Unfortunately, the cosmological constant aka. the 'dark energy' makes up 70% of the energy density of the universe according to boomerang experiment and several supernova studies.
Yeah, but the conclusions of cosmology are notoriously malleable. If you look at everything that's happened to our image of the universe in the last 50 or so years, I think it's pretty obvious that our current interpretation isn't something to be taken as gospel. Not that I could come up with anything better, but I think somebody will in the next decade or two.
Opinion is divided on this point. David Hume said you can never know things about the world for sure, because most of our knowledge (i.e. anything that is not, like some people think maths is, a priori) comes from inductive reasoning, rather than deductive reasoning which is guaranteed to preserve truth.
Deductive reasoning ain't all it's often cracked up to be. Pierre Duhem did a pretty good job of showing that deductive reasoning doesn't actually let you disprove much of anything in practice, because you can never test a single theory in isolation. You can always come up with some sort of auxilliary hypothesis that explains experimental data while preserving your pet theory. (Gaah! I just used something from that philosophy of science course! Good thing the final's on thursday, so I can forget all about this and go back to believing that logic can be applied directly to the real world.)
BZZZZT. Thanks for playing. When every fundamental interaction depends on the speed of light in a vacuum (all interactions are mediated by particles, which can travel no faster than c, within the precision allowed by Heisenberg), time damn well does slow down if c is reduced. Trouble is, c is a constant*. We can't modify it. (We can change the speed of light in a material, but that's just due to the interactions of the material with photons. It doesn't alter the speed of light in a vacuum)
I suggest you get some physics education beyond high school and/. before you go bitching other people out for correctly stating the laws of physics as we know them.
*Well, maybe. The theories which predict otherwise are not terribly well developed at this point, so I'll leave them out of the discussion for now.
I wonder how much of the cluster's processor time is wasted on the overhead for 64 copies of Win2k. It can't be that much, but I'd imagine a more efficient OS could speed things up a bit.
Anyways, did the simulator simply match reality here? Or was there something more to this?
The simulator provided an explanation of an odd aspect of reality, namely, that no small moons with eccentric orbits could be found in a certain large range of orbits around Jupiter.
You could also sit 3 miles away with a scoped.50 BMG rifle (available at your local gun show for under $3000) and put a half-inch hole in the liquid hydrogen tank just before the engines ignite. It looks like the ground restrictions are sufficient to make that impossible this time though.
I have to agree that this is a sensible move. At least watching shuttle launches isn't a constitutional right, like habeas corpus or public trials used to be.
All conductors oppose changes in magnetic fields by having currents induced which try to preserve the original magnetic flux through the conductor. If there was no magnetic field, and you turn one on, a current will be induced so as to oppose the field. In a non-superconductor, those currents don't flow for very long before ohmic resistance does its thing. In a superconductor, they flow until the external magnetic field is removed, and the original flux is achieved without an induced current.
Persistent currents, on the other hand, are created when you cool a superconducting ring (doesn't work with solid chunks because of the Meissner effect) below its critital temperature in a magnetic field, and then remove the external field. The superconductor has to maintain whatever magnetic flux through the ring that was present when it became superconducting, so a current is induced that mimics the external field. That current stays around until either the original external magnetic field is restored or the material ceases to be superconducting.
And I'd damn well better know what I'm talking about, since this will be on the final in 2 weeks.
(I didn't bother with much detail about the Meissner effect and its consequences. If you're confused about the persistent current part, I can explain that in detail.)
(through the ground, through towns, farmers' fields, through the Japanese people...)
More like through the ground, under towns, farmers' fields, under the Japanese people. I'm not sure how far underground the KEK is, but I know the Super-K is quite a ways down, and then there's that whole curved earth thing. The beam doesn't reach the surface until somewhere in the Sea of Japan, I think.
Quantum effects happen over distances that are significantly smaller than the 'diameter' of an electron, a couple orders of magnitude smaller than an atom, much less a 60+ molecule buckytube.
