Thermo/StatMech
drorzel | January 8, 2013The most talked-about physics paper last week was probably Negative Absolute Temperature for Motional Degrees of Freedom (that link goes to the paywalled journal; there's also a free arxiv preprint from which the above figure is taken). It's a catchy but easily misinterpreted title-- Negative absolute temperature! Below Absolute Zero! Thermodynamics is wrong!-- that obscures the more subtle points of what's going on here. So, in the interest of clarity, I'm going to attempt an explanation, over the course of a few posts, but given my schedule these days, that might spread over a couple of…
drorzel | February 27, 2012We're in the home stretch of this term, and it has become clear that I won't actually be using the toy model of the arrow of time I've talked about in the past in my timekeeping class this term. These things happen. Having spent a not-insignificant amount of time playing with the thing, though, I might as well get a final blog post about it, with something that sort-of worked and something that shows why I'm not a computational physicist:
First, the thing that sort-of worked: in thinking about trying to use the code I wrote, I was struggling to come up with a way to quantify the apparent…
drorzel | January 23, 2012I'm fairly certain somebody has already done this, because it's such an obvious idea. It's a little beyond my cargo-cult VPython skills right at the moment, though (I can probably learn to do it, but not right now), and I none of the applets I Googled up seemed to be doing this, so I'm posting this sketchy description because I've spent some time thinking about it, and might as well get a blog post out of the deal.
So, as we said back in the holiday season, one of the most fundamental concepts in the modern understanding of thermodynamics and statistical physics is the notion of entropy. You…
drorzel | January 20, 2012While clearing the cars of frost Thursday morning, I was reminded of a difference of opinion Kate and I have about the best approach to a cold car. I tend to start the car up, and immediately turn the defrost on full blast; she prefers to let the car warm up for a while first, because it just blows cold air for a while. My feeling is that while the air from the defrost is not warm in an absolute sense, it's still passed over the engine on the way in, and thus is warmer than the outside air. Which means that it's going to increase the temperature inside the car somewhat, and thus will heat the…
drorzel | December 19, 2011Moving along in our countdown to Newton's birthday, we come to 1900, and one of the most revolutionary moment in the history of physics, represented in today's equation:
This is Max Planck's formula for the spectrum of the "black-body" radiation emitted by a hot object at temperature T. It's also the equation highlighted on what might be the most famous xkcd cartoon (albeit in different notation).
This is a fitting next step in the countdown not only for reasons of chronology, but also because it's a nice bridge from thermodynamics and statistical mechanics. After all, the red glow of a hot…
drorzel | December 18, 2011As I said yesterday, I'm going to blow through another entire subfield of physics in a single equation, as our march toward Newton's Birthday continues. Today, it's statistical mechanics, a very rich field of study that we're boiling down to a single equation:
This is Boltzmann's formula for the entropy of a macroscopic system of particles, which says that the entropy s is proportional to the logarithm of the number N of microscopic states consistent with that macroscopic state. The constant kB is there to get the units right.
Why does this get to stand in for the whole field of statistical…
drorzel | December 17, 2011Once again, the advent calendar is delayed until late at night by a busy day with SteelyKid-- soccer in the morning, playing with a trebuchet after lunch, then Arthur Christmas at the Colonie mall. We're running low on days to honor great milestones in physics, though, so I don't want to skip a day entirely.
I'm also trying to spread this around to cover a fairly representative set of subfields; having done classical mechanics and E&M at some length, I need to rush through a couple of other subfields quickly. One of these is classical thermodynamics, a field with a rich history and wide…
drorzel | September 19, 2011I got a new comment on an old post asking an interesting question about thermodynamics:
I have a question that bears somewhat on this issue of keeping cars parked in the sun, cooler. You all know those accordion folded/aluminized shades you can put up inside the windshield and back window.
Seems to me putting them INSIDE is the wrong approach. They should be on the OUTside of the window acting as real shades and reflecting away the sun before it gets into the inside of the car.
This involves some of the same physics involved in the ever-popular issue of climate change, so it's worth talking…
drorzel | July 26, 2011One of the benefits of having joined AAAS in order to get a reduced registration fee at their meeting is that I now have online access to Science at home. Including the Science Express advance online papers, which I don't usually get on campus. Which means that I get the chance to talk about the few cool physics things they post when they first become available, without having to beg for a PDF on Twitter. This week's advance online publication list includes a good example of the sort of cool ultra-cold atom physics that I talked about at and after DAMOP, so let's take a look at this paper in…
drorzel | July 21, 2011It's really frickin' hot in much of the US. Fortunately, we have central air at home, A/C in the car, and convenient local businesses with air conditioning and free wi-fi. The inadequate HVAC systems in the Science and Engineering building on campus aren't anywhere near being able to cope with this, so I'm working from home or a cafe until the weather breaks.
I will, however, use this as a shameless plug to re-link a post from last year, where we scientifically tested whether it's better to leave your car windows open or closed on a hot day. The answer: if it's a short stop, closing the…
drorzel | May 2, 2011Last summer, there was a fair bit of hype about a paper from Mark Raizen's group at Texas which was mostly reported with an "Einstein proven wrong" slant, probably due to this press release. While it is technically true that they measured something Einstein said would be impossible to measure, that framing is a little unfair to Einstein. It does draw media attention, though...
The experiment in question involves Brownian motion, and since I had to read up on that anyway for something else, I thought I might as well look up this paper, and write it up for the blog.
OK, so what did they do that…
drorzel | April 26, 2011This paper made a big splash back in November, with lots of news stories talking about it; it even made the #6 spot on Physics World's list of breakthroughs of the year. I didn't write it up then because I was hellishly busy, and couldn't take time away from working on the book-in-progress to figure out exactly what they did and why it mattered. I've got a little space now between handing the manuscript in last week and starting to revise it (probably next week), so while it's a bit late, here's an attempt at an explanation of what all the excitement was about.
So, what's this about, anyway?…
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 10, 2011A seasonally appropriate poll, brought to you by this morning's frigid dog walk (15F/ -9C), and the memory of a newscast back when I was in Maryland that referred to an overnight low temperature of 22F/-6C as "Bitter, bitter cold":
The maximum (daytime high) temperature I would characterize as "bitter cold" would be:survey software
For the purposes of this poll, assume a still day with no significant "wind chill." All of these temperatures are too high for quantum effects to be significant, so you may only choose one answer at a time, not a superposition of multiple answers.
drorzel | October 7, 2010I'm shamelessly stealing this question from James Nicoll, who asked it about science fiction. The question is a play on the famous comment that only of order a thousand people bought the first Velvet Underground record, but every one of them went on to start a band.
So, the question is, who is the Velvet Underground of science? That is, who is the best example of somebody whose work was only read by a tiny number of people, but ended up being incredibly influential on those people and subsequent generations?
The physics example that comes to mind immediately is Sadi Carnot. Carnot wrote a…
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 17, 2010I got forwarded a physics question last night asking about the connection between wind and temperature, which I'll paraphrase as:
Temperature is related to the motion of the atoms and molecules making a substance up, with faster motion corresponding to higher temperature. So why does it feel warmer when the air is still and why does wind make you feel cold?
This is a moderately common point of confusion, so while I responded to the question in email, I'll also appropriate it for a post topic. So, why doesn't "windy" equal "hot," given that wind consists of moving air molecules?
The full…