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Princeton University Press eBooks

What Is an Elementary Particle?

Erwin Schrödinger

Published Dec 8, 2020• 3 citations
Publisher

Abstract

INDIVIDUALAtomism in its latest form is called quantum mechanics.It has extended its range to comprise, besides ordinary matter, all kinds of radiation, including light-in brief, all forms of energy, ordinary matter being one of them.In the present form of the theoiy the "atoms" are electrons, jorotons, photons, mesons, etc.The generic name is elementary particle, or merely particle.The term "atom" has very wisely been retained for chemical atoms, though it has become a misnomer.This essay deals with the elementary particle, more particularly with a certain feature that this concept has acquired-or rather lost in quantum mechanics.I menn this : That the elementary particle is not an individual; it cannot be identified, it lacks "sameness."The fact is known to every physicist, but is rarely given any prominence in surveys readable by nonspccialists.In technical language it is covered by saying that the particles "obey" new-fangled statistics, either Einstein-Bose or Fermi-Dirac statistics.The implication, far from obvious, is that the unsuspected epithet "this" is not quite prop- erly applicable to, say, an electron, except with caution, in a restricted sense, and sometimes not at all.My objective here is to explain this point and to give it the thought it deserves.In order to create a foil for the discussion, let me summarize in sections 2-5 what we are usually told about particles and waves in the new physics.2. CURRENT VIEWS: THE AMALGAMATION OF PARTICLES AND WAVES Our image of the material world had been made up of two kinds of "fittings"-waves and particles.The former were instanced mainly, if not exclusively, by Maxwell's waves of electromagnetic energy, comprising such as are used in radio, light, X-rays, and gamma-rays.Material bodies were said to consist of particles.One

DOI
10.2307/j.ctv173f1xr.17

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WHAT IS AN ELEMENTARY PARTICLE?

INDIVIDUALAtomism in its latest form is called quantum mechanics.It has extended its range to comprise, besides ordinary matter, all kinds of radiation, including light-in brief, all forms of energy, ordinary matter being one of them.In the present form of the theoiy the "atoms" are electrons, jorotons, photons, mesons, etc.The generic name is elementary particle, or merely particle.The term "atom" has very wisely been retained for chemical atoms, though it has become a misnomer.This essay deals with the elementary particle, more particularly with a certain feature that this concept has acquired-or rather lost in quantum mechanics.I menn this : That the elementary particle is not an individual; it cannot be identified, it lacks "sameness."The fact is known to every physicist, but is rarely given any prominence in surveys readable by nonspccialists.In technical language it is covered by saying that the particles "obey" new-fangled statistics, either Einstein-Bose or Fermi-Dirac statistics.The implication, far from obvious, is that the unsuspected epithet "this" is not quite prop- erly applicable to, say, an electron, except with caution, in a restricted sense, and sometimes not at all.My objective here is to explain this point and to give it the thought it deserves.In order to create a foil for the discussion, let me summarize in sections 2-5 what we are usually told about particles and waves in the new physics.2. CURRENT VIEWS: THE AMALGAMATION OF PARTICLES AND WAVES Our image of the material world had been made up of two kinds of "fittings"-waves and particles.The former were instanced mainly, if not exclusively, by Maxwell's waves of electromagnetic energy, comprising such as are used in radio, light, X-rays, and gamma-rays.Material bodies were said to consist of particles.One

