Enthalpy

Only hydrogen isotopes have different chemical properties, because the delocalization of the hydrogen nucleus is big enough to weaken the hydrogen bonds. Elements heavier and without hydrogen bonds differ chemically very little through the neutron count. Few physical properties vary significantly through the isotopic composition. Superconductors depend a bit on isotopes, superfluid helium fundamentally. Diamond freed of 13C conducts heat many times better, silicon improves as well. Hydrogen used to passivate pending bonds at Si-SiO2 interfaces is slowly knocked away by impinging electrons, and deuterium resists longer in this use. Deuterium is better for some gas-spark switches, lamps and some lasers. These significant effects rely all on the individual atoms' mass, not chemical affinity.

Heavy nuclei have also more protons that produce a stronger electric field to deflect the incoming electron. The strong electron deflection radiates stronger X and gamma rays. If the incoming electron is almost as fast as light (energy >> 511keV), then an aligned observer sees the electron begin to deviate, and later the electron end to deviate - but the beginning is observed after a longer propagation delay than the end, and the delay difference almost compensates the duration of the event, so that the deflection event is extremely short for the observer. This makes the emitted X or gamma shorter and of higher energy, stronger and more concentrated in one direction. It's synchrotron radiation. If using magnetic fields, reversed over millimetric period, it's called a free electron laser (which also needs many electrons to synchronize). Protons and charged particles have a finite path in matter - a short path at MeV energies. The number of surviving photons decreases exponentially, so some survive to any distance; this number may become acceptable for health, or negligible, like 0.01 photon over the experiment duration. Protons stopping in matter create gammas only because cosmic rays have huge energies. Deflection by surviving nuclei wouldn't explain it as for electrons. The nature of the collisions differs from electrons and involves the destruction of nuclei, the creation of new particles - including at energies inaccessible to the LHC. Among those brutal events, gammas are created as something banal, during collisions, annihilation of particles... ---------- A radioisotopic generator contains enough radioisotope material to run very hot - only its cooling keeps it solid. In a rocket accident (11km/s for instance) the generator and its cooling would be lost. To keep the radioisotope contained, which means solid and not too hot, it consists of individual pellets, which are small enough (1cm) to cool naturally in air when they're dispersed. Though, dispersed pellets need an individual radiation shield, which aso resists the possible re-entry in Earth's atmosphere (it happened with Apollo 13), the impact on the ground, the corrosion in the Ocean... http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator The alphas emitted by 238Pu are blocked by Pu itself and by few µm shielding, enabling this individual shielding of pellets. Gammas emitted by 90Sr, or rather 90Y and by bremsstrahlung of the emitted electrons, demand a thick shield, too thick for individual pellets. It's a collective shield for the whole generator, hence unsafe in an accident. Some projected Moon probes plan to use radioisotopic generators, and I dislike this idea, since batteries exist and Sunlight is plentiful on the Moon. ---------- Electron-positon: as I imagine to understand it, any photon >1022keV could create them, but it takes an electric field to separate them, so they don't recombine immediately. An additional reason is that photons have some energy-versus-momentum relation, electrons have a different one (slightly different if ultra-relativistic), and simultaneous conservation of energy and momentum demands an additional mass that takes recoil - here the nucleus.

Search keywords "primordial neutrinos".

Simultaneous choice by distant entangled particles does not permit to transmit information. Present formulations of entanglement do not seek a physical means to relate the choice by the particle pair. Superluminal communication would ruin Relativity, since the finite speed of information is its fundamental hypothesis - not a remote consequence of increasing mass or speed addition. As such, tachyons allowing to transmit information would be a serious headache to present physics... But they were - at the time they made the buzz - nothing more than an exotic math solution needed by no observation. By the way, what would the group speed of tachyons be? Are they a usual question of phase versus group speed?

How?

A black hole isn't that different from any mass... it's more concentrated. The path of an object in the field of a black hole is usually as near any mass: it passes by, is deviated and goes away. For an object to be captured by gravitation, it must brake, for instance by a collision, or by the slingshot assistance of a moon. The object gaining kinetic energy loses gravitational energy. I'm not sure at all that mass+energy is conserved when masses coalesce. I've read conflicting opinions about that.

"Computer Science" is a paradox in two words, just like "military music".

