scale the whole project up, and easy access to sunken boats?
Unlikely, for this approach, at least. This experiment is relying on special interactions between the cornstarch suspension and the sound waves passing through it. Water behaves a lot more like an ideal liquid than a cornstarch suspension does, so it's not going to exhibit the same effect.
There are other patterns of motion you could induce in water to get access to sunken ships (e.g. dump in lots of sound and put an antinode in the channel volume, or set up currents to produce a vortex). However, it's probably cheaper just to go down there with submarines and/or scuba gear.
If you need an open-air environment to do the recovery properly (which would actually be _bad_ for most shipwrecks), pressurize an inflatable dome down there (using suitable gas mix instead of ordinary air if at any significant depth). Your divers are already acclimatized to the pressure, so no ultra-strong submarine hull is needed (keep dome pressure slightly over ambient).
Next question: how the heck do you control the spin of individual baryons in a nucleus?
You fire something at the nucleus and isolate the ones where one of the outer-shell nucleons was bumped up to the energy state you want.
If you fire X or gamma rays at the nucleus, you should only be able to excite very short-lived isomers (if it is boosted by absorbing a photon, it can decay by emitting a photon). Firing things like electrons or protons at the nucleus can excite states that don't have a single-photon decay path. These can be metastable.
We do the same thing in HeNe lasers. Helium atoms are excited to a metastable state by electric discharge, and after a while interact with neon atoms, putting them in a state suitable for lasing (target state of neon has almost exactly the same energy as the metastable helium state, so the exchange happens easily).
And perhaps they got a head start on us, and that large percentage of "dark matter" actually consists of Dyson Spheres which capture everything, so are "undetectable" by us.
Two problems with this scenario:
The spheres would be detectable in non-visible bands. Even a Matrioshka style nested dyson sphere would have an outer temperature above the microwave background level (and would in fact stick out like a sore thumb in the microwave background surveys). In practice, you quickly run into diminishing returns with mass vs. power flux, so a Matrioshka's outer shell would be somewhere in the intermediate IR range. So, you'd get what looked like a red supergiant, only more so (IR super-duper-giant).
Most of the dark matter is "non-baryonic". While the astrophysics version of the term has a different meaning than the particle physics version, it still means that whatever dark matter is, most of it is _not_ the kind of matter we're familiar with. It has to be something that doesn't interact via the EM force (hence the focus on EM-neutral particles that interact via the weak nuclear force, mainly because that's the easiest type to detect once EM is ruled out).
It does look very fake. Even stranger is the fact that the thumbnail on the left shows the same picture with a red/white/blue parachute as described in the article. The large picture shows yellow/green/red.
Actually, it looks like it's missing the blue channel from the entire picture for some reason. That explains the color discrepancy, and why the helicopter is such a wonderfully eye-hurting shade of red.
OO.o has a native "export to PDF" feature in the latest builds. You might want to give it another whirl. Might save some (potentially) messy LaTeX work, at least for non-methematical documents.
I use LaTeX by preference. Word is used when I _can't_ send PS or PDF (usually when I'm collaborating on a document with other people who only use Word).
Using OO to build PPT presentations and exporting as PDF is tempting, though.
Why would I change back from a decent, FREE, application like OpenOffice to WordPerfect? If they're planning on selling it on the name, or because people remember WP, it's too late for that now. OpenOffice has taken over, and could soon be challenging MS Office in a big way.
OpenOffice will convince me to abandon Office when it stops mangling fonts and layout for the Word documents people keep sending me. I can read them, but they don't look pretty, and I'm sure as heck not going to _write_ anything in OO while this is a concern.
Bad install? Maybe. But I've run into the problem in two unrelated *nix labs where it was installed. I suppose _both_ admins _could_ be sloppy, but they've been pretty sharp in other regards.
If I'm writing documents in *nix, I use LaTeX and send people postscript or PDF. But when I need to give someone a Word document, or bring a PowerPoint slide show to a conference, I use MS Office.
The biologically equivalent dose for humans, the REM (radiation equivalent in man), is just the dose of radiation times the RBE. So alpha rays have at least ten times the relative biological effectiveness than X-rays.
You are both right.
Alpha particles do more damage, but only if produced by ingested substances. From external sources, they won't penetrate the layer of dead skin on the surface of your body.
Heavy ions behave similarly (at least when in the same energy range).
Betas have a penetration distance of at least several millimetres, so they're definitely an external hazard (first poster was hazy on that).
The real danger at sites of nuclear accidents (or bomb tests, etc) is inhaling radioactive dust. That can get close enough to live tissue to give you lung cancer, and anything soluble can pass into the bloodstream and do more damage.
The danger from nuclear reactors and from long-term waste storage is from soluble radioactives getting into the local water supply and being ingested that way. This is why power plants have multi-stage heat exchange systems and why proposed waste storage sites are at the bottom of mines in non-porus rock, or under a few hundred feet of clay at the bottom of the ocean.
Can someone list any meaningfull danagers of GM food, preferably with something that resembles proof. I'm not trolling for either side here I'm simply curious.
The main reasonable objection I've heard is that, because you're splicing genes from wherever you please, you can no longer tell by inspection whether or not you'll be allergic to any given food. While the "splicing fish genes into vegetables" is an extreme example, it gets the concept across. IMO, this isn't likely to occur accidentally (you know what genes you're copying, and so would know when you're copying something that codes for an allergen). However, it would still occur, and so presents a concern.
A secondary objection is that it's very difficult to grow samples of an engineered crop without it spreading out of the controlled area or cross-pollinating with other nearby compatible plants. This means that if you do, for instance, engineer a strain of wheat that makes anyone with a peanut allergy keel over and die, there's a significant risk of that strain propagating into mundane wheat fields, with un-fun results. Engineered strains are usually specifically designed to be hardier than normal strains (that's why we're engineering them), so they will be competitive with normal strains in the field.
That having been said, I think that genetically engineered crops are inevitable, and mostly beneficial. When this becomes a tried-and-true technology instead of an experimental one, the fuss should die down.
