same. It is one of the principles of the symmetry of physics, the equations seem to show, that if a clock, say, were made of matter on one hand, and then we made the same clock of antimatter, it would run in this way. (Of course, if we put the clocks together, they would annihilate each other, but that is different. An immediate question then arises. We can build, out of matter, two clocks one which is "left-hand"and one which is"right-hand. For example, we could build a clock which is not built in a simple way, but has cobalt and magnets and electron detectors which detect the presence of B-decay electrons and count them each time one is counted, the second hand moves over. Then the mirror clock, receiving fewer electrons, will not run at the same rate. So evidently we can make two clocks such that the left-hand clock does not agree with the right-hand one Let us make, out of matter, a clock which we call the standard or right-hand clock. Now let us make also out of matter a clock which we call the left-hand clock We have just discovered that, in general, these two will not run the same way prior to that famous physical discovery. as thought that they would. Now it was also supposed that matter and antimatter were equivalent. That is, if we made an antimatter clock, right-hand, the same shape, then it would run the same as the right-hand matter clock, and if we made the same clock to the left it would run the same. In other words, in the beginning it was believed that all four of these clocks were the same; now of course we know that the right-hand and left-hand matter are not the same. Presumably, therefore, the right-handed antimatter and the left-handed antimatter are not the same So the obvious question is, which goes with which, if either? In other words, does the right-handed matter behave the same way as the right-handed antimatter? Or does the right-handed matter behave the same as the left-handed antimatter? B-decay experiments, using positron decay instead of electron decay, indicate that this is the interconnection: matter to the"right""works the same way to the“left.” Therefore, at long last, it is really true that right and left symmetry is still maintained! If we made a left-hand clock, but made it out of the other kind of matter, antimatter instead of matter, it would run in the same way. So what has happened is that instead of having two independent rules in our list of symmetries, wo of these rules go together to make a new rule, which says that matter to the ght is symmetrical with antimatter to the left So if our Martian is made of antimatter and we give him instructions to make this"right""handed model like us, it will, of course, come out the other way around What would happen when, after much conversation back and forth, we each have taught the other to make space ships and we meet halfway in empty space? We have instructed each other on our traditions, and so forth, and the two of us come rushing out to shake hands. Well, if he puts out his left hand, watch out! 52-9 Broken symmetries The next question is, what can we make out of laws which are nearly sym metrical? The marvelous thing about it all is that for such a wide range of impo tant, strong phenomena--nuclear forces, electrical phenomena, and even weak ones like gravitation-over a tremendous range of physics, all the laws for these seen to be symmetrical. On the other hand, this little extra piece says, "No, the laws are not symmetrical! How is it that nature can be almost symmetrical, but not perfectly symmetrical? What shall we make of this? First, do we have any other examples? The answer is, we do, in fact, have a few other examples. For instance, he nuclear part of the force between proton and proton, between neutron and neutron, and between neutron and proton, is all exactly the same-there is a symmetry for nuclear forces, a new one, that we can interchange neutron and proton-but it evidently is not a general symmetry, for the electrical repulsion between two protons at a distance does not exist for neutrons. So it is not generally true that we can always replace a proton with a neutron, but only to a good ap proximation. Why good? Because the nuclear forces are much stronger than the 52-11