Feynman_Lectures_on_Physics_Volume_1_Chapter_03.pdf

The Relation of Physics to Other Sciences

3-1 Introduction

Physics is the most fundamental and all-inclusive of the sciences, and has

had a profound effect on all scientific development. In fact, physics is the present-

day equivalent of what used to be called natural philosophy, from which most of

our modern sciences arose. Students of many fields find themselves studying

physics because of the basic role it plays in all phenomena. In this chapter we

shall try to explain what the fundamental problems in the other sciences are,

but of course it is impossible in so small a space really to deal with the complex,

subtle, beautiful matters in these other fields. Lack of space also prevents our

discussing the relation of physics to engineering, industry, society, and war, or

even the most remarkable relationship between mathematics and physics. (Mathe-

matics is not a science from our point of view, in the sense that it is not a natural

science. The test of its validity is not experiment.) We must, incidentally, make it

clear from the beginning that if a thing is not a science, it is not necessarily bad.

For example, love is not a science. So, if something is said not to be a science,

it does not mean that there is something wrong with it; it just means that it is not

a science.

3-1 Introduction

3-2 Chemistry

3-3 Biology

3-4 Astronomy

3-5 Geology

3-6 Psychology

3-7 How did it get that way?

3-2 Chemistry

The science which is perhaps the most deeply affected by physics is chemistry.

Historically, the early days of chemistry dealt almost entirely with what we now call

inorganic chemistry, the chemistry of substances which are not associated with

living things. Considerable analysis was required to discover the existence of the

many elements and their relationships—how they make the various relatively

simple compounds found in rocks, earth, etc. This early chemistry was very

important for physics. The interaction between the two sciences was very great

because the theory of atoms was substantiated to a large extent by experiments

in chemistry. The theory of chemistry, i.e., of the reactions themselves, was

summarized to a large extent in the periodic chart of Mendeleev, which brings out

many strange relationships among the various elements, and it was the collection

of rules as to which substance is combined with which, and how, that constituted

inorganic chemistry. All these rules were ultimately explained in principle by

quantum mechanics, so that theoretical chemistry is in fact physics. On the

other hand, it must be emphasized that this explanation is in principle. We have

already discussed the difference between knowing the rules of the game of chess,

and being able to play. So it is that we may know the rules, but we cannot play

very well. It turns out to be very difficult to predict precisely what will happen in

a given chemical reaction; nevertheless, the deepest part of theoretical chemistry

must end up in quantum mechanics.

There is also a branch of physics and chemistry which was developed by both

sciences together, and which is extremely important. This is the method of

statistics applied in a situation in which there are mechanical laws, which is aptly

called statistical mechanics. In any chemical situation a large number of atoms are

involved, and we have seen that the atoms are all jiggling around in a very random

and complicated way. If we could analyze each collision, and be able to follow

in detail the motion of each molecule, we might hope to figure out what would

happen, but the many numbers needed to keep track of all these molecules ex-

ceeds so enormously the capacity of any computer, and certainly the capacity of

3-1

the mind, that it was important to develop a method for dealing with such com-

plicated situations. Statistical mechanics, then, is the science of the phenomena

of heat, or thermodynamics. Inorganic chemistry is, as a science, now reduced

essentially to what are called physical chemistry and quantum chemistry; physical

chemistry to study the rates at which reactions occur and what is happening in

detail (How do the molecules hit? Which pieces fly off first?, etc.), and quantum

chemistry to help us understand what happens in terms of the physical laws.

The other branch of chemistry is organic chemistry, the chemistry of the

substances which are associated with living things. For a time it was believed

that the substances which are associated with living things were so marvelous

that they could not be made by hand, from inorganic materials. This is not at

all true—they are just the same as the substances made in inorganic chemistry,

but more complicated arrangements of atoms are involved. Organic chemistry

obviously has a very close relationship to the biology which supplies its substances,

and to industry, and furthermore, much physical chemistry and quantum mechanics

can be applied to organic as well as to inorganic compounds. However, the main

problems of organic chemistry are not in these aspects, but rather in the analysis

and synthesis of the substances which are formed in biological systems, in living

things. This leads imperceptibly, in steps, toward biochemistry, and then into

biology itself, or molecular biology.

