Last month, physicists and commentators the world over marked the centenary of the theory of relativity, which gave us everything from GPS to blackholes, and described the machinations of the universe at the largest scales. Despite many struggles by the greatest scientists of our times, the theory of relativity remains incompatible with quantum mechanics, the rules that describe the universe at its smallest, to this day. Yet it persists as our best description of the grand opera of the cosmos.
Incidentally, Einstein wasn’t a fan of quantum mechanics because of its occasional tendencies to violate the principles of locality and causality. Such violations resulted in what he called “spooky action at a distance”, where particles behaved as if they could communicate with each other faster than the speed of light would have it. It was weirdness the likes of which his conception of gravitation and space-time didn’t have room for.
As it happens, 2015 also marks another milestone, also involving Einstein’s work – as well as the work of an Indian scientist: Satyendra Nath Bose. It’s been 20 years since physicists realised the first Bose-Einstein condensate, which has proved to be an exceptional as well as quirky testbed for scientists probing the strange implications of a quantum mechanical reality.
Its significance today can be understood broadly in terms of three ‘periods’ of research that contributed to it: 1925 onward, 1975 onward, and 1995 onward.
The Bose-Einstein condensate (BEC) is a state of matter predicted by Einstein in 1925. The grounds for its hypothesis were laid by Bose. Two years earlier, Arthur Compton had found definitive experimental proof that electromagnetic radiation could behave like waves, as was previously known, as well as particles. In 1924, Bose wrote up a paper about how a population of these ‘light particles’, or photons, might behave under certain conditions and mailed it to Einstein.
Einstein was famously impressed by the paper and translated it into the German until December 1924, and submitted it for publication with some of his notes to the prestigious journal Zeitschrift für Physik. It was published in the same month.
Bose’s thesis was founded on the idea that photons formed a special class of particles, which Paul Dirac later named bosons in his honour. Dirac and Enrico Fermi had separately realised that particles like electrons and protons formed a separate class since called fermions. Bosons and fermions principally differ in the value of a quantum mechanical property of all particles, called spin. The spin describes the values that a particle’s angular momentum can have. All bosons have integer spin (0, 1, 2, 3, …) while all fermions have half-integer spin (1/2, 3/2, …).
Bose had only described the behaviour of photons, which are massless bosons, in his paper. In 1925, Einstein extended Bose’s work to include massive bosons in the theory as well, coming up with a broader framework of rules describing the statistical properties of bosons called Bose-Einstein statistics. And in its aftermath, Einstein quickly realised a peculiar implication. He found that should a gas of bosons be cooled to a particularly low temperature, they would all collectively occupy the lowest quantum energy state.
One way to understand this is in terms of the wave function. In quantum mechanics, the wavefunction is a mathematical problem whose solution yields the chances that a particle is present at a chosen point in space and time. In a cloud of supercooled bosons, Einstein predicted that all the bosons occupying the common state could be described by a single wavefunction – such that no two bosons could be distinguished by their properties. This isn’t possible with fermions. If such a condensation of bosons could be recreated in a lab, physicists knew that it would expose the machinations of an invisibly tiny world on an unprecedented scale.
And that’s what Eric Cornell and Carl Wieman, both from the University of Colorado, Boulder, and Wolfgang Ketterle, from MIT, first perfectly achieved in 1995. The trio’s work was a major boost for physics research as a whole and physicists working with BECs to this day continue to uphold that bearing. To understand why, let’s step back 20 more years to when the idea of a technique called laser-cooling first started to appear in the scientific literature.
There are four known phenomena that describe quantum mechanics in action at the macroscopic scale – a scale at which its effects are otherwise marred by the effects of gravity. The phenomena are BECs, superconductivity, superfluidity, and lasers (the discovery of each of which has received a Nobel Prize). A laser’s defining attributes are “high directionality, monochromaticity, high brightness, and stable intensity”.
