Wikipedia:Reference desk/Archives/Science/2011 August 27

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August 27

Hurricane predictions - too far to the west

I've been watching NOAA hurricane predictions for 9 years. when they are threatening the east coast of the US, the early predictions invariably (or almost invariably) put the track too far to the west, and as a consequence predicting landfall too far to the south. These images show some predictions for Irene.

• 
  Advisory 7
• 
  Advisory 8
• 
  Advisory 9
• 
  Advisory 11
• 
  Advisory 13
• 
  Advisory 15

The first advisory I looked at was # 5. I didn't save it, but it had it tracking farther to the west and impacting Jacksonville. Each successive prediction had it farther to the east and north, until about #15, since when there has been little change. As far as I can remember, this always happens with their predictions for a hurricane like this over the last 9 years. Why don't they realize that there is a systematic error in their model and fix it? Bubba73 ^{You talkin' to me?} 01:35, 27 August 2011 (UTC)

Have you written to NOAA as well as us? HiLo48 (talk) 01:43, 27 August 2011 (UTC)

I don't think it would be useful to write to NOAA without some formal analysis to back up the claim. Hurricane prediction involves a lot of judgement calls, and the people who do it have to integrate large amounts of often contradictory information. NOAA maintains an archive of predictions going back for years, so it ought to be possible for an ordinary (non-academic) person to do a statistical analysis. Looie496 (talk) 01:55, 27 August 2011 (UTC)

No, I have not written to NOAA. I would think that they would compare their predictions with what actually happened and make adjustments to the models, but they don't seem to have done that. Bubba73 ^{You talkin' to me?} 02:46, 27 August 2011 (UTC)

NHC do compare what happened with a TC with what the models say and their predictions based on the Best Track analysis. See the Verification report for more details. Another thing worth pointing out is that NOAA do not own all the models that select the best track from, for example the UK Met Office and the ECMWF run models.Jason Rees (talk) 02:55, 27 August 2011 (UTC)

I just realized that there was a related question on August 24 #Hurricane Irene on Weather Underground: computer models much scarier than 5-day forecast. Bubba73 ^{You talkin' to me?} 03:02, 27 August 2011 (UTC)

Even if they knew the predictions were wrong, it could be a hard thing to fix. These predictions are based on weather models that simulate the whole atmosphere and ocean for the purposes of weather forecasting. Finding the center of the storm and tracking its motion is a highly derived quantity that depends on many other factors, such as wind fields and moisture content and surface temperatures, etc. So, even if they found that the predictions were systematically wrong, it could be very difficult to identify what part of the model needed to be fixed. Dragons flight (talk) 18:15, 27 August 2011 (UTC)

As a frequent reader of the prediction discussions, I can tell you that the forecasters rely heavily on expertise -- they don't just blindly rely on the models. It is actually rare for a forecast to represent a simple consensus of models -- sometimes one model is favored over the others; sometimes (more rarely) a forecaster will simply say that the models don't seem plausible and make a prediction that doesn't match any of them. So if a forecaster noticed that predictions were systematically wrong in some identifiable way, that knowledge would surely be taken into account. Looie496 (talk) 18:33, 27 August 2011 (UTC)

Thanks. They seem to be consistently wrong on ones like this, but I need more data. Bubba73 ^{You talkin' to me?} 21:01, 27 August 2011 (UTC)

You are assuming that expert opinion overriding models actually leads to improvement. In actuarial science, for example, there are many studies showing times where expert judgments actually do worse than mathematical models. This is often the case when a system is sufficiently complicated that humans have trouble grasping all the important aspects and interactions, whereas a well considered computer model may do better at bringing together a multitude of factors. I don't know of any comparable studies of short-term weather forecasting, but with a system as large and complicated as weather, it isn't obvious that expert interpretations will actually make forecasts more accurate. Maybe they do, and maybe they don't. In general though, the more accurate and comprehensive a model ultimately becomes, the less likely that overriding its predictions will be beneficial. Eventually, relying on perceptions of personal expertise is likely to be detrimental, if it isn't already. Dragons flight (talk) 21:38, 28 August 2011 (UTC)

As a point of interest, you can see their older advisories of this hurricane plotted onto google maps here.

As another point of interest, here are the "Spaghetti charts" for hurricane Irine from various dates. Spaghetti Charts. (I'm not sure exactly how they turn this into a single prediction. I think the different models are weighted somehow on past performance.)

This is interesting. But I wonder if you're not data-picking. Storms that travel up the coast are rare anomalies. By identifying them after the fact, aren't you examining only the ones most likely to have defied prediction? APL (talk) 02:05, 28 August 2011 (UTC)

Actually, looking at it closely, it looks like the actual track of the hurricane is within the probability cone of that Advisory #7 image. Just barely, but it's within the cone.

And the cone itself only represents 67% certainty. (I wonder how big the 90% certainty cone is? Does it reach to the Mississippi?) APL (talk) 02:32, 28 August 2011 (UTC)

After a certain point the predictions are very close, but you can see in the early ones that the predicted track is farther and farther east and the point of landfoil is farther and farther north. This seems to be what almost always happens with one that comes up like this one. For instance, advisory #6 would have put landfall right on top of us. Years ago we used to start getting ready to evacuate in a situation like that, but now I know the early predictions have a systematic error. Bubba73 ^{You talkin' to me?} 17:42, 30 August 2011 (UTC)

Squirrel Behavior Question

We have both an American Red Squirrel and two Fox Squirrels sharing our lake shore property. I was told that the American Red Squirrel will tear off the testicles of the Fox Squirrel should it catch it. I have watched the American Red Squirrel "chase" the Fox squirrels, but not catch them. Is this a true "behavior" of an American Red Squirrel?

Thank You!

