## Saturday, April 04, 2020

### Brief items

• From City University of New York, a paper on a bit of the physics relevant to the pandemic - specifically the issue of aerosolized droplets and air circulation in rooms.  The conclusion is that, based on common convection patterns, the best approach to clearing airborne contaminants is a ceiling-mounted suction filter as in surgical operating rooms.  (I suspect that vertical flow ceiling HEPA fan filter units with many air changes per hour as in microfabrication cleanrooms would also work, but it's not like anyone is going to install elevated, gridded flooring everywhere.)  Some of the basic physics of particle suspension is simple enough to teach to high school students, without even getting into viscosity and drag anf real fluid mechanics.  The typical amount of kinetic energy that a would-be suspended particle picks up in collisions with its surroundings is on the order of $k_{\mathrm{B}}T$, or about 26 meV ($4.14 \times 10^{-21}$ J).  For a particle to stay readily suspended, that has to be comparable to the gravitational potential energy that it would cost to elevate the particle by its own typical size.  For a spherical droplet of the density of water, you'd be looking at something like $(4/3)\pi R^{3} \cdot \rho \cdot g \cdot 2R$, where $R$ is the droplet radius, $\rho$ is the density of water, 1000 kg/m3, and $g$ is the gravitational acceleration, 9.807 m/s2.  Setting those equal and solving gives $R \approx 470$ nm.
• The always excellent Natalie Wolchover has a new article in Quanta about how one limiting factor in gravitational interferometers is the quality of the glass used in the dielectric mirrors.  Specifically, the tunneling two-level systems (see here and here) in ordinary amorphous insulating dielectrics at low temperatures are a problem.  It's like I've said ever since my doctoral work:  tunneling two-level systems are everywhere, and they're evil.
• As pointed out by many, this paper has a novel approach to room temperature superconductivity.  This is a bit like my idea of converting my entire lab into ultra-high vacuum workspace.  Sure, personnel would all have to wear special spacesuits, but it would really help preserve samples.
• In these days of social distancing, this was also amusing.
Please stay safe.  I know it's hard to stay positive while all of this is going on, but remember that you're not alone.

Anonymous said...

Solid state physics is yet another field that would benefit from destroying man's age-old nemesis, the Sun.

Mr Burns was right.

DanM said...

That one is funny. I've been at that party. Carrot cake and all.

By the way, I'm in the Houston area for the time being, in case it matters.

Anonymous said...