#include <IAAP.h>
Erm, no. There's really no set limit to the distances over which quantum mechanical behavior can be observed, though it's most often restricted to sub-molecular scales. Quantum mechanics is required to describe pretty much any behavior of single electrons accurately. The 'size' of an atom is determined by the uncertainty in position of its bound electrons. Molecular bonding is a quantum phenomenon. Carbon buckytubes exhibit superconductivity precisely because quantum effects manifest themselves over distances several times the interatomic separation. Bose-Einstein condensates form when all the particles of a piece of matter fall into the same quantum state (i.e. they differ only by position). Still, quantum effects are usually only seen in molecular and smaller scale systems.
As for molecular computing, the molecules in question are mid-sized organic molecules, 2 benzene-type rings with a sulfur atom at each end (according to the diagram in the earlier article). These are large enough that one doesn't have to worry much about each interacting with the next, except at absurdly low temperatures. Even so, each transistor is composed of many molecules in parallel, and the distance scale of the transistor as a whole is much too large to worry about quantum interactions between transistors screwing up your calculations.
How does a collider in Texas require modifications to a collider in Illinois? I find it hard to believe that the SCSC was dependent on Fermilab for a particle source. I think you're thinking of a completely different collider project.
The faster-than-light seeming aspect of it appears disturbing at first. But after a while, you realize that it occurs anywhere in quantum mechanics when a wave function collapses.
Think about it. Consider 2 polarized photons, 2 electron spins, 2 billiard balls, anything entangled such that a particular measurement performed on each always returns opposite results. When the system is set up, each object's probability of being, say, spin up, is 50%. The two spins are described by coupled wave functions, so that the 50% that corresponds to A being spin up also corresponds to B being spin down and vice versa. When one is measured, its wave function collapses into a single eigenstate, and its partner's wave function collapses into the other eigenstate. Thus, the final eigenstate of B is decided by the same measurement that measures the state of A.
This seems disturbing, the instantaneous change of B's wave function an arbitrary distance from A, when only A is being measured. But the simultaneous collapse of 2 coupled wave functions is mathematically no different from the collapse of a single wave function. When you have a particle with a large uncertainty in position, mesuring its position causes it to collapse to a single position eigenstate. If you have 2 detectors some distance apart, and use each to measure the presence or absence of the particle some very short time apart, you know that if you observe it at one, you won't observe it at the other. Say the detectors are 10m apart, and they take their measurements 1ns apart. If you detect the particle at the first one, you KNOW that the second won't detect it. But the 'information' about the wave function's collapse at the first detector would take 33ns to reach the second, if it travelled at the speed of light. So a single wavefunction's instantaneous collapse from all of space to a single point is just as much 'communication' as an entangled particle pair's simultaneous collapse.
So you have a choice: Either the entangled particles' behavior isn't that disturbing, any measurement of a quantum system is really disturbing.
There's also the issue of the resistance of the tire material to separation at the surface. Assume that above some maximum tangent force Ft the surface of the rubber falls apart, leaving part of itself on the road surface. (note the black streaks left on the road from a car moving with locked wheels) Ft is obviously proportional to the contact area, since a larger area must tear off more rubber. If this mechanism for breaking static friction is assumed, then the friction is dependent on area.
For cases when both surfaces remain intact, friction per unit area is dependent on pressure, so total friction is (constant*force/area)*area = constant*force.
If you assume that each surface is a series of circular arcs, instead of a zig-zag, then you get a similar result for static friction. If the surfaces are already moving, then they can't interlock as much as when static, and so the tangent force resulting from the normal force is reduced from that required to start from rest. But then this predicts that the sliding friction is a function of the extent of interlocking during movement, which depends on velocity. Oh well.
You CANNOT send information from one point to another using entanglement. You can generate the same completely random information at 2 separated places, though. The utility of entanglement is in the simultaneous generation and distribution of one time pads or keys for symmetric-key cryptosystems.