Lee — Preceding unsigned comment added by 75.198.53.218 (talk) 01:53, 27 August 2011 (UTC)

Sounds doubtful. Perhaps it tries to kill it by any means available, which might mean ripping off whatever it can get hold of. StuRat (talk) 06:30, 27 August 2011 (UTC)

I'm intrigued to know how the Red Squirrels gained the knowledge of orchidectomy and its effects. This story has a strong anthropomorphic odour. Richard Avery (talk) 07:23, 27 August 2011 (UTC)

American red squirrels are very territorial. Fox squirrels, on the other hand, are not and are quite gregarious. I highly doubt they'd be chasing each other for the express purpose of ripping off testicles though LOL. The American red squirrel is just doing the equivalent of "Git offa mah lawn!"-- Obsidi♠n Soul 12:01, 27 August 2011 (UTC)

I agree it's a suspicious claim, but I'll add that a territorial mammal need not have 'knowledge' of the effects of castration in order to benefit from castration of competitors. wp:or I've seen dogs target testicles in a variety of species, and it doesn't sound completely unreasonable that red squirrels may have picked up this technique over evolutionary time. Still, probably just a colorful embellishment of how they aggressively defend territory :) SemanticMantis (talk) 13:56, 27 August 2011 (UTC)

They just misinterpreted the instructions in the Squirrel Manual: "When you encounter another squirrel, try to rip-off his nuts". StuRat (talk) 16:37, 27 August 2011 (UTC)

This product may contain traces of nuts. Plasmic Physics (talk) 23:48, 27 August 2011 (UTC)

Strings and Wave Packets in Physics

Is a String in string theory the same thing as a Wave packet or something different?Pensioner.bsc (talk) 03:03, 27 August 2011 (UTC)

It's different.

The word "string" in string-theory is used to describe what most physicists would call "a degree of freedom." String theories vary widely, though, so the term is used and abused pretty wildly. Some string theorists propose that a "string" is the fundamental degree-of-freedom for the most elementary of elementary particles; and they have mathematical elaborations to help explain how a string's properties can, when observed more macroscopically, be related to physical properties like mass and charge. Personally, I don't find that any simpler than accepting the elementary properties like mass and charge; nor do I believe it provides any predictive insight; but I am not a string theorist, so I'm not privy to the details of the theories.

A "wave packet" refers to the conceptual idea of a propagating disturbance that has a meaningful degree of spatial locality as it travels. In other words, the simplest idealization of a wave is an infinite plane wave that is totally uniform everywhere. A "wave packet" is a more practical realization that describes a wave disturbance with finite dimensions. You can use a variety of mathematical tools to help understand wave packets and localization - particularly, fourier decomposition.

But, to summarize, "string" and "wave packet" refer to totally different conceptual ideas. Nimur (talk) 03:52, 27 August 2011 (UTC)

Thanks. I've been reading some pop science, Michio Kaku & Jennifer Thompson "Beyond Einstein" OUP 1999.Pensioner.bsc (talk) 20:19, 27 August 2011 (UTC)

Where do the baloons go

When helium baloons rise in the atmosphere and complete their pupose do they continue rising and expand in the diminishing atmospheric pressure untill they burst and fall back to earth? Do they continue on into space and drift around like a bubble? Or do they acheive some sort of equalibrium and drift around in our upper atmosphere? I did search but could find no answer.190.56.16.48 (talk) 05:57, 27 August 2011 (UTC)

When they rise to the point of the pressure of the air outside the balloon being lesser than the pressure of the air inside the balloon, then the balloon pops and falls to the ground. I believe this was illustrated in a Calvin and Hobbes strip. --Σ _{^talkcontribs} 05:58, 27 August 2011 (UTC)

'Twas—Calvin was holding on to the balloon when it began lifting him higher and higher into the atmosphere, when eventualy it pops when he thinks about it doing so and he falls very fast, until he finds his transmogrifier gun and turns himself into a photon so he can get back home. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 20:39, 1 September 2011 (UTC)

What is just as interesting to me is where does the helium go? I believe from things I heard long ago that free helium in the atmosphere does continue to float up and disappear into space. This means that every helium balloon is a loss of helium from the earth. HiLo48 (talk) 06:07, 27 August 2011 (UTC)

The question is "Does the earth's gravity have enough power to indefinitely maintain the atmosphere"? If the answer is no, we've been wasting helium for many, many years. --Σ _{^talkcontribs} 06:18, 27 August 2011 (UTC)

Yes, we have been wasting helium for many, many years. I've done my research now. The Helium article tells us that the helium we use commercially (including in balloons) comes from natural gas, a non-renewable resource, and also that "most helium in the Earth's atmosphere escapes into space by several processes". HiLo48 (talk) 07:19, 27 August 2011 (UTC)

We could always go to Jupiter to get more helium though. Granted it wouldn't be cheap. Googlemeister (talk) 13:58, 29 August 2011 (UTC)

Couldn't help noticing that helium baloons all seem to be much larger than the volume of helium in them. Therefor if the baloon expanded to it's full extent then the pressure inside would be minute and it's concievable that the strength of the material could contain that tiny pressure differential and allow the baloon into space.190.56.16.48 (talk) 06:30, 27 August 2011 (UTC)

There are concerns about a shortage of helium. And this video shows what happens when you send an iPhone up to space with a weather balloon. Dismas|^(talk) 06:48, 27 August 2011 (UTC)

Wow, that is one excellent video! Richard Avery (talk) 07:17, 27 August 2011 (UTC)

Having been party to high altitude baloon experiments I happen to have some first-hand knowlege. The natural latex baloons normally used are manufactured to have a specific strength, so they burst when they reach a specific size (~11m in the case of the experiments I was party to). The amount of helium in the baloon and the weight of the baloon and it's payload would determine at what altitude that happens. I know of one baloon that carried a payload of a little under 1kg to an altitude of over 115 000 feet when it burst. If the baloon were to reach an equilibrium altitude before it reaches its burst point it would drift at that altitude until the latex is degraded by UV radiation, then it bursts (usually a matter of a few hours - provided it's in daylight of course). As the baloons are natural latex they are completely biodegradable and in fact if left outdoors in the sun and weather will completely decompose in a few weeks or months. Roger (talk) 08:32, 27 August 2011 (UTC)

