Oh I don't deny it's worth looking for failures of conservation laws, I mean, while you're doing other stuff. (Personally, I wouldn't hold my breath -- or count on making tenure -- while doing so.)
But I don't think there's any reason to formulate theories that explain the universe in the absence of conservation laws until evidence against them is found. Because conservation laws are pretty much the most useful general way of deriving physical law we have.
Dude...so far as I know a gauge change by definition is not measureable. Roughly speaking, it's just changing where your coordinate origin is, so it can't have an effect as long as conservation laws hold, and we're back to the same iron experimental fact: since conservation laws hold empirically, the laws of physics cannot change in time or over space.
I mean, you can certainly argue that they could change over bazillions of years or light-years, so that's why we "think" conservation laws hold -- that is, they only hold "locally" for some definition of "local" -- but what would be the point of this in the absence of experimental evidence?
Er...the Internet was designed in the 70s. So was Unix. The transistor was designed in the 1950s, ballpoint pens in the 1930s, penicillin in the 1920s...books in the Middle Ages...et cetera.
You seemed to have fallen victim to the classic American marketing myth that good ideas have a best-by date, and "newer" is always by definition "better."
The argument I've found most persuasive, and IIRC correctly from a Berkeley physics seminar umpty years ago by Hawking, shared by at least some first-rank cosmologists, is that the physical laws we have will ultimately prove to be the only possible logically consistent set.
That is, "alternate" universes are ipso facto impossible, because there is no other set of physical laws that are consistent with each other. And imagining them is somewhat like asking whether God can make a stone so heavy he can't lift it, or imagining being your own grandfather via a time-travel machine: a mere exercise in word-play, allowed only by the fact that English is a sufficiently illogical and ambiguous way of communicating that all kinds of nonsense can be put into words and "make sense" grammatically without making the least bit of sense logically.
As far as I know, there's nothing prohibiting a gradual gauge change over time and space.
You bet there is. All the conservation laws (e.g. conservation of energy, or of momentum) rely on the fact that physical laws do not change with time or position in space. If there was a "gradual change" in physical laws, e.g. if the constant in Coulomb's Law or Newton's Law changed slowly from position to position, or over time, then energy and momentum would not be conserved.
And, of course, the fact that energy and momentum are conserved has been verified experimentally in excruciating detail.
First, of all, imagining that one guy at the top is bringing the whole enterprise to its knees is just classic populist wishful thinking. It never works that way. Herbert Hoover didn't cause the Depression, Joe Stalin didn't by himself cause the Cold War, Alan Greenspan didn't cause the dot-com bust or the mortgage meltdown, and your Mikey G isn't by himself blocking all future progress in manned spaceflight.
Figuring out exactly how and why a program craps out is a matter for endless debate among historians, but as a general rule, it's probably reasonable to say that any government enterprise that doesn't enjoy phenomenally (and historically aberrent) high levels of public interest and support always craps out sooner or later.
So the first real problem is not who's heading NASA, but the cold ugly fact that most Americans don't give much of a hoot what NASA is doing, would rather watch American Idol than a manned Moon (or Mars) landing, and aren't much interesting in sending their tax dollars to Huntsville for umpty years so that their grandchildren can watch Right Stuffers frolic on the Red Planet. A plain fact, which most folks in the spaceflight industry strenuously try to avoid dealing with by all different types of denial. (Including, incidentally, the paranoid delusion that one single factor -- or man -- stands in the way of the type of broad and deep public support that the space program enjoyed in the brief and historically unique period between 1945 and 1965.)
But the second real problem is that a government program is almost certainly a dead-end nonroute to the kind of massive social and technological change that spaceflight enthusiasts hope spaceflight will produce. There is, actually, no recorded instance whatsoever in history of a government program doing anything more than starting off (at best) something like the colonization of other planets. The voyages of exploration during the 16th and 17th century, and the colonization of the New World in the 18th century, were weakly and inconsistently supported by national goverments: they were, in general, private enterprises, undertaken by individuals for individual dreams of wealth and glory.
That is what is missing in space exploration. There is no individual -- or small entrepreneurial organization -- path to space, and not much private, materialistic, "greedy" and "selfish" motivation for people to risk their fortunes, lives and honor getting into space. If such a thing were to emerge, then humans would naturally get off the planet, not only without any need for massive government programs, but probably in spite of government efforts to stop them. (It would be like MP3 file sharing. Notice no government program was required to get that going? Because it's intrinsically easy? Or because people really want to do it? I'm guessing the latter.)
