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August 14

Brownian ratchet (again)

Hi, looking at the article Brownian ratchet, it seems to me that if the cog wheel and pawl, as well as the structures fixing them in place, were made out of a hypothetical material that was completely rigid on an atomic scale in the face of thermal noise, then the device would work as advertised in violation of the laws of thermodynamics. Is that true? Do the laws of thermodynamics in themselves make the existence of such a material impossible then? 86.160.83.115 (talk) 01:24, 14 August 2011 (UTC)[reply]

No, it would not work in that case. Since thermodynamics is nothing more than information theory applied to Nature, the best way to see through paradoxes like this one, is to translate it in terms of information. Although it may not always allow you to actually do detailed computations, for that you may need to consider the detailed dynamics (which in this case involves rather complicated non-equilibrium thermodynamics).

So, if a molecule strikes the wheel in the right way, it will move, pushing the pawl. But the pawl (assumed to be rigid) will then oscillate. This motion can cause the wheel to move backward. This motion, being a consequence of the molecule striking it, contains the information that a molecule made the wheel move earlier. For the device to work as advertised, you need to get rid of this information. But the fundamental laws of physics forbid you to erase information completely from the universe. The best you can do is dump the information from the unwanted place (the pawl) to somewhere else. For this to happen by itself requires dissipation of the energy, but that won't happen in thermal equilibrium. Count Iblis (talk) 15:29, 14 August 2011 (UTC)[reply]

A closely related way to see through this, is to consider time reversal symmetry. Under time reversal a final state will evolve back to an initial state, which implies that the final state contains the information present in the initial state. If you consider the process that moves the wheel in the right directon, then you can see how the wheel would move in the wrong direction by applying time reversal to that process. In thermal equilibrium, both processes occur with equal probability. Count Iblis (talk) 15:47, 14 August 2011 (UTC)[reply]

The "Why it fails" section of the article explains that the machine fails because "a ratchet and pawl small enough to move in response to individual molecular collisions also would be small enough to undergo Brownian motion as well. The pawl therefore will intermittently fail by allowing the ratchet to slip backward or fail by not allowing the ratchet to slip forward." Is this explanation incorrect or incomplete? If it is correct and complete then why would my magic material not eliminate the (relative) Brownian motion of the pawl and the cog, thereby preventing the ratchet from slipping? 109.153.232.146 (talk) 20:50, 14 August 2011 (UTC)[reply]

If you assume that the pawl doesn't dissipate energy, then it will flutter hence and forth and that will make the set up to fail to work. So, te given explanation is incomplete. It's similar to the old arguments puporting to show why Maxwells Demon fails. While these analyses are technically correct, they are sort of "Straw Man arguments", i.e. they make assumptions and then one shows that the set up fails, but that is then due to the assumptions that are made. Then, if you make a different assumption, like you did here, it seems that the thing really can work.

Like in case of Maxwell's Demon, the fundamental reason why this doesn't work was given by Landauer: you can't erase information, at best you can move it, and there is an entropy cost to be paid for that (so that won't happen in thermal equilibrium).Count Iblis (talk) 21:20, 14 August 2011 (UTC)[reply]

Thanks. The need for the pawl to dissipate energy may be a way forward for me to understand why my proposal would not work. Unfortunately the "information" argument is beyond me; I don't even begin to understand how it's relevant. If you think the article's explanation is incomplete and ever have the time and inclination to improve it then I'm sure that would be appreciated by readers. 109.153.232.146 (talk) 21:53, 14 August 2011 (UTC)[reply]

The article explanation is fine. You seem to think that your rigid material would be immune to Brownian motion. It would not. Dauto (talk) 21:57, 14 August 2011 (UTC)[reply]

The article said (before I fixed it) "a ratchet and pawl small enough to move in response to individual molecular collisions also would be small enough to undergo Brownian motion as well" [sic]. That makes no sense. "Movement in response to individual molecular collisions" is Brownian motion. I think the OP was misled by that sentence and imagined a device that's immune to one but not the other. -- BenRG (talk) 22:37, 14 August 2011 (UTC)[reply]

My idea was that the rigid material would be immune to "internal" Brownian motion affecting the positions of the pawl and cog relative to one another. I did not imagine the rigidity would prevent the whole assemblage from jiggling about in unison if hit, say, by air molecules. 109.153.232.146 (talk) 00:21, 15 August 2011 (UTC)[reply]