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INDIVIDUALAtomism in its latest form is called quantum mechanics.It has extended its range to comprise, besides ordinary matter, all kinds of radiation, including light-in brief, all forms of energy, ordinary matter being one of them.In the present form of the theoiy the "atoms" are electrons, jorotons, photons, mesons, etc.The generic name is elementary particle, or merely particle.The term "atom" has very wisely been retained for chemical atoms, though it has become a misnomer.This essay deals with the elementary particle, more particularly with a certain feature that this concept has acquired-or rather lost in quantum mechanics.I menn this : That the elementary particle is not an individual; it cannot be identified, it lacks "sameness."The fact is known to every physicist, but is rarely given any prominence in surveys readable by nonspccialists.In technical language it is covered by saying that the particles "obey" new-fangled statistics, either Einstein-Bose or Fermi-Dirac statistics.The implication, far from obvious, is that the unsuspected epithet "this" is not quite prop- erly applicable to, say, an electron, except with caution, in a restricted sense, and sometimes not at all.My objective here is to explain this point and to give it the thought it deserves.In order to create a foil for the discussion, let me summarize in sections 2-5 what we are usually told about particles and waves in the new physics.2. CURRENT VIEWS: THE AMALGAMATION OF PARTICLES AND WAVES Our image of the material world had been made up of two kinds of "fittings"-waves and particles.The former were instanced mainly, if not exclusively, by Maxwell's waves of electromagnetic energy, comprising such as are used in radio, light, X-rays, and gamma-rays.Material bodies were said to consist of particles.One
INDIVIDUALAtomism in its latest form is called quantum mechanics.It has extended its range to comprise, besides ordinary matter, all kinds of radiation, including light-in brief, all forms of energy, ordinary matter being one of them.In the present form of the theoiy the "atoms" are electrons, jorotons, photons, mesons, etc.The generic name is elementary particle, or merely particle.The term "atom" has very wisely been retained for chemical atoms, though it has become a misnomer.This essay deals with the elementary particle, more particularly with a certain feature that this concept has acquired-or rather lost in quantum mechanics.I menn this : That the elementary particle is not an individual; it cannot be identified, it lacks "sameness."The fact is known to every physicist, but is rarely given any prominence in surveys readable by nonspccialists.In technical language it is covered by saying that the particles "obey" new-fangled statistics, either Einstein-Bose or Fermi-Dirac statistics.The implication, far from obvious, is that the unsuspected epithet "this" is not quite prop- erly applicable to, say, an electron, except with caution, in a restricted sense, and sometimes not at all.My objective here is to explain this point and to give it the thought it deserves.In order to create a foil for the discussion, let me summarize in sections 2-5 what we are usually told about particles and waves in the new physics.2. CURRENT VIEWS: THE AMALGAMATION OF PARTICLES AND WAVES Our image of the material world had been made up of two kinds of "fittings"-waves and particles.The former were instanced mainly, if not exclusively, by Maxwell's waves of electromagnetic energy, comprising such as are used in radio, light, X-rays, and gamma-rays.Material bodies were said to consist of particles.One
INDIVIDUALAtomism in its latest form is called quantum mechanics.It has extended its range to comprise, besides ordinary matter, all kinds of radiation, including light-in brief, all forms of energy, ordinary matter being one of them.In the present form of the theoiy the "atoms" are electrons, jorotons, photons, mesons, etc.The generic name is elementary particle, or merely particle.The term "atom" has very wisely been retained for chemical atoms, though it has become a misnomer.This essay deals with the elementary particle, more particularly with a certain feature that this concept has acquired-or rather lost in quantum mechanics.I menn this : That the elementary particle is not an individual; it cannot be identified, it lacks "sameness."The fact is known to every physicist, but is rarely given any prominence in surveys readable by nonspccialists.In technical language it is covered by saying that the particles "obey" new-fangled statistics, either Einstein-Bose or Fermi-Dirac statistics.The implication, far from obvious, is that the unsuspected epithet "this" is not quite prop- erly applicable to, say, an electron, except with caution, in a restricted sense, and sometimes not at all.My objective here is to explain this point and to give it the thought it deserves.In order to create a foil for the discussion, let me summarize in sections 2-5 what we are usually told about particles and waves in the new physics.2. CURRENT VIEWS: THE AMALGAMATION OF PARTICLES AND WAVES Our image of the material world had been made up of two kinds of "fittings"-waves and particles.The former were instanced mainly, if not exclusively, by Maxwell's waves of electromagnetic energy, comprising such as are used in radio, light, X-rays, and gamma-rays.Material bodies were said to consist of particles.One
Introduction It is believed that the basic physical building blocks forming the world we live in may be categorized into particles of matter and carriers of force between matter. All known elementary constituents of matter and transmitters of force are quantized. For example, energy, momentum, and angular momentum take on discrete quantized values. The electron is an example of an elementary particle of matter, and the photon is an example of a transmitter of force. Neutrons, protons, and atoms are composite particles made up of elementary particles of matter and transmitters of force. These composite particles are also quantized. Because classical mechanics is unable to explain quantization, we must learn quantum mechanics in order to understand the microscopic properties of atoms – which, for example, make up solids such as crystalline semiconductors. Historically, the laws of quantum mechanics have been established by experiment. The most important early experiments involved light. Long before it was realized that light waves are quantized into particles called photons, key experiments on the wave properties of light were performed. For example, it was established that the color of visible light is associated with different wavelengths of light. Table 2.1 shows the range of wavelengths corresponding to different colors. The connection between optical and electrical phenomena was established by Maxwell in 1864. This extended the concept of light to include the complete electromagnetic spectrum. A great deal of effort was, and continues to be, spent gathering information on the behavior of light.
