Copenhagen Interpretation of Quantum Physics

Today we will get a basic overview of the Copenhagen interpretation of quantum mechanics.  Then Dr. Bencivenga will discuss a few lessons that can be taken from this interpretation.

The Copenhagen Interpretation.  The main idea is that only when an electron is observed can we fix a position or velocity of the electron.  When a body (such as an electron) is not observed, it is in a superposition, meaning that it has many values for any given parameter.  For example, a body may be in many positions or it may have different velocities.  Each velocity that a body has a weighted value that can be mathematically represented.  For example, an electron, E, may be 20% at location A, 20% at location B and 60% at location C.  It is only when the electron is observed that it becomes a matter of fact what the velocity of the electron is.  The electron will instantaneously and randomly shift from a state of superposition where there are many values for each vector to a state of determinacy where there is only one value for the vector that is being measured.  Before the electron's position is measured, it can be expressed by a formula expressing the probability that the electron is located at any given place.  A probabilistic notion of the mechanics of matter is just as causally necessary as a Newtonian view.

But one feature of quantum mechanics (QM) is that whenever one parameter, such as location, is measured, this means that other parameters, such as velocity, cannot be measured with as much accuracy. This is the indeterminacy principle, or uncertainty principle.  Dr. Bencivenga notes the difference between uncertainty and indeterminacy.  Uncertainty has to do with our knowledge of the world.  Indeterminacy, however, has to do with the way the world is.  It is one thing to claim that we are uncertain about the location of an electron.  Yet it is another thing to claim that the position of the electron is indeterminate.  Heisenberg himself endorsed this stronger, second notion.  He thought that until we measure the position or velocity of an electron, there is no matter of fact about the position or velocity.

Heisenberg then makes some philosophical conclusions based on this innovation in theoretical physics.

We used to think about the world as having some determinate, fixed state that we can learn about with science.  Even if our understanding is limited, we have often thought that there was some concrete, fixed, matter of fact about the world.  But quantum mechanics views a world that is always in a possible state and never in a fixed state until we encounter it. Probability thus viewed is no longer a measure of a degree of likelihood that something will happen.  In quantum mechanics, probability is used to express the potential within a body.

Another important feature of QM is the notion that observation itself changes the physical nature of the observed entity itself.  According to QM, it is only when the position is measured that there is a fact about the position.  Observing a body takes it out of a state of superposition and brings it into a fixed and determinate state.  There is a reaction between reality and the observer.  Quite literally, observers change reality when they observe it.  Scientists are thus not just observers of the natural world, but they are players in nature who take part in making the thing that they are trying to observe.

QM has been criticized by modern scientists.  Einstein, for example, was unhappy with the random view of nature presented by QM.  He thus added to the theory of QM in an attempt to reduce indeterminacy to uncertainty.  He thought that if we could take certain hidden factors into consideration, then we could have a complete and fixed image of nature.  However, Dr. Bencivenga notes that there is not a single set of hidden variables that can be applied consistently in order to complement QM.