But until that kind of broad interest emerges, I don't think any amount of government exploration is going to be anything more than expensive entertainment. (Mind you, I don't object to the entertainment, but that's because I personally would, weirdly, rather watch a manned Moon or Mars landing than every first-class gee-whiz movie that will be made from now to the end of time.)
It's worth asking whether government can prime the pump, so to speak, and make it easier for private enterprise and individual ambitions to get into space, so that people can start to get turned on to the whole business, and a broad and deep urge to go can emerge. Maybe it can. Unfortunately, probably step #1 is to back off the goofy noble selfless we came in peace for all mankind aura that clings to the endeavour nowadays, which merely serves to cut it off from the range of activities normal, non-selfless, non-noble people do everyday and think about doing tomorrow.
Matter is unbelievably thin out there. Roughly 1 atom every 10 cubic centimeters. By contrast the best vacuums we can produce on Earth (around a trillionth of an atmosphere) contain 250 million atoms in every 10 cubic centimeters.
It doesn't damage the spacecraft because, as anyone who has put out a candle flame with his fingers can tell you, it's not temperature that is dangerous but heat. Things with very little heat to transfer -- in this case, some unbelievably tiny amount of matter -- but at very high temperature, are harmless.
An analogous situation exists with respect to electricity: it isn't voltage per se that is dangerous to you but rather charge. Things that are at very high voltages (e.g. the static charge you built up when you scuff your shoes on a dry winter day) can be quite harmless if the amount of charge that can be transferred is very small, e.g. just a little spark.
The confusion exists in part because usually things at high temperature (or high voltage) have plenty of heat (or charge) to transfer, and then they are more dangerous than equivalent reservoirs at lower temperature (or voltage), because they transfer the lethal dose of heat (or charge) much faster.
They have two data points: where V1 and where V2 crossed into the heliopause.
And that's enough. If the heliosphere were a sphere, then by definition every point on its surface would be equidistant from the Sun. Now that they know two points are not equidistant from the Sun, they know it's not a sphere.
What they don't know is how big the "dent" is. It could be no bigger than the spacecraft, of course, although that is extraordinarily unlikely. Or it could be enormous. This is probably what you mean: they can't say exactly what shape the heliosphere is without lots of data points, outlining it.
However, only a few -- in the case of a sphere, just two -- data points are enough to say what shape it isn't.
Uh...that's all very interesting and stuff, but where exactly do you actually disagree with me? As I pointed out, we adjust the spelling of words for a variety of reasons, for reasons of consistency with ancient grammar, for euphony, because pronunciation patterns shift (cf. the changes in English pronouns), and so forth and so on.
My point was that objecting to the "o" in "blogosphere" merely because there was no pre-existing "o" in "blog" or "sphere" is overly pedantic. Do you disagree? Or are you one of those who fight about the correct way to spell "hemoglobin" and whether it's a sign of irredeemable boorishness to spell the plural of "formula" as "formulas?"
Why do I have to call it something? What's wrong with a descriptive phrase, which is what most of us call any particular area of work? What are you working on, son? Why, I'm working on spin polarization currents in conductors and semiconductors, gramps. Could be important stuff someday...
Nice and clear, descriptive, to the point. Don't need to go get the latest jargon dictionary and look up some newfangled buzzword.
Er...it proves that at least one august body charged with the evaluation of the significance of fundamental physics experiments decided that the results from the COBE experiment were far more significant than anything that's come from Hubble.
It's an appeal to authority argument, which means it rests on how well you think Nobel Prizes in physics correlate with the importance of the physics. For myself, I'd say the correlation is not as strong as one might like, but it's moderately strong.
Resistive heating only occurs when you have net charge current.
No, wait...I don't think this is right. You get plenty of heating from eddy currents, which don't cause a net transport of charge. Plus, it's not true that there is zero charge transport going on in a conductor at equilibrium. There are plenty of thermal fluctuations in the population of electrons near the Fermi surface, and these will give rise to short-lived currents as they relax.
However the trick, as I think someone else commented below, is that these fluctuations dissipate exactly as much heat as flows into the electrons from the lattice and surroundings, that is, everything is in thermal equilibrium, so while heat may flow back and forth from lattice to electrons, there is no net macroscopic dissipation of energy.