This has been a research topic some decades ago. Programmes exist, alas. I'm convinced that these programmes have created the infamous websites that contain one page per compound, will all possible names, and whose properties are only estimates without any warning sign.

The part of Wiki's article was obviously written as propaganda in favour of the project. It contains no physical basis, but uses the standard techniques of propaganda. That's why I asked if Airbrush, who develops very similar arguments in this thread, is also the author of the regrettable section at Wiki. Botched articles at Wiki do exist, for instance the one that claims cyclopropane to detonate when liquid or compressed, despite all safety data sheets ignore this, and Wiki's suggestions there (tungsten wool to brake the cyclopropane molecules...) are meaningless. Apparently, the article was botched when Esa considered to develop of a rocket engine burning cyclopropane. As an other example, French articles describing nuclear (quasi-) accidents in France have been cut, and only their English translation is still available. So, yes, use Wiki - but keep in mind it can be false, and sometimes intentionally. ---------- Nasa and Esa do physics. As any human enterprise, they sometimes err, and they also need to catch the public's interest. As spenders of public budget, they also depend on decisions by politicians. From time to time, you can read about worm fossiles in Martian stones before the story is fully assessed, or airships to reach orbit, or superconducting magnetic shields for interplanetary travellers that have clearly indentifyable weaknesses. But I've seen much worse in several companies.

1) and 2) Too few comets are known in-situ. From distance, we observe only the degassing. For lack of better observation, comets are supposed to contain initially much snow, and after some passes close to the Sun, far less snow but still dust. Are there other variations? I bet the question is still open. 3) Heavy celestial bodies, especially Jupiter, when the comet is already that near to the Sun. In the Oort cloud: little known, because it's unexplored. The cloud has objects on very different paths, so if it has enough objects, near passes must deviate some comets towards us. 4) If they contain only snow and sand, no. But if some contain Fe3O4, or just carbon-loaded iron, these may have a small field. 5) I didn't understand "asymmetric rotation". 6) One was observed falling into the Sun. Not very spectacular, but a small gas emission was observed at the Sun's opposite side. 7) Fully evaporate the snow if there is still some. A really close approach may break the comet in parts as Jupiter did for Shoemaker-Levy. 8) The Giotto probe observed that the gas is emitted locally, in this case from several places, by the nucleus. From some distance, the aspect is uniform. Influence on path: of course. That's only a matter of how much. Though, the gas emission must be slow, like 100m/s, so the effect is small. 9) As far as I know, no single comet has ever been deviated by humans up to now. Plans do exist: look for "Near-Earth Objects". Nothing obvious, but one scenario does want to evaporate some material of the object, so the reaction force pushes the object away. But it needs volatile materials at the object (less easy with iron or stone), and depending on the heat source, a rotation slow enough would be easier. If you hoped to have invented the method, sorry. Painting the object is also proposed, as well as a tractor, and nuclear explosions, and impactors... Some methods might work if the object is small enough (...but still detected on time) and the necessary push is small enough and the trajectory accessible to our rockets, and and and. The more general case is pretty much out of reach. 10) Break apart: by the Sun I don't remember any, but by Jupiter yes. This alone is no reason to fall into the Sun: a celestial object continues on its path, as opposed to a shot duck. But the proximity to the Sun can mean that the next pass is a collision.

2) The slower alphas have more time to eject an electron from the molecule. They also act by two protons. 3) Yes and in that, photons differ radically from electrons and ions, which have a finite (though somewhat variable) path in matter. It's a serious annoyance, because when the gamma flux vastly exceeds the acceptable, the thickness increases correspondingly. To protect electronics, cameras or worse, humans in a space vessel, one has to stop a few extremely energetic particles, which may prove impossible within possible mass, say for a 10 months travel to Mars. Worse: stopping the protons creates gammas which cannot be stopped easily. Or think at radioisotope batteries. Everyone designer of such generator would love to use waste from uranium reactors, especially 90Sr. But its child, 90Y, emits a gamma in one disintegration over many, so shielding is difficult and costs thick lead, prohibitively heavy on a spacecraft, especially because these ugly poisons shall please be shielded on an individual pellet basis, not as a whole generator that would break apart in an accident. Result: the only source is 238Pu, a pure alpha emitter, which must be produced on demand, and is scarce and expensive. 4) There are more fundamental reasons favouring heavy elements against X and gammas. Against X, the absorption by a deep electron, typically 1s, removes like 90keV at once and with a high probability. Against gammas over 1022keV, the strong electric field near a heavy nucleus favours the creation of electron-positon pairs, which is an efficient absorption process. So much that a shield equally efficient is lighter if using a heavier element - which the density of electrons wouldn't explain. Beyond lead, you have thorium and natural or depleted uranium, which are very little radioactive and are used (infrequently) as shields. ---------- Complete absorption curves of photons, electrons, protons, alphas... for every element and a few compounds are available at Nist's website, really nice of them.

