Wave Particle Duality z
Post on 11-Jul-2016
DESCRIPTIONThis short paper will address several issues. They are 1) How light, commonly viewed as displaying wave-like behavior, in certain instances, displays the characteristics of a particle; 2) how electrons, likewise, generally thought of as particles, in certain instances, display wave-like behavior; 3) a prominent interpretation of this wave-particle duality of matter, the Copenhagen interpretation; as well as 4) a discussion of the metaphysical implications of the wave-particle duality of matter.
Running head: WAVE-PARTICLE DUALITY OF MATTER
5WAVE-PARTICLE DUALITY OF MATTER
Wave-Particle Duality of MatterUniversity of Alabama
Wave-Particle Duality of MatterThere seems to be an inherent contradiction in the statement x is both a wave and a particle. Yet, experiments have shown that matter, contrary to intuition, displays characteristics of both a wave and a particle (Feynman, 1963). This short paper will address several issues. They are: How light, commonly viewed as displaying wave-like behavior, in certain instances, displays the characteristics of a particle; how electrons, likewise, generally thought of as particles, in certain instances, display wave-like behavior; a prominent interpretation of this wave-particle duality of matter, the Copenhagen interpretation; as well as a discussion of the metaphysical implications of the wave-particle duality of matter.Double-Slit ExperimentThe double-slit experiment can be used to illustrate the particle-like behavior of light waves. In the double-slit experiment, we set up a device, say, a laser beam that can send a beam of light towards a detection screen, which is placed at the opposite side of the experiment. In between the laser and the detection screen is a thin wall that has two parallel slits in it, slit 1 and slit 2. The laser is turned on, and the light travels from the laser through the double-slit wall and is detected at the detection screen. The pattern that shows up on the detection screen is one that would result if waves traveled through the slits, interfered with one another, and then arrived at the detection screen. When I say that the waves interfere with one another, what I am saying is that when the crests, or troughs, of two waves meet, this causes constructive interference, and the amplitude of the wave is intensified. When a crest and a trough meet each other, this causes deconstructive interference, and the amplitude of the wave is mitigated. To visualize the interference pattern that develops, you can think of the pattern that would result if water waves were propagating through the experiment. Hence, light acts as a wave. However, the light shows up on the detection screen in lumps, or discrete pieces of matter (Leclair, 2012). To further display the particle-like behavior of light, one only needs to place detectors at each slit. This would allow the observer to know which slit, 1 or 2, the light actually traveled through on its way to the detection screen. When detectors are place at the slits, the interference disappears and a particle-like pattern is detected (Brukner & Zeilinger, 2002). The pattern that results is the same as if the laser were actually a gun, randomly shooting discrete bullets though the slits towards the detection screen. Therefore, the double-slit experiment provides evidence that light does, in fact, behave like a particle. The double-slit experiment is not simply limited to experiments involving light, however. The experiment can also show the wave-particle duality of electrons.The conception that many of us non-physics students have of electrons is that of a tiny discrete piece of matter orbiting around the nucleus of an atom. Yet, the double-slit experiment yields results that show that electrons exhibit wave-like behavior (Feynman, 1963). To understand this, we simply need to replace our laser with an electron gun. The electron gun fires randomly, so we do not know if the electron will travel though slit 1 or slit 2 on its way to the detector. Now, if the electrons are like particles, we should see a pattern on the detection screen as if we watched a gun randomly fire discrete bullets. When we start the electron gun, though, this is not what we see. We do see the electrons arrive at the detection screen in discrete lumps, but, after a while, we see an interference pattern develop on the screen (Feynman, 1963). Even if the electron guns rate of fire is reduced so as to fire a single electron at a time, the interference, like that observed in water waves, still exists. So, is the electron a particle or a wave? This also leads to the intuitive question, which slit did the electron travel through, 1 or 2? There are various interpretations of what the double-slit experiment is actually telling us, the most popular, perhaps, being the Copenhagen interpretation. I will now give a brief discussion of the Copenhagen interpretation as well as discuss the metaphysical implications of the Copenhagen interpretation. Copenhagen InterpretationFrom my limited understanding of the Copenhagen interpretation, it holds that we cannot, actually, know where a electron is located between the slit and the detector; we can only give the probability of its being in a certain place. That is, there is no fact of the matter whether the electron is in region a, or region b, at a certain time between the wall and the detector. The most that can be said is that there is a so and so percent chance that the electron is in region a. The Copenhagen interpretation is therefore, probabilistic in nature. The electron is, in a sense, in all of the places until it is measured. Once it is measured, the electron collapses on itself at a certain point (Leclair, 2012). The collapse is known as the wave function collapse. Professor Leclair describes the wave function of a particle (or electron) as the amplitude of the particle at a certain location and time (2012). The probability of finding the particle at a certain location, a, at a time t, is | (x, t)|2. Were is simply the wave function. It seems that what is actually real, according to the Copenhagen interpretation, is probability. So, is the electron a particle or a wave? The Copenhagen interpretation would say that the electron has wave-like properties (Leclair, 2012). Which slit did the electron go through 1 or 2? If we observe the electron as it is traveling from the gun to the detector, it will choose one of the slits, but the interference will disappear. If we do not observe the electron, we cannot say which slit the electron took. There seems to be a sense that the Copenhagen interpretation does not care which slit the electron took, only if the probabilities can lead to testable results, which, as I understand the Copenhagen interpretation, it does. Metaphysical ImplicationsIf the Copenhagen interpretation is correct, it is not a meaningful question, physics-wise, to ask which slit the electron took. However, for the metaphysician, she will certainly not be content with it doesnt matter. I, with my major in philosophy, intuitively want to say that the electron took either slit 1, or slit 2. Einstein contended that there were missing variables that, if accounted for, could provide for a complete theory (Leclair 2012). If Einstein had not been proven incorrect, I would likely side with him. There seems that there is an objective reality, which we are only observing, not bringing into being by observing it. A possible remedy for this problem, I contend, may lie with what Professor Leclair stated in a lecture for PHL 480 in the Sp. 12 semester. We have gotten the question backwards. We are trying to assign everyday behavior to material that exists at the quantum level. We want to label an electron as either a wave or a particle, but it is neither. It is an electron. The real question is how everything that exists arises from something that, at its deepest level, is only probabilistic.
ReferencesBrukner C, Zeilinger A (2002). "Youngs experiment and thefiniteness of information".Philos.Trans. R. Soc. Lond.360: 1061-1069.Feynman, R., Leighton, R., & Sands, M. (1963).The FeynmanLectures on Physics: Volume 1(2nd Edition ed., Vol. 1).Boston: Addison-Wesley.Leclair, P. PHL 480 Class Handout. (2012, Spring). PHL 480:Physics and Metaphysics. University of Alabama, Departmentof Physics: Dr. Patrick Leclair.