Maybe what you mean is you get resistive heating when you have nonequilibrium currents, currents driven by distributions of electrons that are out of thermal equilibrium.
But it seems to me that, from a purely thermodynamic point of view, a nonequilibrium flow is a nonequilibrium flow, and must dissipate, and its energy must end up as heat, no matter what it is that's flowing, charge, spin polarization, or whatever.
Which means the only way I can see the dissipation being slower is if it's hard for the order parameter (here, the charge polarization) to decay, because it couples poorly with whatever relaxation mechanisms there are. It's just that I would have thought that flipping electron spins is easy enough that spin polarization would decay just as easily as charge polarization. I suppose I must be wrong, but I wonder why?
Could be. But you're assuming the ship is being used at 100% capacity, and I'm not convinced that's true. No other means of cargo transportation -- airplanes, tractor trailers, trains -- is. They all have substantial down time sitting around in the yard, doing nothing. Which suggests, again, that the cost of just letting the beast sit there is not as high as the cost of a bad schedule that has stuff arriving at the wrong time. Any complex business operation depends more, I think, on precise scheduling than on making sure transportation is as flat-out fast as possible, or that resources are used to exactly 100% capacity.
To be sure. I didn't say it was zero cost to have the cargo hang out in the ocean a little longer; I just said it was probably dwarfed by the cost of having it arrive on the wrong day in port.
If the time in transit was the critical factor, they wouldn't be using boats, would they?
Thanks very much! Wish the PR release and abstract had been more informative.
What's the argument for there being potentially far less dissipative losses with pure spin currents, however? It's still going to interact with the lattice via spin-orbit and spin-spin coupling terms, no? You're still going to get resistive heating, no? Is it just the fact that the magnetic dipole interactions are much shorter range interactions than the Coulomb force? (Except wouldn't it be a screened Coulomb force in the lattice anyway?)
Er, I understand the derivation. I just think it's a silly word. How would one usefully distinguish what these folks are doing -- which as far as I can see just amounts to detecting an electric current in a funky magnetic way -- from working with polarized light, which is, forsooth, detecting the spin current carried by photons? Aren't they both "spintronics?"
I don't see the problem with "blogosphere," by the way -- the extra "o" is just added to make it roll off the tongue easier, and there's plenty of precedent for adding unobtrusive vowels to make combining forms (e.g. prefixes) out of nouns. Hence, Greek "psyche" (soul) becomes "psycho" with an extra "o" when put in front of words that begin with consonants, like "logia" (study) to get "psychology." Latin "crypt" (vault, cavern, from earlier Greek usage meaning secret) adds an "o" to become "crypto" when it's used as a prefix, as in "cryptogram" or "cryptofascist."
You'll want to share your wisdom with the Royal Swedish Academy of Sciences, then, since they awarded the 2006 Nobel Prize in Physics to Mather and Smoot for work carried out with the COBE satellite.
Now, the press release says the exciting thing about "spintronics" (ugh) is that " it frees one from the constraints of capacitive time constants and resistive voltage drops and heat buildup which accompany charge motion."
Well, fair enough; I can readily imagine that if you could get information to flow through a magical material without having to actually make electrons move, that would be great. No more of that pesky knocking into the lattice that they do which converts their motion into heat.
But...um...how exactly do you get a spin current without the electrons actually moving? I mean, given that the spins in question are nailed to the electron? Seems tricky. Like driving down the highway without having your car move...
Furthermore, if we read further down the abstract, we find this:
"NRL scientists first inject a spin polarized electrical current. . ..which generates a pure spin current flowing in the opposite direction. .."
Sounds to me like the existence of their spin current depends on the existence of an old-fashioned charge current. So how's this help? How is this a "key enabling advance" (as the press release calls it), still less a "major breakthrough" as the/. article excitedly and credulously calls it?
There not going to be stolen, they're going to be 'borrowed' by the adults who live with or near the children, which means the anti-theft software won't do a thing.
I think the OP has a reasonable concern. Just the chat and camera functions of these little widgets could make them very valuable to adults in certain very poor communities. Even worse, some of those who can most easily buy off (or threaten) the children and/or their parents, and who might want to use these widgets to conduct business, are the unpleasant types who run local criminal gangs.
There's ample precedent for donations to Third World countries ending up in entirely the wrong hands, unfortunately.