Hello everybody! We read again and again "Face-centered cubic metals are ductile, body-centered are not, while hexagonal close-packed are brittle". Some authors want even to justify it through numbers of slipping planes. Worse, a few ones would curb Nature to their reasoning if only they could, like "Ti-Al5V is brittle because it's a hexagonal alpha alloy" or "Zinc breaks upon bending because hexagonal" - no, I won't tell you who wrote that one. About every metallurgy textbook has a crystallography chapter that reproduces this nonsense, and I'm getting tired of it, so I've just made a table with metals of mechanical use. Most metals go against the claim. Observations : Some very ductile metals like magnesium and zinc are hexagonal close packed. Iron, which can be the origin of the false claim, is very ductile as a body-centered cubic lattice, if pure. More so than as some austenitic alloys. Pure titanium is ductile and hexagonal, as are its alpha alloys. Its alpha+beta alloys are less ductile. All alkaline metals (not for mechanical use) are body-centered and very ductile. Silicon, germanium (not exactly metals) are face-centered cubic and perfectly brittle, as polycrystals as well. Many "brittle" metals become ductile if pure enough. Tantalum is extruded cold. Some demand a very high purity to be less brittle, like beryllium. Maybe some "brittle" metals are just not pure enough? Most metals can be embrittled by some alloying element, like phosphorus in iron of any phase. A partial link can be seen with the fusion temperature, which logically relates with easy propagation of dislocations - but there are exceptions. Marc Schaefer, aka Enthalpy

No physics here. Did you botch Wiki's article yourself?

I wish to take advantage of the present context to express my full and complete agreement with Swansont, as usual.

They have been built with YAG, where one single emitting atom stays indefinitely in the cavity, gets excited and emits. The resonance changes the transition lifetime with this single atom. Exactly as in any laser; call it gain if you wish. Slightly more exotic: antiresonating cavities are used to prevent the de-excitation of atoms. By the way, the image of a bunch of photons travelling through the cavity can hold in superradiant mode, but in most continuous-mode lasers, the coherence is hugely longer than the cavity, which means that the photons just stand in the cavity as a standing wave, and the lasing atoms de-excite at random places without any propagation. The photon is as long as the cavity, because of the mirrors. No worry with that! What I strongly disagree is if someone desires - sorry if I misinterpret - a photon size that differs from the size of the wave packet. Until a photon is absorbed, I see no reason nor means to define a photon size other than by the wave packet - there is no hidden variable. The photon can get shorter when it's absorbed, but not before. And the length of the wave packet, as well as the linewidth, agree fully with the lifetime of the atomic transition. We touch something fundamental in QM here: I dare to claim that the photon is not emitted briefly at a random moment whose distribution is defined by the lifetime of the atomic transition, but that the photon is emitted - or in equivalent words, the wave packet lasts - over the whole duration of the transition. Only this is consistent with with the observed linewidth. Then, this photon can be absorbed in a short time or place.

I expect no magnetic effect of the matrix materials cited (some very special polymers are ferromagnetic). Possible effect: one matrix material has left some magnetic particles touching an other while the other separates them well. For instance if the surface tension differs, or their viscosity. Your nanoparticles are permanent magnets, aren't they? Paraffin has allowed them to stick closer to an other. Other effect: the organic materials have reduced a little bit of the oxide at the particles' surface. Not equally much by both matrices. Or: different casting temperatures have acted differently on the FeOx.

