What Is a Boson? Einstein Was the First to Know

09/01/2012 06:15 pm ET | Updated Nov 01, 2012
  • A. Douglas Stone Carl Morse Professor and Chair of Applied Physics, Yale University

Recently people around the world have been exposed to an unfamiliar scientific term, due to the observation of a new elementary particle by physicists at CERN. The term is "boson," and the particle, the Higgs boson, completes a chapter in the modern sub-atomic description of nature. The word surely sounds odd to non-expert ears, with its echoes of a certain American TV clown, and no obvious Greek or Latin root. What is the origin of this term, and why is it worth specifying that the Higgs is not just any old supersmall particle, but a member of the particular family of fundamental physical objects grouped together as bosons? In fact this terminology goes back almost 90 years, to the dawn of the modern understanding of the atom. It stems from one of the most fortuitous episodes in the modern history of science, and involves intimately the most famous physicist of all time, Albert Einstein.

In June of 1924 Albert Einstein had become not just the best-known scientist of his era, but one of the most recognized names on the planet. The demands on his time and attention had grown exponentially due to the publicity associated with his now experimentally confirmed General Theory of Relativity. Thus when an unknown 30-year old Indian physicist, Satyendranath Bose, sent him an unsolicited manuscript to read, the chances that it would end up anywhere but the circular file were very low. "Respected Sir, I have ventured to send you the accompanying article for your perusal and opinion," the letter began, and after explaining the scientific goal of the paper, it closed with an astonishing request, that Einstein translate the English manuscript into German for publication. "Though a complete stranger, I do not feel any hesitation in making such a request. Because we are all your pupils..." Despite the long odds, in this case Bose hit the scientific lottery. At that time Einstein was deeply involved in the struggle to understand how atoms and light behaved, a 20-year quest that had begun in 1905 when had dared to suggest that light, which had been "proven" to be an electromagnetic wave during the 19th century, consisted of localized particles, which we now call photons. Bose's paper was on this topic and Einstein read it carefully, decided that it "signifies an important advance," and translated it for publication in a top German physics journal. This began a chain of events that ultimately enshrined Bose's name in the modern theory of nature.

Bose had tried to solve a longstanding problem in describing thermal radiation (the electromagnetic energy emitted by any hot object) using Einstein's photon concept. The fundamental law determining how much energy there is in thermal radiation had been found by Max Planck twenty-four years earlier, but up to that point all attempts to deduce this law from the "photon gas" picture, using thermodynamic principles had failed. Somehow Bose, in a terse document of less than two journal pages, had succeeded. But how had he done it?

The key was to count the number of states of motion that a photon can take on, when confined to a certain volume; this would determine the "entropy" of the gas, from which the Planck Law followed. However, in counting the photon states Bose had, apparently unknowingly, counted them differently from all previous physicists, including Einstein. When his new approach gave the right answer (Planck's Law), he simply wrote up the calculation, without any detailed discussion, and sent it to Einstein. Somehow, Einstein intuited that this new counting method was not simply an error by an inexperienced researcher, but represented a correct guess about the bizarre properties of the unobservable atomic domain.

How could something as mundane as an atomic accounting method actually change our view of nature? Well, as any gambler knows, the laws of statistics are also laws of nature. The reason that when we flip two coins we find a heads and a tails half the time (on average) is that the coin is equally likely to land on either side. Moreover there are two ways to get a heads and a tails (coin 1 = heads, coin 2 = tails; coin 1 = tails, coin 2 = heads) and only one way to get either of the other results. But what if we had two really identical coins, and instead of flipping them in the open we jiggled them around in a closed box, and then opened it for each trial? In this case we would not know, when we found a heads and a tails, whether it came from one or the other of the two ways. Would this change the probability that we get a heads and a tails? Absolutely not. These probabilities stem from the fact that each coin is a distinct object with independent properties. But Bose's accounting had essentially denied that this was true of micro-particles like photons.

Bose's reasoning assumes that photons are not like macroscopic coins, and that it makes no sense to ask whether photon 1 is in state 1 and photon 2 is in state 2, or vice-versa. These two states do not separately exist and hence there is only one such configuration of two photons. If we think of photons as "quantum coins," the probability of flipping two of them and getting a tails and a heads is only one third, not one half (and correspondingly the probability of heads-heads or tails-tails is now increased to one third). Note, and here's the mind-bending part, this is not because photons (or atoms) are small and we can't tell which photon is in which state. Unlike macroscopic coins, the quantum coins exist in a single fuzzy combination of heads-tails + tails-heads. While all of this was implicit in Bose's reasoning, he much later admitted that he "had no idea that what I had done was really novel."

Einstein however quickly grasped the enormous implications of this change of viewpoint. By December of 1924 he had understood the meaning of Bose's new statistics and applied them to a conventional gas consisting of atoms. He discovered that at ultra-low temperatures atoms can form a new state of matter, called a Bose-Einstein condensate, which eventually was observed in Nobel prize winning experiments in 1995. Within the next few years, Werner Heisenberg, Erwin Schrodinger and others found the basic equations describing atoms and light, the theory now known as quantum mechanics. It turned out that in addition to particles that obey Bose statistics, now called bosons, there is another category of particles, called fermions, after the physicist Enrico Fermi. These particles are indistinguishable in the Einstein-Bose sense, but also cannot share the same state with each other. In the coin analogy, the states head-heads and tails-tails can't occur. Protons and electrons are fermions, whereas bosons are the force-carrying particles in nature, the Higgs being the newest member of the club. All these force-carriers carry the name of a physicist whose elevation into the physics pantheon hung on the slimmest of chances, that the great man, Einstein, would rescue his groundbreaking paper from obscurity.

A. Douglas Stone is Carl Morse Professor and Chair of Applied Physics at Yale University. His forthcoming book from Princeton University Press is titled, "Einstein and the Quantum: The Quest of the Valiant Swabian."