(©Robert DiSalle 2001)
Einstein’s theories of special and general relativity were parts of a larger picture of the physical world that reflected Einstein’s general philosophical views. Increasingly, in the 1920's and 1930's, Einstein, and other physicists who shared his views, felt uneasy about the growing success of quantum mechanics, which seemed to contradict many of his fundamental assumptions.
Einstein’s view of the physical universe was quite complex, but its basic principles include the following.
1. Matter and energy in spacetime are continuous “fields”: energy varies continuously from point to point. The notion of a field goes back to Newton’s idea of gravity as a “power” of a massive body that is “propagated to the spaces around it,” to accelerate whatever bodies are there. This means that if you are at distance r from the earth you would feel a certain gravitational acceleration; at a distance r + △r, you would feel slightly less, and the accelerative force would vary continuously with the distance. Einstein’s idea of a field is based on the nineteenth-century electromagnetic field theory; it differs from Newton in that fields are dynamical, so that energy propagates through them in a wavelike motion. According to Newton, a change in the mass would instantly change the gravitational field everywhere; according to Einstein, a change would propagate outward at a finite speed. The continuity means that the strength of the field can take any real value, and in increasing, say, from zero to one, must pass through all the real numbers between zero and one.
2. Physical systems are “separable”: we can understand the behavior of one without any reference to others that are spatially distant from it. This means that a physical system can be isolated, in the sense that it can’t be affected by anything except the events in its own past. If something is happening right now in another galaxy, it cannot now affect our galaxy, and can only affect it once it becomes part of our “causal past,” i.e. whenever a signal from that event can reach us, sometime in the future. Therefore I can predict everything about a system by knowing what has happened in its past.
3. The laws of physics are comprehensible by human beings, and independent of human beings. The point of physics, according to Einstein, is to represent the nature of reality by constructing scientific concepts. This assumes that reality has an independent nature, unaffected by us and our capacity to know about it.
4. The physical world is deterministic: knowledge of its state at one time, and the laws of physics, should enable us to deduce with certainty all of its future states. Einstein summed this up in the remark, “God does not play dice with the universe!” In classical physics, we often find that we cannot predict the future precisely, but have to give probabilities for events occurring. (For example, weather prediction is always probabilistic.) We assume, however, that this is because of our ignorance; there are too many factors involved for us to make an accurate prediction. But if someone knew all of the relevant conditions, she could predict with certainty. Dice games are probabilistic, because innumerable small factors affect how the dice will land. But someone (e.g. God) who could know all of these factors could, in principle, predict the outcome of the roll of dice exactly.
5. “Unified field theory”: To the extent that quantum mechanics disagrees with the foregoing principles, it is “incomplete.” It will be completed only when quantum phenomena, i.e. the laws governing fundamental particles, are united with gravity in a continuous classical field theory.
Quantum mechanics, or the “quantum field theory” that has extended quantum principles to the electromagnetic and nuclear forces, offers a different view of the universe, one whose principles can be contrasted directly with Einstein’s:
1. Matter and energy both are distributed in discrete “quanta,” and levels of energy are separated by “jumps”. “Planck’s constant” represents a minimal amount of energy that cannot be subdivided. Thus every field, including the electric and magnetic fields, will behave as if divided into particles. Einstein’s notion of a continuous field does not appear to apply.
2. Physical systems can be “holistic” or “entangled”. A system of particles may be spread out indefinitely in space, and yet behave in an interdependent manner, i.e. their behaviour can be “correlated” even though there is no possible transmission of causal influence. No matter how far apart we are, what I measure on a part of this system can be affected by what you are doing right now.
3. Physical phenomena can be dependent on which experiments the observer decides to do. For example, if I choose a certain experiment, an electron will behave like a particle. If I choose another, it will behave like a wave. If I set my detector at a certain angle and orientation, what you measure at your detector may be affected. Many have interpreted this to mean that any complete description of a system has to include the observer and measuring apparatus.
4. The physical world is indeterministic: physics can give no more than probability distributions for future states. This means that the statistical character of our predictions has nothing to do with our ignorance. Even with a maximal amount of information, our predictions can only be probabilities. Nature is not merely too complicated for precise prediction; it is, at bottom, indeterministic.
5. “Quantum gravity”: General relativity is only approximately correct, and must be replaced by a quantum theory. That is, contrary to Einstein’s expectations, gravity must be understood within the quantum framework, instead of the other way round. This implies that Einstein’s theory is approximately correct for large scales and low energies, but at higher energies or smaller scales it must break down, and these situations supposedly will reveal the quantum effects in the gravitational field. It may even turn out that space and time themselves are not continuous.