Resonant movements follow often - but not always - a sine function because the movements are small, which linearizes the object's behaviour. Mechanical resonance modes are often numerous. By design, you can put the interesting mode at a frequency very different from the others (say, a spring and a mass), rather close to an other (the sound board of a piano), or have two modes at the same frequency (a round bell vibrates NS and EW). Depending on where you provoque the vibration and with what material, you excite more one mode or an other. That's the effect of varied mallet coatings at percussion instruments; the musician also targets different locations to get varied sounds. Vibrations do degrade materials. I expect a resonance to act exactly as any cyclic deformation, for which Woehler curves are known: http://en.wikipedia.org/wiki/Fatigue_(material) - Take margins versus the material's proof stress - The material shall have a proof stress as high as possible and can be brittle - The surface shall be smooth, and the polishing grooves be parallel to the elongation direction In a mechanical vibration of adequate material, the biggest loss uses to be the fastenings, but this improves quickly if holding at the nodes with the proper compliance and material. Then, the second loss is sound radiation, which improves with small, open areas, and with a material with slow sound propagation like bell bronze or invar.

Hello, radiocomm tinkerers and everyone! The Uda-Yagi antenna is the rake you see on roofs to receive television programmes: http://en.wikipedia.org/wiki/Yagi-Uda_antenna High-gain versions have over 20 elements, but sometimes the driven element plus a single reflector or director suffice: I see a reflector almost everytime two elements are used. Though, I believe a director instead has only advantages: The radiation resistance drops less; The gain is marginally better; The backward attenuation improves slightly; The passband is wider; The elements' width and spacing are less critical; The antenna is shorter. I've built antennas of both types, but not for the same use in order to compare them. The comparison stems from experimental data in hobbyist books. Has someone a different experience? If not, I'll consider that the usual practice can improve a little bit. Marc Schaefer, aka Enthalpy

The math behind text compression (hence lossless) uses to be fairly simple, like: - Observe what symbols or strings of symbols appear more often, and define shorter codes for these. - Check if a subsequence appears many times in a text, then spell it only once. What's less obvious: - If no information is available on the data (say, if you ignore it's a text) then no general method can exist. - Some people have misused the opportunity to introduce "amounts of information", entropy and the like. You guessed, these are not the ones who invented nor programmed the methods. As long as you decide to ignore these little useful additions, data compression uses reasonably simple and intuitive maths. Anyway, what you need to program the decompression algorithm is simple.

Even a single electron cannot be too close to the nucleus as a mean value. This is because a more compact electron, as it is a wave, has more kinetic energy. Past the optimum size, which defines the orbital, the kinetic energy increases more than the electric attraction brings.

True and authentic lasers have been built with one single atom in a cavity, and operated. Population inversion is then easier to achieve, as it reduces to "the atom is excited". Concentrated on single-atom lasers, I forgot to add: "an emitter whose population is inverted", which is a condition to lase. My mistake.

You think what you want, but figures are strictly against, whatever the method is. Cathedrals take several lifespans to build, and though, humans do build them, including right now. One argument for cathedrals is that one sees them progress through his work. An other is that people working on them hope to deserve better chance for Paradise. Drawing the parallel, you should invent a new religion that gives some kind of reason like Paradise for people to develop interstellar probes that take centuries to reach their goal, or even for people to board such a vessel. Some countries won't jail you for that. Fine, because religion is more adequate than science to justify such an undertaking.

Until satellites collide, they have very little influence on an other's trajectory, because they're so light as compared with the planet. It's completely different for electrons, whose repulsive force is similar to the attraction by the nucleus. You must forget any "plane orbit", and even trajectories, for electrons on stable orbitals around a nucleus. This image is not only outdated, it is also false and misleading. Electrons occupy some volume near a nucleus. http://winter.group.shef.ac.uk/orbitron/ The orbital gives a probability to find an electron around each possible position around a nucleus, but it does not tell "the electron is here at that time, there later". The historical improvement by quantum mechanics was this. It explains why electrons occupy a permanent volume instead of falling on the nucleus, and give numerically accurate values for the energy differences between the orbitals. With several electrons, you have one single wavefunction that gives the probability to find electron 1 around this position AND electron 2 around that position AND electron 3 ...etc. One wavefunction for all electrons gives the possibility, for instance, that every electron of a spherical orbital can be at any angle around the nucleus with equal probability density, BUT that two electrons are improbably near to an other, because they repel an other. From that single wavefunction for several electrons, one can deduce a probability density to find one electron - or rather any of the electrons - around a position near the nucleus, but this probability density does not include the information about electron correlation - that their repulsion keeps them apart. As far as I know, no algebraic solution exists for two electrons around a nucleus (a helium atom, say). Numerical solutions exist for a few electrons. As the numerical computation complexity must increase very quickly with the number of electrons, I expect algorithms to make simplifying assumptions, for instance that orbitals deep near the nucleus are little influenced by outer shells, or maybe that multi-electron orbitals resemble a weighted sum of few single-electron orbitals, multiplied by a correlation function that represents the repulsion between the electrons... Some trick is mandatory, because just 103 finite volume elements per electron and only 6 electrons for the 2p orbitals would require to solve over 1018 hypervolume elements, which is just impossible.

