Old-School Experiment with Light

Uncertain Principles: Measuring the Angular Momentum of Light

Being a formal and mathematical book, it pretty much leaves the subject there, but my immediate reaction is to look for an experiment that proves the angular momentum is real. So I did a little Googling, and turned up a paper from 1936(!) that does just that. And I talked about it in class, because I think experiments are way cool, and like to bring them in whenever possible. Having looked this up and read it carefully, I figure I might as well write it up for ResearchBlogging while I’m at it.

Not a Peep Out of You!

Peep Research

There are several important implications indicated by these [low pressure] results:

1. Peeps are poorly equipped as fighter pilots, supporting the Supreme Court ruling that banning peeps from the cockpits of F-16 planes in combat does not violate the Anti-Discrimination Act of 1992. (Orville & Wilbur Peep vs. US Government, 1994)
2. Peeps should exercise caution when ascending after deep-sea diving excursions, as sudden decreases in pressure may exceed the structural integrity of visceral parenchyma.
3. This may explain the tragic demise of the Col. Lewis Peep expedition which attempted to reach the peak of Mt. Everest in the spring of 1856. It is important to note that these data do not exclude alternative theories suggesting that the group was devoured by a pack of diabetic mountain lions. (Schroedinger, Heisenberg, & Bohr, 1922).

Smoke but no Mirrors

Optical Trapping and the Momentum of Light

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The explanation talks about the wave-particle duality, but I think that’s a distraction. This is a dipole force phenomenon; the beam’s intensity is greatest at the center, and where the focal point occurs, as shown in the drawing at the end of the post. This gives rise to a gradient in the electric field. If you put a dielectric particle in this region, it will feel a force in the direction of the field maximum, or toward the highest intensity of light.

The Replacements

If it sticks, force it. If it breaks, it needed replacing anyway.

Uncertain Principles: Eucatastrophe in Physics

Sometimes, an equipment failure can be the best thing that happens to an experiment. This is particularly true in labs that rely on short-term labor like post-docs (who are generally hired for about two years) and graduate students (who are in a given lab longer, but typically in charge of the experiment for only a few years), where kludgey short-term solutions implemented in order to get fast results can become locked in as new experiments build on the first one.

I’m proud to say that there is no duct tape in the devices we’ve built. It exists in one or two places in the lab (even involving ductwork). When you’re in it for the long haul, there’s more incentive to do things right. But I remember at TRIUMF, we needed a variable voltage reference, which ended up being a 2 9V batteries in series discharging slowly through a MegaOhm-ish potentiometer acting as a voltage divider. Dial up what you need, and it lasted long enough that it was easier to replace the batteries than do up a proper circuit.

Whoa, Nellie!

Slowed light breaks record

Although this separation process involves distorting the pulse-storing BEC – and hence the nature of the revived pulse – it is completely deterministic, which means that no quantum information is lost. By doing so, the team was able to store the pulse for up to about 1.5 s, shattering the previous record of about 600 ms. Furthermore, the fidelity of the revived pulse – the ratio of output energy to input energy – was more that 100 times better than previous systems.


Another video, reminiscent of the viral popcorn-popped-with-a-cellphone video I discussed a while back

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And, in fact one of the response videos is with popcorn

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Also one involving toast

Objections: One is electrostatic. Matt has been discussing static charge distributions recently (here and here) and it’s very important to note that he’s discussing charge distributions on conductors — the charges can easily rearrange themselves. But in these video examples, the people and the targets are not conductors. So while you might build up some static charge on a person (in a very questionable display of boys gleefully rubbing other boys with balloons. Not that there’s anything wrong with that, if that’s who you are, balloon-fetish-freaks). A discharge to another insulator just isn’t going to send the energy where you want it to. A small discharge will even out the potential difference, and you’re done. A full discharge needs to be to a conductor, preferably a grounded one.

Speaking of sending the energy, how much energy are we discussing here? I’m not sure how much energy it takes for an eggsplosion, but I’m guessing we’re talking well above a few Joules. Accounting for my slight overestimation of the water content in the earlier popcorn analysis, it probably still takes somewhere north of 10 Joules of energy to pop a single kernel. Can we get anywhere close with static charge?

The energy stored in a capacitor is 1/2 CV^2. The capacitance of the human body is a few hundred picofarads. Let’s be generous and say it’s 2,000 picofarads (pico is 10^-12). How much of a potential difference do we need for 10 Joules? Do the math — it’s 100 kV. A few kV makes for a painful spark when discharging to a doorknob. A 5 mm spark between conducting spheres happens at about 16 kV. A realistic spark leaves us at significantly less than a Joule of energy.


Vintage Lab Pictures

I was decreasing the local entropy in a small part of my abode and found a shoebox full of photos which happened to contain a few shots of my grad school lab, in all its glory. We were building an interferometer which would use cold atoms, which means relatively large deBroglie wavelengths and a correspondingly small system. But one has to trap the atoms and cool them down first, and then generate the cold beam of atoms to feed into the diffraction gratings that comprise the interferometer, so the system is still quite complex. Since I was the first PhD candidate in the group, it meant I was involved in the construction of most of the components of the lab apparatus pictured here. It also meant a lot of fumbling in the dark, both figuratively and literally, since the only one with any experience with doing this kind of work was the PI, who had other duties (like teaching, writing grant proposals, etc.) It was a big day when the group finally reached the point where we didn’t want him playing with the experiment, because we knew more about the details than he did. That took a couple of years.

Here’s the vacuum system.


At the bottom of the picture is the oven, which was basically a pipe bomb with a hole in it (under vacuum, of course), and when heated sufficiently, would spray Rubidium atoms out. These were collimated with a second hole a few cm away (the extra Rb was collected on some cold metal and sent back into the oven during a refill cycle we ran each night after running the oven). Underneath is a turbo pump and a roughing pump; the small green hose was for adding dry nitrogen if we needed to open the system up. The nitrogen was supposed to keep the Rb from reacting with water in the air. It never worked.

The long pipe leads down to the next part of the vacuum system, and it has a “wobble stick” that blocks the atomic beam, if desired, and a valve to isolate the two parts of the system. You can also see an ion gauge on the left which was normally off so the light from the filament wouldn’t register on any photodiodes or the photomultiplier tube.

The tube is about a meter or so long, and needed to be so we could slow the atoms down. A laser was sent down the tube from the other end, and “chirped” in frequency to account for the changing Doppler shift — you wanted the laser to track the resonance of the atoms as they slowed down, so they would keep scattering photons. Once the atoms were slow enough, the laser turned off, and the atoms drifted into the bigger chamber, where they were trapped in a two-dimensional trap called an “atomic funnel.” They were able to move along the axis of the funnel, and were forced out to the left into the region where the interferometer would be. I made the gratings for the interferometer, but never got to the point where the apparatus was finished.

Here is an early version of the funnel:


It’s a “hairpin” made of copper tubing so we could water-cool it while sending several amps through it, and it creates a quadrupole field in two dimensions, so that atoms would be trapped into a pencil shape along the central axis, left-to-right. It’s supported from below by two insulated standoffs — you can see the ceramic, i.e. insulating, part — to keep it straight. The dark hole straight through it is the tube leading to the oven and the lighter window on the left would be where one of the six trapping lasers would come in. Up top there is a tube with some lenses in it for imaging the trap, and on the right is a target for aligning the lasers. The target is on a linear feedthrough, and the target could be inserted into the center and all of the lasers sent through it to make sure they all overlapped where they should.

The funnel tubes were originally soldered together. What you see here is version 2, because at one point we lost cooling water while the system was energized, and the solder melted. Which meant that lots of water was introduced to the system and it became a giant fish tank, sans fish. Fortunately, we had interlocks in the system so that if the pressure rose too high the pneumatic valves would shut (that’s what the rest of the green tubing is for in the first picture), and the damage was limited. The white blobs at the ends of the little elbows are torr-seal, which we used to repair the trap. This eventually failed, too, and we replaced it with a much better trap fashioned from a single piece of copper tubing.


I graduated after having built much of this and characterizing the atomic beam. We were able to extract atoms going up to 10 m/s — this was adjustable, depending on the laser frequencies of the different beams, but atoms going too slow would miss the interferometer because of gravity, so we didn’t bother trying to generate a beam going below a couple of m/s. The atoms were also somewhat cold — less than a milliKelvin — so that the beam didn’t spread out too much. At the time, one other funnel had been built, but it had on of its lasers along the axis of the funnel, which precluded putting any sort of target there. This was novel enough for a publication (T. B. Swanson, N. J. Silva, S. K. Mayer, J. J. Maki, and D. H. McIntyre, “Rubidium atomic funnel,” J. Opt. Soc. Am. B 13, 1833-1836 (1996)) and more importantly, a degree.

A Lesson in the Scientific Method

This has it all. A scientist, working on his own, discovering something new (and useful) using proper scientific methodology … and he’s in high school. WCI student isolates microbe that lunches on plastic bags

First, he ground plastic bags into a powder. Next, he used ordinary household chemicals, yeast and tap water to create a solution that would encourage microbe growth. To that, he added the plastic powder and dirt. Then the solution sat in a shaker at 30 degrees.

After three months of upping the concentration of plastic-eating microbes, Burd filtered out the remaining plastic powder and put his bacterial culture into three flasks with strips of plastic cut from grocery bags. As a control, he also added plastic to flasks containing boiled and therefore dead bacterial culture.

Six weeks later, he weighed the strips of plastic. The control strips were the same. But the ones that had been in the live bacterial culture weighed an average of 17 per cent less.

That wasn’t good enough for Burd. To identify the bacteria in his culture, he let them grow on agar plates and found he had four types of microbes. He tested those on more plastic strips and found only the second was capable of significant plastic degradation.

Oh, and yes, he won the top prize at the science fair.

That Giant Sucking Sound

Is coming from sciencegeekgirl’s Hands on Science Sunday: Feeling pressured?

All you need is a big trash bag and an industrial strength vacuum cleaner, and a willing victim (er, “faithful subject of science.”) The victim (aka “subject) gets inside the bag, and once you suck all the air out of the bag with the vacuum cleaner, they’ll feel an intense pressure. SAFETY FIRST! Read this PDF writeup of the activity (from the Exploratorium’s Eric Muller) for all the ins-and-outs and safety factors in doing this with your kids. (Words to the wise — don’t put your head inside the bag!) It’s stunning — try it if you can.

Sorry, Wrong Model

Extreme Ultraviolet Laser Challenges Einstein

No, not really. (Any headline that implies that Einstein might be wrong is invariably incorrect — these are things that have been tested for 100 years)

In the new study, the physicists shot xenon atoms with FLASH, an x-ray laser that uses intense photons in the extreme ultraviolet energy range, about forty times the energy of visible light. The xenon atoms lost a whopping 21 electrons at once, which indicates that it was hit by 50 photons simultaneously. Not only that, but the first electrons to pop off were from an inner region of the atom, like if you peeled an onion starting with the second layer.

Here’s the thing: there are situations where you look at E&M interactions classically. If you put a large electric field around a material, you can ionize it; even though E&M interactions are explained by virtual photons, this is a case where classical physics works out just fine, and a high-intensity laser has a large electric field. Another case is a FORT (far off-resonant dipole force trap), where the intensity profile of a focused laser gives an electric field gradient.

So ionizing 21 electrons is pretty cool, but one needs to be careful in how one phrases these “challenge to Einstein” headlines. You have models of light that are wave-like and particle-like, and you use the model that works. The lesson of the photoelectric effect is NOT that light always exhibits particle properties.