You could just as well ask: "how can an electric field line just stop somewhere?", and thereby conclude that there can be no such thing as an "electric monopole" (a positively- or negatively-charged particle). As long as the universe has no net electric or magnetic charge, all lines will terminate somewhere. If the universe did have a net charge the point is subtle, but that's irrelevant: the paper talks above pairs of opposite-pole monopoles created together, like a particle and its antiparticle. So this argument doesn't hold water.
Monopoles aren't impossible in principle (it would just be an extra term in Maxwell's equations) and are predicted in some theories, but fundamental-particle monopoles have never been observed. The summaries of this paper are confusing a lot of people: the authors are describing a crystal system with excitations that look like monopoles. They are NOT describing discovery of a new fundamental particle, but rather a new kind of solid-state phenomenon.
It's worth noting that Planck doesn't actually use a lot of liquid helium to cool itself down. It's cryogenic system is based upon "cryogen-free" mechanical refrigerators - the satellite launches warm, then cools itself down electrically and by radiating to space. The satellite lifetime isn't limited by running out of liquid helium.
Herschel, in contrast, does have a giant liquid helium tank. It launches full of helium, and eventually warms up when the tank runs out.
The "E-modes" and "B-modes" referred to in the article aren't quite the same as electric and magnetic fields. Here's the basic story.
Suppose you try to map the polarization of the microwave background across the sky. Each direction on the sky has some polarization magnitude and direction, which we can represent by a little headless arrows on the sky (headless because flipping the polarization 180 degrees doesn't change it). A map of the CMB polarization thus looks like a bunch of little line segments of varying sizes and orientations all across the sky.
Now imagine looking at the pattern of polarization directions near some point on the sky. This arrangement of lines can be "curl-free" if the lines are oriented radially or circumferentially around the central point; this is called an "E-mode" pattern. The polarization pattern might instead have a curl component, which is called a "B-mode" pattern. another way of looking at it: an E-mode pattern looks locally the same when mirror-reversed, while a B-mode pattern does not. Any field on the sky can be written as the sum of an E-mode pattern and a B-mode pattern.
This technicality is important because of how polarization is generated in the microwave background. It turns out that all kinds of relatively mundane processes can generate E-modes - they're still very interesting and informative, but we know they're there (and have even detected them). B-mode patterns are much more unusual - it turns out that normal CMB physics cannot generate large-scale B-modes. Inflation, however, generates a background of gravity waves in the early universe that produce a B-mode contribution to the CMB. This is incredibly tiny and difficult to detect, but it's a smoking gun for inflation.
It's worth noting that more than one such telescope hopes to probe CMB polarization on a similar timescale. Caltech and JPL are leading the BICEP2 and SPIDER collaborations (also with NIST), which will also be deploying in a few months (the former at the South Pole, the latter on a high-flying balloon) to probe E-mode and B-mode CMB polarization. The Princeton experiment mentioned in this article isn't that different - it just apparently has better press!
That's true: E=mc^2 is valid for moving particles if m is interpreted as the relativistic mass.
The grumbling comes about because physicists themselves almost never talk about relativistic mass in this sense anymore. Nowadays we usually say that a particle has an invariant mass m (its rest mass) which determines the relationship between its energy and momentum; E^2 = (mc^2)^2 + (pc)^2. That way a particle's mass has a single, well-defined value regardless of how fast it's moving. What you might call the "relativistic mass" I just call E/(c^2).
The two formalisms are completely equivalent, of course, but modern notation has swung toward defining "mass" as the rest mass only.
I'm generally bothered when folks trot out statistics claiming that the news media ran more negative articles/clips on one side of an argument than the other, and thus is hopelessly biased. What law of nature says that "fair" coverage has to have a balance between positive and negative for the two sides? If one side strays farther from reality on verifiable, important things, the news media should call them on that. The media shouldn't pick a side a priori, but it also has a responsibility to speak up when the facts are clear (which, admittedly, they aren't always).
