Abstract: The impact of quantum mechanics on the 20th century philosophy of physics is well known. Although the theory of relativity transformed our conception of the nature of space-time, it was quantum mechanics which was revolutionary in the sense that it pulled the rug off the realist�s feet. But has it really done so? Why is QM considered the antirealist�s strong point? In this paper, we would examine the major features of QM that supposedly pose immense problems to realism. We would also try to suggest ways on how the realist could get around these problems.
In this essay, I'll first try to give a brief historical sketch of quantum theory and I will then turn to the problem of realism. So let�s start.
Historical background
The Story of the Quanta:
Since Newtonian times, �matter� was identified with particles and any form of radiation was considered as waves. This idea was the main problematic in explaining the blackbody radiation, the �first problem� in quantum mechanics and one of Lord Kelvin�s �two clouds on the horizon of twentieth century physics� . The atomic model as suggested by Rutherford could not explain the findings of the discrete spectra of electromagnetic radiation of all elements. In 1900 Max Planck used a collection of radiating harmonic oscillators in thermal equilibrium to produce blackbody radiation. Using statistical mechanical techniques, Planck postulated that the oscillators could emit and absorb energy only in discrete amounts or quanta. The energy of the quantum is proportional to the frequency of the radiation and is expressed as E=h?, where E is the energy, h is the Planck�s constant and ? is the frequency of the radiation. This formula fitted into Planck�s data but Planck himself could not provide a theoretical justification for such a move . He seems to have been inspired by Boltzmann and the interrelation of entropy and probability (Cushing 1998).
At this stage, with Max Planck, the first problem of QM enters twentieth century physics � radiation, defined as �packets� of energy or quantum, thus Planck�s constant laid the foundations of quantum mechanics.
Einstein�s Corpuscular theory of light:
In 1905 Einstein pointed out that in an interaction of light and matter, if light is made incident on a metallic plate, the electrons of the metal could be knocked off depending on the frequency and intensity of the light used. This suggested that energy in light could only exist in discrete packets or �photons�, so light seems to have a particle aspect to it. Light was generally considered to have wavelike properties. Einstein�s suggestion of the particle aspect of light, inspired de Broglie who opined that light can be both wave and particle aspects. According to de Broglie, the particles which make up an atom would, under certain conditions show wave phenomena such a diffraction and interference and will not only have frequency but also as in light energy, have a wavelength. This wavelength ? is inversely proportional to a particle�s momentum p. Thus the de Broglie wavelength is given by
? = h/mv =h/p
With de Broglie�s bold conjecture, the second problematic - wave particle duality enters quantum mechanics.
Schr�dinger�s Wave Equation:
The discovery of wave like behaviour of particles, gave rise to the necessity of a wave theory describing the behaviour of matter on the atomic scale. Erwin Schr�dinger postulated a wave equation to mathematically describe the electron�s behaviour in space-time. The Schr�dinger wave equation is written in the form
?(??/ ?x) / ?x + 8 ?2 m /h2 (E � v)? = 0
here E is the total energy of the particle, v is its potential energy, m is its mass and h is the Planck�s constant. ? is the wave function, the square of which relate to the probability of finding an electron at a particular point. The probability of finding a particle in a volume of space is given by
P = |?|2 ?v
Thus Schr�dinger attempted to describe not only the wave associated with a free electron but also tried to find the probability of where an electron might be located. Yet from the de Broglie wave equation, Werner Heisenberg derived the famous uncertainty relations
?x ?p ? h
?x represents the uncertainty in position of a particle and ?p represents the uncertainty of momentum. Heisenberg�s uncertainty principle limits the accuracy of our knowledge or measurement of simultaneous values of position and linear momentum of a particle or wave. W cannot measure both the position and the momentum of a particle simultaneously with precision. Any attempt to measure the position of a particle will alter its momentum .
The third bone of contention enters quantum mechanics � uncertainty leading to problems of indeterminacy.
Thus we have historically sketched the three major problems for the realist which QM brought into physics along with its development
1. Discontinuity � Problems of Causality
2. Wave-particle duality � The measurement problem
3. Uncertainty � the problem of indeterminacy
Quantum Mechanics and Realism:
Although we have identified three problems for the realist, the problems give rise to associated problems as we have mentioned.
