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Oh come on. What Obama said is not what that mined quote implies he said. Any time you see politicians making a brief quote of something said by their opponents, your first impression should be that that quote was almost inevitably out of context, that what the opponent said was something very different than that out-of-context quote implies. Unfortunately, too many dumb members of the electorate fall for the quote mining; that's why politicians of all ilk use this tactic. Are you one of those dumb members of the electorate, or are you smarter than that? To quote you, Bull Squat. Much more thanks are owed to liberals than conservatives. The groups that work hardest to protect my right to freedom of speech, my right to vote, my right to a fair trial: Liberals. The groups that work hardest to constrain speech, constrain people's right to vote, constrain trials: Far right conservatives. Be careful what you wish for. You might just get it.

Maybe you should have said how that video relates to the topic of the thread. I am leaning toward Obama right now. If you are implying that my thinking of voting for Obama is somehow unpatriotic, I take extreme umbrage at your implication. I've worked on making the scariest weapons in the arsenal of the United States even scarier, and I proudly have two sons in the active military. Liberals and middle of the roaders (you, sir, are no middle of the roader) can be and are patriotic. We just aren't wacked about it.

What in the world does this song have to do with this thread?

Quote mining works. Evidence: Your post. You fell for Republican quote mining hook, line, and sinker. Quote mining is a favorite ploy by campaigners of all political ilk. For example, people in battle ground states almost certainly will see Democratic ads starring Mitt Romney saying he likes firing people. That, too, is quoting out of context. Regarding Obama's statement, you're right, it's bull. Only not the way you think.

What are you trying to use the accelerometers for? Your question is a bit too open-ended; you are asking us to write a book (multiple books!) and teach you the contents of an upper level undergraduate / graduate level class (multiple classes!). Do you know what a Kalman filter is?

Yes, and this is seen by mainstream physics and astronomy as yet another of many solid demonstrations of the existence of dark matter. You won't get much of the sense of that at this forum. This forum probably isn't the right place to ask your question.

What inconsistencies? Other than the fact that relativity is not consistent with Newtonian mechanics, there are no inconsistencies that we know of. With regard to that inconsistency with Newtonian mechanics: It's Newtonian mechanics that is wrong. Demonstrably wrong. As far as concepts tossed around, willy nilly, well this is a video narated mostly by Brian Greene. I'm not a big fan of him, or of Brian Cox, or of Michio Kaku. In my opinion, all three do a big disservice to science by spending a whole lot more time mystifying physics rather than explaining it. A good science popularizer demystifies science. These three do not do this. That's the general relativistic version of the twin paradox. It is demonstrably correct. You don't need a black hole. All you need is a somewhat large body such as the Earth. Take an atomic clock up a mountain and it will run faster; see http://www.wired.com/science/discoveries/news/2007/12/time_hackers/?currentPage=all. Put an atomic clock on an airplane and fly it around the world. The clock will run faster or slower than a ground-based clock depending on whether the plane flies east or west. This is the Hafele–Keating experiment; see http://en.wikipedia.org/wiki/Hafele–Keating_experiment. If you have a GPS receiver, it wouldn't work if the universe behaved in the Newtonian manner you erroneously insist on thinking the universe behaves. No, it doesn't. It is demonstrably false. It happens to be approximately correct on the surface of the Earth because clocks at rest anywhere on the surface of the Earth will tick at nearly the same rate. The gravitational time dilation from the tallest village in Tibet or the Andes to the Dead Sea is only measurable by atomic clock. Just because it is approximately correct on the Earth does not mean it is universal. It isn't.

iNow is correct. You cannot construct a universal "now". Simultaneity is relative.

It is obviously seven minutes of terror for the humans watching remotely from mission control in Pasadena, California. Entry, descent, and landing is a hand wringing time. Not quite so obviously, it is also seven minutes of terror for the humans who designed and built the Mars Science Laboratory entry, descent, and landing systems. Those systems have to be fully autonomous.

The Mars entry software is "just" control theory. You might need a masters degree or more to understand it, but it's not "AI"1. It's just math and physics, sensors and effectors. Guidance system software indicates where the spacecraft should be based on a pre-planned trajectory and navigated state. That navigated state comes from the navigation system software, which indicates where the spacecraft is based on sensor readings and prior navigated state. The navigated and guidance state are never quite in agreement. Getting the two into alignment is the job of the control system software. 1Is this AI? The AI community doesn't think so because it doesn't use techniques invented by the AI community. The guidance, navigation, and controls community however says what's the difference? It's not as if those techniques developed by the AI community are "intelligent". All AI techniques to date are "soft AI" (as opposed to "hard AI"). In the minds of the guidance, navigation, and controls community, those soft AI techniques are just math and physics, sensors and effectors, but just done badly.