Party balloons are usually made from metallised PET. Although more gas tight than latex they never the less are permeable to helium. Helium gas are very small atoms and escapes by atomic diffusion. Temperature and gas gradient affects the speed that this happens. A party balloon also has a large surface to volume ratio so it will lose positive buoyancy faster than a airship (which may also have a secondary barrier) and its lift will not be as great. Indeed the helium mix (its not pure helium) and the weight of the envelope (balloon) will only give sufficient positive lift for a few hundred feet for an ordinary balloon. So a party balloon will just go up to its equilibrium hight then slowly descend again.--Aspro (talk) 10:40, 27 August 2011 (UTC)

Helium in the Earth's atmosphere. ~AH1 ^(discuss!) 22:16, 31 August 2011 (UTC)

Discoloured teeth from drugs/meds

I'm pretty sure that there was a medication that resulted in a lot of people (maybe just in the UK) having discoloured teeth because of either some medication they took or their mother took. It's not fluorosis but something similar. Does this ring a bell with anyone? Thanks. --87.112.226.247 (talk) 09:30, 27 August 2011 (UTC)

Tetracycline#Cautions.2C_contraindications.2C_side_effects. --Aspro (talk) 09:37, 27 August 2011 (UTC)

That's great, thanks for the info Aspro :-) --87.112.226.247 (talk) 15:00, 28 August 2011 (UTC)

Shoddy reporting story???

http://www.huffingtonpost.com/2011/06/29/diet-soda-weight-gain_n_886409.html

How can diet soda (by its own) increase the weight of people if it has no calories? Is it some case of correlation does not imply causation? Seems to me as if this was written with the "I didn't make myself fat, it's the fault of external factors beyond my control" perspective. Raskolkhan (talk) 13:43, 27 August 2011 (UTC)

Something with no calories might still affect your metabolism changing how many calories you retain from other stuff you eat. I am personally very suspicious of artificial sweeteners for that very reason. Dauto (talk) 13:52, 27 August 2011 (UTC)

Its explained here, but there doesn't appear to be a free copy of their actual paper available.[1]--Aspro (talk) 14:00, 27 August 2011 (UTC)

And there are many other ways where consuming things without calories may cause you to gain weight. Here are a few:

1) Consuming salt may cause you to retain water.

2) Consuming things which accumulate in the body will increase your weight, like arsenic or lead. Of course, they will also kill you, at which point you're likely to lose weight.

3) Consuming something which causes you to grow tumors may make you gain weight. StuRat (talk) 16:31, 27 August 2011 (UTC)

I'd like to know how the study was controlled before putting too much weight on it. For example, was the control group drinkers of regular cola? or people who don't like cola? Or people who wanted to drink cola, but were forbidden for the purposes of the study? Besides the cola did they make sure that both groups were eating and drinking the exact same things?

It occurs to me that people who drink a lot of cola might be big fans for sweet foods in general, while people who don't like cola might prefer healthier foods in general. This is certainly true of people I know in my life.

(In short, People who drink a lot of cola might also eat a lot of junk food.)

These sorts of seemingly minor differences in how the study is run can drastically change the meaning of a scientific study, but the news never reports these details. Without reading the study itself you can't draw any conclusions. APL (talk) 21:20, 27 August 2011 (UTC)

In fact, check this out, from the press release :

"The results were adjusted for waist circumference, diabetes status, leisure-time physical activity level, neighborhood of residence, age and smoking status at the beginning of each interval, as well as sex, ethnicity and years of education."

Notice that "Diet" is conspicuously missing from that list. Why is that? That's the obvious thing to adjust for!

Makes you wonder if the creators of the study didn't have an axe to grind. (Or it makes you wonder if they ran the numbers once taking diet into account and got a boring non-publishable result, and wondered "what can we forget to calibrate for that will get us into the news so we'll all have jobs next year?")

Until I know otherwise I'm going to assume that this study just proved that "eaters of Junk Food will get fat even if they drink diet soda". APL (talk) 21:31, 27 August 2011 (UTC)

You're missing the entire point of the study. The idea is that your body expects that something sweet is coming, prepares for it (presumably by releasing insulin), and when it doesn't get it, makes you crave sweets. Thus, you gain weight by giving in to the enhanced cravings. Obviously you can't gain weight if you are denied the additional food you crave as a result of drinking the diet soda. Unless you intend to put diet soda drinkers across the nation in shackles to prevent them from eating more sweets, such a study as you propose would be useless. StuRat (talk) 22:22, 27 August 2011 (UTC)

StuRat, I understand that was the point of the article, but that's not what the press release says that the scientists studied.

The study shows that there is a statistical "link" between drinking diet soda and gaining weight! Well, no kidding! I could have told you that! Do you know any health food nuts that drink soda of any kind? I sure don't. Since the study (apparently) didn't control for type of diet, The "Health Food Nut" demographic would be enough to cause a statistical "link", while telling us absolutely nothing about artificial sweeteners. In addition, the lack of healthy drink alternatives at Fast Food joints would also be enough to cause a statistical "link" between diet cola and weight gain, also without telling us anything useful about artificial sweeteners.

Imagine that you go to McDonalds six times a week and order a Diet Coke with your super value meal. That will make you a "Diet Cola drinker" for the purposes of this study, but is it the cola that's making you fat?

(If Bob only drinks fruit juice he buys with his groceries at "Whole Foods", and Alice only drinks Diet Soda she purchases with her meals at McDonalds, it's safe to say that Alice is more likely to gain weight for reasons that have absolutely nothing to do with the diet soda.)

Until there's some indication that they controlled for this variable (And, as I said, It's conspicuously missing from the list of variables they controlled), then I don't see that this study tells us anything that isn't already obvious.

76.28.67.181 (talk) 00:27, 28 August 2011 (UTC)

To respond more directly to sturat's point, all the scientists' theories about how artificial sweeteners cause weight gain are BS until they demonstrate that artificial sweeteners do cause weight gain, and judging from their own press release (since I can't find a free version of the paper) I'm not convinced that they've proved that at all. They may have simply demonstrated that Cola's are primary drunk by people with poor diets. Which seems likely to me.

