Waves and Particles

Sigma Xi - Phi Beta Kappa lecture, Eugene, May 26, 1934

By Linus Pauling

I Introduction

It was with the feeling of being pretty honored that I accepted your invitation to speak at the Sigma Xi - Phi Beta Kappa installation ceremonies this evening. This was not my only feeling, however. It was accompanied by considerable trepidation even anxiety, consequent to the necessity of preparing a speech. For Professor Stafford and Professor Taylor informed me, very explicitly, that this speech should be interesting, even exciting, if possible to the auditors, and also be generally understandable. Now I have heard speeches of this type given, by men who are able speakers and who know some things of general interest, and it at first seemed to me that all I needed to do was to emulate them.

I have learned of many exciting discoveries recently. There is the great American discovery of the isotope of hydrogen, present in heavy water. This would make a romantic tale, especially since it is entirely American-like, indeed, most discoveries of the last two or three years. there is the story of the positron, discovered a year or so ago by Dr. Carl Anderson in Pasadena. There are the marvelous machines, in Berkeley, Pasadena, M.I.T., and elsewhere, for accelerating particles with several million volts of energy, and disintegrating atomic nuclei, forming new atoms, and accomplishing the ideal of the alchemist - the transmutation of elements. Then, there are the new and interesting discoveries regarding the structure of molecules, involving in particular the idea of resonance, introduced by the new quantum mechanics, and all of the other revolutionary advances in structural chemistry.

And yet, are these things really of general interest? They interest me - I find that my heart beats faster on my learning that not only are there hydrogen atoms twice as heavy as the ordinary ones, but also still heavier hydrogen atoms, three times as heavy as the prototypes, have been discovered. But is this test - my own reaction - a satisfactory criterion? On analysis, it is formed not to be. For the question which I have recently formed most exciting during recent weeks is whether the adjacent carbon atoms in a benzene molecule are 1.39 Å or 1.42 Å apart - 1 Å being 1/100,000,000 cm or about 1/254,000,000 in. - and this question leaves other people's emotions unaffected. This reductio ad absurdum has carried the conviction that too great detail is to be avoided, even though the details may interest me.

In going to the other extreme, and speaking on the development of our concepts of matter and radiation, I may be wrecked on the charybdis of superficiality, in avoiding the scylla of particularity.

II Matter and Radiation

The contents of the physical world are divided into two classes, matter and radiation. The distinction between the classes is the speed with which the entities travels - radiation (light, radio waves, heat waves, x-rays, γ rays, and probably part of cosmic rays)travels with the speed of light, 3x10¹⁰ cm/sec, and matter always travels with a smaller speed.

The efforts of theoretical scientists have been to understand the nature of matter and radiation - to understand what they are, and to predict the future behavior of systems. This consists essentially in discovering mathematical equations. In the past the equations used in the discussion of the elementary entities of matter and radiation were the same as those used in the discussion of macroscopic phenomena. We then could say that we "understood" light and matter, meaning that we formed a picture based on our everyday experience.

Until recently, it was believed that light behaved exactly as though it were transverse waves in the ether, and that the fundamental material entities - electrons, atomic nuclei, molecules, etc. - behaved exactly as though they were particles - minute billiard balls which bounced around exactly as do the balls on a billiard table. In fact, it is first these two concepts of our world of every-day experiences - waves such as those on the surface of the ocean, and particles, such as marbles r billiard balls - which have dominated the development of physical thought.

III The Nature of Light

First stage. Newton Both concepts were early applied to light. In 1672 the great Sir Isaac Newton, after making his experiments with prisms published his "New Theory about Light and Colours". He rejected Huygens' suggestion that light is wave-like in nature, because he found it difficult to account for its rectilinear propagation on that basis, and discussed the properties of light on the basis of the view that it consists of a stream of particles. Despite the trouble that he had to go through to explain the colors of soap bubbles ("Newton's rings"), and despite Huygens' objection that the particles in two intersecting beams of light should collide (contrary to observation) Newton's ideas dominated for over a hundred years.