Hi Greg, The answer A) is yes you would be weightless The answer B) is yes. Greg, ignore Alan. His answer to you was completely wrong. There's a basic problem with your question: Let us suppose that I am in a rocket capable of travelling at .9c, ... What do you mean by this? There's a problem with that 0.9c itself. With respect to what? The way you stated that "let us suppose" makes it appear that you think there is some universal reference against which one can measure velocity. There isn't. Everything is relative. One way to interpret your "let us suppose" is that you have a spaceship that is somehow capable of accelerating to 0.9c relative to the initial state of the spaceship. This acceleration of course takes time. You can't just instantaneously change velocity without violating several laws of physics. Unfortunately, I think you are doing just that (violating known physics) with your parenthetical remark (i.e. the gravitational attraction of the massive body is capable of propelling my rocket at the same speed as the engines). If that is the case, the answer to your question is "who knows?" This is a question along the lines of Suppose I have a device that violates all known laws of physics. What do the laws of physics say will happen when I use this device? The answer is "who knows?" I'm not saying that the known laws of physics are not sacrosanct. They are but an approximation of reality. But when you posit something that goes outside those laws, well, you've gone outside those laws. The known laws of physics no longer apply. The question just said so! I'll try to add some (known physics) realism to your question. One interpretation is that your spaceship was already going at 0.9c relative to this object well before it came into the gravitational influence of the object. (The gravitational influence does extend to infinity, but it's small, really, really small, at great distances.) In this case you'll zip through the object's sphere of influence so quickly that nothing much of interest will happen. Another interpretation is that your spaceship is falling straight into a black hole. Suppose you used your engines to keep the ship at rest with respect to the black hole at some constant distance from the black hole. Suddenly your engines fail and you don't get them back online until your velocity with respect to the black hole is 0.9c. You are screwed. Unless you have engines that violate the known laws of physics, that is. You might be okay if you happen to have engines that violate the known laws of physics (e.g., engines that can instantaneously transfer momentum to somewhere else), but now asking what the known laws of physics say will happen is a bit of a silly question.

In fact, the eruption of water vapor on the Saturn' icy moon, Enceladus is curious. Electromagnetic effect is everything? In fact, you write nonsense. I never said the electromagnetic effect is everything. That is the silly electric universe nonsense. What I did say is that ordinary matter is subject to gravitation and electromagnetism (and also the strong and weak interactions). It is those extra interactions beyond gravitation that enable ordinary matter to form interstellar gas clouds, stars, and planets.

First some words of advice. 1. Don't take anything posted by alpha2cen seriously. He or she spouts nonsense. 2. Stop with all the font silliness. It makes it very hard to quote what you wrote. Now to answer your questions. If 83% of all matter is Dark Matter, then why aren't our bodies,and everything on earth, and in our Solar System, 83% Dark matter? There's a misconception here. Just because dark matter is hypothesized to comprise 83% of all of the mass of the universe does not mean that dark matter comprises 83% of every bit of mass in the universe. Think of it this way: Just because 71% of the Earth's surface is water does not mean that the rock you just picked up is 71% water. If Dark Matter responds to gravity, then why is it distributed so unevenly? This is exactly backwards. It is ordinary matter rather than dark matter that is distributed so unevenly. The reason is that ordinary matter interacts with photons while dark matter does not do so. These electromagnetic interactions are what enables ordinary matter to clump up to form interstellar gas clouds, stars, and planets. Dark matter is distributed much more uniformly than is ordinary matter because dark matter only interacts gravitationally.

Nonsense. There is no centrifugal force in an inertial frame. The centrifugal force is a kind of fictitious force. Fictitious forces exists solely in the mind of a non-inertial observer. Fictitious forces cannot be sensed. Looking at orbits as a balance between centrifugal force and gravity is an absolutely lousy way to explain orbits. It doesn't work. The correct Newtonian explanation of why a uniform gravitational field cannot be sensed is that the gravitational acceleration of a test mass is independent of mass. A uniform gravitational field exerts zero measurable stress or strain on an object. A non uniform gravitational field such as the near-spherical field exerted by a planet on an object can be sensed, but only when the object is large enough / sensors are sensitive enough so as to make tidal gravitational effects observable. Newtonian mechanics is not the ultimate explanation of gravitation. General relativity is much closer to that ultimate explanation. The general relativistic explanation of why a uniform gravitational field cannot be sensed is very simple: Gravitation is a fictitious force in general relativity. Fictitious forces cannot be sensed. This isn't even nonsense. It isn't even a sentence.

