Monthly Archives: April 2008

No, Mr. Bond, I Expect you To Die

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Texans Build World’s Most Powerful Laser. Sort of.

More than a petawatt, but of course it’s pulsed, not continuous. Joules of energy in femtosecond-ish long pulses will yield a petawatt pulse, and it hasn’t earned its title quite yet, but it’s expected to.

The pulse compression, which they do with gratings, can also be done with prisms and chirped mirrors. Pulses tend to disperse, because the speed of light in a medium varies with wavelength, so the blue and red end of the pulse travel at slightly different speeds (normal dispersion will have the blue light going just a little slower than red light). The grating method uses grating pairs in place of mirrors, tilted so the blue end of the pulse travels a shorter distance so it can catch up to the red end; a similar arrangement can be done with pairs of prisms. Chirped mirrors have different reflective layers that allow the red end of the pulse to penetrate further before reflecting. Because this laser is going for high power in addition to short pulse length, they actually stretch the pulse out with gratings in the opposite orientation, before amplifying it (to limit the peak power in the gain medium) and then compressing.

Stop! In the Name of … Physics

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Stopping and Freezing a Bullet

Well, not so much a bullet as pretty much any paramagnetic atom (which means most atoms). Reminiscent of Sisyphus cooling only with really big magnetic pulses instead of polarization gradients.

The principle is similar to coilguns being developed by the military to launch projectiles, only “in reverse,” Raizen says. To those atoms with their dipoles aligned opposite the beam’s direction, the pulses are like hills they have to climb, except that each hill disappears before the atoms have a chance to slide “downhill.”

A Look Back at Laser Cooling

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Physical Review Letters is celebrating its Z=79 anniversary, and highlighting important letters. This week is:

Letters from the Past — A PRL Retrospective: This week’s Milestone Letter was originally published in 1970

Acceleration and Trapping of Particles by Radiation Pressure
A. Ashkin Phys. Rev. Lett. 24, 156 (1970)

This was a description of radiation pressure on transparent latex spheres, which felt a force when placed in laser light that had a gradient — the refraction gives rise to a force, or pressure, because the power is asymmetric across the sphere — the light changes direction, so the sphere must recoil, and the amount of recoil doesn’t balance. This is a precursor to a dipole force trap, which traps atoms at a field maximum (or minimum) of a light field, e.g. from focused laser. It also lays out radiation pressure by near-resonant scattering

The absorption and isotropic reradiation by spontaneous emission of resonance radiation striking an atom results in an average driving force or pressure in the direction of the incident light

which was the idea that led to laser cooling and optical molasses.

There is also a brief summary of the laser cooling history there this month, Landmarks: Laser Cooling of Atoms that takes you through the milestones up until the Nobel prizes for laser cooling and Bose-Einstein condensates.

You Say Potato, I Say Potahto

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You say MRI, I say NMR. These sound almost like William Steig would have used them, but not quite.

Chad over at Uncertain Principles give the lowdown on the phenomenon of Nuclear Magnetic Resonance

[…] Nuclear Magnetic Resonance (NMR), which is the “M” and the “R” in “Magnetic Resonance Imaging”– they ditched the “N” because “nuclear” is scaaaary, and doctors are wusses.

Yeah, but tell your doctor that when he’s got a sharp instrument or weird probe-thingy in his hand.

Igor … Throw the Main Switch!

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I’ve always wanted a huge double-pole, double-throw knife switch in the lab, because you can’t utter that phrase without it, but it’s probably a bad idea.

So you settle (just a little)

dgot.jpg

Because sometimes you just need to run a few hundred amps through something …

Life of Riley

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I’ve seen the movie National Treasure a couple of times (and it was on again recently) and while I enjoyed it, there are a few things that bugged me about it. Two physics-related, and one, not so much.

1. Why use a green laser to set off the temperature sensor alarm? Green is easy to see, and could have been noticed by anyone in the room. What would have been better is an infrared laser of the same power — the heating would have been pretty much identical. But since Riley was looking through the viewfinder of his camcorder, he would have been able to see an IR beam! Camcorders filter IR for the recorded signal, but tend not to do so for the signal going to the viewer. You can try this with a TV remote, and see it flash in the viewfinder of your camcorder or digital camera. So he could have heated the detector with a much lower risk of detection. Riley’s a techno-geek, so he should have known this.

2. They lucked out with the shadow of the steeple of Independence Hall. The sun’s relative location in the sky varies with the time of year, both vertically and horizontally, due to the tilt and orbit of the earth. This means the shadow would have traced out an analemma, which I have previously discussed. That the shadow pointed to the correct brick was fortuitous. There should have been some clue in there about time of year, and some mention of how to correct for that.

3. Jon Voight being so pissed off. His “all it will do is lead to another clue” anger would have been more believable to me if anyone in the family had ever figured out a clue before, and especially if he had failed to solve a riddle where others in the family had succeeded. As it sits, though, it only made sense for him to think the first clue was a fake. But once it was solved, he was proven wrong.

Don't Waste Your Money

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Sunny Kalara at “Talk Like a Physicist” wants a 576 MegaPixel camera, and while the post talks the physicist talk, it doesn’t walk the physicist walk (if there is such a thing).

And the gauntlet has been thrown down —

Next time a person tells me that I don’t need a digital camera with more than 6-10 mega pixel resolution, I am going to hit him/her on the head with the sharp corner of my camera

it appears I am risking photogricide by saying, “But that might actually be true. More pixels do not necessarily make a better picture.”

A further claim is:

Apparently, if you converted the resolution of what an eye can perceive in to mega pixel, it turns out that an eye can see at 576 Mega pixel. So, I want my camera to be at least 576MP camera; is that too much to ask?

When I look out, I see in stereo; with full depth – is it too much to expect that my camera does the same?

I want to take pictures for the unknown technology that will be available to me in 20 years, not for the 3×4 print that can be printed now!

Both the 6-10 MP claim and the 576 MP claim are based on a few assumptions, and as any good physicist knows, you have to make sure these assumptions are not violated in your analysis. So let’s put the Pentax down and talk about this.
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