Wikipedia:Reference desk/Archives/Science/2011 January 22

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January 22

Interstellar travel using asteroid rubble for shielding

One of the steepest obstacles to interstellar travel, whether by generation ship or vitrification freezer based sleeper ship colonization, is exposure to baryon and electromagnetic radiation for which shielding is more economical to take from the rubble of a spherical (loosely gravitationally bound) asteroid, because it is very expensive to launch fabricated shielding from the Earth's surface to space. Therefore, early successful interstellar craft will likely look like spherical asteroids themselves, with habitat modules burrowed under their rough surfaces. Comet fluid propellant tanks, for example mylar balloon based and/or rigid tanks (possibly formed from asteroid rubble) would also need to be shielded from baryons. Why do all the designs in the Interstellar travel article assume that asteroid-based shielding would not be necessary, sufficient, and the most economical shielding solution? 71.198.176.22 (talk) 00:17, 22 January 2011 (UTC)

Your question seems to be based on certain assumptions: (i) it is focused on something hypothetical = interstellar human travel; (ii) if/when interstellar human travel becomes a reality, we will know nothing new about economical radiation shielding; and (iii) the authors of existing WP articles considered and dismissed your proposed solution. If you have evidence to the contrary, please be more clear. I'm not convinced of any of these, but if you'd like to try improving the Interstellar travel article based on reliable sources, please do so. Talk:Interstellar travel would be a great place to discuss that article's content and propose significant changes. -- Scray (talk) 01:45, 22 January 2011 (UTC)

I haven't checked the article but isn't the normal assumption that most of the construction will in fact be done in space with the materials possibly coming from the moon or yes even an asteroid so the cost of launching fabricated shielding from earth may not be relevant anyway? Nil Einne (talk) 04:01, 22 January 2011 (UTC)

Probably, but in practice the ISS wasn't constructed in space so much as assembled from modules constructed on Earth. I don't see any reason to believe that will not continue to be the most economical solution. Fabricating fancy smooth hulls in space from asteroid fragments would be orders of magnitude more expensive than placing habitat and propellant tank modules inside such fragments and fastening them together. 71.198.176.22 (talk) 08:43, 22 January 2011 (UTC)

Well the ISS is orders of magnitudes away from capable of interstellar travel let alone being a generation ship. And we don't have any sort of base on the moon. (From what I've seen the general assumption is we won't still be stuck on earth by the time we begin to seriously consider a generation ship.) So perhaps not the best comparisons... You should perhaps consider where/how you are going to get these asteroid fragments of the right mass for your purposes, what difficulties using something which isn't optimised for the purpose is going to create (e.g. in extra fuel and engine capabilities needed) since this seems to be something you've ignored. In fact you seem to be mainly pointing out albeit indirectly the obvious, that we are still decades, probably centuries away from this being capable of construction such a ship whatever your starting point so this is all very highly speculative and going by our current capabilities a bit flawed. Nil Einne (talk) 12:32, 22 January 2011 (UTC)

Fabricating fancy smooth hulls in space . . . ." Why would you bother to do that with a vessel that will only operate in a vacuum and doesn't therefore require any kind of streamlining? 87.81.230.195 (talk) 13:48, 22 January 2011 (UTC)

The Wikipedia article is, like any other Wikipedia article, based on what is written in reliable sources. There are lots of reliable sources that describe ideas for interstellar vessels that don't involve lots of shielding. There are also some that describe ideas that do - the idea of hollowing out an asteroid and adding an engine is well known (although it doesn't seem to be mentioned in our article - probably an oversight).

However, shielding is only a big issue for fast interstellar vessels (at relativistic velocities, interstellar dust is a big issue). For slower vessels (sleeper ships and generation ships), shielding shouldn't be a big issue. Radiation in interstellar space is much less than near stars, so even over long timescales you don't need to worry too much. A couple of inches of titanium should be sufficient hull. A generation ship would, be neccessity, be very large so you would get a lot of shielding just from the air inside (assuming people don't spend too much time in the outer sections - make sure the living quarters are near the middle). --Tango (talk) 13:44, 22 January 2011 (UTC)

Are you sure interstellar cosmic ray particles and the radiation they produce upon collision with a craft's hull are not harmful over 1000-year time frames without Earth's protective atmosphere? Even at nonrelativistic velocities, dust is likely to have a very large relative velocity so erosion over centuries is neccessary to address. 171.67.128.116 (talk) 23:13, 22 January 2011 (UTC)

... and by the time that these vessels are carrying humans, we will probably have developed "USS Enterprise" style of "shields" that deflect all harmful particles, dust, and even larger debris away from the inner hull. Dbfirs 14:16, 22 January 2011 (UTC)

It's possible. Our current understanding of science doesn't include anything that could be used to make a force field, though. We can deflect charged particles, that's easy, but neutral ones are a problem. --Tango (talk) 15:17, 22 January 2011 (UTC)

Yes, good point! I've watched so many episodes of Star Trek that I've started to believe its pseudo-science! Dbfirs 21:10, 22 January 2011 (UTC)

Except interstellar dust is often (usually?) polar, which also lends itself well to deflection. So really, neutral nonpolar materials are your biggest concern: mostly monatomic and diatomic gases. SamuelRiv (talk) 18:07, 24 January 2011 (UTC)

Do you have a source for that? I don't see why dust would likely have a charge at all. Cosmic ray protons are positively charged, but what kind of power would it take to charge a hull enough to meaningfully deflect or mitigate them at the often relativistic velocities they travel? 71.198.176.22 (talk) 18:54, 25 January 2011 (UTC)

Death by Alzheimer's?