The shuttle was *never* an optimal launch vehicle, even before the safety issues came to light
Ah? What makes you say this? A direct connection to the mind of God? A little bird told you? It's just intuitively obvious?
The only facts we have are that the Shuttle has been able to deliver umpty tons of stuff and men to LEO -- way more than can possibly be lofted by any other launch system in the world -- but at a cost which is staggeringly higher than the original projected cost in the 1970s. From those facts two conclusions are possible:
(1) The Shuttle is a uniquely stupid and expensive way to get stuff to orbit, and [insert your favorite launch candidate here, e.g. expendable rockets, space elevators, linear accelerators on mountaintops, Jetson's flying belts] would be much cheaper, even in its v1.0 implementation.
(2) The cost of getting large chunks of stuff along with men to orbit by any method is a lot more expensive than people thought in the 1970s (or even now), and the Shuttle is merely the first system to demonstrate that ugly fact.
You're entitled to pick conclusion (1), as you have, but you should keep in mind your ideas about the obvious flaws in the Shuttle system are just as theoretical and just as unverified by measured fact as were the original ideas about the obvious superiority of a Shuttle system that its 1970s engineers had.
Personally, I'd say the lesson the wise person would draw from the Shuttle experience is not that folks in the 1970s were surprisingly dumb, but that getting reliable and economical and safe access to space is way trickier than anyone ever thought it would be. Which suggests substantial humility in guessing which clever new launch design is really going to be cheaper and safer than the Shuttle.
It's unlikely the radiation dose ever will be reduced enough because of limitations dictated by the laws of physics.
I call bullshit. What's your argument for this wild statement?
The new model year of a car seems a pretty good analogy.
That's your opinion, fair enough. I think you're wrong. A new model year rarely involves any advance in technology, efficiency, et cetera. Not even the equivalent of bumping up the processor speed by 20%. It's just marketing flash, as anyone whose bought a few cars in their life can tell you.
Furthermore, as I said originally, it's not this particular model of machine that I think represents a significant advance, it's the entire field of endeavor, of which this machine may represent merely a milestone. For people in the business, I'm sure this particular machine is not news, because they are aware of the general trend and this doesn't indicate a big bump or sudden change in the trend.
But my impression of the original comment was that it was made by someone who wasn't even aware of the importance of the general trend of multislice CT machines towards faster, better, and lower-radiation imaging, and thought vaguely that the whole trend was merely towards a more expensive and flashier way to diagnose knee problems in NFL running backs.
But....dude...what I said is that this general area of development is significant and important, even if this particular machine is just a minor milestone on the path. C'mon now, improvement is improvement, progress is progress.
Yes of course the radiation reduction and practically every other benefit is overplayed in TFA. These are journalists, right? They overplay everything; they live their entire lives with the emotion chip overclocked by 200%. That's how they got into a career that consists entirely of writing short, breathless sentences about celebrity abortions, the end of the world, and the paranoid fantasy du jour. This is why they couldn't concentrate on algebra in high school long enough to go into any kind of technical field, right?
Oh aye. That's why I mentioned the Russians. It's certainly a lot cheaper to find a disgruntled Russian corporal who doesn't mind earning ten years' salary in ten minutes by turning a blind eye than mounting a Mission : Impossible operation to steal a warhead.
That's also why I suggest an important use is to backstop the human component of nuclear stewardship. Put one of these guys next to the main gate of your storage depot, essentially. Even if SSgt. Ivan sells his soul for 30 pieces of silver, the alarm bells should go off when the unauthorized truck exits the main gate. The fortunate aspect here is that there already exists a fairly tight transportation network for high-grade fissionables in every nuclear nation, and monitoring at a fairly modest number of checkpoints could significantly improve confidence that nothing has leaked out of the system somewhere.
Imagine, for example, that you have one installed next to the end of the runway at each AFB. You'd never have the folly committed last month, when a bomber was accidentally loaded up with nuclear weapons and flew across the country with them, because little red lights would light up in the control tower as soon as the airplane taxied into position for take-off.
I realize none of this is mentioned in TFA. But why would anyone seriously involved in nuclear proliferation issues tell a fucking journalist his real intentions? Far better to trowel on a little entertaining bullshit about monitoring ports for smuggled suitcase nukes.
