Wikipedia:Reference desk/Archives/Science/2013 January 30

= January 30 =

Existance of solid state variable inductors?
Is there such thing as a non-coil, variable inductor? If so, what sort of frequency ranges are possible with a given unit? Also, can one be constructed from any of the other common electronic components (or some combination thereof) by exploiting parasitic capacitance? 75.196.240.108 (talk) 02:41, 30 January 2013 (UTC)


 * Yes. Inductance can be synthesised by incorporating capacitance in a feedback network.  By the circuit technique known as gyration, very high Q inductance can be created at audio and low RF frequencies.  The use of shunt negative feedback around and amplifier lowers output impedance.  If capacitance is used to reduce the loop gain as fequency increases, the output impedance must then rise with frequency - this is the property of inductance.  In both gyrators and negative feedback amplifiers, the loop gain can be adjusted by means of a potentiometer, or by a control voltage.  Hence the resulting inductance is varied.  Using a gyrator, it is thus readily possible to make a very high Q variable inductor, outperforming a wound component at selected frequencies.  A very good gyrator for audio and low RF can be constructed with a couple of op-amps.  By using discrete transistors the range can be extended to high frequencies.  Gyrator integrated circuits are available.
 * See http://en.wikipedia.org/wiki/Gyrator, however please note that the example single op-amp circuit presented there offers rather poor performance.
 * Similarly, transit time in transistors causes practical amplifiers to drop in gain at high frequencies. Hence, a feedback network can make inductance without any coil and without even any capacitance. However, the stability, accuracy, and quality of such inductance is usually pretty poor.
 * Keit 121.215.78.229 (talk) 03:39, 30 January 2013 (UTC)


 * (I'm not the OP). I'm very impressed with this answer, and left a small q for you on your talk page. 178.48.114.143 (talk) 00:54, 31 January 2013 (UTC)

Menopause for trees
Are there any fruit trees that stop producing fruits after X number of years? Someone described this "menopause for trees" phenomenon to me and I'm very dubious of its veracity.Dncsky (talk) 08:36, 30 January 2013 (UTC)


 * A Google search on "years old and still producing fruit" (with the quotes) turns up enough examples that I doubt that it is true in the general case. There may very well be individual species that do. --Guy Macon (talk) 10:55, 30 January 2013 (UTC)


 * You might like to read How grandma's apple tree shook experts to the core about an apple tree that is thought to have been planted in 1806 and is still producing apples of a type unknown to the National Fruit Collection. Alansplodge (talk) 11:24, 30 January 2013 (UTC)


 * I think the answers above are far too hasty, and are focused on rare exceptions rather than the general trend. It is a little sloppy to call it "menopause for trees", but the analogy isn't terrible... see plant senescence for starters. Trees do age, and fruit yield does tend to decline with age. For instance, this extension service page indicates stone fruit trees only viably fruit for 15-20 years: . Another good source (USA govt. report here: ) gives a "useful life" of fruit trees as 16 years (peach) to 37 years (almond). What you are really looking for is an "age-yield relationship", and there is much research into this topic if you want to dive into some specifics.


 * In short, many species of commercial interest do show declining fruit yield with age. How fast the drop-off is, and how long it takes can vary wildly by species. SemanticMantis (talk) 17:09, 30 January 2013 (UTC)

Thanks a lot, everyone. Dncsky (talk) 21:38, 30 January 2013 (UTC)

Menopause and senescence are not the same thing. Menopause is the gap between reproductive lifespan and actual lifespan. In most animals those are close together, but they have become separated in humans and the gap is progressively lengthening. The adaptation value of menopause has been debated for decades. One of the simplest hypotheses is that the extra years improve the likelihood of reproduction grandchildren, though evidence is mixed. See life history theory. If there is a gap in plants between potential age until death by senescence and an age at which the plant no longer can reproduce itself, it would be fair to compare it to menopause. But it makes no sense and is not useful to simply consider menopause the same as human aging. alteripse (talk) 21:41, 30 January 2013 (UTC)

Name of the diagnostic test shown in the movie Exorcist
What is the name of the diagnostic test performed on Regan MacNeil in the movie The Exorcist (film)? She was undergoing two different tests: one is Pneumoencephalography and what is the other test where the doctors injecting some dye in her neck? --PlanetEditor (talk) 13:10, 30 January 2013 (UTC)


