Monthly Archives: January 2009

I Get No Kick from Champagne

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So tell me why should it be true?
That I get a kick out of you

gg writes up the recent paper on the Abraham-Minkowski controversy

Measuring the ‘kick’ of a photon leaving a fiber!

The difficulty lies in the fact that any discussion of the momentum of light in a medium must properly account for the total momentum of the system, which includes the momentum of the medium itself. When traveling into a medium of refractive index much greater than unity, the light is strongly interacting with the material and it becomes almost arbitrary to distinguish between the momentum of the photon and that of the matter: the two are completely intertwined. With this perspective, one would say that the designation of ‘light momentum’ and ‘medium momentum’ are completely arbitrary, merely different ways to slice ‘total momentum pie’. Differences in experimental results can be explained away as a failure to completely account for the interaction between the light and the medium.

Free the Facts

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Free the Facts A Flickr set by Dave Gray

Facts are an important element of any decision-making process. A fact asserts that something is the case. When we as a society make decisions that affect our future, facts, and conversation or argument about what they mean, is a critical part of those decisions.

But what is a fact, and how do we know that something is a fact? Is there a “keeper of the facts?”

This little thread is an exploration of facts: What they are, how they come to be, who has access to them and why. It’s especially focused on the facts that make up the sum of our scientific knowledge.

via

Are You the Keymaster?

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Gatekeeping at Faraday’s Cage

[I]t bothered her that in the sciences and engineering there are often classes used as “weeders”, the principal being that the “unfit” are not able to survive the rigors of the fundamentals classes and will drop out before too much time and money has been invested by either party.

The teacher called this “gatekeeping”. It’s a concept with which I am familiar because, unfortunately, I’ve been on both ends of it. I also have a lot of mixed feelings on the topic, and it helped me to hear that this philosophy bothers other people.

And I Say, It's Alright

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Here comes the sun, or at least its shadow, at Dot Physics: When is the Sun directly overhead?

There’s a nice little video that accompanies the post which also demonstrates some of the foibles of doing experiments

What I want to do is change the question a little bit. Rhett points out that one of the common answers is “Everyday at noon,” which can never be correct if you are outside of the tropics. But let’s change this to “When is the sun on the N-S line that goes directly overhead?” The answer still isn’t “everyday at noon.”

Why? Because the earth’s orbit isn’t a circle, and we orbit the sun fastest when we are near perihelion (in January) and slowest near aphelion (in July), with the difference being about a kilometer per second. If we define our time in terms of the sun being on that overhead line — i.e. we use a sundial — then the length of the day will vary, and this is why we generally don’t use solar time (when we’re using solar time) without modifying it. What we do is apply the equation of time, which gives rise to the figure-eight-ish analemma (found on globes as well as sundials). This takes into account both the inclination of the sun and the eccentricity, to give a correction to solar time and correct the reading. While this makes our day 24 hours again, it also means that the sun will be on that overhead line as much as 15 minutes or so before or after actual noon, as kept by our clocks.

Wash Your Hands, Jeffrey

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Graphic Encouragement to Wash Your Hands

Each batch of agar contained a little bit of cefoxitin, an antibiotic that should prevent any ordinary bacteria from growing on the plates.

After a little bit of incubation, the first plate (left) was covered in bright red colonies. It provided damming evidence that the infection can easily be spread by hand.

The second plate (right) was completely free of bacteria. It showed that disaster can be averted very easily. By taking just a minute to lather up, anyone who works with patients can fight the spread of antibiotic-resistant bugs.

NEJM abstract

New Quantum Teleportation Result

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Via both Physics and Physicists and Uncertain Principles, I see that there is a new result in quantum teleportation between ions that were about a meter apart. Both posts have short summaries (along with Chad considering doing a more thorough write-up) and other links.

I think the Science Daily or Eureka Alert (which I think are identical) are the better ones, since they actually explain how the entanglement occurs:

You excite the two ions so they well drop back down into one of two complementary states, and in doing so they release photons that would be different in energy if they represent the two different transitions.

Before reaching the beamsplitter, each photon is in a superposition of states. After encountering the beamsplitter, four color combinations are possible: blue-blue, red-red, blue-red and red-blue. In nearly all of those variations, the photons cancel each other out on one side and both end up in the same detector on the other side. But there is one – and only one – combination in which both detectors will record a photon at exactly the same time.

In that case, however, it is physically impossible to tell which ion produced which photon because it cannot be known whether the photon arriving at a detector passed through the beamsplitter or was reflected by it.

Thanks to the peculiar laws of quantum mechanics, that inherent uncertainty projects the ions into an entangled state. That is, each ion is in a correlated superposition of the two possible qubit states

Are They Spherical?

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Scientists make virtual cows to research methane emissions

“As the materials ferment you end up with what we call the poo jars. That is as technical as an engineer would want to get,” says Wood.

Methane gas emissions are monitored.

“Every time the little unit here flicks, we count the flicks for the amount of gas produced,” says Wood.

Surprisingly, the methane that cows release comes from an unexpected source.

“Cows don’t fart methane. 99% of the methane comes from their mouth.”