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The Stern–Gerlach experiment showed that the spatial orientation of angular momentum is quantized. It demonstrated that atomic-scale systems have intrinsically quantum properties, and that measurement in quantum mechanics affects the system being measured. In the original experiment, silver atoms were sent through a non-uniform magnetic field, which deflected them before they struck a detector screen. Other kinds of particles can be used. If the particles have a magnetic moment related to their spin angular momentum, the magnetic field gradient deflects them from a straight path. The screen reveals discrete points of accumulation rather than a continuous distribution, owing to the quantum nature of spin. Historically, this experiment was decisive in convincing physicists of the reality of angular momentum quantization in all atomic-scale systems. The experiment was first conducted by the German physicists Otto Stern and Walther Gerlach, in 1922. ==Basic theory and description== The Stern–Gerlach experiment involves sending a beam of particles through an inhomogeneous magnetic field and observing their deflection. The results show that particles possess an intrinsic angular momentum that is closely analogous to the angular momentum of a classically spinning object, but that takes only certain quantized values. Another important result is that only one component of a particle's spin can be measured at one time, meaning that the measurement of the spin along the z-axis destroys information about a particle's spin along the x and y axis. The experiment is normally conducted using electrically neutral particles or atoms. This avoids the large deflection to the orbit of a charged particle moving through a magnetic field and allows spin-dependent effects to dominate. If the particle is treated as a classical spinning dipole, it will precess in a magnetic field because of the torque that the magnetic field exerts on the dipole (see torque-induced precession). If it moves through a homogeneous magnetic field, the forces exerted on opposite ends of the dipole cancel each other out and the trajectory of the particle is unaffected. However, if the magnetic field is inhomogeneous then the force on one end of the dipole will be slightly greater than the opposing force on the other end, so that there is a net force which deflects the particle's trajectory. If the particles were classical spinning objects, one would expect the distribution of their spin angular momentum vectors to be random and continuous. Each particle would be deflected by a different amount, producing some density distribution on the detector screen. Instead, the particles passing through the Stern–Gerlach apparatus are deflected either up or down by a specific amount. This was a measurement of the quantum observable now known as spin angular momentum, which demonstrated possible outcomes of a measurement where the observable has a discrete set of values or point spectrum. Although some discrete quantum phenomena, such as atomic spectra, were observed much earlier, the Stern–Gerlach experiment allowed scientists to conduct measurements of deliberately superposed quantum states for the first time in the history of science. By now it is known theoretically that quantum angular momentum ''of any kind'' has a discrete spectrum, which is sometimes imprecisely expressed as "angular momentum is quantized". If the experiment is conducted using charged particles like electrons, there will be a Lorentz force that tends to bend the trajectory in a circle (see cyclotron motion). This force can be cancelled by an electric field of appropriate magnitude oriented transverse to the charged particle's path. Electrons are particles. (It must be noted that observation of the Stern-Gerlach effect with free electrons is infeasible.〔Mott, N.F., Massey, H.S.W. (1965/1971). ''The Theory of Atomic Collisions'', third edition, Oxford University Press, Oxford UK, pp. 214–219, §2, Ch. , reprinted in 〕〔http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.81.4772 - Comment on "Stern-Gerlach Effect for Electron Beams"〕) These have only two possible spin angular momentum values measured along any axis, +ħ/2 or −ħ/2, a sheerly quantum mechanical phenomenon. Because its value is always the same, it is regarded as an intrinsic property of electrons, and is sometimes known as "intrinsic angular momentum" (to distinguish it from orbital angular momentum, which can vary and depends on the presence of other particles). To describe the experiment with spin + particles mathematically, it is easiest to use Dirac's bra–ket notation. As the particles pass through the Stern–Gerlach device, they are being observed by the detector which resolves to either spin up or spin down. These are described by the angular momentum quantum number ''j'', which can take on one of the two possible allowed values, either +ħ/2 or −ħ/2. The act of observing (measuring) the momentum along the z axis corresponds to the operator ''J''z. In mathematical terms, the initial state of the particles is :, where constants ''c''1 and ''c''2 are complex numbers. This initial state spin can in fact point in any direction. The squares of the absolute values (|''c''1|2 and |''c''2|2) determine the probabilities that for a system in the initial state one of the two possible values of ''j'' is found after the measurement is made. The constants must also be normalized in order that the probability of finding either one of the values be unity. However, this information is not sufficient to determine the values of ''c''1 and ''c''2, because they are complex numbers. Therefore the measurement yields only the squared magnitudes of the constants, which are interpreted as probabilities. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Stern–Gerlach experiment」の詳細全文を読む スポンサード リンク
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