(If your concern is that drinking diet cola will actually cause someone to switch from salads to cheeseburgers, then the correct course is obviously not to just ignore that variable completely! but to create an experiment that tests that concern. Possibly by restricting the study to people who have recently switched from regular cola to diet.)

76.28.67.181 (talk) 00:41, 28 August 2011 (UTC)

Are you APL ? StuRat (talk) 00:39, 28 August 2011 (UTC)

Oh. Yes. Sorry. APL (talk) 01:19, 28 August 2011 (UTC)

What you could do is ensure that everyone had identical dietary habits before you started the study, then give half of them diet soda and the other half diet soda with the artificial sweeteners removed. However, you obviously would then have to let them eat what they want during the study, as that's the only way to determine if drinking diet soda makes you eat more calories. StuRat (talk) 00:49, 28 August 2011 (UTC)

You seem to be trying to compare regular soda with diet soda. That might be a valid study, but I'd expect that both are far worse than drinking something healthy (like water or unsweetened tea). Since they advertise diet soda as allowing you to lose weight after the switch from regular soda, such a study might dispel that myth. StuRat (talk) 00:57, 28 August 2011 (UTC)

Oh, I agree with that. My point was just that you can't look at "Diet Soda Drinker" as though it was a completely independent variable. In society at large, it's clearly not. It tends to be closely associated with things like "Hamburger Eater" and "Processed food buyer" which will obviously impact a study of waist measurements.

Of course, The scientists involved may well have grouped the people in their study by diet habits, perhaps the press-release writer simply didn't mention it. I should have emphasized that, because I really do hate to criticize scientists based entirely on how their work is reported in the press. I guess that brings us back to the original question of "Shoddy reporting??" APL (talk) 01:19, 28 August 2011 (UTC)

Conspiracy theories that link aspartame to cancer have been debunked again and again by health authorities (I'm refering to the things that cause tumors increase body weight remark)... besides no one forces people to eat more even if they have increased cravings. It's not as if Pepsi Max implants a brain parasite that causes involuntary eating. ADL pretty much elaborate on why this "study"was never published in a legitimate scientific journal. Raskolkhan (talk) 07:21, 28 August 2011 (UTC)

Regarding "no one forces people to eat more even if they have increased cravings": All things being equal, if people crave more food they will eat more and gain weight, it's as simple as that. The "no one forces people" argument could be extended to keeping trans fats in foods, etc., because, hey, nobody is forcing them to eat it. StuRat (talk) 07:31, 28 August 2011 (UTC)

It might not force people to eat more, but if it can be proven to increase cravings that's an important thing for consumers to know. Not enough to ban it or anything, but I think most people would want to know if their "diet" product was going to cause sugar cravings. APL (talk) 08:31, 28 August 2011 (UTC)

We have discussed this study previously at Ref Desk. Again I say that I am very suspicious of the claim that if I am eating a diet which maintains my present weight, and I were to switch from 3 diet sodas a day at 0 calories total to three corn sweetener filled 12 ounce Cokes at 140 calories each, or 420 calories a day, that I would lose weight. Instead, I would gain 43 pounds a year, since 3500 excess calories added to the food input adds a pound of fat. Was this study funded by the "corn sweetener institute?" I deny that drinking artificially sweetened soda causes some bizarre "craving" for sweets. Instead, it decreases the desire for sweets. If the researchers are truly scientists, they would follow their "amazing" findings with feeding studies of rats, pigs, guinea pigs, and other animals, to see if there is a change in exercise, metabolism, or food choices when the beverage is switched from sugar pop to diet pop and back. "Miraculous" scientific results require followup. That is how science works. Edison (talk) 22:59, 28 August 2011 (UTC)

Who made a claim that regular soda would help you lose weight ? I believe they are saying that BOTH regular soda and diet soda are bad for weight loss. To lose weight, switch to something like water or tea (without sugar or artificial sweeteners). That's what I drink, and I have been slowly losing weight since I switched. StuRat (talk) 20:28, 29 August 2011 (UTC)

Firstly, The Huffington Post is notorious for shoddy reporting, quackery and pseudoscience. Second, I can't provide the reference but I thought the general consensus was that people who drink "diet" drinks overcompensate by thinking they can have that extra piece of cake or whatever because they've been "good" by not drinking non-diet soda. Vespine (talk) 00:26, 29 August 2011 (UTC)

space moving

If space can expand then space can move. If space can move then can space move at different speeds and if so do the different speeds result in compression and decompression like sound waves in air and if so could space expansion just be the part of the wave (front) we are in at this moment? --DeeperQA (talk) 14:20, 27 August 2011 (UTC)

YES! space compressional waves can propagate through space itself. See gravitational waves. Dauto (talk) 14:24, 27 August 2011 (UTC)

Is space expansion related to gravity? --DeeperQA (talk) 14:47, 27 August 2011 (UTC)

Yes, absolutely. Space expansion is a particular solution to Einstein's gravity equations. See Einstein field equations. A gravitational wave is a different solution for the same field equations Dauto (talk) 15:01, 27 August 2011 (UTC)

Nevertheless, they are not a good explanation for the observed expansion of space because they don't lead to a uniform expansion in all directions at the same time. Dauto (talk) 14:26, 27 August 2011 (UTC)

Please consider the effect of blowing up a balloon and then not blowing it ups as fast - the traditional way of demonstrating what happens to galaxies if they were dots on the surface of a balloon. --DeeperQA (talk) 14:45, 27 August 2011 (UTC)

The rate of blowing up cannot be arbitrarily increased and decreased at will. The rate changes according to Einstein's field equations. The presence of matter has the effect to decrease the rate of expansion - that's the usual behavior of gravity. The presence of dark energy may have the unusual effect of increasing the rate of expansion - that's unusual but it is still a gravitational effect. Dauto (talk) 15:01, 27 August 2011 (UTC)

