Category Archives: Physics

Colonel Mustard, In the Lab, With a Lead Ingot

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Roman ingots to shield particle detector

Lead is, in principle, a shield against radiation, but freshly mined lead is itself slightly radioactive because it contains an unstable isotope, lead-210. “We could never use it for our experiment, which is exactly about keeping background radioactivity to a minimum,” says Ettore Fiorini, a physicist at the University of Milan-Bicocca and coordinator of the CUORE experiment. After it is extracted from the ground, however, lead-210 decays into more stable isotopes, with the concentration of the radioactive isotope halving every 22 years. The lead in the Roman ingots has now lost almost all traces of its radioactivity.

Fly Me to LEO

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The Big Picture: Journeys to the International Space Station

April 12th marked the 49th anniversary of human spaceflight, when Yuri Gagarin became the first person to orbit the Earth in 1961. At this moment, 13 humans are currently in low-Earth orbit, aboard the International Space Station. Several were already aboard the ISS when a Soyuz TMA-18 brought a fresh crew up from the Baikonur Cosmodrome on April 2nd – they were later joined by the crew of the Space Shuttle Discovery on the 131st shuttle mission to date (only three remaining launches scheduled). NASA recently signed a new deal with Russia for six more round-trips to the ISS, at a cost of $55.8 million per seat. Collected here are recent photos of the Space Station, its current crew, their launch vehicles, and the views from above. (38 photos total)

Here’s a neat picture from the collection (larger version at the link)

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The water bubble acts as a lens and inverts the image, but the air bubble inside also acts as a lens, inverting the image once again. A lens made of air? Well, sort of. Light refracts at the interface of any two media with different indices, so you can look at it as a lens. If we lived in water, we’d probably think of air bubbles in that way. Alternately, you can think of the bubble as dividing the water bubble into two lenses for the light traveling through it, while the light not going through it passes through only one lens.

A variant on this would be if you were to inject a small sphere of a liquid with a larger index inside the water bubble, if you were on the ISS, or use glass with two different indices here on earth.

Should We Dress Them Alike?

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Quantum fun: how identical must “identical photons” be?

The basic idea of the experiment is simple. Take two objects that emit single photons that are identical to each other. Detect these photons after they have been mixed together using a beam splitter. If they are identical, both photons will appear on the same output port of the beam splitter and set off the same detector. If they are different, they can appear on different output ports and set off different detectors.

Something the article really doesn’t delve into is a discussion of what is interfering and why you get his articular correlation. This isn’t the familiar case of two waves that are 180º out of phase giving you destructive interference, which is called second-order interference (by some, anyway, it’s the same authors using the terminology across multiple papers, so I don’t know if it’s standard or them hoping it will catch on). One reason we know that this isn’t interference of this type is that this has been tested with independent sources, such that the beamsplitter where they combine is far enough away that the travel time exceeds the coherence time of the light. There can not a be a well-defined phase in that case, so you will not see an interference pattern emerge from this effect.

This is fourth-order interference, involving Fock states* (photon-occupation states) and this is QM with no classical analogue, so I don’t really have a good ball-and-stick description which I can share to explain why this happens. (That is to say I don’t have one, not that I have one and am precluded from sharing it for some reason of national security. As far as you know.) You have photon occupation states leaving the beamsplitter, for travel in the two directions, A and B. You can have |2,0> or |0,2>, for two photons going to one detector or the other, and there’s a |1,1> state, for a photon going to each detector. But really there are two |1,1> states, that look the same when we can’t tell the difference between the photons. And since you add amplitudes before “squaring” them (multiplying by the complex conjugate), the |1,1> terms cancel if the amplitudes are the same, i.e. if you have a 50/50 beamsplitter, leaving you with the photon pairs as the only possibility, as long the photons are identical. Which then raises the question which the paper is trying to address.

* Yes, really. Innuendo galore. Anything you can think of has probably been done before.

WIkipedia article on the HOM effect, as this is called.

