Wikipedia:Reference desk/Archives/Science/2014 April 8

Science desk
< April 7	<< Mar | April | May >>	April 9 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

April 8

3D Printing using soil

Are there any useful building materials in soil? Like if one day in the future they have such advanced 3D printers that you can just take some soil from your back yard, dump it into the machine, and it's able to build something useful out of it. ScienceApe (talk) 13:06, 8 April 2014 (UTC)

Depending on what type of soil you have, perhaps:

1) Clay: Can be fired to make ceramics.

2) Sand: Can be melted to form glass.

In both cases, you can't use current 3D printing techniques to make objects directly out of those materials, but you might be able to use a mold or form you create using 3D printing to make an object out of clay or sand. StuRat (talk) 13:13, 8 April 2014 (UTC)

Yes, there are useful building materials in soil. You don't need to use a 3D printer. Mudcrete is often used in road construction, while Rammed earth has been used for building for many millennia. As you will see from the top of the page, though, we are unable to provide predictions or speculation about future technological developments. RomanSpa (talk) 13:25, 8 April 2014 (UTC)

Soil has been used as a building material for thousands of years. It is literally the first building material. Besides clay, ceramics, and glass noted above, there's other building materials made essentially out of dirt: brick, adobe, mudbrick, sod, Cob, and dozens more which I am tired of searching for. --Jayron32 14:24, 8 April 2014 (UTC)

If you're imagining a sci-fi "matter converter" where random unsorted matter is dumped in one end and it's automatically sorted, purified, chemically altered, and used for manufacturing, then we're a long way off.

However, if you're looking for just crudely converting available matter into stuff, you might be interested in this Solar Sinter project, or even more basic, but probably more useful, this open-source brick press from the Global Village Construction Set.

APL (talk) 15:04, 8 April 2014 (UTC)

What about stuff like silicon to make electronics? ScienceApe (talk) 15:39, 8 April 2014 (UTC)

Silicon certainly exists in (some) soil. But purifying it would be an ordeal. It's not impossible that in some sci-fi future the silicon is somehow separated for use making chips on demand. That's way in the future though. I don't think anyone even has a good idea how that would work. If we're talking future sci-fi tech, Carbon nanotubes might also be used to make electronics. There's plenty of carbon in most soil.

To be honest, most dirt is probably best used for growing stuff in. Either the old fashioned way, or in some sci-fi way. APL (talk) 16:07, 8 April 2014 (UTC)

That reminds me of another use for dirt, you can put it in a form (perhaps made with a 3D printer) and grow a fungus inside the form to hold it all together, then seal it in with a vinyl covering, to make a cushion. StuRat (talk) 17:10, 8 April 2014 (UTC)

A common ingredient in quartz sand is silicon dioxide, so you can purify silicon from that. StuRat (talk) 17:07, 8 April 2014 (UTC)

Much of the Earth's crust is feldspar, which contains aluminum, silicon, and oxygen. If we had unlimited cheap power, we could make aluminum conductors and structural components, silicon semiconductors, hard corundum pieces out of it. Wnt (talk) 22:10, 8 April 2014 (UTC)

I'd think circuit boards would be first. Layers of circuit traces, via holes and insulators. A 4 layer PCB would be an awesome 3D printer application for multiple element printers. You would then solder the ICs to it. That would be a huge prototyping breakthrough. --DHeyward (talk) 03:02, 10 April 2014 (UTC)

Antibiotic resistance arguments applied to vaccines

If it is recommended that usage of antibiotics be limited to discourage antibiotic resistance, why does it not then follow that vaccine usage be limited to discourage vaccine resistance? (Note: I am not a supporter of the anti-vaccination movement, but I have heard this argument against vaccines advanced by some anti-vaccination partisans and would like to hear a counterargument.) —SeekingAnswers (reply) 13:55, 8 April 2014 (UTC)

Vaccines are far more specific than antibiotics. A typical antibiotic acts against a broad range of bacteria, but a typical vaccine only acts against one very specific type of virus. In fact often the biggest challenge in developing a vaccine to is get it to work against all the different strains of a particular kind of virus. The consequence is that resistance to one type of vaccine does not cause problems in dealing with any type of virus except the specific one that that vaccine is designed for. Looie496 (talk) 14:12, 8 April 2014 (UTC)

