Nature of Quantum Theory
The Descent into the Quantum World
Suppose one were to ask for a scientific description of your hand. Biology could describe it in terms of skin, bone, muscles, nerves, blood etc., and this might seem a completely satisfactory description. However, if you were just a bit more curious, you might ask what the muscle and blood etc. were made of. Here you would descend to a chemical explanation in terms of molecules of protein, water etc. and the reactions and relations between these. If you were still not satisfied with this, you would have to descend into the quantum world. At this level, the solidity and continuity of matter dissolves. The molecules of protein etc. are made up of atoms, but the atoms themselves are mainly vacuum. Most of the mass of the atom lies in a small nucleus, comprised of protons and neutrons, which are themselves made up of smaller particles known as quarks. The rest of the mass of the atom resides in a cloud of electrons orbiting around the nucleus.
The fundamental particles are bound together by the four forces of nature, which are gravity, electromagnetism and the strong and the weak nuclear forces. The strong nuclear force binds together the particles in the nucleus of the atom, and acts only over the very short range of the nucleus itself. Gravity is a long-range force that mediates the mutual attraction of all objects possessing mass. The electromagnetic force is perhaps the force most apparent in everyday life. We are familiar with it in the form of light, micowaves and X-rays. It holds together the atom through the attraction of the opposite electrical charges of the electron and the proton. It also governs the interactions between molecules. Van der Waals forces, a weak form of the electromagnetic force is vital to the conformation of protein and thus to the process of life itself. In contrast to the nuclear forces, gravity and elecromagnetism are conceived of as extending over infinite distance, but with their strength diminishing according to the inverse square law. That is, if you double your distance from an object, its gravitational attraction will be four times as weak. The quanta can be divided into two main classes, the fermions, which possess mass and the bosons which convey energy or the forces of nature. The most fundamental fermions are the quarks making up the nucleus and the circling electrons, while gluons and photons are the most prominent bosons. The gravitons, which may intermediate the gravitational force remain hypothetical.
The Quantum Wave
The quantum particles or quanta are unlike any particles or objects that are encountered in the large scale world. When isolated from their environment they are conceived as having the property of waves, but when they are brought into contact with the environment, there is a process of decoherence, in which the wave function is described as collapsing into a particle. The wave form of the quanta is different from waves in matter in the large scale world, such as the familiar waves in the sea. These involve energy passing through matter. By contrast, the quantum wave can be viewed as a wave of the probability for finding a particle in a specific position. This probability wave also applies to states of the quanta such as momentum. While the quanta remains in its wave form, it is viewed as a superposition of all the possible positions that the particle could occupy. At the peak of the wave, where the amplititude is greatest, there is the highest probability of finding a particle, when the wave eventually collapses. However, the choice of position for each individual particle is completely random, representing an effect without a cause. This comprises the first serious conceptual problem in quantum theory.
The Two-Slit Experiment
The physicist Richard Feynman said that this classic experiement contained all the problems of quantum theory. In the early nineteenth century, an experiment by Thomas Young showed that when a light source shone through two slits in a screen, and then onto a further screen, then a pattern of light and dark bands appeared on a further screen, indicating that the light was in some places intensified and in other reduced or eliminated. Where two waves of ordinary matter, for instances waves in water, come into contact an interference pattern forms, by which the waves are either doubled in size or cancelled out. This appearance of this phenomenon in Young’s experiment demonstrated that light was a wave, contrary to most scientific opinion prior to the experiment.
Later, the experiment was refined. It could now be performed with one or two slits open. If there was only one slit open, the photons or light quanta, or any other quanta used in the experiment behaved like particles. They passed through the one open slit, interacted with the screen beyond and left an accumulation of marks on that screen, signifying a shower of particles rather than a wave. But once the second slit was opened, the traditional interference pattern, indicating interaction between two waves, reappeared on the screen. The ability to generate the behaviour of either particles or waves, simply according to how the experiment was set up, showed that the quanta had a perplexing wave/particle duality.