That said, I'm not going to argue that there is no bias in the media, nor that the recent election cycle was completely fair. If nothing else, Obama had a huge structural advantage in news coverage because he was vastly "newer" in numerous different ways. I'm sure the personal views of the news staff play some role as well. This study of the Washington Post is unusually comprehensive and interesting.
The above should be taken as a more general rant about this kind of tit-for-tat comparison, whether trotted out by Fox News to attack the "liberal media" or in "balanced" science pieces where a crackpot gets as much airtime as legitimate science. I just don't find this general metric for judging bias particularly compelling.
The Standard Model assumes that all three neutrino species (electron, muon, and tau) are massless, and is essentially agnostic about whether neutrinos are their own antiparticles. If a neutrino is (is not) its own antiparticle, we call it a Majorana (Dirac) particle. Most any reaction which could tell the difference between Majorana and Dirac neutrinos can't occur in a Standard Model with massless neutrinos, so the difference is subtly and has no real experimental consequence.
We know the Standard Model is wrong, however. From neutrino oscillations, we know that neutrinos have tiny masses. This suddenly means that there ARE experimental consequences: neutrino-less double beta decay is possible for Majorana neutrinos but not Dirac neutrinos, for example. This is what EXO and many other experiments (GERDA, MAJORANA, CUORE,...) are looking for. Existing results aren't quite sensitive enough to tell the difference, but new ones may be.
Just because something is not part of the Standard Model doesn't mean that it's unpopular - we need to change the Standard Model somehow, after all, since it's wrong about neutrino masses! My impression (as a particle physicist, but not in this sub-field) is that most particle theorists actually expect neutrinos to be Majorana particles. There are very interesting theories based upon a scheme called the "see-saw mechanism" which can simultaneously explain why neutrinos have such tiny masses and why the universe has so much more matter and antimatter. If neutrinos are just boring old Dirac particles, it will be back to the drawing board!
There is definitely such evidence - it comes from Big Bang Nucleosynthesis. The idea is that the light elements (deuterium, helium, lithium) were produced when the early universe had temperatures conducive to fusion. This phase only lasted a few minutes, and the abundance of the light elements today depends sensitively on the conditions during this period. The abundance of deuterium tells us pretty clearly that the total mass of matter (which affected the temperature profile during nucleosynthesis) was much greater than the total mass of ordinary matter (which participated in the fusion process). Similar evidence comes from the cosmic microwave background.
Astrophysicists did not initially want to believe that the missing matter was exotic, but there's some extremely compelling evidence!
A correction to myself: As other posters note, there is no mention in this article of winos. They are possible dark matter candidates in other papers, however.
To clarify slightly, there are two kinds of dark matter. There is "dark ordinary matter", which is just gas and dust that we can't find. We've now found most of that. The vast majority of the missing mass, however, is NOT ordinary matter. This is the mysterious part.
There's a big distinction between the general dark matter theory and particular candidates for dark matter. The general picture is supported by numerous different lines of evidence: not just galactic rotation, but by gravitational lensing, the microwave background, structure formation, etc. It has been much more successful than any modified gravity theory thus far. It's a good model thus far, and we'll drop it if other observations come along.
There are literally hundreds of specific theories of dark matter's composition, however, and those are individually on shakier ground. These are mostly particle physics models emerging from the 1980s. There are an infinitude of papers and preprint suggesting this or that candidate and what signatures it could generate. They do all make predictions, however, and our observations are getting good enough to test many of them. Between astrophysics and particle accelerators we have a real chance of figuring this out (and the PAMELA observation seems unusually interesting!)... but there are a lot of overblown claims in the media in the meantime.
The charged wino would not be a reasonable dark matter candidate for just the reason you give: it would interact with light and we would have detected it by now. The dark matter candidate should be uncharged and thus a partner of an uncharged particle, e.g. a zino or photino.
There's a terminology issue, however (here comes the boring part). The electromagnetic (photon) and weak forces (W+/- and Z) are understood to be aspects of a unified electroweak force. In electroweak theory its more convenient to talk of 3 W fields (+/- and neutral) and one neutral B field. The photon is a mixture of the neutral W and B, the Z is another such mixture.
