wave-particle duality (When Is a Particle Not a Particle?)

wave-particle duality (When Is a Particle Not a Particle?)

In describing the quantum world, we must abandon the classical notion of a particle as an indivisible mass point with an independent existence and well-defined, measurable extrinsic properties. Whatever microscopic entities are, they are certainly not indivisible mass points. We can verify this assertion in the laboratory. In suitably designed experiments quantum "particles" act like classical particles-appearing, for example, as spots on a screen. But in other experiments their transit through space is like that of a wave, manifesting behavior such as diffraction and interference, as though they were diffuse wave fronts. This apparently contradictory behavior is a manifestation of the wave-particle duality that characterizes the domain where quanta dwell.

we'll see why it poses such a challenge to the student of quantum physics: it renders useless the conventional mental images with which we visualize physical phenomena. Thus, an atomic electron is not a tiny "planet" orbiting a "nuclear sun," as in the Rutherford and Bohr models of the atom. But neither is it a "fuzzy" thing, smeared out over a region of space, as it is portrayed in many introductory physics texts. The electron is something else, neither particle nor wave but eerily reminiscent of both.

The dual nature of subatomic particles subverts the classical concept of a particle. But the true nature of microscopic entities is even more nebulous than is implied by wave-particle duality, for the properties of quantum particles are not, in general, well defined until they are measured. In a sense, the physical properties of electrons, protons, and the like are "potential" or valent properties until an experimenter-a macroscopic being-performs a measurement.

You will encounter this disquieting aspect of quantum mechanics if you ask a quantum physicist to predict the value you would obtain were you to measure, say, the position of an electron in a metal. He cannot give you a definite answer, even if he knows fully the state of the electron just prior to the proposed measurement. The inability of quantum theory to provide precise answers to such simple questions is not a deficiency of the theory; rather it is a reflection of its essential nature. We can see this if we look at how quantum mechanics specifies the state of a particle.

Unlike a classical state, a quantum state is a conglomeration of several possible outcomes of measurement of physical properties. At most, quantum physicists can tell you only the possible outcomes and the probability that you will obtain one or another of them. Quantum mechanics is expressed in the language of probabilities, not certainties. It is inherently statistical in nature, describing not definite results of a measurement on an individual system, but rather possible results of measurements on a large number of identical systems. What, then, controls what actually happens in a particular measurement--e.g., which of the possible values of position a particle exhibits when we measure this quantity? Random chance.

Of course, were we to carry out a position measurement on a single particle, we would get a single value. So immediately after the measurement, we can meaningfully talk about the position of the particle-its position is the number we got in the measurement. But what about immediately before the measurement? According to quantum mechanics, the particle does not then have a position. Rather, its position prior to measurement is latent-a mere possibility, waiting to be made actual.

Thus, by the act of measurement, we change the state of the particle from one in which it is characterized by a plethora of possible positions to one in which it has a single, well-defined position. Clearly, measurement will play a more vital role in quantum physics than in classical physics. When we study the microworld, experimentation is not just a way of discovering the nature of external reality, but rather is a way of creating certain aspects of reality! In contrast to the assumptions of classical physics, an observer cannot observe a microscopic system without altering some of its properties.

Intrinsically indeterminate interactions between the observer and the observed are an inevitable feature of the quantum universe, one that applies not just to position, but to all properties of microscopic particles. Some physicists believe that we macroscopic observers "create" the microscopic building blocks of the universe by actualizing via measurement their various physical properties.

This interaction is unavoidable: the effect of the observer on the observed cannot be reduced to zero, in principle or in practice. This fact, which is reflected in the mathematical structure of quantum theory and has been verified by countless experiments, demolishes the philosophical notion of an objective universe, the idea that what we study in physics is necessarily a "real world" external to and independent of our perceptions.

The source:

Michael A. Morrison - Understanding Quantum Physics.

By. Fady Tarek