Wrap-up, Osheroff-fest

The symposium in honor of Doug Osheroff was great fun. It was great to see old friends again, to hear some stories that I didn't know, and to find out what other former group members are up to. The actual talks were generally pretty good, with a number of speakers focusing on how exciting and vibrant the whole field of low temperature physics was in its heyday. There were a total of seven Nobel Laureates there (DDO, Steve Chu, Bob Laughlin, Bob Richardson, Dave Lee, Phil Anderson, and Tony Leggett), and a bunch of other luminaries (Michael Fisher, Daniel Fisher, Bill Brinkman, Ted Geballe, and even a special and unexpected (by me, at least) appearance by Ed Witten). Steve Chu's talk was remarkable in part because he so clearly loved the chance to give an actual technical talk about some of his research, which you get the feeling he doesn't do so much at the DOE. Fun stuff, even when Bob Laughlin was giving me a hard time :-)

Osheroff-fest

I am currently visiting Stanford for my thesis advisor's big birthday bash/retirement festivities. It's really great to see so many former students, postdocs, and collaborators, and it's more than a little surreal to be back here after so long. It's a shame taht a few couldn't make it - they're sorely missed. There is going to be a day-long symposium tomorrow in his honor that should be very interesting. I'll post some brief description of some of the talks, if they seem like they are of general interest.

Excellent talk today + the point of colloquia.

Today I was fortunate to host my department's weekly colloquium, with Prof. Wilson Ho from UC Irvine as the speaker.  He gave a great talk about "Visualizing Quantum Mechanics", in which he showed (using experiments from his own group) how scanning tunneling microscopy can be a great teaching tool for illustrating concepts from undergraduate quantum mechanics.  He covered the exponential dependence of tunneling on distance, imaging of molecular orbitals, the crossover between classical (activated) diffusion and quantum (tunneling-based) diffusion, particle-in-a-box physics in 1d atomic chains, visualization of Fermi's Golden Rule via light emission experiments, and other neat results.  The audience included not just the usual collection of faculty and grad students, but also a bunch of the current undergrad quantum students as well.  

The talk was pretty much a letter-perfect example of what a colloquium is supposed to be.  It was accessible to a general audience, was genuinely educational, had appealing visuals, and contained enough intellectual "meat" to be satisfying for experts, including some not-yet published stuff.  It would be nice if every speaker realized the difference between a colloquium and a seminar....

On an unrelated note, I can't resist commenting on this awful article from Reuters hyping the LHC.  I'm as happy as anyone that the accelerator is running well, but does the CERN press office really need to keep churning out this kind of garbage?  Can't they just have a nice release/article talking about how nicely the experiment is running, and how they're hitting their targets, without making just laughable statements?  I think we can be pretty damned sure that the LHC is not about to discover incontrovertible evidence for parallel universes.   

Buckyball celebration/symposium

In honor of the 25th anniversary of the discovery of C₆₀ at Rice, the university is holding a symposium to celebrate.  In addition to the surviving members of the discovery team (laureates Curl and Kroto; Prof. Heath, Dr. O'Brien), there are many big names in the business (Millie Dresselhaus, Marvin Cohen, Phaedon Avouris, Hongjie Dai).  Andre Geim is going to skype in, apparently, since getting the Nobel Prize this past week has understandably scrambled his travel plans.  Unfortunately I'm flying to Washington, DC later this morning, so I will miss most of the fun, but I'm sure it will be a very interesting and lively event.

2010 Physics Nobel for graphene

The 2010 Nobel Prize in Physics has been awarded to Andre Geim and Konstantin Novoselov for graphene.  Congratulations to them!  Graphene, the single-atomic-layer limit of graphite, has been a very hot topic in consensed matter physics since late 2004, and I've posted about it here and here.  There is no question that graphene is a very interesting material, and the possibility of serious technological applications looms large, but as Joerg Haber points out, overhype is a real danger.  The prize is somewhat unusual in that it was very fast on the scale of these things.  I also find it interesting that only the Manchester group was given the prize, given the impact of the work going on in this area at other places at around the same time (for example, take a look at the first few talks in this session I put together at the 2005 APS March Meeting).  I do hope that those in the British scientific funding establishment take note that future prizes and innovations like this are at severe risk if research and educational funding cuts continue.

