Topics Physics

Question Quantum behaviour says that before a phenomenon is observed there may be a number of possible outcomes. Once observed, the number of possible outcomes becomes one; what actually happened? Surely the present moment consists of an infinity of phenomena which, with the benefit of Quantum hindsight, may be seen to have actually been certainties. Uncertainty exists only in the mind of an imperfect observer; there’s no such thing as foresight outside of a limited, dry mathematical framework. This leads me to think the following; i) That everything is as it is because it could not possibly have been any other way. ii) All the things in the universe whose extremely improbable existence we marvel at and things which everything else depend on who, if they were any other way, lots of other things wouldn’t work either, were actually (in retrospect), absolute certainties. Is this a gross misunderstanding of Quantum theory, an obvious conclusion, or a line of thinking with some mileage? I can see it leading in some interesting directions although I’d like to know if they’re theoretically blind alleys. M. Nicoll

Accepted:
December 12, 2005

Comments

The problem you raise in your first paragraph is called the measurement problem: What happens when a measurement takes place?

Most physicists would not agree with your statement that "Surely the present moment consists of an infinity of phenomena which, with the benefit of Quantum hindsight, may be seen to have actually been certainties." The way most physicists interpret quantum mechanics, the uncertainty about the outcome of a measurement is not "only in the mind of an imperfect observer," but rather in the world itself. For example, before you measure the position of a particle, it simply doesn't have a position. It's not just that we don't know its position, it's that it doesn't have a position.

This interpretation seems to be forced on us by experiments like the famous two-slit experiment. In this experiment, particles are fired at a barrier with two slits in it, and then their positions are recorded when they strike a screen behind the barrier. These positions form an interference pattern, similar to the interference pattern that would result from waves coming through the two slits and then interfering with each other. However, if you cover one of the slits, then the interference pattern goes away. If each particle went through one slit or the other, then you might expect the observed pattern to be a combination of the patterns you get with one slit or the other covered, but that's not what is actually observed. It seems as if each particle somehow goes through both slits--and therefore doesn't have a single precise position when it goes through the barrier. However, when it strikes the screen its position is measured and it is found to be at a particular position. Thus, on this interpretation of quantum mechanics, measurement is a mysterious process in which something that was indeterminate somehow becomes determinate.

However, there are other interpretations of quantum mechanics, sometimes called hidden variable interpretations, in which particles do have precise positions at all times. One such theory is David Bohm's theory. In this theory, the outcome of a measurement is predetermined, and uncertainty exists only in the mind of the observer. One advantage of this theory in my opinion is that it completely solves the measurement problem: A particle has a precise position at all times, and when you measure its position you find out where it is. There is no mysterious process of a position that was indeterminate becoming determinate. (The way Bohm's theory deals with the two-slit experiment is that the particle goes through only one slit, but there is also a wave that goes through both slits and then forms an interference pattern that guides the trajectory of the particle.)

Even in a deterministic theory like Bohm's, I'm not sure I would go along with your statement that "everything is as it is because it could not possibly have been any other way." In a deterministic theory the outcome of an experiment is determined by the initial conditions. The outcome could have been different if the initial conditions were different, so I wouldn't say that no other outcome was possible. It depends on what you mean by "possible", but if different initial conditions are considered to be possible, then different outcomes are possible as well.

A good reference for Bohm's theory: J. S. Bell, Speakable and unspeakable in quantum mechanics, Cambridge University Press, 1987.