Second stage. Young and Fresnel Then, over a hundred years ago, there came the revolution. Thomas Young, Professor at the Royal Institution of London, discovered the phenomenon of diffraction - the interference and reinforcement of coherent light -, a characteristic wave phenomenon, and Augustin Fresnel, a young Frenchman, showed that all the known properties of light could be explained by assuming it to consist of transverse vibrations.

I hope you will allow me to interpolate at this point a short discussion of interference and reenforcement of waves.

Third stage. Einstein and Compton.The wave theory of light was further developed and refined during the whole of the nineteenth century by brilliant men - Hamilton, Maxwell, Hertz, and others, until, like other branches of physics, it was considered by many physicists to have been perfected. And then came the twentieth century to deal it two body blows. The first was due to Albert Einstein. In 1905 he pointed out that a number of newly discovered facts could be simply explained by assuming that light consists of packets - light quanta or photons - containing a given amount of energy; that is, that in some respects it is similar to a stream of particles. As an example, let us take the photoelectric effect. If a beam of light is shining on a piece of matter, it is found that the electrons in the matter either do nothing at all, or else they are ejected with a considerable and constant amount of energy. this is inexplicable on the wave theory - it is as though a number of logs, on the beach of a river, were not to move up and down under the action of the waves, but were individually to remain motionless, and spasmodically jump in the air to the height of exactly ten feet. Stronger waves would not increase the height, but only the number of jumps. moreover, ripples would be more rather than less efficacious than waves of longer wave length.

In 1923 a still more striking discovery was made by AH Compton, causing him to become America's third Nobel prize winner in physics. He found that on collision with electrons light behaved exactly as a marble of mass hv/c₂ colliding with a billiard ball of mass m. This "Compton effect" causes light scattered by electrons to change color.

This is our present state - light behaves in some respects like a stream of particles and in some like transverse waves.

IV. X-Rays and Crystals

X-radiation is exactly like light. After 17 years of uncertainty, this was shown by Max von Laue by interference experiments with crystals.

Also they show Compton effect, etc.

V. The Nature of Electrons

Up to ten years ago, no one doubted that electrons were little particles, of known mass and electric charge (and spin). And then came a surprise - more sudden and astounding than the new discoveries about light. It was found that electrons behave like normal waves, the wave length being h/mv. The first experiments were made by Davisson and Germer of the Bell Laboratories in New York, the first theory (a mystical one) by the Frenchman de Broglie.

Now electron waves are even used as a tool by the chemist for molecular-structure determination. Similar wave-interference experiments have been carried out with protons, hydrogen atoms, hydrogen molecules, helium atoms, etc.

So our conception of matter is just as ambiguous sa that of light. Matter too shows. Both am undulatory and a corpuscular character.

VI. The New Quantum or Wave Mechanics

The sceptic auditor (if not bored into mental inactivity) might well ask "Does not this wave-particle duality make it impossible to make unambiguous predictions? When does the electron or photon know it is to behave as a particle and when as a wave?"

The answer is that we have a set of equations - neither wave nor particle-like - describing with some accuracy the physical world. These - incorporated in quantum or wave mechanics - were discovered by Werner Heisenberg in 1925, when he was 24 years old, and further developed by many people - Schrödinger, Dirac, Born, Pauli, and others. it is of interest that Pauli is Heisenberg's age, Dirac still younger and Schrödinger not much older.

Pauli was particularly precocious. In 1919, at a meeting of the Deutsche Physikalische Gesellschaft, in Berlin, Einstein addressed a body of serious scientists on his latest studies in general relativity. When he was through, the chairman asked if there were any comments (expecting none). An 18-year-old boy arose, and said, "I agree with Professor Einstein in regard to most of his interesting work. On two points, however, he is in error". Pauli at this time wrote the Enz article on general relativity.

VII. Physics and Years

I do not wish to give the impression that modern physicists differ from their fore-runners. Mathematics and theoretical physics are particularly suited to young men, not requiring the extensive experience and wide background necessary for work in the lest exact sciences, and young men have always been at work. thus consider the men we have mentioned.

Newton, when he was 23-24 years old, discovered the binomial theorem, differential and integral calculus, and universal gravitation.

Aristotle said that a young man can be a mathematician, but it takes an old man to be a politician.