"Definitively inaccurate": One more comment about NRC rankings

One last post before the Nobel in physics is announced tomorrow.... As many people in the academic blogosphere have reported, there are some serious issues with the NRC rankings of graduate programs.  Some of these seem to be related to data entry, and others to nonuniform or overly simplistic interpretations of answers to survey questions.  Let me give a couple of examples.  I'm in the physics and astronomy department at Rice, and for several years I've helped oversee the interdisciplinary applied physics graduate program here (not a department - applied physics does not have faculty billets or its own courses, for example).  I filled out faculty NRC paperwork, and I was also in charge (with a colleague) of filling out the "department"-level NRC paperwork for the applied physics program.  I know, with certainty, that some of the stats for the two programs are very very similar, including the allocation of work space to graduate students and the approximate completion rates of the PhD program.  However, while these seem to show up correctly in the applied physics NRC data, they are both skewed bizarrely wrong (and very unfavorably, like the completion rate in the NRC data is too low when compared with reality by at least a factor of two!) in the physics & astronomy departmental NRC data.  Now, overall the department did reasonably well in the rankings, and if one looks particularly at just the research stuff per faculty member, physics and astronomy did quite well.  However, this issue with student data really stinks, because that's what some sites geared toward prospective students emphasize.  It's wrong, there's no fixing it, and it looks like it will be "definitively inaccurate" (to borrow a phrase from Douglas Adams) for at least a decade. 

Reductionism, emergence, and Sean Carroll

In the last couple of weeks, Sean Carroll has made two separate posts (here and here) on his widely read Cosmic Variance blog at Discover magazine, in which he points out, in a celebratory tone, that we fully understand the laws of physics that govern the everyday world.  In a reductionist sense, he's right, in that nonrelativistic quantum mechanics + electricity and magnetism (+ quantum electrodynamics and a little special relativity) are the basic rules underlying chemistry, biology, solid state physics, etc.  This is not a particularly new observation.  Fifteen years ago, when I was a grad student at Stanford, Bob Laughlin was making the same comments, but for a different reason:  to point out that this reductionist picture is in many ways hollow.  I think that Sean gets this, but the way he has addressed this topic, twice, really makes me wonder whether he believes it, since beneath all of the talk about how impressive it is that humanity has this much understanding, lurks the implication that all the rest of non-high energy physics (or non-cosmology) is somehow just detail work that isn't getting at profound, fundamental questions.   The emergence of rich, complex, often genuinely "new" physics out of systems that obey comparatively simple underlying rules is the whole point of condensed matter these days.  For example, the emergence, in 2d electronic systems in semiconductors, of low energy excitations that have fractional charge and obey non-Abelian statistics, is not just a detail - it's really wild stuff, and has profound connections to fundamental physics.  So while Sean is right, and we should be proud as a species of how much we've learned, not everything deep comes out of reductionism, and some fraction of physicists need to stop acting like it does. 

Grad school, rankings, and geniuses

At long last, the National Research Council has finally released their rankings of graduate programs, the first such ranking since 1993. Their methodology is extremely complicated, and the way they present the data is almost opaque. This is a side effect of an effort to address the traditional problem with rankings, the ridiculousness of trying to assign a single number to something as complex and multivariate as a graduate program. The NRC has gone out of their way to make it possible to compare programs on many issues, and that's generally a good thing, but at the same time it makes navigating the data painful. The best aid I've seen in this is this flash app by the Chronicle of Higher Education. It does a great job of showing, graphically, the range of rankings that is relevant for a particular program, and you can do side-by-side comparisons of multiple programs. As I had suspected, most programs have a fairly broad range of possible rankings, except those at the very top (e.g., Harvard's physics department is, according to the "S" rankings, which are those based on the metrics that faculty members themselves identified as important to them, somewhere between 1 and 3 in the country.). One thing to note: the "S" rankings probably mean more about department quality than the pure research "R" rankings, since the "R" rankings will naturally bias in favor of larger departments. The other thing that becomes obvious when playing with this app for a few minutes is that some departments had clear data entry problems in their NRC data. As an example, my own department appears to have "zero" interdisciplinary faculty, which is just wrong, and undoubtedly didn't help our ranking.

In other news, the MacArthur Foundation has released their 2010 list of Fellows, known colloquially as recipients of "Genius Grants". I'm only familiar with some of the ones that touch on physics, and the people involved are all very good and extremely creative, which is exactly the point, I guess! Congratulations, all. Now let the speculation begin on the Nobel Prizes, which are going to be announced next week.

Finally, I wanted to link to this great post by my friend Jennifer Rexford, who has intelligent advice for first-year graduate students.

