## Saturday, February 29, 2020

### APS March Meeting cancelled

Hello all - I have just heard from Dan Arovas, program chair of the APS March Meeting, that the APS has decided to cancel the meeting, which was scheduled to begin tomorrow: "Just finished a Zoom meeting with APS CEO Kate Kirby, APS presidential line, secretary treasurer, counselor. APS is preparing a statement for release to the press. Right now you can help by informing all your students, postdocs, and colleagues. The web site will be updated as soon as possible."

This is a response to COVID19. As I post this, the meeting website has not yet been updated.  I will post more when I learn more.

Update: The text of the APS email: "Due to rapidly escalating health concerns relating to the spread of the coronavirus disease (COVID-19), the 2020 APS March Meeting in Denver, CO, has been canceled. Please do not travel to Denver to attend the March Meeting. More information will follow shortly."

Update: APS website now confirms.

Update: Here is the text of the letter from the APS president and CEO about the decision.
To the Board, the Council and Unit Leaders of APS:
You have probably already heard that on Saturday, February 29, the APS Leadership decided to cancel the 2020 March Meeting in Denver. We are writing to give you some of the details that led to this difficult decision, which was made in consultation with the APS senior management and the March Meeting program chair.
APS leadership has been monitoring the spread of the coronavirus disease (COVID-19) in the days leading up to the meeting. As you know, a large number of March Meeting attendees come from outside the US. Many have already canceled their attendance, particularly those from China, where travel to the meeting is not currently possible. In addition, we had many planning to come from countries where the CDC has upgraded its warning to level 3 as recently as the day of our decision, yesterday February 29. Even more were coming from countries where the virus appears to be establishing itself in the general population, so that the warning level could rise during the course of our meeting, which might significantly delay their return travel or even lead to quarantines.
In this case the safety of the attendees has to be a primary concern. There is a reasonable expectation that in a meeting with many thousands of participants, some will fall ill. This always happens of course, but it presently takes some time to establish whether an illness is seasonal flu or COVID-19, and many attendees who have come into contact might need to be quarantined during the testing. In light of this danger, we realized that ordinary social events such as the evening receptions would have to be cancelled out of caution.
We appreciate the high cost of our decision, both for the APS and also the attendees. We don’t know the actual loss yet, but the APS portion alone is certain to be in the millions of dollars. We want to assure the APS Board, Council, and Unit Leaders, that we have considered this carefully. Our society is strong financially, and we can absorb this loss. The welfare of our community is certainly a greater concern.
We know you have many questions about the path forward following this decision. We will continue to communicate and confer with you regularly in the coming weeks, as we all come to terms with the need to find new ways to maintain strong international science contacts.
Phil Bucksbaum, APS President
Kate Kirby, APS CEO

## Monday, February 24, 2020

### BAHFest 2020 at Rice, Sunday March 8 UPDATE: postponed.

Update:  This event is going to be postponed until the fall semester.

For those in the Houston area:

BAHFest is a celebration of well-argued and thoroughly researched but completely incorrect scientific theory. Our brave speakers present their bad theories in front of a live audience and a panel of judges with real science credentials, who together determine who takes home the coveted BAHFest trophy. And eternal glory, of course. If you'd like to learn more about the event, you can check out these articles from the Wall Street Journal and NPR's Science Friday

Here are some examples from past shows:

Our keynote for this year's event is the hilarious Phil Plait (AKA the Bad Astronomer)! Phil will be doing a book signing of his book "Death from the Skies" before and after the show.

The event is brought to you by BAHFest, and the graduate students in Rice University's Department of BioSciences. Click here for more information about the show, including how to purchase tickets. We hope to see you there!

[Full disclosure:  I am one of the judges at this year's event.]

## Saturday, February 22, 2020

### Brief items

As we head out of a very intense week here and toward the March APS meeting, a few brief items:

• Speaking of the March Meeting, I hear (unofficially) that travel restrictions due to the coronavirus have made a big dent - over 500 talks may be vacant, and the program committee is working hard to explore options for remote presentation.  (For the record, I fully endorse the suggestion that all vacant talks be delivered in the form of interpretive dance by Greg Boebinger.)
• There will be many talks about twisted bilayers of various 2D materials at the meeting, and on that note, this PRL (arxiv version here) shows evidence of "strange metallicity" in magic-angle bilayer graphene at temperatures above the correlated insulator state(s).
• Following indirectly on my post about condensed matter and Christmas lights, I want to point out another example of how condensed matter physics (in the form of semiconductor physics and the light emitting diode) has changed the world for the better in ways that could never have been anticipated.  This video shows and this article discusses the new film-making technique pioneered in the making of The Mandalorian.  Thanks to the development of organic LED displays, infrared LEDs for motion tracking, and lots of processing power, it is possible to create a floor-to-ceiling wraparound high definition electronic backdrop.  It's reconfigurable in real time, produces realistic lighting on the actors and props, and will make a lot of green screen compositing obsolete.  Condensed matter:  This is The Way.
• Superconducting gravimeters have been used to check to see if there are compact objects (e.g., hunks of dark matter, or perhaps microscopic black holes) orbiting inside the earth.  I remember reading about this issue while in college.  Wild creative idea of the day:  Maybe we should use helioseismology to try to infer whether there are any such objects orbiting inside the sun....