Short answer: FPGAs let you build using basic gates and (very small) lookup tables. This lets you build anything you please, and fully optimize the number of functional units of each type that you have, but has a speed and size penalty.
This chip is basically a RISC processor with an FPGA-type fabric bolted on as a co-processor, as far as I can tell from the detail-poor press release. By implementing most of the instruction pipeline as fixed, optimized hardware, it runs without any of the penalties of a purely FPGA-based implementation. When you have a number-crunching task that would benefit from a custom logic implementation enough to offset the performance penalty of implementing it in programmable logic blocks, the compiler configures the programmable logic into a suitable coprocessor which is stuck in as an extra branch of the instruction pipeline.
How much benefit you get from this depends on what you're doing. Modern general-purpose microprocessors have enough vector instructions to handle most DSP-ish tasks without an abysmal speed penalty (just a large size and power penalty over a purely DSP-based implementation). Most computing tasks aren't limited by processing horsepower at all - they're either waiting for memory accesses to complete (even cache accesses are very slow compared to register accesses), or they're waiting for the target address of a branch to be decided (speculation and BTBs don't address this perfectly by a long shot). A reconfigurable processor would suffer from much the same type of problem. While using the programmable logic path for slice processing could remove some of the branching penalties (by following all paths and selecting the desired result), this would be at an even greater area and power cost.
For specialized applications, it would be quite useful, of course.
A quick glossary of terms being thrown around, for anyone confused:
FPGA - Field Programmable Gate Array. This is a combination of lookup tables, sum-of-products combinational logic blocks, and scratch-pad SRAM that you can hook up in nearly arbitrary ways to produce custom circuits at a gate level. Bulky and slow, but good at implementing algorithms efficiently. Configuration information is loaded from a serial PROM chip at startup, letting you change it relatively easily.
CPLD - Complex Programmable Logic Device. Like an FPGA, but stores configuration information internally, so you need to take out the CPLD and burn it to change configuration instead of re-burning the configuration PROM.
PLA/PLD - Programmable Logic Array/Device. Little cousin to CPLD. This is what you played with in second or third year. Typically these are just a sum-of-products combinational logic block with a register stuck on the end to latch the output. Useful as glue logic.
ASIC - Application-Specific Integrated Circuit. This is an integrated circuit that's half-made. A number of gates and registers and so forth have been fabricated on the chip, and the lowest few metal layers have been used for internal routing for these, but you get to define the upper metal layers to form arbitrary connections among these (either as the last fabrication step, or by laser-cutting a pre-fabricated wiring mesh to leave the geometry you want). Works much like a CPLD, but the design is decided at fabrication time and cannot be changed. Faster and less bulky than a CPLD implementation.
Standard cell design. This is a custom-fabricated integrated circuit that uses cells from a standard library of components, usually automatically placed and routed from a VHDL or Verilog description of what you want the chip to do. Faster than an ASIC if you have good place and route software, but more expensive in small quantities because you're making what amounts to a full custom chip. Design time is much less than a fully custom design would be, though (but verifying that the design description is correct is a royal pain).
I hope this clears things up for anyone who was confused.
The problem I see with this bubble stuff is that they detect it by the emission of neutrons. Anything which gives out lots of neutrons is going to have many of the problems of fission - any plant big enough tobe useful will need shielding and will produce nasty waste makeing decomissioning expensive.
ObOldJoke: Some years ago, a student paper allegedly proposed that the fusion reactor architecture most worth pursuing was a Gravitational Confinement design.
The kicker is that he may be right (you could power a state with a solar farm the size of a medium-sized city, if you had cheap enough bulk power storage, while fusion plants are likely to be very expensive to build and maintain). We'll find out in 50 years or so.
Whatever happened to the idea of superconducting power rings? Did the magnetic flux intensity just prove too great for practical use?
The main problem with superconducting storage is that high-temperature superconductors break down at relatively modest [for power storage] field strengths (in the range of 1 Tesla), and liquid helium cooled superconductors are only somewhat better (best I've heard is 8 Tesla for particle accelerator magnets cooled to 1.8K).
It turns out that the situation is even worse, though. Even if you spec magical materials for the field coils - like, say, zero resistance and the strength of carbon nanotubes (120 GPa) - energy per unit coil mass is much worse than the energy density you'd get with a comparable amount of chemical fuel. The same turns out to apply to flywheels (even constructed of magical ultra-tensile materials), and most of the other proposed schemes I've heard of.
This means that chemical storage schemes like fuel cells are likely to win out over more exotic storage mechanisms for the near to intermediate future. While you could build a storage plant using hydrogen fuel cells, the holy grail would be to get something like methanol working as efficiently (you can use it as a fuel easily enough - it's rebuilding it from the combustion products that's the problem).
If there was a demand for these items, even a perceived one, they'd get produced. But there isn't.
Now, a 50% solar cell...?
While demand in the alternative energy market is iffy right now, you'll definitely have demand for more efficient photovoltaics at _any_ price in the space industry.
Lifting mass into space is expensive. If you can get a 2-3x improvement in power to weight ratio of your solar arrays using materials like this, the world will beat a path to your door even if you don't have a way to grow it epitaxially.
As for long-term cost prospects, we're already mass-manufacturing similar highly-mismatched alloys for LEDs (anything with Ga/N/As as constituents, for one), so I'm confident costs will come down eventually.
Not to belittle the man's efforts, but a raytracer is *incredibly* simple, algorithmically... hell, I could (and did) write a basic one that renders the classic reflective sphere on a checkerboard (with shadows) in an afternoon. Sure, making it small requires a few tricks, but, honestly, I've seen much more impressive things.
Sphere on checkerboard, two mirrored spheres, and fractal-textured picture of Saturn. Each of the above in one (long) first-year lecture, on a TI-81 graphing calculator (which has 2k of memory for all programs total). Greyscale implemented using one-dimensional error-diffused dithering for all of the above.:) And yes, Saturn had shadows.