Also, it should be noted that antibiotics act as vaccines for the bacteria they kill. That is, the general idea behind antibiotic resistance is that bacteria are exposed to an agent designed to harm them, and they adapt so the agent isn't harmful anymore. Which is almost exactly what a vaccine does for you. In the first case, you're training bacteria to not be harmed by the antibiotics, and in the second case, you're training yourself to not be harmed by viruses. Furthermore, in science we don't base our understanding of the world by incomplete analogy. We base it by actual data. Antibiotic resistance is an observed phenomenon, and is happening. For most diseases we vaccinate against, we aren't finding any such evidence of that happening. I say most, because there are SOME diseases which show resistance to vaccinations: [1] but as that reference makes clear, most of those are the exception and not the rule. --Jayron32 14:19, 8 April 2014 (UTC)

Sorry, but "antibiotics act as vaccines for the bacteria they kill" is at best misleading. Vaccines work by activating the body's immune system; antibiotics work by attacking bacteria directly. Vaccine resistance is possible, but the only way for a virus to develop resistance is for it to get better at overcoming the immune response, and there is strong evolutionary pressure in that direction even without any vaccine being present. Looie496 (talk) 14:48, 8 April 2014 (UTC)

No, vaccines cause the host organism (you) to develop resistance by exposing them to the harmful agent. You know, like antibiotics cause the host organism (bacteria) to develop resistance by exposing them to the harmful agent. The exposure to an agent which causes resistance to that agent is in common to both how vaccines prevent disease in you, and how antibiotic-resistance works in bacteria. --Jayron32 15:00, 8 April 2014 (UTC)

This sounds like you're thinking of a collection or colony of infecting bacteria as a superorganism or similar. Exposure to antibiotics over time can lead to a colony of resistant bacteria, but any individual either resist, or dies from the antibiotic, right from the start. Over time, only resistant individuals are left. This is rather different than engaging the human immune system. (At least that's the common mode of action... Do you have a ref for acquired resistance to antibiotics within an individual bacterium over time?) SemanticMantis (talk) 16:15, 8 April 2014 (UTC)

That's exactly correct. --Jayron32 16:37, 8 April 2014 (UTC)

Well, vaccines do change over time - for example, influenza vaccine is different every year, and the virus keeps evolving, in part to get around current natural immunity and in part to avoid the vaccines. With many other viruses it's not so much of a problem - for example, smallpox and cowpox parted ways a long time ago, so I doubt smallpox will come up with a way to avoid cowpox-based immunity any time soon (not without help anyway). Then there's HIV, which evolves so damn fast it avoids even the natural immune system, and so far vaccine makers are still struggling. Wnt (talk) 16:35, 8 April 2014 (UTC)

I take your point that influenza quickly evolves, and we make new vaccines every year. But I'm not sure that's "resistance," because the virus is merely scrambling the features that identify it to our immune system, and it would do that just as much even without vaccines, right? Sure, it's a bit of a red queen hypothesis situation, but I don't agree with the semantics of saying "flu resists last year's vaccine." -- It's simply that last year's vaccine loses its effectiveness as the virus mutates on. SemanticMantis (talk) 21:19, 8 April 2014 (UTC)

p.s. (Do we have a WP:TELEOLOGY warning? No? I'm pretty sure you'd agree that the flu has no goal or end in mind ;)

Well, when H5N1 came out, I remember reading stories that it was spreading quickly due to not being affected by existing vaccines. I don't think it's as dramatic an effect as antibiotic resistance simply because we don't give it time to add up - every year the vaccine can be made differently, while the drug remains the same. Wnt (talk) 11:38, 9 April 2014 (UTC)

Viruses don't evolve with natural selection the way other living organisms do. Influenza is a classic virus as it moves from species to species picking up pieces as it goes. Influenza moves between pigs, birds and humans. It's survival is mixing different elements of each and is horizontal gene transfer. Whether a particular strain has more human effects or not is not particularly relevant to its survival. Viruses like small pox and polio generally affect only humans so there is no development or adaptation. Because of that, small pox has largely been eradicated. The biggest threat is when a virus picks up a gene sequence that gives it a route to humans. --DHeyward (talk) 03:54, 10 April 2014 (UTC)