## Thursday, February 13, 2020

### Film boiling and the Leidenfrost point

While setting up my eddy current bounce demonstration, I was able to film some other fun physics.

Heat transfer and two-phase (liquid+gas) fluid flow is a complicated business that has occupied the time of many scientists and engineers for decades.  A liquid that is boiling at a given pressure is pinned to a particular temperature - that's the way the first-order liquid-vapor transition works.  Water at atmospheric pressure boils at 100 C; adding energy to the liquid water at 100 C via heat transfer converts water into vapor rather than increasing the temperature of the liquid.

Here we are using liquid nitrogen (LN2), which boils at 77 K = -196 C at atmospheric pressure, and are trying to cool a piece of copper plate that initially started out much warmer than that.  When the temperature difference between the copper and the LN2 is sufficiently high, there is a large heat flux that creates a layer of nitrogen vapor between the copper and the liquid.  This is called film boiling.   You've seen this in practice if you've ever put a droplet of water into a really hot skillet, or dumped some LN2 on the floor.  The droplet slides around with very low friction because it is supported by that vapor layer.

Once the temperature difference between the copper and the LN2 becomes small, the heat flux is no longer sufficient to support film boiling (the Leidenfrost point), and the vapor layer collapses - that brings more liquid into direct contact with the copper, leading to more vigorous boiling and agitation.  That happens at about 45 seconds into the video.  Then, once the copper is finally at the same temperature as the liquid, boiling ceases and everything gets calm.

For a more technical discussion of this, see here.  It's written up on a site about nuclear power because water-based heat exchangers are a key component of multiple power generation technologies.

## Tuesday, February 11, 2020

### Eddy currents - bouncing a magnet in mid-air

Changing a magnetic field that permeates a conductor like a metal will generate eddy currents.  This is called induction, and it was discovered by Michael Faraday nearly 200 years ago.   If you move a ferromagnet near a conductor, the changing field produces eddy currents and those eddy currents create their own magnetic fields, exerting forces back on the magnet.  Here is a rather dramatic demo of this phenomenon, shamelessly stolen by me from my thesis adviser.

In the video, you can watch in slow motion as I drop a strong NdFe14B2 magnet from about 15 cm above a 2 cm thick copper plate.  The plate is oxygen-free, high-purity copper, and it has been cooled to liquid nitrogen temperatures (77 K = -196 C).   That cooling suppresses lattice vibrations and increases the conductivity of the copper by around a factor of 20 compared with room temperature.  (If cooled to liquid helium temperatures, 4.2 K, the conductivity of this kind of copper goes up to something like 200 times its room temperature value, and is limited by residual scattering from crystalline grain boundaries and impurities.)

As the magnet falls, the magnetic flux $\Phi$ through the copper increases, generating a circumferential electromotive force and driving eddy currents.  Those eddy currents produce a magnetic field directed to repel the falling magnet.  The currents become large enough that the resulting upward force becomes strong enough to bring the magnet to a halt about 2 cm above the copper (!).  At that instant, $d\Phi/dt = 0$, so the inductive EMF is zero.  However, the existing currents keep going because of the inductance of the copper.  (Treating the metal like an inductor-resistor circuit, the timescale for the current to decay is $L/R$, and $R$ is quite small.)  Those continuing currents generate magnetic fields that keep pushing up on the magnet, making it continue to accelerate upward.  The magnet bounces "in mid air".  Of course, the copper isn't a perfect conductor, so much of the energy is "lost" to resistively heating the copper, and the magnet gradually settles onto the plate.  If you try this at room temperature, the magnet clunks into the copper, because the copper conductivity is worse and the eddy currents decay so rapidly that the repulsive force is insufficient to bounce the magnet before it hits the plate.

(Later I'll make a follow-up post about other neat physics that happens while setting up this demo.)

## Sunday, February 09, 2020

I realized it's been several years since I've run a version of this, and it's the right season....

This is written on the assumption that you have already decided, after careful consideration, that you want to get an advanced degree (in physics, though much of this applies to any other science or engineering discipline).  This might mean that you are thinking about going into academia, or it might mean that you realize such a degree will help prepare you for a higher paying technical job outside academia.  Either way,  I'm not trying to argue the merits of a graduate degree.
• It's ok at the applicant stage not to know exactly what you want to do.  While some prospective grad students are completely sure of their interests, that's more the exception than the rule.  I do think it's good to have narrowed things down a bit, though.  If a school asks for your area of interest from among some palette of choices, try to pick one (rather than going with "undecided").  We all know that this represents a best estimate, not a rigid commitment.
• If you get the opportunity to visit a school, you should go.  A visit gives you a chance to see a place, get a subconscious sense of the environment (a "gut" reaction), and most importantly, an opportunity to talk to current graduate students.  Always talk to current graduate students if you get the chance - they're the ones who really know the score.  A professor should always be able to make their work sound interesting, but grad students can tell you what a place is really like.
• International students may have a very challenging time being able to visit schools in the US, between the expense (many schools can help defray costs a little but cannot afford to pay for airfare for trans-oceanic travel) and visa challenges.  Trying to arrange skype discussions with people at the school is a possibility, but that can also be challenging.  I understand that this constraint tends to push international students toward making decisions based heavily on reputation rather than up-close information.