The most common dark matter candidate (the lightest neutralino) is a mixture of the supersymmetric partners of these particles: the neutral bino and neutral wino (and two neutral higgsinos). We could just as well say that we're mixing the photino and zino (and two neutral higgsinos), but bino and wino are more common terminology.
The paper is speaking about a dark matter candidate which is primarily the neutral wino, with a little admixture of the other states. Note that this doesn't mean the dark matter is composed of multiple different particles, just that the one particle it is composed of is "in-between" these labels.
I wouldn't say that the correlation is that bad, actually. It just looks bad the way they plotted it, with no error bars on the red sine wave. If you add the given error bar to that sine wave and imagine wiggling the data around correspondingly, the combined fit probably wouldn't be that bad.
They are extremely difficult to detect, but not impossible. Moreover, nuclear reactors produce quite a lot of them.
As a recent example, the KamLAND neutrino experiment (http://kamland.lbl.gov/) used a 1000 ton detector in Japan to study the flux of neutrinos emitted from dozens of reactors in Japan and Korea, some hundreds of miles away. KamLAND performed precision studies of the propagation of neutrinos over distance, and was also able to detect the rising and falling neutrino fluxes as various reactors powered up and down.
The detection device described in the article is much smaller, but it's located much closer to the reactor. I've heard talks on this, and it seems quite reasonable.
This has actually been a component of nearly every computer for many years. IBM apparently introduced the first commercial GMR-based hard drive in late 1997, a 16.8 Gigabyte model that at the time was among the largest commercially available. Pretty much any gigabyte-scale drive, and so essentially all drives available today, use GMR heads.
A large number of the complaints on this thread seem to amount to "I would never trust the internet for a presentation! Give me my trusty laptop any day!" I think these folks are missing the point of this product.
I work on a scientific collaboration that spans several institutions across the country. We use weekly teleconferences for specialized subgroups and occasional online meetings of the whole group to keep coordinated on what each other are doing. For these occasions we're always dealing with distributing presentations over the internet in a reasonable way. We usually post PDFs or HTML on the web, but we've had problems in the past with our own servers going down during telecons. We're also often editing our talks at the last minute, and we can have problems where someone downloads their PDFs before one of us posts his or her last changes.
For collaborations like ours this is a very intriguing product. I trust Google's servers more than my group's, to be honest, and we can always post backups on our own server. A consistent-looking presentation that I could easily edit right up to the last minute (or even collaboratively) is appealing. I grant that there are other solutions which may be better in some cases and that I'd probably never use this for a conference talk, but it's still intriguing for groups in our situation.
Dark matter is certainly a bizarre hypothesis, and the question you ask is natural - couldn't we just be wrong about gravity? It's somewhat easier to believe that Einstein's general relativity is modified than that the universe is filled with so much mysterious, unseen "stuff". This is the basic idea behind MOND (MOdified Newtonian Dynamics) theories, which has received a good bit of thought among physicists.
MOND doesn't look like the right solution, however. The last 2 or 3 decades have provided an enormous amount of observational data about the structure of the universe (large scale structure, galactic dynamics, gravitational lensing, light element abundances, the cosmic microwave background...), all of which is basically consistent with the simplest dark matter model ("cold dark matter") and inconsistent with any modified gravity theory. We don't need to imagine any particularly exotic properties to the dark matter, it just needs to be something that doesn't interact with electromagnetic forces (just like a neutrino only much heavier). Even very complicated MOND models fail to match observations, however (unless you add in a bunch of dark matter anyway).
Perhaps the most striking example is provided by observations of the Bullet Cluster last year. Basically we've found a pair of colliding galaxy clusters where the collision has separated the dark matter from the ordinary matter somewhat. Skipping over the details, this provides dramatic evidence that dark matter is real "stuff" - in essentially any modified gravity theory without dark matter, the gravitational forces still have to be coming from the same place as the visible matter! This is a very general argument, and observations like this have more or less put the nail in the coffin of MOND theories.
