<text><span class="style10">uantum Theory and Relativity (3 of 4)Uncertainty</span><span class="style7">Werner Karl Heisenberg (1901-76), a German physicist, interpreted wave-particle duality differently. He proposed that when a beam of light is directed at a screen with two slits, the interference pattern formed exists only if we do not know which slit the photon passed through. If we make an additional measurement and determine which slit was traversed, we destroy the interference pattern. Heisenberg showed that it was impossible to measure position and momentum simultaneously with infinite accuracy; he expressed his findings in the </span><span class="style26">uncertainty principle</span><span class="style7"> named after him. This changed the thinking about the precision with which simultaneous measurements of two physical quantities can be made.</span><span class="style10">Particles</span><span class="style7">Matter is made up of vast numbers of very small particles. The behavior of these particles cannot be described by the theories of classical physics, since there is no equivalence to subatomic particles in everyday mechanics. Thus it is not helpful to discuss the behavior of electrons in atoms in terms of tiny `planets' orbiting a `sun'. Louis Victor de Broglie (1892-1987), the French physicist, suggested that if light waves can behave like particles, then particles might in certain circumstances behave like waves. Later experiments confirmed that under appropriate conditions particles can exhibit wave phenomena.</span><span class="style10">Atomic energy levels</span><span class="style7">Quantum systems are described by a mathematical equation known as the </span><span class="style26">Schr├╢dinger equation</span><span class="style7"> after the Austrian physicist Erwin Schr├╢dinger (1887-1961), who first formulated it. In situations such as where a negatively charged electron is bound to the positively charged nucleus of an atom, the Schr├╢dinger equation has solutions for only </span><span class="style26">discrete</span><span class="style7"> or </span><span class="style26">quantized</span><span class="style7"> allowed values of the energy of the electron. The energy of an electron in an atom cannot take a lower value than the least of the allowed values - the </span><span class="style26">ground state</span><span class="style7"> - so the electron cannot fall into the nucleus. If an atom, through the interaction of forces on it, is excited into an allowed state of energy that is higher than the ground state, it can emit a photon and jump into the ground state The energy of the photon is equal to the difference in energy levels of the two states. The energy of the photon is related to the wavelength of the light wave associated with it - thus light can be emitted by atoms only at particular wavelengths.</span><span class="style10">Quantum mechanics</span><span class="style7">Quantum mechanics is the study of the observable behavior of particles. This includes electromagnetic radiation in all its details. In particular, it is the only appropriate theory for describing the effects that occur on an atomic scale.Quantum mechanics deals exclusively with what can be observed, and does not attempt to describe what is happening in between measurements. This is not true of classical theories, which are essentially complete descriptions of what is occurring whether or not attempts are made to measure it. In quantum mechanics the experimenter is directly included in the theory. Quantum mechanics predicts all the possible results of making a measurement, but it does not say which one will occur when an experiment is actually carried out. All that can be known is the probability of something being seen. In some experiments one event is very much more likely than any other, therefore most of the time this is what will be found, but sometimes one of the less probable events will occur. It is impossible to predict which will occur; the only way to find out is by making the appropriate measurement. For example, in an isotope of the element americium, 19% of the nuclei decay purely by alpha-particle emission and 81% decay by alpha emission followed by photon emission. For any individual americium nucleus it is not possible to say which decay will occur, only what will be observed on average. In some experiments the same event can occur in different ways. What is measured depends on whether it is known which of the possible paths was taken. Thus any additional knowledge, which can only be gained by making an additional measurement, changes the outcome of the first experiment.</span></text>
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<text><span class="style10">. The photon nature of light.</span><span class="style7"> The results of two-slit interference after the passage of 50,200 and 2000 photons have passed through. The characteristic pattern is only observed after many photons have passed. The initial results appear random.</span></text>
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<text>ΓÇó TIMEΓÇó MOTION AND FORCEΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó ELEMENTS AND THE PERIODIC TABLE</text>