 * Does anyone know? --PlanetEditor (talk) 15:18, 30 January 2013 (UTC)


 * I'd leave it at least 24 hours (rather than just over two, in this case) before prompting people to respond. Everyone's in different time zones, and has different work commitments and activity cycles. Please just be patient, and I'm sure that if there is an answer, someone will provide it. AlexTiefling (talk) 15:27, 30 January 2013 (UTC)


 * Maybe cerebral angiography.  Sean.hoyland  - talk 15:43, 30 January 2013 (UTC)


 * It will also help if you can specify at what minute of the film these events occur so people don't have to watch the whole film over again. μηδείς (talk) 18:30, 30 January 2013 (UTC)


 * 43-46 minutes. --PlanetEditor (talk) 19:15, 30 January 2013 (UTC)


 * I think the movie called one of the procedures a "spinal tap". StuRat (talk) 19:21, 30 January 2013 (UTC)

Good answers. Pneumoencephalography was a very unpleasant, very low-yield test for brain tumors that was devised in the early 20th century and replaced by the CT scan and MRI in the 1970s. Angiography is still done to look for aneurysms or other interruptions of brain circulation. In the 1970s it was also done to look for brain tumors but these days CT and MRI are superior for that purpose. It involves injecting dye into an artery, not into the spine. A spinal tap is a lumbar puncture and is always done with a long straight needled in the lower back to obtain cerebrospinal fluid without being high enough to risk puncturing the spinal cord itself. I dont remember the scene in the movie, but if it was lower back it was a spinal tap, and if higher up it was angiography. If they made the movie now, they would probably depict a neurometabolic PET scan, which shows differential activity of separate parts of the brain. alteripse (talk) 21:29, 30 January 2013 (UTC)

Dynamics of a planetoid being captured by a star
A rogue planetoid from interstellar space comes from the left (as viewed from a point outside the plane of the path and the star) into the vicinity of a star, but below the star as seen by the viewer. (Or it could be a soon-to-be-moon approaching a large planet.) The star's gravity pulls the planetoid out of its linear path and captures it into orbit. The planetoid swings around above the star and back toward the left, then curves back around to the right, etc.

(1) Is the orbit exactly elliptical as soon as one orbit has been completed?

(2) If not, then how many orbits does it take to become within epsilon in some sense of being elliptical?

(3) How is the orbit's eventual elliptical eccentricity determined by the ratio of the star's mass to the planetoid's mass, the amount by which the planetoid would have missed the star in the absence of the star's gravity, and the planetoid's initial velocity?

(4) Which focus of the ellipse is the star at -- the one nearer where the planetoid came from, or the farther one? Duoduoduo (talk) 19:03, 30 January 2013 (UTC)


 * An object not in orbit will not enter an orbit unless there is some change in momentum. So for example an object alling from infinity to near the sun will follow a parabola shape.  If the object was moving towards the sun already, it would follow a hyperbola shape.  There would have to be some interaction, such as tidal, or an encounter with another planet to reduce the rogue objects speed so that it can slow to enter the orbit.  As this change happens the shape of orbit will transition to an ellipse.  It could also interact and not be slowed to a non escape velocity, then it would continue on a different hyperbolic trajectory.  The star would be at the focus near where the rogue object is moving the fastest. For (3) this would be nothing to do with mass ratio, and since gravitational capture will not happen without something else happening, it depends on what else, such as planets, is going on. Graeme Bartlett (talk) 21:08, 30 January 2013 (UTC)


 * Minor correction: If the object was moving towards the sun it would follow a straight line and hit the sun. If the object was moving in any other direction, than it would follow a hyperbola. Dauto (talk) 04:29, 31 January 2013 (UTC)


 * The planetoid may, for example, nail a small asteroid (one that's orbiting the star) and lose enough momentum in the process. It would then run a risk of getting stuck with the star, as a captured asteroid. Or there could be torque transfer due to a gravitational swing-by with a gas giant - which could suffice if the encounter is close enough and/or the planetoid came in slowly enough. - ¡Ouch! (hurt me / more pain) 21:36, 30 January 2013 (UTC)