Astrophysicists are almost universally convinced at this point that something like dark matter exists. We're starting to map its distribution in detail throughout the universe, and the next major challenge will be determining its makeup - either by production in an accelerator or detection in dedicated experiments.
[Begin off-topic physics rant] The uncertainty principle is no magical addition to physics or mathematics - it's just a consequence of Fourier analysis. Quantum mechanics postulates (extremely successfully!) that "particles" have wavelike properties: a particle is better represented by some distribution function. Its position in space has to do with where the distribution function is greatest, and its momentum has to do with how fast the function wiggles (its Fourier frequency spectrum). The uncertainty principle is just the straightforward mathematical statement that no function can have a definite frequency (momentum) and a definite position at the same time. A function with a definite frequency is a sine wave over all space (no well-defined position), and a sharply-peaked function with a definite position has a broad Fourier spectrum (no well-defined momentum). If you work out the details for a general function you get a minimum value for the product of the spreads in position and momentum. The end - no magic, nothing to do with observers affecting the systems they observe.
It's true, however, that we don't yet understand how wavefunction collapse (the transition between distribution functions and single measurements, what you probably mean by "observer effects") fit into our mathematical models of the world. This doesn't mean it's impossible, however - people are even starting to have ideas. [End off-topic physics rant]
Fair enough - it was late and I threw in a bit of jargon there:) A bit of explanation:
(1) 1 GeV is approximately the proton mass, so this new particle is a bit over 5x the proton mass
(2) "Resonance" in this case means a feature in their data that looks like a new particle. When analyzing data from an accelerator, you basically add up the energies of all the particles coming out of a collision and histogram the result for a lot of collisions. If you see a peak in the histogram, it may mean that something interesting is happening at collisions of a particular energy, and such a peak is a signature that a particle is being created. The rumors related to a peak at ~180 GeV, which means it probably isn't the same peak that led to the discovery of the 5 GeV "cascade B" mentioned in this article.
(3) Dzero (or D0) is one of the two major detectors at the Tevatron particle accelerator (the other is CDF). They are the source of the rumors and of this new discovery.
(4) I say this is probably an "analysis issue", in that the 180 GeV feature could turn out to be an analysis mistake. It's probably being rechecked extensively by the folks working on Dzero, and they'll eventually let us know if it's real.
The article describes a new particle with a mass a bit over 5 GeV. This is interesting, but is very different from the supposed resonance at ~180 GeV appearing in the rumors from the Tevatron. It seems pretty unlikely these are related. We'll still have to wait and hear from Dzero on the original rumors (probably just an analysis issue).
I was at the announcement at the APS April meeting a couple of days ago. My impression and that of the other physicists I've talked to was that this was darn impressive, but in the end disappointing.
This is a project that has been rolling along for four decades. Over that time, many of the things this experiment was designed to test have been indirectly tested using observations about binary pulsars. Now they're getting hit by incredibly subtle systematics in their apparatus (note that the apparatus was not misconstructed or anything, there are just some surprises that were too subtle to measure until the thing actually reached space). The worry is that the experiment is now not so interesting, even if they managed to beat down their error bars through blood, sweat and tears. If they confirm the predictions of GR everyone will say "gee, great". If they don't, people will be concerned about how well they really understand their error bars. Either way, they don't make the splash one might have hoped all those years ago.
It's true that there are multiple scales to the dark matter problem, and that our arguments for exotic dark matter apply on the extra-galactic scale. I don't think theorists seriously argue that baryons solve the galactic dark matter problem, however. The Bullet cluster result (Google for Sean Carroll's excellent piece on this) tells us that the dark matter in galaxy clusters can't be baryonic either (it interacts too weakly with ordinary matter). The numbers we have from various experiments add up best if even galaxies are dominated by dark matter halos.