The Plutonic Relationship

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Hate Mail from Third-Graders

“It’s not easy being a public enemy,” writes Neil deGrasse Tyson in his book The Pluto Files. When Neil’s museum grouped Pluto not among the planets but rather with icy comets in an obscure region called the Kuiper Belt, he heard from thousands of outraged Pluto defenders. It’s tough being called a heartless Pluto-hater, particularly by a dismayed eight-year-old. Below, peruse a few of the letters elementary schoolkids sent Neil, and see how their tone shifted over the years, as the public slowly came to accept Pluto’s fall from planethood.

Size Matters

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Abraham Lincoln was once asked, “How tall should a man be?” “Tall enough that his feet reach the ground,” was his reply.

Building A Shrink Ray? Consult This Grisly Physics Paper About Exploding Horses

Making something giant-sized looks cool in science fiction. Seeing it basically disintegrate under the strain of its own weight wouldn’t look nearly as cool. Okay, maybe it would. But it wouldn’t make for an interesting fight for the protagonist.

The “grisly” paper is On Being the Right Size by J. B. S. Haldane.

An insect, therefore, is not afraid of gravity; it can fall without danger, and can cling to the ceiling with remarkably little trouble. It can go in for elegant and fantastic forms of support like that of the daddy-longlegs. But there is a force which is as formidable to an insect as gravitation to a mammal. This is surface tension. A man coming out of a bath carries with him a film of water of about one-fiftieth of an inch in thickness. This weighs roughly a pound. A wet mouse has to carry about its own weight of water. A wet fly has to lift many times its own weight and, as everyone knows, a fly once wetted by water or any other liquid is in a very serious position indeed. An insect going for a drink is in as great danger as a man leaning out over a precipice in search of food. If it once falls into the grip of the surface tension of the water—that is to say, gets wet—it is likely to remain so until it drowns. A few insects, such as water-beetles, contrive to be unwettable; the majority keep well away from their drink by means of a long proboscis.

Old-School Experiment with Light

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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.

Call Sign: Melanogaster

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Fruit Fly Aerial Maneuver Explained

To study insect maneuvers, Jane Wang and Itai Cohen of Cornell University and their colleagues put several flies in a chamber that was illuminated from three perpendicular directions by bright projectors. When a fly passed through a pair of crossed laser beams, three high-resolution cameras were triggered to film at 8000 frames per second. Each camera recorded the fly’s shadow from one of the projectors. A new computer algorithm developed by the group automatically combined the two-dimensional silhouettes into a three-dimensional reconstruction of the fly.

They're Usually More Interested in Knots

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Mathematics Reveal Universal Properties Of Old Rope

Despite rope’s obvious geometric properties, the art of rope making has been strangely neglected by mathematicians over the centuries. Today, Jakob Bohr and Kasper Olsen at the Technical University of Denmark put that right by proving the remarkable property that ropes cannot have more than a certain number of turns per unit length, a number which depends on the diameter of the component strands.

And that’s just the start. They go on to show that a rope with a smaller number turns than this maximum will always twist in one direction or another under tension.

Lights! Camera! Ticket!

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Math tutor uses numbers to fight red light camera ticket

When his wife received a ticket in the mail recently, the first thing she said was the yellow light was too short.

So Mike, who works with numbers all the time as a math tutor, put it to the test.

“I said, ‘If it’s really short, then you got short-changed and you got a ticket illegally,'” said Mogil.

The speed limit on Collier Boulevard, where she was cited, is 45 mph. According to county guidelines, the yellow light should be 4.5 seconds.

Mogil said he tested it 15 times with an average of only 3.8 seconds.

Good for him, showing with empirical data that the light was wrong. Evidence wins. Maybe it’s just me, though, but the story seems to be hinting that anyone who is not a math teacher/tutor, or perhaps a similarly-accomplished professional, would be incapable of performing this kind of analysis. I really hope that timing a light with a stopwatch and averaging the results is within the capabilities of anyone licensed to drive a car.

Mogil says he’s already checked 65 intersections and found that only seven yellow lights are long enough.

I have read of areas where red-light cameras were installed, and the contractor shortened the length of the yellow light in order to jack up revenue — they get a share of the ticket proceeds. This is either negligence or fraud. I hope they find out which.