I miss my TI-81. I'll dust it off again when I finally kit-bash a holder for D cells on to it (it eats 4 AAAs in about 9 hours when running rendering code).
Re:And electrons ditto. Need ION beams to be subat
on
Pioneer Electron Beam DVD
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· Score: 2, Informative
Electrons also have the wave-particle duality. And being light they're big.
If you want a beam to do etching on an atomic scale you need ions, not electons.
Electron wavelength turns out not to be a problem. Calculate the wavelength, or look at a garden-variety hydrogen atom - you can get sub-angstrom precision with a few dozen eV. Electron beams are typically in the many-keV range (though something small enough to fit in an optical drive's read/write head would likely be hundreds of eV).
Focusing is the biggest problem I can think of offhand (magnetic focusing fields are much less well-behaved than optical lenses).
I guess it depends but I'm just going with what the article says.
The only thing I can think of is that the structure of the foam is such that it deflects radiation in such a way that it just turns it into heat as it bounces around within its structure.
Virtually all shielding mechanisms for high-energy radiation just boil down to charged particles or high-energy photons scattering off the electron coulds (exception is neutron radiation, which scatters off of nuclei only).
While resonances cause exceptions (e.g. Cadmium absorbing neutrons of a certain energy very effectively), for the most part the degree of shielding you get is directly tied to the amount of mass between you and the radiation source. So I wouldn't expect this to shield much more effectively against cosmic rays than water, steel, or lead would (and steel can at least act as your micrometeorite shielding while it's blocking radiation).
For some shielding scenarios, it's better to have light elements (more energy loss in particle/nucleus interactions), but for anything above x-ray energies the detailed arrangement of the atoms doesn't matter. So, a block of graphite (or a carbon composite hull) works just as well.
Water staying in atmospheres.
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Brine on Mars?
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· Score: 4, Interesting
Gases do move into outer space. Gravity slows down the process, but it doesn't stop it. When you get to the outer atmosphere, the velocity of gas atoms and molecules follow a predictable statistical distribution, dependent on their atomic mass and average temperature. Many atoms and molecules will reach escape velocity, and diffuse away from the planet. What do you think happened to the atmospheric helium on Earth?
Molecular weight of helium: 4 Molecular weight of water: 18
Gases escape over geologic time if the mean particle velocity is more than about a tenth escape velocity (if I recall correctly). Light particles at a given temperature (defined by average particle kinetic energy) move faster and so are lost more readily. Heavier particles are moving more slowly, and so are lost at a _much_ slower rate (the tail of the Boltzman distribution is exponential).
The real reason Mars has relatively little water is that water is broken up in the upper atmosphere by interaction with solar UV. While water may not be light enough to escape, hydrogen definitely is (molecular weight 2, and weight of an atomic hydrogen radical formed by a UV event is 1). This mechanism works on all of the planets (especially the inner ones) to strip their atmospheres of hydrogen.
Mars has a less active geology than Earth. We get hydrogen compounds (including water) replenished from volcanic sources. Earth also has a much higher escape velocity, which means that hydrogen is lost less quickly when formed (and has longer to recombine to form chemicals with higher molecular weight).
Both of these help explain why Earth is wet and Mars isn't. On the short term, however, water stays bound in Mars's atmosphere just fine. Those ice caps that migrate seasonally via atmospheric gas transport aren't all CO2, you know.
You can find a number of documents online discussing why Venus did get stripped of most of its water, despite being heavy and having a fairly active geology.
The delta-V is a function of the path; the required delta-V for a typical ion-drive trajectory is actually a bit higher than a two-impulse elliptical transfer.
Let me clarify - you shave 4 km/sec off the delta-v you need to produce using chemical rockets.
An ion drive with an Isp of thousands or better can use even an inefficient transfer scheme and use only a little fuel. Adding 4 km/sec using a rocket with an Isp of 300 gives you another factor of 3+ on the fuel:cargo ratio.
1KG of He3 is worth $6 million. Transportation costs to space and manufacturing processes need to come together...oh yeah, and we need to figure out that whole fusion thing. But once we're there, science knows of no better energy source (outside of anti-matter).
Why not just process He3 out of terrestrial helium? Sure, the ratio's about 1e-8 instead of the 1e-4 or so you have for deuterium, but you don't need a million tonnes of the stuff, and it stands a good chance of being cheaper to process it here than to haul the required large-scale strip-mining equipment to process it out of lunar soil.
D+D also works fine for fusion, with the drawback of your reactor vessel breaking down more quickly. Using D+He3 doesn't completely solve that problem, though, as parasitic D+D reactions occur even if you have a large surplus of He3 in the fuel.
For long-term space sources, we'd probably scoop-mine the atmosphere of Jupiter for helium to process instead of mining trace amounts from rocky bodies, but that's a far future consideration.
If we use fusion at all in the medium term, I suspect it will be D+D.
(D+T is much easier to reach breakeven on, but dumps almost all of its energy into neutron radiation, and requires you to breed tritium to sustain the reaction. This makes it a poor choice for large-scale power generation.)
Correct. But it does help for things that are going to Earth. That is, a base on the Moon would be ideal as a place for docking spacecraft that shuttle back and forth between places (like mining the asteroids, for instance).
Much as with lunar mining, this is only economical if you have a large amount of space traffic to and from Earth's region of space. No such traffic exists, nor will it exist in the near future. Even the arguments about towing metal-rich asteroids around only hold water if you assume the metals will be used in space.
So far, nobody's found a good enough reason to move people and industry into space in sufficient quantity to produce a need for space mining/storage.
Huh? Mars's orbital velocity is ~24 km/s, Earth's is 30 km/s. That's 5 km/s difference, and escape velocity from Earth is 11 km/s, and Mars's escape velocity is 5 km/s. As far as I can tell, if you escape from Mars's orbit, you'll basically fall back to Earth's orbit, free.
You never "fall" between orbits. You have to make two course corrections - one from a circular orbit at Mars radius to a transfer orbit, one from the transfer orbit to a circular orbit at Earth radius - and neither is free.