The latter IS Brownian motion. The first is thermal agitation. Brownian motion is the apparently random jitter of objects due to the action of molecules on them. Dauto (talk) 01:33, 15 August 2011 (UTC)[reply]

Oh, thanks, I think I have been misunderstanding this. I was imagining that the slippage was due to relative motion between the pawl and cog caused by this "thermal agitation", but in fact the whole assemblage jittering about even in unison would jiggle the pawl and allow the ratchet to slip? That seems to make sense; is that right? I guess any spring/gravitational force strong enough to defeat this would also stop the cog rotating in the desired direction? 86.160.84.240 (talk) 11:46, 15 August 2011 (UTC)[reply]

That's right. And that's why the time reversal (or information conservation) argument comes in. One process is the time reversal of the other and you cannot stop one without stopping the other unless the system is out of equilibrium in which case there would be a source of free energy. Dauto (talk) 14:50, 16 August 2011 (UTC)[reply]

Are hummingbirds territorial?

We have three or four ruby-throated hummingbirds which frequent our backyard bushes and feeder. Lately, one female seems to be hanging around longer than most. She will spend most of the day perched or resting in a weiglia bush which is aboutr 20 feet from the feeder. When another humingbird attempts to use the feeder, whe will leave her perch and chase off the "intruder". Is this normal behavior? JDBakerJdbaker5 (talk) 01:44, 14 August 2011 (UTC)[reply]

Yes, extremely. Given their high metabolism, food is a very high value localized commodity for hummingbirds. It's half the fun of watching them. μηδείς (talk) 01:48, 14 August 2011 (UTC)[reply]

We have three feeders, none visible from the others, arranged around the house, and probably at least 6-8 hummingbirds in the neighborhood. They're constantly chasing each other in circles around the house from feeder to feeder. It appears that all get enough food, but the behavior is typical of ruby-throated hummingbirds. Other species seem to be less territorial. Acroterion (talk) 13:29, 14 August 2011 (UTC)[reply]

If they were colonial and had stings like bees they'd be frigging terrifying. (Reminds me of monsters from A Case of Conscience and Beowulf's Children. μηδείς (talk) 19:25, 14 August 2011 (UTC)[reply]

Using gene therapy to change physical characteristics

Is it theoretically possible to use gene therapy to change physical characteristics like... hair or eye color? Something like that? ScienceApe (talk) 02:16, 14 August 2011 (UTC)[reply]

It certainly is, but the symptoms of the infection and the reactions that might occur could be like having the symptoms of HIV and the pox or viral meningitis at the same time. A certain percentage of human obeistiy is likely caused by viral infection. Keep in mind that many traits depend on development, so, for instance, once you've reached adulthood and your bones have stopped growing, a gene for a bigger body frame won't do anything. You will not be the one you want to have such methods tried on first. Read Michael Crichton's Next and its afterword. Read the brilliant Dark Benediction by Walter M. Miller.μηδείς (talk) 02:58, 14 August 2011 (UTC)[reply]

Yes. However, note that the characteristics wouldn't change in a few seconds or minutes, like in your average bad sci-fi movie. More likely they would change as the hair or eye pigment is naturally replaced, so over the course of weeks or months. In the case of hair, this might lead to a "bad dye job" period with different colored roots. They eye color might also be somewhat unattractive when midway between the two colors. StuRat (talk) 03:08, 14 August 2011 (UTC)[reply]

How massive?

How much mass must an asteroid have to have a large influence on tectonic activities when striking earth?--Irrational number (talk) 10:32, 14 August 2011 (UTC)[reply]

I wrote a Spanish article a bit related: es:Crust tsunami (video). emijrp (talk) 10:58, 14 August 2011 (UTC)[reply]

Created a stub: Crust tsunami. emijrp (talk) 11:13, 14 August 2011 (UTC)[reply]

thanks... a bit more?:D--Irrational number (talk) 13:09, 14 August 2011 (UTC)--Irrational number (talk) 13:09, 14 August 2011 (UTC)--Irrational number (talk) 13:09, 14 August 2011 (UTC)[reply]

I can find no sources either at GoogleBooks or GoogleScholar that even mention "crust tsunami" - where have you seen the term used apart from the video? Mikenorton (talk) 21:03, 14 August 2011 (UTC)[reply]

I think Emijrp mis-heard what was on the video. The announcer says crest tsunami, not crust tsunami. At least I think that's what he says. That term doesn't seem to show up on Google Scholar either; it may have been just a nonce term. --Trovatore (talk) 08:55, 15 August 2011 (UTC)[reply]