I'm inclined to say influenza has a range of different alleles in a gene pool like any other population, though its sexuality is a bit strange. It expands into a range of different niche environments like many other species. If you think of its spread in humans as an irrelevancy you can say evolution doesn't apply to what you're not looking at, but I wouldn't take that view. Wnt (talk) 14:08, 11 April 2014 (UTC)

A key distinction between the two is that vaccines don't directly act upon viruses, they act upon the host's immune system, which does all the real "work" in fending off disease. That is why it "doesn't follow." Think about your analogy from the other direction too: there is no analogue to herd immunity in the case of antibiotics and bacteria, because an antibiotic doesn't spread itself from host to host by its own action. SemanticMantis (talk) 22:02, 8 April 2014 (UTC)

Exactly the opposite. Vaccination promotes the survival of humans resistant to viruses. Antibiotics promotes the survival of bacteria resistant to antibiotics. It is probably true that more rare/severe viruses become higher value targets but only because the more common ones are eradicated. --DHeyward (talk) 00:57, 9 April 2014 (UTC)

I think I can fairly safely say that the vast majority of arguments I see or hear against vaccination are built on misinformation, logical fallacies and non-sequiturs. This one is no exception. Antibiotics are compounds that act directly upon bacteria to either kill them or prevent their replication through structural disruption or interference with bacterial metabolic or other intracellular biochemical processes. Antibiotics do not modify the individual's immune response to a bacterium. Bacteria develop resistance to antibiotics largely as a result of evolutionary processes that follow on from inappropriate use of antibiotics, creating a milieu that favours the survival and reproduction of bacteria that possess mechanisms which render them impervious to the mode of action of a particular class of antibiotic. Vaccines do not directly kill or otherwise interfere with the normal functioning of the viruses or bacteria they are directed against. Rather, they are generally composed of a weakened form of a particular organism, which is recognised by the immune system and used to develop or augment an individual's cellular immune system response to that organism. Vaccines aren't always 100% effective because they rely on the individual mounting a sufficiently vigorous immune response to the antigens in the vaccine and forming sufficient antibodies that are specific to the infective organism. There's a good vaccine-car analogy that goes like this: If I take a new car, then rip out the engine, rip out the seats and smash the headlights, you'll still be able to identify it if I leave the 3-point star on the hood. If you're really up on your cars you'll even be able to pick the model and year by the body shape. Now, if I take the same car, and then remove all the badges and do a heap of after-market body modifications, you'll struggle to figure out what make of car it was, let alone the year and model. The antibiotic-car analogy goes something like this: take a car, fill it with C4, detonate it, go find all the pieces and chop those pieces up into little bits until there's no sign of the car. Spectacularly effective unless the car manufacturer comes up with a C4-proofing option. Mattopaedia ^{Say G'Day!} 01:58, 12 April 2014 (UTC)

About a birds name

I have seen a bird in towns in eastern side of India in West Bengal.the birds are big in size have copper coloured wings and rest of the body is black.But unfortunately I dont know the name of this bird.Please tell me the name of this bird and its habits.Please mention if there is any Wikipedia article about this bird.Another type of bird that has come to my notice is even smaller than a sparrow but I am startled and amazed by its colour.There is a greenish lustre or blueish green lustre.It appears as if the bird's feathers act as a diffraction grating.It has very thin carved beak which I think can prominent feature.Please identify this bird and give its name and corresponding Wikipedia article.Pleasemention the habits of these birds and where you have seen them.117.194.229.124 (talk) 14:01, 8 April 2014 (UTC)

It sounds like a member of the Starling or Myna family, many of which are known for their iridescent feathers as you describe. --Jayron32 14:09, 8 April 2014 (UTC)

I see the picture of Starlig but it doesnt match the second type of bird I queried about.Please dont forget to mention about the first bird.117.194.229.124 (talk) 14:14, 8 April 2014 (UTC)

A straightforward Google search finds the Greater Coucal. Looie496 (talk) 14:17, 8 April 2014 (UTC)

There are hundreds of different species of birds in the starling family. Your second type of bird could be any number of them, including the Common Starling, which also has a number of subspecies. Don't just look at one picture in the first article you find. Search through pictures of different kinds of starlings and see if you find any matches. --Jayron32 14:21, 8 April 2014 (UTC)