It would be a good idea to not start a new section for each question if their all about the same topic. I've noticed that you have created three sections, all questions about space-time. Plasmic Physics (talk) 14:51, 27 August 2011 (UTC)

Our minds work differently. --DeeperQA (talk) 15:15, 27 August 2011 (UTC)

shrinking galaxies

Are there any galaxies that are growing smaller? --DeeperQA (talk) 14:49, 27 August 2011 (UTC)

Galaxies usually don't shrink or expand. They orbit their centers spinning around themselves. Dauto (talk) 15:08, 27 August 2011 (UTC)

But on the other hand if it's true that all/most galaxies contain a supermassive black hole at their centre, then whether by accretion or act of super nova, collapse of massive material toward the centre seems intuitively to indicate shrinkage of the entire galactic body.Phalcor (talk) 16:04, 27 August 2011 (UTC)

Not really. Only stars close to the black hole are pulled in. Most stars in the galaxy are in a stable orbit about the galactic core. StuRat (talk) 16:23, 27 August 2011 (UTC)

Which for me begs the question:- Does the collective gravity of the stars near the centre remain the same pre-black hole formation and post black hole formation?Phalcor (talk) 16:44, 27 August 2011 (UTC)

(Pedantic linguistic aside for the interested: Phalcor probably means 'raises the question'; 'begs the question' means something else entirely :) — Preceding unsigned comment added by SemanticMantis (talk • contribs) 17:15, 27 August 2011 (UTC)

You should get a better dictionary. Most modern dictionaries give "raise/elicit the question" as a standard meaning of "beg the question", e.g. Merriam-Webster.[2] --Colapeninsula (talk) 11:04, 29 August 2011 (UTC)

Yes, the mass remains the same. When a black hole swallows a star, it's mass increases by that amount (minus whatever streams out as jets). StuRat (talk) 17:29, 27 August 2011 (UTC)

..and thereby more space is made available or created? --DeeperQA (talk) 17:53, 27 August 2011 (UTC)

If by "made available" you mean that space once occupied by a star is now empty, that's true. The diameter of the black hole's event horizon will also have grown, but I don't think it's increase in volume is as much as the decrease in the star's volume (hopefully somebody else can verify this). StuRat (talk) 18:12, 27 August 2011 (UTC)

DeeperQA, based on this and your previous questions, you seem to want to explain the cosmological expansion as having something to do with individual bodies collapsing into black holes. It doesn't work that way. There was an explosion at the scale of the whole cosmos and matter is still flying apart from that explosion. The expansion is not affected by anything that clumps of matter do locally, such as collapsing into black holes. There is no need for any process to "make space available" for the expansion. All space at a given time is "new" since it is a different set of spacetime points than the space at any previous time. -- BenRG (talk) 19:59, 27 August 2011 (UTC)

So you are saying that prior to the Big Bang space was compressed like air in a tank and that the Big Bang is when the tank burst and the air began filling the vacuum surrounding the tank, ie. space filling the Universe? --DeeperQA (talk) 20:10, 27 August 2011 (UTC)

No, not at all. "Space" and "vacuum" are the same thing, and, as I said above, space at different times is different space. If you were standing in a trapezoidal room, you wouldn't ask how the space from the wide end of the room was compressed to fit in the narrow end, or what the room was expanding into. There's no law of width conservation that says that rooms have to be rectangular. -- BenRG (talk) 20:42, 27 August 2011 (UTC)

So you are saying that time changes space or vacuum and that vacuum is expanding but not due to Black Holes releasing the vacuum which makes up atoms? --DeeperQA (talk) 22:13, 27 August 2011 (UTC)

"Releasing the vacuum", there isn't a bit of sense in that! A vacuum isn't released, for something to be released it must first be contained. Plasmic Physics (talk) 23:43, 27 August 2011 (UTC)

You cannot start at the end of a thread and understand it. Please try again. --DeeperQA (talk) 00:15, 28 August 2011 (UTC)

The expansion of space has nothing to do with black holes. Primarily the Hubble expansion is a simple consequence of the explosive impulse imparted to the universe by the Big Bang. Dark energy adds some extra wrinkles, but for the most part space is expanding simply because of the momentum imparted to all matter during the Big Bang. Dragons flight (talk) 02:18, 28 August 2011 (UTC)

..err, but space is not matter but vacuum which does not explain how a force can impart expansion to it. --DeeperQA (talk) 05:56, 28 August 2011 (UTC)

We don't measure space. We measure the distances between observable objects (e.g. stars, galaxies). If they are moving apart, then we say that the space between them is increasing. If this happens everywhere, then we say that all space is expanding. Nonetheless, what we observe is the matter and we use it to infer properties about "space". The explosion of matter as a consequence of the Big Bang is exactly the process we also describe as the expansion of space. The expansion of space is simply another way of describing the observed evolution of the matter. Dragons flight (talk) 06:17, 28 August 2011 (UTC)

DeeperQA: Do you suppose that I post at the beginning of a thread? Who does that? Plasmic Physics (talk) 04:22, 28 August 2011 (UTC)

The key phrase is "read from" rather than "post at". --DeeperQA (talk) 05:57, 28 August 2011 (UTC)

I did exactly that. Plasmic Physics (talk) 07:00, 28 August 2011 (UTC)

It doesn't change the validity of my statement. Plasmic Physics (talk) 08:31, 28 August 2011 (UTC)

Some regions surrounding supermassive black holes actually see increased star formation, which replaces any stars swallowed. The extremely tiny space taken up by a star would make practically no difference to galaxy size, let alone explain the increasing distances between galaxies—any mass removed near the black hole may just be added to the black hole itself. ~AH1 ^(discuss!) 22:05, 31 August 2011 (UTC)

Largest LED

What is the world's largest light emitting diode and what kind of equipment, material, and skill would it take to make a larger one? 208.54.5.228 (talk) 18:23, 27 August 2011 (UTC)

Are you asking about 1 individual LED cell, or a large screen composed of many LED cells ? StuRat (talk) 22:35, 27 August 2011 (UTC)

