What is the Double-Slit Experiment?
The double-slit experiment is the seminal experiment that cracked modern science open to quantum mechanics. It’s an experiment based on the nature of light itself. The double-slit experiment was designed to determine if light acted as a particle or a wave. And what was discovered was that light behaved as a wave under most conditions. However, when the motion of each photon was observed, then they acted like particles instead.
This caused a bit of a quandary in science. Our previous paradign accepted things as one thing or another, but not both. As either a particle or a wave. However, this experiment determined that the act of observation determined whether light acted as one form or the other. It forced physicists to develop an entirely new model of the universe.
History of the Double-Slit Experiment
Scientists in the 1700’s were divided as to the nature of light. The question, as you may have expected was whether light was a particle or a wave. So, in 1801, a scientist named Thomas Young devised an experiment to determine which was the case. The experiment was simple. Young shot a coherent light beam towards a piece of foil. Two slits were cut in the foil, parallel to one another, and a screen was placed on the other side of the foil.
Basically, if light was particulate in nature, only two marks would appear on the screen. If you refer to the picture below, this would appear as two parallel single-slit images. If, on the other hand, light was wave-like, then a series of interference patterns would appear on the screen. The waves would interfere with one another creating several lines, some of greater intensity and others of lesser intensity.
The result of Young’s experiment was an interference pattern, which provided compelling evidence that light was wave-like in nature. As a result, scientists accepted that light travelled in waves, and that was that, at least for the next century and a half. Then, as a result of new theories coming from Einstein and Max Planck, it was theorized that light acted as both a particle and a wave. The interference pattern of the double-slit experiment would still occur if a mass of tiny particles were travelling together and interacting with one another. The particle form of light was termed a “photon.” What’s more, instruments were developed to fire only a single photon at a time.
Armed with these new developments, scientists repeated the double-slit experiment with greater accuracy. Reason suggested that since only a single photon was fired through the slits at a time, the behavior of light would then be demonstrated as a particle. For instance, if a wave was made of a stream of infinitesimally small particles, then it would still show an interference pattern. However, if only a single particle went through the slits at each time, then there would be no other particles to bounce off of, and thus no interference pattern.
And this is where things became fascinating. Even though only one photon was sent through the slits at a time, the interference pattern still occurred. The results defied logic. If there were no other photons to bounce off of, why was the interference pattern still occurring? The photon was acting as if there were other photons present. It’s as if it somehow “knew” the paths of all the other possible photons and acted accordingly. Furthermore, later experiments showed that the same results occurred when working with electrons and larger particles. But this is only half of the mystery of the double-slit experiment.
Trying to make sense of these results, a later experiment was performed with electrons, and one slit was fit with a “which way” detector. This allowed scientists to determine which slit the photon passed through. Suddenly, the results changed. As soon as the electron detector was added, only two parallel lines began to appear on the screen. The observation of the electrons changed their behavior from wave-like to particle-like. This was unprecedented. It was never before suspected that the observation of an event could have any influence upon the outcome.
Implications of the Double-Slit Experiment
The double-slit experiment is the classical demonstration of quantum mechanics. This is the tendency for microscopic particles to behave differently than macroscopic objects. Thus, the behavior of electrons or photons is different than that of, for example tennis balls in a proportionate situation. Objects that we can see with the naked eye tend to take one single, determined path. However, microscopic particles appear to behave like waves until they are observed. As soon as they are observed, the wave behavior disappears and is replaced by the action of a particle. In an attempt to decipher these results, quantum physicists have come to numerous interpretations.
One way of understanding the behavior of quantum particles is termed the Copenhagen interpretation. This posits that these particles act as probability waves, rather than discrete particles. This is similar to a physical wave in that it has peaks and troughs. However, for a probability wave, these are regions of high and low probability for the occurrence of a specific event. At any given point in the life of a wave, in this instance, between the emitter and the detector, there are regions of higher and lower probability that the particle will occupy a certain point in space along the wave. The interference pattern observed in the double slit experiment is thought to be the result of the regions of highest probability.
Another interpretation, offered by Richard Feynman, is termed the Path-integral formulation. This perspective rejects that a particle has a single trajectory, instead positing a sum of all possible trajectories. All of these possible paths are superimposed upon one another. The final trajectory is then the averaged path. To explain this in terms of the double-slit experiment, a single photon travels all possible paths from the emitter to the screen. The sum total of all of these paths will then fall most strongly in the same interference pattern as a wave. This perspective is similar to the Copenhagen interpretation in result; however, it replaces the idea of a probability wave with an infinite number of superimposed particle trajectories.
Another perspective, the De Broglie-Bohm theory, combines the previous two interpretations. It posits that while a particle has a real placement in space at any given moment, it is guided by a pilot wave, similar to the probability wave described above. According to this theory, the particle itself will travel through only a single slit. However, the guiding wave passes through both, causing the particle to appear in accordance with an interference pattern. On the opposite end of the spectrum is the many-words interpretation. This asserts that the waveform is the reality. All of the possible trajectories occur in one of an infinite number of worlds.
Finally, and perhaps most intriguing, there is the relational interpretation. This perspective holds that the behavior of a quantum particle is a function of the relation between the particle and the observer. This means that there is no physical property inherent in the particle that determines its motion. If the particle is not observed, then its behavior is completely determined by a probability function. However, as soon as the outcome is observed, then the probability waveform collapses, producing a single discrete trajectory. This means that the behavior of a system is observer-dependent.
Quantum mechanics has proved a turning point to the world of physics. Classical physics held that the behavior of all particles was singular and discrete. Furthermore, it operated under the paradigm that external systems operate mechanistically. Put simply, things happen, whether we’re looking or not. Although there are more questions than answers when it comes to quantum physics, it has led us to recognize the possibility of infinite realities, particle-wave duality, and the seemingly mystical connection between perception and outcome.