Searching Google for Greater Coucal I got resuls inn image section purplecoloured sunbird and luckily it was the second type type of bird I asked about.The first type of bird is Greater Coucal. Actually in our locality there is overcrowding of crows a menace with some sparrows so it is a delight to see these wonderful birds.Thanks for your kind help.117.194.229.124 (talk) 14:53, 8 April 2014 (UTC)

Let me just add a pointer to our Purple Sunbird article. Looie496 (talk) 16:05, 8 April 2014 (UTC)

Chemical analysis

Suppose a person stumbles upon a substance and he wants to know the chemical composition of the substance.The substance can be quite complex mixture of organic and inorganic compounds.How will the chemist know about what are the compounds present and in what proportion.How this used to be accomplished when there was no NMR spectra or computers available and how is it done nowadays.Isthis type of analysis required in forensic studies and where else this type of analysis required. Can natural substances like fruit skin,fruit juice and products like soaps perfumes creams ointments can be analysed to dtermine composition and what knowledge and expertise and what level of nhemistry knowledge is required to do this.117.194.229.124 (talk) 14:11, 8 April 2014 (UTC)

The first thing the person would have to do is some form of separation techniques to get the mixture into its component substances. A common method is to combine a separation technique with an analytical technique to then identify the substances as soon as they are separated. Analytical systems like GC-MS or LC-MS (for gas chromatography-mass spectrometry or liquid chromatography-mass spectrometry respectively) are commonly used for that exact purpose. You can also do your separation techniques prior to doing your analysis work; for example using Ion-exchange chromatography and then isolating each product and doing analysis individually. This is commonly done for techniques that don't "marry" well to a gas chromatograph, such as Infrared spectroscopy or Nuclear Magnetic Resonance. --Jayron32 14:35, 8 April 2014 (UTC)

The methods you mentioned require computers or electronics but without these using these gadgets how can the work be done as was the case in 19th century or early 20th century.Or was such analysis was impossible at that time.117.194.229.124 (talk) 14:58, 8 April 2014 (UTC)

Basic chromatography techniques such as thin-layer chromatography and paper chromatography and ion-exchange chromatography could be used to separate mixtures without the use of electronics at all. Prior to the use of electronic methods of analysis, the only methods of chemical analysis were called wet chemistry methods, and basically involved a series of chemical tests to elucidate the structure of unknown compounds. For just a few examples, there's the Baeyer's test, universal indicator, the Van Slyke test, Seliwanoff's test, the iodoform test. --Jayron32 15:07, 8 April 2014 (UTC)

If all matter outside the solar system ceased to exist?

To put the question somewhat more specifically than the header title above puts it, what would be the effects on the solar system if all matter outside a radius of, say, 50 astronomical units from the sun (that is, from approximately outside the Kuiper Belt) spontaneously ceased to exist? This is the only counterfactual; in all else, the laws of physics remain the same. Also to clarify, this is a one-time event; matter that after this event passes across the 50 AU boundary does not cease to exist. (A clarification edit: by "matter", I mean objects with rest mass, so electromagnetic radiation, for instance, would continue to exist outside the boundary.) I'll mention several specific phenomenon, with related questions, in particular.

First, how quickly would scientists realize that this disappearance of matter had occurred, and what pieces of evidence would this conclusion rely upon?

Second, I suspect that without the gravitational effects exerted by matter outside the solar system to counterbalance the gravitational pull of the sun, objects (including planets such as our Earth) would begin to fall toward the sun. Is this supposition correct? And if it is correct, how fast would the fall be (that is, how long from this event before the Earth is swallowed by the sun)?

Third, what would the ultimate fate of the solar system be, given this event? I assume it would be some kind of thermodynamic heat death, but how long would the process take? Would the remaining matter from within the solar system all become packed together forever from gravitational effects, or would some or all of the matter disperse, getting forever further away?

I'd also be interested in hearing about whatever other effects people here can think of.

—SeekingAnswers (reply) 15:18, 8 April 2014 (UTC)

First, 7 hours after the event the Fixed stars disappear from the night sky. It takes about 24 hours for the phenomenon to be confirmed by all earth-bound telescopes but by the 2nd day there are videos on YouTube and TV "expert" commentators explaining whether the End of the World is nigh. A Stock market crash ensues.

