Quantum Electrodynamics

I. Fill in the gaps with the following terms: subatomic, relativistic, magnetic moment, quantum, electromagnetic, atomic physics, interactions, muons, charged particles, field.

1. _______ field theory of the interactions of 2. ___________ with the electromagnetic 3. _______. It describes mathematically not only all 4. _____ of light with matter but also those of charged particles with one another. QED is a 5. ____ theory in that Albert Einstein's theory of special relativity is built into each of its equations. Because the behaviour of atoms and molecules is primarily 6. _______ in nature, all of 7. _____________ can be considered a test laboratory for the theory. Some of the most precise tests of QED have been experiments dealing with the properties of 8 ______ particles known as 9. _____. 10. _____ of this type of particle has been shown to agree with the theory to nine significant digits. Agreement of such high accuracy makes QED one of the most successful physical theories so far devised.

II. Order the paragraphs, explain your choice.

A. QED is often called a perturbation theory because of the smallness of the fine-structure constant and the resultant decreasing size of higher-order contributions. This relative simplicity and the success of QED have made it a model for other quantum field theories. Finally, the picture of electromagnetic interactions as the exchange of virtual particles has been carried over to the theories of the other fundamental interactions of matter, the strong force, the weak force, and the gravitational force.

B. In 1928 the English physicist P.A.M. Dirac laid the foundations for QED with his discovery of a wave equation that described the motion and spin of electrons and incorporated both quantum mechanics and the theory of special relativity. The QED theory was refined and fully developed in the late 1940s by Richard P. Feynman, Julian S. Schwinger, and Tomonaga Shin'ichirō, independently of one another. QED rests on the idea that charged particles (e.g., electrons and positrons) interact by emitting and absorbing photons, the particles that transmit electromagnetic forces. These photons are “virtual”; that is, they cannot be seen or detected in any way because their existence violates the conservation of energy and momentum. The photon exchange is merely the “force” of the interaction, because interacting particles change their speed and direction of travel as they release or absorb the energy of a photon. Photons also can be emitted in a free state, in which case they may be observed as light or other forms of electromagnetic radiation.

C. The interaction of two charged particles occurs in a series of processes of increasing complexity. In the simplest, only one virtual photon is involved; in a second-order process, there are two; and so forth. The processes correspond to all the possible ways in which the particles can interact by the exchange of virtual photons, and each of them can be represented graphically by means of the so-called Feynman diagrams. Besides furnishing an intuitive picture of the process being considered, this type of diagram prescribes precisely how to calculate the variable involved. Each subatomic process becomes computationally more difficult than the previous one, and there are an infinite number of processes. The QED theory, however, states that the more complex the process—that is, the greater the number of virtual photons exchanged in the process—the smaller the probability of its occurrence. For each level of complexity, the contribution of the process decreases by an amount given by α²—where α is a dimensionless quantity called the fine-structure constant, with a numerical value equal to (¹/₁₃₇). Thus, after a few levels the contribution is negligible. In a more fundamental way the factor α serves as a measure of the strength of the electromagnetic interaction. It equals ^e2/_{4πεo[planck]c}, where e is the electron charge, [planck] is Planck's constant divided by 2π, c is the speed of light, and ε_o is the permittivity of free space.

III. Match the halves to form collocations:

IV. Explain the following: muons, the magnetic moment, magnetic equation, emit, violate, devised, incorporate, conservation of energy, in a free state, second-order process, virtual photons, the fine-structure constant, the Feynman diagrams.

V. Say whether the statements are true or false:

1. QED has a strong connection with the general theory of relativity.

2. The magnetic moment of muons proves the accuracy of the theory.

3. Charged particles transmit electromagnetic fields.

4. The photons are called virtual, as their existence violates some basic laws.

5. Photons can change their speed and direction of travel.

6. The Feyman diagram indicates how to calculate the variable.

7. After a few levels the contribution is insignificant.

8. The complexity of the process depends on the speed of virtual photons.

9. There are other quantum field theories based on QED.

VI. Speaking

A. Summarise the article in 4 – 5 sentences.

B. You’re giving an online lecture on QED. Speak about its origin, the theories it is based on and its key principles.

Date: 2015-12-18; view: 848