Velocity at perihelion for the Earth-Mars Hohmann transfer orbit is 32.72 km/s, compared to Earth's orbital velocity of 29.78 km/s. Delta-v of 2.94 km/sec either to or from Mars, on this end. Velocity at aphelion for the Earth-Mars Hohmann transfer orbit is 21.48 km/sec, compared to Mars' orbital velocity of 24.13 km/s. Delta-v of 2.65 km/s. Total delta-v needed for a Earth-Mars or Mars-Earth trip: about 5.6 km/sec, and that's assuming that you start far enough away from either world for the gravitational well of each to be negligeable. That's true on the Earth end of things if you're starting from a high orbit (or a high lunar orbit), but not from the Mars end.
If you're going from Mars surface to Lunar surface, you need to add an extra 5.03 + 2.38 = 7.41 km/sec, for a total delta-v of about 13.0 km/sec. If you're just going from low Mars orbit to low Luna orbit, you only need to add about 1.47 + 0.70 = 2.17 km/sec, for a total of about 7.8 km/sec delta-v. Doing at least one of these is unavoidable if you want to actually carry material from one body to the other
For a discussion of transfer orbit mechanics, go here.
Actually, a brief search online finds that I'm right, as shown here. The DV from Mars C3=0 orbit to Earth C3=0 orbit is 0.9 km/s - virtually nothing!
The delta-v quoted by your source is far lower than the delta-v needed to get into a Hohmann transfer orbit even from free space in a circular solar orbit at Earth's radius (which the C3=0 orbit is the equivalent of). As the Hohmann orbits are the lowest energy transfer orbits that don't require slingshots from other bodies, I question the values on that figure.
I meant that ionic propulsion is not a solution to send people there if the ones who got there 30 years ago had a three days trip...
It may very well be cheaper. Lifting something to low earth orbit is hard. Lifting it to escape velocity (or nearly so, for a lunar transfer orbit) is even harder.
You shave at least 4 km/sec off of your required delta-v if you can use ion drives and have a longer trip.
You're going to keep these astronauts on the moon for years anyways; why not spend the first year or so en route instead of on the lunar surface? The environment isn't much more hostile.
They're gonna bounce the rays off its surface. You know, like you do with visible light? Its exactly the same kind of wave for christ's sake.
All EM waves are not equal.
Low-frequency waves, like RF and up through the microwave range, interact with matter like classical EM fields. They're reflected by conductors, and mostly absorbed by dielectrics.
High-frequency waves, like light (IR through UV), are in about the right energy band to interact directly with electrons in atoms. This process involves both classical and quantum effects. Because the energy structure of most materials is complex, you get complicated patterns of absorption bands and what not giving objects colour and distinctive spectra. In cases where an object has lots of free electrons, like a metal, the system is still close enough to the classical one to give reflection.
Really high frequency waves, like X rays and gamma rays, interact on pretty much an atom by atom basis with matter. Sometimes, especially with the low end of the scale (soft X rays), lattice effects in an ordered material will give diffraction patterns. For soft x-rays, you may also get interactions with inner-shell electrons giving absorption bands (gamma rays do the same thing with nuclei). Most of the rest of the time, though, you get scattering.
In summary, all photons are not equal. Shining an x-ray beam at a surface will give you some backscatter, as another poster pointed out, but what you're *not* going to get is diffuse or specular reflection like you'd get with visible light.
What about launch bases on the moon? Escaping the moon's gravity to get to other locations has gotta be way cheaper than escaping earth's gravity.
In order to get to the moon in the first place, you need to have almost completely escaped the Earth's gravity, so it doesn't help for launch of things that are originally from Earth. Best approach for that is to launch them to as _low_ an orbit as you can (so as to minimize delta-v required of high thrust, low-Isp drives), and to spiral the rest of the way out over a period of months using a low thrust, high-Isp drive.
A network of these on the moon, maybe on one of mars's moons or mars iteself. I know, supplies need to come from somewhere, but shipping water from mars's icecaps to the moon might be cheaper than shipping water from earth to them moon.
This turns out not to be the case, as you need a lot of delta-v to travel from Mars's orbit to Earth's (the sun's gravity well is deep).
If a moon base was constructed, the best way to supply it in the short term would be to lift water and other hard-to-get commodities from Earth, and do your best to conserve them at the base. For long-term supply of extensive lunar or near-earth colonies, you'd probably tow an ice asteroid into one of the stable Earth/Moon Lagrange points, and mine that.
FWIW, a better supply chemical to lift from Earth would be ammonia. You can react it with oxygen from lunar rock to get water, and you can also use the nitrogen as a fertilizer component. Methane would also be a good choice. The moon has lots of oxygen, silicon, and metals, but lacks carbon, hydrogen, and nitrogen (which are all needed in fairly large quantity for earth life).
As for things to do on the moon itself, it's useful as a materials supply yard, but only if the materials are going to be used in space. Given the cost of building a lunar mining facility vs. the cost of lifting material from Earth, we'd only build a mining facility if we were building something Really Big in space for other reasons.
Is not the speed of light through a medium subject to the forces exerted by the atoms in the medium as can be measured in the forms of electrical permittivity and magnetic permiablility?
The speed of propagation of the electromagnetic force is always C.
A light wave (an EM wave composed of many photons) interacts with a medium by acting on and being acted upon by the charges within the medium (mostly the electron shells about atoms). This includes, but is not limited to, the electric field of the wave deforming orbitals, with the resulting charge distribution change acting back on the EM wave, and photons being absorbed and re-emitted, per the original poster's description. The net effect of this is to alter the phase velocity of the EM wave. At all points, the force-carrying interactions propagate at C.
Also your "car analogy" is discredited by the already acknowleged fact of light travelling at a constant rate (either a constant for the given medium as I was taught, or a universal constant as you seem to be arguing). The light does not need to accelerate or decelerate in order to start or stop, thus a car is not like a photon in the slightest.