According to Alzheimer's disease:

"AD is a terminal illness with the cause of death typically being an external factor such as infection of pressure ulcers or pneumonia, not the disease itself."

Does this mean that the disease itself kills people occasionally; and if so, how does this happen? I don't have access to the source for this statement. Nyttend (talk) 01:22, 22 January 2011 (UTC)

The last paragraph of the source states:

After the clinical diagnosis of AD, life expectancy is reduced by one third (Heymen et al. 1987). A long persistence of symptoms, the severity of illness, old age, male sex, and physical disease are major risk factors for mortality in AD (Bowen et al. 1996; Burns et al. 1991; Kurz and Greschniok 1994). Pneumonia followed by myocardial infarction and septicaemia are the most frequent causes of death in AD (Förstl and Hewer 1993).

Smallman12q (talk) 01:58, 22 January 2011 (UTC)

Having sadly watched a few people die with Alzheimer's, I'd suggest that some such people just lose interest in all the things to do with healthy living, including eating at all. OK, they can be semi-force fed in an institution, but I suspect that sometimes they are just allowed to pass on. HiLo48 (talk) 01:59, 22 January 2011 (UTC)

Hazelnuts

Why do hazelnuts taste so much better than other kinds of nuts? Do they have some special chemical that other nuts don't have? If so, what is it? --75.15.161.185 (talk) 01:23, 22 January 2011 (UTC)

Apparently there are a number of relevant compounds, and the most important ones depend on heat to be produced (raw hazelnuts are far less flavorful than roasted ones). This paper gives some information, if you can make any use of it. Looie496 (talk) 02:00, 22 January 2011 (UTC)

All nuts contain somewhat different combinations of "chemicals", and all people have somewhat different tastes. I think brazils taste best.--Shantavira|^{feed me} 07:31, 22 January 2011 (UTC)

No, sorry, it's pistachios that taste the best. Richard Avery (talk) 09:03, 22 January 2011 (UTC)

Macadamias HiLo48 (talk) 11:49, 22 January 2011 (UTC)

You are all mistaken. Pecans taste best, followed by walnuts and then almonds. — Preceding unsigned comment added by Roberto75780 (talk • contribs) 18:28, 22 January 2011 (UTC)

Cashews are best. Scary the way they have to get them for you, though. They get one tiny nut out of a whole false-fruit-true-fruit complex, the only part of which not filled with poison oak poison being the nut itself. Stopped complaining about the cost when I found that out. --Trovatore (talk) 21:17, 22 January 2011 (UTC)

After reviewing the article, I guess it's not quite that bad. One part, the "cashew apple", is apparently edible. Still, it sounds like nasty work for someone getting the nuts out. --Trovatore (talk) 21:46, 22 January 2011 (UTC)

See nutella. ~AH1^(TCU) 01:17, 27 January 2011 (UTC)

gravity waves

i am A.mohammadzade and have some difficulties in understanding gravity waves . and based on my studies i know there is some paradoxes in formulation of that .i reach some of this paradoxes and will list them here . jan 22 2011 --78.38.28.3 (talk) 06:55, 22 January 2011 (UTC) —Preceding unsigned comment added by 78.38.28.3 (talk) 06:48, 22 January 2011 (UTC)

Does our article on gravity waves help you? Do you have a specific question?--Shantavira|^{feed me} 07:33, 22 January 2011 (UTC)

From the clarification (sort of) below, the intended topic appears to actually be gravitational waves, which are a quite different topic from gravity waves, despite the similar names. Red Act (talk) 05:47, 24 January 2011 (UTC)

my questions are about the diagravity and paragravity matters . if there is gravity wave in universe then we ought to have diagravity paragravity matters and so reducing the speed of such wave .and after laying three matters in straight line what will happen . and when we got the central mass such as sun what will happen for waves ... {{ i do apologies these are new words and i have aim of this words diagravity paragravity}}--78.38.28.3 (talk) 08:54, 22 January 2011 (UTC) a. mohammadzade iran 2011

Question reformatted. Richard Avery (talk) 09:05, 22 January 2011 (UTC)

Diagravity and paragravity seem to be terms used by those who study magnetic levitation, or even by those who think that gravity is a form of magnetic field.

I recall attending a lecture by one such "crank" many years ago. He had his arguments well-marshalled, and it was difficult to find chinks in his armour of conviction.

Mr Mohammadzade, please don't take this as a comment on the respectability of your studies (of which I know nothing, and would be unqualified to offer criticism), but may I ask: do you have any reason to believe that the phenomena of diagravity and paragravity ought to exist, or any evidence that they might exist? They are not part of the standard theories of gravity. Dbfirs 10:11, 22 January 2011 (UTC)

all waves product of force and energy and passing through the invirnments will effect the speed of wave . so some fields and invirnments reflect the wave or reduce its speed may be resonate in some so we ought to find such invirnments to gravitation wave . and i dont belive this two phenomena . thank you jan 24 2011 --78.38.28.3 (talk) 05:22, 24 January 2011 (UTC)Link title

Look, A.mohammadzade, there's no point in letting you down gently any more. This is a reference desk. Its purpose is to help people find answers to their questions using Wikipedia articles, or perhaps other methods. This is not the proper venue to push your own pet theories about how the universe works. Its just not appropriate. Please stop now. Look, for your sake I hope you are right. Perhaps you will have tripped upon some fundemental fact about the universe which no one before you discovered, and someday you'll be as famous as Albert Einstein because of that. However, until that day comes, no one here is going to accept your pet theories at face value. It's already well established that I don't know shit about Physics. But I do know that no one that does around here thinks your theories actually hold any water. Please just stop pushing them. --Jayron32 05:44, 24 January 2011 (UTC)