I read the question as asking about one single LED. 82.43.90.90 (talk) 23:14, 27 August 2011 (UTC)

If so, the question is why you would want to build a large single LED when a bank of smaller LEDs would be far more efficient. One large LED makes about as much sense as sandpaper with one large grain on it. StuRat (talk) 23:17, 27 August 2011 (UTC)

The question is not about efficiency, it's about a world record in the electronic industry. Plasmic Physics (talk) 23:34, 27 August 2011 (UTC)

Well then, I suppose they could do like the do with pumpkins, and make a huge one, even though it's of no use other than for winning contests. StuRat (talk) 00:35, 28 August 2011 (UTC)

It would be dangerous to try to make the world's largest LED. You could use spray deposition to create an interface boundary between any two semiconductors, but unless the interface surface is flat you will not get a coherent frequency. The alternative to a coherent frequency is usually just heat, so there would be technical merit in such a competition. However, I would like to see the environmental impact statement first. Please see also this video. 76.254.20.205 (talk) 03:17, 28 August 2011 (UTC)

In theory, there already is a "world's largest" LED somewhere, right? ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:45, 28 August 2011 (UTC)

Depends what you mean bij "largest". Largest quantity of lux? Largest use of precious metals? Largest consumption of energy in manufacture or operation? --VanBurenen (talk) 09:26, 28 August 2011 (UTC)

Largest dimensions? The Wikipedia article on LEDs says the upper range for commercialy obtainable LEDs is 8 mm. Plasmic Physics (talk) 10:36, 28 August 2011 (UTC)

It still depends what you mean by 'LED'. For example, you can get very large LED arrays e.g. BridgeLux makes large ones [3] as do many Taiwanese and Chinese manufacturers [4] [5] [6] [7]. However these are made up of multiple dies which you can probably see on the LED, and without any additional (high power LEDs sometimes have a pre-attached dome or lens over the emitter/s but it isn't intended to be the sufficient) optics you can probably see in the output (e.g. [8]). In other words. if you want fairly uniform lighting, you need properly designed optics (also to avoid loss/waste as always). For the Chinese manufacturers, the so called '300W' ones seem to be the highest they advertise. But if you wanted something larger you may be able to get someone to do a custom for you (considering the price, even for one may be doable). I know some will produce custom colours high powers (colours they normally make but not in that size). Note that dealing with the heat output from such a LED is no small (or even medium) matter. In any case, given these have multiple emitters, perhaps the OP doesn't count them.

Luminus produce large LEDs. These are basically a single die/emitter and the output is fairly even over the surface even without the dome/lens [9]. The SST-90 and CST-90 (I think the emitters are the same, just different packaging [10]) are I believe still their largest 'single' emitter white LED. The emitter surface is 3x3 mm or 9 square mm [11] [12]. I think they are currently the largest commercial available single emitter white LEDs. They also have 12 square mm (4 mm x 3 mm and also 4.6 x 2.6 mm) single emitter R, G, B LEDs [13] which I think are probably the largest commercial available single emitter LEDs. [14]

Cree (who along with Nichia) appear to be leading the edge in white LED efficiency have been increasing LED size as they produce more efficient and higher output LEDs, reaching the level required for general room lighting purposes. Their current leader I believe is still the 2 mm x 2 mm XM-L [15]. (Despite IIRC a large marketshare, Nichia doesn't tend to get so much interest from the enthusiast crowd. I believe they don't have much in the large/high output single emitter area.)

The 'largest' emitter doesn't exactly sound like a record most manufacturers would be that interested in, efficiency and light output are likely to be seen as far more important. So I'm not sure you'll easily find other records. Note that while multiple die/emitter LEDs complicate the optics, it does make getting the desired color tint easier [16]. In case anyone is not aware, LEDs are binned by things like colour temperature, chromaticity, light output (luminous flux) and sometimes forward voltage; e.g. [17]. Producing LEDs in whatever bins your customers want remains a big issue.

So commercially, producing large and expensive single emitters would likely face the normal problems producing large semiconductors tend to cause, there's likely to be greater waste or products you can only sell to cheap manufacturers who don't give a stuff. Plus thermal management (which is very important with high power LEDs) and possibly light extraction would I expect be more complicated compared to multiple die/emitters (see [18] and [19] [20] for example). Then you have to worry about patents, Luminus seem to like to tout their patents for big chip LEDs ([21]).

Nil Einne (talk) 17:06, 28 August 2011 (UTC)

Why do most Jews not look, well, semitic?

Why is it that most Jews, for example this chap on the right have lighter coloured skin than most Arabs from Israel and the surrounding area? I am of course aware that Jews come in all colours (shapes, sizes, etc) but one would have thought that with a religion/culture where, for want of a better way of putting it, breeding within the group had such importance it's curious that your average Jew is relatively light skinned. What's up? Egg Centric 18:48, 27 August 2011 (UTC)

Because Rabinovich is basically Ukrainian. Compare with Jews who were not in the diaspora: Mizrahi Jews and Arab Jews.-- Obsidi♠n Soul 19:02, 27 August 2011 (UTC)

But until recently Jews were supposed to have been descendants from the twelve tribes of Israel, right? It shouldn't matter how long his ancestors lived in the Ukraine (within reason, pedant in me has to point out presumably eventually evolution would act to select a skin colour roughly the same as the rest of the population if it was an adaptive advantage, but certainly not in such a short time period). Even now conversion from outside tribal descent is a really sticky issue AIUI. Or am I horribly misinformed? Egg Centric 19:04, 27 August 2011 (UTC)

I think our article on Ashkenazi Jews will probably answer the question, particularly the section on genetics near the bottom. Looie496 (talk) 19:09, 27 August 2011 (UTC)

I think it's also probably a misconception on your part that all Middle Eastern peoples are dark-skinned. Persian, Levantine, and Central Asian peoples (like the Turks and the Pashtun for example) are usually quite light-skinned. Arabs have a slightly darker skin tone, but that's probably just the desert sun.-- Obsidi♠n Soul 19:22, 27 August 2011 (UTC)