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Oh I don't deny it's worth looking for failures of conservation laws, I mean, while you're doing other stuff. (Personally, I wouldn't hold my breath -- or count on making tenure -- while doing so.)
But I don't think there's any reason to formulate theories that explain the universe in the absence of conservation laws until evidence against them is found. Because conservation laws are pretty much the most useful general way of deriving physical law we have.
I imagine the reply would be "mu."
Dude...so far as I know a gauge change by definition is not measureable. Roughly speaking, it's just changing where your coordinate origin is, so it can't have an effect as long as conservation laws hold, and we're back to the same iron experimental fact: since conservation laws hold empirically, the laws of physics cannot change in time or over space.
I mean, you can certainly argue that they could change over bazillions of years or light-years, so that's why we "think" conservation laws hold -- that is, they only hold "locally" for some definition of "local" -- but what would be the point of this in the absence of experimental evidence?
Er...the Internet was designed in the 70s. So was Unix. The transistor was designed in the 1950s, ballpoint pens in the 1930s, penicillin in the 1920s...books in the Middle Ages...et cetera.
You seemed to have fallen victim to the classic American marketing myth that good ideas have a best-by date, and "newer" is always by definition "better."
The argument I've found most persuasive, and IIRC correctly from a Berkeley physics seminar umpty years ago by Hawking, shared by at least some first-rank cosmologists, is that the physical laws we have will ultimately prove to be the only possible logically consistent set.
That is, "alternate" universes are ipso facto impossible, because there is no other set of physical laws that are consistent with each other. And imagining them is somewhat like asking whether God can make a stone so heavy he can't lift it, or imagining being your own grandfather via a time-travel machine: a mere exercise in word-play, allowed only by the fact that English is a sufficiently illogical and ambiguous way of communicating that all kinds of nonsense can be put into words and "make sense" grammatically without making the least bit of sense logically.
As far as I know, there's nothing prohibiting a gradual gauge change over time and space.
You bet there is. All the conservation laws (e.g. conservation of energy, or of momentum) rely on the fact that physical laws do not change with time or position in space. If there was a "gradual change" in physical laws, e.g. if the constant in Coulomb's Law or Newton's Law changed slowly from position to position, or over time, then energy and momentum would not be conserved.
And, of course, the fact that energy and momentum are conserved has been verified experimentally in excruciating detail.
First, of all, imagining that one guy at the top is bringing the whole enterprise to its knees is just classic populist wishful thinking. It never works that way. Herbert Hoover didn't cause the Depression, Joe Stalin didn't by himself cause the Cold War, Alan Greenspan didn't cause the dot-com bust or the mortgage meltdown, and your Mikey G isn't by himself blocking all future progress in manned spaceflight.
Figuring out exactly how and why a program craps out is a matter for endless debate among historians, but as a general rule, it's probably reasonable to say that any government enterprise that doesn't enjoy phenomenally (and historically aberrent) high levels of public interest and support always craps out sooner or later.
So the first real problem is not who's heading NASA, but the cold ugly fact that most Americans don't give much of a hoot what NASA is doing, would rather watch American Idol than a manned Moon (or Mars) landing, and aren't much interesting in sending their tax dollars to Huntsville for umpty years so that their grandchildren can watch Right Stuffers frolic on the Red Planet. A plain fact, which most folks in the spaceflight industry strenuously try to avoid dealing with by all different types of denial. (Including, incidentally, the paranoid delusion that one single factor -- or man -- stands in the way of the type of broad and deep public support that the space program enjoyed in the brief and historically unique period between 1945 and 1965.)
But the second real problem is that a government program is almost certainly a dead-end nonroute to the kind of massive social and technological change that spaceflight enthusiasts hope spaceflight will produce. There is, actually, no recorded instance whatsoever in history of a government program doing anything more than starting off (at best) something like the colonization of other planets. The voyages of exploration during the 16th and 17th century, and the colonization of the New World in the 18th century, were weakly and inconsistently supported by national goverments: they were, in general, private enterprises, undertaken by individuals for individual dreams of wealth and glory.
That is what is missing in space exploration. There is no individual -- or small entrepreneurial organization -- path to space, and not much private, materialistic, "greedy" and "selfish" motivation for people to risk their fortunes, lives and honor getting into space. If such a thing were to emerge, then humans would naturally get off the planet, not only without any need for massive government programs, but probably in spite of government efforts to stop them. (It would be like MP3 file sharing. Notice no government program was required to get that going? Because it's intrinsically easy? Or because people really want to do it? I'm guessing the latter.)