THE NATURALISTIC APPROACH TO NATURAL SCIENCE THROUGH MOTION AND MATTER THE most fundamental questions which are raised in modern science are concerned with matter and energy. which are said to be different aspects of the same reality. This reality is described in terms of particles and waves, and there is good experimental evidence for both. Particles can be recognized by their tracks in a photographic emulsion or in a cloud chamber, and they can be recorded with a Geiger counter. Diffraction and interference phenomena serve for the study of the continuous field or wave character of matter, which is manifested not only by light but also by electrons, neutrons and protons. Thus particles appear as waves and waves as particles: the two seem inseparable. Yet the difficulty of combining these two aspects of things in a unified picture is so great that it has been called the main stumbling block which prevents the formation of a stable and certain concept of matter. It has been suggested that the particles may be more or less temporary entities within the wave field whose form and behavior are determined by the laws of the waves themselves, which in certain cases permit waves to appear as if they were permanent substantial beings. Or perhaps the wa.ve field is merely a device for computing the probability of finding a particle of given properties at a given position. At any rate most theoreticians will probably admit that the individual particle is not a well-defined and permanent entity whose identity or sameness can be detected experimentally. In a word, the modern analysis of matter leaves us uncertain whether the world is a system o£ waves or particles or both. The only thing which seems certain is that there are no welldefined and permanent entities. This picture of physical reality may at first seem very dis219 WILLIAM H. KANE concerting and quite opposed to ordinary ways of thinking. For reassurance we may take hasty refuge in the common view that nothing is certain in science, nothing finaL New data will surely be uncovered, and new principles will be formulated. After all, science gives us only a tentative and partial view which is always subject to revision and revaluation. But the hypotheses of science are not opposed to previous valid findings. On the contrary, agreement with the data of experience is what validates a scientific principle. When new data exceed the limits of former principles, new hypotheses must be formulated which include as special cases the facts and correlations already established. The primary datum of our sensory experience is the fact of change in the world around us and in us. When we realize that we ourselves were born and will die; that we came into being and will cease to be in this world at least; that not only organisms of all kinds but also the chemical compounds are generated and corrupted, why is it surprising to be told by modern scientists that even the chemical atoms and the elementary particles of which they are composed are not permanent entities but transitory things? Perhaps it is because we had become accustomed to think of the atoms as indestructible building blocks out of which all bodies are made. Perhaps there is also a deeper reason for wonder. If everything is changing, how can there be anything even relatively permanent in the world? ·what are air and water, rocks and trees, animals and men? How can there even be change? Can there be a wave without something waving? How can anything come to be and cease to be in this world? To these questions there is no certain answer in modern science. Yet these are the very questions which the primary data of experience raise in our minds. If we desire a fundamental understanding of natural things we must account for change. We must explain in terms of its own proper principles what a changeable thing is and how it comes into being and ceases to be. It is not customary in modern science to look for basic prin- THE NATURALISTIC APPROACH TO NATURAL SCIENCE 221 ciples. Often we are told by philosophers of science and scientific methodology...
The idea of the photon has stirred the imaginations of physicists ever since 1905 when Einstein originally proposed the use of light quanta to explain the photoelectric effect. This concept is formalized in the quantum theory of radiation, which has had unfailing success in explaining the interaction of electromagnetic radiation with matter, seemingly limited only by the ability of physicists to perform the indicated calculations. Nevertheless, it has its conceptual problems—various infinities and frequent misinterpretations. Consequently an increasing number of workers are asking, “to what extent is the quantized field really necessary and useful?” In fact the experimental results of the photoelectric effect were explained by G. Wentzel in 1927 without the quantum theory of radiation. Similarly most electro-optic phenomena such as stimulated emission, reaction of the emitted field on the emitting atom, resonance fluorescence, and so on, do not require the quantization of the field for their explanation. As we will see, these processes can all be quantitatively explained and physically understood in terms of the semiclassical theory of the matter–field interaction in which the electric field is treated classically while the atoms obey the laws of quantum mechanics. The quantized field is fundamentally required for accurate descriptions of certain processes involving fluctuations in the electromagnetic field: for example, spontaneous emission, the Lamb shift, the anomalous magnetic moment of the electron, and certain aspects of blackbody radiation. (The Compton effect also fits here, but see later under references 8b and c.) Here we will outline how the photon concept originated and developed, where it is not required and is often misused, and finally where it plays an essential role in the understanding of physical phenomena. In our discussion we will attempt to give a logically consistent definition of the word “photon”—a statement far more necessary than one might think, for so many contradictory uses exist of this elusive beast. In particular consider the original coining of the word by G. N. Lewis: “[because it appears to spend] only a minute fraction of its existence as a carrier of radiant energy, while the rest of the time it remains an important structural element within the atom…, I therefore take the liberty of proposing for this hypothetical new atom which is not light but plays an essential part in every process of radiation, the name photon!” (our exclamation point). Clearly the present usage of the word is very different.
Some preliminaries We have, until now, thought of atoms as tiny hard spheres that feel a force of attraction towards one another. We ignored the fact that an atom consists of a nucleus surrounded by one or more electrons. If we stopped at this, we would make little progress towards understanding the properties of solids, other than that the atoms may form crystals. We must therefore go on and take a look at the internal structure of the atom itself, and see how this changes when the atom is surrounded by other atoms in a crystal. We shall then find that we need a description that is somewhat removed from our everyday experience. There is a name for this description, quantum mechanics : mechanics, because it deals with the motion of objects, and quantum, because features of the motion, such as energy, are quantized , meaning that they can only have any one of a set of distinct values, and no other value in between. Imagine for example a car which is so constructed that it can travel only at speeds of 10, or 50, or 100 km per hour, and not at any other speed. Then we could say that the speed of the car is quantized. This idea of quantization is something alien to our usual way of thinking. After all, a car is able to go at any speed we like up to its maximum speed.