Nanostructures as optical antennas

My student (with theorist collaborators) had a paper published online in Nature Nanotechnology yesterday, and this gives me an excuse to talk about using metal nanostructures as optical antennas. The short version: using metal electrodes separated by a sub-nanometer gap as a kind of antenna, we have been able to get local enhancement of the electromagnetic intensity by roughly a factor of a million (!), and we have been able to determine that enhancement experimentally via tunneling measurements.

As I've discussed previously, light can excite collective excitations (plasmons) of the electronic fluid in a metal. Because these plasmons involve displacing the electrons relative to the ions, they are associated with local electric fields at the metal surface. When the incident light is resonant with the natural frequency of these modes, the result can be local electromagnetic fields near the metal that can significantly exceed the fields from the incident light. These enhanced local fields can be useful for many things, from spectroscopy to nonlinear optics. One way to get particularly large field enhancements is to look at the region separating two very closely spaced plasmonic structures. For example, closely spaced metal nanoparticles have been used to enhance fields sufficiently in the interparticle gap to allow single-molecule Raman spectroscopy (see here and here).

A major challenge, however, has been to get an experimental measure of those local fields in such gaps. That is where tunneling comes in. In a tunnel junction, electrons are able to "tunnel" quantum mechanically from one electrode to the other. The resulting current as a function of voltage may be slightly nonlinear, meaning that (unlike in a simple resistor) the second derivative of current with respect to voltage (d²I/dV²) is non-zero. From a simple math argument, the presence of a nonlinearity like this means that an AC voltage applied across the junction gives rise to a DC current proportional to the nonlinearity, a process called "rectification". What we have done is turned this around. We use low frequency (kHz) electronic measurements to determine the nonlinearity. We then measure the component of the DC current due to light shining on the junction (for experts: we can do this with lock-in methods at the same time as measuring the nonlinearity). We can then use the measured nonlinearity and photocurrent to determine the optical-frequency voltage that must be driving the tunneling photocurrent. From the tunneling conductance, we can also estimate the distance scale over which tunneling takes place. Dividing the optical frequency voltage by that distance gives us the optical-frequency electric field at the tunneling gap, which may be compared with the field from the incident light to get the enhancement.

It's not at all obvious on the face of it that this should work. After all, the analysis relies on the idea that the tunneling nonlinearity measured at kHz frequencies is still valid at frequencies nearly 10¹² times higher. Experimentally, the data show that this does work, however, and our theorist colleagues are able to explain why.

When you think about it, it's pretty amazing. The radiation intensity in the little nanogap between our electrodes can be hundreds of thousands or millions of times higher than that from the incident laser. Wild stuff, and definitely food for thought.

Interesting links - nonphysics, mostly.

Nothing as interesting as this happens around here (at least, not to my knowledge), and I'm kind of glad. 

xkcd has once again done a far better job demonstrating some aspect of my existence than I ever could have myself.

Fascinating photography of nuclear weapons explosions here.

Tangentially related to nuclear weapons, I got a big kick out of Stephen Colbert's Dr. Strangelove tribute. 

Gravity

There has been a good deal of talk lately about gravity. We're all taught early on in our science education about the remarkable insight of Isaac Newton, that the force that causes, e.g., apples to fall from trees is, in fact, the same force that keeps the moon in orbit about the earth (or rather about a common center of gravity relatively close to the center of the earth). The Newtonian gravitational constant, G, is the least precisely known of all the fundamental constants, in part because gravity is a shockingly weak force and therefore difficult to measure. (As I demonstrated to my freshmen students, gravity is so weak that even with the feeble muscles in my legs I can jump up in the air in defiance of the opposing force of the gravitational pull of the entire earth.) More frustrating than the difficulty in precision measurement of G is the fact that different research groups using different techniques come up with experimental estimates of G that differ by surprisingly large amounts. This paper (published last week in Phys. Rev. Lett.) is another example. The authors sweated over the details of their systematic uncertainties for two years before publishing this result, which disagrees with the "official" CODATA value for G by 10 sigma (!). This is a classic showcase for the art, elegance, and necessary attention to detail required in precision measurement physics.