Slashdot Mirror
(Disclosure: I'm a physicist)
You could just as well ask: "how can an electric field line just stop somewhere?", and thereby conclude that there can be no such thing as an "electric monopole" (a positively- or negatively-charged particle). As long as the universe has no net electric or magnetic charge, all lines will terminate somewhere. If the universe did have a net charge the point is subtle, but that's irrelevant: the paper talks above pairs of opposite-pole monopoles created together, like a particle and its antiparticle. So this argument doesn't hold water.
Monopoles aren't impossible in principle (it would just be an extra term in Maxwell's equations) and are predicted in some theories, but fundamental-particle monopoles have never been observed. The summaries of this paper are confusing a lot of people: the authors are describing a crystal system with excitations that look like monopoles. They are NOT describing discovery of a new fundamental particle, but rather a new kind of solid-state phenomenon.
It's worth noting that Planck doesn't actually use a lot of liquid helium to cool itself down. It's cryogenic system is based upon "cryogen-free" mechanical refrigerators - the satellite launches warm, then cools itself down electrically and by radiating to space. The satellite lifetime isn't limited by running out of liquid helium.
Herschel, in contrast, does have a giant liquid helium tank. It launches full of helium, and eventually warms up when the tank runs out.
The "E-modes" and "B-modes" referred to in the article aren't quite the same as electric and magnetic fields. Here's the basic story.
Suppose you try to map the polarization of the microwave background across the sky. Each direction on the sky has some polarization magnitude and direction, which we can represent by a little headless arrows on the sky (headless because flipping the polarization 180 degrees doesn't change it). A map of the CMB polarization thus looks like a bunch of little line segments of varying sizes and orientations all across the sky.
Now imagine looking at the pattern of polarization directions near some point on the sky. This arrangement of lines can be "curl-free" if the lines are oriented radially or circumferentially around the central point; this is called an "E-mode" pattern. The polarization pattern might instead have a curl component, which is called a "B-mode" pattern. another way of looking at it: an E-mode pattern looks locally the same when mirror-reversed, while a B-mode pattern does not. Any field on the sky can be written as the sum of an E-mode pattern and a B-mode pattern.
This technicality is important because of how polarization is generated in the microwave background. It turns out that all kinds of relatively mundane processes can generate E-modes - they're still very interesting and informative, but we know they're there (and have even detected them). B-mode patterns are much more unusual - it turns out that normal CMB physics cannot generate large-scale B-modes. Inflation, however, generates a background of gravity waves in the early universe that produce a B-mode contribution to the CMB. This is incredibly tiny and difficult to detect, but it's a smoking gun for inflation.
It's worth noting that more than one such telescope hopes to probe CMB polarization on a similar timescale. Caltech and JPL are leading the BICEP2 and SPIDER collaborations (also with NIST), which will also be deploying in a few months (the former at the South Pole, the latter on a high-flying balloon) to probe E-mode and B-mode CMB polarization. The Princeton experiment mentioned in this article isn't that different - it just apparently has better press!
That's true: E=mc^2 is valid for moving particles if m is interpreted as the relativistic mass.
The grumbling comes about because physicists themselves almost never talk about relativistic mass in this sense anymore. Nowadays we usually say that a particle has an invariant mass m (its rest mass) which determines the relationship between its energy and momentum; E^2 = (mc^2)^2 + (pc)^2. That way a particle's mass has a single, well-defined value regardless of how fast it's moving. What you might call the "relativistic mass" I just call E/(c^2).
The two formalisms are completely equivalent, of course, but modern notation has swung toward defining "mass" as the rest mass only.
I'm generally bothered when folks trot out statistics claiming that the news media ran more negative articles/clips on one side of an argument than the other, and thus is hopelessly biased. What law of nature says that "fair" coverage has to have a balance between positive and negative for the two sides? If one side strays farther from reality on verifiable, important things, the news media should call them on that. The media shouldn't pick a side a priori, but it also has a responsibility to speak up when the facts are clear (which, admittedly, they aren't always).
That said, I'm not going to argue that there is no bias in the media, nor that the recent election cycle was completely fair. If nothing else, Obama had a huge structural advantage in news coverage because he was vastly "newer" in numerous different ways. I'm sure the personal views of the news staff play some role as well. This study of the Washington Post is unusually comprehensive and interesting.
