But what kind of matter doesn't interact with electromagnetics? Any elementary particle that is electrically neutral.
Have we ever physically obtained any? Synthesized any? If by "any" you mean "electromagnetically non-interacting", then there are neutrinos. If you mean "the kind of dark matter that is needed to account for astronomical observations", then no, we haven't.
And supposedly something like 70% of the dark "matter" is energy. How does non-dark energy interact with electromagnetism, where the dark "stuff" does not? What is "non-dark energy"? Photons? I don't understand the question.
Why is it that scientists think that dark matter exists simply because the observed galaxies don't conform to Newton's Laws? Wouldn't a simpler solution be to take a step back and consider that, maybe, Newton's Laws are flawed? You want a solution that is simple enough to explain the facts, but no simpler. Modifying the laws of gravity runs into difficulty explaining everything that dark matter can, although you can get it to explain some things (such as galactic rotation curves).
Can someone explain to me why dark matter is the prevalent theory? In short, because it works and none of the alternatives people have proposed over the decades work as well. I can get into details if you want, but you should probably just start at Wikipedia.
Or perhaps why something like MOND is always ignored? MOND isn't ignored. Go to the astro-ph arXiv or the Smithsonian/NASA ADS Abstracts and search for MOND papers. You'll find them, along with criticisms of MOND. Here is a nice but somewhat outdated set of slides on how well MOND fares against the evidence, and a more recent blog post by the same author discussing newer evidence that tightens the screws on MOND even further.
As I said, I don't know what is right, but it just seems like a hack-job to me. I don't know why all the hate for dark matter. Screwing around with the laws of gravity isn't any more elegant, and there are plenty of plausible candidate particles for dark matter lying around in various extensions to the Standard Model.
What standard theories? The standard theories of large-scale structure formation in the early universe, which is mediated by non-baryonic dark matter.
Dark matter does not exist, as least not as far as anyone (except astronomers with good imaginations) knows. Wow, that's a compelling counterargument. However, it neglects the decades worth of observational evidence in favor of dark matter in the form of galactic rotation curves, the motions of satellite dwarf galaxies, gravitational lensing, measurements of galactic gas temperatures (which depend on the local gravitational neighborhood), anisotropies in the CMBR, the rate and structure of large-scale cosmological structure formation, etc.
There is a very nice (and complete!) standard model of physics, and dark matter holds no place. Actually, one of the leading dark matter candidates is the axion, which was introduced into the Standard Model to resolve the strong-CP problem. However, the astronomical evidence indicates that the Standard Model of particle physics is most likely not complete, and that at least one new weakly-interacting massive particle is needed.
Regular matter, that is simply dark - i.e. cold, and not emiting light, does not bother me. But making up particles no one has ever seen just because you don't understand what you are seing is fitting facts to the data. There is nothing wrong with "making up particles no one has ever seen" in order to explain discrepancies in either theory or observation. It's rather the point of science, to frame new hypotheses. Historically, see the prediction of the positron, on the basis of theoretical consistency between quantum mechanics and relativity, or the prediction of the neutrino, on the basis of apparent non-conservation of energy.
Scientists often discuss new theories, etc, and in that context dark matter has it's place, but to claim it exists - as this story does - without being able to actually measure anything is quite silly and premature. If you don't understand something, say so, don't invent plausable explanations that have nothing supporting them except your lack of knowledge. Dark matter is a plausible explanation precisely because it is supported so well by numerous disparate observations. There are other ways one can attempt to explain various discrepant observations (e.g., by modifying the laws of gravity), but dark matter is far and away the most successful, as it passes all known independent tests. There's no reason why an ad-hoc patch designed to explain galactic rotation curves should also end up explaining, say, cosmological expansion, or large-scale structure. And it's silly to claim that we cannot measure anything: we can measure the gravitational effects of dark matter.
Sure, everyone would love it if we could detect dark matter particles directly — and if they interact non-gravitationally, we hopefully will someday. But what's silly is to claim that we have little reason to believe that dark matter particles exist.