Second, the nett pull of the rest of the Universe on our solar system is virtually zero which is why we don't accelerate madly in any particular direction. Gravitational force of the Sun remains the dominant factor keeping the planets in their present orbits unchanged.

Third, if you can explain why the solar system (Address: Local Interstellar Cloud, Local Bubble, Orion–Cygnus Arm, Milky Way) was specially preserved through this singularly destructive event, you probably have the basis for a Belief system that gives a comforting answer to any remaining questions. 84.209.89.214 (talk) 15:46, 8 April 2014 (UTC)

Good and basically comprehensive answer, 84. But I am not so sure there wouldn't be a huge number of people concluding that it was not the rest of the universe that had been destroyed, but we sinners who had been snatched into the void. μηδείς (talk) 16:26, 8 April 2014 (UTC)

Regarding the fixed stars disappearing after 7 hours: In my original question, when I mentioned "matter", I was specifically referring to objects with rest mass. So fixed stars wouldn't disappear from our view after 7 hours, since photons from beyond 50 AU that were emitted before the stars ceased to exist would continue to stream toward us. So fixed stars disappearing that quickly couldn't be what tips everyone off that something has happened.

And on the third part, still, what would happen eventually in the end? Would everything be drawn together or disperse?

Also, if I'm understanding your answer, basically, other than human panic, there would be no real negative effects on Earth?

—SeekingAnswers (reply) 16:50, 8 April 2014 (UTC)

IP 84 has already answered your answer correctly, regardless of your further qualification. You seem to know that the light doesn't disappear if the light doesn't disappear, so your question would seem to be resolved. μηδείς (talk) 20:51, 8 April 2014 (UTC)

cosmic rays would greatly decrease, possibly leading to a decrease in cancer, though the health risk or benefit of low level radiation is unclear; see hormesis. We would also be freer to send out interstellar spacecraft, apart from one small problem. Interest in the Pioneer and Voyager spacecraft would suddenly explode as people became curious if their impact with or passage through the "barrier" could be detected. I think most of us would assume the barrier was some sort of alien quarantine or embargo, or a failure in an existing system meant to simulate an unpopulated universe. Wnt (talk) 16:41, 8 April 2014 (UTC)

Seems like Mach's principle would be relevant. Katie R (talk) 16:51, 8 April 2014 (UTC)

If light and other EM radiation kept coming in from the former stars, it would take 4.3 years for the nearest stars to go dark. So, we would know something serious was going on then. I'm not sure if we would detect anything before that, such as the lack of dust clouds just outside the border, or the lack of gravitational perturbation on Kuiper belt objects from nearby stars (this would result in fewer comets, eventually, but this could take thousands of years to become noticeable). The solar system no longer orbiting the galactic center wouldn't be noticeable, as the gravity from the galactic core was exactly counterbalanced by our orbit. StuRat (talk) 17:21, 8 April 2014 (UTC)

For one thing, the Voyager spacecraft would both stop communicating at the same time. Eris and Sedna would disappear from telescope images. --Bowlhover (talk) 18:57, 8 April 2014 (UTC)

As other people already pointed out, there wouldn't be any noticeable perturbation in planetary orbits. But there would be an interesting effect when the last light of a given star reaches us: There is a lot of electromagnetic radiation within a star that is normally prevented from escaping quickly by the matter (plasma) of the star. And that would then escape at once, so for the last few seconds before becoming invisible, a star would become a lot brighter (for a timespan of its diameter divided by the speed of light, so e.g. 0.65 seconds for Proxima Centauri and 8 seconds for Sirius A).

And a similar thing must happen with the gravitational field of stars, which should relax and dissipate as gravitational radiation (someone will think of Birkhoff's theorem (relativity): A star is often approximately spherically symmetric, and a spherically symmetric metric is static and thus non-radiating according to General Relativity, but General Relativity isn't really compatible with things suddenly disappearing; and neither is Maxwell's electromagnetism). So gravitational wave detectors will finally detect something a few years after the event.