But until that kind of broad interest emerges, I don't think any amount of government exploration is going to be anything more than expensive entertainment. (Mind you, I don't object to the entertainment, but that's because I personally would, weirdly, rather watch a manned Moon or Mars landing than every first-class gee-whiz movie that will be made from now to the end of time.)
It's worth asking whether government can prime the pump, so to speak, and make it easier for private enterprise and individual ambitions to get into space, so that people can start to get turned on to the whole business, and a broad and deep urge to go can emerge. Maybe it can. Unfortunately, probably step #1 is to back off the goofy noble selfless we came in peace for all mankind aura that clings to the endeavour nowadays, which merely serves to cut it off from the range of activities normal, non-selfless, non-noble people do everyday and think about doing tomorrow.
Matter is unbelievably thin out there. Roughly 1 atom every 10 cubic centimeters. By contrast the best vacuums we can produce on Earth (around a trillionth of an atmosphere) contain 250 million atoms in every 10 cubic centimeters.
It doesn't damage the spacecraft because, as anyone who has put out a candle flame with his fingers can tell you, it's not temperature that is dangerous but heat. Things with very little heat to transfer -- in this case, some unbelievably tiny amount of matter -- but at very high temperature, are harmless.
An analogous situation exists with respect to electricity: it isn't voltage per se that is dangerous to you but rather charge. Things that are at very high voltages (e.g. the static charge you built up when you scuff your shoes on a dry winter day) can be quite harmless if the amount of charge that can be transferred is very small, e.g. just a little spark.
The confusion exists in part because usually things at high temperature (or high voltage) have plenty of heat (or charge) to transfer, and then they are more dangerous than equivalent reservoirs at lower temperature (or voltage), because they transfer the lethal dose of heat (or charge) much faster.
They have two data points: where V1 and where V2 crossed into the heliopause.
And that's enough. If the heliosphere were a sphere, then by definition every point on its surface would be equidistant from the Sun. Now that they know two points are not equidistant from the Sun, they know it's not a sphere.
What they don't know is how big the "dent" is. It could be no bigger than the spacecraft, of course, although that is extraordinarily unlikely. Or it could be enormous. This is probably what you mean: they can't say exactly what shape the heliosphere is without lots of data points, outlining it.
However, only a few -- in the case of a sphere, just two -- data points are enough to say what shape it isn't.
Uh...that's all very interesting and stuff, but where exactly do you actually disagree with me? As I pointed out, we adjust the spelling of words for a variety of reasons, for reasons of consistency with ancient grammar, for euphony, because pronunciation patterns shift (cf. the changes in English pronouns), and so forth and so on.
My point was that objecting to the "o" in "blogosphere" merely because there was no pre-existing "o" in "blog" or "sphere" is overly pedantic. Do you disagree? Or are you one of those who fight about the correct way to spell "hemoglobin" and whether it's a sign of irredeemable boorishness to spell the plural of "formula" as "formulas?"
Why do I have to call it something? What's wrong with a descriptive phrase, which is what most of us call any particular area of work? What are you working on, son? Why, I'm working on spin polarization currents in conductors and semiconductors, gramps. Could be important stuff someday...
Nice and clear, descriptive, to the point. Don't need to go get the latest jargon dictionary and look up some newfangled buzzword.
Er...it proves that at least one august body charged with the evaluation of the significance of fundamental physics experiments decided that the results from the COBE experiment were far more significant than anything that's come from Hubble.
It's an appeal to authority argument, which means it rests on how well you think Nobel Prizes in physics correlate with the importance of the physics. For myself, I'd say the correlation is not as strong as one might like, but it's moderately strong.
Resistive heating only occurs when you have net charge current.
No, wait...I don't think this is right. You get plenty of heating from eddy currents, which don't cause a net transport of charge. Plus, it's not true that there is zero charge transport going on in a conductor at equilibrium. There are plenty of thermal fluctuations in the population of electrons near the Fermi surface, and these will give rise to short-lived currents as they relax.
However the trick, as I think someone else commented below, is that these fluctuations dissipate exactly as much heat as flows into the electrons from the lattice and surroundings, that is, everything is in thermal equilibrium, so while heat may flow back and forth from lattice to electrons, there is no net macroscopic dissipation of energy.