Randomness does play a role, but it is not pure chance as is badly portrayed by young earth creationists. That portrayal is a straw man. Nobody says that abiogenesis and evolution happen purely by chance. That's a good question. There are lots of candidates, but finding evidence to test them is hard. Such evidence would require seeing signs of very small organisms preserved for about four billion years. Nobody has said randomness is not involved. However, nobody but those foolish young earth creationists are saying that it was pure randomness. What is organizing "them" is chemistry.

Try again. The first sentence does not make sense, and the latter is false if you mean zero gravitational acceleration.

I would say we haven't looked and that we don't have many places to look. The only planets in the Sun's habitable zone are Venus, Earth, and Mars. There's a lot more to habitability than merely being in a star's habitable zone. Venus has too thick an atmosphere. Mars, too thin. Neither Venus nor Mars has an active plate tectonics system. Mars might have been habitable long ago, and Mars does show some signs of having borne life long ago. It shows some signs that it still bears life right now. We do see life on the one planet that is habitable, and that life originated very shortly after conditions became hospitable for life. Judging from a sample size of one, maybe two, (which is all we've got), it appears that primitive life is very likely to arise if conditions are right. That said, judging from a small sample size is always an iffy proposition. We need more data.

You appear to be confused, alpha2cen, confused about the difference between rotation and revolution, confused about the difference between angular velocity and translational velocity, confused about superposition, and confused about what we do and don't "feel". I'll go over these one at a time. Rotation verus revolution Suppose you have a friend stand stationary, say facing north. You stand in front of your friend and spin in place. From your friend's perspective, you are rotating but you are not revolving. Next suppose you walk in a circle around your friend, but you always face north during this walk. Now your friend sees you as revolving but not rotating. Finally, imagine combining the two motions, spinning while walking around your friend. Now your friend will see you as both rotating and revolving. You are using the word "rotation" when you should be using "revolution" or "orbit". Rotation and revolution are very distinct concepts. Angular versus translational velocity The Earth's rotation rate is one 360 degree revolution per sidereal day. This rotational rate, 360 degrees per day, has units of angle divided by time. The Earth revolves about the Sun, completing an orbit (360 degrees) in one year. The solar system revolves about the Milky Way galaxy, completing an orbit in about 230 million years. These orbital rates, like planetary rotation, have units of angle divided by time. Angle divided by time is angular velocity. You answered swansont's question about angular velocity with translational velocity. The angular velocity of the solar system as it orbits the Milky Way is very, very small. Superposition of force NASA has occasionally placed satellites in orbit about the Moon. A pair of such satellites are orbiting the Moon right now. That these satellites are orbiting the Moon does not mean that the gravitational influences of the Earth, Sun, Milky Way don't influence the orbits of these satellites about the Moon, nor does it mean that they aren't also orbiting the Earth, the Sun, or the Milky Way. Those other bodies do perturb the orbits of those satellites about the Moon, and those satellites are in a sense orbiting the Moon, and the Earth, and the Sun, and the Milky Way. Forces are subject to the superposition principle in Newtonian mechanics, which means that forces add as vectors. What we "feel" Whether we feel gravitation depends on what you mean by "feel". If by "feel" you mean "subject to the force of", then the answer is yes. It's the superposition principle. If by "feel" you mean "sense", the answer is no. We don't "feel" gravitational force. We feel everything but gravity. The astronauts and cosmonauts aboard the International Space Station feel weightlessness even though the Earth's gravitational force on those astronauts and cosmonauts is about 90% of that on the surface of the Earth. What you are feeling as weight is the normal force of the Earth's surface pushing up on you. Take away this upward force such as by taking one of those zero-g rides at an amusement park and you too will feel weightless.

This is pretty much nonsense. Hubble's discoveries had absolutely no influence on Einstein's developments of his theory of special relativity or his theory of general relativity for the simple reason that this knowledge didn't exist at the time Einstein developed those theories. Einstein published his theory of special relativity in 1905. A bit more than a decade later he finally published his theory of general relativity in 1916. Up until the early 1920s, the Milky Way was assumed to be the universe. Edwin Hubble discovered that the Milky Way is one of many galaxies in the early 1920s. It wasn't until the late 1920s that Hubble also discovered that these remote galaxies are receding from ours, a decade after Einstein published his theory of general relativity. Also nonsense. The weak force is mediated by the W and Z bosons. The residual strong force, the force that binds neutrons and protons to form atom nuclei, is mediated by mesons. These weak bosons and mesons are massive; they cannot propagate at the speed of light. Getting back to the original post, the speed of light is the same to all inertial observers. This is one of the two postulates (aka assumptions) that form the backbone of Einstein's theory of special relativity. So what in the world motivated Einstein to make such an apparently ridiculous assumption, one that is in direct conflict with Newtonian mechanics? The answer is Maxwell's equations. Maxwell's electrodynamics theory and Newtonian mechanics strongly disagreed with one another. This disagreement was the Great Schism of late 19th century physics. Other physicists of that time attempted to whitewash away this disagreement. Einstein's insight was to simply take Maxwell's equations at face value: The speed of light is the same to all observers. This means that time and distance are not the absolute quantities envisioned by Newton. Instead, time and distance must be relative to the observer. It also means that velocities don't add vectorially. They merely appear to do so when velocities are small. The speed of the observer with respect to the source of an electromagnetic signal does come into play in that the frequency (and hence wavelength) of the observed signal is not necessarily that of the emitted signal.

Newtonian mechanics implicitly makes some assumptions about various physical behaviors that simply are not true. So what are these invalid assumptions? Just as a starter, Displacement is absolute. Newtonian mechanics assumes that a meter is a meter, which is not true. Length contraction is key consequence of special relativity. Time interval is absolute. Newton went into detail on absolute versus relative time in his scholium to his definitions, the portion of Newton's Principia that precedes his laws of motion. Newton's concept of relative time is still absolute. He assumes two observers equipped with ideal clocks will always agree on time intervals. They won't. The Hafele–Keating experiment demonstrated that this Newtonian assumption is false. Velocities are additive. That the speed of light is the same to all observers makes zero sense in the framework of Newtonian mechanics. Something had to give. That velocities add vectorially is one of the things that had to give. Mass can be broken into infinitesimally small pieces. Newton and Leibniz invented the calculus as an aid to advancing physics. The assumptions implicit in differentiation and integration don't jibe with the quantum nature of the universe. Position and velocity are knowable to infinite precision. You cannot know both position and velocity (momentum) to infinite precision. The Uncertainty Principle gets in your way. For each action, there is an equal but opposite reaction. Newton's third law fails subtlety with various electrodynamic forces, and fails miserably in the quantum domain. The very concept of "force" pretty much fails miserably in the quantum domain. With all these problems, why is it that Newtonian mechanics does work so well in our mundane world? The answer is these assumptions are very close to correct in the limit of largish positions (so as to avoid quantum mechanics), smallish velocities (so as to avoid special relativity), and smallish masses (so as to avoid general relativity). We don't see the deviations between reality and Newtonian mechanics in our mundane, everyday world because these deviations are incredibly small in this domain. They're hidden in the noise.

Just because people who don't know what pi is / how pi is defined ask this question doesn't mean that the question isn't nonsense. It is. Yes. The value of pi has absolutely nothing to do with the physical universe. I think what you are asking is "What if the circumference of a physical circle in our physical universe was 3*d rather than pi*d?" This is still a nonsense question, but it is getting close to being a very good question. The reason it's still nonsense is because space-time is a pseudo-Riemannian manifold and space is a Riemannian manifold. Space locally appears to be Euclidean. Here is the question you should be asking: "What if the circumference of a physical circle in our physical universe was not always pi*d?" or even better, "What is the shape of the universe?" The evidence to date says that our universe is flat, or very, very close to it. Whether it must be flat, whether some other universe might be curved: Nobody knows. It's a very good question.

+1 rep for such a nasty question. It's the nasty questions that test things. I think the answer is "it depends". If I go out and buy life insurance today, only to commit suicide tomorrow (or in most states, within two years), the life insurance company can dispute the claim. The same applies to my dying of cancer tomorrow. The life insurance company can still dispute the claim on the grounds that I withheld key information from them. On the other hand, if the life insurance policy has been in place for more than two or three years (depending on jurisdiction), even plain ordinary suicide is a covered death. How does assisted suicide change this equation?

