Atoms and Molecules
drorzel | January 18, 2011The latest snowstorm is wreaking some havoc on my plans for the day, which means I'm going to lift another question and answer from the Physics Stack Exchange, with some modification. This one is a question about thermal radiation:
What are the quantum mechanisms behind the emission and absorption of thermal radiation at and below room temperature? If the relevant quantum state transitions are molecular (stretching, flexing and spin changes) how come the thermal spectrum is continuous? What about substances (such as noble gases) which don't form molecules, how do they emit or absorb thermal…
drorzel | January 16, 2011I was at a meeting of the Committee on Informing the Public of the American Physical Society at the tail end of last week, so it seems appropriate to post a couple of APS-related announcements here on my return:
1) The APS has just created a Forum on Outreach and Engaging the Public. You may have read about this in the monthly APS News, but in case you missed it, there is a new organization with APS to bring people interested in outreach together:
"The forum provides a venue for people to congregate, provide best practice manuals...and disseminate things that work so people don't have to…
drorzel | January 12, 2011Another response copied/adapted from the Physics Stack Exchange. The question was:
What are the main practical applications that a Bose-Einstein condensate can have?
Bose Einstein Condensation, for those who aren't familiar with it, is a phenomenon where a gas of particles with the right spin properties cooled to a very low temeprature will suddenly "condense" into a state where all of the atoms in the sample occupy the same quantum wavefunction. This is not the same as cooling everything to absolute zero, where you would also have everything in the lowest energy state-- at the temperatures…
drorzel | December 29, 2010Most of what would ordinarily be blogging time this morning got used up writing a response to a question at the
Physics Stack Exchange. But having put all that effort in over there, I might as well put it to use here, too...
The question comes from a person who did a poster on terminology at the recently concluded American Geophysical Union meeting, offering the following definition of "data":
Values collected as part of a scientific investigation; may be qualified as 'science data'. This includes uncalibrated values (raw data), derived values (calibrated data), and other transformations of…
drorzel | December 28, 2010It's the last week of the (calendar) year, which means it's a good time to recap the previous twelve months worth of scientific news. Typically, publications like Physics World will publish a list of top ten physics stories of 2010, but we're all Web 2.0 these days, so it seems more appropriate to put this to a poll:
What is the top physics story of 2010?survey software
I've used the Physics World list as a starting point, because you have to start somewhere. I added a few options to cover the possibility that they left something out, and, of course, you know where the comments are.
This…
drorzel | December 15, 2010I've mentioned before that I'm answering the occasional question over at the Physics Stack Exchange site, a crowd-sourced physics Q&A. When I'm particularly pleased with a question and answer, I'll be promoting them over here like, well, now. Yesterday, somebody posted this question:
Consider a single photon (λ=532 nm) traveling through a plate of perfect glass with a refractive index n=1.5. We know that it does not change its direction or other characteristics in any particular way and propagating 1 cm through such glass is equivalent to 1.5 cm of vacuum. Apparently, the photon…
drorzel | November 17, 2010The big physics-y news story of the moment is the trapping of antihydrogen by the ALPHA collaboration at CERN. The article itself is paywalled, because this is Nature, but one of the press offices at one of the institutions involved was kind enough to send me an advance version of the article. This seems like something that deserves the ResearchBlogging Q&A treatment, so here we go:
OK, what's the deal with this paper? Well, the ALPHA collaboration is announcing that they have created antihydrogen atoms-- that is, a single antiproton orbited by a single positron-- at low temperatures, and…
drorzel | November 12, 2010As mentioned in yesterday's post on ion trapping, a month or so back Dave Wineland's group at NIST published a paper in Science on using ultra-precise atomic clocks to measure relativistic effects. If you don't have a subscription to Science, you can get the paper for free from the Time and Frequency Division database, because you can't copyright work done for the US government.
This paper generated quite a bit of interest when it came out, because it demonstrates the time-slowing effects of relativity without any need for exotic objects like black holes or particle accelerators-- they deal…
drorzel | November 11, 2010One of the many physics stories I haven't had time to blog about recently is the demonstration of relativistic time effects using atomic clocks. I did mention a DAMOP talk about the experiment, but the actual paper was published in Science (and is freely available from the NIST Time and Frequency Division (PDF file), because you can't copyright work done at government labs) a month and a half ago, and generated a bit of buzz at the time.
Given the delay between publication of the article and me blogging about it, I feel obliged to provide a little more detail than you'll get from the news…
drorzel | November 5, 2010Regular commenter onymous left a comment to my review of Warped Passages that struck me as a little odd:
The extended analogy between the renormalization group and a bureaucracy convinced me that she was trying way too hard to make sophisticated concepts comprehensible. Also, I'm not really sure that analogies are the best way to explain concepts to people without using mathematics.