The wave/particle duality was shocking enough, but there was worse to come. Technology advanced to the point where photons could be emitted one at a time, and therefore impacted the screen one at a time. What is remarkable is that with two slits open, but the photons impacting one at a time, the pattern on the screen formed itself into the light and dark bands of an interference pattern. Somehow the photons ‘knew’ to arrange themselves into a pattern indicative on the interaction of waves. The question arose as to how the photons emitted later in time ‘knew’ how to arrange themselves relative to the earlier photons in such a way that there was a pattern of light and dark bands, indicative of interacting waves.
The obvious solution was to place photon counters at the two slits in order to monitor what the photons were up to. However, as soon as a photon is registered by a counter, it collapses from being a wave into being a particle, and the wave related interference pattern is lost from the further screen. The most plausible way to look at it may be to say that the wave of the photon passes through both slits, or possibly that it tries out both routes, and after doing this the divided wave interfers with itself.
The EPR Experiment and the Copenhagen Interpretation
Einstein disliked the inherent randomness involved in the collapse of the wave function. This was despite the fact that his revival of the idea of light in the form of discrete particles or quanta had contributed to the foundation of quantum theory. He sought repeatedly to show that quantum theory was flawed, and in 1935 he seemed to have produced a masterstroke in the form of the EPR (Einstein, Podolsky, Rosen) experiment. At the time this was only a ‘thought experiement’, a mental simulation of how a real experiment might proceed, but since 1982 it has been possible to perform this as a real experiment.
The challenge to quantum theory presented by the EPR experiment hinges on the concepts of locality and non-locality. Locality comprises the idea of normal cause and effect under which objects or particles move or change as a result of being impacted by other objects or particles, or of being directly acted on by energetic forces such as the electromagnetic force. It is local because the object or force producing the action or change has to be in direct contact with the object or particle acted on. Moreover, where a force emitted by one object acts on another distant object such as light emitted from the Sun acting on the Earth, the force passes between the two objects at a speed not greater than that of light. By contrast, non-locality involves the ability of one particle to determine the behaviour of another distant particle instantaneously, and without any matter or energy passing between the two. Einstein termed this ‘spooky action at a distance’.
With the EPR experiment it was shown, that as it stood, quantum theory violated the principle of locality, which is normally regarded as basic to scientific thinking and even to common sense. Quantum theory indicated that when two quanta had been closely related to one another, for instance in the same electron orbital, they could be regarded as quantum entangled. In this state, certain aspects of their behaviour in relation to one another became fixed. For instance, quantum particles have a property of spin, which is partly analgous to the spinning of large-scale objects. Quanta can have the property of spin-up or spin-down. In an entangled state particles could have the relationship that when one had spin up, the other would always have spin down. However, as quanta, while they remained in a wave form, they both represented a superposition of spin-up and spin-down, and therefore neither of them had a defined spin.
The EPR experiment proposed that two such wave-form particles are moved apart. This could be a few metres along a laboratory bench or to the other side of the universe. The relevant consideration is that the two locations should be out-of-range of a signal travelling at the speed of light. Now, if an observation is made on one of the particles, its wave function collapses, and it acquires a defined spin, let’s say spin-up in this case. Now when an observation is made on the other particle, it will always be found to have the opposite spin. This defies the normal expectation of classical physics that a random choice of spin would produce approximately 50% the same spin and 50% different. Therefore, there is seen to be some non-local connection between the two particles, although it is not possible to describe or detect this in terms of a normal physical transfer of energy or matter. This non-locality and the randomness of the outcome of the wave function collapse constitute the two main puzzles in quantum theory.
Copenhagen after the EPR Experiment: – ‘There is no quantum world, there is no deep reality’
The idea of non-locality, which appeared to deny much of what the science of the previous three hundred years had been trying to extablish, was as repugnant to the leaders of the quantum movement, such as Neils Bohr, as it was to Einstein as an opponent of quantum indeterminism. Some modern analysis suggests that Bohr changed his own view of the quantum world in a crucial manner after encountering the EPR challenge. Bohr’s interpretation is known as the Copenhagen Interpretation, and the form of this that emerged after 1935 essentially denied the objective existence or reality of the quantum wave. Bohr said that there was no quantum world, there was no deep reality. The quanta only achieved objective reality when they were the subject of an experiment or observation.
