There will be no rescue shuttle for the Atlantis flight. It is being flown with a skeleton crew that can live in the ISS if needed and await evacuation by Soyuz. You would have been right, but the plan changed. Atlantis is going to be prepped for rescue of Endeavour. Since most of the cost of an Atlantis launch will have to be spent in those preparations anyway, it's just being launched with supplies and the small crew if a rescue of Endeavour is not needed.

Thanks for posting that pdf, but your bandwidth limit is exceeded. Do you have a mirror?

Moon dust should not be so quickly dismissed. It isn't like sand on Earth which has its edges blunted by wind and water erosion, because there is no such erosion on the moon. Moon dust is essentially microscopic shards of broken glass with very sharp edges. It's really nasty stuff, it sticks to everything like barbs. Managing moon dust will have to be a major practical consideration for a lunar colony.

This would be the slowest method possible of sorting isotopes. Unless you really really want just a single atom of a particular isotope floating in free space without electrodes or magnets around, there are better methods. For bulk separation you can only really go the centrifuge or gaseous diffusion route. If you have enough power available there are electromagnetic methods. If you want very small samples of ions you can use penning traps, as are used in several labs around the world for super precise mass measurements. It would be easy to make one with enough resolution to separate out isotopes of an elemental sample, some labs can even separate out different excited states of a single isotope from each other, but only a few ions at a time. Still better than using this optical method.

There is not, they will annihilate at any temperature. For specific arrangements there can be states that live longer than other states. For example, positronium, the bound state of an electron and a positron (basically hydrogen but with the protron replaced by a positron), lives much longer before annihilation when it is in a higher energy state. The ground state has an annihilation lifetime of only 125 ps, compared to 1.1 us for the 2S state.

Why doesn't slashdot display the mu ascii character?

Okay, now I've RTFA, and it is one of the worst science articles I've ever read outside New Scientist or Conservapedia. Let us delve in:

But the isotope, helium-3, like many rare Earth elements, has been in high demand with only limited supply.

Helium is not a rare earth element. I have a feeling this line was inserted just to pitch the link below it.

The gas is part of the leftovers that come from cooking up a hydrogen bomb: you know two parts uranium; one part tritium

No idea where that ratio came from. It's not true and irrelevant.

While there are other ways of decaying tritium without needing to build a bomb to do it...

Is the author fully ignorant of nuclear physics or is she gearing up for some kind of scam where she sells "Tritium Decayers" to the government?

But if a patient takes a breath of helium-3, the resulting MRI is so bright it looks as though the patient inhaled a light bulb.

Not as bad, but misses a great opportunity to explain HOW He3 helps lung imaging. He-3 doesn't exist in any significant quantities in the body, so you can tune the MRI to look for that nucleus and bam, you can see the shape of whatever you fill with it.

Until the FDA approves the recycled helium for humans...

The FDA needs to approve this? That's odd, I wonder why. Too bad you didn't explain why or tell us what stage of approval its in.

For a party that suddenly saw the balloons all pop, despite the warnings, everyone jumped.

wat

There are large amounts of He3 being made in heavy water reactors that is not being collected. Until now there has been little motivation to go through the trouble and expense of modifying these reactors to extract it, but it's not THAT hard. At some point it will just be done and then we'll be fine. This is only a short-term problem. DNRTFA, of course.

Thanks!

Very useful post, but it doesn't address my concern. The geometry and coherency are not the issue, it is the stimulated emission. I believe your description of a traditional laser is not quite right: once the initial spontaneous light starts passing through the rod it induces stimulated emission from the other electrons. It is true that mirrors are not necessary, as in the NIF system for example.

As I understand it the electrons going through the undulators are not undergoing stimulated emission, but are emitting synchrotron radiation as they are accelerated by the undulator's magnetic field. If the electrons are bunched they emit in phase and thus the light is coherent. You're saying that somehow the previously emitted photons stimulate emission from other electrons? What energy do the electrons have that they can emit in stimulated emission? I don't see how it can be their kinetic energy because that would violate conservation of momentum.

Please do explain, because if I'm wrong I'd be quite fascinated to learn what's really going on.

Thanks for the comment.

"Stimulated emission" has a specific meaning in physics: an electron (or in theory some other charged particle) is in an excited state, and a photon passing by causes that electron to drop down to a lower energy level and emit a photon of its own. In the case of the FEL we're certainly doing something to the electrons to make them emit photons, but it's not "stimulated emission" in the correct sense.

The nature of stimulated emission is what requires population inversion or something else similar. Population inversion just refers to getting all the electrons in the system up into that excited state. Perhaps you don't have to go to the extreme, but the point is you get the electrons to store up energy, then de-excite them by passing photons nearby.

Unfortunately the true nature of FEL's isn't universally known outside of the x-ray field. I'm a nuclear physicist and went to a talk on FELs and what they can do for nuclear physics, and afterwards about half the physicists I talked to from the audience didn't know, even after the talk, exactly how the FELs worked. The name "laser" gave them pre-conceptions about the fundamental principles that must govern the FEL so that when the description of its operation came all that was left was confusion.

I also looked at that wikipedia article before making my previous post. It seems okay for wikipedia, but am considering making a note on the page about this. I may want to consult with laser experts first though. Maybe that's why I posted on slashdot instead.

I am a physicist-in-training (grad student), and when I first heard of "free electron lasers" I was extremely impressed, because getting electrons into a multi-keV energy state that can lase without atoms involved sounded nearly impossible. Turns out it is, because these are not actually "lasers" the sense of Light Amplification by Stimulated Emission of Radiation. There is no population inversion as in real lasers.

The name of this specific FEL, the "Linac Coherent Light Source", is a much more correct name. They shoot electrons through a wiggler, and as they wiggle they emit coherent photons. Coherency is they key property of laser light, but the name refers to the method of light creation more than the actual output. I don't know why the x-ray community has felt the need to use this misleading name.

The USS Jimmy Carter already exists, as an attack submarine. wikilink

Tribes / Tribes 2 are the greatest FPS's ever made (reasonable people can disagree about the relative ranking of those two). Their popularity was hindered by lack of advertisement and a steep learning curve. If a major studio did a remake right (made by people who love it) and advertised it well, then maybe it could really take off.

For any of you who haven't tried it or haven't played for a while and want to get back in, there's still a breath of life. Go to TribesNext, download their client and patch and you can play Tribes 2.

It really is like no other game. If you have never seen it I suggest the videos below that show off its epicness. I don't know how anyone can see these and not want to play it forever.

Tribes 1: Legacy Part 1, Part 2, Part 3

Once you've played it no other FPS will match up unless it is made in Tribes' image. "For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return. "

I used to do simulations of galaxy collisions, and dark matter is the easiest part to put into the simulation because it is assumed to be collisionless. It just goes wherever the gravitational potential points it (of course, calculating that gravitational potential can be somewhat complicated if you want to smooth over the rough spots caused by the finite number of particles simulated). The hard part is the gas, which has much more complicated physics due to the collisions, heating, cooling, and radiation on top of the gravitational physics. This also removes the scale-independence from dark-matter-only simulations. These are some of the reasons why early galaxy simulations were largely of elliptical galaxies, because they contain negligible amounts of gas.

Yes, and the difference in isotope is very important here. Si-32 is a beta emitter, which is the type of decay that one might possibly expect to be affected by neutrinos if they had any effect at all, because neutrinos are emitted along with the beta. Ra-226 and Pu-238 are both alpha emitters, which makes the seasonal variation in Ra-226 even stranger because neutrinos are not involved at all in alpha decay.