Consider the car example again. I don't need to spend much time accelerating or decellerating - I'm either driving at 50 km/hr (a photon propagating in space), or I'm stopped at an intersection (a photon that's been absorbed). Average velocity for the car is still less than 50 km/hr, as I spend some of the time at 0 km/hr. When the photon exists, it's travelling at C. The time it spends not travelling (i.e. when the photon is absorbed) causes the average velocity of the wavefront to be less than C.
Slashdot Mirror
scale the whole project up, and easy access to sunken boats?
Unlikely, for this approach, at least. This experiment is relying on special interactions between the cornstarch suspension and the sound waves passing through it. Water behaves a lot more like an ideal liquid than a cornstarch suspension does, so it's not going to exhibit the same effect.
There are other patterns of motion you could induce in water to get access to sunken ships (e.g. dump in lots of sound and put an antinode in the channel volume, or set up currents to produce a vortex). However, it's probably cheaper just to go down there with submarines and/or scuba gear.
If you need an open-air environment to do the recovery properly (which would actually be _bad_ for most shipwrecks), pressurize an inflatable dome down there (using suitable gas mix instead of ordinary air if at any significant depth). Your divers are already acclimatized to the pressure, so no ultra-strong submarine hull is needed (keep dome pressure slightly over ambient).
Next question: how the heck do you control the spin of individual baryons in a nucleus?
:).
You fire something at the nucleus and isolate the ones where one of the outer-shell nucleons was bumped up to the energy state you want.
If you fire X or gamma rays at the nucleus, you should only be able to excite very short-lived isomers (if it is boosted by absorbing a photon, it can decay by emitting a photon). Firing things like electrons or protons at the nucleus can excite states that don't have a single-photon decay path. These can be metastable.
We do the same thing in HeNe lasers. Helium atoms are excited to a metastable state by electric discharge, and after a while interact with neon atoms, putting them in a state suitable for lasing (target state of neon has almost exactly the same energy as the metastable helium state, so the exchange happens easily).
I hope this helps
Two problems with this scenario:
I hope this clears things up.
It does look very fake. Even stranger is the fact that the thumbnail on the left shows the same picture with a red/white/blue parachute as described in the article. The large picture shows yellow/green/red.
Actually, it looks like it's missing the blue channel from the entire picture for some reason. That explains the color discrepancy, and why the helicopter is such a wonderfully eye-hurting shade of red.
OO.o has a native "export to PDF" feature in the latest builds. You might want to give it another whirl. Might save some (potentially) messy LaTeX work, at least for non-methematical documents.
I use LaTeX by preference. Word is used when I _can't_ send PS or PDF (usually when I'm collaborating on a document with other people who only use Word).
Using OO to build PPT presentations and exporting as PDF is tempting, though.
Why would I change back from a decent, FREE, application like OpenOffice to WordPerfect? If they're planning on selling it on the name, or because people remember WP, it's too late for that now. OpenOffice has taken over, and could soon be challenging MS Office in a big way.
OpenOffice will convince me to abandon Office when it stops mangling fonts and layout for the Word documents people keep sending me. I can read them, but they don't look pretty, and I'm sure as heck not going to _write_ anything in OO while this is a concern.
Bad install? Maybe. But I've run into the problem in two unrelated *nix labs where it was installed. I suppose _both_ admins _could_ be sloppy, but they've been pretty sharp in other regards.
If I'm writing documents in *nix, I use LaTeX and send people postscript or PDF. But when I need to give someone a Word document, or bring a PowerPoint slide show to a conference, I use MS Office.
Your mileage may vary.
The biologically equivalent dose for humans, the REM (radiation equivalent in man), is just the dose of radiation times the RBE. So alpha rays have at least ten times the relative biological effectiveness than X-rays.
You are both right.
Alpha particles do more damage, but only if produced by ingested substances. From external sources, they won't penetrate the layer of dead skin on the surface of your body.
Heavy ions behave similarly (at least when in the same energy range).
Betas have a penetration distance of at least several millimetres, so they're definitely an external hazard (first poster was hazy on that).
The real danger at sites of nuclear accidents (or bomb tests, etc) is inhaling radioactive dust. That can get close enough to live tissue to give you lung cancer, and anything soluble can pass into the bloodstream and do more damage.
The danger from nuclear reactors and from long-term waste storage is from soluble radioactives getting into the local water supply and being ingested that way. This is why power plants have multi-stage heat exchange systems and why proposed waste storage sites are at the bottom of mines in non-porus rock, or under a few hundred feet of clay at the bottom of the ocean.
Can someone list any meaningfull danagers of GM food, preferably with something that resembles proof. I'm not trolling for either side here I'm simply curious.
The main reasonable objection I've heard is that, because you're splicing genes from wherever you please, you can no longer tell by inspection whether or not you'll be allergic to any given food. While the "splicing fish genes into vegetables" is an extreme example, it gets the concept across. IMO, this isn't likely to occur accidentally (you know what genes you're copying, and so would know when you're copying something that codes for an allergen). However, it would still occur, and so presents a concern.
A secondary objection is that it's very difficult to grow samples of an engineered crop without it spreading out of the controlled area or cross-pollinating with other nearby compatible plants. This means that if you do, for instance, engineer a strain of wheat that makes anyone with a peanut allergy keel over and die, there's a significant risk of that strain propagating into mundane wheat fields, with un-fun results. Engineered strains are usually specifically designed to be hardier than normal strains (that's why we're engineering them), so they will be competitive with normal strains in the field.
That having been said, I think that genetically engineered crops are inevitable, and mostly beneficial. When this becomes a tried-and-true technology instead of an experimental one, the fuss should die down.
Short answer: FPGAs let you build using basic gates and (very small) lookup tables. This lets you build anything you please, and fully optimize the number of functional units of each type that you have, but has a speed and size penalty.