The Schwarzschild radius isn't "emitted" from anything. It is simply a location in space, or rather a distance that defines a location. You can define a Schwarzschild radius for any object, even yourself. A black hole (roughly speaking) is a body whose matter has all collapsed into a region smaller than its own Schwarzschild radius. The Earth is not a black hole, because its matter is not contained within a region a centimeter across. Anyway, at a distance of 1 (current) Earth radius, a cm-sized black hole's gravity would not be any stronger than the current Earth's gravity; black holes don't suck things in more strongly than the objects they formed from. (Of course, the reason we are not falling into the center of the Earth is because we are standing on a solid surface; that would not be true for a black hole.)
It's not a problem of getting money, it's a problem of relatively unbiased money drying up for anyone who says, "there's no actual certainty here, and many variables to account for." That turns out not to be the case. The quantification of uncertainty has been one of the hottest subfields in climate science in the last 5 years or so, and they're still hiring heavily.
The ultimate Unix editor
on
The Birth of vi
·
· Score: 4, Funny
I see so-called "hardcore" Unix geeks advocating 'ed'. Nonsense! The ultimate Unix editor is "cat >filename". All the others are for indecisive wimps who don't know what they're going to write, or incompetent losers who make mistakes.
It's already sitting in water. [The Arctic polar sea ice] melting wouldn't have a lot of effect on humans. It wouldn't directly raise global sea levels. However, it's hasty to conclude that it definitely wouldn't affect us much. The albedo of the Earth would decrease, accelerating global warming. (To what extent, I haven't calculated.) And melting all the Arctic ice would quite possibly shut down the North Atlantic thermohaline circulation (especially if it's in conjunction with melting of the Greenland ice), which would have a noticeable climate impact: along with subsidiary effects, it would cool Europe relative to the rest of the world. (Which might be a good thing, if so much global warming happens that the whole North Pole melts.) There might be other more indirect effects that I haven't thought of, but probably someone has studied this.
Citation: Zwally et al., Science297, 218 (2002):
The near coincidence of the [Greenland] ice acceleration with the duration of surface melting, followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface. However, Bindschadler, Science311, 1720 (2006) states that, although this effect does exist,
Penetration of surface meltwater to the glacial bed in Greenland can lead to seasonal flow acceleration, but the annually averaged increase in speed is only a few percent. In the case of Helheim Glacier, the relative intensities of warm summers were not associated with the observed changes in glacier speed. And surface melting is uncommon for any of the Antarctic glaciers cited here. On the other hand, surface meltwater is implicated in ice sheet fracture, due to its pressure as it seeps into crevasses. (Ice sheet thinning is also involved, of course.) I'd have to dig up the references, though. I recall modeling which indicates that filling a crevasse as shallow as 6 meters deep with meltwater can cause the sheet to crack all the way through to the bottom.
The credits to "An Inconvenient Truth" were overlaid with a series of "things you can do to help" type suggestions, along with a URL for more information.
Regarding the breakup of the Antarctic Larsen ice shelf, Wikipedia has the following to say: "The leading ideas involve enhanced ice fracturing due to surface meltwater and enhanced bottom melting due to warmer ocean water circulating under the floating ice."
Ice is melting all over the arctic it seems, and there are tentative links to global warming. However no-one has proven that these are not natural events slightly speeded up. With respect to this particular ice shelf, TFA says: "The researchers suspect climate change may have played a role in the collapse but said they cannot definitively say it is a result of global warming."
However, if you look at the Arctic (and Antarctic) in general, it is indisputable that the rate of melting increased substantially starting in the last 100-150 years, coinciding with global warming.
that in 20-30 years ice this thick must have melted (as a result of global warming)... Puhlease.... It takes more than 20 years for ice this thick to melt to a shelving point... As another poster pointed out, global warming has been going on for longer than 20-30 years, it's closer to 100. And as another article on this event noted, the Canadian ice shelves have decreased in size by 90% over the last century.
People don't forget the Sun. Certainly solar forcing is a major factor in climate models. However, variations in solar output alone can't explain the warming trend we currently see. See, for instance, this review.
I hope Part 2 remembers to cover Alternate Reality: The City (1985) and The Dungeon (1987) (Wikipedia). Those games were amazing for their time. AR had a raycasting engine 7 years before Wolfeinstein 3D, animated background scenery, weather and sun systems, great music with synchronized sing-along lyrics, character alignments, it tracked hunger/thirst/encumbrance/temperaturee/etc. The series had an ambitious Matrix-esque 6-game plot scripted out (only the first of which was made, in two parts). It even implemented garbage collection in a literal sense: if your inventory exceeded your free RAM, the Devourer came and ate some of your items at random. A review of the City tells more.
