Category Archives: Physics

Spheropalooza

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A couple of twitter posts showed up close together that wouldn’t seem to be related at first blush: earthquakes and water droplets in space. But they are, because they’re both showing vibrational effects on liquid spheres. The earth is more like a fluid on time scales longer than when you fall and take a tumble.

Here’s a normal mode animation, depicting the earth (and exaggerated by a factor of about a gazillion) showing the various ways it might ring after an earthquake

But as far as vibrational modes go, there’s nothing special about the earth. I know I’ve posted the Alka-Seltzer video, but this video has extra demonstrations and I can’t recall if I’ve linked to it as well

You need to a flashplayer enabled browser to view this YouTube video

The first bulk oscillation looks something like a (0,6,0) mode, or something close.

With the bubbles inside the drop, and droplets inside of that, it’s neat how the combining of droplets (~1:35) gives you more ringing; the total amount of energy in the surface tension goes down as the drop gets bigger — the volume increases faster than the surface area or, put another way, bigger spheres have less curvature per unit area, so they store less energy. That energy has to go somewhere , and that’s into a bulk vibration of the drop. There are a couple of instance of this effect in the video.

Of Course There's an App for That

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“Einstein’s Pedometer” App Measures How Special Relativity Affects Your Daily Activity

The iPod app, designed by a Japanese developer, uses the iPhone’s GPS capabilities and Lorentz transformation equations to calculate [time dilation]. The Lorentz transformation is a set of equations that relate one observer’s space and time coordinates to those of another observer.

via @JenLucPiquant

No Problem for Frogs, but a Problem for Time

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The One-second War (What Time Will You Die?)

Leap seconds make sure the sun is due south at noon by adjusting noon to happen when the sun is due south at the reference location. This very important job is handled by the IERS (International Earth Rotation Service).

Leap seconds are not a viable long-term solution because the earth’s rotation is not constant: tides and internal friction cause the planet to lose momentum and slow down the rotation, leading to a quadratic difference between earth rotation and atomic time. In the next century we will need a leap second every year, often twice every year; and 2,500 years from now we will need a leap second every month.

On the other hand, if we stop plugging leap seconds into our time scale, noon on the clock will be midnight in the sky some 3,000 years from now, unless we fix that by adjusting our time zones.

Good summary of the problem, but falls short in explaining why we have leap seconds. The common explanation is that the earth is slowing down, but that’s not quite right. You have leap seconds because the earth has slowed down. Even if tidal friction were to magically stop, you would still need to insert leap seconds. The earth, as a clock, is running slow compared to atomic time, so it lags farther and farther behind the longer you wait — which means adding leap seconds to get them to approximately agree. The fact that tidal friction will still be with us means this discrepancy is accelerating — that’s why leap seconds will have to be added at shorter and shorter intervals, should we decide to continue to make the adjustments.

No, He's Not the Guy Who Wrote those Fairy Tales

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The birth of electromagnetism (1820)

It is oddly fitting that the birth of electromagnetism, and an entirely new direction in physics, started with the tiniest twitch of a compass needle. In the year 1820, Danish physicist Hans Christian Oersted (1777-1851) observed the twitch of said compass needle in the presence of an electric current, providing the first definite evidence of a link between electricity and magnetism that would set the tone for much of modern physics.

The story of Oersted’s experiment is the stuff of physics legend, but like most legends it is often misremembered and exaggerated. Nevertheless, it is a fascinating piece of work and a piece of scientific history worth recounting.

Before TMI meant TMI

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TMI: Fear, Fukushima and Facts

A critique of the xkcd radiation dose chart I linked to, with some more details and caveats, some of which I recognize as true from by background (but wasn’t going to post on my own because it’s too far away from my areas of competence). Randall’s shortcoming is the mixing of chronic and acute exposure doses (long-term and short-term), which are not equivalent, i.e. a dose spread out over a period of time (e.g. months) does not have the same biological effect of the same dose that happens in a period of minutes or hours or days. Giving the body a chance to repair itself matters.

via fine structure

Radioactive Data Part II

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My analysis of beta counts in California was from some simple EPA data. Here is a more detailed analysis of samples from Seattle: Fission Products in Seattle Reveal Clues about Japan Nuclear Disaster

By measuring the energy of the gamma rays from the filters, these guys have identified exactly which fission products have made their way across the Pacific. And this in turn allows them to make a number of interesting inferences about what has gone wrong at Fukushima.

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[T]here are a huge number of possible breakdown products from nuclear fission in a reactor and yet the Seattle team found evidence of only three fission product elements–iodine, cesium and tellurium. “This points to a specifific process of release into the atmosphere,” they say.

Cesium Iodide is highly soluble in water. So these guys speculate that what they’re seeing is the result of contaminated steam being released into the atmosphere. “Chernobyl debris, conversely, showed a much broader spectrum of elements, reflecting the direct dispersal of active fuel elements,” they say.

However, this comes from an analysis of just the first five days after the fission products were detected (data collected on Mar 17-18), so it does not reflect more recent events.