It looks like "paragravity" and "diagravity" are part fringe science unification of gravity and magnetism, such as in this thing, and part science fiction device, such as in this. Getting back to actual science, there are some approaches toward relating gravity and magnetism at a macroscopic scale that aren't total nonsense; see gravitomagnetism, which pertains to frame dragging, and Kaluza–Klein theory, which unifies the Einstein field equations with Maxwell's equations. Red Act (talk) 16:49, 25 January 2011 (UTC)

Comparing gravity "waves" to the waves of light brings up refraction. Gravitational refraction would be also known as gravitational microlensing. Your question about taking three of gravity and putting into a straight line sounds slightly similar to hypotheses about interstellar travel via folding of spacetime or hyperspace or through (unproven) strings. ~AH1^(TCU) 01:16, 27 January 2011 (UTC)

gravity did not form the universe

I wrote some were : 'it is not correct that we say any thing about all over the world when we have many shortages in calculating and formulation of the data and observations '. what is my meaning ? I want to show that basic subjects of astrophysics shows that gravity is not only r.....a. mohammad zade ( i will write the continue)--78.38.28.3 (talk) 08:07, 22 January 2011 (UTC) --78.38.28.3 (talk) 08:04, 22 January 2011 (UTC)

Question display reformatted. Richard Avery (talk) 08:55, 22 January 2011 (UTC)

How are we supposed to know what you mean? You wrote it... --Tango (talk) 13:52, 22 January 2011 (UTC)

Wait for the continue. --Sean 14:43, 24 January 2011 (UTC)

Different brand of growth hormone

What is the difference between the different brands? Is there a reason why usually old people with a deficiency use a specific brand, bodybuilders or HIV victims (Serostim I believe) use another brand and short children use a different brand? I thought they were all the same product. —Preceding unsigned comment added by 76.169.33.234 (talk) 10:05, 22 January 2011 (UTC)

We have articles on growth hormone and growth hormone treatment. You did not specifically give us the names of the brands that you associate with various users, but I suspect that the answer to your question is that there are versions of the hormone that are approved by the FDA (in the USA at least) specifically for "medical" uses, while others may be manufactured and used outside of the established medical system (or simply approved for use in a different country and thus not generally available in all parts of the world). Another possibility is that manufacturers may use slightly different methods for producing the synthetic hormone and/or different delivery methods. --- Medical geneticist (talk) 14:40, 22 January 2011 (UTC)

I've seen the articles, they do not answer my question, I wouldn't ask if they did :-) Yes, I am talking about FDA approved growth hormones. It seems like Genotropin is used mostly in children (idiopathic short stature and other issues), Serostim is used by HIV victims and bodybuilders, and Norditropin for older people who have low levels. I've read that for example Norditropin is also approved for idiopathic short stature and Genotropin is approved for deficiency in adults, but why is it that each group almost always uses one specific brand? —Preceding unsigned comment added by 76.173.30.220 (talk) 02:31, 23 January 2011 (UTC)

I'm guessing that they're not just different brands, but maybe different doses, concentrations, carriers, and other quirks that make them work differently. As an anology, wine and whiskey both have ethanol as an "active ingredient" but they're also distinct. One other matter with approved products that the manufacturer has to do studies to prove the stuff actually works for the intended purpose. Doctors are free to prescribe the "wrong" drug (so-called "off label" prescriptions), so the hurdle that some companies aim for is just to get one approved use so that they can legally market the drug, they just can't advertise it for the "wrong" use. Having every drug approved for every use would require a lot of costly clinical studies, so most don't bother except for drugs where recouping the investment is likely. SDY (talk) 04:12, 23 January 2011 (UTC)

How to identify used-up batteries with a multimeter?

I have eight identical AA 1.5v alkaline batteries. Four of them I've just removed from a radio after they have been used up. Unfortunately I've mixed these up with four other batteries, two or three of which have been used to varying extents.

How can I best identify the batteries which have the most energy left in them? I would like to identify the most energetic four batteries to put in my digital radio. Altghough I have a multimeter, I've never used it and I only have a superficial understanding of the dials and markings etc. Thanks 92.29.126.0 (talk) 11:38, 22 January 2011 (UTC)

what you want to do is set the multimeter to measure DC voltage, usually it is the V with straight lines, not wavy lines. Then just put the probes on the battery terminals. It doesn't really matter which way, they'll give you the same reading, just ignore whether it is +ive or -ive. Then just pick the ones with the highest numbers... From memory, a "full" battery will give a reading of about 1.8v a flat one will be about 1.3 or less. Vespine (talk) 13:07, 22 January 2011 (UTC)

You're thinking of a digital meter. Often analog meters can't display negative values, so if you hook it up and nothing happens, or if the needle pegs to the left, then try doing it the other way around. APL (talk) 01:51, 23 January 2011 (UTC)

It is hard to help you with out knowing what kind of multimeter it is. The best option is if the multimeter has some battery testing range that tests the battery under load. In other case you can measure the no load voltage, this should give you some indication but is not reliable when comparing batteries of different manufactures or different models, all batteries need to have roughly the same temperature. You need to find a setting for measuring voltage up to at least 2 V, connect the testing cables correctly to the multimeter for that setting and connect the testing cables to the ends of the battery with the correct polarity. The higher the voltage the better battery, you should get something like 1.4 V to 1.6 V. Small differences in voltage can indicate large differences in remaining energy, 1.55 V can be fully charged while 1.50 V can be almost empty, it depends on the exact type of battery. An more reliable indication could be the short circuit current but measuring that could degrade the battery.--Gr8xoz (talk) 13:07, 22 January 2011 (UTC)