Maybe what you mean is you get resistive heating when you have nonequilibrium currents, currents driven by distributions of electrons that are out of thermal equilibrium.
But it seems to me that, from a purely thermodynamic point of view, a nonequilibrium flow is a nonequilibrium flow, and must dissipate, and its energy must end up as heat, no matter what it is that's flowing, charge, spin polarization, or whatever.
Which means the only way I can see the dissipation being slower is if it's hard for the order parameter (here, the charge polarization) to decay, because it couples poorly with whatever relaxation mechanisms there are. It's just that I would have thought that flipping electron spins is easy enough that spin polarization would decay just as easily as charge polarization. I suppose I must be wrong, but I wonder why?
Could be. But you're assuming the ship is being used at 100% capacity, and I'm not convinced that's true. No other means of cargo transportation -- airplanes, tractor trailers, trains -- is. They all have substantial down time sitting around in the yard, doing nothing. Which suggests, again, that the cost of just letting the beast sit there is not as high as the cost of a bad schedule that has stuff arriving at the wrong time. Any complex business operation depends more, I think, on precise scheduling than on making sure transportation is as flat-out fast as possible, or that resources are used to exactly 100% capacity.
To be sure. I didn't say it was zero cost to have the cargo hang out in the ocean a little longer; I just said it was probably dwarfed by the cost of having it arrive on the wrong day in port.
If the time in transit was the critical factor, they wouldn't be using boats, would they?
Thanks very much! Wish the PR release and abstract had been more informative.
What's the argument for there being potentially far less dissipative losses with pure spin currents, however? It's still going to interact with the lattice via spin-orbit and spin-spin coupling terms, no? You're still going to get resistive heating, no? Is it just the fact that the magnetic dipole interactions are much shorter range interactions than the Coulomb force? (Except wouldn't it be a screened Coulomb force in the lattice anyway?)
Er, I understand the derivation. I just think it's a silly word. How would one usefully distinguish what these folks are doing -- which as far as I can see just amounts to detecting an electric current in a funky magnetic way -- from working with polarized light, which is, forsooth, detecting the spin current carried by photons? Aren't they both "spintronics?"
I don't see the problem with "blogosphere," by the way -- the extra "o" is just added to make it roll off the tongue easier, and there's plenty of precedent for adding unobtrusive vowels to make combining forms (e.g. prefixes) out of nouns. Hence, Greek "psyche" (soul) becomes "psycho" with an extra "o" when put in front of words that begin with consonants, like "logia" (study) to get "psychology." Latin "crypt" (vault, cavern, from earlier Greek usage meaning secret) adds an "o" to become "crypto" when it's used as a prefix, as in "cryptogram" or "cryptofascist."
You'll want to share your wisdom with the Royal Swedish Academy of Sciences, then, since they awarded the 2006 Nobel Prize in Physics to Mather and Smoot for work carried out with the COBE satellite.
Now, the press release says the exciting thing about "spintronics" (ugh) is that " it frees one from the constraints of capacitive time constants and resistive voltage drops and heat buildup which accompany charge motion."
.which generates a pure spin current flowing in the opposite direction. . ."
/. article excitedly and credulously calls it?
Well, fair enough; I can readily imagine that if you could get information to flow through a magical material without having to actually make electrons move, that would be great. No more of that pesky knocking into the lattice that they do which converts their motion into heat.
But...um...how exactly do you get a spin current without the electrons actually moving? I mean, given that the spins in question are nailed to the electron? Seems tricky. Like driving down the highway without having your car move...
Furthermore, if we read further down the abstract, we find this:
"NRL scientists first inject a spin polarized electrical current. . .
Sounds to me like the existence of their spin current depends on the existence of an old-fashioned charge current. So how's this help? How is this a "key enabling advance" (as the press release calls it), still less a "major breakthrough" as the
Nonsense. COBE was far more significant. There's much more to science than pretty pictures!
There not going to be stolen, they're going to be 'borrowed' by the adults who live with or near the children, which means the anti-theft software won't do a thing.
I think the OP has a reasonable concern. Just the chat and camera functions of these little widgets could make them very valuable to adults in certain very poor communities. Even worse, some of those who can most easily buy off (or threaten) the children and/or their parents, and who might want to use these widgets to conduct business, are the unpleasant types who run local criminal gangs.
