What is a Photon?

PhotonIn 1905, in the first of his annus mirabilis  (“miracle year”) papers, Albert Einstein carried the quantum revolution a step further – a “quantum leap”, we might say (inside joke). Picking up where Planck left off, Einstein looked at the absorption of EM radiation, especially the photoelectric effect – another unexplained “loose end.” The photoelectric effect, first observed by Heinrich Hertz in 1887, refers to the way electrons are ejected from a metal when ultraviolet light strikes it. It was clear that the energy of the absorbed light is transferred to the electrons, but increasing the intensity of light did not increase the energy of the ejected electrons – it only caused more electrons to be ejected. On the other hand, raising the frequency of radiation increased the energy but not the number of electrons. Einstein pointed out that this behavior is to be expected if the radiation is absorbed in the same discrete “chunks” or quanta derived by Planck from the radiation spectrum. This is the paper for which Einstein received the Nobel Prize in 1921, his theory of relativity being too controversial at the time.

Einstein’s photon. However Einstein’s view of the EM quantum was not the same as Planck’s. Einstein felt that if a quantum of light is emitted by a single atom at one location and then absorbed by a single atom at another location, surely it must be confined to a small region of space during its journey. That is, it must be a particle, as the great Newton had believed some two centuries earlier. Einstein later recanted this view, but by then the damage had been done.  In any case, whether they are particles or spread out fields, EM quanta are called photons), symbolized by the Greek letter gamma (γ),

Fields vs. particles (Round 1).  This was the first round in the ongoing battle between fields and particles. Maxwell’s equations do not permit a field quantum to remain localized, yet photons act as if they are localized. Can it be that photons do not obey Maxwell’s equations? And if they don’t, how can we explain the interference effects that had been observed for over 200 years?

What are photons? This dilemma led many physicists to the concept of wave-particle duality, or even to give up hope of ever comprehending reality. Einstein, however, could not do this. He believed there is a reality and that it must make sense. He spent the last half of his life searching for a unified field theory that would explain (among other things) the peculiar behavior of the photon, and shortly before his death he wrote:

All these fifty years of pondering have not brought me any closer to an­swering the question, what are light quanta? – A. Einstein 

The answer. There is an answer, at least according to QFT. To put it simply, Planck was right and Einstein’s first idea was wrong. In QFT the photon is a spread-out field, and the particle-like behavior occurs because each photon, or quantum of field, is absorbed as a unit. When a spread-out photon is absorbed by an atom, the entire field vanishes and all its energy is deposited into the atom. This process, known as field collapse, is the answer to Planck’s question, how can the photon be in a position to “concentrate its energy upon a single point in space”? The answer is, it isn’t. There is a big whoosh, so to speak, and the quantum is gone, like an elephant disappearing from a magician’s stage.

Field collapse. Field collapse is not an easy concept to accept – perhaps more difficult than the concept of a field. Here I have been working hard, trying to convince you that fields are a real property of space – indeed, the only reality – and now I am asking you to believe that this field quantum, spread out as it may be, suddenly disappears into a tiny absorbing atom. Yet it is a process that can be visualized without inconsistency. In fact, if a photon is an entity that lives and dies as a unit, field collapse must occur. A quantum cannot split and put half its energy in one place and half in another, or live here and die there. That would violate the basic quantum principle. While QFT does not provide an explanation for when or why collapse occurs, some day we may have a theory that does. In any case, field collapse is necessary and has been demonstrated experimentally.

A minority view. I should warn you that most physicists today do not accept this view. They either follow Feynman in believing that photons are particles, or they believe in wave-particle duality, paradoxical as these views may be.  And many, like Stephen Hawking, just don’t worry about the problem. To me, the persistence of and insistence on a belief in particles or in wave-particle duality, despite the phenomenal success of QFT, is in itself a bit of a paradox.

Posted By: Rodney Brooks 


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