Let There Be Light

Light And ‘The Illusion Of Knowledge’

Often, the familiar hides the deepest mysteries. This is certainly the case with light, one of the take-it-for-granted physical phenomena that surrounds us in everyday life. We wake up to it, we turn it on and off, rarely thinking of what it really is. A good thing, for even the greatest physicists pause before talking about the nature of light.

Undisclosed Location

Big storm. Tree down where I originally parked last evening, but I moved my car into better shade. Power out. Fire alarm (luckily false, because the fire department too a long time to show). Hot. Eventually, too hot.

Fled the state in search of an internet connection and juice, yes, the precious juice. Hoping things have been restored by this evening. Enjoy your leap second tonight.

Handy, but Not Deep Thoughts

A few weeks ago Doug Natelson had a post about handy numbers to know … if you’re doing some kind of physics involving liquid nitrogen or liquid helium, or anything else in which a condensed-matter/nanoscale physicists might be involved. (I assume this is in addition to knowing basic constants)

But I do atomic physics. A few favorite things that help me out if I’m away from a calculator or reference book, with some additions from my colleagues. I probably knew more of these, once upon a time.

— The speed of light can be written as 30 GHz-cm. Thus a 30 GHz signal has a wavelength of 1 cm. 1 Ghz means 30 cm.

— 1 nanosecond is 1 foot (light travel in a vacuum)

— a 1 eV photon is 1240 nm

— Room temperature is 1/40 eV (kT)

— Planck’s constant is 0.4 amu-micron-meters/second (useful for deBroglie wavelength calculations)

— There are about $latexpi$  x 10^7 seconds in a year

 

General reference:

– \(sqrt{g} = pi \) (to about 0.3%) Handy for Pendulum problems — just cancel or combine the two values.

I Love it When a Plan Comes Together

Ancient text gives clue to mysterious radiation spike

An eerie “red crucifix” seen in Britain’s evening sky in ad 774 may be a previously unrecognized supernova explosion — and could explain a mysterious spike in carbon-14 levels in that year’s growth rings in Japanese cedar trees. The link is suggested today in a Nature Correspondence by a US undergraduate student with a broad interdisciplinary background and a curious mind.