Recently in 2008 Science Category

Cool

It would seem that I am just bad at blogging this year. Maybe the world has finally surpassed the level of ridiculous that I even feel able to comment on. That, or I'm just doing other stuff at the moment.

However, I would like to take a moment to mention that my alma mater just scored a 28th Nobel Prize in Physics.

Also, I think it's pretty neat that we're starting to spot relatively ordinary meteoroids before they hit the atmosphere. If I read the ephemerides right, it was still something like three times as distant as the Moon when discovered -- the robo-scopes are getting pretty good if they can pick out a truck-sized boulder at that distance. No pictures of the fireball yet, but spaceweather is reporting now that the explosion was picked up by an infrasound array in Kenya and maybe seen by a jet liner over the Sahara.

Easy

If you've ever gotten into a discussion about nuclear weapons with a physicist, at some point it was probably remarked upon at some point that non-proliferation is hard, because at the end of the day, once a country has the requisite weapons-grade fuel, a few grad students could likely build one. As it turns out, this is true, and the US government has experimentally verified it.

I ran across this article in the Guardian that describes a classified project in the '60s, in which two physics PhDs with no knowledge of nuclear weapons were given only access to public libraries and made head researchers of a simulated weapons lab. In under two years, they had produced an (also simulated) working plutonium bomb that could be built in a machine shop. This is not especially surprising, really -- in reality, the biggest risk would be that they'd have an accident and die of radiation poisoning before they finished.

As an aside, doesn't this sound like a fascinating RPG? Manhattan Project RPG: "I redesign the firing pin to be 2 mm longer and have the trigger re-machined." The GM replies, "You didn't replace the beryllium housing. 1d4 technicians have perished in a neutron burst."

Interesting Stuff

Continuing link dumpage, this time from my reading in the DIY scene. I wonder if I can make EGAD nearly-daily again if I just give up on covering the mid-week.

3-D printers (aka robotic prototypers, rapid fabricators ... a whole category of machines that take in 3-D engineering design files and turn them into physical objects by various means) are trickling down to the mass consciousness and the amateur level. By now I've seen probably dozens of examples of people whipping together such devices, typically winding up with something that is a grotesque hybrid of a moveable stage and a hot-glue gun. One of my favorites will render your designs in Cheez Whiz.

However, the most delightful project -- in a "we're gonna have to listen to techno" sort of way -- RepRap is now self-hosting (mostly). As in, one RepRap can make another RepRap. That is so cool. I want.

Okay, what else?

Here's a short video and write-up about a form of tempered glass that's been known about for centuries and has some quite odd properties -- Prince Rupert's Drops are nearly indestructible until you tweak the end of the tail. Then they violently explode.

If I had more time, I would totally enter the Underhanded C Contest.

Evil Mad Scientist labs has been around for twenty millicenturies.

Josh Marshall ran across and posted about a neat article in The Atlantic about GM's crash program to be first to market with a practical electric car. They're aiming for basically now, in car time, but given the state the company is in it's not clear that even that is enough time.

A Cosmic Focus Knob

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There's a general problem in astronomy, which is that we almost totally lack depth perception. This is not to say that we don't know the distances to things, for we frequently do. That's what the cosmic distance ladder is for. However, except for quite nearby things, for which parallax measurements can be done, you're quite some ways up the ladder and you often have only approximate distances only for certain types of objects.

Case in point, at some journal club talk last year a number of interesting conclusions hinged on whether a chunk of radio emission was coming from a particular galaxy however many Megaparsecs distant, or else was far in front of or behind it. For the galaxy, the distance is reasonably well known (certainly via the cosmological redshift, possibly via other means as well). For the radio emission, not so much. It was a continuum source, which means there were no spectral lines to give a redshift, and it wasn't a galaxy, which rules out most of the other higher rungs of the distance ladder.

With present techniques, the Magellanic Clouds are the only galaxies for which one could really conceive of getting parallax measurements. This would have to be done using long-baseline interferometry of radio point sources, of course, using something like the VLBA. The limit is set by the angular resolution of your instrument, since the parallax is nothing more than measuring a (tiny) angle on the sky. For the VLBA, observing at 10 cm from stations around 10,000 km apart, you can get about a milli-arcsecond. Using the Earth's orbit as your separation, that gets you out to a few kiloparsecs.

What could you conceive of building, anyway? If you want a super-long baseline, you need to stick to radio techniques, where you record the waveforms and feed them into a correlator elsewhere. Electronics are getting better, so I can imagine that working up to several hundred GHz, so millimeter waves. We're pretty good at chucking things into solar orbits, and at powering things off solar energy at Earth-like distances from the sun, too. So I can conceive of building a millimeter wavelength interferometer array with a baseline of a couple of AU. And that gets you to about a nanoarcsecond. With this kind of resolution, you could just resolve a penny held up to the sun by an astronaut at Saturn. (Or, someone else with this telescope could see the city lights of Earth from halfway across the Galaxy.)

If you consider that the Solar System moves at about 220 km/s around the galactic center, if you're willing to wait a year as with traditional parallaxes, you get a baseline of 50 AU or so. In principle, you can then measure a parallax out to basically the edge of the universe, 50 Gigaparsecs or so, although you'll have trouble defining a fixed background if you do that. However, this probably wouldn't help with the sort of diffuse source that I started out discussing.

I often wonder if this would work. Measure distance by defocusing an interferometer. By this I mean, interferometric correlators work on the assumption that the incoming wavefronts are flat. The constant-phase surface of a radio signal leaving a point source is actually a sphere (usually), but at a distance of light years, you don't especially care. But I guarantee that you have seen this effect before.

Turn the focus knob of a pair of binoculars, or one of those old cameras that actually made you focus it yourself. Objects at one distance will appear crisply, while objects in the foreground and background become fuzzy. The optics of the focus mechanism are compensating for a specific amount of this wavefront curvature. You would be correct in imagining that this could be used to measure distance, but only out to a certain maximum.

If you push the focus knob all the way to one end, generally marked as "infinity", then everything beyond some distant point will be in focus. Past that is the far field of your device, beyond which the wavefront curvature doesn't matter, and because of which nobody worries about depth of field when photographing landscapes or nebulae. The far field distance is roughly the square of your aperture size divided by wavelength. For your binoculars (3 cm, 550 nm) it's a kilometer or so. For the VLBA (10,000 km, 10 cm) this is about a tenth of a light year, but for our really ambitious yet conceivable telescope (2 AU, 1 mm) this becomes a few Gigaparsecs.

Now, I think an algorithm based on this technique would probably work, even on the diffuse cloud discussed above. You just have to adjust the depth of field until the cloud is at its smallest. You also wouldn't need a fixed background against which to compare (usually distant quasars today, but they probably wouldn't be distant enough for this kind of work). Now, it's rather complex to make a good interferometric image of a spread-out thing, but my understanding is that it's possible. Maybe some of the radio astronomers reading this will set me straight if not.

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