Basic Physics

Notes from "Chapter 2: Basic Physics", from The Feynman Lectures on Physics: Volume I.

2.1 Introduction

The universe is like a great game played by the gods. We don’t know what the rules of the game are, but we’re allowed to watch the game unfold.

The ultimate goal of physics is to see nature as different aspects of one set of phenomena. But due to discoveries like x-rays and mesons, our attempts at unification are far from complete.

Historically, we’ve been able to unify some fields:

• Heat and mechanics: temperature is determined by the motion of atoms.
• Electricity, magnetism, and light: all different aspects of the electromagnetic field.

2.2 Physics before 1920

Our understanding of physics was largely based on laws set by Isaac Newton in the 1700s. We understood that objects in space changed in a medium called time, and a few principles:

• Objects in motion remain in motion unless a force acts on it (inertia)
• A force binds atoms together.
• Another, much weaker force, pulls mass together (gravity)

We discovered that the short range force between atoms was electrical, and that certain combinations bond because their charges fit well together. Most things are in electrical equilibrium—they feel very little force at a distance because of this balance.

In the center of an atom, there are protons and neutrons of about the same weight, both very heavy compared to electrons. The chemical properties of an element are determined only by the number of electrons it has. Hypothetically, we could have called carbon “element 6” instead of carbon.

We learned that the existence of a positive charge, in some sense distorts, or creates a condition in space, where negative charges will feel a pulling force towards the positive charge. This is an electric field. When we put an electron in an electric field, we say it is “pulled.” We then have two rules:

1. Charges make a field.
2. Charges in fields have forces on them and move.

Magnetic influences have to do with charges in relative motion, so magnetic forces and electric forces can really be attributed to one field, as two different aspects of the same thing. If we oscillate a charge, it will create waves in the electromagnetic field. Different rates of oscillations, or frequencies, correspond to different types of waves, e.g. light or radio.

2.3 Quantum Physics

At much higher frequencies, waves in the electromagnetic field behave like particles. In fact, no phenomenon directly involving a frequency has been detected above approximately $10^{12}$ cycles per second. We only deduce the higher frequencies from the energy of the particles, by a rule which assumes that the particle-wave idea of quantum mechanics is valid.

From this, we have a new kind of particle to add to the electron, the proton, and the neutron, called a photon. The field that describes the interaction between these particles is called quantum electrodynamics. This field also predicted the existence of the positron, a positively-charged counterpart to the electron of the same mass. It turns out that this can be generalized to all particles, e.g. antiprotons and antineutrons.

One could say the quantum mechanics unifies the idea of a field, its waves, and particles. Before the 1920s, we thought of space and time as separate entities. However, Einstein’s work combine these into space-time, which is further complicated by gravity—the curvature of space-time.

We soon discovered that Newton’s laws—concepts like inertia and forces—do not hold on the scale of individual atoms. This feels very unnatural to us, because we cannot observe the behavior of objects on such a small scale in our day to day lives.

One strange aspect of quantum mechanics is the uncertainty principle. In short, we cannot know both where something is and how fast it is moving. Because of this, at a small scale, it is impossible for us to predict what will happen in the way that we can with Newtonian physics.

2.4 Nuclei and particles

Nuclei are held together by enormous forces, but do not fully understand yet how these forces work. As a part of this research, we have discovered about thirty new particles, classified as baryons, mesons, and leptons.

There seems to be just four kinds of interaction between particles which, in the order of decreasing strength, are the nuclear force, electrical interactions, the beta-decay interaction, and gravity.

This then, is the horrible condition of our physics today. To summarize it, I would say this: outside the nucleus, we seem to know all; inside it, quantum mechanics is valid—the principles of quantum mechanics have not been found to fail. The stage on which we put all of our knowledge, we would say, is relativistic space-time; perhaps gravity is involved in space-time. We do not know how the universe got started, and we have never made experiments which check our ideas of space and time accurately, below some tiny distance, so we only know that our ideas work above that distance. We should also add that the rules of the game are the quantum-mechanical principles, and those principles apply, so far as we can tell, to the new particles as well as to the old. The origin of the forces in nuclei leads us to new particles, but unfortunately they appear in great profusion and we lack a complete understanding of their interrelationship, although we already know that there are some very surprising relationships among them. We seem gradually to be groping toward an understanding of the world of subatomic particles, but we really do not know how far we have yet to go in this task.