The summary claims the British government is seeking to "lock up people" who haven't committed any crimes. The article itself, however, says no such thing. It only says:
... the National DNA Database being built up by the British government (which includes material from many innocent people), would already allow the identification of those with milder predispositions to anger and violence. It then goes on to speculate about what the government could do with that information, but please note that the article says nothing about the British government seeking to jail people who haven't committed crimes. I am wondering what this claim is based on.
But what about when the distance is expressed in points of a given size? What does it do then? Unless you increase the size of the array, there's no way to account for the new increase in magnitude. Point i is expressed by it's position in the array (x,y) and point j is expressed by it's position in the array (x1,y1) and thus the distance between them is (x1-x, y1-y). Increase the magnitude so that the distance between them is now (2x1-x, 2y1-y) and you've effectively added points in space between them. That doesn't bear any resemblance to the concept of space used in general relativity. In GR, space is as I described it: a set of points, along with the distances between points, given by a geometric quantity called the metric tensor. The distances (metric tensor) may change, but the set of points (underlying spacetime manifold) remains the same. "New space" is never created or destroyed in general relativity, regardless of whether spacetime is inflating, expanding, contracting, or whatever.
I like the other answer better, which is more like cell division than actual expansion- the value in point x,y becomes divided into points x1,y1 and x2,y2 and the value in point x,y ceases to exist. You may like the other answer better, but it doesn't have anything to do with the physical theories you're purportedly discussing.
Think of the universe as a three dimensional array- with each point in the universe being a unique element in the array. The problem with inflation is that it adds elements to the array with information in them (even if that information is null) without it apparently coming from anywhere. Not a good analogy. Neither inflation nor ordinary non-accelerating expansion adds new points of space; it just changes the distances between existing points. Think of a NxN array which gives the distance between point i and point j; expansion just increases the magnitudes of the values in the array. It doesn't increase the size of the array.
Several very respectable general relativists have proposed quantizing spacetime at the smallest scales in an effort to reconcile GR and QM, and there are a number of efforts to analyze Planck scale spacetime and select among a number of competing theoretical frameworks making strong hypotheses about its nature and geometry. Among these are loop quantum gravity, discrete Lorentzian quantum gravity, quantum field theory in curved spacetime + black hole thermodynamics and string theory, QFT in curved spacetime doesn't really probe the Planck scale behavior of gravity.
in order from strongest, most precise and most testable to less strong, less precise and less testable. I would reverse that order. LQG shows many signs of having all the non-predictability problems that other direct attempts to quantize GR show, as artifacts of its non-renormalizable nature. They just show up in a different form, in the guise of infinitely many quantization ambiguities. Lorentzian dynamical triangulations is somewhat better, but is less developed so is less understood. String theory actually is very well formulated and makes specific predictions about things like scattering amplitudes, etc. It doesn't tell you which vacuum state is ours, but then, quantum field theory doesn't tell you which QFT (e.g., the Standard Model) is ours, either.
n fact, given how useful lattice models have been for QM-scale physics in allowing for computer simulation (Lattice QCD especially) and exploring a number of areas not readily accessible to perturbation theory, it is probably worth a great deal of possibly ultimately fruitless effort to explore the quantization of GR-scale spacetime, particularly in models which do not require counterterms, avoid the experience of pestilential divergences, do not require abandoning the discretization of quantum fields into individual particles, and so forth, all in order to remain accurate in the presence of gravity fields of varying strentghs. You can't avoid divergences just by introducing a discrete lattice cutoff; you do have to recover the continuum limit — either by removing the cutoff in an appropriate limit, which defeats the purpose to an extent, or by summing over configurations — and therein lies the problem with all existing lattice-like approaches. (I'm also not sure what you're referring to when you mention "abandoning the discretization of quantum fields into individual particles".)
Bound systems resist expansion. The expansion of space is derived under the assumption that matter in space has the same density everywhere, equal to the (very low) average density of the universe. Systems such as atoms, stars, or galaxies are much denser than that, and space inside them does not locally expand like intergalactic space does.
However, when you say that theories exist that the Universe has a specific shape, it sort of derails that theory to me. To me, a very simple, very logical conclusion is that if an object has a shape, it has dimensions. Okay.
