If Cinderella Were a Physicist: On Satyandra Bose and the Bose-Einstein Condensation

If you take a group of bosons and cool it down to very, very close to absolute zero, all of the bosons will at once lose almost all of their energy. In this situation they will also lose their personal identity.

In 1995, a pair of researchers from the University of Colorado in the USA, Eric Cornell and Carl Weinman, faced a difficult challenge. On the experimental table in front of them stood a group of rubidium atoms, and Cornell and Winman's goal was to cool this sample to an almost unimaginable temperature: 170 billionths of a degree above absolute zero. If Cornell and Weinman succeed in their mission, they may turn the cluster of rubidium atoms into a new type of material, the likes of which has never been seen before.

To understand what the pair of researchers were trying to achieve in 1995, one must go back to the beginning of the twentieth century. Satyendra Bose was born in 1894 in Calcutta, India. Even from his youth he stood out as a particularly brilliant mathematician, and advanced within the Indian Academy to the position of physics lecturer at the University of Dhaka. A tremendous scientific revolution took place in Europe at the same time: Einstein, Bohr, Schrödinger and others turned the Newtonian picture of the world upside down. But distant India was somewhat cut off from the mainstream and only a few Indian scientists were able to attend conferences, trainings and the other opportunities for cross-fertilization that are so important to the progress of science. Bose was very interested in quantum theory, but he was largely forced to work alone.

One day Bose lectured his students about certain aspects of quantum theory. Bose tried to prove to the students that the theory was still not perfect: there were physical phenomena that the quantum theory did not correctly describe. But as happens to every lecturer from time to time, Bose miscalculated at the blackboard. When he reached the end of the calculation, he discovered, to his surprise, that the theory he had written on the board actually described the desired physical phenomena very well. Bose went back several steps in the calculation, and located his error. The accepted theory treats each photon of light as a separate and independent particle: each photon has a set of properties and an appropriate experiment can always be designed to differentiate between any two photons based on these properties. Bose, mistakenly, treated photons as particles whose properties are all exactly the same and are indistinguishable from each other.

In a flash of inspiration, Bose realized that his mistake was not a mistake at all: to explain the desired physical phenomena, one must assume that there are situations in which the photons are completely identical and indistinguishable from one another. After analyzing the idea in depth and convinced of its correctness, Bose wrote a scientific article on the matter in 1925 and sent it for publication in the leading scientific magazines of his time. You can guess what happened next. An anonymous physicist from a remote third world country claims to have discovered an error in quantum theory. All the magazines rejected him outright.

But Bose did not give up. He took the article, folded it neatly into an envelope and sent it directly to the most important scientist in the world - Albert Einstein. If Cinderella was a theoretical physicist, this is exactly how her legend should have looked: Einstein read Bose's paper and immediately understood that it was a dramatic revision of the theory. He translated Bose's article into German and sent it, in Bose's name of course, to a scientific magazine. You can guess what happened next. The most famous genius of his time puts his hands on an important correction to quantum theory. The magazine published the article immediately.

Bose became a very desirable personality in Europe. He spent a year in visits and conferences, working with Einstein and other scientists and when he returned to India he was quickly appointed a university professor. This was very unusual as Satindra Bose was not even a Dr. That's how it is when someone like Einstein recommends you, it turns out that it can help.

Einstein continued and developed Bose's ideas several steps further. He hypothesized that not only photons can behave as completely identical particles - also a certain type of matter particles called 'bosons' (scornfully, of course) are able to do so. If you take a group of bosons and cool it down to very, very close to absolute zero, all of a sudden a kind of 'atomic avalanche' will occur: all the bosons will lose almost all of their energy at once. In this situation they will also lose their personal identity. All the physical properties that normally distinguish one atom from another will be reset. The group of bosons will become, in effect, one big atom. This is a completely new state of matter: it is not a gas, it is not a liquid and it is not a solid - it is...something else. This phenomenon is called 'Bose-Einstein condensation'. It is important to emphasize that bosons must not lose all their energy: if a particle loses all its energy, it stops in place - then we will know for sure its position and speed, and this will violate Heisenberg's uncertainty principle. This principle states that it is impossible, in any way, to know the position and speed of a particle with perfect accuracy. The bosons, therefore, will slide to the minimum energy level possible for the particle.

Bose-Einstein condensation was for many years the holy grail of quantum physics. Quantum phenomena are very difficult to measure in experiments: they occur in very small particles, and for very short periods of time. But Bose-Einstein condensation creates a kind of super-atom: a large particle that is made up of many perfectly identical particles. Because the energy content of atoms is so low, all quantum phenomena take a long time (relatively, of course) and are easier to observe experimentally. In other words, the quantum world leaves the micro realm for a brief moment and moves into the macro realm. All of this may occur, if the calculations of Eric Cornell and Carl Weinman are correct, at a temperature of 170 nanokelvins above absolute zero.

The method by which we intended to reach this breakthrough temperature was particularly innovative: cooling using a laser beam. The photons in the laser beam collide with atoms and slow them down, and as you remember, the amount of energy in an atom is reflected in its speed: a slow atom is a cold atom. The temperature of the group of atoms dropped dramatically. The researchers then activated a strong magnet and directed its action so that the fast, high-energy atoms were able to escape from the sample - and only the slow, cold atoms remained behind. This is similar to how a cup of coffee cools: the fast, hot atoms leave the cup as steaming steam, and the coffee left behind gets colder.

That's all it took. Just as Einstein and Bose had predicted almost seventy years earlier, when the last of the fast atoms left the sample there was a sudden 'avalanche': all the atoms fell as one to their lowest energy level. The group of atoms in the sample became one big atom - a new type of matter was created in the universe, and a new Nobel Prize was born.

[The article is taken from the program Making history!', a podcast about science, technology and history].