in New Orleans and look at Chicago, and Florida is to our right(when our feet are on the ground: ) So we can define"right " and"left"by geography. Of course the actual situation in any system does not have to have the symmetry that we are talking about; it is a question of whether the laws are symmetrical--in other words hether it is against the physical laws to have a sphere like the earth with"left handed dirt"on it and a person like ourselves standing looking at a city like Chicago from a place like New Orleans, but with everything the other way around so Florida is on the other side. It clearly seems not impossible, not against the physical laws, to have everything changed left for right. Another point is that our definition of"right ""should not depend on history An easy way to distinguish right from left is to go to a machine shop and pick up a screw at random. The odds are it has a right-hand thread--not necessarily, but it is much more likely to have a right-hand thread than a left-hand one. This is a estion of history or convention, or the way things happen to be, and is again not a question of fundamental laws. As we can well appreciate, everyone could have started out making left-handed screws! So we must try to find some phenomenon in which"right hand"is involved fundamentally. The next possibility we discuss is the fact that polarized light rotates its plane of polarization as it goes through, say, sugar water in Chapter 33, it rotates, let us say, to the right in a certain sugar solution.That is a way of defining"right-hand, "because we may dissolve some sugar in the water and then the polarization goes to the right. But sugar has come from living things and if we try to make the sugar artificially then we discover that it does not rotate the plane of polarization! But if we then take that same sugar which is made artificially and which does not rotate the plane of polarization, and put bacteria in it ( they eat some of the sugar)and then filter out the bacteria, we find that we still have sugar left(almost half as much as we had before), and this time it does rotate the plane of polarization, but the other way! It seems very confusing, but is easily explained Fig. 52-1.(a)L-alanine(left), and(b) -alanine (right) Take another example: One of the substances which is common to all living creatures and that is fundamental to life is protein. Proteins consist of chains of amino acids. Figure 52-1 shows a model of an amino acid that comes out of a protein. This amino acid is called alanine, and the molecular arrangement would look like that in Fig. 52-1(a) if it came out of a protein of a real living thing. On the other hand, if we try to make alanine from carbon dioxide, ethane and am monia(and we can make it, it is not a complicated molecule), we discover that we re making equal amounts of this molecule and the one shown in Fig. 52-1(b) The first molecule, the one that comes from the living thing, is called L-alanine The other one, which is the same chemically, in that it has the same kinds of atoms and the same connections of the atoms, is a"right-hand"molecule, com pared with the"left-hand"L-alanine, and it is called D-alanine. The interesting thing is that when we make alanine at home in a laboratory from simple gases we get an equal mixture of both kinds. However, the only thing that life uses is L-alanine. (This is not exactly true. Here and there in living creatures there is a Now if we make both kinds, and we feed the mixture to some animal which hi special use for D-alanine, but it is very rare. All proteins use L-alanine exclusivel to"eat, or use up, alanine, it cannot use D-alanine, so it only uses the L-alanine; that is what happened to our sugar-after the bacteria eat the sugar that works