Thanks. So when I read about various natural satellites having been probably captured by the planet, it's always the case that there had to be some third body involved? What would that have been -- another existing moon? Duoduoduo (talk) 22:00, 30 January 2013 (UTC)
 * In theory, it could lose momentum with a grazing hit to the planet. That only requires two bodies. Very unlikely, of course. Or it can lose momentum by plowing through some gas. --Guy Macon (talk) 02:32, 31 January 2013 (UTC)
 * Another thing to happen is gas could be given off to give a jet power. Since this is more likely on the hot sunlit side, it could slow down the approach to the sun.  Another possibility is that this could break up when heated, some parts may end up slower, and other faster, so that some is left in orbit. Graeme Bartlett (talk) 10:43, 31 January 2013 (UTC)

Okay, so let's go back to my original four questions, with the caveat that a minor collision or gas encounter has slowed down the incoming object. (1) After a single orbit, is the orbit exactly elliptical? (2) If not, how do we characterize the process of asymptotically approaching an ellipse? (3) What determines the eccentricity of the ellipse (and is there a formula for it)? (4) Sure, the star is at the focus nearest where the planetoid is fastest -- but is this focus the one nearer the first loop-around, or is it the focus nearer where the planetoid came from? Duoduoduo (talk) 14:40, 31 January 2013 (UTC)

Also, is it really true that there needs to be a collision that slows down the smaller object? The article asteroid capture (which is a terrible article in general and may well contain mistakes) says "An asteroid with too much velocity will pass by the planet with a hyperbolic orbit, shooting it out into space. Asteroids that do not have enough velocity will fall into the planet in an impact event." Aren't some asteroids captured simply because on rare occasions they have the right in-between velocity relative to their approach angle? Duoduoduo (talk) 15:41, 31 January 2013 (UTC)

Also, the article asteroid capture currently says The properties of an asteroid that are most significant to this process [asteroid capture] ''are its mass and its relative velocity toward the planet in question. The mass of the planet in question is also a key variable''. But just above Graeme Bartlett's comment suggests to me that this is not right, that the two masses are irrelevant to capture. Is the article wrong about this? It seems to me that for a given trajectory and velocity, a rogue object say ten times more massive than the other object would be less likely to go into orbit than would a smaller rogue object. Duoduoduo (talk) 16:43, 31 January 2013 (UTC)

National Geographic's raving review of biochar
According to, small cookstoves made out of ordinary buckets that make biochar are the hottest new thing in Kenya. They cook food without killing people with smoke said to be the greatest environmental hazard in terms of deaths; they produce biochar as useful fertilizer; they can run on anything; they even say that "A family cooking a pot of beans will use 40 percent less wood with the Estufa Finca than with an open-fire stove, said SeaChar President Art Donnelly, who designed the stove."

Now I don't know about you, but I'm getting high on the fumes here. Surely this stuff can't all be true - higher efficiency, wider fuel range, less fuel consumption, and leftover reduced carbon - simply by rearranging your cooker a little to exclude oxygen from a fire? Can it...? Wnt (talk) 20:02, 30 January 2013 (UTC)


 * I agree with your skepticism. One error I suspect they make is in saying something like "One kg of fuel burned openly produces X amount of unwanted byproducts, while one kg burned this way produces less".  They should compare the waste per energy produced for cooking, not per kg of fuel.  If you use less oxygen and thus get incomplete combustion, then you produce less energy.  If they then increased the amount of fuel burned as biochar, in order to produce the same amount of energy for cooking, I suspect the advantages would largely disappear. StuRat (talk) 20:15, 30 January 2013 (UTC)
 * I don't think it's about reducing the oxygen, it's not letting the oxygen reach the material itself but only burn the fumes from pyrolysis: they are basically making charcoal, heating the wood and burning the gas produced, which is a mixture of carbon monoxide, hydrogen and other stuff. It's how coal gas was made, and in the second world war they even drove cars with wood gas generators. I don't doubt it's an efficient stove, but I personally wouldn't put that biochar in the ground. That charcoal can be burned in a normal stove, or a barbeque or whatever. Would take away the "green" aspect I guess. Ssscienccce (talk) 21:34, 30 January 2013 (UTC)


 * Leftover carbon and reduced O2 means it is operating at lower efficiency. However, I suspect that is (or at least could be) a good thing, in that it locks up carbon in long-residence time soil carbon, rather than dumping it all into the air with a normally aspirated wood stove. Perhaps they are oversimplifying the results of a life-cycle analysis? That would allow them to sweep "equivalent" units under the rug, e.g. "less fuel" means more fuel, but the wider range means that a lot of it would not have been considered fuel for an ordinary stove, so they can call it "less" in some sense. SemanticMantis (talk) 21:23, 30 January 2013 (UTC)


