For example, quantum theory says that each of the states corresponding to spin about the x -axis is a superposition of the two states with spin about the y -axis. There is no way to visualize this; it has absolutely no classical counterpart. One simply has to accept the result as a consequence of the axioms of the theory. Suppose that, as in Figure 3 , the atoms in beam 1 are passed into a second magnet B, which has a magnetic field along the y -axis perpendicular to x.
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The atoms emerge from B and go in equal numbers through its two output channels. The information about S x is lost because of the disturbance caused by magnet B in the measurement of S y. The observables discussed so far have had discrete sets of experimental values. On the other hand, the position of a particle or the linear momentum of a free particle can take continuous values in both quantum and classical theory.
The mathematics of observables with a continuous spectrum of measured values is somewhat more complicated than for the discrete case but presents no problems of principle. An observable with a continuous spectrum of measured values has an infinite number of state functions. Measurements can be made of position x of a particle and the x -component of its linear momentum, denoted by p x. These two observables are incompatible because they have different state functions.
Differential Equations and Linear Algebra 2nd Edition Recognized for its real-world purposes that seize readers' curiosity, this confirmed reference bargains a whole remedy of linear algebra. Technical Math For Dummies. To interpret his results, Compton regarded the X-ray photon as a particle that collides and bounces off an electron in the graphite target as though the photon and the electron were a pair of dissimilar billiard balls. Dirac showed that an electron has an additional quantum number m s. A microprocessor is built entirely of these logic circuits synchronized to each other.
The phenomenon of diffraction noted above illustrates the impossibility of measuring position and momentum simultaneously and precisely. If a parallel monochromatic light beam passes through a slit Figure 4A , its intensity varies with direction, as shown in Figure 4B. The light has zero intensity in certain directions. The experiment can be repeated with a stream of electrons instead of a beam of light.
According to de Broglie, electrons have wavelike properties; therefore, the beam of electrons emerging from the slit should widen and spread out like a beam of light waves. This has been observed in experiments. If the electrons have velocity u in the forward direction i. Consider p x , the component of momentum in the x -direction.
Exactly where an electron passed through the slit is unknown; it is only certain that an electron went through somewhere. More exact analysis shows that the product has a lower limit, given by. This is the well-known Heisenberg uncertainty principle for position and momentum. It states that there is a limit to the precision with which the position and the momentum of an object can be measured at the same time. Depending on the experimental conditions, either quantity can be measured as precisely as desired at least in principle , but the more precisely one of the quantities is measured, the less precisely the other is known.
The uncertainty principle is significant only on the atomic scale because of the small value of h in everyday units.
Such a limitation is hardly worrisome. The above reasoning leading to the uncertainty principle is based on the wave-particle duality of the electron. When Heisenberg first propounded the principle in his reasoning was based, however, on the wave-particle duality of the photon. He considered the process of measuring the position of an electron by observing it in a microscope. Diffraction effects due to the wave nature of light result in a blurring of the image; the resulting uncertainty in the position of the electron is approximately equal to the wavelength of the light.
To reduce this uncertainty, it is necessary to use light of shorter wavelength—e. However, in producing an image of the electron, the gamma-ray photon bounces off the electron, giving the Compton effect see above Early developments: As a result of the collision, the electron recoils in a statistically random way. The resulting uncertainty in the momentum of the electron is proportional to the momentum of the photon, which is inversely proportional to the wavelength of the photon. So it is again the case that increased precision in knowledge of the position of the electron is gained only at the expense of decreased precision in knowledge of its momentum.
A detailed calculation of the process yields the same result as before equation . When a large body is observed, photons still bounce off it and change its momentum, but, considered as a fraction of the initial momentum of the body, the change is insignificant. The only exception is in the ground lowest energy state. Instead, the energies of the states are spread over a small range. The spread arises from the fact that, because the electron can make a transition to another state, the initial state has a finite lifetime. The transition is a random process, and so different atoms in the same state have different lifetimes.
This energy spread is manifested in a spread in the frequencies of emitted radiation. Therefore, the spectral lines are not infinitely sharp. Some experimental factors can also broaden a line, but their effects can be reduced; however, the present effect, known as natural broadening, is fundamental and cannot be reduced.
The resulting quantum field theory is known as quantum electrodynamics, or QED. QED accounts for the behaviour and interactions of electrons, positrons, and photons. It deals with processes involving the creation of material particles from electromagnetic energy and with the converse processes in which a material particle and its antiparticle annihilate each other and produce energy.
Initially the theory was beset with formidable mathematical difficulties, because the calculated values of quantities such as the charge and mass of the electron proved to be infinite. However, an ingenious set of techniques developed in the late s by Hans Bethe , Julian S. Feynman, and others dealt systematically with the infinities to obtain finite values of the physical quantities. Their method is known as renormalization.
The theory has provided some remarkably accurate predictions. According to the Dirac theory, two particular states in hydrogen with different quantum numbers have the same energy. QED, however, predicts a small difference in their energies; the difference may be determined by measuring the frequency of the electromagnetic radiation that produces transitions between the two states. This effect was first measured by Willis E. Its physical origin lies in the interaction of the electron with the random fluctuations in the surrounding electromagnetic field.
These fluctuations, which exist even in the absence of an applied field, are a quantum phenomenon. The accuracy of experiment and theory in this area may be gauged by two recent values for the separation of the two states, expressed in terms of the frequency of the radiation that produces the transitions:. Again, the physical origin of the QED correction is the interaction of the electron with random oscillations in the surrounding electromagnetic field.
This greatly enhances the sensitivity of the experiment. The most recent results for the value of a are. Since a itself represents a small correction term, the magnetic dipole moment of the electron is measured with an accuracy of about one part in 10 One of the most precisely determined quantities in physics, the magnetic dipole moment of the electron can be calculated correctly from quantum theory to within about one part in 10 We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.
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Next page The interpretation of quantum mechanics. Learn More in these related Britannica articles: The first two decades of the 20th century left the status of the nature of light confused. That light is a wave phenomenon was indisputable: One of the intrinsic properties of an electron is its angular momentum, or spin.
The energy states shown in Figure 1 are of that kind. A transition between such states depends not only on the availability e. The preceding discussion has outlined the general approach to covalent bonding and has shown how it is still widely employed for a qualitative understanding of molecules. It is incomplete in many respects, however. First, the role of the electron pair remains…. More About Quantum mechanics 51 references found in Britannica articles Assorted References history of mathematics In mathematics: Mathematical physics importance of Solvay Conferences In Solvay Conferences major references In principles of physical science: Rise of quantum mechanics In light: Quantum mechanics In philosophy of physics: Quantum mechanics philosophy of science In physical science: Quantum mechanics physical infinities In infinity: Physical infinities spectroscopy In spectroscopy: Basic properties of atoms In spectroscopy: Historical survey time uncertainty In time: Time in microphysics View More.
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