Category Archives: Physics

Sigh

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iPhone city San Francisco is first in U.S. to demand radioactivity warnings on mobiles

The home city of the iPhone has passed a law requiring warning radioactivity warning labels on new mobiles.

San Francisco retailers will soon have to provide information on the specific absorption rate (SAR) of all handsets stocked.

Repeat with me: “Radiation” and “radioactive” are not the same thing.
The specific absorption rate in question is of radiofrequency radiation, which is non-ionizing, and in no way implies that the source is radioactive (i.e. comes from a spontaneous nuclear reaction), because it doesn’t.

On the other hand, it’s the Daily Mail. They apparently handle science no better than Robert Green handles weak shots-on-goal by Americans. (Bang!)

As far as the legislation goes, I think it’s antiscience being sold as informing consumers. But what information is being provided? I think specific absorption rate is being abused here, because it’s not being explained. If I have a mass of 100 kg, does a phone with an SAR of 1.6W/kg mean it is emitting 160 Watts? And for a user who has a mass of 70 kg, the power magically drops to 112 Watts? No. SAR is measured using a calibration standard of one gram of tissue (in the US; in Europe it’s 10 grams) meaning the gram of tissue absorbs 1.6 milliwatts of radiation from the source, under some geometry. The actual power emitted by a cellphone is of order a Watt. But even that information is almost useless without context; the human body radiates somewhere around 800-900 Watts in a more-or-less blackbody spectrum. Is that a cause for concern?

Leon's Getting Larger

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Fact or Fiction: The Days (and Nights) Are Getting Longer

Forces from afar conspire to put the brakes on our spinning world—ocean tides generated by both the moon and sun’s gravity add 1.7 milliseconds to the length of a day each century, although that figure changes on geologic timescales. The moon is slowly spiraling away from Earth as it drives day-stretching tides, a phenomenon recorded in rocks and fossils that provides clues to the satellite’s origin and ultimate fate. “You’re putting energy into the moon’s orbit and taking it out of the Earth’s spin,” says James Williams, a senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

The Banana Equivalent Dose

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I’ve used bananas as examples of radiation sources before, in terms of equivalent dose and the knee-jerk reaction to the mention of “radiation” in news stories.

Well, it turns out that the banana equivalent dose is a more common unit for dose comparisons than I had suspected — it even has its own wikipedia page

The average radiologic profile of bananas is 3520 picocuries per kg, or roughly 520 picocuries per 150g banana.[3] The equivalent dose for 365 bananas (one per day for a year) is 3.6 millirems.

I had given the radiation level for a banana as about 300 picocuries, so obviously I was estimating the banana as being 80 – 100 g, rather than the 150 g used here.

Before You Add the Flash

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On properly heating your pan

How to keep food from sticking in a stainless-steel pan. The first explanation about pores biting into the food sounds hokey, but then we get to the Liedenfrost effect, which is demonstrated in the video.

The water “hovering” over the stainless steel pan like mercury happens due to the phenomenon known as the Leidenfrost effect. You can read more about it on wikipedia, but the basic idea is this: at a certain temperature known as the Leidenfrost point (roughly around 320˚F for water, but varying with surface and pressure), when the water droplet hits the hot pan, the bottom part of the water vaporizes immediately on contact. The resulting gas actually suspends the water above it and creates a pocket of water vapor that slows further heat transfer between the pan and the water. Thus it evaporates more slowly than it would at lower temperatures. At the proper temperature, a similar effect happens with the food you place in the pan, preventing the food from sticking.

I Get No Kick from Champagne

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Or did you mean a real song, like the Camptown Ladies?

Why Black Holes Slow Down

Two black holes that are close enough will mutually orbit and eventually spiral inward toward each other, sending off ever-stronger gravitational waves (ripples in spacetime), until they collide and merge into a larger black hole. If the gravitational waves radiate mostly in one direction at the time of the merger, they “kick” the new black hole in the opposite direction. But some simulations have shown an “anti-kick” following the initial kick–the new black hole shoots away but soon slows down. Researchers haven’t had a clear physical explanation for the anti-kick.