The above should be taken as a more general rant about this kind of tit-for-tat comparison, whether trotted out by Fox News to attack the "liberal media" or in "balanced" science pieces where a crackpot gets as much airtime as legitimate science. I just don't find this general metric for judging bias particularly compelling.
The Standard Model assumes that all three neutrino species (electron, muon, and tau) are massless, and is essentially agnostic about whether neutrinos are their own antiparticles. If a neutrino is (is not) its own antiparticle, we call it a Majorana (Dirac) particle. Most any reaction which could tell the difference between Majorana and Dirac neutrinos can't occur in a Standard Model with massless neutrinos, so the difference is subtly and has no real experimental consequence.
We know the Standard Model is wrong, however. From neutrino oscillations, we know that neutrinos have tiny masses. This suddenly means that there ARE experimental consequences: neutrino-less double beta decay is possible for Majorana neutrinos but not Dirac neutrinos, for example. This is what EXO and many other experiments (GERDA, MAJORANA, CUORE, ...) are looking for. Existing results aren't quite sensitive enough to tell the difference, but new ones may be.
Just because something is not part of the Standard Model doesn't mean that it's unpopular - we need to change the Standard Model somehow, after all, since it's wrong about neutrino masses! My impression (as a particle physicist, but not in this sub-field) is that most particle theorists actually expect neutrinos to be Majorana particles. There are very interesting theories based upon a scheme called the "see-saw mechanism" which can simultaneously explain why neutrinos have such tiny masses and why the universe has so much more matter and antimatter. If neutrinos are just boring old Dirac particles, it will be back to the drawing board!
I stand corrected - I misunderstood the plot initially.
There is definitely such evidence - it comes from Big Bang Nucleosynthesis. The idea is that the light elements (deuterium, helium, lithium) were produced when the early universe had temperatures conducive to fusion. This phase only lasted a few minutes, and the abundance of the light elements today depends sensitively on the conditions during this period. The abundance of deuterium tells us pretty clearly that the total mass of matter (which affected the temperature profile during nucleosynthesis) was much greater than the total mass of ordinary matter (which participated in the fusion process). Similar evidence comes from the cosmic microwave background.
Astrophysicists did not initially want to believe that the missing matter was exotic, but there's some extremely compelling evidence!
A correction to myself: As other posters note, there is no mention in this article of winos. They are possible dark matter candidates in other papers, however.
To clarify slightly, there are two kinds of dark matter. There is "dark ordinary matter", which is just gas and dust that we can't find. We've now found most of that. The vast majority of the missing mass, however, is NOT ordinary matter. This is the mysterious part.
There's a big distinction between the general dark matter theory and particular candidates for dark matter. The general picture is supported by numerous different lines of evidence: not just galactic rotation, but by gravitational lensing, the microwave background, structure formation, etc. It has been much more successful than any modified gravity theory thus far. It's a good model thus far, and we'll drop it if other observations come along.
There are literally hundreds of specific theories of dark matter's composition, however, and those are individually on shakier ground. These are mostly particle physics models emerging from the 1980s. There are an infinitude of papers and preprint suggesting this or that candidate and what signatures it could generate. They do all make predictions, however, and our observations are getting good enough to test many of them. Between astrophysics and particle accelerators we have a real chance of figuring this out (and the PAMELA observation seems unusually interesting!)... but there are a lot of overblown claims in the media in the meantime.
No, there is no evidence that dark matter repels normal matter. In fact, it seems to attract normal matter through the same gravitational laws.
The charged wino would not be a reasonable dark matter candidate for just the reason you give: it would interact with light and we would have detected it by now. The dark matter candidate should be uncharged and thus a partner of an uncharged particle, e.g. a zino or photino.
There's a terminology issue, however (here comes the boring part). The electromagnetic (photon) and weak forces (W+/- and Z) are understood to be aspects of a unified electroweak force. In electroweak theory its more convenient to talk of 3 W fields (+/- and neutral) and one neutral B field. The photon is a mixture of the neutral W and B, the Z is another such mixture.
