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For example, quantum electrodynamics (QED) is the theory that describes the electromagnetic force as being carried by photons. These photons may be real, or may only exist briefly on energy that can be borrowed because of the uncertainty principle (discussed in Chapter 8). These photons that live on borrowed energy are called virtual photons. The fact that the photon is massless leads to the electromagnetic force being long range. QED has been tested in many ways to very high accuracy, and is a very successful theory. The theory was developed in the 1940s by a number of physicists. One of the leaders in the field was Richard Feynman who came up with a way of visualizing the theory that also allowed him to carry out what had previously been very difficult calculations, which could be tested experimentally. Feynman shared the 1965 Nobel Prize in Physics for this work.

It is speculated that the gravitational interaction is carried by a massless particle, called the graviton. It is presumed to be massless, because gravity has the same long range behavior as electricity. However, no gravitons have ever been detected, and this theoretical framework is still being developed. We will see that the absence of quantum mechanical theory of gravity provides a limitation on how far back we can go in probing the big bang.

The strong nuclear force is carried by the pions. Of course, we now know that the pions themselves are not fundamental. They are each made of a quark and an antiquark. Since the pion has a mass, the strong nuclear force has a short range. In fact, the mass of the pion can be inferred from the range of the force (see Problem 21.9). The more massive the carrier, the greater the energy that must be temporarily 'borrowed' to produce the virtual particle. This means that the virtual particle lives a shorter length of time, and can travel a shorter distance in that lifetime.

The weak nuclear force, also a short range force, is carried by three particles. Two are the positive and negative W (for weak), and the third is a neutral particle, called the Z. These particles are much more massive than the pion, and the weak force is also a short range force. The W and Z particles have recently been detected with masses 80 and 90 times that of the proton.

21.3.3 The role of symmetries

In any study of particles and forces, symmetries play a very important role. Symmetries are important in many areas of physics. When we say that a system has a particular symmetry, we mean that the system looks the same after a certain transformation. For example, spherical symmetry means that the system looks the same, even if we rotate it through any angle, about any axis through one particular point (the center of the sphere). Recognizing symmetries can greatly simplify the solving of a problem. If a problem has a certain symmetry, then the solution must have the same symmetry. For example, in Fig. 21.19, we show a spherically symmetric charge distribution. If we want to find the electrical forces around it, those forces must have the same symmetric appearance.

Symmetries have an even deeper importance in physics. Whenever there is a symmetry, there is some quantity that is constant throughout the problem. This means that there is a conservation law. For example, the fact that the laws of physics cannot be changed by rotating our coordinate system leads to the conservation of angular momentum. The fact that the laws of physics are not changed by the translation of the origin of a coordinate system leads to the conservation of linear momentum. The fact that the laws of physics don't change with time leads to the conservation of energy.

Wrong Correct

Fig 21.19.

Correct

Fig 21.19.

Symmetry in an electricity problem. Suppose we wish to calculate the electric field due to the spherical charge distribution in (a).The resulting field must have the same symmetry as the charge distribution.The result in (b) is clearly wrong because, if we rotate the page, the charge distribution still looks the same, but the field direction changes.The type of distribution in (c) has the proper sym-metry.A consideration of symmetry allows us to eliminate unreasonable answers.

We can understand the various forces by understanding what symmetries they have, or, equivalently, what conservation laws they obey. In general, a process will take place as long as it doesn't violate some conservation law. If a reaction that you think should take place does not, it means that there is some conservation law that you might not be aware of, and that the reaction would violate that conservation law. For example, before the quark theory had been proposed, there was a group of particles that should have decayed by the strong nuclear force, but did not. Because of this strange behavior, these particles were called 'strange' particles. It was proposed that there must be some property of these strange particles which had to be conserved. The particular decays would then have violated this conservation law. When the quark theory was proposed, the strange quark(s) was included to incorporate this property.

One interesting property of the weak interaction is that it doesn't obey all the conservation laws that the other forces do. Before this was realized, it was thought that conservation laws were absolute. The first symmetry found to be broken was that concerning parity, which has to do with the behavior of a system under a mirror reflection. It was realized by T. D. Lee and C. N. Yang in 1957 that it is possible to set up a beta decay experiment (beta decay taking place via the weak interaction) and the mirror image experiment, and achieve different results. This experiment was carried out by C. S. Wu a year later, and Yang and Lee shared the 1959 Nobel Prize in Physics for their prediction. Two other symmetries violated by the weak interaction are charge conjugation (the interchange of particles and antiparticles) and time reversal.

Sometimes, we find situations which are inherently symmetric, but somehow lead to an asymmetric result. They are called spontaneous symmetry breaking. For example, suppose we toss a coin in the air. As the coin is spinning, it has an equal probability of being heads or tails. As long as the coin stays in the air, the situation is symmetric between heads and tails. However, once the coin falls, the symmetry is broken. It is either heads or tails. Another example is a ferromagnet, shown in Fig. 21.20. If the magnet is heated above its critical temperature, it has spherical symmetry. There is no preferred direction. When we cool the material, it becomes a permanent magnet. Until it cools, all directions are equally probable. Once it cools, one direction is selected and the whole magnet cools, pointing in that direction. This is an example of a situation that is symmetric as long as it is hot enough, but cooling the system breaks the symmetry. Some circumstances in nature are symmetric as long as there is enough energy.

21.3.4 Color

We have already mentioned two problems with the quark theory. One is that there was no explanation for why the only allowed combinations are three quarks or one quark and one antiquark. The other is that we have not been able to detect

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