Solving the Resolving

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Bad Astronomy: Resolving the iPhone resolution

In other words, at 12 inches from the eye, Jobs claims, the pixels on the new iPhone are so small that they exceed your eye’s ability to detect them. Pictures at that resolution are smooth and continuous, and not pixellated.

However, a display expert has disputed this. Raymond Soneira of DisplayMate Industries, was quoted both in that Wired article and on PC Mag (and other sites as well) saying that the claims by Jobs are something of an exaggeration: “It is reasonably close to being a perfect display, but Steve pushed it a little too far”.

This prompted the Wired article editors to give it the headline “iPhone 4’s ‘Retina’ Display Claims Are False Marketing”. As it happens, I know a thing or two about resolution as well, having spent a few years calibrating a camera on board Hubble. Having looked this over, I disagree with the Wired headline strongly, and disagree (mildly in one case and strongly in another) with Soneira. Here’s why.

Jobs’s claim is 300 dpi at 12 inches. I remember this as 600 dpi at the nearpoint of ~6 inches (15 cm), which is the same angular resolution. Closer than this and most adults can’t focus; your nearpoint is generally larger if you are older. Which is the same claim, and an explanation as to why 300-600 dpi is generally considered photo quality for images that are printed, and 1200 dpi is the highest resolution you’d ever need.

What Superman Sees

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X-Ray of speech

This is me (female subject) saying “både” (“both”). The sequence is an excerpt from a 20 second X-Ray film registred at the Danderyd Hospital in Stockholm in March 1997.

In this sequence I noticed that the lips form an interesting image as the mouth opens; I assume it’s from lipstick of uneven thickness and application, which can be seen when the mouth is fully open. I wonder what kind of heavy elements are in there that help screen higher-energy photons. The effect is absent for the male subject (and apparently “pion” in Swedish means “peony” rather than “meson made of up and down quarks.” Silly Swedes.)

Magic Magic

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I’m confounded by magic.

Radioactive isotope of tin confirmed to have doubly magic nucleus

Magic proton or neutron numbers give the nucleus greater stability and stronger binding, and are therefore usually more common than nuclei with unfilled orbital shells. In doubly magic nuclei both proton and neutron shells are filled, leading to even stronger binding and stability. The outer shells of doubly magic nuclei are rigidly spherical.

OK, here’s an instance where someone is using inconsistent and/or confusing terminology. If magic numbers refer to filled shells, then Sn-132, having 50 protons and 82 neutrons, is doubly-magic by definition. The only thing you have to confirm is that it’s Sn-132. The Tin isotope whips out its ID card, and you’re done.

But no, apparently that’s not enough.

Other confirmed doubly magic nuclei include helium-4, oxygen-16, lead-208, calcium-49, and nickel-48, which are abundant and stable, and nickel-56, which was discovered in 1998 and is less stable than the others, having a half-life of just 5.9 days. Tin-132 is even more unstable with a half-life of only four seconds, which has made confirmation of its doubly magic nature difficult. It has 50 protons and 82 neutrons, and is the first confirmed doubly magic isotope that is both neutron-rich and radioactive.

The scientists investigating this seem to already know it has magic numbers of both, but that doesn’t confirm the doubly-magic nature of the isotope. They had to verify that it is spherical as well, by looking at Sn-133 and saw that it behaved as expected of having a single excess neutron (I assume in terms of a quadrupole moment). But if it hadn’t, would the nucleus still be doubly-magic? Or would it be that the model of doubly-magic nuclei was wrong? I think it’s the latter; magic numbers refer to the numbers (hence the name), and models of nuclear shape are something separate.

On a related note, I wonder if anyone is looking at Sn-100, which is also doubly-magic (using my definition). It’s listed on the table of nuclides as having a half-life of 0.94 seconds, which implies it’s been made in the lab and studied to some extent. Then again, the table of nuclides lists Sn-132 has having a half-life of 39.7 seconds, which is an order of magnitude longer than what’s given in the article. So I’m thoroughly confused. But as a Gemini, I never know what to expect.