 * See biochar section on slash and burn for why it is a good idea to use it as fertilizer. Dmcq (talk) 00:00, 31 January 2013 (UTC)
 * It's not very convincing. Brazil doesn't lose 7,000 km2 rain forest per year because people cook food with it.
 * Switching from slash and burn to slash and char techniques in Brazil can decrease both deforestation of the Amazon basin and carbon dioxide emission, as well as increase crop yields.
 * It won't: Deforestation of the Amazon Rainforest shows that deforestation decreased when soya prices fell and rose when the price went up. Increasing yield will make it more profitable to burn down forests. These "green" initatives are more about publicity, marketing, promotion than they are about results. The people aren't choking from the smoke anymore, but that's the only significant effect. And they get paid for the charcoal, not because it's valuable, but because that's how you get them to use the stoves. I mean, it's not like we lacked the technology of smokeless cooking until now; there's chimneys for example. It's simply that people don't change their ways unless you give them a reason to do so.
 * Biochar is an answer to a problem that doesn't exist: It's not poor people growing food for their family that causes deforestation; in Brazil it's cattle farming, charcoal production(!), and soyabeans for export and biofuel. You can come up with green technologies, durable farming techniques and so on, it won't change the fact that more farm land means more profit. Deforestation will always be lucrative: it will only stop when they can't sell their products on the international market. Ssscienccce (talk) 12:01, 31 January 2013 (UTC)
 * I'm not talking about people cutting the forests down for lumber or ranching, I'm talking about the business of the soil being rather thin and people only using it for a few years after they slash and burn then moving to another area. Using charcoal makes the land last much longer, it's a way of getting settled populations instead of doing slash and burn. This was done by the natives in Brazil before Europeans came and was far more efficient and better for the soil than what was done in Africa. Dmcq (talk) 18:27, 31 January 2013 (UTC)
 * See Terra preta about this practice. Dmcq (talk) 18:32, 31 January 2013 (UTC)
 * My mistake, in these discussions it's not always clear whether someone is referring to the science, or the claimed social/environmental impact. I had read the Terra preta article, but I was reacting mainly to the rosy picture painted by the biochar article. Bit ironic, that's exactly what I usually reproach others: reacting to what they assume was implied... Ssscienccce (talk) 14:40, 1 February 2013 (UTC)

measuring decarboxylase activity
What are simple ways to measure the CO2 output of a decarboxylase enzyme, without having to use pyruvate as a reagent (preferably not NAD+ or NADH, due to interference)? How would air-free UV absorbance work? 137.54.28.86 (talk) 20:13, 30 January 2013 (UTC)


 * Well, the most interesting (and easy for the consumer) would be a variant of - i.e., under the right circumstances, to look at gas evolution.  But you still need some reagent (some source of the CO2 to be emitted) if you want to actually output CO2, and which reagent would depend on exactly which enzyme from which source. Wnt (talk) 20:19, 30 January 2013 (UTC)


 * Yes, I have the CO2 source (alanine decarboxylase) covered. This is for a upper-level biochemistry laboratory where we have to come up with our own protocols for assaying the activity of an unstudied enzyme (a putative valine-pyruvate transaminase). 137.54.28.86 (talk) 21:01, 30 January 2013 (UTC)


 * Well, let's riddle out the question here. There's an l-alanine decarboxylase in Camellia sinensis (tea).  Alanine is CH3CH(NH2)COOH versus pyruvate CH3COCOOH.  So presumably you're aminating pyruvate using valine as the ammonia source, creating alanine, which is decarboxylated to, I suppose, ethylamine? Wnt (talk) 21:24, 30 January 2013 (UTC)

Sick
I'm sixteen and has only been sick 4 times in the last 10 times, and there has always only lasted a few days, and intervals of several years. The rest, of my family is just as often sick as usual. Someone who has an idea why? -- 80.161.143.239 20:52, 30 January 2013 (UTC)


 * I believe Jayron incorrectly removed this Q as a request for medical advice. Your writing is confusing, but I take your question to be "Why do I get sick less often that the rest of my family ?".  Since this isn't asking us to diagnose or treat any current medical problem, we can answer:


 * Genetic variation causes individuals, even in a family, to have different immunity to each disease. Also, general health will affect how susceptible each individual is.  Those with poor diets, little exercise, and with other medical conditions, will be more susceptible to disease than healthy people.  Finally, while you might assume that you and the other members of your family are all exposed to the same diseases, this isn't necessarily true.  Some diseases are only spread by types of contact that can be avoided.  Young children tend to be far sloppier with hygiene, and thus can get diseases that adults usually don't, like intestinal worms. StuRat (talk) 22:55, 30 January 2013 (UTC)


 * What utter bullshit. You jobsworths make me sick. — Preceding unsigned comment added by 78.150.26.47 (talk • contribs)

13.7 billion years
If the universe is 13.7 billion years old, does that mean that the farthest thing away from you is 13.7 billion light years away? 203.112.82.129 (talk) 21:53, 30 January 2013 (UTC)
 * No, that would only be true if you are at the center of the universe. (You can't be, since I am)   ;)   ~:74.60.29.141 (talk) 22:07, 30 January 2013 (UTC)


 * Due to the metric expansion of space, objects can be farther than 13.7 bn ly away. We note at size of the universe that the theoretically observable universe is a sphere roughly 47 bn ly in radius.  Note further that the universe may be infinite in size, and thus objects (though not observable objects) could be infinitely far away.  However, the "center of the universe" joke above is completely wrong.  Measurements from any point in the universe are consistent with that point being the "center". &mdash; Lomn 22:13, 30 January 2013 (UTC)
 * "Measurements from any point in the universe are consistent with that point being the "center"..." if and only if the Hubble expansion is homogeneous and isotropic. This is a reasonably good approximation that matches what we observe so far, and what we expect for a simple universe; but it's an evidence-based hypothesis, and not a perfect fact.  There are lots of ways that cosmologists re-phrase this condition: by describing the geometry of space-time, and so on.  In fact, this is what our article calls the idealized version of Hubble's Law.  Nimur (talk) 18:22, 31 January 2013 (UTC)
 * Right, you are at the center of the universe, and so am I! SemanticMantis (talk) 22:17, 30 January 2013 (UTC)


 * How far then can be the farthest distance from me? 203.112.82.2 (talk) 22:19, 30 January 2013 (UTC)


 * From what Lomn has said, the furthest observable object will be 47 gigalight-years away from you, and the furthest thing that we have not observed is unknown, and could be an infant distance away from you.Dja1979 (talk) 22:58, 30 January 2013 (UTC)
 * Or infinite, even. Evanh2008 (talk&#124;contribs) 23:40, 30 January 2013 (UTC)
 * It could be an infant distance away if it is a larger than average infant. Bus stop (talk) 00:38, 31 January 2013 (UTC)


 * Now, as far as "the farthest thing that we actually have observed" goes, the answer is about 13.4 billion light years. My understanding, though, is that those sort of distance records are corrected for metric expansion, so that it's really more a statement of "we see something 13.4 billion years old" as opposed to "we see something currently 13.4 billion light years away".  For "we see things X years old", 13.7 billion really is the limit.  The cosmic microwave background radiation dates from about 300,000 years after the big bang, and prior to that point, the universe was opaque to EM radiation.  We can see the CMBR, but nothing prior. &mdash; Lomn 01:59, 31 January 2013 (UTC)


 * This is rather simple. If the early universe had been transparent, the oldest light you could see would be 13.7 billion years old.  The objects that had transmitted that oldest light would now be even further away, due to the metric expansion mentioned above.  See redshift.  But the early universe was opaque, see cosmic background radiation. So you cannot see 13.7 billion years back, nor objects that are that far away plus metric expansion. μηδείς (talk) 03:54, 31 January 2013 (UTC)


 * ... but the universe is about 13,772,000,000 years old. Subtract 380,000 years from that (when the universe first allowed light to propagate), and there is still the possibility of light from 13,771,620,000 years ago reaching us, isn't there?  In reality, of course, we have to ask "distance when?" and it is thought that, if the standard model is correct, the greatest current distance from which light will eventually reach us is about 16,000,000,000 lightyears.  Similarly, observers more than that distance away from us now will never be able to see us.  This ignores the question of whether there will be any measurable photons left after travelling that distance.    D b f i r s   09:40, 31 January 2013 (UTC)