The concept of reality or objective existence is here taken to mean that something exists even when it is not being observed by anyone. The Copenhagen Interpretation denies that sort of reality to the wave form of the quanta. The wave was to be seen only as an abstract mathematical expression allowing one to predict the probable position of a particle. If the wave form had no real existence, EPR type situations did not involve any physical action at a distance, and the problem could be deemed to have gone away.
The Aspect Experiment
The question returned to the fore in the 1980’s as technology overtook the orginal EPR thought experiement. In 1969 John Bell’s Theorem had shown mathematically how EPR could be tested, and in 1982 Alain Aspect’s experiment demonstrated the physical reality of EPR. The Aspect experiment did not invalidate Copenhagen, but it transferred the whole debate fom the hypothetical to the scientifically tested level. It presented physics with a stark choice. Either one could accept the Copenhagen Interpretation in which the locality of interactions was preserved, but the components of matter and energy were unreal, or one could have a world that was real, but in part governed by non-local influences, Einstein’s dreaded ‘spooky action at a distance’.
In fact, recent decades have seen a growing challenge to the orthodoxy of Copenhagen. This leaves us without a generally agreed interpretation of quantum theory. The Copenhagan Interpretation preserved us from non-locality, but the concept of the quanta as mathematical abstractions that suddenly produced physical particles may be viewed as troubling. It seems to propose a sort of dualism, comparable to the relationship between spirit stuff and physical stuff. How could mental constructs, such as mathematical formula, become physcial without having had some physical reality in the first place.
Other interpretations have come more to the fore in recent decades. Decoherence has become particularly popular as a substitute for the traditional ‘measurement’ always referred to in the Copenhagen version. In decoherence, the collapse of the wave function happens of its own accord, as a result of the wave becoming entangled with the rest of the environment. In some recent versions, it is suggested that there is no collapse, the information in the wave simply gets lost in the larger scale environment. In some quarters, this is argued to provide a connection to the ‘Many Worlds’ interpretation. In this, there is also no collapse, but a branching of reality into separate universes. So in the Schrodinger cat paradox, for instance, the universe splits into one universe with a live cat and one with a dead cat.
Quantum Gravity & the Search for Reality
The success of quantum theory, which describes matter and energy, and of relativity, which describes space and time, have both been marred by the incompatibility of these two key theories. Relativity describes gravity as the smooth, continuous curvature of space under the influence of massive objects, while quantum theory is based on the idea of energy and matter coming in discreet discontinous units. Mathematically these contrasting features lead to infinities, indicating that something is wrong. The attempt to overcome these problems has led to new theories, such as string theory, and loop quantum gravity.
String theory proposes that the fundamental particles are not point particles, as had been assumed, but one-dimensional strings extending into higher dimensions, beyond the normal four dimensions. The extra dimensions are usually deemed to have been rolled up very small in the Big Bang, which accounts for them never having been detected. The manner in which the strings vibrate determines the nature of the particle involved. The analogy is that of the strings of a violin, where the vibration of the string determines the nature of the note. While this may appear both speculative and improbable, it has the advantage of being described by mathematics that would allow quantum theory and relativity to be compatible.
The two main criticisms of string theory are that it produces 10^500 possible universes, and that it operates against the background of a fixed spacetime, a concept that relativity showed to be invalid. An alternative approach is provided by loop quantum gravity (LQG). This approaches the problem from the direction of relativity and concepts of spacetime, in contrast to string theory, which approaches from the the point of view of particles and quantum theory.
LQG proposes that spacetime is quantised or in discrete units. Spacetime is suggested to be created out of a network, or a lattice, or a series of loops. This theory has drawn on the earlier spin network theory developed by Roger Penrose, and moves towards viewing particles and spacetime as dual aspects of the same thing.
Problems and Opportunities in Quantum Theory
We have emphasised three problematic aspects of the theory, a causality in the randomness of the wave function collapse, a causality in the non-local influences demonstrated by EPR type experiments, and the resulting lack of agreement as to the underlying reality of the physical universe. At the quantum level, we find properties of mass, charge and spin that are given properties of the universe lacking cause or explanation. If we ask, what is the charge on the electron, what is it, not what does it do, the answer will be a resounding silence. The quanta and related spacetime appear to be the only level of the physical universe where it might be possible for science to insert consciousness as an additional fundamental property.