In 1801, the English encyclopaedist scientist Thomas Young conducted an experiment that provided very strong evidence that matter and energy can have both the characteristics of waves and particles. (Later, this experiment also became the basis for the study of the probabilistic nature of quantum mechanical phenomena.)
The experiment generally consists of the following: a coherent light source (of constant wavelength and constant phase, e.g. a laser) illuminates a thin plate with two parallel slits. The light passing through the slits falls onto a screen behind the plate.
The result of the experiment will be quite different in the case that the light is a wave or a particle. Thus, if we assume that light consists of particles (according to Newton’s corpuscular theory), then only two parallel bands of light passing through the slit of the screen should be seen on the projection screen. In this case, the pattern of the light bands will match the size and shape of the slits, and between them (the bands of light) the projection screen will be practically unlit.
If the light is a wave, the results should be different. First, the projection on the screen is much wider than the dimensions of the slits, and second, it is a picture of interference… When the monochromatic light reaches the slits, the points in the slits become secondary sources of spherical waves, and an interference picture of light and dark bands appears on the projection screen. (Such a result cannot be obtained if the light consists simply of particles.)
The results of the experiment prove that light consists of waves
This experiment thus played a vital role in the acceptance of the wave theory of light in the early nineteenth century, opposing it to the corpuscular theory proposed by Isaac Newton (and the generally accepted model of light in the seventeenth and eighteenth centuries). Later, however, the discovery of the photoelectric effect (Heinrich Hertz, 1887) showed that under various circumstances light could behave as if it were composed of individual particles. These seemingly contradictory discoveries help to go beyond classical physics, to the quantum nature of light.
Young's experiment in electronic version
The results of the experiment prove that light consists of waves
In 1989, an experiment similar to Young’s was conducted (only with electrons), which brilliantly confirmed de Broglie’s hypothesis of the dual corpuscular-wave nature not only of photons, but of elementary particles in general. Instead of a stream of solar rays passing through two parallel slits, we put an electron-beam gun in a glass tube in which all the air is drawn, i.e. there is a vacuum so that nothing affects the path of the electrons. The opposite end of the tube has a coating of luminophore (a substance that emits light when ‘activated’ by a stream of electrons – luminescence). Each electron on striking the luminophore leaves a luminous dot on it, thus fixing its arrival as a particle. Interestingly, again an interference picture is formed. This experiment has been conducted many times and in different ways with electrons and other particles, with the same result.
The conclusion from all the experiments is that a single particle must pass through the slits simultaneously – something that contradicts our everyday experience with discrete objects. This phenomenon is also evidence that electrons, protons, neutrons and even larger microparticles – fullerenes (Zeilinger, 1999), molecules about 0.7 Nm in diameter (almost half a million times larger than a single proton) – all have wave behavior and even their own specific frequencies.
The illustration shows the change in the wave function of the electron as it passes through the two slits. The degree of gray represents the probability density of the electron’s presence. The actual size of the electron is actually much smaller than the probability region of its presence. It is clearly seen that the electron “interferes with itself”: the interference bands are clearly noticeable when passing through the two slits, both after the barrier and before it. To investigate the electron’s behaviour even further, the scientists performed a number of variations of the experiment:
Firing the electrons one by one
As the electrons are fired one by one, we will see the electron tracks seemingly randomly distributed. After a while, although we continue to send the electrons out one at a time, interference fringes begin to form. Thus the conclusion is that an electron must in a sense pass through both slits simultaneously and then interfere with itself as it moves toward the screen. The luminescence detector fixes the picture of the electron distribution or, which is the same, the probability of the presence of a particle at some point.
So in the quantum world we encounter a very strange behavior: the electron is a wave until it is localized, and the act of detecting it turns it into a particle.
With particle detectors in the slits
What would happen if we put particle detectors near each of the slits and tried to “catch the parts” of the electron that pass through the corresponding hole? In that case, the quantum would always be “caught” coming out of one of the slits, but never both (which makes sense, given that the quantum is indivisible by definition). The interference pattern disappears, replaced by the normal distribution.
And what happens if we install just one detector near one of the slits? What happens is that even if the quantum is not detected by the detector (it has passed through the other slit), the interference picture on the screen still disappears. It appears that the quantum has somehow “learned” that it is being detected at the other slit and refuses to interfere, “pretending” to be a particle.
In order to register which of the slits one or other electron has passed through, it is necessary to place a measuring instrument, which, however, works with the same electromagnetic waves. In other words, in order to register the electron in space, it is being affected. And by affecting it, we change its state and as a consequence, the interference pattern (it disappears).
If we reduce the impact to a minimum, interference starts to occur immediately, but it is impossible to tell which hole the electron has passed through…
Sources:
Part of the article is borrowed from http://bgchaos.com/560/fractals/quantum-mechanics/опитът-с-двата-процепа-на-юнг/
Chad Orzel, Watching Photons Interfere: Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer, ScienceBlogs
Clintberg, Young’s Double Slit Experiment, Department of Lifelong Learning.
Scientific American, New ‘Double Slit’ Experiment Skirts Uncertainty Principle, Nature
Walter Scheider, Do the “Double Slit” Experiment the Way it Was Originally Done, The Physics
Teacher 24, 217-219, 1986
The secret lives of photons revealed , physicsworld.com
Physics World reveals its top 10 breakthroughs for 2011, physicsworld.com
The Feynman Double Slit, Department of Physics University of Toronto