 * I question the casual use of "infinite" here. Mathematically speaking, infinity is a concept, not a physical reality. ←Baseball Bugs What's up, Doc? carrots→ 12:05, 31 January 2013 (UTC)
 * Nonsense. &mdash; Lomn 14:42, 31 January 2013 (UTC)
 * Only half. We cannot see things infinitely far away, but that does not necessarily mean that true infinity does not exist, either in space or in time. --Stephan Schulz (talk) 17:10, 31 January 2013 (UTC)
 * My math and science teachers told me that "infinity" is not a number or a quantity. But what did they know? According to Lomn, what they taught me was "nonsense". So, who should I believe? My teachers? Or some character on the internet? Well, let me ask this, Mr. Wizard: How is it possible to have an "infinite" quantity of mass in the universe? Wouldn't that render the law of conservation of mass and energy pretty much irrelevant? Because no matter what you try to add or subtract from infinity, you still get infinity back again. ←Baseball Bugs What's up, Doc? carrots→ 21:40, 31 January 2013 (UTC)
 * Bugs, I have linked articles that discuss the potentially infinite nature of the universe. Either provide references for your crackpot theories or don't post them at all. If it helps, see also this interview with the Head of Astrophysics at Oxford, or many of the other trivially-available search results for "universe infinite size". &mdash; Lomn 22:37, 31 January 2013 (UTC)
 * The notion that there's an infinite amount of mass in the universe is the only "crackpot theory" I'm seeing here. ←Baseball Bugs What's up, Doc? carrots→ 01:23, 1 February 2013 (UTC)
 * Breaking it down for you, then: as linked above, there is good reason to suspect that the universe may be infinite in size. There is good reason to suspect that the universe is homogeneous. These two concepts, which are considered reasonable possibilities by the scientific community, logically suggest the reasonable proposition that the universe may also be infinite in mass.  Now, to be fair, this does depend on your definition of "universe" encompassing things that do not now and cannot ever exist within the observable universe, but again, that is not a particularly fringe definition.  The observable universe is, of course, finite in both size and mass. You are welcome to provide referenced counterarguments. &mdash; Lomn 03:15, 1 February 2013 (UTC)
 * My teachers would say that it's ridiculous, for a simple reason: There is no such number or quantity as "infinity". And I reiterate that I resent you referring to my junior high, high school and college teachers as "crackpots". You want references? Consult any math book for a definition of "infinity". ←Baseball Bugs What's up, Doc? carrots→ 04:31, 1 February 2013 (UTC)
 * Your teacher probably simplified things for you. "Infinite" is not a number in the number sets usually handled in primary education (Natural numbers, integers, rationals, reals, or complex numbers). But of course that does not imply that "infinity" does not exist. A bunny is not a number either, and plenty of bunnies exist. Secondly, there is a whole branch on mathematics dealing with "infinite numbers". See ordinal number. As for the physical reality of infinity, our best hypothesis is that the universe is finite but unbounded in space, but, as far as I understand it, infinite in time. And, as Lomn said, an open (infinite) universe is not ruled out. --Stephan Schulz (talk) 08:19, 1 February 2013 (UTC)
 * And it's not sufficient to consult any math text for a definition of infinity. My eighth-grade algebra textbook from Houghton Mifflin Publishers has a quite different definition than my undergraduate text, A Transition To Advanced Mathematics, by Smith, Eggen, et al., and a different still explanation and definition in my graduate level analysis and calculus-with-analysis texts.  Mathematics can be understood on many levels.  It is very common to provide simplifications of concepts.  As Stephan Schulz has correctly pointed out, infinity (and bunnies) can simultaneously "not be a number" and still manifest in actual physical reality.  This does not mean the junior-high explanation is wrong or crackpot.  It's just only useful for a different domain of problem-solving.  And, we should not accept a particular definition, or theory, just because it's printed in a textbook, unless the definition also makes sense.  The moral of this story is, a mathematician should consult many sources of prior knowledge, but ultimately apply critical thinking to arrive at a correct conclusion.  I am perpetually astonished at how many cosmology-enthusiasts forget to do so.  Perhaps it is easier to canonize Stephen Hawking or Brian Greene, and take their word as gospel.  Nimur (talk) 08:41, 1 February 2013 (UTC)
 * Hmmm, I just read that the homogeneous universe is in question over some new observations.  But I should not pretend to understand the quality of the work or the reliability of its conclusions. Wnt (talk) 04:04, 1 February 2013 (UTC)
 * For Lomn's argument, there only needs to be a limited degree of homogeneity (basically, that on large scales, the density of the universe has a lower bound). I don't think the new discovery (if it is confirmed) puts that into question. --Stephan Schulz (talk) 08:19, 1 February 2013 (UTC)