I'm going to be quite contrarian here. Everyone is wrong. Well, almost everyone. Captain Panic's post #2 was correct. No, they don't. They have a different value for the ratio of the circumference of a circle to it's diameter. That ratio is not [math]\pi[/math]. Pi is a mathematical constant, not a physical constant. Mathematicians don't give a hoot whether the universe is Euclidean. By definition, [math]\pi[/math] is the ratio of the circumference of a circle to it's diameter in the Euclidean plane using the Euclidean norm. Alternatively, [math]\pi[/math] is the principal value of the inverse cosine of -1, or twice the principal value of the inverse sine of 1, or [math]4\int_0^1 \frac{dx}{1+x^2}[/math], or ... There are a bunch of different ways to express/calculate [math]\pi[/math]. Not a single one of these approaches involves the curvature of the physical universe. This is a nonsense question. Pi is not a physical constant. Ask a nonsense question and you will get a nonsense answer. My answer is 42.

It's not so much a straw man so much as unstated assumption that intelligent life is rare. Binyamin Tsadik made this assumption explicit in a subsequent post. I hold a similar view, that (extant) intelligent life is rare, exceedingly rare. It is, I think, the best answer to Enrico Fermi's question, Where are they?, and to Stephen Hawking's question, Why would they [uFOs] appear only to cranks and weirdos? It explains why we don't see any Dyson spheres, why we see stellar nurseries (what better place is there for von Veumann probes to reproduce?), why after fifty years of looking SETI has found nothing that is even close to definitive. Pessimistic? Why? It is perhaps pessimistic if intelligent life arises frequently but is inevitably doomed to a quick demise. However, if intelligent life arises but rarely, then this is a good sign. It means the galaxy is our oyster. It is often said? By whom? That's a "citation needed" kind of statement.

Chlorofluorocarbons are blamed for the creation of the ozone holes because they are the key culprit. That they do not occur naturally means we are the cause. Ozone chemistry 101: The ozone-oxygen cycle starts with high energy ultraviolet light splitting an ordinary oxygen molecule into a pair of free oxygen atoms. These free oxygen atoms quickly combine with other ordinary oxygen molecules to form ozone. Ultraviolet light, even low energy UV light, can cause an ozone molecule to split into a free oxygen atom and an ordinary oxygen molecule. The free oxygen atom once again quickly combines with ordinary oxygen to reform an ozone molecule. Ozone can also combine with a free oxygen atom to form two ordinary oxygen molecules. The rates at which these reactions occur drives the balance of ordinary oxygen, ozone, and free oxygen in the ozone layer. The free oxygen + ordinary oxygen reaction is very fast. This means that an ozone molecule will effectively hang around for a long time before recombining with a free oxygen atom to form a pair of O2 molecules. There are other sinks besides the O3+O -> 2O2 path. Ozone is very reactive. It can oxidize just about anything. This means that even under natural conditions, the ozone concentration in summertime will reach a steady state level in which ozone production balances ozone destruction. Now let's turn out the lights (i.e., wintertime). This removes the source of those free oxygen atoms. The ozone concentration will naturally deplete in wintertime. The problem with chlorofluorocarbons is that they greatly enhance the depletion of ozone. Sunlight can split off a free halogen radical from a chlorofluorocarbon molecule. It is that free halogen radical that does the damage. It can react with ozone, stealing a oxygen atom to form a halogen monoxide molecule and an ordinary oxygen molecule. This halogen monoxide molecule in turn reacts with a free oxygen atom to form a halogen radical and an ordinary oxygen molecule. Neither of these reactions requires sunlight. Note that the end result is halogen+O3+O->halogen+2O2. The halogen catalyzes the depletion of ozone and free oxygen, and it does it without sunlight. This reduces the peak ozone level in summertime but more importantly exacerbates the wintertime depletion (which peaks in early spring). So why is it worse in the southern hemisphere? The first thing to realize is that while the sources of chlorofluorocarbons are mostly in the northern hemisphere, the atmosphere is fairly well mixed at the top of the troposphere, even more so in the stratosphere. It doesn't matter where the sources are on the ground. Concentrations aren't the answer. The answer lies in the very different winter weather patterns over the Arctic and Antarctic regions. Antarctica is an isolated continent, completely surrounded by ocean. The Arctic is an isolated ocean, mostly surrounded by continents. A long-lived southern polar vortex sets up every winter over Antarctica. Air doesn't flow in, doesn't flow out. This vortex extends up into the stratosphere. The lack of mixing means that even under natural conditions, wintertime ozone depletion over Antarctica is much greater than it is over the Arctic. Add the enhanced depletion caused by chlorofluorocarbons and this natural depletion becomes severe. The northern hemisphere does occasion see ozone holes form because the northern polar vortex does set up every once in a while.