Also making many waves during 2010 is this paper by Erik Verlinde. The claim of this paper is that gravity is emergent, rather than a "real" force. It's been argued since Einstein published general relativity that gravity is different at a deep level than traditional forces. GR says that we should think of gravity as a deformation of spacetime due to the presence of stress/energy. Freely falling particles always travel on geodesics (locally straight lines), and those geodesics are determined by the distribution of mass and energy (including that due to spacetime deformation). In the appropriate limit, GR reduces to Newtonian gravity. Verlinde, striking out in a completely different direction, argues that one can start from very general considerations, and gravity emerges as an "entropic" force. An entropic force is an apparent force that results from the tendency of matter and energy to explore all available microscopic states. For example, a polymer will tend to ball up because there are many more microscopic states that describe the polymer wadded up than extended. Pulling on the two ends of the polymer chain to straighten it out will require overcoming this entropic tendency, and the result is a tension force. Verlinde argues that gravity arises similarly. I need to re-read the paper - it's slippery in places, especially on what underlying background assumptions are made about time and space, and what really plays the role of temperature here. Still, intriguing food for thought, and it's elegant that he can get both something GR-like and something Newtonian to fall out of such an analysis.

Regardless of how you may feel about Verlinde's speculations and the difficulty of measuring G, at least you can laugh in shocked disbelief that these people are serious.  (I should be careful making jokes.  Knowing Rick Perry, they'll start pushing this in Texas public schools next year.)

Two for the price of one.

I had noticed (and it was also pointed out by a colleague) the essentially simultaneous publication of this paper and this paper (which appear to have been submitted within a week of each other as well).  In both papers, the authors have created short-channel graphene-based transistors in a clever way. They take a conductive nanowire (doped GaN in the Nano Letters paper; CoSi in the Nature paper), coat it with thin aluminum oxide via atomic-layer deposition, and then lay it down on top of a piece of exfoliated graphene. Then they evaporate Pt on top of the device. On either side of the nanowire, the Pt lands on the graphene, making source and drain electrodes. The nanowire shadows part of the graphene (the channel), and then the nanowire itself acts as the gate. This is a nice, self-aligned process, and the resulting graphene devices appear to be very fast (the Nature paper has actual high frequency measurements). Looks like they managed to get two papers in good journals for the price of one technique advance.

Arguing from authority? Hawking, you're supposed to be better than that.

In Saturday's Wall Street Journal, there was an article by Stephen Hawking and Leonard Mlodinow clearly designed as a naked promotion of their new book.  In the article, they argue that modern physics removes the need for a divine being to have created the universe.  Religious arguments aside (seriously, guys, is that particular argument even news anymore?), one thing in the article especially annoyed me.  Toward the end, the authors state:

As recent advances in cosmology suggest, the laws of gravity and quantum theory allow universes to appear spontaneously from nothing. Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist. It is not necessary to invoke God to light the blue touch paper and set the universe going.

Our universe seems to be one of many, each with different laws.

You know what's wrong with this? It states, as if it is established fact, that we understand cosmology well enough to declare that universes spontaneously self-create. It states that the multiverse is a prediction of "many" theories, implying strongly that it's on firm ground. The problem is, this isn't science. It's not falsifiable, and in its present form it's not even close to being falsifiable in the foreseeable future. Seriously, name one PREdiction (as opposed to retrodiction) of these cosmological models, or more seriously, the multiverse/landscape idea, that is testable. Don't claim that our existence is such a test - the anthropic principle is weak sauce and is by no means evidence of the multiverse. Man, it annoys me when high profile theorists (it always seems to be theorists who do this) forget that physics is actually an experimental science that rests on predictive power.

This won't end well, because it's blindingly idiotic.

According to the Chronicle of Higher Education, my Texas A&M colleagues up the road in College Station now get the privilege of being evaluated based on their bottom-line "financial value" to the university.  Take how much money the professor brings in (including some $ from tuition of the number of students taught), subtract their salary, and there you go.  This raises problematic points that should be obvious to anyone with two brain cells to rub together.  First, I guess it sucks to be in the humanities and social sciences - you almost certainly have negative value in this ranking. Congratulations, you leeches who take salary and don't bring in big research funding!  Second, it firmly establishes that the service contributions of faculty to the university are worthless in this ranking scheme.  Third, it establishes that the only measure of your educational contribution is how many students you teach - purely quantity, so if you teach large intro classes you're somehow valuable, but if you teach smaller upper division courses, you're less valuable.  Gee, that's not simplistic at all.  Now, the article doesn't actually say how these rankings will be used, but I'm having a hard time imagining ways that this metric is a good idea.

Silicon oxide and all that.