Discontinuity:
The foundations of realism is based on the fact that 1. Objects exist 2. Objects exist independent of us in the external world 3.We have some direct access to reality.
In the classical sense, the existence of objects would mean existence in a continuous manner. Although classical mechanics could explain discontinuous change, a causal relation was always maintained. A small change in one body would cause a proportional change in the other. Powers writes that particle mechanics cannot be taken to give the true picture of the physical world. He discusses field theory to show that when particles are surrounded by fields, the problem of discontinuity and �sharp� collision could be avoided (1982). Thus the concept of field meant that energy could spread though space and the particles could also carry energy in a localised manner. To the realist, discontinuity brought in the problem of causality as cause and effect is a continuous sequence of events. In classical mechanics knowing initial conditions, one could always predict the outcome of a system according to Newtonian equations. Prediction is a tool of the realist and classical mechanics allowed it. In so far as in QM the realist could not explain that objects exist continuously and could not explain the gaps in the states of the objects, realism was in a problem with its first premise.
Wave-particle (WP) duality �
The problem of Schr�dinger�s cat and the measurement problem is a direct fallout of the W-P duality. The realist claim 2 � the world exists independent of our judgment is knocked down by an argument given to explain the wave particle duality by Bohr. In the 1920s, Niels Bohr developed, along with his disciples Heisenberg and Pauli, what is known as the Copenhagen interpretation of QM where he claimed that the wave and the particle aspects of matter are complementary aspects depending on the experimental situation. This principle known as complementarity suggests that we can measure the wave aspects or the particle aspects of an electron if we set up the experimental situation accordingly. Under no circumstances can we get both the wave and the particle aspects together. Thus the electron is either a wave or a particle depending on how we choose to see it. Bohr also leaned on the Heisenberg uncertainty relations where we can either measure the position or momentum, but never both according to the experimental set up.
So Bohr�s suggestion is often interpreted as a clear statement against realism, the world doesn�t exist independent of our measurement forcing Einstein to say �Does the moon exist only when we look at it�? This also had implications for measurement for using the Schr�dinger wave equation, the wave function collapses when we perform a measurement or carry out an observation. The wave-particle duality is only explained (?) by using the collapse postulate leading to a bigger problem � the measurement problem.
Uncertainty principle (UP):
The realist�s last claim � we can know certain aspects of reality with accuracy - has been shaken out by Heisenberg�s uncertainty principle which shows that it is in principle impossible to measure the momentum and position of an electron with accuracy. In classical mechanics when we deal with macroscopic particles it is possible and even in fact necessary to know both the position and the momentum of a particle to predict its subsequent motion. By showing that in microscopic situation it is impossible to do so, UP brings in indeterminacy in QM and also in realism � we cannot know certain absolutely important aspects of reality with arbitrary accuracy - measuring one aspect precludes the measurement of another.
A serious epistemic constraint seems to have dug up the philosophical grave for the realist�but does the realist step into it?
The Realist�s trick for Survival:
QM has brought in great problems for realism and realism stands in a conceptual dilemma. Realism in the quantum era can survive only if it loosens its adherence to traditional premises. There are several routes suggested - the realist claim that objects exist can be done away with (Ladyman, 1998), the realist can hold onto structures instead of objects (French, 2003) or relations between structures (Russell, 1954). That objects exist independently, the basic premise of realism, can also be moulded in a different way - Bohr can be considered as a context-dependent realist � a realist giving emphasis to wholeness or interconnectedness of mind-matter. A science of wholeness does not have to abandon realism . Thus the independence of objects can be abandoned to a certain extent, yet that does not necessitate the surrender of realism.
The epistemic question - whether we can know certain aspects of reality can be answered as - yes we can, but there are limits to knowledge which the realist will have to accept. Here realism has to change its basic postulates; reality in its microstate might be different from reality in its macrostate. Although the laws of nature are same in every level, the limitations of knowledge are different. The realist can survive only by changing his concepts with the changing demands of the new physics and yet find solace in the conviction that even if God does play dice , He keeps the underlying reality of all (or some things) unchanged.
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