This chip is basically a RISC processor with an FPGA-type fabric bolted on as a co-processor, as far as I can tell from the detail-poor press release. By implementing most of the instruction pipeline as fixed, optimized hardware, it runs without any of the penalties of a purely FPGA-based implementation. When you have a number-crunching task that would benefit from a custom logic implementation enough to offset the performance penalty of implementing it in programmable logic blocks, the compiler configures the programmable logic into a suitable coprocessor which is stuck in as an extra branch of the instruction pipeline.
How much benefit you get from this depends on what you're doing. Modern general-purpose microprocessors have enough vector instructions to handle most DSP-ish tasks without an abysmal speed penalty (just a large size and power penalty over a purely DSP-based implementation). Most computing tasks aren't limited by processing horsepower at all - they're either waiting for memory accesses to complete (even cache accesses are very slow compared to register accesses), or they're waiting for the target address of a branch to be decided (speculation and BTBs don't address this perfectly by a long shot). A reconfigurable processor would suffer from much the same type of problem. While using the programmable logic path for slice processing could remove some of the branching penalties (by following all paths and selecting the desired result), this would be at an even greater area and power cost.
For specialized applications, it would be quite useful, of course.
A quick glossary of terms being thrown around, for anyone confused:
This is a combination of lookup tables, sum-of-products combinational logic blocks, and scratch-pad SRAM that you can hook up in nearly arbitrary ways to produce custom circuits at a gate level. Bulky and slow, but good at implementing algorithms efficiently. Configuration information is loaded from a serial PROM chip at startup, letting you change it relatively easily.
Like an FPGA, but stores configuration information internally, so you need to take out the CPLD and burn it to change configuration instead of re-burning the configuration PROM.
Little cousin to CPLD. This is what you played with in second or third year. Typically these are just a sum-of-products combinational logic block with a register stuck on the end to latch the output. Useful as glue logic.
This is an integrated circuit that's half-made. A number of gates and registers and so forth have been fabricated on the chip, and the lowest few metal layers have been used for internal routing for these, but you get to define the upper metal layers to form arbitrary connections among these (either as the last fabrication step, or by laser-cutting a pre-fabricated wiring mesh to leave the geometry you want). Works much like a CPLD, but the design is decided at fabrication time and cannot be changed. Faster and less bulky than a CPLD implementation.
This is a custom-fabricated integrated circuit that uses cells from a standard library of components, usually automatically placed and routed from a VHDL or Verilog description of what you want the chip to do. Faster than an ASIC if you have good place and route software, but more expensive in small quantities because you're making what amounts to a full custom chip. Design time is much less than a fully custom design would be, though (but verifying that the design description is correct is a royal pain).
I hope this clears things up for anyone who was confused.
The problem I see with this bubble stuff is that they detect it by the emission of neutrons. Anything which gives out lots of neutrons is going to have many of the problems of fission - any plant big enough tobe useful will need shielding and will produce nasty waste makeing decomissioning expensive.
ObOldJoke: Some years ago, a student paper allegedly proposed that the fusion reactor architecture most worth pursuing was a Gravitational Confinement design.
The kicker is that he may be right (you could power a state with a solar farm the size of a medium-sized city, if you had cheap enough bulk power storage, while fusion plants are likely to be very expensive to build and maintain). We'll find out in 50 years or so.
(about 99% of light - don't know how this translates to efficiency, though - article not too technical)
[...]
manufacturing panels 400mm x 500mm @ 20W
That says they're speccing them to about 10% efficiency.
Whatever happened to the idea of superconducting power rings? Did the magnetic flux intensity just prove too great for practical use?
The main problem with superconducting storage is that high-temperature superconductors break down at relatively modest [for power storage] field strengths (in the range of 1 Tesla), and liquid helium cooled superconductors are only somewhat better (best I've heard is 8 Tesla for particle accelerator magnets cooled to 1.8K).
It turns out that the situation is even worse, though. Even if you spec magical materials for the field coils - like, say, zero resistance and the strength of carbon nanotubes (120 GPa) - energy per unit coil mass is much worse than the energy density you'd get with a comparable amount of chemical fuel. The same turns out to apply to flywheels (even constructed of magical ultra-tensile materials), and most of the other proposed schemes I've heard of.
This means that chemical storage schemes like fuel cells are likely to win out over more exotic storage mechanisms for the near to intermediate future. While you could build a storage plant using hydrogen fuel cells, the holy grail would be to get something like methanol working as efficiently (you can use it as a fuel easily enough - it's rebuilding it from the combustion products that's the problem).
If there was a demand for these items, even a perceived one, they'd get produced. But there isn't.
Now, a 50% solar cell...?
While demand in the alternative energy market is iffy right now, you'll definitely have demand for more efficient photovoltaics at _any_ price in the space industry.
Lifting mass into space is expensive. If you can get a 2-3x improvement in power to weight ratio of your solar arrays using materials like this, the world will beat a path to your door even if you don't have a way to grow it epitaxially.
As for long-term cost prospects, we're already mass-manufacturing similar highly-mismatched alloys for LEDs (anything with Ga/N/As as constituents, for one), so I'm confident costs will come down eventually.
Very nifty technology.
Clarification - one 2-hour lecture _per_ program, not all 3 in one lecture. I'm not Johnny 5 :).
Not to belittle the man's efforts, but a raytracer is *incredibly* simple, algorithmically... hell, I could (and did) write a basic one that renders the classic reflective sphere on a checkerboard (with shadows) in an afternoon. Sure, making it small requires a few tricks, but, honestly, I've seen much more impressive things.
:) And yes, Saturn had shadows.
Sphere on checkerboard, two mirrored spheres, and fractal-textured picture of Saturn. Each of the above in one (long) first-year lecture, on a TI-81 graphing calculator (which has 2k of memory for all programs total). Greyscale implemented using one-dimensional error-diffused dithering for all of the above.
I miss my TI-81. I'll dust it off again when I finally kit-bash a holder for D cells on to it (it eats 4 AAAs in about 9 hours when running rendering code).
Electrons also have the wave-particle duality. And being light they're big.
