Neither particle nor wave The information about the particle behaviour and the wave behaviour of light, presented in the preceding chapter, forms an extremely peculiar combination when taken together. And yet both are true: the experiments have been done as described, with the outcome as stated. There are experiments that tell us clearly that light consists of waves; there are others that tell us, equally clearly, that light consists of particles. Somehow we must find a description of light that incorporates both kinds of behaviour. This is a formidable undertaking, but there is one cheerful fact: in absolutely none of the experiments that have ever been done did the particle and the wave behaviours occur simultaneously . Because this is a fact that covers the entire history of physics, we feel justified in assuming that this is a general feature of Nature, and that there cannot ever be an experiment in which the wave and particle behaviours both occur at the same time. This is a generalization, for we can never expect to do all possible experiments. This generalizing hypothesis was first announced by Bohr, who had the wonderful combination of daring and insight to postulate that the observation of certain quantities excludes the simultaneous observation of certain others.
Neither particle nor wave The information about the particle behaviour and the wave behaviour of light, presented in the preceding chapter, forms an extremely peculiar combination when taken together. And yet both are true: the experiments have been done as described, with the outcome as stated. There are experiments that tell us clearly that light consists of waves; there are others that tell us, equally clearly, that light consists of particles. Somehow we must find a description of light that incorporates both kinds of behaviour. This is a formidable undertaking, but there is one cheerful fact: in absolutely none of the experiments that have ever been done did the particle and the wave behaviours occur simultaneously . Because this is a fact that covers the entire history of physics, we feel justified in assuming that this is a general feature of Nature, and that there cannot ever be an experiment in which the wave and particle behaviours both occur at the same time. This is a generalization, for we can never expect to do all possible experiments. This generalizing hypothesis was first announced by Bohr, who had the wonderful combination of daring and insight to postulate that the observation of certain quantities excludes the simultaneous observation of certain others.
Arguments about particles or waves gave way to a recognized need for both particles and waves in the description of radiation. The confusion over particle versus wave properties was resolved by Louis de Broglie. He postulated that since light has a dual wave–particle nature, matter also has this nature. The idea was to perform a diffraction experiment with electrons. An interference pattern was demonstrated not only with electrons, but also with many kinds of material particles like protons and neutrons. However, any attempt to observe an interference pattern with neutral atoms was found difficult by the fact that atoms carried no charge as electrons do and could not penetrate through condensate matter like neutrons. The de Broglie wavelength of an electron in the smallest orbit turns out to be exactly equal to the circumference of the orbit predicted by Bohr. The interpretation of the interference phenomenon becomes more subtle when the light beams are replaced by individual photons or particles.
At the beginning of the twentieth century, it became clear that microscopic particles do not obey Newton's laws of classical mechanics. The idea of light quanta was at odds with the established wisdom that light is a wave phenomenon, of course, but no other explanation of the photoelectric effect seemed possible. The next step in the development of quantum mechanics was taken by Niels Bohr, who introduced postulates about the discreteness of the energy of the bound electrons in an atom similar to the discreteness of the energy of light quanta. Modern quantum mechanics was founded by Werner Heisenberg, Max Born, Erwin Schroedinger, Paul Adrian Maurice Dirac, Wolfgang Pauli, Enrico Fermi, and others. In quantum mechanics, it is in general fundamentally impossible to determine the exact outcome of an experiment; one can only determine the probability of any given outcome.
Having just described electromagnetic radiation as a wave, it may seem a bit peculiar to embark on a particle description of that radiation. Perhaps it will help to recall from Chapter 2 that our perception of something depends in part on the means we use to perceive it and that we used a specific experiment (interference) to define the wave property. There is no reason to believe that one human-invented experiment is the only one that can be used to describe electromagnetic radiation. In fact, the Einstein photoelectric effect, described next, is consistent with light being a particle.
In quantum mechanics, light cannot be described in terms of waves and particles at the same time. However, the question about light "as it really is" implies a description that is independent from the constraints of measurement. It is rather preferable to incorporate both aspects in a causal ontological theory, especially if it simplifies explanation and improves predictions. Hence, the photon can be described as an extended longitudinal association of discrete indivisible constituents, which are held together by the effect of their own waves on each other. The same waves can then be used to interpret the force of interactions between different pulses of light, or between light and matter. The concept of wavemediated interaction between sub-photonic entities is entirely consistent with the observations of quantum mechanics. The key is to assume that elementary sources of waves never collide, and that they have constant parameters. Novel experimental implications follow from this approach. For example, the speed of propagation of changes within magnetic or electrostatic fields may not be equal to the speed of light. Also, interference fringes should be detectable beyond the volume of overlap of two non-entangled beams, but only in special experimental settings.
An experiment with electrons The preceding chapters described the quantum behaviour of Nature in a general way. In order to show quantum uncertainty at work, it is useful to look at an actual experiment in detail. This demonstrates the practical consequences of the particle–wave duality. To keep things uncluttered, the experiment is only an idealized example, but it will serve to show when quantum interference occurs (wave behaviour) and when it does not (particle behaviour). I will try to demonstrate the intimate connection between the properties of waves and linear superposition, and the consequent uncertainty in the motion of quanta. For once I will try to be precise, so this exposition will be rather more technical than elsewhere. Those who wish to do so, may skip to Chapter 6 without much damage to their education. Before we begin, let me repeat the warning about the generality of analyses such as the one to be given here: I speak about ‘experiments’ or ‘observations’ only in order to make a connection with what a real person could do in a real laboratory, and not because the particle–wave duality is only a laboratory phenomenon that requires a conscious observer. Even though an electron does not wear a lab coat, it is subject to the uncertainty relations when it ‘observes’ other quanta.
In the double slit experiment - - central to quantum mechanics and thus to all of reality - - when you LOOK at the particle, it always passes through ONLY ONE slit - - EITHER the right slit OR the left slit, acting like a particle. However, when you AREN’T looking at it, it somehow always passes through BOTH slits - - the right slit AND the left slit - - at the same time, acting like a wave. All of nature is now KNOWN to be both wave and particle simultaneously - - actually, one entity (“wavicle”), not two - - at the most fundamental level of reality. However, no one seems to have ever asked the question: How can anything, much less everything, be in TW0 places at the same time AND ALSO SIMULTANEOUSLY be in ONLY ONE place at the same time? The known laws of physics make this absolutely impossible. Therefore, at least one of our laws must be wrong. In some fundamental mathematical and physical sense, 2 MUST ALSO BE (- - EQUAL- -) 1: 2 AS 1. The right and left slits, seemingly opposite, must at the deepest level of reality be “married” to each other - - one slit, not two. Extending this argument to Schrodinger’s cat, life and death, seemingly opposite, must at the deepest level of reality be “married” to each other, 2 AS 1. Just as all apparent opposites, it must be true that ultimately, life and death must be one thing, not two. This, then, is an all-optimistic physics - - probably the first such theory ever. But it is based entirely on firm scientific fact.