3-3 Biology

Thus we come to the science of biology, which is the study of living things.

In the early days of biology, the biologists had to deal with the purely descriptive

problem of finding out what living things there were, and so they just had to

count such things as the hairs of the limbs of fleas. After these matters were worked

out with a great deal of interest, the biologists went into the machinery inside the

living bodies, first from a gross standpoint, naturally, because it takes some effort

to get into the finer details.

There was an interesting early relationship between physics and biology in

which biology helped physics in the discovery of the conservation of energy, which

was first demonstrated by Mayer in connection with the amount of heat taken in

and given out by a living creature.

If we look at the processes of biology of living animals more closely, we see

many physical phenomena: the circulation of blood, pumps, pressure, etc. There

are nerves: we know what is happening when we step on a sharp stone, and that

somehow or other the information goes from the leg up. It is interesting how that

happens. In their study of nerves, the biologists have come to the conclusion that

nerves are very fine tubes with a complex wall which is very thin; through this

wall the cell pumps ions, so that there are positive ions on the outside and nega-

tive ions on the inside, like a capacitor. Now this membrane has an interesting

property; if it "discharges" in one place, i.e., if some of the ions were able to move

through one place, so that the electric voltage is reduced there, that electrical

influence makes itself felt on the ions in the neighborhood, and it affects the

membrane in such a way that it lets the ions through at neighboring points also.

This in turn affects it farther along, etc., and so there is a wave of "penetrability"

of the membrane which runs down the fiber when it is "excited" at one end by

stepping on the sharp stone. This wave is somewhat analogous to a long sequence

of vertical dominoes; if the end one is pushed over, that one pushes the next,

etc. Of course this will transmit only one message unless the dominoes are set

up again; and similarly in the nerve cell, there are processes which pump the ions

slowly out again, to get the nerve ready for the next impulse. So it is that we know

what we are doing (or at least where we are). Of course the electrical effects

associated with this nerve impulse can be picked up with electrical instruments,

and because there are electrical effects, obviously the physics of electrical effects

has had a great deal of influence on understanding the phenomenon.

The opposite effect is that, from somewhere in the brain, a message is sent

out along a nerve. What happens at the end of the nerve? There the nerve branches

3-2

out into fine little things, connected to a structure near a muscle, called an end-

plate. For reasons which are not exactly understood, when the impulse reaches

the end of the nerve, little packets of a chemical called acetylcholine are shot off

(five or ten molecules at a time) and they affect the muscle fiber and make it con-

tract—how simple! What makes a muscle contract? A muscle is a very large num-

ber of fibers close together, containing two different substances, myosin and

actomyosin, but the machinery by which the chemical reaction induced by acetyl-

choline can modify the dimensions of the molecule is not yet known. Thus the

fundamental processes in the muscle that make mechanical motions are not known.

Biology is such an enormously wide field that there are hosts of other problems

that we cannot mention at all—problems on how vision works (what the light does

in the eye), how hearing works, etc. (The way in which thinking works we shall

discuss later under psychology.) Now, these things concerning biology which

we have just discussed are, from a biological standpoint, really not fundamental,

at the bottom of life, in the sense that even if we understood them we still would

not understand life itself. To illustrate: the men who study nerves feel their work

is very important, because after all you cannot have animals without nerves.

But you can have life without nerves. Plants have neither nerves nor muscles,

but they are working, they are alive, just the same. So for the fundamental prob-

lems of biology we must look deeper; when we do, we discover that all living

things have a great many characteristics in common. The most common feature

is that they are made of cells, within each of which is complex machinery for doing

things chemically. In plant cells, for example, there is machinery for picking up

light and generating sucrose, which is consumed in the dark to keep the plant

alive. When the plant is eaten the sucrose itself generates in the animal a series

of chemical reactions very closely related to photosynthesis (and its opposite

effect in the dark) in plants.