If you want a beam to do etching on an atomic scale you need ions, not electons.
Electron wavelength turns out not to be a problem. Calculate the wavelength, or look at a garden-variety hydrogen atom - you can get sub-angstrom precision with a few dozen eV. Electron beams are typically in the many-keV range (though something small enough to fit in an optical drive's read/write head would likely be hundreds of eV).
Focusing is the biggest problem I can think of offhand (magnetic focusing fields are much less well-behaved than optical lenses).
I guess it depends but I'm just going with what the article says.
The only thing I can think of is that the structure of the foam is such that it deflects radiation in such a way that it just turns it into heat as it bounces around within its structure.
Virtually all shielding mechanisms for high-energy radiation just boil down to charged particles or high-energy photons scattering off the electron coulds (exception is neutron radiation, which scatters off of nuclei only).
While resonances cause exceptions (e.g. Cadmium absorbing neutrons of a certain energy very effectively), for the most part the degree of shielding you get is directly tied to the amount of mass between you and the radiation source. So I wouldn't expect this to shield much more effectively against cosmic rays than water, steel, or lead would (and steel can at least act as your micrometeorite shielding while it's blocking radiation).
For some shielding scenarios, it's better to have light elements (more energy loss in particle/nucleus interactions), but for anything above x-ray energies the detailed arrangement of the atoms doesn't matter. So, a block of graphite (or a carbon composite hull) works just as well.
Gases do move into outer space. Gravity slows down the process, but it doesn't stop it. When you get to the outer atmosphere, the velocity of gas atoms and molecules follow a predictable statistical distribution, dependent on their atomic mass and average temperature. Many atoms and molecules will reach escape velocity, and diffuse away from the planet. What do you think happened to the atmospheric helium on Earth?
Molecular weight of helium: 4
Molecular weight of water: 18
Gases escape over geologic time if the mean particle velocity is more than about a tenth escape velocity (if I recall correctly). Light particles at a given temperature (defined by average particle kinetic energy) move faster and so are lost more readily. Heavier particles are moving more slowly, and so are lost at a _much_ slower rate (the tail of the Boltzman distribution is exponential).
The real reason Mars has relatively little water is that water is broken up in the upper atmosphere by interaction with solar UV. While water may not be light enough to escape, hydrogen definitely is (molecular weight 2, and weight of an atomic hydrogen radical formed by a UV event is 1). This mechanism works on all of the planets (especially the inner ones) to strip their atmospheres of hydrogen.
Mars has a less active geology than Earth. We get hydrogen compounds (including water) replenished from volcanic sources. Earth also has a much higher escape velocity, which means that hydrogen is lost less quickly when formed (and has longer to recombine to form chemicals with higher molecular weight).
Both of these help explain why Earth is wet and Mars isn't. On the short term, however, water stays bound in Mars's atmosphere just fine. Those ice caps that migrate seasonally via atmospheric gas transport aren't all CO2, you know.
You can find a number of documents online discussing why Venus did get stripped of most of its water, despite being heavy and having a fairly active geology.
The delta-V is a function of the path; the required delta-V for a typical ion-drive trajectory is actually a bit higher than a two-impulse elliptical transfer.
Let me clarify - you shave 4 km/sec off the delta-v you need to produce using chemical rockets.
An ion drive with an Isp of thousands or better can use even an inefficient transfer scheme and use only a little fuel. Adding 4 km/sec using a rocket with an Isp of 300 gives you another factor of 3+ on the fuel:cargo ratio.
1KG of He3 is worth $6 million. Transportation costs to space and manufacturing processes need to come together...oh yeah, and we need to figure out that whole fusion thing. But once we're there, science knows of no better energy source (outside of anti-matter).
Why not just process He3 out of terrestrial helium? Sure, the ratio's about 1e-8 instead of the 1e-4 or so you have for deuterium, but you don't need a million tonnes of the stuff, and it stands a good chance of being cheaper to process it here than to haul the required large-scale strip-mining equipment to process it out of lunar soil.
D+D also works fine for fusion, with the drawback of your reactor vessel breaking down more quickly. Using D+He3 doesn't completely solve that problem, though, as parasitic D+D reactions occur even if you have a large surplus of He3 in the fuel.
For long-term space sources, we'd probably scoop-mine the atmosphere of Jupiter for helium to process instead of mining trace amounts from rocky bodies, but that's a far future consideration.
If we use fusion at all in the medium term, I suspect it will be D+D.
(D+T is much easier to reach breakeven on, but dumps almost all of its energy into neutron radiation, and requires you to breed tritium to sustain the reaction. This makes it a poor choice for large-scale power generation.)
Correct. But it does help for things that are going to Earth. That is, a base on the Moon would be ideal as a place for docking spacecraft that shuttle back and forth between places (like mining the asteroids, for instance).
Much as with lunar mining, this is only economical if you have a large amount of space traffic to and from Earth's region of space. No such traffic exists, nor will it exist in the near future. Even the arguments about towing metal-rich asteroids around only hold water if you assume the metals will be used in space.
So far, nobody's found a good enough reason to move people and industry into space in sufficient quantity to produce a need for space mining/storage.
Huh? Mars's orbital velocity is ~24 km/s, Earth's is 30 km/s. That's 5 km/s difference, and escape velocity from Earth is 11 km/s, and Mars's escape velocity is 5 km/s. As far as I can tell, if you escape from Mars's orbit, you'll basically fall back to Earth's orbit, free.
You never "fall" between orbits. You have to make two course corrections - one from a circular orbit at Mars radius to a transfer orbit, one from the transfer orbit to a circular orbit at Earth radius - and neither is free.
Velocity at perihelion for the Earth-Mars Hohmann transfer orbit is 32.72 km/s, compared to Earth's orbital velocity of 29.78 km/s. Delta-v of 2.94 km/sec either to or from Mars, on this end. Velocity at aphelion for the Earth-Mars Hohmann transfer orbit is 21.48 km/sec, compared to Mars' orbital velocity of 24.13 km/s. Delta-v of 2.65 km/s. Total delta-v needed for a Earth-Mars or Mars-Earth trip: about 5.6 km/sec, and that's assuming that you start far enough away from either world for the gravitational well of each to be negligeable. That's true on the Earth end of things if you're starting from a high orbit (or a high lunar orbit), but not from the Mars end.