Icek (talk) 19:28, 8 April 2014 (UTC)

If we're going with the premise that the light remains but not the matter, note that it takes something like 10,000 to 170,000 years for gamma rays to escape the Sun as visible light. So in that second the star would get something like a trillion times brighter, all in hard radiation. Alpha Centauri starts at magnitude 1.3, so it would come out at something like 3 orders of magnitude brighter than the sun, in X-rays. Not a nice surprise. :) But then again, how much of the lifespan of the gamma rays inside the Sun is counted as "associated with" matter enough that it would be disappeared with the matter, and how much as "free"? (I don't think you can frame the premise meaningfully really??) Wnt (talk) 20:12, 8 April 2014 (UTC)

If you ask the question of what happens if a charge (like an electron or proton in the star) disappears from the viewpoint of Maxwell's equation, you run into a contradiction: Assuming a spherical charge distribution, and the charge becoming less with time, the $\mathbf {E}$ field changes according to

$\nabla \cdot \mathbf {E} ={\frac {\rho }{\varepsilon _{0}}}$

Then everywhere tangential magnetic fields should be created according to

$\nabla \times \mathbf {B} =\mu _{0}\left(\mathbf {J} +\varepsilon _{0}{\frac {\partial \mathbf {E} }{\partial t}}\right)$

where $\mathbf {J}$ is zero (no current).

But that's not quite possible in a spherically symmetric problem.

The only more fundamental theory is quantum field theory, and I guess renormalization won't be possible anymore if you just take out all the massive particles at a particular point in time.

Icek (talk) 21:08, 9 April 2014 (UTC)

Could I get an answer on the third part of the original question: in this hypothetical universe, in which the only the only matter remaining is that in the solar system, which of the various theories about the ultimate fate of the universe would likely be correct? —SeekingAnswers (reply) 19:07, 9 April 2014 (UTC)

The solar system doesn't have enough mass to collapse to anything denser than a white dwarf. And in the long run, as we seem to be in a universe with accelerating expansion and the cosmic background radiation's temperature becomes lower and lower, everything would cool and evaporate.

By the way, the mass density equivalent of the cosmic background radiation is about 4.64*10^-31 kg/m³, or about half an electron mass per cubic meter. So within 4.3*10⁶⁰ m³ of otherwise empty intergalactic space there is about one solar system's mass of about 2*10³⁰ kg. It's only a ball with a bit less than 11000 light years radius. Icek (talk) 21:08, 9 April 2014 (UTC)

Minor point: the x-ray burst from alpha Centauri wouldn't be a surprise at all. Because of this. - ¡Ouch! (^{hurt me} / _{more pain}) 06:26, 10 April 2014 (UTC)

I'm pretty that the burst would be significantly different than anything observed so far, and would end up with tons of telescopes pointed at it. And then of course when the burst is over and there's no star left behind we'll probably know something bad happened. Katie R (talk) 18:41, 10 April 2014 (UTC)

You're pretty? Wow. My real point was that the disappearing Alpha Centauri wouldn't be a surprise because it would be months after Proxima would have disappeared from our sky. Proxima is a red dwarf which is not visible to the unaided eye, but even that would have emitted quite a burst. - ¡Ouch! (^{hurt me} / _{more pain}) 07:49, 11 April 2014 (UTC)

The OP's added clarification that incoming photons from distant stars would continue after the stars themselves are magically deleted suggests that the event would for many years go undetected here. However astronomers would not long miss the steadily accumulating number of position hops by the fixed stars; this is due to disappearances of gravity lensing by intervening stars, although they might be puzzled about the cause. I am already puzzled to think how the OP's destructive phenomenon might interact with a Black hole and wonder if anything in the proposition rules out the possibility of its arbitrary border slicing a black hole in half. The "laws of physics remain the same" condition seems to call for the Impedance of free space to be unpeturbed even at the concave destruction limit but if this cannot be guaranteed, we would see at least some clouding or distortion of all the fixed stars at time E+7 hours (due to partial reflection and refraction) followed by a sudden great brightening of the whole night sky at E+14 hours due to internal reflection of our own Sun. Such a scenario takes away the luxury we have enjoyed of having time to ponder over a likely Ultimate fate of the universe and we would find ourselves more than usually lonely and ill prepared to predict how such a cosmic vacuum metastability event plays out on our doorstep. 84.209.89.214 (talk) 23:26, 10 April 2014 (UTC)

Do not see. I'd think the bursts would come in at the very moment the grav lensing ceases; thus we'd not get an early warning unless there's a massive non-radiating body (which could be anything from brown dwarf to black hole) within 4.something light years. A rogue planet might qualify, too, but it'd have to be really close to the line of sight to cause observable lensing.