In the cells of living systems there are many elaborate chemical reactions,

in which one compound is changed into another and another. To give some im-

pression of the enormous efforts that have gone into the study of biochemistry,

the chart in Fig. 3-1 summarizes our knowledge to date on just one small part of

the many series of reactions which occur in cells, perhaps a percent or so of it.

Here we see a whole series of molecules which change from one to another

in a sequence or cycle of rather small steps. It is called the Krebs cycle, the respira-

tory cycle. Each of the chemicals and each of the steps is fairly simple, in terms

of what change is made in the molecule, but—and this is a centrally important

discovery in biochemistry—these changes are relatively difficult to accomplish in a

laboratory. If we have one substance and another very similar substance, the one

does not just turn into the other, because the two forms are usually separated by

3-3

an energy barrier or "hill." Consider this analogy: If we wanted to take an object

from one place to another, at the same level but on the other side of a hill, we could

push it over the top, but to do so requires the addition of some energy. Thus

most chemical reactions do not occur, because there is what is called an activa-

tion energy in the way. In order to add an extra atom to our chemical requires

that we get it close enough that some rearrangement can occur; then it will stick.

But if we cannot give it enough energy to get it close enough, it will not go to com-

pletion, it will just go part way up the "hill" and back down again. However,

if we could literally take the molecules in our hands and push and pull the atoms

around in such a way as to open a hole to let the new atom in, and then let it snap

back, we would have found another way, around the hill, which would not require

extra energy, and the reaction would go easily. Now there actually are, in the cells,

very large molecules, much larger than the ones whose changes we have been de-

scribing, which in some complicated way hold the smaller molecules just right, so

that the reaction can occur easily. These very large and complicated things are

called enzymes. (They were first called ferments, because they were originally

discovered in the fermentation of sugar. In fact, some of the first reactions in

the cycle were discovered there.) In the presence of an enzyme the reaction will go.

An enzyme is made of another substance called protein. Enzymes are very

big and complicated, and each one is different, each being built to control a certain

special reaction. The names of the enzymes are written in Fig. 3-1 at each reaction.

(Sometimes the same enzyme may control two reactions.) We emphasize that the

enzymes themselves are not involved in the reaction directly. They do not change;

they merely let an atom go from one place to another. Having done so, the enzyme

is ready to do it to the next molecule, like a machine in a factory. Of course, there

must be a supply of certain atoms and a way of disposing of other atoms. Take

hydrogen, for example: there are enzymes which have special units on them which

carry the hydrogen for all chemical reactions. For example, there are three or four

hydrogen-reducing enzymes which are used all over our cycle in different places.

It is interesting that the machinery which liberates some hydrogen at one place

will take that hydrogen and use it somewhere else.

The most important feature of the cycle of Fig. 3-1 is the transformation

from GDP to GTP (guanadine-di-phosphate to guanadine-tri-phosphate) because

the one substance has much more energy in it than the other. Just as there is a

"box" in certain enzymes for carrying hydrogen atoms around, there are special

energy-carrying "boxes" which involve the triphosphate group. So, GTP has more

energy than GDP and if the cycle is going one way, we are producing molecules

which have extra energy and which can go drive some other cycle which requires

energy, for example the contraction of muscle. The muscle will not contract

unless there is GTP. We can take muscle fiber, put it in water, and add GTP,

and the fibers contract, changing GTP to GDP if the right enzymes are present.

So the real system is in the GDP-GTP transformation; in the dark the GTP

which has been stored up during the day is used to run the whole cycle around the

other way. An enzyme you see, does not care in which direction the reaction goes,

for if it did it would violate one of the laws of physics.

Physics is of great importance in biology and other sciences for still another

reason, that has to do with experimental techniques. In fact, if it were not for the

great development of experimental physics, these biochemistry charts would not

be known today. The reason is that the most useful tool of all for analyzing this

fantastically complex system is to label the atoms which are used in the reactions.