Ideally one should use a load in the form of a resistor but if you have never used a MM before, this should be good enough. Check that Black probe is plugged into MM socket COM. Red probe into VωmA socket. Turn the center control to point to 20 in the V direct voltage section (usually left-hand side top). There should be a straight line to the right of the V symbol with dots underneath it. If it's a V with a wavy line next to it, it is the wrong sector. Touch the red probe on the battery + and black probe on other end. If it reads about 1.5 or more, then there is life in the battery. If it is 1.3 or below it is probably very nearly dead. I would throw below this, as your getting to the point where it will start leaking,--Aspro (talk) 13:19, 22 January 2011 (UTC)

The multimeter I've got is this one, the DMB30B: http://www.e-sun.cn/EN/d.aspx?pht=835 92.24.191.47 (talk) 14:21, 22 January 2011 (UTC)

Turn the centre control (from position in photo) two more places anti-clockwise to position 20. That means it will indicate up to 20 volts DC. Bottom right-hand socket (COM) for plugging in black probe. Red Probe plug in next socket up (VωmA). Then: Follow instructions as per my first reply. Touch the red probe on the battery + and black probe on other end. If it reads about 1.5 or more, then there is life in the battery. If it is 1.3 or below it is probably very nearly dead. I would throw below this, as your getting to the point where it will start leaking, Afterwards return control to straight up (OFF). Adopting this habit will extend the internal battery life. I guessed that 19 times out of twenty, if anyone has a MM and doesn't know how to use it, then the chances are, it will have the ubiquitous layout of inexpensive MM's, which are common through out the world – as was, in this case.--Aspro (talk) 14:52, 22 January 2011 (UTC)

To sort out used up batteries from good batteries, test the voltage under a normal load, by placing across the battery a resistor which draws some current. The "normal" load current will be greater for, say a D cell (perhaps 300 ma) than for an AA cell (lower current). You might use 100 ma for an AA cell. R=E/I, or 1.5/.100 or 15 ohms for an AA cell. Mileage may vary: some applications will draw less current, some will draw more. For a tired cell, the voltage will drop swiftly, while for a newer cell, the voltage will remain high longer. Edison (talk) 05:46, 23 January 2011 (UTC)

I always use the 10A setting and check what is the maximum amps the battery is producing. I connect it only for one second, because this shorting of the battery is damaging the battery. My AAAs give 3A when they are fresh and 1A or less if they are old.--Stone (talk) 18:56, 23 January 2011 (UTC)

No offense but I think you guys are really over complicating the issue. The OP clearly stated he has 8 identical batteries. In this case you really don't need to worry about putting them under load or shorting them by using the current meter setting, which I think is a particularly bad idea, especially if you go on and try it on something like a lithium polymer battery. If you just measure the voltage and compare the values like my 1st reply suggested, that will be more then adequate to pick out the batteries that have the most charge left. Vespine (talk) 22:07, 23 January 2011 (UTC)

Volume of sound at cold temperatures

When the outside temperature drops below -40 °C/°F the sound of the local Learjet 25 seems much louder. I live about 2.5 kilometres (1.6 mi) north of the Cambridge Bay Airport and at current temperatures it sounds as if the plane is flying right over the house and may even cause dishes to rattle slightly (I should have checked on the pressure at departure to see if that was affecting the rattle). This does not seem to happen when the temperature is higher but still below 0 °C (32 °F). I did do some searching on the Internet and came across this but wasn't sure if it was correct. The bit about the density I mean. I know the Speed of sound decreases as the temperature decreases. So is the sound louder or just higher pitched and seems louder at cold temperatures? CambridgeBayWeather (talk) 12:12, 22 January 2011 (UTC)

It is a very common phenomena caused by temperature inversion, which is also responsible for sometimes making things visible even thought they are over the horizon. See:Jets in the Living Room? Blame Inversions--Aspro (talk) 12:41, 22 January 2011 (UTC)

I would assume that it is things under the horizon that becomes visible, things over the horizon can normally be seen with out an inversion.--Gr8xoz (talk) 12:47, 22 January 2011 (UTC)

Before or above the horizon but not over. As in Over-the-horizon radar.--Aspro (talk) 13:24, 22 January 2011 (UTC)

Very quick answer. Thanks. In case you were interested this is how the street in front of my house looks around this time of year. CambridgeBayWeather (talk) 13:23, 22 January 2011 (UTC)

'Tis never too late to move and follow the Sun south! :-)--Aspro (talk) 13:29, 22 January 2011 (UTC)

An inversion layer acts as a form of waveguide. Mikenorton (talk) 15:50, 22 January 2011 (UTC)

Don't forget that we have an article on atmospheric inversion. ~AH1^(TCU) 01:03, 27 January 2011 (UTC)

E=mc²

Using this Rule .. Can we conclude that Energy Have mass or weight ?? It's proved that Energy = mass multiply the speed of light square does this mean that energy contain mass ? — Preceding unsigned comment added by Meshmesh micho (talk • contribs) 13:55, 22 January 2011 (UTC)

See Mass–energy equivalence, which answers this very clearly in the opening paragraphs: "The equation E = mc² indicates that energy always exhibits mass in whatever form the energy takes". --Mr.98 (talk) 14:25, 22 January 2011 (UTC)

Sigh. We can argue endlessly about this. What the equation really says is that a particle of mass m has energy E = mc² when it is at rest. The reverse statement can be made for composite systems where internal motions give the system an effective mass (when the system as a whole is at rest) but it is useless and misleading for particles without internal structure because it seems to imply that mass and energy are the same thing, which they are not. Mass is an intrinsic propery of a particle, energy describes the state of a particle. --Wrongfilter (talk) 15:06, 22 January 2011 (UTC)

My intent is not to argue endlessly about this (or anything) — just to point the OP in the direction of the article which they should really read first before coming back here for further discussion (endless or otherwise). This question is pretty directly discussed in the article. --Mr.98 (talk) 15:43, 22 January 2011 (UTC)

(EC) But rest energy is that same property? So mass is the same as energy surely? Just that it can have different origins. The statment is surely most explicit in natural units, where c=1 with no dimensions. So the relation becomes E=m. —Preceding unsigned comment added by 129.67.37.227 (talk) 17:21, 22 January 2011 (UTC)

As has been pointed out on this desk numerous times this week, the use of the term "mass" to refer to rest-mass, or effective gravitational mass, varies between physicists. If you want to debate whether "mass" and "energy" are the same, you're waging a language-battle better suited to the language desk. A more useful question is, "does an increase in energy affect the gravitational force?" The answer is an easy "yes." Let's not devolve into semantic quibbling. We should steer interested people to investigate the physical realities and phenomena, without fighting about the various different names that other scientists use for the exact same phenomena. Nimur (talk) 19:37, 22 January 2011 (UTC)

Nonsense. That equation really means that (relativistic) mass and energy are the same thing. It also means that (rest) mass and rest energy are the same thing. rest mass is an intrinsic property of the particle while the (relativistic) mass describes the state of the particle. You could ask "Isn't that redundant?". Yes it is redundant. That's why, by convention, the word mass is used to describe rest mass (and/or rest energy) while the word energy is used to describe relativistic mass (and/or energy). Dauto (talk) 17:18, 22 January 2011 (UTC)

Precisely, thus photons feel gravity. —Preceding unsigned comment added by 129.67.37.227 (talk) 17:23, 22 January 2011 (UTC)

No, photons take the path they do near massive objects because photons travel in straight lines, and spacetime in the vicinity of a massive object is curved. Viewing photons as "feeling" (i.e., being accelerated by) a gravitational "force" is a Newtonian picture of gravity, which works quite poorly when dealing with light. If you try to use E=mc² and Newtonian gravity to calculate how much deflection of light there should be as it passes near a massive object, you wind up with an answer that's only half of the actual deflection. See Tests of general relativity#Deflection of light by the Sun. Red Act (talk) 21:11, 22 January 2011 (UTC)

Spacetime is just a mathematical object designed to explain Einstein's theory of gravitation, it would be awkward to suggest that it actually exists (In a similar way to a force in Newtonian mechanics). Gravitation, one would, I suppose, describe as the dynamical interactions of massive bodies. Since photons are indeed affected by the presence of massive bodies, one would be inclined to say that they are thus affected by gravity. Anyone arguing the converse, would rather than discussing anything of physical significance, be beleaguering what is in fact no more than a discrepancy of definition. —Preceding unsigned comment added by 129.67.37.227 (talk) 14:03, 23 January 2011 (UTC)

I presume you're meaning that the notion of curved spacetime was designed to explain general relativity. Spacetime in the broader sense of the unification of space and time of course existed as a concept for years before general relativity ever came into being. It's just that with general relativity, spacetime went from being modeled as a Minkowski space to being modeled as a four-dimensional Lorentzian manifold.

I don't see why it's awkward to suggest that spacetime actually exists. There appears to be something that exists even in places and times where there aren't any particles anywhere nearby (not counting the fleeting particles in the vacuum state), so it's not really adding anything questionable to simply label the large-scale structure of that somethingness as being "spacetime". The mathematical description of that somethingness, such as the metric tensor that's ascribed to each part of it, is of course something that doesn't physically exist; it's just a mathematical model of something that does physically exist. And spacetime is a description of large-scale physical structure, so sure, it might break down at Planck scale, but arguing that that means that spacetime doesn't actually exist would be a lot like arguing that physical spheres don't actually exist, because if you look too closely at for example a billiard ball, it actually has a horribly complicated shape, that you can't even describe adequately in three dimensions.

Your statement that it's awkward to suggest that spacetime actually exists could also be taken to mean that perhaps you think that space is Euclidean, and time is an independent, unrelated dimension, but I sure hope that's not what you mean.

Yes, even from a general relativity perspective, a photon can be said to be "affected by gravity", if by "gravity" you mean the curvature of spacetime, and by "affected by" you mean that it alters what the photon's world line is. But the photon isn't "affected" in the same sense that a force would affect it. I.e., there's nothing acting on the photon locally, that produces an acceleration.

My main objection to your statement "precisely, thus photons feel gravity" is the implication that the reason why photons are affected by gravity is because they have a relativistic mass, because they have energy and E=mc². There isn't a way I can think of to look at the idea that photons are affected by gravity because they have a relativistic mass, without that idea being inaccurate.

Yes, in Newtonian gravity, a particle needs to have a nonzero mass in order for the equation for gravitational force to produce a nonzero force. But you just can't use Newtonian gravity when dealing with light, and hope to get an answer that's anywhere near accurate. For many purposes, the Newtonian approximation to gravity is "close enough", and bringing up the general relativity perspective of gravity would amount to little more than pedantic quibbling. But there are a couple situations in which the difference between Newtonian gravity and general relativity isn't just pedantic quibbling. One of those situations is when dealing with an object that's so massive that gravitational time dilation is no longer insignificant. And another situation is when dealing with a particle that's travelling so fast (like a photon), that even a tiny amount of quibbling over exactly what time things occur due to gravitational time dilation, is equivalent to a big difference spatially.

