Mathematics Is More Than Just A Language- It Is Language Plus Logic
There is no model of the theory of gravitation
today, other than the mathematical form.
It this were the only law of this character it would be interesting and rather
annoying. But what turns out to be true is that the more we investigate, the
more laws we find, and the deeper we penetrate nature, the more this disease
persists. Every one of our laws is a purely mathematical statement in rather
complex and abstruse mathematics. Newton's statement of the law of gravitation
is relatively simple mathematics. It gets more and more abstruse and more
and more difficult as we go on. Why? I have not the slightest idea. It is
only my purpose here to tell you about this fact. The burden of the lecture
is just to emphasize the fact that it is impossible to explain honestly the
beauties of the laws of nature in a way that people can feel, without their
having some deep understanding of mathematics. I am sorry, but this seems
to be the case.
You might say, "All right, then if there is no explanation of the law,
at least tell me what the law is. Why not tell me in words instead of symbols?
Mathematics is just a language, and I want to be able to translate the language."
In fact I can, with patience, and I think I partly did. I could go a little
further and explain in more detail that the equation means that if the distance
is twice as far the force is one fourth as much, and so on. I could convert
all the symbols into words. In other words I could be kind to the layman as
they all sit hopefully waiting for me to explain something. Different people
get different reputations for their skill at explaining to the layman in layman's
language these difficult and abstruse subjects. The layman then searches for
book after book in the hope that he will avoid the complexities which eventually
set in, even with the best expositor of this type. He finds as he reads a
generally increasing confusion, one complicated statement after another, one
difficult-to-understand thing after another, all apparently disconnected from
one another. It becomes obscure, and he hopes that maybe in some other book
there is some explanation... The author almost made it- maybe another fellow
will make it right.
But I do not think it is possible, because mathematics is not just another
language. Mathematics is a language plus reasoning; it is like a language
plus logic. Mathematics is a tool for reasoning. It is in fact a big collection
of the results of some person's careful thought and reasoning. By mathematics
it is possible to connect one statement to another. For instance, I can say
that the force is directed towards the sun. I can also tell you, as I did,
that the planet moves so that if I draw a line from the sun to the planet,
and draw another line at some definite period, like three weeks, later, then
the area that is swung out by the planet is exactly the same as it will be
in the next three weeks, and the next three weeks, and so on as it goes around
the sun. I can explain both of those statements carefully, but I cannot explain
why they are both the same. The apparent enormous complexities of nature,
with all its funny laws and rules, each of which has been carefully explained
to you, are really very closely interwoven. However, if you do not appreciate
the mathematics, you cannot see, among the great variety of facts, that logic
permits you to go from one to another.
It may be unbelievable that I can demonstrate that equal areas will be swept
out in equal times if the forces are directed towards the sun. So if I may,
I will do one demonstration to show you that those two things really are equivalent,
so that you can appreciate more than the mere statement of the two laws. I
will show that the two laws are connected so that reasoning alone will bring
you from one to the other, and that mathematics is just organized reasoning.
Then you will appreciate the beauty of the relationship of the statements.
I am going to prove the relationship that if the forces are directed towards
the sun then equal areas are swept out in equal times.
Fig1
We start with a sun and a planet (Fig. 1), and we imagine that at a certain time the planet is at position 1. It is moving in such a way that, say, one second later it has moved to position 2. If the sun did not exert a force on the planet, then, by Galileo's principle of inertia, it would keep right on going in a straight line. So after the same interval of time, the next second, it would have moved exactly the same distance in the same straight line, to the position 3. First we are going to show that if ther is no force, then equal areas are swept out in equal times. I remind you that the area of a triangle is half the base times the altitude, and that the altitude is the vertical distance to the base. If the triangle is obtuse (Fig. 2), then the altitude is the vertical height AD and the base is BC. Now let us compare the areas which would be swept out if the sun exerted no force whatsoever (Fig. 1).
Fig2
The two distances 1-2 and 2-3 are equal, remember. The question is, are the two areas equal? Consider the triangle made from the sun and the two points 1 and 2. What is its area? It is the base 1-2, multiplied by half the perpendicular height from the baseline to S. What about the other triangle, the triangle in the motion from 2 to 3? Its area is the base 2-3, times half the perpendicular height to S. The two triangles have the same altitude, and, as I indicated, the same base, and therefore they have the same area. So far so good. If there were no force from the sun, equal areas would be swept out in equal times. But there is a force from the sun. During the interval 1-2-3 the sun is pulling and changing the motion in various directions towards itself. To get a good approximation web will take the central position, or average position, at 2, and say that the whole effect during the interval 1-3 was to change the motion by some amount in the direction of the line 2-S. (Fig. 3).
Fig3
This means that though the particles
were moving on the line 1-2, and would, were there no force, have continued
to move on the same line in the next second, because of the influence of the
sun the motion is altered by an amount that is poking in a direction parallel
to the line 2-S. The next motion is therefore a compound of what the planet
wanted to do and the change that has been induced by the action of the sun.
So the planet does not really end up at position 3, but rather at position
4. Now we would like to compare the areas of the triangles 23S and 24S, and
I will show you that those are equal. They have the same base, S-2. Do they
have the same altitude? Sure, because they are included between parallel lines.
The distance from 4 to the line S-2 is equal to the distance from 3 to line
S-2 (extended). Thus the area of the triangle S24 is the same as S23. I proved
earlier that S12 and S23 were equal in area, so we now know S12 = S24. So,
in the actual orbital motion of the planet the areas swept out in the first
second and the second second are equal. Therefore, by reasoning, we can see
a connection between the fact that the force is towards the sun, and the fact
that the areas are equal. Isn't that ingenious? I borrowed it straight from
Newton. It comes right out of the Principia, diagram and all. Only the letters
are different, because he wrote in Latin and these are Arabic numerals...
Mathematics, then, is a way of going from one set of statements to another.
It is evidently useful in physics, because we have these different ways in
which we can speak of things, and mathematics permits us to develop consequences,
to analyse the situations, and to change the laws in different ways to connect
the various statements. In fact the total amount that a physicist knows is
very little. He has only to remember the rules to get him from one place to
another and he is all right, because all the various statements about equal
times, the force being in the direction of the radius, and so on, are all
interconnected by reasoning.
-Richard Feynman