Category Archives: Physics

One More Thing . . .

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The other thing that struck me about bait-and-switch was this

I gravitated toward a scientific life with fantasies of sci-fi movies running through my head, with large machines emitting lightning at the flip of a huge Frankenstein-type switch, or several people poring over softly-glowing computer screens as an experiment produces fantastic data in real-time, and great discoveries are made. I thought this kind of thing actually happened even as I started grad school (even if I had never seen it in my various research summers…)

It doesn’t happen often, but it does happen (depending on your definition of “great”). Back in my first postdoc, at TRIUMF, we trapped radioactive potassium atoms for nuclear-decay tests of the standard model. Or, more precisely, we planned to do this, since the research had progressed only to the point where stable potassium had been trapped when I started working there. Not too long after my arrival we were scheduled for a few stretches of beam time, with an appropriate target to produce the radioactive isotopes we were trying to trap.

Since these were radioactive isotopes, the exact frequency for trapping them was unknown, though the presence of stable isotopes meant (in principle) that the isotope shift could be calculated to some degree of accuracy and narrow down the range of frequencies for the trapping and repump interactions. Since the linewidth of the transition is somewhere around 5 MHz, and you should be able to see a trap with a laser detuning of somewhere between a half a linewidth and several linewidths to the red of resonance, we set up to scan in discrete steps of several MHz, pausing at each step to look for fluorescence at the center of the trapping region — literally looking: we integrated the output from a CCD camera and displayed it on a computer screen, along with a graph of the total fluorescence.
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You'll Learn the Interesting Stuff Later

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Bait and Switch, and observation from Entropy Bound. Peter’s argument is in the context of “the lab” being more mysterious when you don’t know what’s going on (the bait) but by the time you get to work in one you’re doing actual science which is (one hopes) quite interesting, even if the apparati have lost their mystique.

But when you get down to it, it’s a bait-and-switch: when you are growing up, no-one ever tells you that things aren’t so colorful and mysterious, so by the time you finally realize that it’s not, you’ve found a much more interesting — albeit prosaic –real world to ponder.

I can certainly identify with this, and also see a related effect along another tangent: are we using the right bait? You take physics classes (and this probably holds true for other disciplines, though I have little empirical data for comparison) and there is this sometimes spoken, sometimes unspoken promise of “I know this is basic stuff and may seem boring, but I promise if you learn this, we’ll get to some interesting stuff later on.” Whether that holds true or not depends on what you’re doing, who’s teaching and what your threshold of “interesting” is. I now wonder if this is part of the hurdle to get more students interested in physics — do we bore them to death learning basic kinematics, thermodynamics and E&M? Does this drive some students away who might otherwise be interested if they were doing physics discovered after 1900? At least in biology there is the prospect of dissecting something even in introductory courses (which is why I shied away from biology. Dissection, moi? Not only no, but fuck no). In chemistry you play with chemicals. In physics we’re sliding blocks down an incline. (My undergraduate experience did have one bonus, though. Since we were a small school and could only support one sequence per year in general physics, it was designated a sophomore-level course, so that everyone taking it could have calculus as a pre- or co-requisite. In order to make sure they physics majors had something to do, we had a course in basic optics and relativity and another in electronics that were engaging, but then anyone following the normal sequence regressed to the yawn-fest)

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Life Imitates Art

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Well, Art is busy, so life imitates a cartoon instead.

The other night on the Colbert Report, he interviewed George Johnson, author of The Ten Most Beautiful Experiments (which I’ve mentioned before), and the idea for which was stolen from Chad. The interview was standard Colbert schtick, and Johnson doesn’t really explain what’s going on with the physics, but the last 30-45 seconds is great, and becomes the XKCD cartoon “The Difference,” about science and pain.

UPDATE: if you are getting frustrated with the Comedy Central player, there’s an embedded link here that seem worked better for me. (I can’t embed the video myself, alas)

Oh, Now They Tell Me

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A while back I bought a radio-controlled helicopter to fly around the apartment — it isn’t something designed to withstand much more than the gentlest of breezes — and broke it in almost record time. A harsh learning curve. I strayed into enemy airspace smashed into the lights above the dining-area table and snapped one of the rotor spokes. Oh, well. I suppose it’s fixable, but I haven’t gotten around to it yet. Add it to the list.

Now arXiv tells me why tiny helicopters are so hard to fly

[M]oments of inertia drop in proportion to the fifth power of vehicle size. This gives small helicopters quicker response times, making them more agile. But the real killer is that the main rotor tip speed in a small helicopter is the about the same as it is for a large helicopter. So the ratio of the rotor moments to the moments of inertia can become huge and unmanageable.

So it’s all because of scaling. Curse you, scaling laws! A disproportionately large curse!

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Oh Dear, Have You Put On Some Mass?

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The topic comes up, as it sometimes does, of the mass-energy equivalence from relativity. There are different tangents to this — what does the equivalence really mean, can you really turn energy into mass, does a photon have rest mass, what is the difference between relativistic mass and rest mass, and is the use of relativistic mass grounds for justifiable homicide, or is one compelled to stop at maiming?

E = mc2 is the equation everyone knows, but what many don’t know is that the equation already assumes one is at rest. The actual equation is E2 = p2c2 + m2c4, which reduces to the more familiar form when the object is at rest. The implications of this are that photons have no mass, the mass term for massive particles doesn’t change when you move — that energy is in the kinetic term, (which renders relativistic mass moot) and also that the mass will increase if you add energy that does not appear in the kinetic term, i.e. extra energy in the center-of-momentum frame appears as mass.

The last concept showed up at Cosmic Variance recently, in the context of the mass of a spinning top

The spinning gyroscope has more energy than the non-spinning one. As a test, we can imagine extracting work from the spinning gyroscope — for example, by hooking it up to a generator — in ways that we couldn’t extract work from the stationary gyroscope. And since it has more energy, it has more mass. And the weight is just the acceleration due to gravity times the mass — so, as long as we weigh our spinning and non-spinning gyroscopes in the same gravitational field, the spinning one will indeed weigh more.

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Mini-Hoops

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Over at Popsci, the physics of tossing a ping-pong ball into a beer cup, with videos of, well, tossing a ping-pong ball into a beer cup, under various conditions you’d find in a dorm (starting with the unlikely presence of beer cups).

These guys are pretty amazing. And the nonchalance with which they accomplish each trick shot adds a certain understated humor to this entertaining video. But though the guys seem to be developing a seemingly useless (if highly impressive) skill in their spare time, there’s quite a bit of complex science at play.

I think being able to edit out the misses tempers that amazing/impressive just a little. Mostly it reminds me of how much free time I had in college, even though it didn’t seem that way at the time.

You Spin Me 'Round

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From arXiv, rotation of a thin film of water when subjected to perpendicular electric fields.

The question is: what’s causing the rotation? The team can easily control the direction and speed of rotation by varying the relative angle and direction of the electric fields, which rules out the possibility that convection is causing the rotation (something that is seen when a field is applied to some thin films of liquid crystals). Neither does adding salt to water change the effect, ruling out the possibility that ion movement directs the flow.

Movies

Doomed to Fail

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A while back I posted some links to anti-relativity sites and gg suggested that it would be fun to debunk the claims. Sometimes that’s fun, but often — and especially after doing it a number of times — I find that it’s tedious. An error is present, and one has to find it in a morass of often awkwardly defined and unnecessarily complicated scenarios (hey, let’s use three trains, and multiple clocks which will be juggled by a clown on each train!) set up by the author. Sometimes with some horrific ASCII “art,” to boot, though some do have fancy animated gifs.

The reason one knows that an error is present in these thought experiments is because a contradiction has been found. One might think that this is a dogmatic BESS (Because Einstein Said So) argument, but it isn’t — the issue here is that the ultimate authority, and the only authority one is allowed to quote, is absent from the problem: nature. These are thought experiments, and it all boils down to doing coordinate transformations and calculations. Special relativity consists of Lorentz transformations, which are derived from the hypothesis that the speed of light is invariant; all inertial reference frames will measure the same value. This has the admittedly strange consequence (especially to the uninitiated) of time and length not being absolute quantities, which runs counter to most peoples’ everyday experience. We think in Galilean terms which serves us reasonably well in everyday experience, and the differences presented by Lorentz transformations are not apparent to us under these conditions.

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Classic Physics: Light is an EM Wave

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Classic Science Paper: Otto Wiener’s experiment (1890) at Skulls in the Stars.

By 1890, then, scientists were interested in seeing whether similar results held for light waves: it seems that a number of scientists remained unconvinced that light truly was just another manifestation of electromagnetic waves! One big obstacle stood in the path of such studies: the smallness of the wavelength of light. Hertz’s radio waves had a wavelength of meters, but visible light has a wavelength on the order of 500 nanometers, or 500 billionths of a meter! Such distances cannot be directly observed with the naked eye, so experimental ingenuity was required – and Otto Wiener provided it.

(My own summary of some “classic” physics is progressing)