Can anyone answer my question...?--Irrational number (talk) 08:23, 15 August 2011 (UTC)[reply]

Well, the Spanish source, taken for what it is, says "centenares de kilometros" which means "hundreds of kilometers", which certainly would splash the crust. Anything that big won't leave witnesses. Leaving aside the reliability of the sources, the Spanish article specifies crust tsunami, whereas crest tsunami simply makes no sense in the context. μηδείς (talk) 15:57, 15 August 2011 (UTC)[reply]

See "Meteor Pelting May Have Triggered Plate Tectonics" μηδείς (talk) 16:01, 15 August 2011 (UTC)[reply]

I'm not sure how much support that idea has, but it is interesting that the onset of subduction (and thereby plate tectonics) has recently been dated to 3 billion years ago from the observation of inclusions in diamonds [1]. Mikenorton (talk) 17:14, 15 August 2011 (UTC)[reply]

Time it takes solar system to orbit around the galactic centre.

I have read a lot of articles that tell me that its 225 million years.

I would like to know how astronomers have measured this, also does this mean that the deep sky objects will "move" into other areas of the sky ?

thx :) — Preceding unsigned comment added by 92.30.186.14 (talk) 12:15, 14 August 2011 (UTC)[reply]

The movement of stars in the night sky relative to each other and very distant objects is called proper motion. That's caused by the relative motion of the stars and our solar system. The estimate of the length of a galactic year (and it's a pretty rough estimate) is got by measuring the distance to the centre of the galaxy (see cosmic distance ladder for how distances are measured), using that to estimate the length of the Sun's orbit and then measuring the speed of the sun through the galaxy to get how long it will take to complete one orbit. I'm not quite sure how the orbital speed is measured, though... it's easy enough to work out how we're moving relative to nearby stars, but they are also orbiting the centre of the galaxy, so that doesn't help much. See Sun#Motion and location within the galaxy for some more details. --Tango (talk) 13:45, 14 August 2011 (UTC)[reply]

One has to identify objects that belong to the halo of the Milky Way, not the disk. The halo shows no net rotation, so one can determine the Sun's velocity relative to the mean of these objects. Candidates are globular clusters and so-called high-velocity stars. If one plots their velocity vectors, they show an asymmetric distribution with a centroid at around −220 km/s (so-called "asymmetric drift"). --Wrongfilter (talk) 18:15, 14 August 2011 (UTC)[reply]

Identify (?) bomb

This is from RAF Cowden, an aerial bombing test range, possibly a bomb. But is it actually a bomb or something else? (I didn't take the photograph so I don't know the scale but it must be ~1.5ft long) - looks odd for a bomb.. What is it? Thanks. Imgaril (talk) 16:40, 14 August 2011 (UTC)[reply]

It is quite possibly an older aerial bomb which has lost its tail fins. I'd watch my step in that area. Beeblebrox (talk) 21:03, 14 August 2011 (UTC)[reply]

Such things are common in former artillery ranges, even those which supposedly have been "cleared." In the U.S. at least one has been turned into a park (Tobyhanna State Park in Pennsylvania) which issues warnings about finding old ordnance to visitors.

Compare the images on this page. Possibly the front half of a practise bomb. Nanonic (talk) 23:47, 14 August 2011 (UTC)[reply]

This is what's called a shell, which may or may not carry an explosive payload which might be called a bomb. (The mistake would be analogous to showing a picture of an envelope and calling it a letter. An envelope is not a letter and may or may not contain a letter.) What sort of shell this is I haven't a clue. μηδείς (talk) 01:34, 15 August 2011 (UTC)[reply]

Thanks - Nanonic's images of the 'BDU-33' nailed it - looks the same - (though I though the one here looks bigger).Imgaril (talk) 02:06, 15 August 2011 (UTC)[reply]

Fly stickers - do they work?