If you're going from Mars surface to Lunar surface, you need to add an extra 5.03 + 2.38 = 7.41 km/sec, for a total delta-v of about 13.0 km/sec. If you're just going from low Mars orbit to low Luna orbit, you only need to add about 1.47 + 0.70 = 2.17 km/sec, for a total of about 7.8 km/sec delta-v. Doing at least one of these is unavoidable if you want to actually carry material from one body to the other
For a discussion of transfer orbit mechanics, go here.
Actually, a brief search online finds that I'm right, as shown here. The DV from Mars C3=0 orbit to Earth C3=0 orbit is 0.9 km/s - virtually nothing!
The delta-v quoted by your source is far lower than the delta-v needed to get into a Hohmann transfer orbit even from free space in a circular solar orbit at Earth's radius (which the C3=0 orbit is the equivalent of). As the Hohmann orbits are the lowest energy transfer orbits that don't require slingshots from other bodies, I question the values on that figure.
I meant that ionic propulsion is not a solution to send people there if the ones who got there 30 years ago had a three days trip...
It may very well be cheaper. Lifting something to low earth orbit is hard. Lifting it to escape velocity (or nearly so, for a lunar transfer orbit) is even harder.
You shave at least 4 km/sec off of your required delta-v if you can use ion drives and have a longer trip.
You're going to keep these astronauts on the moon for years anyways; why not spend the first year or so en route instead of on the lunar surface? The environment isn't much more hostile.
They're gonna bounce the rays off its surface. You know, like you do with visible light? Its exactly the same kind of wave for christ's sake.
All EM waves are not equal.
Low-frequency waves, like RF and up through the microwave range, interact with matter like classical EM fields. They're reflected by conductors, and mostly absorbed by dielectrics.
High-frequency waves, like light (IR through UV), are in about the right energy band to interact directly with electrons in atoms. This process involves both classical and quantum effects. Because the energy structure of most materials is complex, you get complicated patterns of absorption bands and what not giving objects colour and distinctive spectra. In cases where an object has lots of free electrons, like a metal, the system is still close enough to the classical one to give reflection.
Really high frequency waves, like X rays and gamma rays, interact on pretty much an atom by atom basis with matter. Sometimes, especially with the low end of the scale (soft X rays), lattice effects in an ordered material will give diffraction patterns. For soft x-rays, you may also get interactions with inner-shell electrons giving absorption bands (gamma rays do the same thing with nuclei). Most of the rest of the time, though, you get scattering.
In summary, all photons are not equal. Shining an x-ray beam at a surface will give you some backscatter, as another poster pointed out, but what you're *not* going to get is diffuse or specular reflection like you'd get with visible light.
What about launch bases on the moon? Escaping the moon's gravity to get to other locations has gotta be way cheaper than escaping earth's gravity.
In order to get to the moon in the first place, you need to have almost completely escaped the Earth's gravity, so it doesn't help for launch of things that are originally from Earth. Best approach for that is to launch them to as _low_ an orbit as you can (so as to minimize delta-v required of high thrust, low-Isp drives), and to spiral the rest of the way out over a period of months using a low thrust, high-Isp drive.
A network of these on the moon, maybe on one of mars's moons or mars iteself. I know, supplies need to come from somewhere, but shipping water from mars's icecaps to the moon might be cheaper than shipping water from earth to them moon.
This turns out not to be the case, as you need a lot of delta-v to travel from Mars's orbit to Earth's (the sun's gravity well is deep).
If a moon base was constructed, the best way to supply it in the short term would be to lift water and other hard-to-get commodities from Earth, and do your best to conserve them at the base. For long-term supply of extensive lunar or near-earth colonies, you'd probably tow an ice asteroid into one of the stable Earth/Moon Lagrange points, and mine that.
FWIW, a better supply chemical to lift from Earth would be ammonia. You can react it with oxygen from lunar rock to get water, and you can also use the nitrogen as a fertilizer component. Methane would also be a good choice. The moon has lots of oxygen, silicon, and metals, but lacks carbon, hydrogen, and nitrogen (which are all needed in fairly large quantity for earth life).
As for things to do on the moon itself, it's useful as a materials supply yard, but only if the materials are going to be used in space. Given the cost of building a lunar mining facility vs. the cost of lifting material from Earth, we'd only build a mining facility if we were building something Really Big in space for other reasons.
Is not the speed of light through a medium subject to the forces exerted by the atoms in the medium as can be measured in the forms of electrical permittivity and magnetic permiablility?
The speed of propagation of the electromagnetic force is always C.
A light wave (an EM wave composed of many photons) interacts with a medium by acting on and being acted upon by the charges within the medium (mostly the electron shells about atoms). This includes, but is not limited to, the electric field of the wave deforming orbitals, with the resulting charge distribution change acting back on the EM wave, and photons being absorbed and re-emitted, per the original poster's description. The net effect of this is to alter the phase velocity of the EM wave. At all points, the force-carrying interactions propagate at C.
Also your "car analogy" is discredited by the already acknowleged fact of light travelling at a constant rate (either a constant for the given medium as I was taught, or a universal constant as you seem to be arguing). The light does not need to accelerate or decelerate in order to start or stop, thus a car is not like a photon in the slightest.
Consider the car example again. I don't need to spend much time accelerating or decellerating - I'm either driving at 50 km/hr (a photon propagating in space), or I'm stopped at an intersection (a photon that's been absorbed). Average velocity for the car is still less than 50 km/hr, as I spend some of the time at 0 km/hr. When the photon exists, it's travelling at C. The time it spends not travelling (i.e. when the photon is absorbed) causes the average velocity of the wavefront to be less than C.