 * Yes, for this particular purpose, all that really needs to be said is that the average density (matter and/or energy) of the universe does not approach zero. So long as an infinite universe does not tend towards absolute emptiness, its mass will likewise be infinite. Even if it does tend towards absolute emptiness, its mass may still be infinite, but then the math is harder than I like to mess with in my free time. &mdash; Lomn 14:47, 1 February 2013 (UTC)
 * Sorry, but you're wrong. The bunny reference is irrelevant. There are many bunnies in the world, and taking a census of them could be difficult or impractical, but there is still only a finite number of bunnies in the world. It is not possible for there to be an infinite quantity of any object, be it bunnies or galaxies. ←Baseball Bugs What's up, Doc? carrots→ 15:08, 1 February 2013 (UTC)
 * You missed the point. The bunnies referred to what I took as one branch of your argument (paraphrased as "Infinity is not a number, therefore no infinity exists"). This is wrong both in the assumption ("Infinity", or, more concretely many different infinities, are indeed numbers in suitable number systems) and in the inference (there are things that are not numbers, and they exist, therefore the argument is wrong). This invalidates your argument, not your conclusion. The conclusion could still stand. But there are independent arguments that make the conclusion at least doubtful. Consider, again, the time from now to eternity. And while you repeat with some force that there cannot be infinitely many galaxies, you have not supported that claim by any argument. If the universe is spatially infinite, there is good reason to believe that there are indeed infinitely many galaxies (although, with Einstein, the concept of "are" becomes iffy, since it assumes some simultaneous "now"). --Stephan Schulz (talk) 15:27, 1 February 2013 (UTC)
 * You miss the point. It is not possible for an infinite quantity/number/count of any object to exist. Infinity exists as a concept in math, but there is no numerical value associated with it. The time argument is irrelevant. All of time does not exist all at once - only a single point in time exists at once (unless you're arguing for God, who supposedly exists outside of time). So whether the universe will last forever or whether it has an endpoint (which we can only guess about), does not violate the rule. ←Baseball Bugs What's up, Doc? carrots→ 15:37, 1 February 2013 (UTC)
 * I'm with Bugs on this. Actual infinite physical quantities probably don't exist; (if you find one, it's merely a "zero quantity" which you have mathematically taken the inverse of) in the past where an infinite quantity has caused paradoxical results it has led to elimination of the infinite quantity by modification of the theory (i.e. Ultraviolet catastrophe).  The use of an infinite quantity merely represents the limits of a theory, not a physical reality.  Saying that the universe may be infinite is just a shorthand for saying that the universe is larger than our current theories can account for, so there's nothing mathematically preventing a universe with infinite mass.  That is certainly not the same thing as saying that an infinite mass universe physically exists, or that the current theories require an infinite mass universe.  It's merely a concession that the current theories cannot eliminate, via purely mathematical methods, the possibility of an infinite universe.  Per George E. P. Box, "All models are wrong, some are useful" applies here: All human created models are always an approximation of physical reality, and all models break down at some point.  Predictions of things like "infinite mass" represents that point in any modern theory.  That doesn't mean the theory isn't highly useful in explaining all sorts of observable phenomena in the universe, merely that at that limit, it's probably inadequate, and represents a locus of future development.  -- Jayron  32  15:54, 1 February 2013 (UTC)

... unless the cosmologist really really really means "infinite" when they say infinite. Cosmology is perhaps the one instance when a physicist's model of infinity really really really does mean infinity, and does not represent a breakdown of the model. As I keep reiterating, a proper physics explanation will actually formally state what is infinite; derive a consequence; and show how an observation provides evidence of that consequence. This is totally dissimilar to the breakdown of a simple model (like when I facetiously used a linear approximation of pressure to find the top of the Earth's atmosphere earlier this week). In those cases, the model is being used outside of its intended purpose, and calculation of an infinity indicates an error in the application of arithmetic. That is categorically not what is happening when a cosmologist or mathematical physicist posits a universe with infinite mass, or infinite volume, or so forth. Nimur (talk) 16:38, 1 February 2013 (UTC)
 * There are an infinite number of different kinds (i.e. masses) of photon, aren't there? If each photon travels a single wavelength without returning to its source, then this defines an infinite amount of space, right? Wnt (talk) 03:34, 3 February 2013 (UTC)