The study of the nature of light is an important research area in modern physics. Many, including the theoretical physicist Albert Einstein, have contributed to theories involving light. Of these, the wave-particle duality is arguably the most strange and noteworthy concept in the field. Throughout history, some physicists have argued that light behaves as a wave, such as Christiaan Huygens and others, such as Isaac Newton have proposed that light consists of particles (Wave-Particle Duality, March 2010).
Today, as stated in the wave-particle duality, light is said to exhibit wave-like and particle-like properties. And still today, physicists are troubled by understanding this concept. The concept of wave-particle duality is one of the most confusing concepts in physics because it isn’t like anything we see in the everyday world. In the 18th and 19th century, there was a huge debate among physicists studying light about whether light was made of particles shooting around like bullets, or waves washing around like waves (Wave-Particle Duality, January 2010).
There are times when light seems to act as both. At times, light appears to only travel in a straight line, as if it were made of particles. Yet other experiments show that light has a frequency or wavelength, just like a sound wave or water wave (Wave-Particle Duality, January 2010). “Until the 20th century, most physicists thought that light was either one or the other, and that the scientists on the other side of the argument were simply wrong. ” (Wave-Particle Duality, January 2010) “If light travels as particles we can imagine particles of light (photons) as bullets fired from a rifle.
Imagine a brick wall with two holes in it, each the same size and large enough to fire bullets through, with a second wall behind where the bullets will strike (The Double Slit Experiment, 2008). ” After shooting enough bullets, you would see a cluster of bullets behind the two holes (The Double Slit Experiment, 2008). Since this result is indeed what you get with bullets, a similar outcome is expected with photons. If we instead imagine light as a wave instead of particles, we can imitate its behavior with a water tank.
When the wave spreads out from its source, it would get to both holes at the same time and each hole would then act as a new source (The Double Slit Experiment, 2008). The waves would then spread out again from each of the holes, in phase. “As the waves moved forward, spreading as they go, they would eventually interfere with one another. Where both waves are lifting the water surface upward, we get a more pronounced crest (constructive interference); where one wave is trying to create a crest and the other is trying to create a trough the two cancel out and the water level is undisturbed (destructive interference) . (The Double Slit Experiment, 2008) If this system were repeated with light instead of water and if light travels as waves, the second wall would display an interference pattern of alternate light and dark bands across the wall (The Double Slit Experiment, 2008). Particles, however, would create two distinct areas of light, like where the bullets would hit (The Double Slit Experiment, 2008). In 1909, a scientist named Jeffrey Taylor attempted to settle the argument once and for all. He used an experiment previously invented by a man named Thomas Young.
In this experiment, light was shone through two holes right next to each other. When light was shone through these two small holes, it created an interference pattern that seemed to show that light was actually a wave (Wave-Particle Duality, January 2010). However, Taylor furthered this experiment by photographing the film coming out of the holes with a film that is uncommonly sensitive to light. “When bright light was shined through the holes, the film showed an interference pattern, just like Young showed earlier (Wave-Particle Duality, January 2010). Taylor then turned down the light to a very dim level and when the light was dim enough, Taylor’s photographs showed a dotted pattern of light behind the holes. According to this, it seemed that light behaved as a particle. “If Taylor allowed the dim light to shine through the holes for long enough, the dots eventually filled up the film to make an interference pattern again. This demonstrated that light was somehow both a wave and a particle. ” (Wave-Particle Duality, January 2010) To make matters even more confusing, Victor de Broglie suggested that matter might act the same way (Wave-Particle Duality, January 2010).
Scientists then performed these same experiments with electrons, and found that electrons too are somehow both particles and waves. Today, experiments such as this have been made in so many different ways by countless people that scientists just accept that both matter and light carry the wave-particle duality and are somehow both a wave and a particle. For the most part, scientists admit that they don’t fully understand how this could be, but from this experimental data, they are certain it must be true. Although it seems impossible to understand how anything can be both a wave and a particle, scientists do have a number of equations for describing these things that have variables for both wavelength (a wave property) and momentum (a particle property). ” (Wave-Particle Duality, January 2010) This experiment has in fact been carried out a multitude times, with the same results every time, and the results are nothing less than amazing. When the experiment is set up with both slits open, the resulting interference pattern clearly shows that light behaves as a wave. If this were the only thing to it, we could all go home happy campers.
This is where the word strange can be fit in. If the experiment is set up to fire individual photons, so that only one photon at a time goes through the setup, the same interference pattern would not be expected. It would be expected that a single photon would go through one of the holes and not both at the same time to create an interference pattern (The Double Slit Experiment, 2008). But what happens is not what’s expected. After waiting until enough individual photons have passed through to create a pattern (which takes millions of photons), the results do not show two clusters opposite the two holes (The Double Slit Experiment, 2008).
They show the same interference pattern! It seems as if each individual photon somehow ‘knows’ that both holes are open and yields that result (The Double Slit Experiment, 2008). Each photon will place itself on the wall in a position that when enough photons have passed through, they collectively create an interference pattern when there is no known interference (The Double Slit Experiment, 2008)! If this experiment is repeated, except this time with only one hole open, individual photons act normally and all cluster round a point on the detector screen behind the open hole, as expected (The Double Slit Experiment, 2008).
Conversely, when the second hole is opened, the photons pass through the holes to form an interference pattern (The Double Slit Experiment, 2008). From these outcomes, it is concluded that not only does a photon passing through one of the holes ‘know’ of the other hole, but also perceptive of it being open. To understand why this is, scientists have tried watching the photons to see which hole the photon goes through and if it goes through both holes at once or if half a photon goes through each ole (The Double Slit Experiment, 2008). Detectors are placed at the holes to record the passage of the electrons through each of the holes. However, when this experiment is staged, the photons now behave like normal particles (The Double Slit Experiment, 2008). The results show that the photon travels through one hole and not the other, never both, and the pattern that is created matches the pattern for bullets, with no sign of interference (The Double Slit Experiment, 2008).
Additionally, the same result occurs in there is a detector by just one of the holes. This means that when a photon passes through a hole, it can tell if the other hole is open or not and if the other hole is being watched. The double slit experiment establishes many things. First, it appears as if each photon starts out as a single particle, arrives at the detector as such, but seems to go through both holes at once, interfered with itself and placed itself in the correct location to form the overall interference pattern.
This conduct brings bout many questions. . “Does the photon go through both holes at the same time? How does a photon go through both holes at the same time? How does it know where to place itself on the detector to form part of the overall pattern? Why don’t all the photons follow the same path and end up in the same place? ” (The Double Slit Experiment, 2008) According to Albert Einstein, “the most comprehensible thing about the universe is that it’s comprehensible. This can be understood; maybe we just need to look at it in a different light!
References
The Double Slit Experiment. (2008). Retrieved February 25, 2010 from website: http://www. quantum-theories. com/theories/double-slit-experiment. php Wave-Particle Duality. (2010, January 27). Retrieved February 25, 2010, from website: http:// simple. wikipedia. org/wiki/Wave-particle_duality Wave-Particle Duality. (2010, March 19). Retrieved February 25, 2010, from website: http://en. wikipedia. org/wiki/Wave–particle_duality
