2012 (UTC)

Or a weak transmitter on board an airplane, or near the up-link which probably is the weakest system point. With a huge (state) budget one could employ satellites for the job. Electron9 (talk) 11:32, 10 June 2012 (UTC)