There's ample precedent for donations to Third World countries ending up in entirely the wrong hands, unfortunately.
The shuttle was *never* an optimal launch vehicle, even before the safety issues came to light
Ah? What makes you say this? A direct connection to the mind of God? A little bird told you? It's just intuitively obvious?
The only facts we have are that the Shuttle has been able to deliver umpty tons of stuff and men to LEO -- way more than can possibly be lofted by any other launch system in the world -- but at a cost which is staggeringly higher than the original projected cost in the 1970s. From those facts two conclusions are possible:
(1) The Shuttle is a uniquely stupid and expensive way to get stuff to orbit, and [insert your favorite launch candidate here, e.g. expendable rockets, space elevators, linear accelerators on mountaintops, Jetson's flying belts] would be much cheaper, even in its v1.0 implementation.
(2) The cost of getting large chunks of stuff along with men to orbit by any method is a lot more expensive than people thought in the 1970s (or even now), and the Shuttle is merely the first system to demonstrate that ugly fact.
You're entitled to pick conclusion (1), as you have, but you should keep in mind your ideas about the obvious flaws in the Shuttle system are just as theoretical and just as unverified by measured fact as were the original ideas about the obvious superiority of a Shuttle system that its 1970s engineers had.
Personally, I'd say the lesson the wise person would draw from the Shuttle experience is not that folks in the 1970s were surprisingly dumb, but that getting reliable and economical and safe access to space is way trickier than anyone ever thought it would be. Which suggests substantial humility in guessing which clever new launch design is really going to be cheaper and safer than the Shuttle.
It's unlikely the radiation dose ever will be reduced enough because of limitations dictated by the laws of physics.
I call bullshit. What's your argument for this wild statement?
The new model year of a car seems a pretty good analogy.
That's your opinion, fair enough. I think you're wrong. A new model year rarely involves any advance in technology, efficiency, et cetera. Not even the equivalent of bumping up the processor speed by 20%. It's just marketing flash, as anyone whose bought a few cars in their life can tell you.
Furthermore, as I said originally, it's not this particular model of machine that I think represents a significant advance, it's the entire field of endeavor, of which this machine may represent merely a milestone. For people in the business, I'm sure this particular machine is not news, because they are aware of the general trend and this doesn't indicate a big bump or sudden change in the trend.
But my impression of the original comment was that it was made by someone who wasn't even aware of the importance of the general trend of multislice CT machines towards faster, better, and lower-radiation imaging, and thought vaguely that the whole trend was merely towards a more expensive and flashier way to diagnose knee problems in NFL running backs.
But....dude...what I said is that this general area of development is significant and important, even if this particular machine is just a minor milestone on the path. C'mon now, improvement is improvement, progress is progress.
Yes of course the radiation reduction and practically every other benefit is overplayed in TFA. These are journalists, right? They overplay everything; they live their entire lives with the emotion chip overclocked by 200%. That's how they got into a career that consists entirely of writing short, breathless sentences about celebrity abortions, the end of the world, and the paranoid fantasy du jour. This is why they couldn't concentrate on algebra in high school long enough to go into any kind of technical field, right?
Oh aye. That's why I mentioned the Russians. It's certainly a lot cheaper to find a disgruntled Russian corporal who doesn't mind earning ten years' salary in ten minutes by turning a blind eye than mounting a Mission : Impossible operation to steal a warhead.
That's also why I suggest an important use is to backstop the human component of nuclear stewardship. Put one of these guys next to the main gate of your storage depot, essentially. Even if SSgt. Ivan sells his soul for 30 pieces of silver, the alarm bells should go off when the unauthorized truck exits the main gate. The fortunate aspect here is that there already exists a fairly tight transportation network for high-grade fissionables in every nuclear nation, and monitoring at a fairly modest number of checkpoints could significantly improve confidence that nothing has leaked out of the system somewhere.
Imagine, for example, that you have one installed next to the end of the runway at each AFB. You'd never have the folly committed last month, when a bomber was accidentally loaded up with nuclear weapons and flew across the country with them, because little red lights would light up in the control tower as soon as the airplane taxied into position for take-off.
I realize none of this is mentioned in TFA. But why would anyone seriously involved in nuclear proliferation issues tell a fucking journalist his real intentions? Far better to trowel on a little entertaining bullshit about monitoring ports for smuggled suitcase nukes.