The Solar thermal rocket enables an extensive mission around Saturn. This first part describes up to the capture by a first moon. http://en.wikipedia.org/wiki/Saturn http://en.wikipedia.org/wiki/Moons_of_Saturn http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn A Delta IV Heavy shall place 8808kg at 3162m/s (10km2/s2) above Earth's gravity. The eighteen 4.57m Solar engines add 7176m/s, leaving 4944kg heading to Saturn in 5+ years. Acceleration can take more than 12 days, so lighter engines are better, with diaphragms or a small secondary mirror to limit the power at one Sun-Earth distance. At Saturn's 9.582UA, each engine pushes 29mN and uses 0.2kg/day hydrogen. The necessary inclination of the capture orbit at Saturn already constraints the launch window, so no gravitational assistance is taken from Jupiter, but smarter people could use Earth and possibly Venus to save on the launcher or extend the probe; both are welcome for this high-energy mission. Outer moons have too inclined orbits, so the first target is the puzzling two-tone Iapetus. http://en.wikipedia.org/wiki/Iapetus_(moon) http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Iapetus&Display=Facts&System=Metric Preferring Nasa over Wiki, the orbit has 3561Mm radius and 8.313° inclination versus Saturn's equator; lower moons are equatorial. The probe arrives with 5760m/s above Saturn's gravity and brakes by 4596m/s (502 days) before a 4195m/s pass at 3813Mm from Saturn's center. 276m/s more braking (30 days) reach a capture orbit of 14490Mm*3561Mm covered in 320 days - make science meanwhile. Two peri-Saturn kicks totalling 872m/s (95 days) achieve the circular 3561Mm orbit covered in 79.3 days. Well, at least if believing my spreadsheet: SaturnWeakBrake.zip Gravitational assistance by Iapetus (escape 573m/s) or an other moon might help. The probe reaches a polar Iapetus orbit to study it, with periapsis 1.2Mm from Iapetus' center (the moon has 735km radius) and apoapsis 15Mm in Iapetus' orbital plane (Iapetus' influence reaches 36Mm against Saturn). Computed from 200m/s above Iapetus' gravity, the manoeuvre passes with 342m/s at 1.92Mm from the moon's center, and reaches the final orbit in a single 169+23m/s (11.9+1.6 days) brake consuming 192m/s. IapetusWeakBrake.zip Smarter people might enter the moon's gravity where Saturn's one makes it easier - if any feasible. The described capture by Saturn then the moon consumes 5744-8=5736m/s, leaving 3116kg orbiting Iapetus. Marc Schaefer, aka Enthalpy

Finally, sketches of the machines with many nozzles and pipettes. Real machines will have many more than sketched to be compact and fast. The cooling gas can be air. The water can be pre-cooled around the freezing point. The batch machine can let the head oscillate to spread water drops better, while the continuous process machine can offset the pipettes over the rows. Experiments will tell better if drops splattering and rolling need these measures at all; black ice does not. Injectors would create smaller droplets than pipettes do, but the water must not freeze before touching the existing ice. The continuous machine may have elastomer wipers between the watery and cooling zones. Elastomer moulds, especially silicone, ease the removal. The conveyor belt may perhaps be a continuous elastomer, preferably reinforced with fibres over a small thickness; or be thin continuous metal, preferably covered with nickel with embedded Pfte; or have joints. Stereolithography may help manufacture the head. A (disk) saw would cut the blocks more precisely than the sketched guillotine. Real-time local thickness measurement and feedback looks useful. Marc Schaefer, aka Enthalpy