If you're forced to abandon Newtonian gravity when dealing with photons, the alternative is to use general relativity. And in general relativity, a particle's mass is irrelevant as to whether or not or how much the particle is "affected by gravity". There is no "gravitational force" that gets computed, and the particle's mass doesn't enter into the calculation in any other way (unless the "particle" is so massive that it, too, affects the curvature of spacetime). All that matters in computing the particle's world line is the particle's initial event and four-velocity. A hypothetical particle that has no relativistic mass, but travels at the speed of light, would have the same world line as a photon, given the same initial conditions. (Of course, such a particle would have multiple theoretical problems with it; I'm not suggesting that such a particle might one day actually be discovered. For one thing, how could such a particle even be detected, if it has no energy to affect anything?) Particles are "affected by gravity" simply because they're traveling in straight lines (geodesics) on a curved spacetime, not because they have relativistic mass. Red Act (talk) 00:51, 24 January 2011 (UTC)

This formula prove that energy and mass is equal. Essentially all mass is a form of energy. Plus, all "mass" have a potientially energy if it is fully converted to energy. A food for thought. --Tyw7  (☎ Contact me! • Contributions)   Changing the world one edit at a time! 18:00, 22 January 2011 (UTC)

Not only can energy have mass, most of the mass you deal with day-to-day is energy. The mass of protons and neutrons are much greater (by about eighty times) than the summed masses of the constituent quarks. It's the energy associated with bringing the quarks together that accounts for the rest. Most of the mass of protons and neutrons (and by extension of most things) is due to energy, rather than (rest) mass of fundamental particles. -- 174.21.229.4 (talk) 18:50, 22 January 2011 (UTC)

See here for another perspective Count Iblis (talk) 22:56, 22 January 2011 (UTC)

Generally, rest energy is the energy an object at rest would liberate if annihilated at 100% efficiency, typically by antimatter of the same mass without counting the energy released by antimatter. In this context, the energy-mass equivalence implies that a certain amount of energy is equal to the maximum amount of energy that could be released by a certain mass. ~AH1^(TCU) 01:01, 27 January 2011 (UTC)

Discovery of link between food and energy

I am trying to re-find the name of the man who effectively discovered the link between food consumption and its use to provide the human body with the energy we need to exist as human beings. This man may have operated in the Middle Ages because I remember seeing a photograph of an oil painting of him sitting on a primitive weighing machine in a darkened, candle-lit, almost windowless room, wearing long robes. But as this might have been seen as heretical in this era, perhaps he was active in the early days of The Enlightenment. Basically what this man did was to weigh himself at frequent intervals, to weigh everything he ate and drank and, in the interests of science (!), he weighed everything he excreted. Not sure how he factored in sweating though. After many observations he consistently found that the sum of his beginning weight plus the weight of food consumed over a period, less the weight of all excretions during that period, did not equal his actual weight at the end of the period. His weight was always less than this figure. And the difference, of course, was the fact that the body had been consuming some of this food to provide all the energy our bodies need to function. I would be curious to learn more about this man, but searches across the web and within Wikipedia reveal nothing. And yet the photo I saw, along with the above brief description of his work, was in a book about the greatest discoveries in medical history, alongside such luminaries as Jenner on inoculation, Simpson on anaesthesia and Fleming on penicillin. Can anyone throw light on this? —Preceding unsigned comment added by 86.139.150.81 (talk) 15:05, 22 January 2011 (UTC)

Antoine Lavoisier? See http://www.nature.com/nature/journal/v468/n7327_supp/full/468S16a.html. Exxolon (talk) 15:20, 22 January 2011 (UTC)

If that's not it, it is likely Jan Baptist van Helmont, see Jan_Baptist_van_Helmont#Observations_about_digestion. He also made similar studies about trees (though arrived at the wrong conclusion). He's a very important early scientist; though his predecessor Roger Bacon and his followers Robert Boyle and Antoine Lavoisier get a lot better pub, van Helmont was a prescient scientist in his dilligence and adherance to the scientific method. --Jayron32 16:52, 22 January 2011 (UTC)

That sounds a lot like Van Helmont's work, yes, although I don't know of him doing quantitative work on human digestion. Physchim62 (talk) 17:36, 22 January 2011 (UTC)

I remember seeing the painting or reading about this. The robes could have been from long ago or from the Middle East. He might have been the same possibly Turkish person who, during a plague, thought that dipping things under water would be a protection. 92.24.182.196 (talk) 20:30, 22 January 2011 (UTC)

Leonardo da Vinci per Nutrition#From antiquity to 1900. 171.67.128.116 (talk) 23:15, 22 January 2011 (UTC)

Sorry, neither of those two articles indicate it was Leonardo da Vinci. 92.24.184.8 (talk) 15:02, 23 January 2011 (UTC)

Is it Sanctorius? Here's a picture of him in his weighing chair. Karenjc 15:17, 23 January 2011 (UTC)

Yes, I think it probably is, although I'm not the OP. I seem to recall an oil painting of him - have not been able to find that, although there is a woodcut of him sitting in his large scales. 92.24.184.8 (talk) 21:01, 23 January 2011 (UTC)

