Bubbles, (amazing soap ones. Not sure if they’re tiny.) over at Talk Like a Physicist
more bubbles (original link)
The Photon Push-Me Pull-You
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
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
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!
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?'
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.”
Waterfalls
There is something about waterfalls that fascinates me.
Crime Scene Investigation Investigated
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.
While I'm on the Subject . . .
gg announces that a compendium of classic science, being dubbed The Giant’s Shoulders, has been created and given its own niche in the ether.
The first huddling-up of said Giants will be at Blog Around the Clock
"Classic" Timekeeping, Part III
Timekeeping measurements always rely on the comparison of two oscillators; when you check to see if your clock or watch is running fast or slow, you do this by comparing it to another clock. Finding disagreement between two clocks won’t tell you a priori which one is the culprit, just as in the adage that a man with two clocks is never sure what time it is. But comparing three clocks allow pair-wise comparisons, and begin to allow one to assign a stability to the individual clocks.
Comparisons are what the scientists did in the second paper in my review, “Time, Analysis of records made on the Loomis chronograph by three Shortt clocks and a crystal oscillator.” The quartz crystal oscillator gave the input to the Loomis chronograph, and the three Shortt clocks were compared to crystal, and could then be compared with each other by differencing the data, which removes the crystal from the measurement.
The interesting (to me) part of the paper begins a few pages in, where they begin discussing the influence of the moon. The moon should give rise to a change in amplitude that would occur over an interval of 24h 50m, and should be distinguishable from diurnal terms present in the pendulum clocks. Two different time series were analyzed, one having a duration of 54 days, and the other having a duration of 146 days. This was long enough to average out noise terms, since the preliminary estimate of the effect was 153 microseconds per half-period of oscillation (i.e. one second)
The theory of the effect of direct attraction is presented in terms of tidal potentials, and it, of course, ends up depending on the angular position of the moon and the latitude of the observer. There are secondary effects as well. The tidal effect of the moon is not only on the water, but on the solid earth as well, though because it is not particularly elastic, the earth’s deflection is smaller, and this changes the radius by a small amount. There is a redistribution of mass when this occurs. Further, there are the local effects of the depression of the ocean bed and coast at high tide (as this was fairly near new York City), and the change in mass that occurs because of the water. It turns out that these indirect effects very nearly cancel, and the results should be close to the 153 microseconds predicted by direct attraction.
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