If it has dimensions, it has a beginning and an end. No. Consider a Euclidean plane. It has two dimensions, but no beginning or end.
How can the universe be infinite if it has a beginning and an end? Are you confusing infinite space with infinite spacetime? Spacetime has a beginning, but that doesn't mean that space can't be infinite.
We can theorize that there is no end to the universe, that I can transmogrify into a photon and pick a direction and travel that direction forever (another sticky term...), but we can't verify or quanitify that. It bothers me that there are no theories (that I know of) that can really answer this, but it seems we have reached the limit of physics as we know it. Most forms of inflationary cosmology predict an infinite universe.
I kind of get the balloon analogy, but isn't there still a central point we're going away from. No. In the balloon analogy, the surface of the balloon is all of space. We can embed that sphere in a 3D space for purposes of visualization, but that space doesn't actually exist — it's just how humans need to visualize curved surfaces.
Isn't the size of the universe a measure of where the matter begins and ends (finite) and everything else is a vacuum (infinite)? No. The balloon's surface has no "edge", and the balloon is uniformly filled with matter. It's not correct to think of the matter-filled balloon expanding within some higher-dimensional vacuum. A space doesn't have to be embedded in anything larger in order to be curved, or even in order to expand — the geometry and expansion of a space can be defined by measurements made completely intrinsically to that space, without reference to any external space.
Here is a mapping that takes the reals (-infinity,+infinity) to the interval (0,1) in 1-1 correspondence: arctan(x)/pi + 1/2. (The inverse tangent maps (-infinity,+infinity) into the finite interval (-pi/2,+pi/2), and the rest is just rescaling and shifting to fit into (0,1).)
So TFA mentions enormous black holes. What happens to them? What's the lifecycle? They sit around, and get bigger every once in a while whenever something falls into them. Same as ordinary stellar black holes.
At some point do they get big enough to suck themselves into their own little inaccessable chunk of spacetime? There is no upper limit on the size of a black hole, but at some point it has sucked up most of the matter nearby, and doesn't thus grow much after that.
Or does Hawking radiation manage to eventually make a black hole evaporate away? That's possible, but supermassive black holes radiate very weakly. (The bigger the hole, the less it radiates.) They can't evaporate at all until the cosmic background radiation cools to below the black hole's Hawking temperature; otherwise, they absorb energy from the cosmic background and grow. A supermassive black hole will take about a googol (!) years to evaporate (see here) — assuming that black holes can completely evaporate out of existence.
While I'm at it, is there any evidence that black holes attract dark matter? Not directly, no, since we can't directly detect dark matter. We know that galaxies attract dark matter, but we can't localize the attraction well enough to attribute specific parts of the attraction to black holes. However, since dark matter is attracted by gravity in general, there is no reason why it shouldn't be attracted by a black hole's gravity.
So light travels along the sphere, not through it? Yes, in this analogy.
How is such a structure infinite? It's not. An infinite space would be the infinite elastic string, or an infinite 2D sheet, or whatever. You can choose whichever you want, although the infinite case is currently regarded as more likely.
If the expansion is accelerating (as other posts have suggested) how could light catch up to us? The expansion has only recently begun accelerating. However, an accelerating expansion doesn't mean that you can't see the early universe, it just means that we can see less and less of the objects in the early universe.
The issue I'm having is that it seems pretty convenient that we're seeing light from a specific time (say the beginning of the universe). We can see light from all times (or rather, all times after the photon decoupling that created the cosmic background radiation, because the universe was opaque before then). Light from earlier times was emitted from points farther away. Light from the earliest times was emitted from the farthest points we can see, and spent the entire history of the universe traveling to us; we can't see points that are even farther away, because the light from them has not yet reached us.
That would seem akin to saying that if I look real hard I would be able to see the sun when it was created. To see something that happened 5 billion years ago (when the Sun was created), you would have to be able to look at objects that are billions of lightyears away, ones whose light is just now reaching us.
Slashdot Mirror
And supposedly something like 70% of the dark "matter" is energy. How does non-dark energy interact with electromagnetism, where the dark "stuff" does not? What is "non-dark energy"? Photons? I don't understand the question.
Sure, everyone would love it if we could detect dark matter particles directly — and if they interact non-gravitationally, we hopefully will someday. But what's silly is to claim that we have little reason to believe that dark matter particles exist.
