Category Archives: Physics

Deconstructing BJ Thomas

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Myth of raindrop formation exploded

“Myth” may be overstating it, which is par for the course. But someone actually observed what happens to large raindrops, to see why we get the distribution of raindrop sizes.

The pair got the idea from the unusual but well known transformations of fuel droplets travelling at high speed in diesel engines. As they travel, drops flatten from a sphere into a pancake-like disc; this catches passing air and inflates like a liquid parachute that eventually explodes in a shower of smaller droplets.

New Tool for the Stealth Paparazzi

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Invisible flash takes photos without the glare

Although the dark flash gives a crisp image without disturbing those in the picture, the results have an odd colour balance that looks like a view through a night-vision scope.

To give the pictures more normal hues, Krishnan and Fergus used colour information from a brief, flash-free photograph of the same scene taken quickly after the dark flash image.

The second image is dim and blurry, and so it lacks some of the fine detail of its dark flash twin. However, the researchers use software to combine the sharp detail from the first image with the natural colours from the second image, resulting in a remarkably natural end result.

The Magnetic DeeJay

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Mix Master Magnets

A rotating magnetic field can induce tiny beads in a fluid to link up into spinning strings, researchers report in the July Physical Review E. The team measured the effects of different field configurations to find the most efficient way to use the beads to stir the fluid. These “self-assembled stir bars” could mix even the hard-to-stir fluids in micron-sized channels within miniaturized chemical analysis chips that are in development.

Surely You're Doodling, Mr. Feynman

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Landmarks: Powerful Pictures

Every popular explanation of particle physics is liberally illustrated with cartoon-like pictures of straight and wiggly lines representing electrons, photons, and quarks, interacting with one another. These so-called Feynman diagrams were introduced by Richard Feynman in the Physical Review in 1949, and they quickly became an essential tool for particle physicists. Early on, Feynman struggled to explain the meaning of the diagrams to his fellow physicists. But using them, he came up with easy answers to difficult problems in quantum mechanics and ultimately won a share of the Nobel Prize.

Projecting

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Chad over at Uncertain Principles has been doing a great series of interviews about career paths other than academia, in The Project for Non-Academic Science.

One of the difficulties with trying to broaden the usual definition of scientists is that there’s not a lot of press for non-academic science. Academic culture is so strongly focused on academic careers that people don’t hear a lot about careers outside the usual Ph.D-postdoc-tenure-track-job track. Which helps feed the stress and angst regarding the job market.

There is a listing of interviews with more to come.

I didn’t see a point in volunteering, since I have my own (albeit smaller) platform, and I’ve already given a career path summary this hits most of the main points in those interviews. A few details that are missing are about how I got my current job and how someone else would go about getting a similar job. I got it through informal networking — I already had met my current boss because atomic physics is a small, and therefore incestuous, community, and I got an email that was forwarded a few times advertising the position. I had already responded to an earlier advertisement for a position, which was later withdrawn due to funding issues, but used that to finagle a visit to the Observatory after a conference in DC. So when a job opened up for real, I was essentially “pre-interviewed” for it, and since I had the requisite background in laser trapping, I jumped on to the short list immediately. Since there really aren’t academic programs that do timekeeping, the prep work is all in the atomic physics for atomic clock R&D.

If I were going to do a full interview, I would interview myself like William Hurt did in The Big Chill

So you went to Oregon State University to enter the doctoral program in physics. And you just had to finish that dissertation.

I didn’t have to. I’m not hung up on this completion thing.

Then you had several jobs, all of which you quit.

What are you getting at? They’re called postdocs. I was evolving. I’m still evolving.

But your real claim to fame came as a cartoonist in Physics Today

I wouldn’t call it fame, exactly. I was a few cartoons, and I may have had a small, deeply disturbed following.

What are you doing now? Or I should say, what have you evolved into now?

Oh, I’m in research.

What are you researching?

Umm, an atomic device.

What kind of atomic device?

I … don’t … have to answer that.

Sorry, gotta go.

