1905 - A Year to Remember in Physics
Consider that he laid the mathematical foundation for molecular motion (in describing Brownian motion); quantum mechanics (explaining the photoelectric effect); and revolutionizing our understanding of space and time. Oh, and he also came up with E = mc2. Each of these discoveries is worthy of scientific immortality, but Einstein wrote the articles largely in a four-month period. There are countless articles and books explaining Einstein’s science and life, so it is not my place to repeat what has already been said.
What I want to point out are a couple of science nuggets Einstein worked on that are not as well known as special and general relativity and the photoelectric effect. Two examples are stimulated emission and Bose-Einstein condensation. The theories for each of these phenomena were developed by Einstein via quantum mechanics, which is ironic since he spent a good portion of his career trying desperately to develop an alternative to Quantum mechanics and its probabilistic handling of the subatomic universe.
Stimulated Emission is the ‘se’ segment of laser, which stands for Light Amplification by Stimulated Emission of Radiation. Einstein was able to reason (around 1917) that in certain types of systems, population inversion is possible. This means there are more atoms in an excited state than in a ground state. Photons, or ‘particles’ of light (hypothesized by Einstein in 1905 to explain the photoelectric effect), that are in the system are therefore more likely to stimulate the emission of other photons from an excited atom than to be absorbed by an atom. It turns out that a photon that is emitted by stimulated emission will have the same frequency (i.e. be the same color) and have the same phase as the original photon. This is called coherent light, and it is the reason laser light is so intense and remains focused over large distances. Presently lasers are used in everything from laser pointers to communications to price scanners to leveling tools to surgery. They are also used in a process called laser cooling, which makes use of a photon’s momentum (yes, relativity says things with no mass can have momentum!) to slow atoms to almost zero speed; this means the temperature can approach one-billionth of a Kelvin, which is nearly absolute zero!
Ultra-cold temperatures are just what the doctor ordered to make a strange state of matter called a Bose-Einstein condensate. These gentlemen predicted back in the early 1920s that certain particles called bosons, or particles with integer values of spin, can occupy the same energy states when cooled down. Einstein and Bose made use of the Heisenberg uncertainty principle (you know a theory is weird when a rule of nature is uncertainty) in quantum mechanics to reason that at very low temperatures, where motion almost ceases to exist, the momentum of the boson is well-known. The uncertainty principle states that is the momentum is well-known, the position cannot be well-known. Because quantum mechanics treats particles as waves, and waves can overlap and interfere with each other, the wave functions of the bosons are spread out and overlap, meaning we can think of multiple bosons as occupying the same volume of physical space. Think of moving your fists towards each other, and instead of hitting each other and stopping, your fists merged together and occupied the same space like a ghost’s fists might do. Sounds weird, no?! This goes against almost everything we were taught in our science classes. The trouble is this does happen. Over the past decade, as ultra-low temperatures became possible, scientists have created Bose-Einstein condensates. A possible use of this phenomenon may lie in quantum computing, but we will almost certainly see applications in the near future.
We are still testing Einstein’s theories fifty years after his death. This year has been designated the World Year in Physics to commemorate the centennial of Einstein’s big year.