The most common dark matter candidate (the lightest neutralino) is a mixture of the supersymmetric partners of these particles: the neutral bino and neutral wino (and two neutral higgsinos). We could just as well say that we're mixing the photino and zino (and two neutral higgsinos), but bino and wino are more common terminology.
The paper is speaking about a dark matter candidate which is primarily the neutral wino, with a little admixture of the other states. Note that this doesn't mean the dark matter is composed of multiple different particles, just that the one particle it is composed of is "in-between" these labels.
I wouldn't say that the correlation is that bad, actually. It just looks bad the way they plotted it, with no error bars on the red sine wave. If you add the given error bar to that sine wave and imagine wiggling the data around correspondingly, the combined fit probably wouldn't be that bad.
They are extremely difficult to detect, but not impossible. Moreover, nuclear reactors produce quite a lot of them.
As a recent example, the KamLAND neutrino experiment (http://kamland.lbl.gov/) used a 1000 ton detector in Japan to study the flux of neutrinos emitted from dozens of reactors in Japan and Korea, some hundreds of miles away. KamLAND performed precision studies of the propagation of neutrinos over distance, and was also able to detect the rising and falling neutrino fluxes as various reactors powered up and down.
The detection device described in the article is much smaller, but it's located much closer to the reactor. I've heard talks on this, and it seems quite reasonable.
Air is pretty darn useful for carrying away heat by conduction and convection flows. So the XBox 360 should have more overheating problems in vacuum.
Physicists can't resist this kind of question...
This has actually been a component of nearly every computer for many years. IBM apparently introduced the first commercial GMR-based hard drive in late 1997, a 16.8 Gigabyte model that at the time was among the largest commercially available. Pretty much any gigabyte-scale drive, and so essentially all drives available today, use GMR heads.
A large number of the complaints on this thread seem to amount to "I would never trust the internet for a presentation! Give me my trusty laptop any day!" I think these folks are missing the point of this product.
I work on a scientific collaboration that spans several institutions across the country. We use weekly teleconferences for specialized subgroups and occasional online meetings of the whole group to keep coordinated on what each other are doing. For these occasions we're always dealing with distributing presentations over the internet in a reasonable way. We usually post PDFs or HTML on the web, but we've had problems in the past with our own servers going down during telecons. We're also often editing our talks at the last minute, and we can have problems where someone downloads their PDFs before one of us posts his or her last changes.
For collaborations like ours this is a very intriguing product. I trust Google's servers more than my group's, to be honest, and we can always post backups on our own server. A consistent-looking presentation that I could easily edit right up to the last minute (or even collaboratively) is appealing. I grant that there are other solutions which may be better in some cases and that I'd probably never use this for a conference talk, but it's still intriguing for groups in our situation.
Dark matter is certainly a bizarre hypothesis, and the question you ask is natural - couldn't we just be wrong about gravity? It's somewhat easier to believe that Einstein's general relativity is modified than that the universe is filled with so much mysterious, unseen "stuff". This is the basic idea behind MOND (MOdified Newtonian Dynamics) theories, which has received a good bit of thought among physicists.
MOND doesn't look like the right solution, however. The last 2 or 3 decades have provided an enormous amount of observational data about the structure of the universe (large scale structure, galactic dynamics, gravitational lensing, light element abundances, the cosmic microwave background...), all of which is basically consistent with the simplest dark matter model ("cold dark matter") and inconsistent with any modified gravity theory. We don't need to imagine any particularly exotic properties to the dark matter, it just needs to be something that doesn't interact with electromagnetic forces (just like a neutrino only much heavier). Even very complicated MOND models fail to match observations, however (unless you add in a bunch of dark matter anyway).
