Archive for September, 2008

The World Will Not End, Thanks to a Technicality

I’m sorry, this is abuse. You want 12A, next door.

Day of reckoning for doomsday lawsuit

Basically the decision came down to an issue of jurisdiction: Wagner and a co-plantiff made their claim under the National Environmental Protection Act (NEPA). But NEPA only applies to “major federal actions,” and the judge said that the US contribution to the LHC (US$531 million or about 10% of the overall cost) was too small to constitute a major federal project.

Viva Las Vegas

One of the questions one asks when trapping atoms in a magneto-optic trap (MOT) is “What shall we do with the atoms?” You often have an idea before you do the trapping — it’s not like we’re trophy hunters, trapping just to have something on the wall. Trapping in and of itself hasn’t been the goal for quite some time now, at least in experimental labs; one wants to do some kind of experiment with the atoms. Some of the time that can be done in the trap, but quite often it involves moving the atoms somewhere else. Sometimes you actually wanted an atomic beam of some sort, instead of a collection of atoms just sitting there, suspended in space — the trapping environment involves bright, near-resonant laser light and magnetic fields and those could be undesirable. The atom beam gets you away from this, and if you look at the beam from a perpendicular direction, the Doppler shift is very small. Perhaps you want low-speed collisions, and tuning the speed of the beam allows you to do your experiment. There are also a number of atom-optics experiments that can be done, e.g. sending the atoms through transmission gratings comprising an interferometer. The problem could also be the relatively high vapor pressure of the gas in your vapor cell giving you excessive background signals, or collisions with that background vapor could be the problem, limiting the trap lifetime. So you need to move the atoms, transporting them to a region that is better-suited for the experiment you are doing.

When I was at TRIUMF, the problem was the background and trap lifetime. We were trapping radioactive atoms, and the idea was that when an atom decayed, the beta would go one way and the atom would recoil, and each could be detected. But a vapor-cell MOT captures only the small percentage of atoms stupid enough moving slowly enough to get trapped, leaving the majority of the zipping around in the cell or sticking to the walls (or worse, attaching themselves to detectors). Not only did this mean they would be swamping the signal from the trapped atoms, the signals would be coming from different directions and originating from different points.

About the time we started fretting about this problem (you have to trap them first before you worry about the next step, and nobody had trapped these isotopes before) we got a visit from Zheng-Tian Lu, then at JILA/NIST, and he had come up with an ingenious method of generating a low-velocity atomic beam and shared the details with us (the paper was in the pipeline but had not yet been published at the time)

A typical vapor-cell MOT uses three beams along the cartesian axes, and it’s possible to do this by retroreflecting each of these beams — the vapor is dilute, so with decent mirrors there isn’t a large drop in intensity (any imbalances will push the trap slightly off-center as the effect of the magnetic field compensates). You get the proper polarization of the beams by placing a quarter-wave plate in front of the retroreflection mirror (this changes the circular light to linear and then back to circular of opposite helicity; if you started with linear it would circularize it and change it back to linear, perpendicular to the original. Ah the fun you can have with waveplates)

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Does the Moon Orbit the Earth, or the Sun?

The Moon that went up a Hill but came down a planet over at Bad Astronomy

[Y]ou have to look at something called the Hill sphere. Basically, it’s the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits.
OK, in English — and more pertinent to this issue — it’s the volume of space around the Earth where the Earth’s gravity is more important than the Sun’s. If something is orbiting the Earth inside Earth’s Hill’s sphere, it’ll be a satellite of the Earth and not the Sun.

This is About Physics, Not Statistics

Stealing bases – head or feet first?

I Don't See Tina Turner Anywhere

Oh, wait, that was Thunderdome.

The Baikonur Cosmodrome at The Big Picture

When NASA’s last scheduled Space Shuttle mission lands in June of 2010, the United States will not have the capability to get astronauts into space again until the scheduled launch of the new Orion spacecraft in 2015. Over those five years, the U.S. manned space program will be relying heavily on Russia and its Baikonur Cosmodrome facility in Kazakhstan. Baikonur is an entire Kazakh city, rented and administered by Russia. The Cosmodrome was founded in 1955, making it one of the oldest space launch facilites still in operation. Here are collected some photographs of manned and unmanned launches from Baikonur over the past several years. (26 photos total)

Did Jules Verne Write This?