The genetics certainly support the common group of ancestors theory, ruling out intermarriage as a cause - and yes I know that Turks, Lebanese etc are pretty light skinned - but I really don't think they're as light skinned as the Jews I have in mind. A theory just entered my own head - perhaps the lighter skinned ones were more likely to emigrate north while the darker skinned ones were more likely to head south. Any research on this? Ethiopian and (the remaining) Sudanese Jews are pretty dark, I know this. Egg Centric 19:39, 27 August 2011 (UTC)

IMO, highly unlikely that migration patterns would follow very minor variations in skin color. :/ More likely that those who went south are darker skinned due to either the climate or intermarriage with local populations and that the original groups were light-skinned (compare peninsular Arabs with sub-Saharan Arabs). The taboo against intermarriage or conversion might not be present or more relaxed in non-European Jewish populations. The article on Arab jews specifically points out that they probably were of Arab ethnicity for example.-- Obsidi♠n Soul 20:00, 27 August 2011 (UTC)

Before we resort to some very crude ad hoc speculations, let's note Jewish ethnic divisions and Genetic studies on Jews. To summarize the results, while Jews indeed have tended to remain apart as an ethnic group, matters of the heart are not so readily predicted; though there is a remarkable degree of preservation of the male lineages, there are signs of gene flow in every population. Note that conversion to Judaism is possible, for example. I don't think people would be people if they could live side by side on a continent for a thousand years without visible evidence of union. Wnt (talk) 20:21, 27 August 2011 (UTC)

Exactly.-- Obsidi♠n Soul 20:48, 27 August 2011 (UTC)

Egg Centric says above that "until recently Jews were supposed to have been descendants from the twelve tribes of Israel, right?" Wrong. Jews would have included converts. And Egg Centric says "Even now conversion from outside tribal descent is a really sticky issue…" How? Why? I'm not aware of anything "sticky". It is straightforward: anyone who wants to convert to Judaism can convert to Judaism. There is little proselytization, so the conversion option lacks popularization. Bus stop (talk) 03:30, 28 August 2011 (UTC)

"Anyone who wants to convert to Judaism can convert to Judaism" - but it's strongly discouraged isn't it? I heard (from a Jewish friend) that rabbis must turn potential converts away three times before permitting them to convert. Pais (talk) 05:54, 28 August 2011 (UTC)

Not exactly -- AFAIK there's nothing in the Torah that requires rabbis to turn away potential converts (and indeed it wouldn't make much logical sense, given that any religion benefits from gaining converts). However, since circumcision is mandatory for all male converts to Judaism, obviously such a conversion is a very serious decision and not to be taken lightly. This in itself serves to limit the number of converts from other religions, without any need for anyone actively turning converts away. 67.169.177.176 (talk) 01:43, 29 August 2011 (UTC)

That's not a well-thought-out statement. An aversion to getting oneself circumcised would not be a deterrent to conversion for at least 65% of the world's potential converts. The circumcision requirement would be moot for 100% of the world's women, and moot for the 30% of the world's men who are already circumcised, the vast majority of whom are not already Jewish. Only about 0.2% of the world's population are Jewish, so the number of Jews in the world could in principle be increased by a factor of about 325 through conversions, without a single circumcision being required. Red Act (talk) 06:53, 29 August 2011 (UTC)

It's not moot for the 30% of non-Jewish men already circumcised (see Brit milah). In general, circumcision of all male converts to Judaism is required, even if they are already circumcised: in this case the circumcision ritual is performed and at least a drop of blood is obtained from the penis. I think it isn't too far-fetched to believe that some might find this a reason to think twice about conversion. - Nunh-huh 07:32, 29 August 2011 (UTC)

Oops, my mistake. I didn't know about Hatafat dam brit. Red Act (talk) 08:09, 29 August 2011 (UTC)

Sorry to say this

But I don't think anyone has really answered my question. Having said that, the stuff above is certainly interesting so it is no waste of time - indeed it's more knowledge, and 'tis impossible to have too much knowledge. But regarding my skin colour question.... I am still looking for an answer. Sorry. Egg Centric 22:18, 30 August 2011 (UTC)

Jewish is basically someone born from a Jewish mother. That gives plenty of opportunity to mix up with local, light skinned people and change your color in just one generation. Quest09 (talk) 22:30, 1 September 2011 (UTC)

Weird maple disease

The Norway maples in our yard seem to be afflicted with some weird disease. The leaves have multiple large black spots about the size of my thumbnail, that have some tiny yellow specks inside them and a yellow rim. They are distributed more or less randomly over the leaves. Do you have any clue what this could be? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 19:39, 27 August 2011 (UTC)

Tar spots on sycamore

Hmmm... might be tar spots caused by Rhytisma acerinum.-- Obsidi♠n Soul 19:54, 27 August 2011 (UTC)

That looks exactly like what they have. Thanks! Whoop whoop pull up ^{Bitching Betty | Averted crashes} 20:14, 27 August 2011 (UTC)

Light reflecting off light

Can a light beam reflect off the photons in another light beam? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 20:21, 27 August 2011 (UTC)

Yes, photon-photon scattering is possible, but it is highly inefficient at typical energies so you need a very, very large number of photons in order to have an observable number of scattering events. At the human scale, two beams of light are always going to appear as if they pass right through each other. Dragons flight (talk) 21:02, 27 August 2011 (UTC)

Think of it like firing two guns across the path of one another. It's possible the bullets will strike each other, but highly unlikely. StuRat (talk) 22:28, 27 August 2011 (UTC)

What about the Double-slit experiment?-- Obsidi♠n Soul 22:44, 27 August 2011 (UTC)

That's completely different; that's wave interference. It has nothing to do with photons scattering off one another. --Trovatore (talk) 22:48, 27 August 2011 (UTC)

But wouldn't it mean that there is a possibility that photons are pure waves and thus collision would behave differently than if they were particles? I mean, it's unknown of course and I only have a very basic understanding of physics, but... -- Obsidi♠n Soul 23:04, 27 August 2011 (UTC)