Perhaps the most striking example is provided by observations of the Bullet Cluster last year. Basically we've found a pair of colliding galaxy clusters where the collision has separated the dark matter from the ordinary matter somewhat. Skipping over the details, this provides dramatic evidence that dark matter is real "stuff" - in essentially any modified gravity theory without dark matter, the gravitational forces still have to be coming from the same place as the visible matter! This is a very general argument, and observations like this have more or less put the nail in the coffin of MOND theories.
Astrophysicists are almost universally convinced at this point that something like dark matter exists. We're starting to map its distribution in detail throughout the universe, and the next major challenge will be determining its makeup - either by production in an accelerator or detection in dedicated experiments.
[Begin off-topic physics rant]
The uncertainty principle is no magical addition to physics or mathematics - it's just a consequence of Fourier analysis. Quantum mechanics postulates (extremely successfully!) that "particles" have wavelike properties: a particle is better represented by some distribution function. Its position in space has to do with where the distribution function is greatest, and its momentum has to do with how fast the function wiggles (its Fourier frequency spectrum). The uncertainty principle is just the straightforward mathematical statement that no function can have a definite frequency (momentum) and a definite position at the same time. A function with a definite frequency is a sine wave over all space (no well-defined position), and a sharply-peaked function with a definite position has a broad Fourier spectrum (no well-defined momentum). If you work out the details for a general function you get a minimum value for the product of the spreads in position and momentum. The end - no magic, nothing to do with observers affecting the systems they observe.
It's true, however, that we don't yet understand how wavefunction collapse (the transition between distribution functions and single measurements, what you probably mean by "observer effects") fit into our mathematical models of the world. This doesn't mean it's impossible, however - people are even starting to have ideas.
[End off-topic physics rant]
Fair enough - it was late and I threw in a bit of jargon there :) A bit of explanation:
(1) 1 GeV is approximately the proton mass, so this new particle is a bit over 5x the proton mass
(2) "Resonance" in this case means a feature in their data that looks like a new particle. When analyzing data from an accelerator, you basically add up the energies of all the particles coming out of a collision and histogram the result for a lot of collisions. If you see a peak in the histogram, it may mean that something interesting is happening at collisions of a particular energy, and such a peak is a signature that a particle is being created. The rumors related to a peak at ~180 GeV, which means it probably isn't the same peak that led to the discovery of the 5 GeV "cascade B" mentioned in this article.
(3) Dzero (or D0) is one of the two major detectors at the Tevatron particle accelerator (the other is CDF). They are the source of the rumors and of this new discovery.
(4) I say this is probably an "analysis issue", in that the 180 GeV feature could turn out to be an analysis mistake. It's probably being rechecked extensively by the folks working on Dzero, and they'll eventually let us know if it's real.
The article describes a new particle with a mass a bit over 5 GeV. This is interesting, but is very different from the supposed resonance at ~180 GeV appearing in the rumors from the Tevatron. It seems pretty unlikely these are related. We'll still have to wait and hear from Dzero on the original rumors (probably just an analysis issue).
I was at the announcement at the APS April meeting a couple of days ago. My impression and that of the other physicists I've talked to was that this was darn impressive, but in the end disappointing.
This is a project that has been rolling along for four decades. Over that time, many of the things this experiment was designed to test have been indirectly tested using observations about binary pulsars. Now they're getting hit by incredibly subtle systematics in their apparatus (note that the apparatus was not misconstructed or anything, there are just some surprises that were too subtle to measure until the thing actually reached space). The worry is that the experiment is now not so interesting, even if they managed to beat down their error bars through blood, sweat and tears. If they confirm the predictions of GR everyone will say "gee, great". If they don't, people will be concerned about how well they really understand their error bars. Either way, they don't make the splash one might have hoped all those years ago.
It's true that there are multiple scales to the dark matter problem, and that our arguments for exotic dark matter apply on the extra-galactic scale. I don't think theorists seriously argue that baryons solve the galactic dark matter problem, however. The Bullet cluster result (Google for Sean Carroll's excellent piece on this) tells us that the dark matter in galaxy clusters can't be baryonic either (it interacts too weakly with ordinary matter). The numbers we have from various experiments add up best if even galaxies are dominated by dark matter halos.