The Schwarzschild radius isn't "emitted" from anything. It is simply a location in space, or rather a distance that defines a location. You can define a Schwarzschild radius for any object, even yourself. A black hole (roughly speaking) is a body whose matter has all collapsed into a region smaller than its own Schwarzschild radius. The Earth is not a black hole, because its matter is not contained within a region a centimeter across. Anyway, at a distance of 1 (current) Earth radius, a cm-sized black hole's gravity would not be any stronger than the current Earth's gravity; black holes don't suck things in more strongly than the objects they formed from. (Of course, the reason we are not falling into the center of the Earth is because we are standing on a solid surface; that would not be true for a black hole.)
I see so-called "hardcore" Unix geeks advocating 'ed'. Nonsense! The ultimate Unix editor is "cat >filename". All the others are for indecisive wimps who don't know what they're going to write, or incompetent losers who make mistakes.
The credits to "An Inconvenient Truth" were overlaid with a series of "things you can do to help" type suggestions, along with a URL for more information.
Regarding the breakup of the Antarctic Larsen ice shelf, Wikipedia has the following to say: "The leading ideas involve enhanced ice fracturing due to surface meltwater and enhanced bottom melting due to warmer ocean water circulating under the floating ice."
However, if you look at the Arctic (and Antarctic) in general, it is indisputable that the rate of melting increased substantially starting in the last 100-150 years, coinciding with global warming.
People don't forget the Sun. Certainly solar forcing is a major factor in climate models. However, variations in solar output alone can't explain the warming trend we currently see. See, for instance, this review.
I hope Part 2 remembers to cover Alternate Reality: The City (1985) and The Dungeon (1987) (Wikipedia). Those games were amazing for their time. AR had a raycasting engine 7 years before Wolfeinstein 3D, animated background scenery, weather and sun systems, great music with synchronized sing-along lyrics, character alignments, it tracked hunger/thirst/encumbrance/temperaturee/etc. The series had an ambitious Matrix-esque 6-game plot scripted out (only the first of which was made, in two parts). It even implemented garbage collection in a literal sense: if your inventory exceeded your free RAM, the Devourer came and ate some of your items at random. A review of the City tells more.
... the National DNA Database being built up by the British government (which includes material from many innocent people), would already allow the identification of those with milder predispositions to anger and violence. It then goes on to speculate about what the government could do with that information, but please note that the article says nothing about the British government seeking to jail people who haven't committed crimes. I am wondering what this claim is based on.Bound systems resist expansion. The expansion of space is derived under the assumption that matter in space has the same density everywhere, equal to the (very low) average density of the universe. Systems such as atoms, stars, or galaxies are much denser than that, and space inside them does not locally expand like intergalactic space does.
In most theories, space is taken to be uncountable. There are some discrete theories in which space is countable, but they don't have as much support.
Here is a mapping that takes the reals (-infinity,+infinity) to the interval (0,1) in 1-1 correspondence: arctan(x)/pi + 1/2. (The inverse tangent maps (-infinity,+infinity) into the finite interval (-pi/2,+pi/2), and the rest is just rescaling and shifting to fit into (0,1).)
What happens to them? What's the lifecycle? They sit around, and get bigger every once in a while whenever something falls into them. Same as ordinary stellar black holes. At some point do they get big enough to suck themselves into their own little inaccessable chunk of spacetime? There is no upper limit on the size of a black hole, but at some point it has sucked up most of the matter nearby, and doesn't thus grow much after that. Or does Hawking radiation manage to eventually make a black hole evaporate away? That's possible, but supermassive black holes radiate very weakly. (The bigger the hole, the less it radiates.) They can't evaporate at all until the cosmic background radiation cools to below the black hole's Hawking temperature; otherwise, they absorb energy from the cosmic background and grow. A supermassive black hole will take about a googol (!) years to evaporate (see here) — assuming that black holes can completely evaporate out of existence. While I'm at it, is there any evidence that black holes attract dark matter? Not directly, no, since we can't directly detect dark matter. We know that galaxies attract dark matter, but we can't localize the attraction well enough to attribute specific parts of the attraction to black holes. However, since dark matter is attracted by gravity in general, there is no reason why it shouldn't be attracted by a black hole's gravity.