Just answer that last question! (muffled struggle, fade out)

Taking Umbrage

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And Penumbrage, I guess. The Big Picture: The longest solar eclipse of the century

Earlier today, the moon passed directly in front of the sun, causing a total solar eclipse that crossed nearly half the Earth – through India, Nepal, Bangladesh, Bhutan, Myanmar and China. Today’s was the longest total solar eclipse of the 21st century, lasting as much as 6 minutes and 39 seconds in a few areas. Despite cloudy skies in many of the populated areas in the path, millions of people gathered outside to gaze up and view this rare event. Collected here are a few images of the eclipse, and those people who came out to watch. (33 photos total)

Here’s what it looked like from the ISS

Putting Your Thermoregulation Where Your Mouth Is

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Toucan Beak Is New Kind of ‘Heating Bill’

[U]sing infrared thermography, a type of temperature-sensing video originally developed by the U.S. military, scientists have tracked the pattern of heat distribution across the toucan’s body under changing outside temperatures. When the bird got too hot, it released heat by sending blood to its highly vascular but uninsulated beak. In cooler weather, the toucan constricted blood vessels in its beak to conserve heat and stay warm.

Vintage Lab Pics: The Laser System

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I already showed the vacuum system from my grad school days. This is the laser system that drove it. Slowing and trapping a thermal beam of atoms and then creating a new cold beam requires several lasers at different frequencies.

This first picture is a diode laser system, obviously home-built; this pre-dates any sort of commercially available system by several years, and perhaps a decade. On the far left are the electrical connections and the on/off switch. Power is needed for the laser, a piezoelectric transducer stack and the thermoelectic cooler, and a signal from a thermistor is fed back for coarse temperature tuning/stabilization of the wavelength. The diode is mounted in the thin rectangular block and has a collimating lens mounted in the thicker one; as I recall the lens position was adjusted with an external jig and then glued down.

osu-diode-laser

Light leaves the diode and hits the grating, reflecting off to the bottom, but the grating is blazed — the lines are angled, and in this position one of the diffraction orders is reflected back into the laser, which forces the laser to operate at that frequency. Thus, by changing the angle slightly, the wavelength can be tuned over some small range, perhaps a few nanometers. The grating is mounted on a small kinematic mount, and this obscures a gap between two parts of the mount. At the lower right you can see the gap where the piezo stack is, and at the upper left, near the screw, is the pivot point.

The entire block is mounted on a thermoelectric cooler to stabilize the temperature. Laser diodes are coarsely temperature-tunable, so the temperature is chosen to get you close to the desired wavelength (780.24 nm for Rb-85). When operating, this would be covered with a plexiglass housing to act as a thermal barrier and a baffle for air currents, and on later designs contained a tray for a desiccant to help prevent condensation on the cold laser.

Here is the table, with a couple of lasers in the foreground. Light goes out of the side of the boxes and hits the turning mirrors; some of the light is picked off and sent into the spectroscopy cells visible just past the lasers (the one on the left is closer) This ensured the lasers were on resonance.

osu-diode-laser-setup

This is the whole laser table, showing the vacuum system on the right. Much of the equipment is on shelves above the optical table, and this design ensured the attraction of shorter personnel to the lab, as might be predicted by Murphy’s law.

osu-laser-table

The blue boxes on the center-line of the table are fast photodiodes; we used a beat between lasers to generate a locking signal for those not locked to the spectroscopy signals, and could tune the other lasers several MHz away using this technique. This allowed us to tune the trapping laser beams such that the trap was not stationary in the lab frame. The atoms would feel a force to eject them from the atomic funnel, and the lasers would become equal due to the Doppler shift, once they atoms were moving at the right speed.

Goosing the Droplet

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Squirting Water without a Gun

Fluid jets are normally made by forcing liquid through a nozzle, such as in a squirt gun or a syringe. But in the 10 July Physical Review Letters, researchers report a way to induce a fluid jet to burst from an isolated droplet. The team placed a liquid droplet on a surface and blasted it with focused surface acoustic waves–nano-sized versions of the ground-shaking waves from earthquakes–causing the droplet to shoot upward in a narrow stream. The researchers believe the technique could be useful in drug delivery, biomedical research, and inkjet printing.