Journey to the Center of the Neutron

A neutron contains three quarks, and nuclear physicists don’t completely understand how these move within the particle. Last year, an analysis revealed a negative charge at the center of the neutron, and now an article in the Rapid Communications section of the September Physical Review C attributes this negative core to very fast moving “down” quarks. The results elaborate on an emerging three-dimensional view of these fundamental particles and their proton cousins.

The Value of Sports Statistics

Chad’s got a post up about how Baseball Statistics Are Crap. I’ve got a different beef.

(There are, certainly, a lot of dubious statistics in baseball. I just don’t agree that things are as bad as Chad says but maybe it’s just that I’m used to the idiosyncrasies. I do understand the infield fly rule, after all. If that weirdness makes sense, maybe the weird statistics do, too.)

Anyway, my objection is that even with these simplified statistics, the sportscasters and writers read too much into them. They don’t understand what the statistics are saying, and the value of statistics is to be able to compare players. In baseball it’s not so bad — even if the stats are flawed, a player hitting .356 is objectively a better hitter, by this measure than one who is hitting .290. But what does “by this measure” mean? In baseball, you can hit for average or for power — there are different skills and abilities useful to the team, and you want to find the statistic that is appropriate to the skill you are trying to quantify.

In this regard, I think, football is an example where the reporters are a great abuser of statistics. And this goes beyond saying “turnover ratio” when “differential” is meant (one thing that’s gotten better over the years). The main abuse, I think, is saying that accuracy is measured by completion percentage, and this seemingly happens all the time.

Accuracy is your ability to hit a target, and if you want to compare apples-to-apples, the target should be the same one. A stationary target at 10 yards is easier to hit than a moving one at 40 yards. A better receiver, who can get open, is easier to hit, and also affects the ability for other receivers to get open. You can have a receiver who drops the ball even though it’s “right at the numbers,” or one who catches everything thrown his way. When nobody’s open and he’s trapped, a quarterback can take a sack or throw the ball away, giving him an incompletion. All of that affects completion percentage, and none of it reflects accuracy.

Chad Pennington is touted as an “accurate quarterback” by many sports journalists, who, in the next breath, mention he has a weak arm and dumps the ball off quite often. Short passes. Connection? I think so!

My favorite example is Donovan McNabb. When Terrell Owens was about to join the team, analysts were all cautious about how Owens would tolerate the inaccurate McNabb, who had never completed 59% of his passes. Until that year, when he completed 64%, and everyone was saying how accurate he had become. Owens leaves, and the completions percentage drops back down. (It’s up again this season, and last — he’s got better receivers, and he dumps the ball off to Westbrook when he has to)

Ballistic and Nonballistic Trajectory: Career Path

I’ve been adopted by three high school groups (so far). Last time I did this, there was a list of questions, so I got a head start on answering the ones I thought might be asked. The answers seem to have tunneled into the ether, however, but since questions about career path are likely to come up (and I haven’t done a post on that), here I go.

I’m not tempted to say “I didn’t take the typical career path” because I don’t think there is any such thing. Some might propose that the typical path is grad school to postdoc to university teaching and research position, but since only about a third of doctorates work in academia, and there are positions other than the university research professor (teaching-only positions, liberal-arts or community colleges) that’s not really “typical,” though it may be perceived to be.

I went to Hartwick College, a small liberal-arts school in the middle of nowhere Oneonta, NY, which was about two hours away from home (less now, since they put in a highway while I was there). I ended up there because the financial aid offered by my main choice, Cornell, was a tad less than I needed: they offered zero. (I was put on a waiting list and the money ran out before the candidates did). Between student loans, work-study, scholarships and grants, Hartwick was possible. I had AP credits for physics and calculus and overloaded my schedule one term to graduate early. (3.5 years)
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Pointing the Way

Why government MUST invest in fundamental research

Basic research is not a place where “the market” tends to show interest.

Problem number one is that it typically takes 20-30 years – in best case scenario to see “returns” on fundamental discoveries. Often discoveries pave the way to other discoveries, and so forth, which eventually trickle down to technological applications – often unintended or unforeseen.

Find me a company that is willing to invest in a project with no hope of return for 20-30 years and I have a bridge in Alaska I want to sell you.

Not Part of the Curriculum

Physics for Future Presidents apparently doesn’t include reflections.

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