Black hole slicing: since the mass of a black hole is at a single point, it's hit or miss. Even if a black hole is a fuzzball (string theory), I'd rule out that it can be cut in half if photons are unaffected.

I wonder how refraction and internal reflection mix with it. The refractive index difference would be so tiny that we probably wouldn't detect it, given that we'd look at the surface at about 90 degrees, where refraction is generally at its weakest. Maybe the pulse frequency of pulsars would change. - ¡Ouch! (^{hurt me} / _{more pain}) 07:49, 11 April 2014 (UTC)

The non-indent of my post is because I haven't much to say about Proxima Centauri's flares that you brought up in the thread flirting with pretty Katie. The size of a Black hole to us is the finite, optically observable diameter of its event horizon, not the abstraction of a point singularity. The OP's proposition effectively partitions the Universe into our local factual part and a momentarily counterfactual surrounding in which mass spontaneously vanished. It supposes a massive superluminal violation of the physical law of Mass–energy equivalence and leaves us clinging with whatever sanity remains to an easy assurance that all physical laws are thereafter re-established. But devoid of mass there can be no Gravitational constant which at a stroke invalidates what Isaac (1687) and Albert (1915) laboured to elucidate. My post speculates a visible Schlieren at the factual-counterfactual interface; actually anything might happen there including the Dark matter (more than 1/4 of our Universe) turbulently boiling away into pre-existent nothingness. The alarming implication of your belief that we would get no early warning of this is that it could already have started. ¡Hush!. 84.209.89.214 (talk) 14:58, 11 April 2014 (UTC)

Is the technic of the suction cup - base on surface tension?

I've read our article about, but it's not clear for my question. 5.28.177.131 (talk) 18:58, 8 April 2014 (UTC)

No, a suction cup works on the principle of differential pressure. There's lower pressure inside, higher pressure outside, so the difference in pressure keeps it pressed against whatever it is sticking to. --Jayron32 19:01, 8 April 2014 (UTC)

What Jayron32 said. When the suction cup is not stuck on something (i.e. when it's like most other objects you interact with) it's surrounded by air, so the atmospheric pressure we experience here on earth kind of balances out. When you remove all of the air from one side, however, the only pressure is what's pushing down on it from the other side. The strength of the suction is basically a measure of how easily air can sneak back in. This is also why suction cups can't work in space, where there is no atmospheric pressure. --— Rhododendrites ^talk |  20:44, 8 April 2014 (UTC)

Perhaps that air not sneaking back in quickly is what this Q is about, as that is the interesting part, at least to me. There's obviously a pressure differential, yet the vacuum doesn't immediately suck the air back in along the edges. Adhesion of the suction cup material to the material on which it's mounted gets the credit there, I believe. StuRat (talk) 00:52, 9 April 2014 (UTC)

There is definitely something going on that we haven't explained yet, but I don't think we need to invoke adhesion or any other chemical or electrostatic forces. I think it's all to do with the elasticity of the cup. Imagine the situation with a rigid vessel, like the Magdeburg hemispheres. It's obvious what's happening there, because somebody used a vacuum pump to suck out the air. Now imagine that the copper hemispheres are replaced with rubber ones. You don't need a vacuum pump any more: you just squash the two hemispheres together, expelling the air, and then let them expand by their own elasticity. They could have called them the "Magdeburg sink plungers". The suction cup works like that, only with one hemisphere replaced by a flat surface. A suction cup is its own vacuum pump, using its elasticity to expand a sealed volume against atmospheric pressure. Thus it causes the atmosphere to exert a force on it, and that force pushes the rubber against the wall to create the seal (and incidentally, to create the friction that stops it from sliding down the wall). --Heron (talk) 18:30, 9 April 2014 (UTC)

Well, the grease was needed in the hemisphere case, and water helps to keep a suction cup stuck, so I believe those liquids are an important part of the equation, with adhesion serving to plug the gaps with those fluids. StuRat (talk) 23:11, 9 April 2014 (UTC)

Every kid with the suction cup darts know they work better when darts are licked. My guess is the liquid on the rubber has an adhesion to the rubber and the pressure on the liquid pulls the rubber tight on the surface through its adhesion to the liquid. --DHeyward (talk) 09:52, 10 April 2014 (UTC)