It's been a busy week work-wise; hence the low rate of blogging.  However, I would be remiss if I failed to talk about the science behind a paper (on which I am a coauthor) that was mentioned on the front page of the New York Times yesterday.  A student, Jun Yao, co-advised by my colleagues Jim Tour and Lin Zhong, did a really elegant experiment that has gotten a lot of attention, and the science is pretty neat.  Here's the deal.  Lots of people have done experiments where they've seen what appears to be nonvolatile switching of the electrical resistance in various nanoscale systems (e.g., junctions in nanotubes and other nanomaterials).  That is, what is observed is that, with the use of voltage pulses, the electrical resistance of a device may be programmed to be comparatively high or comparatively low, and that state is preserved for a looooong time.  Long story short:  sometimes this behavior has nothing in particular to do with the nanoscale system being studied, and really results from the properties of the underlying or nearby silicon oxide, which is generally treated as inert and boring.  Well, as people in the Si industry can tell you at length, it turns out that silicon oxide isn't necessarily inert and boring.   What Jun showed via some elegant cross-sectional transmission electron microscopy is that when big voltage pulses are applied across small distances, it is possible to modify the oxide, effectively doing electrochemistry, and turning some of the oxide back into Si nanocrystals.  When those nanocrystals give a hopping path from one electrode to the other, the device is "on".  When that path is broken, the device is "off".  The resulting nanocrystals themselves are quite small, on the order of a few nm.  Hence the excitement about possibly using this approach for very dense, nonvolatile memory.  There are, of course, a great many engineering issues to be overcome (there's no need to tell me about that in the comments....), but it is definitely a pretty science result. 

The wisdom of combining complementary techniques

In the September issue of Nature Materials, I have a News and Views piece about a really neat article by Sakanoue and Sirringhaus of the Cambridge University organic electronics group. My apologies to those without subscriptions - here's a brief summary:

Transport in organic semiconductors is generally poor when compared with that in inorganic semiconductors. Disorder and purity are major concerns, and electronic conduction (parametrized by the mobility of the charge carriers) very often is thermally activated, so that decreasing temperature leads to an exponential worsening of charge transport. This is in contrast to the situation in clean, nice materials like Si or GaAs, when lowering T leads to improving mobility, as scattering of carriers by thermal phonons is reduced. The Cambridge investigators have successfully made transistors from high quality spin-cast films of TIPS-pentacene, a small molecule organic semiconductor. These films actually do show improving conduction as T is reduced down to 140 K. At high source-drain electric fields and high carrier densities, transport becomes pretty temperature independent down to cryogenic temperatures.

Most importantly, however, the Cambridge group has also done "charge modulation spectroscopy" - optical spectroscopy measurements on the films as well as on the molecules in solution. By combining the optical measurements with the transport experiments, they are able to make rather strong statements about how localized the charge carriers are. They can thus rule out exotic physics or voltage-driven metal-insulator transitions as the origin of the good conduction regime.

This work shows the power of combining complementary techniques. Relying only on transport, we had made similar arguments here. However, the addition of the optical data greatly enhances the scientific arguments - what we had argued as "consistent" is totally nailed down here, thanks to the additional information from the spectra.

Deep thoughts....

Pondering introductory mechanics has made me think again about some foundational issues that I've wondered about in the past.  Mach's Principle is the idea, put forward by Ernst Mach, that the inertial properties of matter depend somehow on the distribution of matter at far away points in the universe.  The classic thought experiment toted out to highlight this idea is "Newton's bucket".  Imagine a bucket filled with water.  Start rotating the bucket (relative to the "fixed stars") about it's central axis of symmetry.  After transients damp away due to viscosity of the water, the water's surface will have assumed a parabolic shape.  In a (non-inertial) frame of reference that co-rotates with the bucket, an observer would say that the surface of the liquid is always locally normal to the vector sum of the gravitational force (which wants to pull the liquid down relative to the bucket) and the (fictitious, and present because we're working in a rotating frame) centrifugal force (which is directed radially outward from the rotation axis).  [In an inertial frame of reference, the water has arranged itself so that the gradient in hydrostatic forces provides the centripetal force needed to keep the water rotating about the axis at a constant radius.]  This rotating bucket business, by the way, is a great way to make parabolic mirrors for telescopes.

Mach was worried about what rotation really means here.  What if there were no "fixed stars"?  What if there were no other matter in the universe than the bucket and liquid?  Moreover, what if the bucket were "still", and we rotated the whole rest of the universe about the bucket?  Would that somehow pull the liquid into the parabolic shape?  This kind of thinking has been difficult to discuss mathematically, but was on Einstein's mind when he was coming up with general relativity.  What does acceleration mean in an otherwise empty universe?  There seems to be reason to think that what we see as inertial effects (e.g., the appearance of fictitious forces in rotating reference frames) has some deep connection with the distribution of matter in the far away universe.  This is very weird, because a central tenet of modern physics that physics is local (except in certain very well defined quantum mechanical problems).    

