Category Archives: Experiments

Rolling, Rolling, Rolling

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A few days ago I was relating the cans-in-a-blanket problem, and retelling the vacuum joke and story to someone who had not yet heard them. One of my colleagues commented on a problem he had been given during an interview, also involving cans of soda:

You have two cans, one filled with ice and the other with liquid, but otherwise identical. The cans are rolled down an incline. Which one reaches the bottom first?

Much like the previous problem, I think there is a common misconception at play here for some people who get the answer wrong, and I’ll get to the explanation below. One of the people in the conversation said his first impulse was the wrong answer, but when we discussed the physics, we all agreed on the solution.

I set up to do a demonstration, though my first attempt was thwarted — I filled up a can with water and popped it in the freezer, hoping the can would be strong enough to hold together and have the ice expand vertically. It wasn’t.

first-attempt

I think the problem being that since ice will freeze from the top down and outside-in, the ice adhered to the can too well to let it expand upward as much as I hoped. (BTW — Black Cherry Citrus? Blecch. I bought it by accident when they redesigned their color scheme and introduced the flavor)

So I did it again, adding a little bit of water and letting that freeze, repeating the process several times until it was full, and it worked. Here is the experiment to investigate the problem given above:

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For those who think that the liquid-filled can will roll more slowly, I think I know what the misconception is: most of us have seen or done the experiment with spinning an egg, and a hard-boiled egg spins readily while the unboiled egg doesn’t. So the intuition is that since liquids don’t spin readily, the liquid-filled can won’t want to roll very fast. And, as we can see, that’s wrong.

The reason the intuition is wrong is from a misinterpretation of the reason the unboiled egg doesn’t spin — it’s because it’s difficult to transfer energy and angular momentum to the liquid by spinning the container; the coupling between them is weak. And angular momentum tells you the tendency for something to spin — it only changes when you apply a torque. With the soda cans it means that the work being done, adding energy (gravity acts on it, and there is a torque from the friction of the treadmill causing rotation)but this energy isn’t being added to the liquid, so it must be going into the can itself, which isn’t very massive — almost all of the energy goes into translational kinetic energy. The frozen water, though, does rotate with the can, so the gravitational potential energy has to be shared between translation and rotation of the can + ice system, so the translational kinetic energy (and therefore speed) is smaller.

Phone a Friend, or Ask the Audience?

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Welcome to Who Wants to be an Atomic Physicist?

Is It a Gas, Fluid, Solid, or All of the Above?

Because rubidium is magnetic, however, Stamper-Kurn and his Berkeley colleagues thought that the magnetic interactions between rubidium atoms in the gas might nudge them to adopt a type of regular spacing like atoms in a solid.

To look for this ordering, Stamper-Kurn’s team used a conventional laser trapping technique to confine a gas of millions of rubidium atoms in an oblong, surfboardlike trap. They then cooled the sample to below 500 nanokelvin. Lastly, they hit their collection of rubidium atoms with a beam of circularly polarized light, which is reflected differently by atoms with a different magnetic orientation and can, therefore, reveal the magnetic orientation of the atoms in the sample. What they saw was that within their optical trap, the rubidium atoms ordered themselves into an array of 5-micrometer-square domains, inside which all of the atoms adopted a similar magnetic orientation. What’s more, these domains adopted a crystalline-like ordering, with alternating domains with different magnetic directions. This ordering wasn’t perfect like the regular lattice of sodium and chlorine atoms in table salt. But it’s not random either (see picture). “There is some emergent order which shows up in this system,” Stamper-Kurn says.

Once the Berkeley researchers spotted the ordered makeup of the atoms, they decided to check whether the gas was coherent as well. Using another laser, they nudged two groups of rubidium atoms already in their trap. They found that the atoms interfered with each other in the same way that two coherent light beams create an interference pattern of light and dark stripes, an unmistakable sign of their wavelike quantum nature.

via

ZZ Top Physics

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Electrons in Rydberg states exhibiting behavior like classical orbits — Lagrange L points.

Rumour spreadin a-round in that Texas town
’bout that shack outside Lagrange

An astronomical solution to an old quantum problem

When a small satellite moves in a sun-earth system there are five stable points at which the satellite remains fixed with respect to the rotating sun-earth system. These are the famous Lagrange L points. In 1994 Bialynicki-Birula et al. showed that stable Lagrange points could be produced in the atomic electron problem by applying a circularly polarized microwave field rotating in synchrony with an electron wave packet in a highly excited state (a so-called Rydberg atom). The electron wave packet then remains localized near the Lagrange point while circling the nucleus indefinitely. Effectively the atom is made to behave quite classically.

via Zz

You're the Top

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Fermilab collider experiments discover rare single top quark

Previously, top quarks had only been observed when produced by the strong nuclear force. That interaction leads to the production of pairs of top quarks. The production of single top quarks, which involves the weak nuclear force and happens almost as often as the strong force production, is harder to identify experimentally. Now, scientists working on the CDF and DZero collider experiments at Fermilab achieved this feat, almost 14 years to the day of the top quark discovery at Fermilab in 1995.

Those Kinky Alkali Atoms

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Cross-dressing Rubidium May Reveal Clues For Exotic Computing

In their experiment, they cause a gas of rubidium-87 to form an ultracold state of matter known as a Bose-Einstein condensate. Then, laser light from two opposite directions bathes or “dresses” the rubidium atoms in the gas. The laser light interacts with the atoms, shifting their energy levels in a peculiar momentum-dependent manner. One nifty consequence of this is that the atoms now react to a magnetic field gradient in a way mathematically identical to the reaction of charged particles like electrons to a uniform magnetic field. “We can make our neutral atoms have the same equations of motion as charged particles do in a magnetic field,” says Spielman.

Cross-Dressing? Someone has broken into the liquor cabinet again.

Michael Faraday's GUT

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Michael Faraday, grand unified theorist? (1851)

The common thread of many of [Faraday’s] discoveries is their goal: demonstrating that all the physical forces of nature are but different manifestations of a single, ‘universal’ force. This idea was a surprisingly modern one for Faraday’s time, and is known today as a unified field theory. Such research was likely on the minds of many researchers of that era, however: once Ørsted discovered that a magnetic compass needle could be deflected by an electric current, the notion that all forces might be related was a tantalizing dream. Faraday went further than any of his contemporaries in realizing that dream, and experimentally cemented the link between electricity and magnetism and light. Faraday was by no means done, however, and in 1851 he published the results of his attempts to demonstrate that electricity and gravity are related!