If you think of a series of coin flips being used to generate a key or one time pad, entanglement basically allows 2 coins to be made, such that when simultaneously flipped, they always land with opposite sides up. You can't control which side yours will land on, so you can't control which side the other will land on. You do know, however, that every time yours lands on heads, the other one landed on tails. So you and your friend each take a coin, and whenever you need to communicate, you both start flipping. One of you bitwise NOTs your data, then you encrypt and send the message. Your friend can then easily decrypt it with his key.
One pair of entangled particles can only be used for one flip, however. So if you want a real key, you need a continuous stream of entangled particle pairs from a single source. Small modifications to this system allow the easy detection of anyone eavesdropping on the entangled particle stream.
Re:Drone's defence capabilities
on
Robots Go To War
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· Score: 1
Yeah, but then they'd have to manufacture it. And set up a satellite communication system to support it. And have enough control of the airspace to give them useful service lifetimes. And have a large enough military to do something with the information from it. Any country with all of those already has the technology necessary to build one.
Besides, I don't think there's really a hell of a lot of ultra-modern technology in one of them. The reason they weren't developed earlier is just the military's sluggish adoption of anything new that isn't fast, shiny, and really fscking expensive.
First off, the current design holds 2 missiles. Secondly, it's not intended as a major battlefield weapon. This is intended to allow the UAVs to attack small targets of opportunity, which would otherwise disappear before attack forces could be called in. If you have more than a couple tanks in one place, you call in the big guns (or in this case, the AH-64s and F-15s). Chances are that a large group of tanks would have a hard time finding a hiding place before the attack force gets there (hiding one tank is hard enough, let alone 10 of 'em). However, you can bet that in 10 years or so there will be a UAV designed specifically for significant-sized attacks.
What puzzles me about the whole thing is how the military plans to apply it to Afghanistan. They don't have any tanks to speak of; their entire military is infantry. They probably have a few captured Russian tanks and APCs, but they're practically useless in that kind of terrain anyway. Using a laser-guided anti-tank missile to kill one guy with a 40 year old rifle seems an inadvisable way to conduct an attack. Oh well, I guess the military just wants to play with their new toys.
In the middle of the Pentagon courtyard, there is (or at least was) a hot dog stand called "Ground Zero." Does anybody know if that stand is intact, or is it buried in rubble, or something in between? I haven't been able to find any pictures of the Pentagon to indicate the state of the courtyard, so I can't tell.
Of course, my concern and condolances go out to all affected by this, but I can't help but wonder what happened at Ground Zero.
That's precisely what has me scared shitless. I'm just hoping that Bush and company can keep a level head through this, and not just start the traditional military dick-waving contest.
What's the difference between a viola and a trampoline?
You take your shoes off to jump on a trampoline.
Why do violists stand for long periods outside people's houses?
They can't find the key and they don't know when to come in.
Why do so many people take an instant dislike to the viola?
It saves time.
Why do violists leave their instruments on the dashboards of their cars?
So they can park in "handicapped" parking places.
Why shouldn't you drive off a cliff in a car with three violas in it?
You could fit in at least one more.
A violist in an orchestra was crying and screaming at the oboe player sitting directly behind him. The conductor asked, "What
are you so upset about?"
The violist replied "The oboist reached over and turned one of the pegs on my viola and now it's all out of tune!"
The conductor asked "Don't you think you're overreacting?"
The violist replied "I'm not overreacting! He won't tell me which one!"
In case you couldn't tell, I have an acquaintance who plays viola. These jokes, however, were plagarized from here.
Re:Tell us what we want to know!
on
Tractor Beam?
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· Score: 4
Well, the principle of operation of this (which only rotates objects) and optical tweezers (which move objects but can't rotate them) requires that the object refract light. Strictly speaking, there is no upper bound. But in order to produce a noticable force on a macroscopic object, you'd need far more light (several orders of magnitude) than would be necessary to incinerate said object.
The force results from the object changing the direction of propagation of photons, and thus changing their momentum. The recoil from this change is what moves the particle. The force is on the same order of magnitutde as the radiation pressure on an opaque object.