Thus, if we could introduce into the cycle some carbon dioxide which has a

"green mark" on it, and then measure after three seconds where the green mark

is, and again measure after ten seconds, etc., we could trace out the course of the

reactions. What are the "green marks"? They are different isotopes. We recall

that the chemical properties of atoms are determined by the number of electrons,

not by the mass of the nucleus. But there can be, for example in carbon, six

neutrons or seven neutrons, together with the six protons which all carbon nuclei

have. Chemically, the two atoms C 12 and C 13 are the same, but they differ in

weight and they have different nuclear properties, and so they are distinguishable.

3-4

By using these isotopes of different weights, or even radioactive isotopes like C 14 ,

which provide a more sensitive means for tracing very small quantities, it is pos-

sible to trace the reactions.

Now, we return to the description of enzymes and proteins. All proteins are

not enzymes, but all enzymes are proteins. There are many proteins, such as the

proteins in muscle, the structural proteins which are, for example, in cartilage and

hair, skin, etc., that are not themselves enzymes. However, proteins are a very

characteristic substance of life: first of all they make up all the enzymes, and

second, they make up much of the rest of living material. Proteins have a very

interesting and simple structure. They are a series, or chain, of different ammo

acids. There are twenty different amino acids, and they all can combine with

each other to form chains in which the backbone is CO-NH, etc. Proteins are

nothing but chains of various ones of these twenty amino acids. Each of the amino

acids probably serves some special purpose. Some, for example, have a sulphur

atom at a certain place; when two sulphur atoms are in the same protein, they

form a bond, that is, they tie the chain together at two points and form a loop.

Another has extra oxygen atoms which make it an acidic substance, another has

a basic characteristic. Some of them have big groups hanging out to one side, so -

that they take up a lot of space. One of the amino acids, called prolene, is not

really an amino acid, but imino acid. There is a slight difference, with the result

that when prolene is in the chain, there is a kink in the chain. If we wished to

manufacture a particular protein, we would give these instructions: put one of

those sulphur hooks here; next, add something to take up space; then attach some-

thing to put a kink in the chain. In this way, we will get a complicated-looking

chain, hooked together and having some complex structure; this is presumably

just the manner in which all the various enzymes are made. One of the great tri-

umphs in recent times (since 1960), was at last to discover the exact spatial atomic

arrangement of certain proteins, which involve some fifty-six or sixty amino acids

in a row. Over a thousand atoms (more nearly two thousand, if we count the

hydrogen atoms) have been located in a complex pattern in two proteins. The

first was hemoglobin. One of the sad aspects of this discovery is that we cannot see

anything from the pattern; we do not understand why it works the way it does.

Of course, that is the next problem to be attacked.

Another problem is how do the enzymes know what to be? A red-eyed fly

makes a red-eyed fly baby, and so the information for the whole pattern of enzymes

to make red pigment must be passed from one fly to the next. This is done by a

substance in the nucleus of the cell, not a protein, called DNA (short for des-

oxyribose nucleic acid). This is the key substance which is passed from one cell

to another (for instance, sperm cells consist mostly of DNA) and carries the

information as to how to make the enzymes. DNA is the "blueprint." What does

the blueprint look like and how does it work? First, the blueprint must be able

to reproduce itself. Secondly, it must be able to instruct the protein. Concerning

the reproduction, we might think that this proceeds like cell reproduction. Cells

simply grow bigger and then divide in half. Must it be thus with DNA molecules,

then, that they too grow bigger and divide in half? Every atom certainly does not

grow bigger and divide in half! No, it is impossible to reproduce a molecule

except by some more clever way.

The structure of the substance DNA was studied for a long time, first chemi-

cally to find the composition, and then with x-rays to find the pattern in space.

The result was the following remarkable discovery: The DNA molecule is a pair

of chains, twisted upon each other. The backbone of each of these chains, which

are analogous to the chains of proteins but chemically quite different, is a series

of sugar and phosphate groups, as shown in Fig. 3-2. Now we see how the chain

can contain instructions, for if we could split this chain down the middle, we would

have a series BAADC . . . and every living thing could have a different series.

Thus perhaps, in some way, the specific instructions for the manufacture of pro-

teins are contained in the specific series of the DNA.

Attached to each sugar along the line, and linking the two chains together, are

certain pairs of cross-links. However, they are not all of the same kind; there are

3-5

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: