Daily Archives: June 18, 2008

The Photon Push-Me Pull-You

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A few weeks ago, over at Built on facts, I threw Matt a bit of a knuckleball in the comments.

[C]onsider a solid bar of the same index [as water]. You send in the pulse of light (assume a really good AR coating so there’s no reflection). What happens to the speed of the bar?

This was sneaky because it is one of the unsolved issues in physics (I feel no remorse for doing this, and Matt realized that something was up) — the theory is complicated enough that it’s really easy to miss out on some of the subtleties and end up with an invalid answer. There are two schools of thought: Minkowski, who had taken the approach that the photon’s momentum in the medium should be nE/c, and Abraham, whose approach gave the momentum as E/nc. Clearly, the results are at odds, and this came to be known as the Minkowski-Abraham momentum controversy.

I found a number of articles on the topic, but perhaps the best one is a review article from Reviews of Modern Physics. Momentum of an electromagnetic wave in dielectric media by Pfeifer et. al, No. 4, October–December 2007 pp. 1197-1216. (link is to a pdf file) The article points out that this isn’t a simple problem, because a photon in a medium can’t be naively treated as just a photon — both solutions have merit, but must include the interactions with the medium, which are obviously different depending on the approach you take — in the end there can be only one you can only have one answer for the momentum of the system.
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Wisdom From Inigo

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You keep using that word. I do not think it means what you think it means.

Newly born identical twin stars show surprising differences

The identical twins were discovered in the Orion Nebula, a well-known stellar nursery, that is 1,500 light years away. The newly formed stars are about 1 million years old. With a full lifespan of about 50 billion years, that makes them equivalent to one-day-old human babies.
[…]
By measuring the difference in the amount that the light dipped during the eclipses, the astronomers were able to determine that one of the stars is two times brighter than the other and calculate that the brighter star has a surface temperature about 300 degrees higher than its twin. An additional analysis of the light spectrum coming from the pair also suggests that one of the stars is about 10 percent larger than the other, but additional observations are needed to confirm it.
“The easiest way to explain these differences is if one star was formed about 500,000 years before its twin,” says Stassun. “That is equivalent to a human birth-order difference of about half of a day.”

So they have different brightness, surface temperature and possibly size. Maybe we shouldn’t be using the baby analogy (it’s not like they share DNA) and should stop calling them identical twins.

Update: Scientific Blogging does a better job, calling them fraternal twins, but the link has an auto-starting video with no means (that I can find) to turn it off!

The Necessity of Mathematics

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Awesome megapost over at Science after Sunclipse, covering many overlapping issues on the topic.

To use mathematics in the natural sciences, we first decide how we wish to represent some aspect of the world in mathematical form. We then take the diagrams and equations we’ve written and manipulate them according to logical rules, and in so doing, we try to make predictions about Nature, to anticipate what we’ll see in places we have not yet looked. If additional observations corroborate our expectations, then we’re on the right track. (It’s rarely so clean-cut as that — the process can spread across thousands of people and multiple generations of activity — but that’s the gist of it.) Several skill sets are involved: one must know how to idealize the world, and then how to work with that idealization. Remarkably enough, our schools fail to teach either skill.

It's So Big! It's Just So Big!

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A splash of astronomy news. Astronomers create first four-continent telescope

Astronomers have long combined observations from individual telescopes. The process, called interferometry, produces the same resolution as a single dish as wide as the distance between the antennas.

Recently, the Arecibo Observatory in Puerto Rico joined a project called Electronic Very Long Baseline Interferometry (e-VLBI), which can make temporary radio telescopes that rival the size of the Earth.

What's an Appropriate Font for 'Anal?'

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Indiana Jones and the Fonts on the Maps

Whenever Indy is traveling great distances, which happens in all the films, there is a montage of the airplane or boat superimposed over an animated map showing the route. It’s an old-fashioned convention, an homage to the movies of the Thirties and Forties. Unfortunately, the typefaces would be more at home a few decades later.

We all know getting the physics right is the only thing that matters. Everything else gets filed under “willing suspension of disbelief.”

Crime Scene Investigation Investigated

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I stumbled across a dead body couple of posts over at Quantum Moxie on the thermodynamics of post-mortem cooling of a body: Mistakes were made . . . and the followup, Post-mortem body cooling in variable environments.

[T]he standard post-mortem body cooling method used to estimate time of death (TOD) does not take into account a varying environmental temperature (i.e. it assumes a constant ambient temperature when applying Newton’s Law of Cooling).

And since were on the subject of physics and dead bodies, Zapperz notes a brouhaha about a physics exam question involving a gunshot victim and bullet trajectories.