It may be worth pointing out that the idea is sound but that such a measurement would be all noise and no signal. At a fundamental level, conservation of mass-energy requires that everything that goes in either stay in or come back out. If we want, we can make a semi-arbitrary distinction and say that the heat emitted by the body is energy "used up" by it rather than mass excreted, but that works out to about 2 milligrams in a lifetime. So what you would be measuring would be principally evaporated sweat and water lost in the breath (the breath I believe is something on the order of a kilogram per day). --Tardis (talk) 19:50, 23 January 2011 (UTC)

Yep, our article on fluid balance pegs those 'insensible losses' (so called because they are difficult to measure) to perspiration and humid breath at between 500 and 800 mL per day per adult. One more point — even if you account for water lost this way, you'll still be missing about 250 grams (half a pound) per day from your mass balance. Your exhaled breath contains about one kilogram of carbon dioxide; while three quarters of that mass comes from inhaled oxygen, the carbon content is from ingested food. TenOfAllTrades(talk) 20:25, 23 January 2011 (UTC)

Head-splitting volume of fire and other alarms

I can almost understand why alarms are loud in public places or large building, and why they continue their mind-numbing racket for long after surely even the dead have been advised, but why do the ones manufactured for homes also do this? The noise is so great that it is painful and it so fills my head that I cannot think while it is screaming. In a house with two people, within 2 feet of one another, this seems not just overkill (so to speak) but actively unhelpful as it blocks any ability for a reasoned response. Is there such a thing as a remote silencer that allows you to shut off the racket while you work out the safest path out of the house? (We have done the route mapping in advance, but there are many possibilities depending on where a fire starts and which access points are blocked to a wheelchair.) I don't mind the volume at first, to ensure we are awake and paying attention, but why is it necessary after that? (Our insuror says that any tampering with the alarm itself may lead to the cancellation of our insurance.) Bielle (talk) 16:49, 22 January 2011 (UTC)

That is indeed true, if you tamper with your firealarm, CO detector, etc, the insurance can be canceled. They are loud for one purpose: To make sure you know the danger, and so you can get out of the house/building. I don't know of any remotes that will silence them, Sorry. Tofutwitch11 ^(TALK) 16:51, 22 January 2011 (UTC)

They are loud as they may have to be heard through closed multiple intervening doors by people who may be heavy sleepers, intoxicated, drugged or a combination of all three. The are designed to galvanise the person out of sleep with a "fight or flight" response so they can instantly start escaping. Exxolon (talk) 17:21, 22 January 2011 (UTC)

Yes. The annoyance factor comes from the fact that the alarms almost certainly only get activated once with good reason (when your house is about to burn down) and any number of seemingly malicious false alarms that seem to prefer going off at 3 in the morning (my CO detector did that three or four times in a month and never before or since). While it's undeniably a pain the... ears, the volume overkill is almost certainly responsible for saving my house and all my possessions and perhaps even saving the lives of my family. We heard the shriek through the walls of our freehold condo and were able to react to a fire in our neighbour's kitchen. That, of course, is only small consolation when your eardrums gets pushed so far into your skull that they nearly meet in the middle, but there you go... Matt Deres (talk) 17:37, 22 January 2011 (UTC)

Actually, my alarms have been activated many times, never for a good reason (usually because my toaster was acting up). I think that's part of the difficulty — you're going to have far more false positives than you will emergency situations, and by definition the alarm can't distinguish between the two. --Mr.98 (talk) 22:47, 22 January 2011 (UTC)

Which is exactly what I said, but thanks for the support. :-) Matt Deres (talk) 23:16, 22 January 2011 (UTC)

The way I would put it is that you would set your telephone or your doorbell to a volume at which you're pretty certain to hear it but at which you can ignore it if you feel like it. You set your alarm to a volume at which you can't ignore it even if you want to! Physchim62 (talk) 17:47, 22 January 2011 (UTC)

In-laws should come with similar settings, and be set to the lower one... Physchim62 (talk) 17:49, 22 January 2011 (UTC)

In theory it should be possible to opt for more expensive insurance and quieter alarms, with a slightly greater risk of death and less annoyance. Consumer choice, and all that. In practice, I don't suppose such products are on the market. Googling "quieter fire alarm" didn't turn up anything promising. Probably it's illegal to choose not to be annoyed. 213.122.5.253 (talk) 17:53, 22 January 2011 (UTC)

Only if your house isn't anywhere near mine! APL (talk) 02:45, 24 January 2011 (UTC)

This looks like what you want - if you're not in canada, then it isn't much use, but it might lead you to others wherever you do live - I was searching for "fire alarm silencer remote". I can assure you from my OR that the volume is indeed a good thing as Exxolon has pointed out - it can take a while to wake you if you are suitably (or unsuitably) inebriated but they do work eventually! SmartSE (talk) 19:25, 22 January 2011 (UTC)

Some fire alarms have a hush button on them, I think it's more common on the mains powered ones. Mine peeps after it is hushed until the smoke goes away. If it gets a lot thicker, then it starts again - I've never tried to hush it a second time. CS Miller (talk) 20:24, 22 January 2011 (UTC)

Even at such a volume, smoke alarms have been found less than adequate at waking children leading to special products like: "square wave" [1] and the screaming parent's voice model[2] Rmhermen (talk) 21:41, 22 January 2011 (UTC)

I will just add that one of the flaws that the review committee on the Three Mile Island accident found was that the alarms were too loud, too shrill, and could not be turned off in the middle of the disaster, which made it more difficult to actually deal with the accident as it was having. My point is just that the engineering logic that goes into making an alarm is not necessarily the same logic that actually tells you about how the alarm will work in the real world. --Mr.98 (talk) 22:47, 22 January 2011 (UTC)