Got myself a recurring fly/midge problem in my kitchen bin this summer. Those coloured, flower-shaped window stickers that claim to kill flies (that you can find really cheap on eBay). Do they really work as well as is claimed? --Kurt Shaped Box (talk) 20:24, 14 August 2011 (UTC)[reply]

Taking out the garbage works great. Looie496 (talk) 21:26, 14 August 2011 (UTC)[reply]

Sounds like a cuter version of flypaper, which works fine but is not at all pleasant to look at. Beeblebrox (talk) 21:29, 14 August 2011 (UTC)[reply]

Naw man, I *do* take out the garbage when the bag is full. However, I have loads of flies in my bin again about a day later when there's very little in there (just some mandarin peel and a couple of apple cores today - opened bin to drop in pizza box and flies erupted). I'm not sure where they're coming in from. They're not regular houseflies - these are much smaller (maybe 4mm long). --Kurt Shaped Box (talk) 21:36, 14 August 2011 (UTC)[reply]

Almost certainly part of the Drosophila family attracted by, err, mandarin peel and apple cores. Nothing an appropriately sized seagull couldn't sort out, I'll be bound. I've used the window stickers to try to kill other sorts of flies: no joy. --Tagishsimon (talk) 21:41, 14 August 2011 (UTC)[reply]

Seagulls, hehe. μηδείς (talk) 21:47, 14 August 2011 (UTC)[reply]

I wish that this bird was my gull-friend. --Kurt Shaped Box (talk) 21:52, 14 August 2011 (UTC)[reply]

There may be something else attracting them, a lingering smell in the bin perhaps? Keeping in mind that many bugs find their food almost entirely by smelling for it, you may want to consider scrubbing the inside of the bin with bleach or something. Even if you can't smell it, they apparently can. Beeblebrox (talk) 21:43, 14 August 2011 (UTC)[reply]

Yes, almost certainly fruit flies, given the examples. If you eat a lot of fruit, you might consider throwing the cores and peels and whatever outside immediately. They're biodegradable anyway :P -- Obsidi♠n Soul 21:46, 14 August 2011 (UTC)[reply]

I moved to Georgia (US) last summer from parts further north and both this and last summer I've had a similar problem with fruit flies in the trash. Once they take hold it's hard to get rid of them. If there's any sort of food waste in the trash they're back with in a day or two of taking out the garbage. One thing I've found that helps a lot is to scrub out the trash bin. The flies lay eggs on the inside of the lid and inside of the bin itself (outside of the bag). You'll see a lot of fly maggots hanging out here (the eggs themselves are hard to see). Cleaning the bin is not guaranteed to fully eradicate them, depending on how thorough you are, but it will at least keep the numbers in check. I haven't heard of those stickers so I don't know if they work. I did read online that you can make a trap with vinegar and some detergent, and it was completely ineffective. Rckrone (talk) 22:13, 14 August 2011 (UTC)[reply]

Keep garbage (kitchen waste, as opposed to trash) in the freezer until you take it to the curb. μηδείς (talk) 22:19, 14 August 2011 (UTC)[reply]

Freezer is one option. Garbage disposal is another. Flushing is a third. Putting them in the outside trash is a fourth. A sealable "slop jar", such as an old coffee tin, is a fifth option. There's just no need to leave fruit fly food out in your home where fruit flies can get to it. StuRat (talk) 23:35, 14 August 2011 (UTC)[reply]

As for your current population, one method which can work is to leave out plates filled with lemon-scented (hand) dish-washing detergent. The lemon scent attracts them, and the detergent traps and kills them. StuRat (talk) 23:38, 14 August 2011 (UTC)[reply]

Keeping everything in the fridge or freezer, including grain food from the pantry and food scraps from the garbage (assuming, as in NYC, garbage disposals are illegal or you have a septic tank) will work against mice, rats, house flies, fruit flies, roaches and A Flock of Seagulls. Flushing helps too but leaves a mess between kitchen and toilet. μηδείς (talk) 00:51, 15 August 2011 (UTC)[reply]

Thanks for the tips, guys. I've never ever had this problem before. Sure, I've had insects get in my house (who hasn't?), wasps nests, flying ant day crap, etc. but never an infestation of the little bastards on this scale. --Kurt Shaped Box (talk) 00:59, 15 August 2011 (UTC)[reply]

I'd guess the core of the problem is that a pregnant female found it's way to your rotting garbage. StuRat (talk) 03:01, 15 August 2011 (UTC)[reply]

I had a problem with fruit flies and those stickers were very helpful (they become more effective once a few flies are stuck to them - that seems to attract other flies more than the smell). The other thing I found useful was a vacuum cleaner. I would use it to literally suck them out of the air. It took several weeks to get rid of them completely, but they're gone now. --Tango (talk) 11:34, 15 August 2011 (UTC)[reply]

What's wrong with automatic free of charge waste disposal by letting your waste be converted to flies? Count Iblis (talk) 14:50, 16 August 2011 (UTC)[reply]