The Quantum Physics Of Vampires

(AP Photo/Vadim Ghirda)

Via forbes.com by Chad Orzel

I've spent an inordinate amount of time thinking about the quantum physics of vampires... Having put that time in, though, I might as well get a blog post out of it.

The central physics idea that I'm about to grossly overthink is that vampires are somehow distinguishing sunlight from other forms of light. They're perfectly capable of appearing in brightly lit rooms to attack ordinary humans, but sunlight reduces them to ash in seconds. But in physics terms, one photon is just like another. So what could possibly distinguish sunlight from other forms of light?

In physics, we can describe any individual photon of light in terms of two related numbers, the frequency and the wavelength (they're related through the speed of light-- frequency times wavelength is equal to the speed-- which is a universal constant; for this reason, physicists will frequently switch between the two, opting for whichever is most convenient at a given moment). To characterize a source of light, though, we need to know the full spectrum of frequencies it puts out-- what's the intensity of light emitted (how many photons per second) at a particular wavelength.


Most sources that generate a significant amount of light are either thermal sources or atomic line sources. A thermal source is just an object that's emitting light because it's very hot-- the heating element in a toaster, say, or the filament of an incandescent bulb. An atomic line source, on the other hand, consists of a collection of atoms of a particular element that are then induced to emit light at one of the characteristic frequencies associated with those atoms-- a neon light, or those yellowish sodium-vapor streetlights, say. For these purposes, lasers are a special case of an atomic line source-- they emit only a single narrow range of frequencies (though in the case of semiconductor lasers, these aren't actually coming from atomic states).

The light from a thermal source has a very broad spectrum, emitting a wide range of different frequencies, which might seem like a total mess, but it turns out there's a simple way to characterize these. Hot objects emit light in what's called a "black-body spectrum," a particular distribution of intensities vs. wavelength that depends only on the temperature. the physics of black-body radiation was first explained by Max Planck in 1900, and Planck's theory is what gives us the term "quantum" for a unit of energy.

So, if you're looking for a distinction between sunlight and candlelight or sunlight and an incandescent bulb (for more modern vampires), the key distinction between them is the temperature. A candle flame is pretty hot in human terms, but only around 2000K (reminder: Kelvin temperatures are measured starting at absolute zero, and one kelvin is one degree Celsius; room temperature is a little bit less than 300K), while a really hot light bulb filament might hit 3000K. The Sun's spectrum closely matches a black-body at something like 5600K.

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