I'm not talking about the implication that making sophisticated concepts comprehensible is not worth doing, but rather the negativity toward analogies. It's odd because, if you think about it, a huge chunk of…
drorzel | September 14, 2010Not long ago, a new preprint on the fine structure constant got a bunch of press, nicely summed up by the Knight Science Journalism Tracker last week. I meant to say something about this last week, but what with it being the first week of classes and all, I didn't find the time.
I still think it's worth writing about, though, so after a reproduction of the key figure, we'll have the usual Q&A-format explanation of why I don't quite trust this result:
So what's this all about? The preprint in question is the latest in a series of attempts to measure possible changes in the fine structure…
drorzel | September 13, 2010A fairly straightforward question: quantum physicists divide the world into two categories of things, fermions and bosons. What's your favorite object having integer spin?
What's your favorite boson?online survey
Superpositions of answers, while allowed in properly symmetrized wavefunctions, are not valid responses to this poll.
drorzel | September 8, 2010The theory of relativity takes its name from a very simple and appealing idea: that the laws of physics should look the same to moving observers as to stationary ones. "Laws of physics" here includes Maxwell's equations for electricity and magnetism, which necessarily means that moving observers must see the same speed of light as stationary observers (Einstein included the constancy of the speed of light as a second postulate in his original relativity paper, but it's redundant-- the constancy of the speed of light is a direct consequence of the principle of relativity). This leads directly…
drorzel | September 3, 2010Over at Confused at a Higher Level, Melissa offers an alphabetical list of essential supplies for a condensed matter experimentalist at a small college. This is a fun idea for back-to-school time, so I'll steal it, and offer the following alphabetical list of essentials for Atomic, Molecular, and Optical physics at a small college, kind of a condensed version of the three part series I did a few weeks ago.
A is for Acousto-optic modulator This is a device that uses sound waves in a crystal to deflect light and shift its frequency. It's essential for rapid control of laser properties.
B is for…
drorzel | August 30, 2010Last week, John Baez posted a report on a seminar by Dzimitry Matsukevich on ion trap quantum information issues. In the middle of this, he writes:
Once our molecular ions are cold, how can we get them into specific desired states? Use a mode locked pulsed laser to drive stimulated Raman transitions.
Huh? As far as I can tell, this means "blast our molecular ion with an extremely brief pulse of light: it can then absorb a photon and emit a photon of a different energy, while itself jumping to a state of higher or lower energy."
I saw this, and said "Hey, that's a good topic for a blog post…
drorzel | August 25, 2010The Joerg Heber post that provided one of the two papers for yesterday's Hanbury Brown Twiss-travaganza also included a write-up of a new paper in Nature on Mott insulators, which was also written up in Physics World.
Most of the experimental details are quite similar to a paper by Markus Greiner's group I wrote up in June: They make a Bose-Einstein Condensate, load it into an optical lattice, and use a fancy lens system to detect individual atoms at sites of the lattice. This lattice can be prepared in a "Mott insulator" state, where each site is occupied by a definite number of atoms. As…
drorzel | August 24, 2010Two papers in one post this time out. One of these was brought to my attention by Joerg Heber, the other I was reminded of when checking some information for last week's mathematical post on photons. They fit extremely well together though, and both relate to the photon correlation stuff I was talking about last week.
OK, what's the deal with these? These are two papers, one recent Optics Express paper from a week or so ago, the other a Nature article from a few years back. The Nature paper includes the graph you see at right, which is a really nice dataset demonstrating the Hanbury Brown and…
drorzel | August 23, 2010I'm a big fan of review articles. For those not in academic science, "review article" means a long (tens of pages) paper collecting together the important results of some field of science, and presenting an overview of the whole thing. These vary somewhat in just how specific they are-- some deal with both experiment and theory, others just theoretical approaches-- and some are more readable than others, but typically, they're written in a way that somebody from outside the field can understand.
These are a great boon to lazy authors, or authors facing tight page limits ("Ref. [1] and…
drorzel | August 20, 2010It's nearly time for classes to resume, which means it's time for a zillion stories about Beloit College's annual Kids These Days List, listing off a bunch of things that this year's entering college class, who were mostly born in 1992, have always taken for granted. A sample:
1. Few in the class know how to write in cursive.
2. Email is just too slow, and they seldom if ever use snail mail.
3. "Go West, Young College Grad" has always implied "and don't stop until you get to Asia...and learn Chinese along the way."
4. Al Gore has always been animated.
5. Los Angelenos have always been trying…
drorzel | August 19, 2010At the tail end of Tuesday's post about wind and temperature, I asked a "vaguely related fun bonus question:"
If the air molecules that surround us are moving at 500 m/s anyway, why isn't the speed of sound more like 500 m/s than 300 m/s?
This is another one that people are sometimes surprised by. The answer is simply that in a sound wave, the air molecules don't really go anywhere. When something creates a sound-- say a foolish dog barking at a perfectly harmless jogger going by outside, to choose an example completely at random-- there isn't any actual thing that travels from the noisy dog…