One of the joys of the arXiv is that anyone can submit anything to the website. Cranks and kooks can publish to their hearts' content in the theoretical physics section. Their work will remain there, read only by those searching for casual amusement. Yet somewhere between all the excellent science and slapstick comedy are scientists who just get stuff flat out wrong.
This is the story of how two respected physicists failed to understand photon angular momentum. Don't worry, they're not alone. Every physicist who has given the subject any thought has lost sleep working it out (and has had nightmares involving Jackson's Classical Electrodynamics). Since I lost sleep over it, I figured I would ensure that you all lose some sleep too.Spinning photons and rotating electric fields
The fundamental confusion arises from the fact that there are two equivalent ways of describing the angular momentum of a photon. A cursory inspection of nature, however, seems to reveal that one is more natural than the other.
A new study shows that the asteroid 4 Vesta may have a different internal structure than previously thought. Vesta, the second largest body in the asteroid belt after the dwarf planet Ceres, is notable for two gigantic craters, so big that they partly overlap despite being on opposite poles of the asteroid.
The first, chronologically speaking, is called Venenia (Named for a priestess of the goddess Vesta in Roman mythology), the result of an impact some 2 billion years ago. The crater is 395 kilometers in diameter, but only penetrated about 25 kilometers deep into the surface of Vesta. And then there’s Rheasilvia. Also named for a priestess of Vesta, Rheasilvia is a whopping 505 km in diameter (Vesta is only 525km in diameter), and the rim of the crater is also one of the tallest mountains in the solar system. Rheasilvia was probably created about one billion years ago, and it obliterated part of Venenia where the two overlap.
The impact penetrated so deep that it’s thought to reach down through the asteroid’s crust to its mantle. The new study, however, shows that, while it did reach about 60-100 km, it did not penetrate to the mantle, suggesting the mantle begins deeper than previously thought.