The Copenhagen Interpretation
29 esteemed scientists, 17 of which have or someday would receive the Nobel Prize, walk into a room in October 1927 for the Fifth Solvay International Conference on Electrons and Photons. The most notable physicists are the titans of their time, Albert Einstein, and Niels Bohr. Albert Einstein became famous in 1916 when he, in a series of lectures, revolutionized his the idea of gravity with his discovery of relativity. Einstein had claimed the Nobel Prize in 1921 for separate work with the photoelectric effect. Bohr, on the other hand, claimed his fame for modeling the atomic structure in 1913, receiving the Nobel Prize for this in 1922. Despite a strong friendship Einstein and Bohr had an almost equally long history of disagreement. This disagreement would lead to one of the most important scientific discoveries in the history of physics.
The Photoelectric Effect
The photoelectric effect “proves” that light is a particle through a simple experiment involving light that when shone onto
sodium in a vacuum releases electrons proportional to the frequency of the light. This showed that the photon had transferred all of its energy into the electrons, similar to Max
Planck’s discovery of blackbody radiation
pot never boils.” In 1923, physicist Louis de Broglie hypothesized wave-particle duality, describing particles as coins. On one side the particle behaves as a quantum (a boson or force carrier), taking up tangible space and interacting only when directly interacted with. On the other side,
Davisson-Germer Experiment
The Davisson-Germer Experiment “proved” that light was a wave by shooting electrons at a refractive crystal. Some of the electrons were reflected off the surface of the crystal while others continued further, reflecting off of other layers. When analyzed, the data showed a pattern of bright and dark fringes
There was only one explanation of this pattern: the electron must have interfered with one another. This phenomenon would not be possible with “tangible” particles as the refraction would have kept them mostly in the order they were fired. The electrons would have to be waves.
The two experiments were almost exactly the same, yet gave opposite results. This gave rise to a fundamental question: Are fermions and light particles, waves, or both? Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and others thought they had the solution: The Copenhagen Interpretation. Copenhagen stressed that the universe ceased to function if not being interacted with similar to the proverb "a watched
the particle is a wave, taking up no single position but instead a range positions that interact with their surrounding environment. For example, when a stone is dropped in a pond it will release waves in a circle around where the stone was dropped. The waves cause the water to bob up and down as well as the leaves that may be on the surface. Particle-waves interact in a similar way, the water described in the analogy acting as spacetime and the leaves acting as other particles/waves. Every time a particle (most commonly a photon) interacts with other particles it flips this coin and behaves as one of the outcomes.
Copenhagen also stresses that the position and momentum of a particle cannot be precisely measured (observed) at the same time. An easily observed fact is that, as object's get smaller, it becomes more difficult to distinguish what they are. For example, when looking at a piano from 3 meters away, it is fairly easy to determine that it is a piano, however, when looking at a termite from 3 meters away, it is difficult to determine what it is and one may end up thinking the termite is an ant or a tick. This same concept can be applied to the nanoscopic world. As one scales down the dimensions of measurement, it becomes more and more difficult to observe whether a particle is behaving as a wave or a particle.
This makes it increasingly difficult to define the parameters of the particle’s whereabouts. Thus, Heisenberg deemed that, without interacting (transferring energy) with the particle, it is impossible to know with absolute certainty where the particle is. Instead, its whereabouts are represented by a wavefunction (shown above), a wave of probabilities of the particle’s location. The following figure displays a simple wavefunction. One thought experiment that is crucial to the understanding of superposition (having a wavefunction) is Schrodinger's Cat (shown below). It goes something like this: Erwin enjoys killing cats. He is constantly thinking up new ways to do away with the cute mammals
without the humane society getting involved. One day he learns that uranium decays. He decides to try a contraption that would kill a cat if uranium decays. He places uranium with a one hour half life in a box and puts a detector near it. If the uranium decays the detector will release a current to a machine that then fires a bullet at the cat. In an hour he comes back to his contraption to find that he accidentally spilled glue on the box sealing it. Erwin cries, now he’ll never get to find out if the cat suffered. The cat could be dead or alive, no one will ever know unless he can find a way to open the box.
The cat in Erwin’s story is in a superposition. It is neither dead nor alive, just a string of probabilities that get greater and greater as time passes. Although it did justify some of the weird things that were happening
in these experiments, Einstein would not have any of it. Einstein believed in the strict rules assembled by Charles Pierce and Carl Popper. Often he and Bohr would meet and he would give Bohr a complex thought experiment. For the next few days, Bohr would think through the thought experiment and he always found a solution. At the Solvay Conference Einstein told Bohr “God does not play dice.”
Bohr replied, “Stop telling God what to do.”
The Copenhagen Interpretation, after several years of debates, became the most widely used interpretation of quantum physics. Despite some alternate explanations (ie. many worlds and pilot-wave), Copenhagen still stands. In a period of five years, Bohr and team truly did revolutionize our perception of the universe.