Re:You do not have the slightest clue
on
Fission in a Box
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· Score: 2
W-42 THERMONUCLEAR WARHEAD INSTRUCTIONS
BEFORE USE:
1. Remove cranium from anal cavity
2. Take a physics course or two ...
U-238 is fissionable. It is not fissile, so it is not used for fuel in reactors, except to be bred into Pu-239. Neutrons at the energies produced by fission reactions cannot induce fission in U-238. At that energy, neutrons either don't interact at all, or are captured, leading to beta decay. Neutrons from fusion do have enough energy to induce fission, though. When you look at the reactions, you see that a few grams of deuterium-tritium mixture can produce enough high-energy neutrons to induce fission in several hundred grams of U-238. I can't comment on the exact numbers (not a nuclear engineer), but the given figures definitely fit in order-of-magnitude estimates.
This is why H-bomb schematics always show a cylinder of U-238, with a rod of U-238 down the middle, and the intervening space full of deuterium and tritium, with a complete fission bomb at one end. I think there's usually a beryllium casing outside the uranium cylinder, but I'm not positive.
It is possible to create a bomb which derives almost all its energy from fusion, but such weapons are heavy and large for their yield. They have almost the same design as described above, but the U-238 jacket is replaced with tungsten or lead. One mostly-fusion device, the Tsar Bomba, derived 97% of its yield from fusion. It was intended, however, to have a U-238 jacket, to produce an even higher yield.
Most of this info was taken from the Nuclear Weapons FAQ, section 1.5. Here's one link, I'm sure Google can find more if you want.
I could see that causing cancer if you ate it or implanted it under your skin, but Am-241 emits alpha particles (no danger if outside the body), some low-energy x-rays (pretty low intensity), and a pretty wide selection of mid-low energy gamma rays (very low intensity). That's not too significant a hazard unless you wear it as jewelry or somethimg.
Slashdot Mirror
Yeah, but the conclusions of cosmology are notoriously malleable. If you look at everything that's happened to our image of the universe in the last 50 or so years, I think it's pretty obvious that our current interpretation isn't something to be taken as gospel. Not that I could come up with anything better, but I think somebody will in the next decade or two.
Well, there's that whole nuclear power thing for starters. It's kinda hard to design a reactor if you don't know how the hell it works.
Deductive reasoning ain't all it's often cracked up to be. Pierre Duhem did a pretty good job of showing that deductive reasoning doesn't actually let you disprove much of anything in practice, because you can never test a single theory in isolation. You can always come up with some sort of auxilliary hypothesis that explains experimental data while preserving your pet theory. (Gaah! I just used something from that philosophy of science course! Good thing the final's on thursday, so I can forget all about this and go back to believing that logic can be applied directly to the real world.)
I suggest you get some physics education beyond high school and /. before you go bitching other people out for correctly stating the laws of physics as we know them.
*Well, maybe. The theories which predict otherwise are not terribly well developed at this point, so I'll leave them out of the discussion for now.
Anyways, did the simulator simply match reality here? Or was there something more to this?
The simulator provided an explanation of an odd aspect of reality, namely, that no small moons with eccentric orbits could be found in a certain large range of orbits around Jupiter.
I have to agree that this is a sensible move. At least watching shuttle launches isn't a constitutional right, like habeas corpus or public trials used to be.
Persistent currents, on the other hand, are created when you cool a superconducting ring (doesn't work with solid chunks because of the Meissner effect) below its critital temperature in a magnetic field, and then remove the external field. The superconductor has to maintain whatever magnetic flux through the ring that was present when it became superconducting, so a current is induced that mimics the external field. That current stays around until either the original external magnetic field is restored or the material ceases to be superconducting.
And I'd damn well better know what I'm talking about, since this will be on the final in 2 weeks.
(I didn't bother with much detail about the Meissner effect and its consequences. If you're confused about the persistent current part, I can explain that in detail.)
More like through the ground, under towns, farmers' fields, under the Japanese people. I'm not sure how far underground the KEK is, but I know the Super-K is quite a ways down, and then there's that whole curved earth thing. The beam doesn't reach the surface until somewhere in the Sea of Japan, I think.
#include <IAAP.h>
Erm, no. There's really no set limit to the distances over which quantum mechanical behavior can be observed, though it's most often restricted to sub-molecular scales. Quantum mechanics is required to describe pretty much any behavior of single electrons accurately. The 'size' of an atom is determined by the uncertainty in position of its bound electrons. Molecular bonding is a quantum phenomenon. Carbon buckytubes exhibit superconductivity precisely because quantum effects manifest themselves over distances several times the interatomic separation. Bose-Einstein condensates form when all the particles of a piece of matter fall into the same quantum state (i.e. they differ only by position). Still, quantum effects are usually only seen in molecular and smaller scale systems.
As for molecular computing, the molecules in question are mid-sized organic molecules, 2 benzene-type rings with a sulfur atom at each end (according to the diagram in the earlier article). These are large enough that one doesn't have to worry much about each interacting with the next, except at absurdly low temperatures. Even so, each transistor is composed of many molecules in parallel, and the distance scale of the transistor as a whole is much too large to worry about quantum interactions between transistors screwing up your calculations.
And is it just me, or are the trolls lazy today?
How does a collider in Texas require modifications to a collider in Illinois? I find it hard to believe that the SCSC was dependent on Fermilab for a particle source. I think you're thinking of a completely different collider project.
Anything that results in fewer of 'em down here is a plus in my book! And send the the damn telephone sanitizers and hairdressers with them!
Think about it. Consider 2 polarized photons, 2 electron spins, 2 billiard balls, anything entangled such that a particular measurement performed on each always returns opposite results. When the system is set up, each object's probability of being, say, spin up, is 50%. The two spins are described by coupled wave functions, so that the 50% that corresponds to A being spin up also corresponds to B being spin down and vice versa. When one is measured, its wave function collapses into a single eigenstate, and its partner's wave function collapses into the other eigenstate. Thus, the final eigenstate of B is decided by the same measurement that measures the state of A.
This seems disturbing, the instantaneous change of B's wave function an arbitrary distance from A, when only A is being measured. But the simultaneous collapse of 2 coupled wave functions is mathematically no different from the collapse of a single wave function. When you have a particle with a large uncertainty in position, mesuring its position causes it to collapse to a single position eigenstate. If you have 2 detectors some distance apart, and use each to measure the presence or absence of the particle some very short time apart, you know that if you observe it at one, you won't observe it at the other. Say the detectors are 10m apart, and they take their measurements 1ns apart. If you detect the particle at the first one, you KNOW that the second won't detect it. But the 'information' about the wave function's collapse at the first detector would take 33ns to reach the second, if it travelled at the speed of light. So a single wavefunction's instantaneous collapse from all of space to a single point is just as much 'communication' as an entangled particle pair's simultaneous collapse.
So you have a choice: Either the entangled particles' behavior isn't that disturbing, any measurement of a quantum system is really disturbing.
For cases when both surfaces remain intact, friction per unit area is dependent on pressure, so total friction is (constant*force/area)*area = constant*force.
If you assume that each surface is a series of circular arcs, instead of a zig-zag, then you get a similar result for static friction. If the surfaces are already moving, then they can't interlock as much as when static, and so the tangent force resulting from the normal force is reduced from that required to start from rest. But then this predicts that the sliding friction is a function of the extent of interlocking during movement, which depends on velocity. Oh well.
If you think of a series of coin flips being used to generate a key or one time pad, entanglement basically allows 2 coins to be made, such that when simultaneously flipped, they always land with opposite sides up. You can't control which side yours will land on, so you can't control which side the other will land on. You do know, however, that every time yours lands on heads, the other one landed on tails. So you and your friend each take a coin, and whenever you need to communicate, you both start flipping. One of you bitwise NOTs your data, then you encrypt and send the message. Your friend can then easily decrypt it with his key.
One pair of entangled particles can only be used for one flip, however. So if you want a real key, you need a continuous stream of entangled particle pairs from a single source. Small modifications to this system allow the easy detection of anyone eavesdropping on the entangled particle stream.
Besides, I don't think there's really a hell of a lot of ultra-modern technology in one of them. The reason they weren't developed earlier is just the military's sluggish adoption of anything new that isn't fast, shiny, and really fscking expensive.
What puzzles me about the whole thing is how the military plans to apply it to Afghanistan. They don't have any tanks to speak of; their entire military is infantry. They probably have a few captured Russian tanks and APCs, but they're practically useless in that kind of terrain anyway. Using a laser-guided anti-tank missile to kill one guy with a 40 year old rifle seems an inadvisable way to conduct an attack. Oh well, I guess the military just wants to play with their new toys.
On the 12th of September, 2001
The 107th Congress of the United States of America hereby declares a state of war against, ummm, somebody.
In the middle of the Pentagon courtyard, there is (or at least was) a hot dog stand called "Ground Zero." Does anybody know if that stand is intact, or is it buried in rubble, or something in between? I haven't been able to find any pictures of the Pentagon to indicate the state of the courtyard, so I can't tell.
Of course, my concern and condolances go out to all affected by this, but I can't help but wonder what happened at Ground Zero.
That's precisely what has me scared shitless. I'm just hoping that Bush and company can keep a level head through this, and not just start the traditional military dick-waving contest.
You take your shoes off to jump on a trampoline.
Why do violists stand for long periods outside people's houses?
They can't find the key and they don't know when to come in.
Why do so many people take an instant dislike to the viola?
It saves time.
Why do violists leave their instruments on the dashboards of their cars?
So they can park in "handicapped" parking places.
Why shouldn't you drive off a cliff in a car with three violas in it?
You could fit in at least one more.
A violist in an orchestra was crying and screaming at the oboe player sitting directly behind him. The conductor asked, "What are you so upset about?"
The violist replied "The oboist reached over and turned one of the pegs on my viola and now it's all out of tune!"
The conductor asked "Don't you think you're overreacting?"
The violist replied "I'm not overreacting! He won't tell me which one!"
In case you couldn't tell, I have an acquaintance who plays viola. These jokes, however, were plagarized from here.
The force results from the object changing the direction of propagation of photons, and thus changing their momentum. The recoil from this change is what moves the particle. The force is on the same order of magnitutde as the radiation pressure on an opaque object.
BEFORE USE:
1. Remove cranium from anal cavity
2. Take a physics course or two
U-238 is fissionable. It is not fissile, so it is not used for fuel in reactors, except to be bred into Pu-239. Neutrons at the energies produced by fission reactions cannot induce fission in U-238. At that energy, neutrons either don't interact at all, or are captured, leading to beta decay. Neutrons from fusion do have enough energy to induce fission, though. When you look at the reactions, you see that a few grams of deuterium-tritium mixture can produce enough high-energy neutrons to induce fission in several hundred grams of U-238. I can't comment on the exact numbers (not a nuclear engineer), but the given figures definitely fit in order-of-magnitude estimates.
This is why H-bomb schematics always show a cylinder of U-238, with a rod of U-238 down the middle, and the intervening space full of deuterium and tritium, with a complete fission bomb at one end. I think there's usually a beryllium casing outside the uranium cylinder, but I'm not positive.
It is possible to create a bomb which derives almost all its energy from fusion, but such weapons are heavy and large for their yield. They have almost the same design as described above, but the U-238 jacket is replaced with tungsten or lead. One mostly-fusion device, the Tsar Bomba, derived 97% of its yield from fusion. It was intended, however, to have a U-238 jacket, to produce an even higher yield.
Most of this info was taken from the Nuclear Weapons FAQ, section 1.5. Here's one link, I'm sure Google can find more if you want.
I could see that causing cancer if you ate it or implanted it under your skin, but Am-241 emits alpha particles (no danger if outside the body), some low-energy x-rays (pretty low intensity), and a pretty wide selection of mid-low energy gamma rays (very low intensity). That's not too significant a hazard unless you wear it as jewelry or somethimg.