The thing that's been knawing away at the back of my mind when thinking about this is the following.  There is a big overall dipole moment in the cosmic microwave background.  That means, roughly speaking, that we are moving relative to the center-of-mass frame of reference of the matter of the universe.  We could imagine boosting our velocity just so as to null out the dipole contribution to the CMB; then we'd be in an inertial frame co-moving with the overall mass distribution of the universe.  If inertial properties are tied somehow to the overall mass distribution in the universe, then shouldn't the center-of-mass frame of reference of the universe somehow be special?  Some high energy theorist may tell me this is all trivial, but I'd like to have that conversation.   Ahh well.  It's fun that basic undergrad physics can still raise profound (at least to me) issues. 

Memories and The Mechanical Universe

As I get ready to teach honors mechanics to first-year undergrads, I have been scouting the web for various resources.  I ran across the complete series run of The Mechanical Universe (streaming for residents of the US and Canada), a great show that I remember watching on PBS occasionally when I was in high school.  It's based on first-year physics at Cal Tech, and each episode opens and closes with David Goodstein lecturing to a class in an auditorium.  It's very well done, and the computer animation was exceptionally good and informative, considering it was produced in the mid-1980s.  Thanks, Annenberg Foundation, for making this show available!  (Funny sequel of sorts:  I actually had the pleasure of meeting Prof. Goodstein in 2003, and for some irrational reason I was surprised that he didn't look exactly the same as he had in 1984....)

What I missed, plus book recommendations

I'm finally back from travel, just in time to immerse myself in prep for the upcoming semester. It's hard to believe that classes start in 10 days.

While I was away from blogging, it looks like I missed some fun posts. For example, the Japanese group that made the first major discovery of the iron pnictide superconductors has found that sake (or something in sake) boosts superconductivity in a related compound.  Chad Orzel did a pretty nice job posting about superconductivity as well, though I might do a different post later about this.  He also had a post prompted by a reader demanding to know why all statistical physics courses are lame.  (The answer is, of course, that the reader had never taking stat mech from me :-).  Ahem.  Perhaps not.)  Along related lines, Charles Day at Physics Today has started a blog, which I will add to the blogroll at right.  Glad to see that he leaps into discussing why he likes condensed matter physics.  I also missed the excitement about the proposed proof that P != NP.  The discussion online about the would-be proof is very impressive - it's always nice to see Fields medalists blogging, especially when they write as well as Terence Tao. 

One final remark for now.  I strongly recommend reading The Alchemy of Air and The Demon Under the Microscope.  These are terrific, interesting books, and they really do a great job of making science (in this case chemistry) as exciting as any novel.  Many thanks to Paul Chirik for recommending them to me.

A cool application and more travel

Apologies for the long break between posts.  It's been an incredibly hectic summer, and I'm about to go on a last big trip before the school year starts (and I get to teach honors intro mechanics to ~ 90 frosh - should be exciting, at least).

Before I go, I wanted to point out a very cool application of micromachining and computing power.  There are many consumer electronic devices now that contain within them a little 3-axis accelerometer made by micromachining techniques, like this one.  The basic gadget consists of a micromachined "test mass" (typically a block of Si) suspended on (silicon) springs.  When the whole device is accelerated, the test mass "lags behind" because of its inertia, just as you get pushed back into the seat of your car when the car accelerates forward.  Through (often) capacitive sensing, the displacement of the test mass can be transduced into a voltage that the chip then outputs.  If the displacement can be detected along three axes, voila, you have a 3-axis accelerometer.  This is the widget that tells the Nintendo Wii how you've been swinging the controller, and it tells iPhones and other similar toys how to orient their displays.  With added sophistication, it's also possible to make micromachined gyroscopes.  They aren't true gyros that spin.  Rather, they're micromachined resonators (like tuning forks of particular shapes), and rotation leads to Coriolis forces that twist the resonator in a way that can be detected.  (For Wii aficionados, that is how the "Wii Motion Plus" works.)  Then you can get angular accelerations, too.

What is the point of this discussion?  Well, some people at Microsoft Research had a great insight.  You can put a sensor like this on a digital camera.  If the acceleration data is logged when a picture is snapped, then it is possible to retroactively unblur photos (at least, pictures that were blurry because the camera was moving).  This is the slickest thing I've seen in a while!