I have to agree with the logic here. But there is something very wrong with how noise issues are handled in the U.S., and I honestly think that the manufacturer's/owners/insurer's motivation is more to declare that they are better than you (and your neighbors) than anything else. Routinely noise complaints (from any source) are not merely ignored, but derided and actively resented. The U.S. federal government pretends to have no power, leaving everything to local noise ordinances, even for vehicles used in interstate commerce - yet the moment that some obscure groups complained that hybrid cars were producing too little noise, there was no obstacle whatever to plans to make them produce more noise. Either it is a monkey instinct for authorities to beat their breasts from the highest tree, or else an orc's heartfelt desire to destroy every peaceful natural symphony in the cosmos, from the deepest sea to the furthest island. Wnt (talk) 05:11, 23 January 2011 (UTC)

The most likely time for an unnoticed fire is at night when everyone is asleep. When the OP has a fire when they are sound asleep behind closed door(s), then they will be grateful for the loudness. 92.24.184.8 (talk) 15:05, 23 January 2011 (UTC)

The OP could also buy incredibly loud and slightly inaccurate alarms that detect knives, strangers, wild animals, poisons and nuclear isotopes. These may be intensely annoying on a typical day when they go off by mistake, but should any one of these alarms actually ever do its job, the OP will be grateful for the relentless aggravation. 213.122.56.117 (talk) 00:19, 26 January 2011 (UTC)

Depending mostly on your building, some alarms can be privately installed onto wall fixtures that use batteries. However, there is also the temptation to take out the battery, which can be hazardous in the event of a real fire. Some home systems such as ADT have alarms detecting fire and multiple emergencies such as burglery and which contact a local response centre. ~AH1^(TCU) 00:55, 27 January 2011 (UTC)

Weight

In the book it shows that weight can be calculated by the formula w=m1m2/d^2. How in the world do you drive this formula? As long as I know, the formula to find out the weight is the same as the force of the grivational attraction between two objects (formula Gm1m2/d^2). How can you calculate the weight (first formula) without the gravitational constant G? --Tyw7  (☎ Contact me! • Contributions)   Changing the world one edit at a time! 17:58, 22 January 2011 (UTC)

you do need to know the gravitational constant to calculate the weight. You can obtain the gravitational constant from cavendish experiment 83.134.145.153 (talk) 18:00, 22 January 2011 (UTC)

How did the text book derive the first equation? --Tyw7  (☎ Contact me! • Contributions)   Changing the world one edit at a time! 18:05, 22 January 2011 (UTC)

That's clearly a typo. Dauto (talk) 18:29, 22 January 2011 (UTC)

It somewhat depends on the context. The first formula is valid if everything is in a natural units system where the gravitational constant has been set to one. Technically speaking, the first equation is valid for all unit systems, it's just that you need the factor G to convert the somewhat useless (mass)^2/(distance)^2 units to more conventional (force) units. -- 174.21.229.4 (talk) 18:37, 22 January 2011 (UTC)

It may have been done with dimensional analysis. 92.24.182.196 (talk) 20:35, 22 January 2011 (UTC)

Are stromatolites our direct ancestors?

This was mentioned in an NGC documentary, but the stromatolites article doesn't mention this. Count Iblis (talk) 23:01, 22 January 2011 (UTC)

It would be more accurate to say that stromatolites and humans share a common ancestor. HiLo48 (talk) 23:10, 22 January 2011 (UTC)

Just to a bit more explicit, anything that's alive right now can't really be our ancestor, only a very distant cousin (parents and grand-parents excepted, of course). It would be like me walking up to some French boy in a Paris schoolyard and calling him my father since I (partially) come from French extraction - as related as we might be, "ancestor" isn't an appropriate word. The stromatolites alive today are the result of millions of previous generations, just as we are. Matt Deres (talk) 23:29, 22 January 2011 (UTC)

Stomatolites aren't really lifeforms, they are made by micro-organisms. It is those micro-organisms that are the lifeforms. There are various different micro-organisms that can create them (generally, the fossil record only preserves the stromatolite, not the bacteria that created it, so it's hard to tell precisely what made them), but I not sure whether any of them are in our line of descendants. If they are, you have to go back at least a billion years. --Tango (talk) 00:01, 23 January 2011 (UTC)

(ec) While that is all undoubtedly correct, it is kind of dodging the question: are ancient stromatolites in direct human ancestry? That is, did a stromatolite forming cyanobacteria give rise to the Eucarya (which is the line to which humans belong). According to Tudge's mammoth work on classification the ancestral Eucarya were formed from a co-operation of at least three different lines. The host cell was not (according to Tudge) a cyanobacter, but the cell plastids may have well have been. However, Tudge suggests that this cyanobacter was something like Synechococcus and gives no suggestion that it was stromatlolite forming. SpinningSpark 00:20, 23 January 2011 (UTC)

The notion of ancestry is actually pretty fuzzy for bacteria, because they are capable of swapping genes, even across species that differ pretty widely in structure. Thus, it is likely that many, if not all, of the bacterial types that existed billions of years ago are to some degree our ancestors. But there is really at present no way of pinning this down. Looie496 (talk) 00:26, 23 January 2011 (UTC)

I see, so this question already starts to lose its meaning here well after life first arose. Count Iblis (talk) 16:32, 23 January 2011 (UTC)

Not to mention some viruses that have added codes to our human genome over time. ~AH1^(TCU) 00:48, 27 January 2011 (UTC)