In 1975, two pairs of physicists – Theodore Hänsch + Arthur Schawlow and David Wineland + Hans Georg Dehmelt – published papers on how lasers could be used to cool gases to near-zero temperatures. They drew on developments from 1933 and 1972. In the former, Otto Frisch used radiation from a lamp to deflect a beam of sodium atoms. In the latter, German and French physicists repeated the feat with lasers.
Starting 1978, William Phillips would devise more reliable techniques to cool atoms with lasers. The working principle was simple: the radiative force of the laser would be used to dampen the random thermal motions of the atoms, thereby reducing their energy and temperature. Over the 1980s, Phillips and others would also devise a method to first slow down the atoms, trap them atoms in a cage of lasers and then cool them.
On June 5, 1995, Cornell, Wieman and their team started with a cloud of rubidium atoms at room temperature in a lab at the Joint Institute for Lab Astrophysics in Boulder. They first slowed the atoms with lasers and trapped them by bombarding them with photons from all directions. Then, they were cooled to about 10 microkelvin and the lasers were turned off.
The atoms were next held in place by a cage composed of magnetic fields, which held onto the atoms by interacting with the tiny magnetic fields generated by their electrons. The scientists now used evaporative cooling – kicking out the hotter atoms and taking away some of the heat from the cloud – while a technique Cornell had developed was used to keep the atoms in place.
Over time, the team produced a BEC of about 2,000 rubidium atoms that lasted for 15-20 seconds (almost exactly 70 years after Bose’s letter to Einstein).
Four months later, Wolfgang Ketterle improvised on the Cornell-Wieman technique to produce a BEC that was over 100x denser. His journey to achieving the BEC also contains a gem of an episode where his mentor’s sacrifice contributed to his success. As Physics Today reported in 2001,
In 1993, [Dan] Pritchard stepped aside in a remarkably magnanimous gesture. Ketterle was then a candidate for an MIT professorship, and wanted to continue working toward a BEC. To save Ketterle from having to compete in the shadow of his mentor, Pritchard bowed out of the BEC project and turned his laboratory over to Ketterle so that the former postdoc could continue the quest. Pritchard told Ketterle at the time, “I’m giving you the keys to the family car because I know you can drive faster than I can.”
In 1996, Ketterle achieved an even more impressive feat: create two BEC clouds in the same container, overlap them and study the interference patterns. The result was conclusive proof that a BEC demonstrated quantum mechanical behaviour at a larger scale, without letting gravity get in the way.
When William Phillips, among the pioneers of laser-trapping and -cooling atoms, started out in the field in 1978, he hadn’t anticipated its application in developing BECs. And after winning the Nobel Prize for physics in 1997, he said, “I hesitate to predict where the field of laser cooling and trapping will be even a few years from now. Such predictions have often been wrong in the past, and usually too pessimistic.”
David Wineland, who was among the two pairs of physicists who proposed laser-cooling in 1975, went on to use the technique to advance the fields of spectroscopy, atomic timekeeping and quantum entanglement (the “spooky action” Einstein alluded to). More recent advances have allowed scientists to use lasers as if they were tweezers to hold and move around individual atoms.
Similarly, because of BECs’ obeisance to quantum mechanical rules, scientists have used their atoms as proxies for individual particles like electrons and then simulate their interactions. Ketterle has suggested (paywall) that such simulated systems could be used to tease out the properties of topological insulators, Majorana fermions and new superfluids – all being lucrative areas of research in contemporary condensed-matter physics.
The BEC was also among Einstein’s last major papers, and Bose played a significant role in making it happen. It’s unclear if Einstein was aware of the potential for radiation to be used in such a mechanical way alongside matter – to push and pull on it (although James Maxwell’s electromagnetic theory had allowed for it since the previous century) – or that the same tool that would hone “spooky action” would also help realise BECs. But despite his reservations, the idea itself has proved enduring, almost as much as its kindred theory, of relativity, a decade short of a century on.