Just because they vibrate does not make them waves. Electrons vibrate and thus can appear like waves, but are they waves? No! Whoop whoop pull up ^{Bitching Betty | Averted crashes} 23:11, 27 August 2011 (UTC)

Note that the smallest thing anyone has ever actually seen is an atom. We can only posit whether anything smaller than that is a wave or a particle by their behavior. And as discovered from the aforementioned experiment, photons (and electrons and apparently all matter) behave both as a particle and a wave, either like a single bullet or like ripples in a pond (not sinusoidal wiggling vibrations like I think you just described electrons as). Though not being a physicist, I don't know if that's accurate as well. That has been a point of debate for the some time and the current scientific consensus is that they can be one or the other, some physicists call them wavicles, some like I mentioned earlier stick by a wave-only hypothesis for everything (it's actually the older theory, until Einstein demonstrated that light behaved as individual particles as well).

Anyway *shrugs* :S I'm actually also asking a question, heh, not giving an answer (as I've confessed I'm completely ignorant in this field). If they can behave both as a particle and a wave, when photons do collide, would it be as particles or as waves?-- Obsidi♠n Soul 23:59, 27 August 2011 (UTC)

Unfortunately normal language and intuition does not cover the situation well. QM is just weird at a very fundamental level, and you can't get around the basic weirdness by choice of words.

QM and relativity showed up at about the same time, and both broke people's intuitions. Even today, students may study them at about the same time, and experience the intuition-breaking at around the same time. So it takes a while to realize that relativity's weirdness is something you can come to terms with just by adjusting some of your expectations, but QM is weird at a much deeper level. QM challenges metaphysical realism itself. There are interpretations of it that save some form of realism, but always at a cost.

A lot of treatments of how surprising QM is focus on indeterminism, which I think is a mistake. Indeterminism is not counterintuitive. Most people believe in a form of free will that requires indeterminism. If you break realism, though, that's much harder to come to terms with. --Trovatore (talk) 23:28, 27 August 2011 (UTC)

If you have light reflection off light, that is nonlinear behaviour. It may be possible for two high energy gamma ray photons to collide and make an electron positron pair. Also if you shine light through a non linear optic material you can get many interesting interactions. Graeme Bartlett (talk) 23:38, 27 August 2011 (UTC)

A quick search pointed me to [22], which makes all kinds of fascinating statements I don't comprehend. It begins by saying "in high-energy reactions the photon exhibits a complex hadronic structure..." and goes on in great detail - quarks, gluons, partons ... my guess is that if you have enough energy in a photon it spawns various particles with rest mass that can interact with one another, thus allowing two photons to interact - but whether my source confirms or denies that, I can't really say... Wnt (talk) 03:31, 28 August 2011 (UTC)

That means that inside a photon you will find a complex structure of virtual particles that includes all sorts of hadrons. The fact that those particles usually have rest mass doesn't keep them from showing up inside that photon internal structure because they are all virtual particles which means they are allowed to be off shell. Dauto (talk) 14:25, 28 August 2011 (UTC)

So if two photons are too low energy to create pairs of quarks, gluons, etc., do they still scatter from one another? Wnt (talk) 22:33, 28 August 2011 (UTC)

Photons are never too low on energy to create virtual pairs of quarks, gluons, electrons, and whatnot because all those particles are virtual and can be off shell. They are even allowed to have negative energy. Dauto (talk) 15:20, 29 August 2011 (UTC)

Yes, the scattering cross section for photon-photon scattering in the low energy limit is given by

$\sigma ={\frac {973}{22275\pi }}\left({\frac {E}{m}}\right)^{6}{\frac {\alpha ^{4}}{m^{2}}}$

where E is the photon energy (we have two beams of photons of energy E meeting head on) and m is the electron mass. Count Iblis (talk) 23:45, 28 August 2011 (UTC)

Hey, count Iblis, do you have a reference for that equation? I would like to read up alittle on scattering of light by light. Dauto (talk) 15:23, 29 August 2011 (UTC)

It's derived in the book "Quantum Field Theory", by Itzykson & Zuber, in Chapter 7. They don't derive the complete expression, just the low energy result directly, using the effective Euler-Heisenberg Lagrangian. Count Iblis (talk) 14:47, 30 August 2011 (UTC)

Boinnng! Whoop whoop pull up ^{Bitching Betty | Averted crashes} 00:44, 29 August 2011 (UTC)

Multiplying the necessary powers of $\hbar$ and $c$ to Count Iblis' formula, that would be

$\sigma ={\frac {973}{22275\pi }}\left({\frac {E}{m}}\right)^{6}{\frac {\alpha ^{4}}{m^{2}}}{\frac {\hbar ^{2}}{c^{14}}}$

or numerically in SI units:

$\sigma =1.199548\cdot 10^{-105}{\frac {1}{\lambda ^{6}}}$

It would be interesting to see the average time between 2 collisions, given 2 beams each with power P and beam cross section A:

$t={\frac {hc}{P\lambda }}{\sqrt {\frac {A}{\sigma }}}$

numerically in SI units:

$t=5.735455\cdot 10^{27}{\sqrt {A}}{\frac {\lambda ^{2}}{P}}$

For violet light (400 nm) with 1 mm² beam cross section and 1 W power it would be roughly one collision every 29 thousand years. Icek (talk) 09:11, 29 August 2011 (UTC)

I have to correct myself, because I made the unreasonable assumption that photons essentially have an infinite coherence length. With finite coherence length l (or a length along which the photons can potentially collide), the formula should be:

$t={\frac {h^{2}c^{3}}{lP^{2}\lambda ^{2}}}{\frac {A}{\sigma }}$

In the example I mentioned above, with 1 meter interaction length, it's about only 1 collision in $8\cdot 10^{24}$ years!

Icek (talk) 18:38, 29 August 2011 (UTC)

According to wave-particle duality, electrons too can be seen as waves. ~AH1 ^(discuss!) 22:00, 31 August 2011 (UTC)