![]() ![]() For example, you can measure the position of a moving electron by scattering light or other electrons from it. Information can be lost, and in some cases it is impossible to measure two physical quantities simultaneously to exact precision. How does knowing which slit the electron passed through change the pattern? The answer is fundamentally important- measurement affects the system being observed. If you do not observe which slit the electron goes through, you obtain a double-slit pattern. ![]() Knowing the particle went through one slit forces a single-slit pattern. There is no escape by using another method of determining which slit the electron went through. If you determine that the electron went through one of the slits, you no longer get a double slit pattern-instead, you get single slit interference. What is observed is that an electron always goes through one slit or the other it does not split to go through both. One possibility is to have coils around the slits that detect charges moving through them. But it is a fair question, and so we should look to see if the electron traverses one slit or the other, or both. ![]() Does this also mean that the electron goes through both slits? An electron is a basic unit of matter that is not divisible. The same interference pattern builds up! This implies that a particle’s probability distribution spans both slits, and the particles actually interfere with themselves. To test this, you can lower the intensity until there is never more than one electron between the slits and the screen. You might imagine that the electrons are interfering with one another as any waves do. ![]() This can be observed for photons or electrons-for now, let us concentrate on electrons. First, we note that these patterns are identical, following d sin θ = mλ d sin θ = mλ, the equation for double-slit constructive interference developed in Photon Energies and the Electromagnetic Spectrum, where d d is the slit separation and λ λ is the electron or photon wavelength.īoth patterns build up statistically as individual particles fall on the detector. Consider the double-slit patterns obtained for electrons and photons in Figure 29.22. Let us explore what happens if we try to follow a particle. It is somewhat disquieting to think that you cannot predict exactly where an individual particle will go, or even follow it to its destination. Those who developed quantum mechanics devised equations that predicted the probability distribution in various circumstances. There is a certain probability of finding the particle at a given location, and the overall pattern is called a probability distribution. After compiling enough data, you get a distribution related to the particle’s wavelength and diffraction pattern. However, each particle goes to a definite place (as illustrated in Figure 29.21). The idea quickly emerged that, because of its wave character, a particle’s trajectory and destination cannot be precisely predicted for each particle individually. Both patterns are probability distributions in the sense that they are built up by individual particles traversing the apparatus, the paths of which are not individually predictable.Īfter de Broglie proposed the wave nature of matter, many physicists, including Schrödinger and Heisenberg, explored the consequences. Figure 29.22 Double-slit interference for electrons (a) and photons (b) is identical for equal wavelengths and equal slit separations. ![]()
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